The metal leaves a itchy rash on the skin
It is the the chrome chain collar that is the problem, but the base metal under it that the Chrome is attached to. Small holes appear in the Chrome as it begins to wear exposes the metal to your dogs skin. For many this leaves a very itchy rash. Wearing a Stainless Steel collar is an option but they are much heavier.

The result is that you slave can no longer wear the chain as much as he wants, the more exposure the worse it gets.
Some slaves have no reaction and can wear the chain till it rusts off their neck.

This rash can be anywhere on the body, as was more common for 'cheap' cock rings

Prevention^

Dogs normally have hair as a barrier to the metal. Higher quality (often heavier) chains use Brass as a base metal, so there is no nickel to react to. Some Masters in fact like seeing Brass around the slaves neck, and this is quite common for pony boys, as Ponies are know for the brass and leather that make up their harnesses.

So what can you do?

• One course of action is to supply your own barrier. This typically takes the form of using a moisturizing crème such as "Curél" (common in the USA), so wash the neck well and put a layer of this where ever the chain would make contact, even under where the dog disk would sit.

• Sadly, change the chain when you notice the itch. I know it is horrible having your chain removed, but it is for your own good. Your Master is likely to keep a good supply in, so he can changed it every few months.

• Dont have it too tight. Some Masters like the notion that it is almost garroting the slave, but this will cause a reaction in its own right, compounding the slaves problem - You dont want to make it too hard to be a slave :)

• You can also have breaks from wearing your collar, something that the Master would not like (let alone many slaves). Wearing your leather collar might be ok for all the time you are at home and the chain when you are not with/separated from your Owner.