Pitting corrosion destructive forms

Pitting represents an extremely locahzed attack that produces holes in the metal or alloy. It is one of the most destructive, locahzed forms of corrosion. The pits are small cavities or holes with a depth greater than or equal to the surface diameter. They penetrate the metal, causing equipment failure due to preformation with minimal weight loss. The severity of pitting corrosion is controlled by environment, chloride concentration,... 

Pitting and crevice corrosion are types of localized corrosion. This form of corrosion can be observed for instance on passive metals especially austenitic stainless steels in the presence of certain anions (chloride, bromide). Pitting corrosion (Fig. l-9b) results from a local destruction of the protective passive film, with the formation of a small corrosion anode. Pitting can start at faults in the passive film or at non-metallic inclusions like sulfides in stainless steel. Because of hydrolysis of the corrosion prod-... 

Intercrystalline corrosion is accompanied by a modification of the structure of the barrier layer that tends to crystallise. This form of corrosion leads to exposure of grains and swelling of the metal. Pitting corrosion is also observed. Above 250 a sharp increase in corrosion is found the kinetics changes to a linear law (Figure D. 1.13). A rapid destruction of the metal is observed. 

Pitting is one of the most destructive forms of corrosion it causes equipment to fail because of perforation with only a small percentage weight loss from the entire structure. It may be considered as the intermediate stage between general overall corrosion and complete corrosion resistance. This is shown diagramatically in Figure 2.17. 

Dry abrasive blast cleaning should be used on new steelwork where the main contaminant is mill scale. For heavily rusted and pitted steelwork, increased durability can be obtained by the use of wet abrasive blasting where this is practicable. The water will be more effective in removing the potentially destructive and corrosive soluble iron-corrosion products that form at the bottom of corrosion pits. 

It is a form of localized corrosion of a metal surface where small areas corrode preferentially leading to the formation of cavities or pits, and the bulk of the surface remains unattacked. Metals which form passive films, such as aluminum and steels, are more susceptible to this form of corrosion. It is the most insidious form of corrosion. It causes failure by penetration with only a small percent weight-loss of the entire structure. It is a major type of failure in chemical processing industry. The destructive nature of pitting is illustrated by the fact that usually the entire system must be replaced. 

Pitting A form of localized corrosion which leads to destruction of the passivity by chloride ions and creation of pits on the surfoce. 

At low current densities corrosion can take the form of pitting, while at higher current densities considerable destruction of the metal can occur. Other than uniform dissolution in an active chemical, this is one of the two main causes of unexpected, very rapid corrosion of aluminum (an uncommon event). The other is the presence of mercury ions in the environment (see the preceding section on Deposition Corrosion ). 

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