Machinery's Handbook, 31st Edition
Corrosion 549 oxide layer. Areas of oxygen deprivation can become anodic, resulting in hidden pockets of accelerated corrosion. Wet and damp/atmospheric corrosion occur when water and contaminants in the en- vironment form an electrolyte liquid or damp film on the surface of a part, facilitating electrochemical corrosion. Wet corrosion rates generally are highest, while atmospheric corrosion rates depend on the amount and type of moisture. To minimize electrochemical corrosion, the best course of action is to avoid mixing different metals in an assembly and match the potentials of the part(s) to the environment. The next lines of defense are barriers and protective area ratios (see Galvanic Corrosion , below). When those methods are not enough, it is common practice to add one or more auxiliary anodes or cathodes to the system. Current from an external DC power supply may be applied to change the electrical potential of the target material and control the flow of electrons. Some systems can monitor and optimize conditions in the cell by using a reference electrode. Cathodic Protection: Adding one or more sacrificial anodes (either a part or a coating) to a system can protect a part that otherwise would be the anode in an electrochemi- cal reaction. Buried or submerged metal pipes are commonly protected passively by the addition of a sacrificial magnesium anode placed nearby and connected by a wire. An impressed current system includes a DC power supply to supply current, which improves system performance and extends anode life. Cathodic protection is broadly applicable for all metals and alloys, but it is often not suitable for corrosive environments and can accelerate hydrogen embrittlement. Anodic Protection: This newer method employs additional cathodes and an applied current to shift the target material’s potential into passive range. Anodic protection ap - plies to corrosive environments and can be achieved with much lower current density than cathodic protection would require. This method is limited to materials that exhibit active-passive behavior (material surfaces can change from active to passive when ex - posed to oxidizers or applied current). Steel, titanium, and nickel alloys are included in this group. Galvanic Corrosion.—In this electrochemical corrosion process, sometimes called bi- metallic corrosion, dissimilar metals or alloys interact when electrically coupled by a conductive fluid. If the two metals or alloys have different electrochemical potentials, current will flow from the more active (anodic) metal to the more noble (cathodic) metal. In this galvanic couple, the anode will experience dissolution (usually oxidation), while the cathode will experience reduction. In practical terms, the anode will corrode faster or differently than it would alone in the electrolyte, and the cathode usually will be pro- tected. The metals or alloys usually are in direct contact with each other, but this is not necessary for corrosion to occur—it can expand into the entire wetted area if the fluid is highly conductive. Parts in direct contact will experience the most corrosion; as separa - tion between parts increases, corrosion decreases. Intensity of the corrosion process depends on differences in electrochemical poten- tial between two metals, distance between the metals, wetted surface areas of the parts, and conductivity of the surrounding fluid. Other factors include oxygenation, fluid pH, metal composition and variations, presence of films or deposits, passive film stability, and exposure of metals to processes such as cold-working or welding. Corrosion often will intensify as oxygen levels in the electrolyte drop, and oxygen depletion frequently occurs in crevices and other stagnant areas. Galvanic Compatibility: The US military published empirical compatibility informa- tion in MIL-STD-889B, “Dissimilar Metals.” Data in Table 1 represents those results. Because many factors influence galvanic corrosion, this data should be used only as a general guide.
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