Why are my stainless steel nuts rusting?
Stainless steel fasteners are often used in highly corrosive environments. When stainless nuts begin to rust, this can occur when hot forged nuts that are manufactured only to a material grade such as 18-8, SS304, and SS316 or to the ASTM A194 Grade 8 or 8M specifications. None of these steel grades or nut specifications require solution annealing after hot forging which is the culprit. The reason that stainless doesn’t rust is because of the grain structure. Hot forging stainless steel breaks the grain boundaries and that allows the iron present in the stainless to rust. Carbide solution treating restores the grain boundaries and therefore the corrosion resistance.
The ASTM F594 nut specification, as well as A194 Grades 8A and 8MA, require a solution annealing process after hot forging to restore the grain boundaries. This ensures that the nuts will not rust. Corrosion should not be of concern for cold-formed nuts since they start with solution annealed material and are not subjected to heat during the manufacturing process.