I have just joined this forum - to make some suggestions
Have worked in swimming pool industry for many years dealing with stainless steel parts
Stainless steel is only stainless when i has access to oxygen. The oxygen reacts with the metal additives (molybdenum I believe) to create a protective film which prevents further corrosion. If this film is scratched, the protective coating reforms.
Without oxygen, the material can erode. This is sometimes called crevice corrosion. It is particularly prevalent with $304 and lesser grades of stainless steel. #316 does not appear to have this problem.
When we fit round O rings to stainless parts
, the narrow space between the O ring and flat stainless steel surface becomes de-oxygenated, causing corrosion where the o ring touches the stainless. The trick here is to use square section O rings.
When the stainless was covered with sikaflex, it may have created a region deficient in oxygen.
I hope this helps
The corrosion occurs within the metal, and does not require stray electrical current
or disimilar metals to form. From metallurgical viewpoint, the dissimilar metals may be the protected shiny surface, and the unprotected corroded surface. The corrosion occurs within the metal part, and not to an external component.