Download Cathodic Protection of Tower Anchors Presented by Tom McKigney

Cathodic Protection of
Tower Anchors
Presented by Tom McKigney
Brance-Krachy Co., Inc.
Houston Texas
to The Society of Broadcast Engineers
September 10, 2002
THE CORROSION CELL
When two dissimilar metals are connected in an electrolyte such as soil, a
corrosion cell is formed. One metal becomes the anode and suffers
corrosion while other metal becomes the cathode and corrosion is
prevented.
Anodic and cathodic areas exist on the surface of all steel structures due to
slight variations in material. Composition, local stresses, differences in
coating condition and availability of oxygen are all factors in creating this.
In the absence of cathodic protection corrosion will eventually occur.
ELEMENTS OF A CORROSION CELL
▪
▪
▪
▪
ANODE & CATHODE
ELECTRICAL POTENTIAL
ELECTROLYTE
METALLIC PATH
THE GALVANIC SERIES
Anchor Corrosion Cell
Stray Current Corrosion
ANCHOR CORROSION MITIGATION
▪1. Galvanizing and Epoxy Coatings
▪2. Concrete Encasement.
▪3. Electrical Isolation.
▪4. Cathodic Protection.
▸ Galvanic Anodes
▸ Impressed Current
▪When used in conjunction with Cathodic Protection the other methods
▪ can be beneficial in mitigating corrosion problems.
Galvanizing and Epoxy Coatings
Not a solution
▪ Galvanizing depletes rapidly underground
▸Only effective against atmospheric corrosion
▪ Epoxy coatings can accelerate failure if used alone
▸Holidays give rise to the small anode large cathode
effect
Concrete Encasement
▪
▪
▪
▪
Used for structural strength
May provide some corrosion protection
Corrosion can cause concrete failure
Cracks in concrete may set up corrosion cells
Electrical Isolation
CATHODIC PROTECTION SYSTEMS
▪ The cathodic protection of metals has been around since 1824.
▪ Sir Humphrey Davy attached zinc plates on the hulls of British naval
vessels to retard corrosion of the copper sheathing.
▪ Cathodic Protection continues to be an effective method for mitigating
the destructive action of corrosion.
▪ The principal of cathodic protection involves the introduction into the
electrical circuit a metal that is more electro-negative than the existing
anodic and cathodic areas.
▪ This additional metal becomes the anode and will corrode while
providing current to the metal it is protecting, thereby overcoming the
local anodic area and making them cathodic.
CATHODIC PROTECTION
GALVANIC SYSTEMS
▪ The classic Cathodic Protection solution to a galvanic corrosion
problem is to introduce an anode of a suitable alloy of magnesium or
zinc which will suffer corrosion and can be sacrificed in protecting the
cathode. This approach is used in galvanic systems.
Anchor Cathodic Protection
IMPRESSED CURRENT SYSTEMS
As an alternative solution, the protective current needed to make
the structure cathodic may be obtained by applying low voltage
Direct Current derived from normal ac mains supply.
This is achieved by transformer /rectifiers which supply DC to
specially designed anodes which will dissipate large currents
without themselves suffering significant wastage.
IMPRESSED CURRENT SYSTEM
Cathodic Protection System
Design Requirements
Anode Selection depends on:
Soil Resistivity
Current Requirement
Design Life
Maintenance and Record Keeping
Check system and record
Potential Readings on a
regular schedule.
This should be done at
least once a year.