Solution and Procedure for Depositing a Protective Coating on Galvanized Steel Parts, and Solution Regeneration Procedure

Corrosion is often observed in hot and cold water distribution systems or appliances which are made of galvanized steel. This corrosion, which is particularly frequent in the first months of service, is undesirable whenever it occurs; it is particularly undesirable when the galvanized steel is in the plumbing of buildings.

The presence and degree of corrosion is closely connected with certain factors, including water temperature, degree of water hardness, and the presence of traces of copper in the water. There is a considerable acceleration of the corrosion in hot water between 60.degree. C. and 80.degree. C. Corrosion is also more severe with a low degree of water hardness, and with water which contains copper, even at very low concentrations.

The present invention concerns a technique for protecting galvanized surfaces intended to be in contact with water, including hot water. An aqueous solution is contacted with the galvanized surface, to deposit a coating having such properties as good abrasion resistance, good mechanical behavior, and insolubility in water. This solution may be regenerated following use, by adding to it a regenerating solution which replaces those components consumed in the formation of the protective coating on the galvanized surfaces. Regeneration avoids having to discard the used solution, and permits the retention and reuse of the unconsumed components in the solution.

The inventors have discovered that a coating of hydrated zinc pyrophosphate, Zn.sub.2 P.sub.2 O.sub.7 . 3H.sub.2 O, has the desirable properties. This coating may be obtained from an aqueous solution containing hexametaphosphate and metasilicate; using the sodium salts of each of these, the solution is made up according to the following formula, with quantities of solute given per liter of final solution.

10 to 70 g. of sodium hexametaphosphate

1 to 40 g. of sodium metasilicate

15 to 40 ml. of orthophosphoric acid (density = 1.71 g/ml)

10 to 50 g. of anhydrous zinc chloride

calcium carbonate to bring the pH value to between 2.0 and 3.0

Nickel ion in the coating solution serves as an accelerator for the depositing of the coating on the galvanized part. The amount of nickel ion added to the solution may be varied in accordance with the desired rate of deposition. For example, from 0.5 to 20 grams of hexahydrated nickel chloride per liter of final solution may be added to the above-described solution.

A solution was made up according to the following table, with amounts of solute given per liter of final solution:

35 g. of sodium hexametaphosphate

5 g. of sodium metasilicate

15 ml. of orthophosphoric acid (density = 1.71 g/ml)

20 g. of anhydrous zinc chloride

5 g. of crystallized hexahydrated nickel chloride

calcium carbonate to bring the pH to about 2.8

This solution, used at an average temperature of 65.degree. C., leads to the formation of a protective coating on galvanized steel by circulation or immersion, the solution being filtered and stirred continuously.

The rate of formation of the protective coating, and the temperature at which the coating may be obtained, are affected by the amount of chlorate added to the initial solution. Up to 20 grams of sodium chlorate per liter of final solution may be added to the solution described previously in Table I. A deposit may be obtained in 3 days at 40.degree. C. with a small amount of sodium chlorate, or in 8 days at room temperature, at a more acid pH value, with a greater added amount of sodium chlorate. Thus, the treatment of galvanized parts where the circulation of a hot solution is impossible, such as through a cold water distribution system which is not heat-insulated, is now feasible. If the solution contains chlorate, the preferred pH range is from 2.5 to 3.0. A solution containing no chlorate is preferably used at a pH value of from 2.7 to 3.0. Solutions containing either nickel or chlorate, or both, may be used at a temperature of from 10.degree. C. to 70.degree. C. Solutions containing neither nickel nor chlorate should be used at from 40.degree. C. to 70.degree. C.

Since the formation of the pyrophosphate is necessary for the proper coating on the galvanized part, it is important to avoid hydrolysis of pyrophosphate to orthophosphate. Therefore, the presence of nitrate ions in the coating solution should be avoided.

The regeneration of used coating solution may be accomplished in the following series of steps. First, the amounts of phosphorus and zinc which must be replaced are determined by analysis of the used coating solution. The regenerating solution should contain an amount of metaphosphoric acid, HPO.sub.3, corresponding to the amount of phosphorus which must be replaced in the coating solution. Metaphosphoric acid can be formed in the regenerating solution by reacting a corresponding amount of a metaphosphate salt with a strong acid. For example, the desired amount of sodium hexametaphosphate can be dissolved in water, and reacted with a sufficient amount of sulfuric acid to convert the hexametaphosphate to metaphosphoric acid, according to the following equation:

zinc chloride and zinc oxide are next added to the regenerating solution, in a sufficient combined amount to supply the necessary amount of zinc to the used coating solution. Sufficient zinc oxide must be added with the zinc chloride so that the pH value of the regenerating solution is maintained at between 3.0 and 3.3. Excessive addition of zinc oxide may cause a neutralization of the regenerating solution. If the solution is allowed to have very high concentrations of both zinc and phosphorus, zinc phosphate may precipitate.

The regenerating solution thus formed is added to the spent coating solution, and the pH value of the resulting solution is then adjusted to between 2.0 and 3.0.

Tests have shown that regeneration of the same initial solution about 30 times led to the formation of deposits having the same general properties as those obtained after the first use of the initial solution. Corrosion tests made on galvanized pipes treated with different solutions regenerated according to this process were carried out at a water temperature of 80.degree. C., a total water hardness of zero, a copper content of 1 ppm, and at a water renewal of one-third of the total volume per day. The pipes treated in accordance with the present invention showed an excellent corrosion resistance compared with untreated pipes; the properties of pipes treated with regenerated solutions were comparable to those of pipes treated with a fresh solution. After 14 months of testing under the above conditions, the treated pipes had only some rust pits whereas the reference pipes presented a general corrosion.