If the feed water contains lime salts, a deposit will be formed in the economizer and feed connection which will more or less effectually protect the pipes from internal corrosion ("rusting" or "eating away"). If, however, the water is very free from lime, and air is introduced by the feed pump, internal pitting (small hollows) will be formed. Considerable damage may then be done before the danger is discovered and steps taken to prevent further mischief.
External corrosion does not as a rule give much trouble, but under certain conditions the combined action of heat and moisture on asbestos pipe-covering will set up pitting. This, however, can be prevented by painting the pipes with any good graphite paint before the covering is applied.