Examples of corrosion of austenitic stainless steel joint surfaces attacked by liquid media penetrating gaskets are described. The author postulates liquids of high electrical conductivity containing weak oxidizing and reducing agents are principally responsible for the pitting attack of the gasket bearing surfaces. Presence of chloride salts or organic chlorides in the gasketing material may contribute to stress corrosion cracking of the joint surface. Graphite seems to have no major influence although in some cases it may tend to aggravate the attack.
Corrosion is attributed to pools of liquid wicked through the gasket via fibers, as in gaskets with asbestos fiber fillers. Wicking by short fiber fillers is less than by long fibers. The small pools of liquid create active-passive and/or oxygen concentration cells with the entire inside surface of equipment as cathodic areas. This imbalance tends to accelerate pitting.
Control is achieved by using gasket materials without wicking constituents or which have continuous exterior surfaces of non-wicking material. Alteration of the process chemicals and application of a barrier coating between the gasket and the bearing surface of the joint were considered as control measures but not tested. Tests using 37 gasket materials compressed in racks suspended in the process stream showed non-fibrous materials to be superior.
Correct joint design is essential to reduce crevices which permit process liquid to seep between the gasket and the bearing surface. The author recommends that all mating surfaces be machined, that design prevent bolt-load from deforming gasket surfaces and that built-up stainless steel mating surfaces on carbon steel joints be avoided. Examples of correct and incorrect joints are shown. Testing gasket materials before use is recommended.
