The seal or packing removes the oxide layer as the shaft or sleeve rubs against it. The oxide reforms when the active metal is exposed and the process starts all over again. This corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications. The low PH salt water attacks the active layer that is exposed Because of the lack of fluid flow over the attacked surface oxygen is not available to re passivate the stainless steel.
Corrosion continues unhampered under the rubber and tight fitting clamp.
The inside of the O-ring groove experiences the same corrosion as the shaft or sleeve. The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The liquid tries to replace the missing elements as it flows through the system. The un-dissolved metals often coat them selves on the mechanical seal faces or the sliding components and cause a premature seal failure. Heat accelerates the process.