PITTING CORROSION P016
This is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others.
It occurs when the corrosive environment penetrates the passivated protective film in only a few areas as opposed to the overall surface.
Be aware that halogens will penetrate passivated stainless steel. Referring to the galvanic chart you will note that passivated 316 stainless steel is located nine lines from the bottom and active 316 stainless steel is located thirteen lines from the top.
Pit-type corrosion is therefore simple galvanic corrosion as the small active area is being attacked by the large passivated area. This difference in relative areas accelerates the corrosion causing the pits to penetrate deeper.
The electrolyte fills the pits and prevents the oxygen from passivating the active metal so the problem gets even worse.
This type of corrosion is often called concentrated cell corrosion. You will also see it under rubber parts that tend to keep oxygen away from the active metal parts, retarding its ability to form the passivated layer.
To prevent this type of corrosion in salt waer applications, we put a layer of zinc oxide between the rubber part and the stainless steel component.