I once was involved in the manufacturing of motorcycle carburators made from cast aluminum. At least once that I remember, we had problems with the aluminum carb bodies developing rusty spots. The problem stemmed to a vendor tumbling the castings in media that had also been used for ferrous parts. Ferrous residue imbedded itself in the aluminum and later rusted. Perhaps some SS frames have been introduced to tumbling or blasting media that had been previously been used on regular steel parts...