In our first post in our series on the forms of corrosion, we introduced uniform corrosion and galvanic corrosion. In this post we explore three additional forms of corrosion: passivity, pitting, and crevice corrosion.
“Passivity is the enabler of our metals-based civilization” D.D. Macdonald Pure Appl. Chem., 71 (1999)
Several metals such as iron, chromium, and aluminum are inherently reactive because they occur in nature as their ores rather than in elemental form. However, these metals and their alloys are widely used in industry because, under particular environmental conditions, they lose their chemical reactivity and become inert. This phenomenon, which is known as passivity, is a result of the reaction of the metal with water and/or oxygen to form a highly adherent and very thin oxide film on the metal surface. Familiar examples of passive alloys are stainless steel, which is used in different applications from kitchenware to the petrochemical industry. Passivity affects the kinetics of the corrosion process (i.e., corrosion rate) by producing a protective film that acts as a barrier against corrosion attacks. However, passive films are not perfect and certain conditions lead to the breakdown of passivity, and hence, localized corrosion. Pitting, crevice and stress corrosion cracking are the main forms of localized corrosion, which directly or indirectly cause corrosion failures of industrial assets.
Without passivity, most of the technologies that depend on the use of metals in any society could not exist. Passivity is a critical element in controlling the corrosion phenomena.
Once the passive breakdown happens, although the remainder of the passive film remains intact and continues to protect the metal against general corrosion, the localized corrosion proceeds at a considerably high rate because it is concentrated at a fixed area. Furthermore, compared to general corrosion, the presence of localized corrosion is more difficult to detect. The high corrosion rate and difficulty of detection, make localized corrosion dangerous as it can lead to catastrophic failure of the metal piece.
The word “passivity” was first used by C. Schonbein in 1836. Early researchers found that the dissolution of iron in certain acid solutions stopped after first undergoing active dissolution. Oxide or passive films on metals are often very thin and are not visible to the eye. Alloys such as stainless steel tend to have passive films tens to hundreds of angstroms (Å) in thickness.
A schematic of the corrosion pit. Once the passive film breaks down a corrosion pit starts to grow. While the rest of the passive film remains intact and supports the cathodic reaction, the anodic reaction takes place inside the pit with a high dissolution rate.
Pitting is one of the most destructive and treacherous forms of corrosion. During pitting corrosion of passive metals and alloys, local metal dissolution occurs leading to the formation of cavities within a passivated surface area, which causes an asset to fail because of perforation with only a small percent weight loss of the entire structure. Corrosion pits are sometimes isolated or in a cluster and can be small or large in diameter. In practice, pitting corrosion of nearly all passive metals and alloys is commonly observed in the presence of chlorides or other halides. Among engineering alloys, stainless steel suffers the most from pitting corrosion, although pitting has been frequently reported for Ni alloys, Al alloys, Cu alloys, and even galvanized steel. Also, pitting corrosion is one of the main corrosion forms that threaten the integrity of a pipeline. Furthermore, pitting corrosion can trigger other types of corrosion, stress corrosion cracking for example. Metallurgical factors, such as alloy chemical composition and microstructure, as well as environmental variables, like temperature and concentration of chlorides, directly affect the pitting corrosion resistance of an alloy. Because corrosion pits are often covered with corrosion products, it is generally difficult to detect them.
Pitted case of a wristwatch. Sweat contains chlorides which can cause pitting corrosion of the stainless steel case of the watch. High ambient temperature and relative humidity accelerate the pitting corrosion process.
Crevice corrosion is a form of localized corrosion that takes place within narrow clearances (i.e., crevices) or under shielded metal surfaces exposed to a corrosive environment. Although pitting corrosion and crevice differ in their initiation mechanisms, the propagation mechanism for these two forms of localized corrosion is similar. Crevice corrosion is associated with a small volume of stagnant solution (also known as the occluded solution) caused by geometrical clearances, such as under gaskets or seals, between overlapping metal sheets, or under bolt heads.
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