My stainless steel boat hook is suffering from pitting. Why is this?

Jack Sparrow, Jamaica

Sorry to hear that, Jack. Pitting corrosion is a form of localised corrosion, which can occur on most metals in specific environments. It can cause leakage or rupture of pipework or vessels and may leak to the development of stress corrosion cracks.

Pitting takes place in two stages; pit initiation and pit propagation. During the pit initiation stage, very little is visible to the naked eye, although localised breakdown of protective surface films takes place at anodic sites. This is often temperature critical, with pit initiation not occurring below a certain temperature. During the pit propagation or growth phase, local cell conditions become established as chloride and other anions are drawn into the pit site and the solution within the pit becomes acidic. The metal area surrounding the pit is cathodic to the pit and corrosion here is often negligible. 

Growing pits may become stifled by corrosion products and may become inactive or inactive pits may become active again as conditions change. Pits usually grow faster on upward facing surfaces such as the 6 O’clock position in horizontal pipes, as in this situation the aggressive localised solution remains easier within the pit.

Aluminium or stainless steels are susceptible to pitting in chloride-containing waters, as the small Cl- ion is able to penetrate and break down the passive oxide film. However, the concentration of chloride needed to cause pitting depends very much on the alloy type and composition. For instance, super-duplex stainless steels will resist pitting in sea-water, while 304 austenitic stainless steels may pit in waters containing < 100mg/L Cl-. Pitting of low alloyed steels and iron occurs in oxygen rich waters under corrosion tubercles. Copper can undergo pitting in different water compositions depending on temperature (more about this later).

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