I work in silverware so I am well out of my depth here in terms of chemistry, and I have a question regarding one of the cleaning processes we use in the workshop.
Background: we use warm sulphuric acid (diluted to 10%ish strength) or Sodium Bisulphate to clean any oxides or flux remnants (typically sodium tetraborate or similar is what we use) from the surface of sterling silver (an alloy which is on average 925 parts silver and the rest Copper) after it has been heated to soften its structure or solder parts together. Usually this yields a fine matte surface where fine silver is drawn to the surface to cover a light grey copper oxide that forms beneath (though this is subject to the presence of oxygen during heating and so may not be the case).
The Issue: occasionally, while the "pickle" (that's is what we call the acid solution) is still apparently fine, we find that upon removal there is a greyish matte coating on any silver that has been in there, that is tough to remove. This would also occur if one was to leave the silver in the solution for several weeks by accident instead of the usual ten mins, in fact it becomes an almost black colour. I do not know if these two reactions are the same or not. I did speculate that it could be Titanium as occasionally people put pieces with tippex into the solution (we use it as a stop for soldering), but I do not know if this is possible. The problem happened again after several weeks with a fresh batch which definitely had no contact with Titanium.
My suspicion that it is a plating comes from the common knowledge that if iron comes in contact with the solution then a pink plating will appear on the surface of the silver, but this is as far as my knowledge goes.
The question (s): is anyone able to give a definitive answer as to what the substance is(if it is a plating!), how to prevent it, how I could investigate to see what the issue is myself (I know there are a lot of variables), and any relevant reading material that would be suitable for the layman who only got as far as GCSE chemistry?