Most silver plating is "fine" silver. And proper plating is quite complex starting with a copper layer, then the silver layer. Back in the day when coal was the main fuel of choice, and there was no differentiation between high and low sulphur coal, plate-silver used to tarnish (not corrode) quickly due to high levels of SO2 in the air. Hence the references in Victorian times to "polishing the silver" as a full time job. Also back in the day, the underlying materials were often brass or bronze, so quite stable in itself.. Further, electroplating was not yet an established science so "plate" was made using a dipping process with sterling silver. Sterling silver is far more prone to tarnish than fine silver, and, why the 'very best' plate used fine-silver for surfaces exposed to food or drink. It does not tarnish under normal conditions.

Today, the underlying materials might be anything from steel to brass to copper to many other things. The copper layer is still necessary for the silver to adhere, and the silver coating will be the minimum amount absolutely necessary. The silver will be fine silver from this method and very resistant to tarnish.

So, here is what happens. As the plating ages, especially if there are heat/cool cycles, the silver coating will get tiny holes - right through the copper to the base material. Now, electrolysis can occur as O2 and moisture gets between the copper and the base material. This causes tiny flaking (granular) such as you describe. At this point, the silver actually makes things worse by using the copper as a sacrificial electrode.

At home, take a common nail and a bit of copper wire. Wrap the wire around the nail and drop both into a glass of tap water. Give it a week.

If you have a bit of silver wire (must be fine silver), do the same with both wrapped around the nail.

Peter Wieck
Melrose Park, PA