Molybdenum alloy addition provides the increased resistance to pitting corrosion in 3XX stainless steel. If you really want to attempt to understand how molybdenum improves pitting resistance, I would suggest locating the following book - "Corrosion of Stainless Steels" by A. John Sedricks. There is a chapter devoted to pitting corrosion of stainless steels. There is reference to the formation of a ferrous molybdate layer in the outer regions of the passive film which contribute to increased pitting resistance along with a change in the pitting potential. The book contains graphs showing increased pitting resistance with increasing molybdenum content (which needs to be limited because molybdenum additions increase the risk of sigma and chi phase precipitation).
There are a few other good refs also
Stainless Steel, by R.A. Lula, from ASM. Nice and thin and concise.
In 3xx SS there are two issues that both must be balanced. One is phases, keeping it austenitic. This requires a balance between the elements that form austenite (Ni, Mn, Cu, N, C) and those that promote ferrite (Cr, Mo, Fe).
Then corrosion resistance. Cr, Mo, N, W all enhance pitting resistance.
