The prevention of corrosion in once-through water systems has long been of great concern to both the private and the public sectors. These once-through systems include municipal water systems and industrial once-through water cooling systems.
In the past, in muncipal water systems, corrosion inhibition has been directed toward the protection of mild steel, galvanized steel and cast iron found in these systems. Now that copper piping and fittings are increasingly being used in homes and at other points in the municipal water system, it becomes important to deal with potential copper corrosion problems. This is an especially important concern in view of environmental regulations shortly to come into effect.
According to current municipal water testing regulations, the metal ion content of municipal water systems is measured at points short of the delivery location (i.e. at fire hydrants). Since copper piping or fittings are generally not present in the system prior to or at these points, present steel and iron corrosion treatments suffice to prevent corrosion. However, under new regulations which will soon come into effect, municipal water testing will be carried out at the delivery site (i.e. the home tap). When testing is carried out at this point, undesirably higher copper levels will be encountered unless the water is pretreated with a copper corrosion inhibitor such as the treatment described herein. Thus, the present invention becomes especially important.
In addition to its use in municipal water systems, copper* is found in industrial water systems at such points as the cast copper fittings into which black iron pipings may be fitted. At these points, corrosion of the copper fittings may be a problem. In addition, there is a likelihood of corrosion due to galvanic coupling at the point of interconnection. FNT *Copper as used herein also includes alloys of copper such as brass and admiralty metal.
Typical corrosion inhibitors employed in once-through water systems such as those described above have included sodium hexametaphosphate and zinc salt-polyphosphate combinations. These treatments have been used to prevent steel and iron corrosion, but they have not been used to prevent copper corrosion. Indeed, the most common treatment -- sodium hexametaphosphate -- will not prevent copper corrosion and may, in some cases, increase the corrosion rate.
I have now discovered a particular polyphosphate composition which offers important advantages over polyphosphate-zinc compositions generally as a corrosion inhibitor and which has outstanding ability to prevent or reduce copper and mild steel corrosion rates.