Pitting is a classic form of localized corrosion that often is found in a wide range of applications. It can occur on plain carbon steels but it is found much more frequently on alloys that form robust protective (or passive) films. Examples of these materials include many stainless steels, plus many copper, aluminum and nickel alloys. The passive films on such alloys normally prevent uniform or general corrosion but when they breakdown at specific spots the substrate metal is exposed. Pitting attack can then initiate if the damaged film cannot repair itself quickly.

Carbon steels also form passive films but they are weak in comparison to the films formed on the previous classes of alloys. Typically carbon steels experience total and not localized film breakdown and repair is rare. Thus general corrosion can occur and that form of attack is much more common on carbon steels than pitting.

The electrochemical reactions in pitting consist of the anodic oxidation reaction within each pit and one of the possible cathodic reduction reactions on the area immediately surrounding the pit. The typical first step in the process is passive film breakdown by local contact with specific aggressive ions. The most common of these aggressive species is chloride ions. Local conditions such as higher concentrations of these ions or other halides (bromides, fluorides or iodine) or higher local temperature conditions accelerate damage to passive films. The films can also be damaged mechanically due to surface scratches on the metals. Once initiated the pitting process is said to be autocatalytic because electrochemical conditions inside each pit get worse with time and the rate of attack increases.

Unlike general corrosion, pitting corrosion occurs on small, segregated spots and thus detection and monitoring to indicate the degree of penetration is difficult. This makes it more dangerous. For example, attempting to assess the presence or extent of pitting damage inside a pipe by making ultrasonic measurements of the pipe’s wall thickness from the outside is a hit and miss process. Different pits initiate at different times and penetration into the metal varies so finding the deepest pits by the same external NDE method is uncertain.

Detection of pits is somewhat aided by knowing the areas where they typically occur. They are most likely on the lower, horizontal portions of wetted facilities such as in piping, storage tanks and pressure vessels. This orientation promotes the pitting process and metal penetration. Pitting is also favored in areas where the corrosive medium is stagnant or has a very low fluid velocity. Areas of normal velocity discourage pitting.

Control of pitting can take different forms.  Alloy selection to provide inherent resistance is frequently used. The ability of an alloy to have and to be able to maintain its passive film in the presence of aggressive conditions largely will depend on its composition. For example, in the commonly used austenitic stainless steels, the more chromium, nitrogen and especially molybdenum that are present the more likely the alloy will be resistant to pitting in solutions containing chloride ions. The addition of chemical corrosion inhibitors to the corrosive medium can be an effective control method but maintaining the correct concentration in all areas of a system is critical. Having a too small concentration of particular inhibitors can increase pitting rates above the rates that occur with zero inhibitor present. If feasible other control methods include eliminating “dead leg”, stagnant areas in piping systems, limiting service temperature or concentration of aggressive ions and providing for complete drainage of all residual corrosive liquid during prolonged shutdowns. Pitting often initiates during shutdowns in system areas that cannot be fully drained.         

 

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