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Transcript
Indiana University of Pennsylvania
Knowledge Repository @ IUP
Theses and Dissertations
5-2015
Industrial Zinc Plating Processes
Christopher Eric Sierka
Indiana University of Pennsylvania
Follow this and additional works at: http://knowledge.library.iup.edu/etd
Recommended Citation
Sierka, Christopher Eric, "Industrial Zinc Plating Processes" (2015). Theses and Dissertations. Paper 1283.
This Thesis is brought to you for free and open access by Knowledge Repository @ IUP. It has been accepted for inclusion in Theses and Dissertations
by an authorized administrator of Knowledge Repository @ IUP. For more information, please contact cclouser@iup.edu.
INDUSTRIAL ZINC PLATING PROCESSES
A Thesis
Submitted to the School of Graduates Studies and Research
in Partial Fulfillment of the
Requirements for the Degree
Master of Science
Christopher Eric Sierka
Indiana University of Pennsylvania
May 2015
Indiana University of Pennsylvania
School of Graduate Studies and Research
Department of Chemistry
We hereby approve the thesis of
Christopher Eric Sierka
Candidate for the degree of Master of Science
___________________
____________________________________
Keith Kyler, Ph.D.
Professor of Chemistry, Advisor
____________________________________
Jonathan Southard, Ph.D.
Professor of Biochemistry
____________________________________
Nathan McElroy, Ph.D.
Professor of Chemistry
____________________________________
Jean-Pierre Habets.
President of H&W Global Industries
ACCEPTED
___________________________________
Timothy P. Mack, Ph.D.
Dean
School of Graduate Studies and Research
_____________________
ii
Title: Industrial Zinc Plating Processes
Author: Christopher Sierka
Thesis Chair: Dr. Keith Kyler
Thesis Committee Members:
Dr. Jonathan Southard
Dr. Nathan McElroy
Mr. Jean-Pierre Habets
Electrodeposition is a process, which uses an electrical current to reduce cations of a desired
material from a solution and coat that material as a thin film onto a conductive substrate surface. In this
case, zinc cations will be reduced and coat the steel substrates acting as a sacrificial coating to provide
corrosion resistance. Three widely used industrial zinc plating processes will be examined, including acid
chloride, alkaline cyanide, and alkaline non-cyanide. These processes will be discussed in terms of
throwing power, cathode efficiency, hydrogen embrittlement, deposition mechanisms and salt spray
testing. After the zinc plating line is operational, training methods must be documented to adhere to ISO
9001 quality level.
iii
TABLE OF CONTENTS
Chapter
I
Page
INTRODUCTION ........................................................................................................................ 1
1.1 Electroplating Background………………………………………………………………….…………………….....1
1.2 Electroplating Deposition Mechanisms ........................................................................... 10
II
ZINC PLATING PROCESSES ..................................................................................................... 12
2.1 Cyanide Zinc Plating ......................................................................................................... 12
2.2 Acid Chloride Plating........................................................................................................ 15
2.3 Alkaline Non-Cyanide Plating .......................................................................................... 17
2.4 Zinc-Alloy Plating ............................................................................................................. 19
III ACID CHLORIDE PROCESS STEPS ............................................................................................ 21
3.1 Chemicals and Equipment ............................................................................................... 22
3.2 Alkaline Cleaning and Electrocleaning............................................................................. 22
3.3 Acid Pickling ..................................................................................................................... 26
3.4 Acid Chloride Zinc Plating ................................................................................................ 27
3.5 Acid Dip / Chromates ....................................................................................................... 33
3.6 Waste Treatment Management ...................................................................................... 34
IV TESTING AND ANALYSIS .......................................................................................................... 38
4.1 Salt Spray ......................................................................................................................... 38
4.2 Hull Cell Analysis .............................................................................................................. 41
4.3 Plating Tank Analysis ....................................................................................................... 44
V TRAINING METHODOLOGIES AND QUALITY ASSURANCE...................................................... 47
5.1 Introduction to Training Methodologies and Quality Assurance .................................... 47
VI COST ANALYSIS....................................................................................................................... 56
6.1 Zinc Plating Chemicals ..................................................................................................................... 56
VII SUMMARY AND CONCLUSION ............................................................................................... 59
REFERENCES…………………………………………………………………………………………………………….……... 60
iv
LIST OF TABLES
Table
Page
1
List of Metals that can be Electroplated................................................................................ 2
2
Cathode Current Efficiencies of Various Plating Solutions .................................................... 5
3
Electromotive Force Series .................................................................................................... 6
4
Alkaline Cyanide Zinc Plating Bath Constituents ................................................................. 12
5
Acid Chloride Zinc Plating Bath Constituents ...................................................................... 16
6
Alkaline Non-Cyanide Bath Constituents............................................................................. 18
7
Miscellaneous Requirements .............................................................................................. 18
8
Surfactant Composition ....................................................................................................... 24
9
Salt Spray Hourly Requirements .......................................................................................... 40
10 Salt Spray Hourly Requirements.......................................................................................... 40
11 Hull Cell Zinc Plating Testing................................................................................................ 43
12 Hull Cell Process Sequence .................................................................................................. 43
13 Sample Work Instruction for Zinc Plating with Type II Colored Chromate Coating ........... 51
14 Sample Work Instruction for Zinc Plating with Type III Colorless Chromate Coating ........ 52
15 Plating Time Calculations for Various Zinc Coating Thicknesses and ASF ........................... 53
16 Chemical Costs..................................................................................................................... 57
17 Zinc Plating Chemical Costs ................................................................................................. 57
18 Maximum Square Footage for 250 Ibs of Zinc Ball Anodes................................................. 58
v
LIST OF FIGURES
Figure
Page
1
Cathode efficiencies of plating processes.1 ......................................................................................... 4
2
Nernst Diffusion Layer2 ........................................................................................................................ 9
3
Step Edge Ion Transfer……………………………………………………………………………………………..……………….11
4
Step Edge Transfer and Diffusion……………………………………………………………………………………………..…..11
5
Terrace Ion-Transfer .......................................................................................................................... 11
6
Simultaneous Metal Deposition. ....................................................................................................... 20
7
Acid Chloride Process Steps. .............................................................................................................. 21
8
Pourbaix diagram of Iron at 25˚C....................................................................................................... 26
9
Pourbaix diagram of Aluminum at 25˚C. ........................................................................................... 26
10
Tank Design. ...................................................................................................................................... 28
11
Polypropylene tank (left) and Stainless Steel tank (right). ................................................................ 29
12
pH Adjustment Tank. ......................................................................................................................... 37
13
Semi-Automatic Waste Treatment System. ...................................................................................... 37
14
Salt Spray Corrosion Theory. ............................................................................................................. 39
15
A Kocour 267 mL Hull Cell. ................................................................................................................ 41
16
Hull Cell Ruler. ................................................................................................................................... 42
17
Structure of metal/EDTA complex..................................................................................................... 44
18
Zinc Plating with Yellow Passivate Coating........................................................................................ 54
19
Zinc Plating with Clear Passivate Coating .......................................................................................... 55
vi
CHAPTER I
INTRODUCTION
1.1 Electroplating Background
Many electroplating processes exist to change or enhance existing properties of metals.
Almost all metals can be plated, if the correct sequence of steps is taken, Table I gives a list of
all possible metals that can be electroplated. The concentration of this research is
predominantly on zinc electroplating. Three types of electroplating chemical processes
predominantly exist for Zinc plating. These processes are (1) alkaline cyanide, (2) alkaline noncyanide, and (3) acid chloride. The main component of each electroplating chemical process
includes a dominant Zn2+ complex which gets reduced to Zn0 at the cathode. Zinc coatings are
predominantly coated onto carbon steel cathodes. Furthermore, successful zinc plating does
not occur without proper pre-cleaning and post chemical treatments. A whole plating industry
exists for commercial and military purposes, and H&W Global Industries is a metal finishing
facility interested in installing a moderate size zinc plating line to meet the demands. Areas of
my research include; the basics of electroplating, the Zinc Plating process, equipment selection,
Analytical Testing, and training of chemical line operators. First and foremost, the basics of
electroplating will be explained to understand underlying principles that can be beneficial in an
industrial setting. The first equation to be explained in detail will be Faraday’s Law, followed by
Cathode Efficiency, Nernst Equation, Throwing Power, and Ion Mobility.
1
Table 1. List of Metals that can be Electroplated
Group Number
Metals
6
7
8
9
10
11
12
13
14
15
16
Cr
Mn, Tc, Re
Fe, Ru, Os
Co, Rh, Ir
Ni, Pt, Pd
Cu, Ag, Au
Zn, Cd, Hg
In, Ti
Sn, Pb
As, Sb, Bi
Se, Te
A. Faraday’s Law
The definition of electroplating is the “electrodeposition of an adherent metallic coating
upon an electrode for the purpose of securing a surface with properties of dimensions different
from those of the basis metal1” Electroplating is an electrolytic process which follows the
principles of Faradays laws. Faraday’s 1st law states the amount of chemical change produced
by an electric current is proportional to the amount of electricity that passes. By measuring the
quantity of electricity that passes, one can measure the amount of chemical change that will be
produced. Faraday’s 2nd law states the amounts of different substances liberated by a given
quantity of electricity are proportional to their chemical equivalent weights. If the equivalent
weight of a metal is known (E) then one can predict the amount of substance that can be plated
on a specific substrate indicated by Eq 1.
G= lET/96000
(1)
2
where G = grams of substance reacting, I = Current (amps), E = chemical equivalent weight, and
T = Time (sec). The equation commonly employed in the electroplating industry is Eq 2.
ASF = Amps X Surface Area
(2)
The acronym ASF stands for Amps per Square foot. Most often industry utilizes this
acronym for ease of learning. A lesser known measurement used is ADM, which stands for
Amps per square decimeter. Surface Area is always measured in ft2 because it is used for larger
plating areas used most frequently in the plating industry. Another important parameter to
determine the best zinc plating process is Cathode Efficiency represented by Eq 3, denoted as
CE. By industry standards, cathode efficiency translates to the speed of a plating solution. For
example, the acid chloride process has 95% CE and that of the alkaline cyanide process has 5080% CE. Before taking CE into consideration, both plating processes would take the same
amount of time for plating; however cathode efficiency must be accounted, resulting in the acid
chloride process to plate Zinc faster than the alkaline cyanide process. By industry standards,
plating times are vital to determine if a plating process is worth the monetary risk.
B. Cathode Efficiency
CE = 100 X Actual/Theoretical
(3)
Figure 1 shows several cathode efficiencies of varying zinc plating baths. As depicted by Figure
11, the Acid Chloride process shows constant cathode efficiencies above 90% as current density
increases, while the cathode efficiencies of the non-cyanide zincate plating solution decrease
below 60% as the current density increases. Competing reactions are limited at the cathode for
the acid chloride process, while many competing reactions occur at the cathode of the non-
3
cyanide zincate process. This pattern of current efficiencies is seen with other metals as well.
For example, Table 2 shows plating efficiencies for a range of different plating metals, such as
Gold and Tin. As portrayed, an acid chloride bath shows above 95% cathode current
efficiencies. As Cathode efficiency increase, throwing power will decrease.
Acid Chloride
Acid Sulfate
Cyanide
Noncyanide
Zincate
Figure 1. Cathode efficiencies of plating processes.1
4
Table 2. Cathode Current Efficiencies of Various Plating Solutions1
Metal Deposit
Ag
Au
Cd
Cr
Electrolyte
CN
Acid, Neutral, CN
CN
Cu
Acid SO4,
CN (High efficiency)
CN (Low efficiency)
P2O7
Fe
In
Acid
Acid
CN
Acid
Acid
Acid
Acid
Alkaline
Acid
CN
CrO3 /H2SO4
CrO3 /SO4-F
Ni
Pb
Rh
Sn
Zn
Cathode Efficiency (%)
100
50-100
85-95
10-15
18-25
97-100
30-45
90-95
100
90-98
30-50
30-50
93-98
95-100
10-50
90-95
70-95
95
50-80
C. The Nernst Equation important in plating of metals.
E = E0 + (RT/nF) lna
(4)
i. E = electrode potential
ii. E0 = constant characteristic of material of electrode. Standard
electrode potential
iii. R = gas constant = 8.3143 J/kmol
iv. T = absolute temperature in Kelvins
v. F = faraday
vi. N = valence change
vii. A = activity of the metal
E0 = -0.76 volts for zinc without complex
Zn2+ + 4(CN)- → [Zn(CN)4]
E0 = -1.1 volts for zinc with cyanide complex
When the unit activity a; is found to be 1, which rarely happens in nature, then E = E0.
When this occurs, the standard electrode potentials can be calculated and tabulated as in Table
5
3. The EMF series is an arrangement of various metals in order of their electrochemical
activities based on their standard oxidation-reduction potentials (E0). The most active metals in
the series will have high negative E0 values and are located on the bottom of the EMF series
table. The noble metals will have positive E0 values, located on the top of EMF series table. For
instance, if two metals are coupled together in the EMF series, the species with a larger
negative standard potential will act as the anode and corrode compared to the other metal
species with a positive standard potential. Several exceptions exist due to environmental
factors and passivity. Aluminum exhibits higher corrosion resistance despite having a larger
negative E0 due to the presence of an Al2O3 layer present on the surface. As a consequence of
the electrode potentials in this series, the Nernst Equation is born.
Table 3. Electromotive Force Series
Reaction
Au3+ + 3e = Au
Pt2+ + 2e = Pt
O2 + 4H+ + 4e = 2H2O
Pd2+ + 2e = Pd
Ag+ + e = Ag
O2 + 2H2O + 4e =4OHCu2+ + 2e = Cu
Sn3+ + 2e = Sn2+
2H+ + 2e = H2
Pb2+ + 2e = Pb
Sn2+ + 2e = Sn
Ni2+ + 2e = Ni
Co2+ + 2e = Co
Cd2+ + 2e = Cd
Fe2+ + 2e = Fe
Cr3+ + 3e = Cr
Zn2+ + 2e = Zn
Al3+ + 3e = Al
Mg2+ + 2e = Mg
Na+ + e = Na
E0,V(SHE)
+1.42
+ 1.2
+1.23
+0.83
+0.799
+0.401
+0.34
+0.154
0.00
-0.126
-0.140
-0.23
-0.27
-0.402
-0.44
-0.71
-0.763
-1.66
-2.38
-2.71
6
D. Throwing Power1
The metal deposit distribution is affected by the variation of the cathode efficiency with current
density. In plating solutions in which the cathode efficiencies decrease rapidly as current
density increases, excess deposits will plate on edges and corners. This phenomenon is coined
throwing power. In this case, little throwing power is available. If throwing power is minimal
then longer plating times are necessary to achieve the minimum coating thicknesses in
recesses. In plating solutions in which the cathode efficiencies increase as current density
increases, excess deposits will plate more evenly, resulting in high throwing power. Less plating
time is required to achieve minimum coating thicknesses in low current density areas. The next
factor that influences throwing power is the overall geometry of the plating system. Plating
tanks can be made to accommodate varying part geometries by moving the anodes to the
contour of the parts. Higher cathode polarizations at higher current densities results in
decreased cathode efficiency, resulting in higher throwing power. Cathode polarization is
commonly known as overpotential; which is the difference in electrode potential between its
equilibrium potential and its operating potential when a current is flowing2,10. Throwing power
is mistakenly used to describe covering power. Covering power applies to the lowest applied
current density at which a plating bath produces a deposit. The inability to plate in low current
density areas can be improved by applying a high current density strike to initiate plating into
recesses, and then normal current densities may be used to finish plating.
E. Ion Mobility
The mobility is the ability of ions to move in an electric field. Under the influence of an applied
voltage, ions move toward electrodes; cations move toward cathode, anions move toward
7
anode. Each particular ion moves at a particular rate characteristic of that ion, is called its
mobility or ionic conductivity. Electrical forces between ions interfere with mobility, along with
solvent impediment, solution viscosity and retarding effects of ions of opposite charge. Ion
mobility is the velocity that an ion attains per unit of electric field. The ion mobility is given by
the following eq2:
Ui = |Zi|e/ (6πηri)
(5)
Where z = ion charge, e = electronic charge, η = solvent viscosity, and r = ion radius.
Electrical conductivities will be highest for highly charged small ions in solvents of low viscosity.
The consumption of electroactive species close to the electrode results in a concentration
gradient and diffusion of the species towards the electrode from the bulk may become ratedetermining as described by Figure 2. Therefore, a large overpotential is needed to produce a
given current. For any reaction to proceed, and overpotential is required to overcome the
potential barrier at the electrode/surface barrier. Cathodic activation overpotential shifts the
energy level of the ions right outside the Nernst diffusion layer nearer to the potential barrier,
so more ions can cross the potential barrier at a given time, producing a deposit.
8
Figure 2. Nernst Diffusion Layer2
Whereas COX(X) is the concentration of ions at the surface of the metal and COX is the
concentration of ions in the bulk of solution surrounding the metal surface. At the maximum
current density, the metal species is reduced as soon as it reaches the electrode. The
concentration of the Zn2+ is non-existent at the electrode surface, and is determined by the rate
of transport of the ions to the metal surface. If the current density introduced is greater than
the maximum current density, the double layer becomes further charged and other processes
will occur other than reduction of the Zn2+.
F. The Plating Bath3
All plating baths have the basic features. The first part of a plating bath is to provide a source of
metal to be deposited. The second feature is to form complexes with ions of the depositing
metal. Complex formations are not always required; however deposits formed from complex
salts are far superior to those of simple salts (ZnCl2). The next feature of a plating bath is to
provide conductivity. Many metal salts are poor conductors (low ionic mobilities), so
conducting salts are added to increase conductivity of solution. Two examples of conducting
9
salts are KCl and NH4Cl. The next feature in a plating bath is compounds that stabilize the
solution against hydrolysis. When metal salts are subject to hydrolysis, the corresponding metal
hydroxides are insoluble in solution. In some alkaline baths, absorption of CO2 from the air
would precipitate the metal compounds in solution, unless a CO2 acceptor was present such as
NaOH. Another feature in a plating solution is compounds that act as a buffer. pH is critical in
neutral pH ranges of 5-8. pH is not critical in plating solutions with high and low pH. The last
feature in a plating bath is compounds to aid in dissolving anodes to replenish metal
concentrations without having to add Zinc.
1.2 Electroplating Deposition Mechanisms:
It is hypothesized that electro deposition occurs in two phases. The first phase involves
initiation of a few metal atoms adhering to the substrate. This phase has been proposed to
occur by two generally accepted mechanisms. The first mechanism can occur by two
pathways4. The mechanism will be determined by the inhomogeneity of the surface. The first of
these mechanisms is the Step-Edge Ion-Transfer. An M-adion is an ion adsorbed onto a surface.
The first case “ involves a direct transfer to the kink site, as portrayed by Figure 3; the M-adion
is in the half crystal position, where it is bonded to the crystal lattice with one half of the
bonding energy of the bulk ion, thus the M-adion belongs to the bulk crystal, but it is still
hydrated.” The second pathway involves a transfer to the step-edge site and diffuses along the
step-edge until it reaches a kink as portrayed by Figure 4. However, in both cases the M-adion is
incorporated into the metal crystal lattice.
10
Metal Surface
Figure 3. Step Edge Ion Transfer
Figure 4. Step Edge Transfer and Diffusion
The second proposed mechanism is a Terrace Ion-Transfer Mechanism, 4 as portrayed by
Figure 5. The metal ion is transferred from the solution to the flat face of the terrace region. At
this position, the metal ion is in the adion state having almost all the water of hydration. The M
adion goes to lower energy and diffuses into the kink site.
Metal Surface
Figure 5. Terrace Ion-Transfer
11
Following this initial first phase is a second phase where crystal growth occurs from the
few initially adhering atoms. Two basic mechanisms exist to explain the growth mechanism of
plated deposit; Layer growth and three dimensional crystallite growths. In a “layer growth
mechanism a crystal enlarges by a spreading of discrete steps one after another across a
substrate.” Several growth forms can be made from this method including columnar, whiskers
and fiber texture. The second method involves formation of “isolated nuclei and their growth to
three dimensional crystallites and then the coalescence of the crystallites then a formation of
the linked network and then a formation of a continuous deposit.”
12
CHAPTER II
ZINC PLATING PROCESSES
2.1 Cyanide Zinc Plating
For plating zinc onto a substrate, the cyanide Zinc plating process was the first
commercially available zinc plating chemistry. However, due to strict environmental
regulations, the use of “cyanide” zinc plating has decreased exponentially over the years being
replaced by other processes. Nevertheless, a brief description of the chemistry of the process is
warranted. In the “cyanide” process the reduction of Zn2+ ions occurs by the following this
series of reactions.5 The main bath constituents that make the reduction of Zn possible are
stated in Table 4.
[Zn(CN)4]2- + 2OH-
[Zn(OH)2] + 4CN-
(6)
[Zn(OH)2] + e-
[Zn(OH)2]-
(7)
[Zn(OH)2]-
Zn(OH) + OH-
(8)
ZnOH + e-
Zn + OH-
(9)
Table 4. Alkaline Cyanide Zinc Plating Bath Constituents
Chemical
Regular (g/L)
Mid (g/L)
Low (g/L)
Zn(CN)2
60
30
10
NaCN
NaOH
Na2CO3
40
80
15
20
75
15
8
65
15
NaxSy
2
2
-
Brightener
1-4
1-4
1-4
12
A key advantage of utilizing this process over the others is the capability of zinc plating
parts with low current density areas such as tubes. The reason this process works well is
because it has higher throwing power than the other two processes.
Another significant issue with the cyanide zinc process is a problem known as Hydrogen
Embrittlement11. Only the chloride process seems not to be plagued by Hydrogen
Embrittlement and this latter process will be discussed in detail later. Hydrogen Embrittlement
is a process in which high strength steels become brittle and fractures after exposure to
hydrogen. The introduction of hydrogen to a plating bath is accomplished with the following
equation, which occurs at the cathode.
2H+ + 2e-
H2
Two proposed models exist to explain the hydrogen absorption onto a metal substrate.
Both models incorporate the idea that atomic hydrogen adsorbs onto the surface of the metal
substrate, and diffuses into the metal, resulting in brittleness of the metal. The first model
suggests that the intermediate stage through which electrolytic hydrogen passes on entry to
the metal substrate is the adsorbed state (MHads)) and is identical to hydrogen evolution
mechanism. The reaction sequence is as follows below: The first step introduces hydrogen into
solution as H3O+ which then adsorbs onto the metal surface (MHads), releasing water as a
byproduct as shown below.
H3O+ + M + e-
MHads + H2O
The adsorbed hydrogen then diffuses through the metal surface, forming a metal-hydrogen
complex (MHabs) as depicted below.
13
MHads
MHabs
Adsorbed hydrogen still on the surface of the metal will be released under equilibrium controls,
in which hydrogen evolution occurs as shown below.
MHads + MHads
H2 + 2M
The second model suggests the atomic hydrogen enters the metal lattice the same way
it is discharged, and that the intermediate stage in which hydrogen enters the metal lattice is
not identical to the adsorbed hydrogen mechanism, which results in hydrogen evolution. The
first step depicts hydrogen absorbing through the metal surface (MHabs).
H3O+ + M + e-
MHabs + H2O
At the same time hydrogen is adsorbing to the metal surface as shown below.
H3O+ + M + e-
MHads + H2O
Both mechanisms end with the same result in which hydrogen evolution occurs as shown
below.
MHads + MHads
H2 + 2M
Secondly, this process has decreased cathode efficiencies compared to the others, which leads
to higher plating times. Lastly, the costs of alkaline cyanide chemical process are steep,
resulting from the waste treatment needed to reduce cyanide.
Cyano-complexes are used to plate Cu, Cd, Au, Ag, Zn and In. All cyano plating solutions
are alkaline in nature. If acid is added to a cyanide plating solution, poisonous cyanide gas
would be produced. One exemption is the cyano-gold [cyanoaurate (I)] complex, which is
stabilized in low pH solutions.
14
Hydrogen embrittlement may occur as a result of acid pickling, electroplating, and
aqueous corrosion, which involve the discharge of hydrogen ions. One key way to remove
atomic hydrogen is derived from its mobility at high temperatures. A bake cycle is employed
between the temperatures of 350-400◦F. This allows the diffusion of the hydrogen out of the
metal or metal-alloy.
2.2 Acid Chloride Plating
Zinc may be plated from several types of acid solutions, based on zinc sulfate or zinc
chloride complexes. The acid chloride process is relatively new in comparison with the alkaline
cyanide and alkaline non-cyanide zinc plating processes. Acid Chloride process has rapidly
changed the zinc plating industry and constitutes about 50% of all zinc baths in most developed
nations. Cast iron, malleable iron, and carbonitrided are readily plated in acid zinc plating
processes and not the others. The reduction of Zn2 at the cathode occurs in the following
manner5.
ZnCl2 + 2KCl
K2ZnCl4
(10)
K2ZnCl4
2K+ + ZnCl4
(11)
ZnCl2
Zn2+ + 2Cl-
(12)
The first advantages for selecting the acid chloride process is the high cathode
efficiencies, resulting in less side reactions and faster plating times. Secondly, minimal waste
treatment is needed dependent on the Acid Chloride process selected as depicted by Table 5.
The only chemical that can cause waste treatment issues is an abundant amount of NH4Cl.
15
A disadvantage of this process is the corrosive nature of the chemical used, resulting in
solution laying in recesses, which could be detrimental to the coating if proper rinsing
procedures are not followed. Three types of chloride baths exist; all NH4Cl, mixed NH4Cl / KCl,
and all KCl. Each has a distinct advantage over the other; however the mixed NH4Cl / KCl bath
gets the best of both worlds. The proprietary ingredients in zinc plating solutions include Carrier
Brighteners, Primary Brighteners, and surfactants/carriers. Primary Brighteners reduce the
roughness of zinc plated surface to maximize optical reflecting power (ammonium bisulfitezetaplus maintenance). These brighteners tend not to be highly soluble in solution. Carrier
Brighteners keep Primary Brighteners from crashing out of solution (Sodium Benzoate-Zetaplus
Merit Make-up). Most chloride systems contain at least 2 primary brighteners and 4-8
surfactants. Surfactants are compounds that lower the surface tension of a liquid and a solid,
which allow the Primary and Carrier Brighteners to reach the surface of the substrate easily.
Table 5. Acid Chloride Zinc Plating Bath Constituents5
Chemical
All NH4OH
(g/L)
All KCl
(g/L)
Mixed
Bath-KCl
(g/L)
Mixed
Bath-KCl
(g/L)
Zn
15-30
22-38
15-30
15-30
NH4Cl
120-180
-
30-45
30-45
KCl
-
185-225
120-150
-
NaCl
-
-
-
120
H3BO3
-
22-38
-
-
Carrier
brightener
4% b/v
4% b/v
4% b/v
4% b/v
Primary
brightener
0.25% b/v
0.25% b/v
0.25% b/v
0.25% b/v
16
2.3 Alkaline Non-Cyanide Plating
The last process which bears some discussion is the alkaline non-cyanide process which
still finds current use in Industry today. The alkaline non-cyanide zinc plating process is a
reliable, cost efficient method, similar to that of the acid chloride zinc plating process. The
electrodeposition reaction of plating zinc on a steel substrate occurs in the pathway shown
below.5
[Zn(OH)4]2-
[Zn(OH)3]- + OH-
(13)
[Zn(OH)3]- + e-
[Zn(OH)2] + OH-
(14)
[Zn(OH)2]-
Zn(OH) + OH-
(15)
ZnOH + e-
Zn + OH-
(16)
The first drawback of an alkaline non-cyanide process is the presence of high levels of
carbonates found in solution. Carbonate build-up occurs in solution due to elevated levels of
CO2 entering the solution by the following reaction.
2NaOH + CO2
Na2CO3 +H2O
(17)
Carbonates increase with an increase in solution agitation and solution temperature,
which leads to a decrease in solution conductivity, thus hindering the electrodepostion process.
The maximum amounts of carbonates allowed in solution range from 50-100 g/L which is
customary with absorption of CO2 from air. A number of methods exist to crash carbonates
out of solution. The first technique is to cool the solution to 5-10°C, until the carbonates freeze,
then the carbonates can be filtered out of solution. A less frequent method employed is the
precipitation of carbonates by calcium hydroxide.
17
Table 6. Alkaline Non-Cyanide Bath Constituents
Chemical
Range (g/L)
Zn metal
6.0-17.0
NaOH
75-112
Additives
As recommended by
Manafacturer
The three processes that were presented; cyanide, acid chloride, and alkaline noncyanide require other additional conditions in order to be successful. A comprehensive
discussion of all the conditions required for each process is beyond the scope of this paper.
Table 7 provides a condensed version of some of the specific requirements for each process.
For example, the acid chloride process required air agitation while the others do not. Air
agitation would be detrimental to the zinc coating for the cyanide and alkaline non-cyanide
processes due to CO2 being produced with air agitation.
Table 7. Miscellaneous Requirements
Requirements
Anode Polarization
Conduct. of Bath
Sol’n
Air Agitation
Heating Required
Filtration Required
pH Adjustment Req’d
Purifier needed to
treat impurities
Chromate Receptivity
Waste Treatment
Iron treatment by
Oxid.
Acid Chloride Zinc
Alkaline NonCyanide Zinc
Cyanide Zinc
Rarely
Excellent
Yes
Dependent on C.D.
Yes
Fair
Required
Required
Yes
Yes
No
Not required
Required
Yes
No
Yes
Not required
Required
No
No
Yes
Good
Simple
Yes
Dependent on C.D.
Simple
No
Excellent
Complex
No
18
Furthermore, the acid chloride solution has greater conductivity due to KCl. The
conductivity of the alkaline non-cyanide plating solution is poor at low current density regions
and excellent at high current density regions. pH adjustment of the acid chloride process
solution is required to keep Fe2+ in solution. However, the initial plating solution is in need of
oxidation treatment with 30% H2O2, followed by filtration to discard any Fe2+ impurities in KCl,
ZnCl2, and NH4Cl.
2.4 Zinc-Alloy Plating
Recent demands for higher quality finishes, more specifically, longer lasting coatings,
have lead into a newer process known as zinc-alloy plating. The most notable zinc alloying
elements are Iron, Cobalt, Nickel, and Tin. Two metals may be plated simultaneously if the
potentials are similar, and if the ionic activities of two dissimilar metals are different. Alloys
cannot be deposited unless the activity of the more noble metal ion in a solution is greatly
decreased by a stable complex formation. The current density vs potential graphs will look
similar for alloying elements as depicted by Figure 6. As stated earlier, the single deposition
potentials are similar, but the ionic activities must be different in order for simultaneous
deposition to occur. So, M1 and M2 may be plated simultaneously.
Zinc-nickel alloys4 can be plated from acid or alkaline non-cyanide solutions. Typically,
the acid solution provides a nickel content of 10-14% while the alkaline non-cyanide solution
provides 5-8% nickel or 10-17% nickel content. The corrosion resistance increases as nickel
content increases to 17%, after this level the zinc-nickel deposit becomes nobler than the
substrate, losing its corrosion properties. The zinc-nickel alloy plating process is a quite bit more
19
expensive than all other zinc-alloy plating processes, however the added corrosion resistance
more than makes up for the increase in cost.
Zinc-Cobalt alloys are employed as acid chloride solutions with little or no ammonium
present. The cobalt content is generally 1%. The acid chloride bath has increased cathode
efficiencies and plating speeds.
Figure 6. Simultaneous Metal Deposition.
20
CHAPTER III
ACID CHLORIDE PROCESS STEPS
A series of zinc plating steps is required to successfully apply a metallic coating to a steel
substrate. Figure 7 depicts a typical zinc plating sequence. Each chemical process shown will be
explained in detail with the mixing procedures.
Figure 7. Acid Chloride Process Steps.
21
3.1 Chemicals and Equipment
All equipment and chemicals were purchased by H&W Global Industries. Almost all
chemicals were purchased through Trichem Technologies. All necessary equipment for a pilot
scale zinc plating line including Industrial Tanks (Polypropylene and SSTL), Heaters, Filtration
Systems, Air Agitation lines, Rectifiers, Fume Hoods, Cranes, Racks, Load bars, Copper Cathodes,
Zinc Anodes, and Anode baskets were provided by H&W Global Industries. Furthermore, a
Kocour 267 mL Hull Cell with built-in heater and air agitation was purchased by H&W Global
Industries. Also, commodity chemicals such as Hydrochloric acid and Nitric Acid were purchased
from Interstate Chemicals in large quantities. All titration reagents such as Ammonium
Hydroxide, Ammonium Chloride, Sodium Cyanide, 10% Formaldehyde, 0.153 N Silver Nitrate,
and Eriochrome Black T Indicator were purchased through Trichem Technologies without
further purification.
3.2 Alkaline Cleaning and Electrocleaning
Cleaning Variables include Time, Temperature, Concentration, pH, and agitation. A
cleaning process needs to balance these variables to obtain optimal cleaning at a reasonable
cost. For example, longer time of exposure to the cleaning solution will typically produce a
cleaner surface. But the customer may only have limited time in which to clean. Likewise,
higher temperatures usually help clean better. But higher temperature will increase customer
energy costs, and can damage some substrate. This is true as well for cleaner bath
concentration. A more concentrated cleaning bath may remove soils better, but will add to
chemical costs, and strongly alkaline or strongly acidic chemicals may attack the substrate. At
higher concentrations, cleaning chemicals may be more difficult to rinse off and present a
22
higher risk of contaminating subsequent stages. Spray application cleans better but may not get
into complicated areas of fabricated metal parts. Immersion application can penetrate
complicated shapes but has less mechanical energy acting on the soils.
Chemical cleaners can be divided into solvent-based and aqueous. Solvent-based
cleaners can be chlorinated hydrocarbons, glycol ethers, alcohols, etc. Due to VOC and safety
issues, the popularity of solvent-based cleaners is diminishing, although for organic soils and
greases they can be highly effective. Aqueous cleaning can be further divided into acid cleaning,
neutral cleaning and alkaline cleaning. Acid cleaners are good at removing oxides and corrosion
products, but can attack and dissolve the substrate, so they have limited use. Cleaners
dedicated for aluminum are frequently acidic. Neutral cleaners are generally mild and soap-like;
they don’t usually attack the base substrate, due to their neutral pH, but this also makes them
less aggressive to soils. Alkaline cleaning is the most popular type of cleaning. Like acid cleaning,
however, they can attack certain substrates, particularly zinc and aluminum. Other methods
include vapor degreasing, sand blasting, electrocleaning, and ultrasonic cleaning. Blasting
removes bulk soils on the surface, however care must be taken so no substrate contamination
occurs. Aqueous cleaners contain one or more soap-like chemicals6, known as “surfactants”
which include Anionic, Cationic, and Non-ionic compounds, referred to in Table 8. In addition,
they usually contain other materials known as “builders”; these can be phosphates, silicates,
hydroxides, carbonates, or acids, which form insoluble compounds with metals that would
adversely affect the electroplating process such as Iron. Many aqueous cleaners will also
contain small amounts of solvents, such as glycol ethers.
23
Alkaline cleaner’s work in a series of steps6: 1) Displacement, which removes soils from
the part physically. 2) Emulsification and Dispersion, which chemically absorbs soils into
solution and disperses the soil throughout and prevents re-depositing, back onto the part. 3)
Saponification, which makes a chemical “soap” out of organic acids in the oils and caustic from
the cleaner. 4) Dissolution, which removes a small layer of metal oxides by the alkaline strength
of the cleaner.
Table 8. Surfactant Composition6
Anionic (ionic)
(Carboxylic Acids and Salts,
Sulfuric Acids Derivates, Sulfonic
Acids and Salts, Phosporic Acids
Esters and Salts and, Acylamino
Acids and Salts)
Negatively charged. Oldest and most common surfactant.
Excellent dispersive action. Sensitive to water hardness
ions. Produced in high volumes. The majority are
inexpensive. Used in most detergent systems. Sulfates and
sulfonates are common anionic surfactants.
Cationic (ionic)
Positively charged. Prompt to change surface properties
making a hydrophilic surface act as a hydrophobic and vice
(Alkyl Amines, Alkylimidazolines,
versa. Poor dispersive action. Used as germicides. Great
Quaternary Ammonium
emulsifying capacity. Used mostly as fabric softeners.
Compounds, Ethoxylated Alkyl
Amines and, Esterified
Quaternaries)
Amphoteric (ionic)
Can act as cationic or anionic detergents, depending on the
pH of the solution. Work best at neutral pH. Used in
(Acyl Ethylenediamines and
combination with Anionic or Cationic surfactants to enhance
Derivatives, N-Alkyl Amino Acids
certain properties (foam or detergency). Commonly used in
or Imino Acids)
personal care products (shampoos, foam baths, etc).
Electrocleaning chemistries work the same way as alkaline cleaning chemistries, except
electrocleaning processes tend to create much more foam, formed by electrolysis, which is
increased by solution agitation. Certain compounds are placed in the tank to lessen the amount
24
of soap bubbles formed. Some cleaner chemistry allows the alkaline cleaner and electrocleaner
to be used interchangeably, which saves time, space and money. Two methods of
electrocleaning exist, anodic and cathodic. Oxygen evolution occurs with anodic electro
cleaning which helps loosen any particles that may still be on the surface. Hydrogen evolution
occurs with cathodic electro cleaning which consequently leads to hydrogen embrittlement
issues. A quick method to determine if a part is clean is by the water break test. A mist of
water is sprayed onto part and a visual check is done. If water beads on the surface, then the
part fails the water break test and must be re-cleaned. Water beading means oil is still present
on the surface. If the water forms a continuous streak then the surface of the part is deemed to
be a water break free surface. Surface Oxides that form on the surface of different metals in
cleaners are pH dependent. A common example is a steel part, in which cleaners operate within
a pH window of passivation of 2-14. Therefore, highly alkaline chemicals may be used, such as
NaOH as shown by Figure 8. Aluminum cleaners cannot use caustic chemicals in their
formulation, because the part would corrode, in reference to Figure 9. However, less caustic
chemicals may be used, including carbonates, or acids.6
25
Figure 8. Pourbaix diagram of Iron at 25˚C.
Figure 9. Pourbaix diagram of Aluminum at 25˚C.
3.3 Acid Pickling7
After cleaning the part to be plated, next step in the pre-plating process is acid pickling.
Acid pickling is an important step that removes oxides and rust, as well as activating the surface
for plating. Pickling activates the surface by neutralizing and solubilizing the residual alkaline
films and micro-etching the surface1. Several acids can be used for pickling, depending
specifically on the substrate. Sulfuric acid and Hydrochloric acid are predominantly used for
26
acid pickling. The concentration of acids can vary as long as the tanks are made of appropriate
materials. Pickling tanks require no heat, because they work well enough without them. The
last parameter for pickling is immersion times. The immersion times differ from 30 seconds to 5
minutes. Lower immersion times are used when no rust is on the surface of the part, and higher
immersion times are used for parts that have elevated amounts of rust present. The mixing
procedure for creating a pickling bath was as follows. Approximately 50% of the tank was filled
with DI water, then 48.75 gallons of 20 degree Hydrochloric acid was added, which equated to
25% by volume hydrochloric acid. The tank was filled to the operating range with DI water. Tank
is now ready for production.
3.4 Acid Chloride Zinc Plating
Tank Schematic:
A schematic of a plating tank is shown below. Several key factors that need examined
for a plating tank; include A) tank design B) tank material C) solution agitation D) filtration E)
cathode/anode placement F) heating elements, G) source of DC current and H) Plating racks.
27
G
D
F
B
E
C
Figure 10. Tank Design.
A. Tank Design
Tank design is the most important part of development of a plating line. Tank design
parameters include size, location, and set-up of plating tanks. The size of the plating tank is
beneficial in determining the amount of plating that can be achieved in an 8 hour shift. For
instance, smaller plating tanks are adequate for running small batches of parts, however if the
parts are large in size, then less can be plated at a time, which leads to longer lead time for
customers. The area of the plating tank is also important in determining the type of parts that
can fit. If the plating tank 8’ X 2’ then only long parts can fit in the tank, and if the plating tank is
6’ X 4’ then only wide parts can fit in the tank. The size of the tank is based on customer needs,
which is why each plating facility has different size specifications. The next important step in
the development of a zinc plating tank is the design, which includes location of anodes,
28
cathodes, air lines, filter pump, and heater. Plating tanks can be designed in a variety of ways,
but the wrong design can lead to devastating results. For instance, if the anodes are placed too
far apart in the plating bath, then the coating thicknesses of the parts will differ greatly from
one end to the other. If the heater is not large enough to heat the solution, then it will take
more time to operate heat up the plating solution.
B. Tank Material
Tank material is selected by two criteria. A first criterion is the pH of solution being used, for
example, alkaline or acidic. Polypropylene tanks can withstand acidic solutions, but can only
withstand alkaline solutions to a certain degree. A second criterion is the temperature of the
solution, for example SSTL tanks cannot withstand excessive temperatures, whilst
polypropylene tanks can. A mixture of these two criteria will afford the correct tank material.
The two most notable tank materials are polypropylene and SSTL 302 or 316 series. Figure 11
below depicts the two types. The left is polypropylene and the right is a SSTL series. In this case
of zinc plating, a polypropylene tank is the best choice. Stainless Steel tanks have the chance of
stray currents which affect the plating process and could be a potential danger.
Figure 11. Polypropylene tank (left) and Stainless Steel tank (right).
29
C. Solution Agitation
The main objective of solution agitation is to replenish ions to the substrate by one of
two ways; air agitation or mechanical agitation. Most facilities utilize air agitation because of its
simplicity and low cost. The air agitation lines in Figure 10 give good solution agitation close to
the substrate. Size and spread of holes in air agitation is important for size of bubbles (want
small, large quantities of bubbles). The specific gravity of solution, chemical, and temperature
play key roles in determining air agitation requirements. The specific equations used to
determine air agitation holes and spread are beyond the scope of this paper.
D. Filtration
The purpose of filtration is to discard any unwanted material present in the plating bath.
Unwanted material can unknowingly deposit on the part causing adhesion failures. The
selection of filtration pump systems is selected by two variables; solution turnover and filter
size. Solution turnover refers to the number of gallons of solution filtered per hour. A larger
filter pump is required for larger tanks and vice versa. Filter size refers to the micron rating of
filters. Micron ratings can range from 5 microns – 200 microns. Most plating solutions will
utilize this micron range, however larger micron ratings are not as effective as removing
metallic impurities as smaller micron ratings. The type of material being filtered must be
known to determine the type of filtration required.
E. Cathode/Anode Placement
The type of anode material is indicative of the metal being plated. First and foremost
the anode completes the electrical circuit and introduces current into the plating bath. In the
instance of zinc plating, titanium baskets work to hold the anodes. The anode material is high
30
purity zinc ball anodes which replenish the plating bath, eliminating zinc metal additions, when
a current is applied.
F. Heating Element
Most plating tanks require heat to speed up plating operations. The # of kilowatts
required for the heater is based on the chemical composition, required temperature of
solution, ambient temperature at which the tank will be used, total cubic feet of tank, total #
gallons, and heat up time desired, as shown by Eq 18, however a surface temperature loss
factor must be added. The surface temperature loss factor is based on the temperature of the
plating solution, which can be found in any engineering handbooks. The equation below is the
example calculation for determine kilowatts of heating a solution of water.
K = A x 1.0* x 8.35** x B
(18)
3412 x C
A = Total gallons of solution
B = Temperature difference between ambient temperature and expected temperature
C = # of hours required to heat up solution
1.0* = Specific heat of water or chemical solution
8.35** = Specific weight of water or chemical solution
After the # of kilowatts is determined, the correct heater can be purchased.
G. DC current
A direct current is required for the plating operation to succeed. The tool that converts
an AC current to a DC current is a rectifier. A rectifier comes in a wide range of Amps and Volts,
and is chosen based on the size of the plating tank and the total surface area that can be plated
and type of plating solution used. As mentioned earlier by Eq 2, the amount of Amps can be
31
determined by the surface area and current density. In this way, the size of the rectifier can be
determined. After the size of the rectifier is found, the + feed must be connected to the anodes,
and the – feed must be connected to the cathode, as shown in Figure 10.
H. Plating Racks
Next, size and types of racks are important to prohibit solution entrapment and for cost
efficiency. The rack material is important, because racks could be a potential source of
contaminants in plating baths if the right material is not chosen correctly. Zinc plating racks can
be Steel or Stainless Steel. Not only are tanks and materials needed for the zinc plating tank,
but every step in the process has different requirements.
Zinc Plating Tank Mixing Procedure15
Before adding chemicals to the polypropylene tank, a pretreatment of the plating tank
was necessary. The tank was cleaned and leeched with 1-2% hydrochloric acid for 2-4 hours,
and then cleaned with DI water. The plating tank was then filled with water approximately to
75% of its final volume. While stirring, 63.4 pounds of ZnCl2 was added, this equates to 39.0
grams per liter. After the ZnCl2 was dissolved, 271.9 pounds of KCl was added, this equates to
167.3 grams per liter. Next, while stirring, 84.1 pounds of NH4Cl was added, this equates to
51.75 grams per liter. All chemicals were completely dissolved in solution. The bath was then
diluted to approximately 90% of its final volume with cold water, and mixed to ensure uniform
composition of the solution. The pH of zinc plating solution was adjusted to 5.0 – 5.3. Next,
737.1 milliliters of 30% H2O2 was added, which was 0.1% of the bath concentration. The
purpose of H2O2 was to oxidize any iron impurities that were present in the Potassium
Chloride. The solution was filtered for 3-4 hours to remove the oxidized iron. Next, 530 mL
32
Zetaplus Merit Maintenance was added and mixed into solution. The purpose of Zetaplus
Maintenance is to brighten the zinc deposit. Then, 9.75 gallons of Zetaplus Merit Make-up was
added to solution. The purpose of the Zetaplus Merit Make-up is too solubilize the brightener in
solution. The bath was diluted to its final volume and mixed well. The solution was ready for
production use.
3.5 Acid Dip / Chromates
Directly after the plating step, a brown blume is present on the parts, which decreases
the brightness of the parts. A post plating nitric dip is required to remove the brown blume,
resulting in an increase in deposit brightness. The nitric acid step not only removes the
brownish tint to the coating, it also lowers the pH of the coating to activate it for the
chromate/passivate.
The scope of this research was based on the use of the trivalent passivates. The use of
hexavalent chromates is beyond the scope of this paper, however a little detail on the matter is
warranted due to its importance in the past.
Acid Dip Tank Mixing Procedure
The last tank needed for the zinc plating line is the nitric acid dip tank. Add 75% DI water
to tank and add 185.2 mL of 42◦ nitric acid. Add the final volume of DI water to tank. Tank was
ready for production.
Clear Passivate Tank Mixing Procedure
The next tank that required chemicals was the clear passivation step9. Approximately
50% of DI water was added to the tank, and with constant agitation 15% of Lanthane 316 or
33
29.25 gallons was added. Lanthane 316 is composed of Cr3+ complexes and cobalt which
increase the corrosion resistance of the zinc deposit. The final operating level was filled with DI
water, and the pH was adjusted to 2 with dilute Nitric acid or Sodium Hydroxide. The
temperature of the solution was verified to be within the operating range. The clear passivation
tank was ready for production.
Yellow Passivate Tank Mixing Procedure
The next tank was the yellow passivation tank9. Approximately 50% DI water was added
to tank, and with constant agitation, 10% by volume of Lanthane 316 or 19.5 gallons was
added. 0.5% of Tri-Yellow Dye or 0.975 gallons was added. DI water was added to fill solution
level to optimum operating
Hydrogen embrittlement may occur as a result of acid pickling, electroplating, and
aqueous corrosion, which involve the discharge of hydrogen ions. The hydrogen is then
chemisorbed onto the metal surface, and if the hydrogen is not evolved as a molecular product,
can enter the metal. One key way to remove atomic hydrogen is derived from its mobility at
high temperatures. A bake cycle is employed between the temperatures of 350-400◦F. This
allows the diffusion of the hydrogen out of the metal or metal-alloy.
3.6 Waste Treatment Management
The waste treatment system is an important part of the surface finishing industry. Many
state and government regulations exist to limit the amount of harmful substances being
introduced into the environment. Three major factors influence the size, complexity, and cost
of conventional wastewater treatment systems. The type of pollutant substance plays a key
34
role in the complexity of the wastewater system. Hexavalent chromium reduction and cyanide
oxidation complicate the water treatment. The second factor that influences conventional
wastewater systems is the size of the plant. The use of larger tanks and more chemical
processes would require bigger set-ups, furthermore the water flow rate and volume of water
treated plays a huge impact on the size of the set-up. The last factor is strict environmental
regulations. Facility location and environmental laws governing that area will ultimately affect
the type of wastewater treatment system.
The main purpose of the waste treatment system at H&W Global is to limit the amount
of metals that get introduced into the environment. Wastewater is introduced into the 2000
gallon holding tanks found in Figure 12. After the holding tanks reach their highest capacitance,
wastewater is transferred to a smaller 1400 gallon tank where the pH adjustment occurs
depicted by Figure 12. The pH of the wastewater must be adjusted to the range of 6.5-8.5,
before further steps are taken. After the pH is adjusted, 200 gallons of wastewater at a time is
introduced into the semi-automatic flocculent system depicted by Figure 13. The flocculent is
then added to the wastewater to bind any metals present and precipitates them out of
solution. The wastewater is then transferred on a polypropylene filter media in which water is
gravity filtered out and transferred to the final holding tank. The water is now clean and can be
placed into the environment or reused. The metal bound flocculent material is placed in the
garbage for disposal.
Hexavalent chrome is a known carcinogen and must be reduced to trivalent chromium
before further processing can occur. Hexavalent chromium can be reduced with SO2 gas,
35
sodium metabisulfite, ferrous sulfate, ferrous chloride, or ferrous hydrosulfide. The reaction of
hexavalent chromium and sodium metabisulfite must occur at a pH range of 2.5-3. A pH higher
than 4 slows the reaction to impractical limits. Another drawback of the reaction is the noxious
acidic vapors. Ferric sulfate will reduce hexavalent chromium near neutral pH, and the ferric ion
becomes an excellent coagulant to bind other metals. H&W Global Industries utilizes a
proprietary blend primarily made of ferric sulfate.
Many plating shops still utilize cyanide in their chemical processes in spite of strict
environmental regulations. Sodium hypochlorite or chlorine gas is used to reduce cyanides to
carbon dioxide and nitrogen. First stage Cyanide oxidation is carried out at pH of 10.5 or higher
to reduce Cyanide to Cyanate. The reaction ceases below pH 9. Second stage Cyanide oxidation
is carried out at pH 8.0-8.5 and additional chlorine is added to complete conversion to CO2 and
Nitrogen. Second stage cyanide oxidation is rarely used in Industry because most
environmental regulations do not require total oxidation of Cyanide.
36
Figure 12: pH Adjustment Tank (Courtesy of H&W Global Industries, inc).
Figure 13: Semi-Automatic Waste Treatment System (Courtesy of H&W Global Industries, inc).
37
CHAPTER IV
TESTING & ANALYSIS
An analysis of the plating process is important to ensure consistent plating results.
Several analysis procedures include Salt Spray testing, Hull Cell Analysis, and titrimetric
determination of plating bath constituents; including Zinc metal concentration and total
chlorides in solution. Salt spray testing is a determination of the corrosion protection of the
plated deposit. The Hull Cell gives rapid information on the brightness levels, uniformity,
throwing power, and plating bath chemistry. Titrimetric determination of solution constituents
gives an idea of how close the concentrations of Zinc and chlorides are to the optimal ranges.
Titrimetric determination and Hull Cell Analysis will be explained in detail later. Salt spray
testing theory is covered in detail below.
4.1 Salt Spray
A very well defined set of tests exist to check the corrosion properties of many finishing
processes, including Anodizing, Chemical Conversion Coatings, Passivation, Black Oxide, Zinc
Phosphate, Zinc Plating, and paint. The most commonly employed test is the Salt Spray test.
Other less common tests include Humidity testing, Acetic Acid Salt Spray, Copper Accelerated
Salt Spray, Corrodkote, and Sulfur Dioxide testing. Each test mimics a certain aspect of the
environment at an accelerated pace. However, since salt spray testing is used predominantly
on coupons, not the parts themselves, it will be explained in depth.
38
If a water droplet rests on metal surface, there exists a difference in the volume of
oxygen, available to specimen relative to position of metal surface within the water
droplet. At center of drop, metal is in contact with dissolved oxygen in water droplet
and with oxygen in OH- due to water dissociation.
—Frank Altmayer 12
At edge of drop, additional oxygen from air is available; an [O2] gradient is produced
from edge of drop to center. The difference in oxygen creates an oxidation potential of 0.3 V.
Center of drop/metal surface become anodic, as metal dissolves from water, leaving behind
electrons that flow to the edge of drop. Metal ions form at center of water droplet, and a pit
develops depicted by Figure 11.
Figure 14. Salt Spray Corrosion Theory.
Each chemical process such as Anodizing, Chemical Conversion, and Plating require
coupons to be subject to salt spray solution for a given standard amount of hours until first
signs of corrosion are seen. Some typical salt spray times are shown in Tables 9 and 10. For
example, anodizing requires 336 hours of salt spray (MIL-A-8625F). Zinc plating salt spray hours
with a clear chromate conversion coating requires a minimum of 96 hours of salt spray before
39
signs of corrosion occur as stated by Table 9. Several parameters must be followed in
accordance with ASTM B11713. The specific gravity, pH, collection rate, chamber temperature,
cabinet temperature, and pressure must be recorded daily and kept within range. The salt spray
solution must be kept between 4-6%. A benefit of salt spray testing is the troubleshooting
capabilities. Problems in the process such as in the pre-cleaning, chemical treatment, and posttreatment steps can be identified and eliminated. If test coupons fail the salt spray test, one
variable at a time can be changed until the test coupons pass salt spray. A possible deterrent to
the salt spray test is the evident time factor. Anodize test coupons take up to two weeks in the
salt spray. If the test fails, all production parts must be recalled until a solution is found and the
test coupons re-tested.
Table 9. Salt Spray Hourly Requirements IAW ASTM B 633-13
Type
I
Description
As-plated without
supplementary treatments
With colored chromate
coatings
With colorless chromate
conversion coatings
With Phosphate conversion
coatings
With colorless passivate
With colored passivate
II
III
IV
V
VI
Minimum Salt Spray
(hours)
NA
96
12
NA
72
120
Table 10. Salt Spray Hourly Requirements
Chemical Process
Salt Spray Hours
Anodizing
Chemical Conversion Coating
Black Oxide
Primer
336 hours
168 hours
96 hours
336 hours
40
4.2 Hull Cell Analysis
Analysis of any plating bath can be analyzed by what is known as a Hull Cell. A Hull Cell is
a miniature version of a plating bath with the same parameters. Most Hull Cells come with
equipped air agitation, heat, and timers. A Hull cell is shown in Figure 15, which includes a heat
source and air agitation. The Hull Cell was patented by R.O. Hull in 19391. A Hull Cell is
described as a “Trapezoidal box of a non-conducting material; an anode is laid against the
sloping side, connected to a current source with alligator clips”. A current is passed through the
Hull Cell; the current density along the sloping side varies in a known manner. The character of
the Hull Cell Panel varies with varying current densities. The highest current density is
ascertained when the panel is closest to the Anode. The lowest current density is ascertained
when the panel is farthest away from the Anode. A Hull cell ruler is used to determine the
plating characteristics for a range of current densities as depicted in Figure 16.
Figure 15. A Kocour 267 mL Hull Cell.
41
One end of Hull Cell
lined up here
One end of Hull Cell
line up here
Figure 16. Hull Cell Ruler.
A Hull Cell can either hold 267 ml or 1000 ml of plating solution. Chemical additions or
pH adjustments can be made in the Hull Cell, and then scaled up to be made in the actual
plating tank. Two grams of material added to the sample corresponds to a 1 ounce per gallon
addition to the plating tank. The current density of at any point on the cathode is represented
by Equation 6, where L = length (cm) along the cathode, I = Current in amperes, and the current
density can be found in Amps per meter squared, which is converted to Amps per foot squared
because of its simplicity in Industry. Eq 19 is not used readily to determine the current density
in industry; however a tool that is readily used is a Hull Cell ruler. After the Hull Cell panel is
plated, it is lined up along the Hull Cell as depicted by Figure 16, and the current density range
is shown at 2 amps.
Current Density (A/m2) = 100I (5.102 – 5.24 log L)
(19)
The Hull Cell produces a deposit that is a true reproduction of the plating chemistry
obtained at various current densities within the operating range of a particular system.
42
Specifically, it mimics operational variables in the actual plating bath, such as pH, current
density, temperature, and air agitation2. Other plating factors that can be monitored include
organic and metallic contamination, addition agents, covering power, and brightness range of
plating deposit. Adjustments can be made in a Hull Cell before chemical additions are made to
the plating bath. Table 11 shows a typical Hull Cell sequence of steps. A dull, hazy hull cell panel
will be evident if no brighteners are added. However, after numerous additions of Zetaplus
Merit Make-up and Zetaplus Merit Maintenance (brightener), the Hull Cell panel was found to
be bright throughout, including the low and high current density areas.
Table 11. Hull Cell Zinc Plating Testing
Panel #
1
2
3
4
ZetaPlus Merit Make-up Hull
Cell Additions (mL)
None
1.5
1.5
1.5
ZetaPlus Merit Maintenance
Hull Cell Additions (mL)
None
None
1.0
1.5
Table 12. Hull Cell Process Sequence
Process
Time (sec)
Temperature (◦F)
Strip (HCl)
Rinse
Alkaline Cleaner
Rinse
Pickle (HCl)
Rinse
Zinc Plating
Rinse
Acid Dip (HNO3)
Rinse
Chromate Coating
Rinse
60
30
300
30
60
30
300
30
30
30
60
30
Ambient
Ambient
150
Ambient
Ambient
Ambient
87
Ambient
Ambient
Ambient
85
Ambient
43
4.3 Plating Tank Analysis
The zinc metal concentration of the plating bath is mostly determined by EDTA
titrations. EDTA is particularly valuable as a titrant because the reagent combines with metal
cations in a 1:1 ratio. This hexadentate ligand is shown in Figure 17. EDTA is not only valuable
because it forms chelates with metal cations, but also that it is stable for titrations, which
makes it useful for the analysis of zinc.
Figure 17. Structure of metal/EDTA complex.
The reaction between zinc and EDTA is mentioned below. As you can see, the formation
constant for the complex is very large at 3.2 X 1016, which means this reaction will take place
rapidly.
Zn2+ (aq) + EDTA4- (aq)
Zn (EDTA)2- (aq)
(20)
Kf = 3.2 × 1016
The actual titration procedure is stated below.
1. Pipette 5 milliliters of plating solution into 250 milliliter Erlenmeyer flask.
2. Add 50 milliliters of DI water
3. Add approximately 25 milliliters of buffer solution to produce pH of 5.15. (To ensure
consistency of results, buffers are used to stabilize the pH).
a. Buffer solution- 90 grams of NaOAc / 500 milliliters of DI water,
44
b. 15 milliliter of Acetic Acid & Dilute to 1.0 liter
4. Add a small amount of Xylenol indicator to produce violet color.
5. Titrate immediately with 0.0575 M EDTA to a yellow endpoint.
6. Calculate concentration of zinc metal. (oz/gal)
The second titration to determine the zinc metal concentration is stated below.
1. Pipette 2 milliliters of zinc plating solution into a 250 milliliter flask and dilute with
100 milliliter of DI water.
2. Add 20 milliliter of Ammonium Hydroxide/Chloride Buffer.
a. 53.5 grams Ammonium Chloride
b. 10 grams Sodium Cyanide
c. 350 milliliters Ammonium Hydroxide
d. Diluted to 1000 milliliter with DI water.
3. Add 10 milliliter of 10% Formaldehyde and 0.5 grams of Eriochrome Black T
Indicator.
4. Titrate with 0.0575M EDTA to a blue endpoint.
5. Calculation: mLs 0.0575M EDTA X 7.5 = g/L Zinc Metal
Total chlorides in the zinc plating bath is important because the conductivity of the
solution increases with chlorides, specifically KCl and NH4Cl. Several ways exist to determine
the total chlorides in the plating bath, however a precipitation reaction is the most feasible. A
specific precipitation reaction is known as the Mohr method, in which sodium chromate is used
as the indicator. The specific sequence of reactions is outlined below.
45
Ag+ + Cl-
AgCl (s)
2Ag+ + CrO42-
(21)
Ag2CrO4(s)
(22)
After all the chlorides have been precipitated as Silver Chloride, the first excess of Silver
Nitrate results in the formation of Silver Chromate precipitate, which indicates the endpoint.
The actual titration procedure is outline below.
1. Pipette 1.0 milliliter sample of zinc plating solution into a 500 milliliter flask and
dilute with 100 milliliters of DI water.
2. Add 10 milliliters of 10% sodium chromate solution.
3. Titrate with 0.153 N AgNO3 to a reddish-brown endpoint.
4. Factor: milliliters of 0.153 N AgNO3 X 7.5 = g/L Total Chlorides
46
CHAPTER V
TRAINING METHODOLOGIES AND QUALITY ASSURANCE
5.1 Introduction to Training Methodologies and Quality Assurance
The next step after the development of a plating process is the training methodologies
and quality assurance. Proper training procedures and quality documents must be
implemented for chemical line operators to follow for consistent results. A quality
management system must be in place to comply with ISO 9001. This standard is based on a
number of quality management principles, which include strong customer focus, the process
approach, and continual improvement. Utilization of ISO 9001 helps ensure that customers get
consistent, high quality products and services. A big part of ISO 9001 is training. Operators
must have procedures to follow to obtain high quality products and services.
Two types of documents that control a process are general work instructions and
specific work instructions. General Work Instructions take information directly from the
process specification; in this case, ASTM B633-13. This standard specification is used across the
country as an industry standard for zinc plating. A customer would typically refer to this
specification with vital information that includes coating thickness requirements14, type of
chromate coating14, and plating times14. Furthermore, it is not feasible for a line operator to
look over a specification in great detail due to time restrictions. This time restriction ultimately
leads to the creation of General Work Instructions and Specific Work Instructions that follow
industry standards. An example of a General Work Instruction includes 1) reference to
specification 2) purpose of General Work Instruction 3) General procedure for chemical line
47
operators to follow for a specific plating process, as well as general guidelines that govern a
process, which include 4) calculations, and 5) level of quality expected. A more detailed
oriented work instruction is referred to as specific work instructions, which clearly define each
process step, indicated by Tables 13 & 14. The time, temperature, and amps are clearly defined in each
step (if applicable).
Sample General Work Instruction
GENERAL WORK INSTRUCTIONS FOR ZINC PLATING ON STEEL
SUBSTRATES
IAW ASTM B633-13
1) Specification
2) Purpose of General Work Instruction
Purpose: To electrodeposit zinc coatings on iron or steel parts to protect them from
corrosion.
General Procedures:
3
1. The work order will specify one of four standard thickness classes and the type of
finish required as indicated in Table 1 and Table 2 attached.
2. Work Instructions xxxx are for zinc plating with clear chromate coatings at four
varying thicknesses.
3. Work Instructions xxxx are for zinc plating with yellow chromate coatings at four
varying thicknesses.
4. After the Work Instruction to be used is selected, the surface area must be
determined for the parts and racks to be ran through the production line. The surface
area can be measured in square inches and then converted to square feet.
5. A Hull Cell must be run prior to production to verify the Zinc plating bath constituents
are correct.
6. Fill out the Amps, Date, Load #, and Immersion Time on paper to develop a chemical
addition baseline. (Zetaplus Merit Make-up & Maintenance)
Example:
48
X in2 / 144 in2 = ft2
X = wetted surface area in square inches
5. The immersion time in the zinc plating TK-X must be determined by the following
equation:
Immersion Time = (Desired Thickness x 14.3 x 60 min) / ASF / 0.97
ASF range = 20-40 ASF
Desired Thickness = 0.2- 1.0 mil (5-25 µM), as specified on Work Order
6. The total Amperage must be calculated.
Example:
ASF X ft2 = Amperage
7. The Work Instructions can now be followed.
Table 1: Thickness Classes for Coatings per ASTM B-633-13
Service Condition
Classification Number &
Conversion Coating Suffix
Thickness Range in Mils (µM)
SC 4 (Very Severe)
Fe/Zn 25
1-2 (25)
SC 3 (Severe)
Fe/Zn 12
0.5-1 (12)
SC 2 (Moderate)
Fe/Zn 8
0.3-0.5 (8)
SC 1 (Mild)
Fe/ Zn 5
0.2-0.3 (5)
Table 2: Finish Type and Corrosion Resistance Requirements per ASTM B-633-13
Type
Description
Minimum Salt Spray (Hours)
I
As-Plated without
supplementary treatments
NA
II
With Colored chromate coatings
96
III
With colorless chromate
conversion coatings
12
4
49
Plating Time Calculations for Various Zinc Coating Thicknesses and ASF
Example Calculations: Assume 97% cathode efficiency for acid chloride process.
20 ASF at varying thicknesses:
1. (1 mil X 14.3 X 60 min) / 20 ASF = 42.9 minutes /0.97 = 44.2 minutes
2. (0.5 mil X 14.3 X 60 min) / 20 ASF = 21.5 minutes /0.97 = 22.1 minutes
3. (0.3 mil X 14.3 X 60 min) / 20 ASF = 12.9 minutes /0.97 = 13.3 minutes
4. (0.2 mil X 14.3 X 60 min) / 20 ASF = 8.6 minutes / 0.97 = 8.8 minutes
25 ASF at varying thicknesses:
1. (1 mil X 14.3 X 60 min) / 25 ASF = 34.3 minutes /0.97 = 35.4 minutes
2. (0.5 mil X 14.3 X 60 min) / 25 ASF = 17.2 minutes /0.97 = 17.7 minutes
3. (0.3 mil X 14.3 X 60 min) / 25 ASF = 10.3 minutes /0.97 = 10.6 minutes
4. (0.2 mil X 14.3 X 60 min) / 25 ASF = 6.9 minutes / 0.97 = 7.1 minutes
30 ASF at varying thicknesses:
1. (1 mil X 14.3 X 60 min) / 30 ASF = 28.6 minutes /0.97 = 29.5 minutes
2. (0.5 mil X 14.3 X 60 min) / 30 ASF = 14.3 minutes/0.97 = 14.7 minutes
3. (0.3 mil X 14.3 X 60 min) / 30 ASF = 8.6 minutes/0.97 = 8.8 minutes
4. (0.2 mil X 14.3 X 60 min) / 30 ASF = 5.7 minutes/0.97 = 5.9 minutes
40 ASF at varying thicknesses:
1. (1 mil X 14.3 X 60 min) / 40 ASF = 21.5 minutes / 0.97 = 22.1 minutes
2. (0.5 mil X 14.3 X 60 min) / 40 ASF = 10.7 minutes / 0.97 = 11.1 minutes
3. (0.3 mil X 14.3 X 60 min) / 40 ASF = 6.4 minutes / 0.97 = 6.6 minutes
4. (0.2 mil X 14.3 X 60 min) / 40 ASF = 4.3 minutes / 0.97 = 4.4 minutes
Coating Requirements:
5
50
1. Unless specified otherwise by the purchaser, a bright, semi-bright, or dull finish shall
be acceptable.
Table 13. Sample Work Instruction for Zinc Plating with Type II Colored Chromate Coating (Courtesy of
H&W Global Industries, inc)
SAMPLE
Spec:
ASTM B633-Latest Revision
Customer:
Type:
Part #:
SC #:
Type II (Colored Chromate)
SC 1
Zinc Plating
0.2 - 0.3 mil
20-40 ASF
WI #:
Surface Area:
Coating:
Qty./ Rack:
Thickness:
Racks/Load:
ASF:
Qty./Load:
SEQUENCE
#
TANK
SOLUTION
1
5
Alkaline Cleaner
2
5
Electrocleaner
3
4
Electrocleaner
Rinse
4
6
Pickling
5
7
Pickling Rinse
6
8
Zinc Plating
7
9
Zinc Plating Rinse
8
10
Acid Dip
9
11
Acid Dip Rinse
10
14
11
12
TIME
Dwell Time = Time in
solution
TEMP ° F
Set Point = control
setting
5-10 minutes
140 - 180
Set Point: 150
30 sec-2 minutes
140 - 180
Set Point: 150
1-2 minutes
AMBIENT
1-5 minutes
AMBIENT
1-2 minutes
AMBIENT
4-9 minutes
65-95
Set Point: 90
1-2 minutes
AMBIENT
30 sec-1 minute
AMBIENT
AMBIENT
Colored
Chromate
45 sec-1.5 minutes
Optimal: 60 sec
68-86
Set Point: 75
Colored
Chromate Rinse
1-2 minutes
AMBIENT
15
Compressed Air
Dry
Varies
AMBIENT
X
REMARKS
1-2 minutes
Date:
Date Created
By:
Rev. Date:
By:
Approved By:
51
REMARKS
15-20 ASF
3-6 Volts
Remove all rust on parts-if
applicable
20-40 ASF
3-9 Volts
Table 14. Sample Work Instruction for Zinc Plating with Type III Colorless Chromate Coating (Courtesy
of H&W Global Industries, inc)
SAMPLE
WI #:
Spec:
Customer:
Type:
Part #:
SC #:
Surface Area:
Coating:
Qty./ Rack:
Thickness:
Racks/Load:
ASF:
ASTM B633-Latest Revision
Type III (Colorless Chromate)
SC 1
Zinc Plating
0.2 - 0.3 mil
20-40 ASF
Qty./Load:
SEQUENCE
#
TANK
SOLUTION
1
5
Alkaline Cleaner
2
5
Electrocleaner
3
4
Electrocleaner
Rinse
4
6
Pickling
5
7
Pickling Rinse
6
8
Zinc Plating
7
9
Zinc Plating Rinse
8
10
Acid Dip
9
11
Acid Dip Rinse
TIME
Dwell Time = Time in
solution
TEMP ° F
Set Point = control
setting
5-10 minutes
140 - 180
Set Point: 150
30 sec-2 minutes
140 - 180
Set Point: 150
1-2 minutes
AMBIENT
1-5 minutes
AMBIENT
1-2 minutes
AMBIENT
4-9 minutes
65-95
Set Point: 90
1-2 minutes
AMBIENT
30 sec-1 minute
AMBIENT
1-2 minutes
AMBIENT
45 sec-1.5 minutes
Optimal: 60 sec
68-86
Set Point: 75
12
Clear Chromate
AMBIENT
13
Clear Chromate
Rinse
1-2 minutes
11
AMBIENT
X
Compressed Air
Dry
Varies
12
REMARKS
10
Date:
Date Created
By:
Rev. Date:
By:
Approved By:
52
REMARKS
15-20 ASF
3-6 Volts
Remove all rust on parts- if
applicable
20-40 ASF
3-9 Volts
Using the General Work Instruction for reference, a chemical line operator can utilize
the calculations given to determine the plating time in the bath, derived by Faraday’s Law. The
line operator would not be responsible for determining the plating time using Faraday’s law,
but would have to determine the plating time based on the current density. The calculation
derived by Faraday’s Law is shown by Eq 23. Where Time (min) = Plating time required for a
specific thickness and current density, Factor (F) = Faraday’s Factor which is 14.3 for zinc plating
processes, Thickness (mils) = Required thickness in accordance with ASTM B 633, Current
Density = Amps/ft2 which varies from 20-40, and C.E = Cathode Efficiency of a specific plating
process, which is 97% for an acid chloride process.
Plating time calculations for various coating thicknesses within a range of Current
Densities are shown in Table 15, as expected, coating thickness increases with plating time.
Furthermore, increases in current density, decrease plating times.
Time = (Factor (F) x Thickness x 60) / Current Density (ASF) / C.E
(23)
Table 15. Plating Time Calculations for Various Zinc Coating Thicknesses and ASF
Amps/Ft2 (ASF)
20
Plating Thickness (mils)
0.2
0.3
0.5
1.0
Plating Time (min)
8.8
13.3
22.1
44.2
25
0.2
0.3
0.5
1.0
7.1
10.6
17.7
35.4
30
0.2
0.3
0.5
5.9
8.8
14.7
53
40
1.0
29.5
0.2
0.3
0.5
1.0
4.4
6.6
11.1
22.1
The quality assurance aspect of the zinc plating process is ultimately to make a good
looking part that is capable of meeting the requirements of ASTM B-633. Figure 18 depicts a
zinc plated part with a yellow passivate coating, and Figure 19 depicts a rack of zinc plated parts
with clear passivate coatings.
Figure 18. Zinc Plating with Yellow Passivate Coating.
54
Figure 19. Zinc Plating with Clear Passivate Coating
55
CHAPTER VI
COST ANALYSIS
6.1 Zinc Plating Chemicals
The acid chloride zinc process is a relatively low cost plating process15, as compared to
other processes such as Nickel plating, Silver plating, and Gold Plating. Zinc is a low cost metal
that requires very few supplementary additions. Tables 16 and 17 gives a cost analysis of each
chemical and the total cost of all the chemicals. Zinc chloride would only need to be purchased
for the initial charge, because the zinc ball anodes replenish the zinc metal ions in solution.
Furthermore, ammonium chloride and potassium chloride require seldom additions, because
the only loss of chemical is due to drag-out. Constant additions of Zetaplus Make-up and
Zetaplus Maintenance are required because the chemicals are plated onto the substrate, which
result in a larger chemical use. These chemical prices are for the initial charge only, these costs
do not include the maintenance of the plating bath.
56
Table 16. Chemical Costs
Chemical
Concentration
Cost per
Pound/Gallon
Cost ($)
$14.55
Size of
Number of
Container Pounds/Gallons
Needed
5 gal
15 gal
$11.57
5 gal
5 gal
$57.9
39.0 g/L
51.75 g/L
$3.27
$0.69
50 #
50 #
100 #
100 #
$327.2
$69.2
167.3 g/L
$0.75
55 #
330 #
$248.0
15% b/v
10% b/v
0.5% b/v
$17.63
$17.63
$30.35
5 gal
5 gal
5 gal
35 gal
20 gal
5 gal
$617.0
$352.5
$151.8
NA
$1.55
250 #
NA
$387.6
NA
NA
NA
NA
$2429.5
ZetaPlus Merit 5% b/v
Make-up
ZetaPlus Merit 0.07% b/v
Maintenance
Zinc Chloride
Ammonium
Chloride
Potassium
Chloride
Lanthane 316
Lanthane 316
+ Tri-Yellow
Dye
Zinc ball
Anodes (99%
Purity)
Total Cost
Table 17. Zinc Plating Chemical Costs
Chemical
ZetaPlus Merit Maintentance
Cost ($)
57.85
Container size
5 Gal
ZetaPlus Merit Make-up
72.77
5 Gal
Lanthane 316
88.14
5 Gal
Tri-Yellow Dye
151.77
5 Gal
Zinc Chloride
163.60
50 #
Ammonium Chloride
34.59
50 #
Potassium Chloride
41.34
55 #
Zinc Ball Anodes
387.55
250 #
57
$218.3
Zinc Ball Anodes Cost Make-up:
Table 18. Maximum Square Footage for 250 Ibs of Zinc Ball Anodes
SC level
Plating Thickness (mils) Weight per Area (g/ft2)
Maximum Square
footage Allowed (ft2)
1
0.2
3.3
34,293.5
2
0.3
5.3
21,405.9
3
0.5
7.93
13,702.4
4
1.0
16.6
6,851.2
Eq 24 is used to determine the weight (g) per square foot of zinc electro deposition. Zinc ball
anodes are a cost that must be accounted. The Density of metallic zinc = 7.14 g/cm3, the zinc
ball anodes come in increments of 250 Ibs, which costs $387.5. The amount of zinc ball anodes
used does not account for plating solution lost due to drag-out.
Weight (grams) = Density (g/cm3) X Area (ft2) X Thickness (mil)
(24)
Where Density = 7.14 g/cm3 = 445 Ib/ft3 = 201,848.44 g/ft3, Thickness = 25 microns (1 mil) =
8.202 X 10-5 ft, Area = 1 ft2, Weight = 16.6 g/ft2. It would take approximately 6,851.2 ft2 plated
at 1 mil to use 250 pounds of Zinc ball Anodes as portrayed by Table 18. These results would be
included in the total price for running customer parts.
58
CHAPTER VII
SUMMARY AND CONCLUSION
All three zinc plating processes have pros and cons; however the acid chloride process is
perhaps the best in terms of brightness, cost and plating times. Acid chloride processes make
up approximately 50% of the three processes. The alkaline cyanide process is still being used
today, but is slowly getting phased out, due to strict environment regulations. The alkaline noncyanide process is the cheapest process, because only NaOH is used, however the finish is much
duller than the acid chloride process. Also, the cathode efficiencies for the alkaline non-cyanide
and alkaline cyanide are low, which leads to competing reactions, resulting in slower plating
times. The other aspect of my research was the pretreatment and post-treatment. Each plating
process requires a clean surface, or adhesion issues will arise. Also, a post-treatment is a
necessity to add corrosion resistance. Each process requires the correct immersion time,
concentration, temperature, and pH to be successful. All these key parameters play a pivotal
role in operating a zinc plating line to ultimately make a profit.
59
References
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2. Lou, H; Huang, Y. “Electroplating” Department of Chemical Engineering, Lamar University,
Beaumont, Texas, U.S.A, 2003, pgs 1-10.
3. Schwartz, M. Deposition from Aqueous Solutions: An Overview. Handbook of Deposition
Technologies for Films and Coatings - Science, Technology and Applications, 2nd edition.
Bunshah, R.F. Ed.; Park Ridge, New Jersey: Noyes Publications, pg 506.
4. Mordechay, S. “Modern Electroplating” Wiley & Sons, 2010. 20-35.
5. Winand, R. “Electrodeposition of Zinc and Zinc Alloys.” John Wiley & Sons, Inc. 2010, 285302.
6. Sylvester, K. Cleaning and Pretreatment. Presented at PPG Industries Coating Tech,
Allison Park, PA, June 19, 2014.
7. Rudy, S.F. Picking and Acid Dipping. In Metal Finishing [Online]; Tucker, R., Ed.; Elsevier, Inc:
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8. Altmayer, F. AESF Foundation, Piitsburgh, PA. Zinc plating personal online class, 2013.
9. Lanthane 316 Technical Data Sheet. Coventya.
10. West, D.M.; Holler. J.F.; Crouch, S.R. Chapter 20: A Brief Look at Some Other
Electroanalytical Methods. Analytical Chemistry: An Introduction, 7th Ed; Skoog, Douglas
A. Thomson Learning, 2000. pp 511-14.
11. Barnoush, A. Hydrogen Embrittlement. Ph.D Dissertation, Saarland University, 2011.
12. Altmayer, F. Critical Aspects of the Salt Spray Test. Plat. Surf. Finish. 1985. pp 36-40.
60
13. ASTM B117-11, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM
International, West Conshohocken, PA, 2011, www.astm.org
14. ASTM B633-13, Standard Specification for Electrodeposited Coatings of Zinc on Iron and
Steel, ASTM International, West Conshohocken, PA, 2013, www.astm.org
15. Zetaplus Merit Technical Data Sheet. Coventya.
61