The primary factor is the water which contains dissolved oxygen and dissolved salts. Each municipality has a water supply that is unique from the standpoint of its composition and specific corrosiveness. Because no part of the fresh water supply is recirculated, the supply of oxygen is not depleted as in a hot water heating system, nor is it as convenient or economical to add chemical inhibitors to mitigate corrosion. Where the water supply is classed as hard the water may be softened but this often results in increasing the corrosiveness of the water because some of the scale-forming constituents are removed. Scale usually reduces corrosion since it may act as a protective coating.
Temperature is an important factor in corrosion by fresh water.
Thus storage tanks for hot water are subject to the most severe corrosion. Those most commonly affected are the galvanized steel tanks, which are economical and under favourable conditions of service with certain types of water do give satisfactory performance. However, in many localities the corrosion of galvanized steel tanks is a serious problem and much study bas been directed toward finding a solution. Copper piping should be kept out of a galvanized tank and for a distance of several feet adjacent to it. The use of corrosion-resistant materials such as copper does not always guarantee that the system will be free from corrosion.