Question about Littleton

It's quite simple, pitting on the surface of a coin is usually due to corrosion. And a silver coin that has pitting is commonly said to be corroded. But then that's because it is corroded. Corrosion has literally eaten into the surface of the coin and caused that pitting.

It's not necessary for a coin to disintergrate completely for it to be called corroded. And it doesn't matter if the coin is silver or copper. Large cents for example are commonly found with pitting, and every one of them is said to be corroded. Same goes for silver.

All metals, with the possible exceptions of gold and platinum, will corrode. And toning on a silver coin if left alone and allowed to progress will absolutely cause pitting on the surface of that coin. Toning itself is corrosion when it comes right down to it and that's why it will eventually destroy the luster on a coin if left to continue.

Silver corrodes, it's as easy as that.