GENERAL CORROSION G006
This type of corrosion occurs when there is an overall breakdown of the passive film formed on stainless steel. It is the easiest to recognize as the entire surface of the metal shows a uniform “sponge like” appearance.
The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C) of either the product or the metal part.
If the rotating portion of a mechanical seal is rubbing against some stationary component, such as a protruding gasket or fitting, the protective oxide layer will be polished off and the heat generated will increase the corrosion as noted above. This explains why corrosion is often limited to only one portion of the metal case.
There are many good publications available to help you select the proper metal for any given mechanical seal application. As a general rule, if the wetted parts of the equipment are manufactured from iron, steel, stainless steel or bronze, and they are showing no signs of corrosion, grade 316 stainless is acceptable as long as you do not use stainless steel springs or metal bellows because of chloride stress corrosion problems.