An Introduction to Forms of Corrosion
An introduction to the general definition of corrosion of metals and alloys was provided in the previous blog post. Different forms of corrosion will be introduced in a series of posts, starting from uniform and galvanic corrosion.
The first step to mitigating corrosion is to identify what form of corrosion is involved. Often, only one form of corrosion causes the failure of a metallic asset. Sometimes, however, several corrosion forms are involved. Further, some corrosion attacks can be triggered by one form of corrosion and propagate as another form. For example, a failure due to stress corrosion mostly initiates from corrosion pits.
Uniform or General Corrosion
Uniform corrosion, which perhaps is the most common form of corrosion, happens when the entire exposed surface of a metal is attacked with equal intensity. This form of corrosion often leaves behind a scale (e.g., rust). At a microscopic level, the oxidation and reduction reactions occur randomly over the surface without any localization. The atmospheric corrosion of iron or steel in aggressive outdoor environments (i.e., rusting of steel) and the tarnishing of silverware are some familiar examples of uniform corrosion. Among different forms of corrosion, uniform corrosion the least objectionable because it can be predicted and designed with relative ease. Designing the corrosion allowance based on the possible metal thickness loss is one of the simplest mitigation methods with uniform corrosion attacks.
Uniform corrosion occurs when the flux of metal ions from the surface and the flux of cathodic reactants to the surface are uniform to atomic dimensions. General thinning of the metal continues until failure. This is the most important form of corrosion with regard to tonnage metal loss.
Gate to the Northwest Passage, a steel sculpture located in Kitsilano, Vancouver. The sculpture is made of weathering steel demonstrates an interesting example of uniform corrosion. The iron rust uniformly covered the entire structure.
Galvanic corrosion occurs when two dissimilar metals or alloys are electrically coupled and exposed to a corrosive environment. The dissimilarity of the two metals could be due to having different chemical compositions or microstructures. Because each metal tends to dissolve (i.e., different corrosion resistance), when two dissimilar metals are joined together there will be a potential difference between them. The more reactive metal (i.e., less corrosion-resistant metal) in the particular environment will experience corrosion, while the more noble metal (i.e., more corrosion-resistant metal) will be protected from corrosion. In other words, the former will be anode and the latter will be the cathode. Generally, the cathode corrodes very little or not at all in this form of couple. A familiar example of a galvanic attack is the corrosion of a steel screw in contact with a stainless steel kitchen sink. Depending on the nature of the solution, one or more of the reduction reactions will occur at the surface of the cathode material. The most practical method to preventing galvanic corrosion is to avoid direct contact of dissimilar metals or coupling metals with relatively similar corrosion resistance.
A familiar example of galvanic coupling: corrosion of a new bike chain in contact with old rusty gears. Although both gears and chain are made of carbon steel, the rusted gears have more corrosion resistance than the new chain, which makes them more noble (i.e., cathode) compared to the chain. While the chain suffers from corrosion, the gears are protected from corrosion attacks.
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