pitting+and+crevice+corrosion

=Pitting Corrsoion and Crevice Corrosion  =


Pitting corrosion is a localized type of corrosion that may either be shallow or deep but is often difficult to identify [1]. By the time pitting corrosion is visible to humans, the damage is often extensive [1]. Stainless steel is affected by pitting corrosion rather than uniform corrosion due to the passivation layer.

Mechanism
It occurs inside small holes (pits) on the surface of a metal's passive oxide or hydroxide layer, which is attacked locally by, among other species, chloride ions. Increasing the chloride ion concentration, increasing the temperature, and decreasing the pH will all contribute to an increased rate of corrosion [1].



Background
Crevice corrosion is a similar type of corrosion that occurs in stainless steel; it consists of a localized corrosive attack [2]. It takes place between two substances (most commonly two metals) in narrow spaces or openings. Bolted connections, as well as the presence of dirt, increase susceptibility [3].

The driving force of this type of corrosion is the difference in level of oxygen between the internal crevice and the external surface [3]. The crevice is depleted of oxygen and becomes the anode. This creates a highly corrosive environment inside the crevice. The low-oxygen area corrodes, acting as the anode, while the high oxygen area is protected, acting as the cathode.

This phenomenon is demonstrated in the following image:

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Reference:
[1] Metallurgical Consultants, “Pitting Corrosion.” [Online]. Available: [] [Accessed: November 28, 2009]. [2] Amtec Consultants, “Corrosion Guide.” [Online]. Available: []. [Accessed: November 28, 2009].

[3] Metallurgical Consultants, "Crevice Corrosion." [Online]. Available: []. [Accessed: November 29. 2009].