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Cathodic Protection Theory Lecture 2 Summer 8A 2016 METL 2441 Cathodic Protection Cathodic protection • We have learned that wherever direct current flows from an underground or submersed structure into an electrolyte, the metal in the structure will be consumed by corrosion. • If an insulating barrier were to be placed between the metal and the electrolyte, current could not flow. Cathodic protection • In theory this is sound, but not practical in the real world. • Such insulating barriers, or coatings, must be perfect when applied and must remain perfect throughout the life of the structure. Cathodic protection • It is not practical to apply a coating that will meet this stringent criterion. • In fact, a coated structure often will suffer leaks before a structure without coatings. • Because current density is higher at the discharge points, coating pinholes, of a coated structure than it is with uniform corrosion on a bare structure. Cathodic protection • Metal loss from corrosion is less on a coated structure, but the effects are more detrimental. • Cathodic protection is a practical corrosion prevention method for most external surfaces of structures submersed in an electrolyte. • With coated structures the cost for cathodic protection is considerably less than on bare structures. Cathodic protection • We learned that corrosion on the surface of a metal is due to voltage difference between two points on the metal surface. • The more active, or anodic point will discharge current from the metal surface to the electrolyte, resulting in corrosion. • If this current is reduced or eliminated, then corrosion is reduced or eliminated. Cathodic protection • The theory of cathodic protection can be viewed from different perspectives. • Cathodic protection is actually a polarization phenomena. • However, polarization is a complex subject and outside of the scope of this course. • It will be mentioned again when discussing criteria for adequate cathodic protection. Cathodic protection • In this course we will take a simplistic view of cathodic protection as a reduction in the voltage difference, or driving force of a corrosion cell. • We have learned from Ohm’s Law that voltage and current are directly proportional to one another. • If we reduce the voltage, then current will also be reduced and so will corrosion. Cathodic protection • By electrically connecting an external electrode that is more active, or negative, to a structure, current is impressed onto the structure. • Impressing current onto the structure will result in a negative potential shift on the structure. Cathodic protection • The first points on the structure to be affected by the current are the more noble points. • Because of the greater voltage difference between these points and the external electrode. • A cathodic protection system is an intentional corrosion cell. Cathodic protection • The four parts of the intentional corrosion cell are: • Anode – External electrode • Cathode – The structure • Metallic Path – Electrical wire connection between the external electrode and structure • Electrolyte – Conductive medium between the external electrode and structure Cathodic protection • In a cathodic protection system, direct current flows from the cathodic protection anode into the electrolyte. • Through the electrolyte onto the surface of the structure. • Down the structure to the return wire. Cathodic protection • Through the return wire back to the cathodic protection anode. • Which completes the electrical circuit. • Once the entire exposed metal surface of the structure is collecting current, the entire structure becomes a cathode, and corrosion ceases. Cathodic protection • There are primarily two methods of providing cathodic protection current to a structure. • Galvanic anode system • Impressed current system CP GALVANIC ANODE SYSTEMS • Galvanic (or sacrificial) cathodic protection makes practical use of dissimilar metal corrosion. • It is important to note that there must be a substantial potential difference, or driving voltage. • Between a galvanic anode and the structure to be protected. CP GALVANIC ANODE SYSTEMS • The galvanic anode is connected directly to the structure it is protecting. • There are several metals commonly used as galvanic anodes and are as follows. • Aluminum • Magnesium • Zinc Sacrificial Anode Installation (1) Sacrificial Anode Installation (2) CP GALVANIC ANODE SYSTEMS • ALUMINUM • Aluminum anodes are used primarily in seawater applications • They are produced in a variety of alloys, of which the mercury and indium alloys are the most common. FPSO Ships Propeller Offshore Anode Installation CP GALVANIC ANODE SYSTEMS • Indium alloy has a slightly higher corrosion potential, but is less efficient. • Aluminum is preferred for seawater applications, where anode volume is not a constraint. • Aluminum has a low consumption rate. • Aluminum anodes require chloride ions to function, so they are not used in fresh water. CP GALVANIC ANODE SYSTEMS • MAGNESIUM • Magnesium anodes are available in two alloys. • A high potential alloy having a nominal corrosion potential of -1.75 to -1.77 volts referenced a copper/copper sulfate electrode. • A low-potential alloy having a nominal corrosion potential of -1.55 volts referenced a copper/copper sulfate electrode. CP GALVANIC ANODE SYSTEMS • Magnesium is normally used in soils and fresh waters. • Although magnesium can be used in seawater, its consumption rate is high. • The higher potential has a tendency to cause overprotection on certain structures (e.g., aluminum ship hulls). CP GALVANIC ANODE SYSTEMS • At 50%, the efficiency of magnesium anodes is significantly less than the efficiency of other anodes. • This is due primarily to the activity of local-action corrosion cells on the anode surface, which results in self corrosion of the anode. • Although magnesium is relatively inefficient, its high driving potential makes it the preferred anode in high-resistivity soil applications. CP GALVANIC ANODE SYSTEMS • ZINC • Zinc anodes also are commercially available in two alloys. • One for use in soils and fresh waters and the other for seawater applications. CP GALVANIC ANODE SYSTEMS • Zinc may undergo rapid intergranular corrosion at temperatures above 120°F (49°C). • The “high” amp alloy material generally should not be used in stagnant aqueous environments. • Because the primary activating element in the material is cadmium, which may react badly in such environments. CP APPLICATION OF GALVANIC ANODES • Principal cathodic protection system when relatively small increments of current are required and/or low resistivity electrolyte exist. • Local cathodic protection to provide current to a specific area on a structure. CP APPLICATION OF GALVANIC ANODES • Some operators install galvanic anodes at each location where a leak is repaired, rather than installing a complete cathodic protection system. • Such practices may be encountered on bare metal or very poorlycoated systems, where complete cathodic protection may not be feasible due to cost. CP APPLICATION OF GALVANIC ANODES • Typical applications include: • Poorly or incompletely coated buried-valve installations. • Shorted casings that cannot be cleared • Isolated sections where the coating has been badly damaged. • Areas where electrical shielding impairs effective current distribution from remotely located impressed-current systems. CP APPLICATION OF GALVANIC ANODES • In cases of cathodic interference, if conditions are suitable, galvanic anodes can be used at discharge points on the foreign line to safely return interfering currents. • They can also be used to provide protection to structures located near many other underground metallic structures. CP APPLICATION OF GALVANIC ANODES • Galvanic anodes can be used where conditions makes it difficult to install impressed-current systems without creating stray-current interference problems. • Galvanic anodes can be an economical choice for cathodic protection under the above conditions. CP APPLICATION OF GALVANIC ANODES • Galvanic anodes can be used where additional current is needed at problem areas. • Some structures with overall impressed-current cathodic protection systems may have isolated points where additional current in relatively small amounts is needed. • Those requirements can be met with galvanic anodes. CP ADVANTAGES OF GALVANIC ANODES • No external power source required. • Low maintenance requirements. • Small current output resulting in less stray-current interference. • Easy to install. • Easy to add anodes in most cases. • Provide uniform distribution of current. • Minimum right-of-way/easement costs. CP DISADVANTAGES OF GALVANIC ANODES • Low driving voltage/current output. • Many anodes may be required for poorly coated structures. • May be ineffective in high-resistivity environments. • Higher cost per unit ampere than impressed current due to lower efficiency (self-consumption). CP GALVANIC ANODE EFFICIENCY • The efficiency of a galvanic anode depends on the alloy of the anode and the environment in which it is installed. • The consumption of any metal is directly proportional to the amount of current discharged from its surface. CP GALVANIC ANODE EFFICIENCY • For galvanic anodes, part of the current discharge is due to the cathodic protection current provided to the structure and part is caused by local corrosion cells on it surface. • Anode efficiency is the ratio of metal consumed producing useful cathodic protection current to the total metal consumed. • For magnesium, anode efficiency is as low as 50%.