Procedure for Depositing a Protective Precoating on Surfaces of Zinc-Coated Ferrous Metal Parts Against Corrosion in Presence of Water

This invention relates to a technique for protecting the surfaces of zinc-coated ferrous metal parts against corrosion in the presence of water and especially inner surfaces of zinc-coated steel piping, as well as to the procedure for obtaining such deposits.

It is common practice to protect ferrous metal parts immersed into water and especially water distribution piping by a zinc coating; such coatings are usually called galvanized.

Zinc, being more electronegative than steel, protects the latter against corrosion by preferentially dissolving itself. This dissolution is delayed and slowed down by a layer of insoluble salts formed by displacement of mineral salts in solution in water. However, the formation of these protective salts is slow and does not always ensure the complete protection of zinc, especially if the water is hot, if it has a low mineral salts content (degree of water hardness near zero) or if it contains metals such as copper which act as a catalyst for the attack on zinc. It has been stated that corrosion is considerably accelerated by water having a temperature between 60.degree. and 80.degree. C. and that traces of copper even at a concentration as low as 1 ppm (part per million) have a pronounced catalytic corrosive effect. Moreover, the more and more frequent use of water softeners, which lowers the degree of hardness by ion exchange, in the water distribution systems, increases the danger of premature corrosion of galvanized parts.

In order to obviate these dangers of corrosion, so-called water "vaccination" techniques are sometimes used which consist in adding continuously measured quantities of silicates or metaphosphates. Most of these salts remain in the distributed water and their presence, in particular that of phosphates, in water intended for consumption by human beings raises questions about hazards to the health of the people who drink such water.

The applicant has discovered that it is possible to bring about a continuous deposit, insoluble in hot or cold water which efficiently protects zinc against corrosion, even in hot water having a low degree of hardness and a significant copper content, by bringing the galvanized parts to be protected into contact with a solution containing silicates or metaphosphates in a phosphoric medium having a low pH value, in the presence of zinc ions; the deposit can be rapidly achieved and gives lasting protection.

Thus, this invention provides a solution intended for rapidly depositing a coating which protects the surface of zinc-coated ferrous metal parts against corrosion in the presence of water, and is characterized by the fact that it is built-up by adding to water, per liter of final solution, from 1 to 40 grams of sodium metasilicate, 14 to 40 milliliters of phosphoric acid of a density of 1.71 g/ml, from 1 to 40 grams of sodium nitrate and 10 to 50 grams of anhydrous zinc chloride; the pH value of the solution being adjusted to a value between 2.3 and 3.8 by the addition of calcium carbonate.

The solution may also contain, per liter of final solution, metasilicate equivalent to 1 to 40 grams of sodium metasilicate, phosphate equivalent to 14 to 40 milliliters of phosphoric acid having a density of 1.71 g/ml, nitrate equivalent to 1 to 40 grams of sodium nitrate, and zinc equivalent to 10 to 50 grams of anhydrous zinc chloride, with the pH of this solution having a value of between 2.3 and 3.8. This solution may be contacted with the parts to be coated for a time and at a temperature sufficient to form the desired coating, after which the parts are rinsed with water.

In a variant, the solution is characterized by the fact that it contains in addition up to 70 grams of sodium hexametaphosphate per liter of final solution.

These solutions are used in a process of depositing a protective coating on zinc-coated ferrous metal parts characterized by the fact that a solution of the type that precipitates at room temperature is made up, that this solution is filtered after adjustment of the pH value, heated to a working temperature ranging from 15.degree. to 75.degree. C., that this solution is brought into contact with the aforesaid parts for a period ranging from 20 to 72 hours, at the aforesaid working temperature, and that the aforesaid part is then rinsed with water.

According to an advantageous form of the process of depositing a protective coating on the inner surface of zinc-coated steel piping, the foresaid solution is brought into contact with the aforesaid inner surface by circulation, preferably intermittent, of the aforesaid solution, and a filter is placed in the aforesaid circulating system.

According to another advantageous form of the process of treating the parts before mounting them, these zinc-coated ferrous metal parts are immersed into the aforesaid solution contained in a vessel, this solution is stirred by its being recycled and filtered.

The characteristics and advantages of this invention appear from the following examples:

A solution is made up according to the following formula for one liter of solution:

35 g of sodium hexametaphosphate

5 g of sodium metasilicate

15 ml of phosphoric acid (dens. = 1.71 g/ml)

14 g of sodium nitrate

5 g of crystallized nickel chloride (NiCl.sub.2.6H.sub.2 O)

20 g of anhydrous zinc chloride

calcium carbonate to bring to a pH value of about 2.8

A solution is made up, according to the following formula for 1 liter of solution:

20 g of sodium metasilicate

17 ml of phosphoric acid (dens. = 1.71 g/ml)

20 g of sodium nitrate

5 g of crystallized nickel chloride (NiCl.sub.2.6H.sub.2 O)

20 g of anhydrous zinc chloride

calcium carbonate to bring the pH value of the solution to 3.6.

A solution is used according to Example 1.

Working temperature of the solution: 65.degree. to 75.degree. C.

Time necessary for the formation of the deposited coating: 20 hours or preferably 24 hours.

Mode of formation: By continuous, or preferably intermittent circulation of the solution, after passing through a glass wool filter

Nature of the coating: Zinc phosphate.

A solution is used according to Example 2.

Working temperature of the solution: between 15.degree. and 25.degree. C.

Time of formation of the deposited coating: 72 hours

Mode of formation: By continuous or preferably intermittent circulation of the solution.

Nature of the coating: Mixture of phosphates and zinc silicates.

The tests have been carried out on test pieces which have been treated according to example 3 as well as on untreated reference test pieces, in the following conditions:

The deposits obtained are continuous, hard, and resistant to abrasion and shocks without flaking. The protection of zinc obtained is efficient in the most unfavorable corrosion conditions, namely in hot water having a low degree of hardness and a significant copper content. The formation of the protective deposit is rapid. When a water distribution system of a new building is put into service, the treatment of the pipings does not cause an appreciable delay to the completion of the assembly. The coating obtained remains hard and adherent in hot and cold water and does not contaminate thse waters, which remain suitable for consumption by human beings.

From 0.5 to 20 grams per liter of final solution of nickel chloride added as a hexahydrated proved an efficient deposit accelerator. It is obvious that its presence is not indispensable and that moreover the addition of other deposit accelerators to the solution of the present invention is included within its scope.