Galvanic Corrosion is the third type of corrosion and occurs when one alloy is

Galvanic corrosion is the third type of corrosion and

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Galvanic Corrosion is the third type of corrosion, and occurs when one alloy is electrically coupled to another and is exposed in a conductive liquid. Several factors affect the rate of galvanic corrosion, such as the conductivity of the fluid, where seawater has less conductivity than fresh water; the ratio of the area of the coupled metals; the negative potentials of the metals; and the use of coatings to hinder the galvanic corrosion process [11]. The fourth type of corrosion is called Stress Corrosion Cracking (SCC). This form of corrosion is dangerous since it is not usually detected until it has advanced to a stage that can cause catastrophic failure. Although uncommon in most pumps, this type of failure can occur in several classes of material and thus the pump designer should be aware of its potential. The factors aiding SCC include tensile stress (residual or applied), a susceptible material, an environment capable of causing stress corrosion, and time [11]. Hydrogen embrittlement is the next type of corrosion that results from the combination of hydrogen and a residual or applied tensile stress. This type of damage results in cracking, blistering, hydriding, or a loss of ductility. Damage done by hydrogen embrittlement is occasionally found in pumps as a result of plating processes, such as chrome plating, that is used to rebuild pump shafts [11]. Microbiologically induced corrosion is a result of microbiological activity, which is mostly found in stagnant water. This type of corrosion occurs most frequently when stagnant water remains in the pump when shut down for an extended period of time. Sulphate producing bacteria, which are found in many waters, form tubercles (slimy, reddish hemispherical shaped mounds or colonies) on carbon steel or cast iron. Upon scraping off, they leave behind a saucer-shaped pit, inside of which will be a wet, black deposit. The pitting is a result of sulphuric acid excreted by the bacteria; however, this will not result in premature failure. In cases such as this, biocides have been found on occasion to help alleviate the problem. Finally, the decay of biological organisms can also generate hydrogen sulphide, which affects the protective oxide film on copper base alloys. This biological activity can impair the corrosion resistance of bronzes [11]. The final type of corrosion is called intergranular corrosion. This infrequent type of corrosion is usually caused by local chemical differences, such as the chrome-depleted regions of an austenitic stainless steel plate. Bronze alloys susceptible to this type of corrosion include aluminium brasses, silicon bronzes, muntz metal, and admirality metal. This type of corrosion often leads to corrosion-assisted fatigue cracks when cyclic loading is applied to the metal [11].
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