This depends upon the environment that the air silencer operates in. For corrosive environments choose stainless steel as it will have a greater resistance. The units themselves see very little mechanical wear as it is only the air flowing in and out of the valve. The most common occurence is that they get blocked with dirt from the surrounding environment. They are meant to do this as they are protecting that dirt from going into the valve. They can usually be cleaned but may sometimes need to be replaced (it is a lot cheaper to replace these than your valve which is why they should be used).