The different types of corrosive attack, especially for coatings can be classified as
(i) General corrosion, corresponding to about 30% of failure, where the average rate of corrosion on the surface is uniform and as.
(ii) Localized corrosion, corresponding to about 70% of failures.
The latter comprises:
- Galvanic corrosion occurring when two dissimilar metals are in contact with each other in a conductive solution (electrolyte), the more anodic metal being corroded, while the more cathodic one is unaffected. The electrolyte plays a key role, as well as the relative surface contact area; the smaller the anodic to cathodic area ratio is, the more severe is the anodic metal corrosion. For example, the protection of low-carbon iron part from atmospheric corrosion by a coating may use either an anodic coating (nickel) or cathodic one (aluminum or zinc). In the first case, no discontinuity in the coating can be tolerated, while it has no importance with the cathodic coating.
- Inter-granular corrosion, occurring when a chemical element is depleted during the coating or bulk material manufacturing, e.g. during heat treatment.
- Pitting, which is a localized corrosion characterized by depression or pit formation on the surface. It occurs for example when stainless steel is corroded by chloride-containing solutions.
- Transgranular corrosion is mainly due to high static tensile stress in the presence of a corrosive environment. It can be intergranular but also transgranular when cracking occurs. The coating material and its microstructure play an important role in this type of corrosion.
Thus coatings can be used against corrosion:
- As sacrificial coatings (cathodic behavior relatively to ions, for example, Zn or Al on steel): the thicker they will be the longer will be the protection (typical thickness varies between 50 and 500 µm, the most frequent one being around 230 µm),
- As dense as possible (even sealed) if they have an anodic behavior and used against either atmospheric or marine corrosion, and high-temperature corrosion: oxidation, carburization, nitriding, sulfidation, molten salt, molten glass…
Corrosive wear occurs when the effects of corrosion and wear are combined, resulting in a more rapid degradation of the material surface. A surface that is corroded or oxidized may be mechanically weakened and more likely wear out at an increased rate. Furthermore, corrosion products, including oxide particles, which are dislodged from the material surface can subsequently act as abrasive particles. Stress corrosion failure results from the combined effects of stress and corrosion. At high temperatures reactions with oxygen, carbon, nitrogen, sulfur or flux result in the formation of oxidized, carburized, nitrided, sulfidized, or slag layer on the surface.
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