IE44108B1 - Solution and procedure for depositing a protective coating on galvanized steel parts, and solution regeneration procedure - Google Patents

Abstract

1529380 Coating galvanized steel parts INTERNATIONAL LEAD ZINC RESEARCH ORGANIZATION Inc 9 Dec 1976 [17 Dec 1975 27 April 1976] 51468/76 Heading C7U An aqueous solution for rapidly depositing a protective coating on the surface of galvanized parts e.g. galvanized steel pipe containing per litre of final solution, an amount of hexametaphosphate equivalent to 10 to 70 grams of sodium hexametaphosphate, an amount of metasilicate equivalent to 1 to 40 grams of sodium metasilicate, an amount of orthophosphoric acid equivalent to 15 to 40 cc's of orthophosphoric acid having a density of 1À71 zinc chloride in an amount equivalent to 10 to 50 grams of anhydrous zinc chloride, and sufficient calcium carbonate to adjust the pH of said solution to a value between 2 and 3. The solution may further contain an amount of chlorate equivalent to up to 20 grams of sodium chlorate and nickel in an amount equivalent to 0À5 to 20 grams of hexahydrated nickel chloride per litre of final solution. The solution may be regenerated by adding metaphosphoric acid, zinc oxide and zinc chloride.

Description

The present invention relates to a solution and procedure for depositing a coating, comprising hydrated zinc pyrophosphate, on the surfaces of galvanized steel parts, to protect against corrosion in the presence of water, and a composition for providing the solution.

According to the present invention there is provided an aqueous solution for rapidly depositing a protective coating on the surface of galvanized parts, containing per litre of final solution, an amount of hexametaphosphate equivalent to 10 to 70 grams of sodium hexametaphosphate , an amount of metasilicate equivalent to 1 to 40 grains of sodium metasilicate, an amount of orthophosphoric acid equivalent to 15 to 40 millilitres of orthophosphoric acid having a density of 1.71 2inc chloride in an amount equivalent to 10 to 50 grams of anhydrous zinc chloride, and sufficient calcium carbonate to adjust the pH of said solution to a value between 2.0 and 3.0.

Nickel, equivalent to 0.5 to 20 grams per litre of final solution of hexahydrated nickel chloride, and chlorate, equivalent to up to 20 grams of sodium chlorate per litre, may be added. The solution is used by contacting it with the parts to be treated for a time determined on the one hand by the solution temperature (between 10°C and 70°C) and on the other hand by the desired deposit thickness. The deposit is hydrated zinc pyrophosphate; it has good abrasion resistance, good mechanical behaviour and is insoluble in water.

The invention further provides a composition which may be combined with water to provide the above solution, said composition comprising an amount of hexametaphosphate equivalent to 10 to 70 parts by weight of sodium hexametaphosphate, an amount of metasilicate equivalent to 1 to 40 parts by weight of sodium metasilicate, an amount of orthophosphate equivalent to 25.65 to 68.4 parts by weight of orthophosphoric acid having a density of 1.71, an amount of zinc chloride equivalent to 10 to 50 parts by weight of anhydrous zinc chloride, and sufficient calcium carbonate to adjust the pH of said solution to a value between 2.0 and 3.0.

The invention further provides a process for adding zinc and phosphate ions to a protective solution, containing per litre of final solution, an amount of hexametaphosphate equivalent to 10 to 70 grams of sodium hexametaphosphate, an amount of metasilicate equivalent to 1 to 40 grams of sodium metasilicate, an amount of orthophosphoric acid equivalent to 15 to 40 millimetres of orthophosphoric acid having a density of 1.71. zine chlorate in an amount equivalent to 10 to 50 grams of anhydrous zinc chloride, and sufficient calcium carbonate to adjust the pH of said solution to a value between 2.0 and 3.0,comprising adding thereto an aqueous regenerating solution contain44108 ing an amount of metaphosphoric acid equal in moles to the number of moles of phosphate ion to be added to said protective solution, containing also zinc oxide and zinc chloride in a combined number of moles equal to the number of moles of zinc ion to be added to said protective solution, and including sufficient zinc oxide to adjust the pH of said regenerating solution to a value between 3.0 and 3.3.

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°C and 80°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 behaviour, 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 - 5 “ 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 5 hydrated zinc pyrophosphate, Zn2P2O73H2O, 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 litre of final solution.

TABLE I to 70 g. of sodium hexametaphosphate to 40 g. of sodium metasilieate 15 15 to 40 ml. of orthophosphoric acid (density = .171) 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 galvanised 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 litre 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 litre of final solution: - 6 TABLE II g. of sodiuni hexametaphosphate g. of sodium metasilicate ml. of orthophosphoric acid, (density=l.71) g. of anhydrous zinc chloride g. of crystallized hexahydrated nickel chloride calcium carbonate to bring ihe pH to about 2.8.

This solution, used at an average temperature of 65°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 litre of final solution may be added to the solution described previously in Table I. A deposit may be obtained in three days at 40°C with a small amount of sodium chlorate, or in eight 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 1O°C to 70°C. Solutions containing neither nickel nor chlorate should be used at from 40°C to 70°C, 4410 8 - 7 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 coatings solutions 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, HPOj, 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;(NaP03)g+3H2SO4+6HPO3+3Na2SO4 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. 441« 8 _ 8 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 1° different solutions regenerated according to this process were carried out at a water temperature of 80°C, a total water hardness of zero, a copper content of 1 ppm, and at a water renewal rate of one-third of the total volume per day. The pipes treated in accordance with the pre15 sent 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 fourteen months of testing under the above conditions, the treated pipes had only some rust pits whereas the reference pipes presented a general corrosion.

Claims (23)

1. An aqueous solution for rapidly depositing a protective coating on the surface of galvanized parts, containing, per litre of final solution, an amount of 5 hexametaphosphate equivalent to 10 to 70 grams of sodium hexametaphosphate, an amount of metasilicate equivalent to 1 to 40 grams of sodium metasilicate, an amount of orthophosphoric acid equivalent to 15 to 40 millilitres of orthophosphoric acid having a density of 1.71, zinc 10 chloride in an amount equivalent to 10 to 50 grams of anhydrous zinc chloride, and sufficient calcium carbonate to adjust the pH of said solution to a value between 2.0 and 3.0.

2. A solution according to claim 1 further compris15 ing an amount of chlorate equivalent to up to 20 grams of sodium chlorate per litre of final solution.

3. A solution according to claim 1 or 2 in which the pH is adjusted to a value between 2.5 and 3.0.

4. A solution according to claim 3 in which the pH 20 is adjusted to a value between 2.2 and 3.0.

5. A solution according to any preceding claim, further comprising nickel in an amount equivalent to from 0.5 to 20 grams of hexahydrated nickel chloride per litre of final solution, said nickel chloride being added 25 to the solution before the adjustment of the pH value,

6. A process for depositing a protective coating on galvanized steel parts, comprising filtering the solution of any preceding claim, and contacting said solution with the parts to be treated with the temperature of said 30 solution maintained at between 10°C and 70°C. 441° 8 10

7. A process according to claim 6, in which said parts are immersed in said solution,and said solution is continuously stirred and filtered.

8. A process for depositing a protective coating 5 on the inside faces of galvanized steel piping, comprising filtering the solution of any of claims 1 to 5, and intermittently circulating said solution through said piping, wherein the temperature of said solution is maintained at between 1O°C and 70°C.

9. 10 9. A process according to claims 6,7 or 8, wherein the temperature of said solution is maintained at between 40°C and 7G°C. 10. A galvanized steel part on which has been deposited a protective coating by the process of claims 6,7 15 or 9.

10. 11. Galvanized steel piping on which has been deposited a protective coating by the process of claims 8 or 9.

11. 12. A process for adding zinc and phosphate ions to a 20 protective solution containing per litre of final solution, an amount of hexametaphosphate equivalent to 10 to 70 grams of sodium hexametaphosphate, an amount of metasilicate equivalent to 1 to 40 grams of sodium metasilicate, an amount of orthophosphoric acid equivalent to 15 to 40 25 millilitres of orthophosphoric acid having a density of 1.71, zinc chloride in an amount equivalent to 10 to 50 grams of anhydrous. zinc chloride, and sufficient calcium carbonate to adjust the pH of said solution to a value between 2.0 and 3.0, comprising adding thereto an aqueous - η regenerating solution containing an amount of metaphosphoric acid equal in moles to the number of moles of phosphate ion to be added to said protective solution, containing also zinc oxide and zinc chloride in a combined number of moles equal to the number of moles of zinc ion to be added to said protective solution, and including sufficient zinc oxide to adjust the pH of said regenerating solution to a value between 3.0 and 3.3.

12. 13. A process according to claim 12, in which the metaphosphoric acid is formed in said aqueous regenerating solution by reacting a metaphosphate salt with a strong acid.

13. 14. A process according to claim 13, in which said metaphosphate salt is sodium hexametaphosphate, and said strong acid is sulfuric acid.

14. 15. A process according to claims 12, 13 or 14, in which said protective solution contains also an amount of chlorate equivalent to up to 20 grams of sodium chlorate per litre of final solution.

15. 16. A process according to any of claims 12 to 15, in which said protective solution contains also nickel in an amount equivalent to from 0.5 to 20 grams of hexahydrated nickel chloride per litre of final solution, said nickel chloride being added to the protective solution before the adjustment of the pH value.

16. 17. A composition which may be combined with water to produce the solution of any of claims 1 to 5, said composition comprising an amount of hexametaphosphate equivalent to 10 to 70 parts by weight of sodium hexametaphosphate, an amount of metasilicate equivalent to 1 to 44i« 8 - 12 40 parts by weight of sodium metasi.licate, an amount of orthophosphate equivalent to 25.65 to 68.4 parts by weight of orthophosphoric acid having a density of 1.71, an amount of zinc chloride equivalent to 10 to 50 parts by weight of anhydrous zinc chloride, and sufficient calcium carbonate to adjust the pH of said solution to a value between 2.0 and 3.0.

17. 18. A composition according to claim 17, further comprising an amount of chlorate equivalent to up to 20 parts by weight of sodium chlorate.

18. 19. A composition according to claims 17 or 18, further comprising nickel in an amount equivalent to from 0.5 to 20 grams of hexahydrated nickel chloride.

19. 20. An aqueous solution accordinj*. to Claim J for rapidity depositing a protective coating on the surface of galvanized parts, substantially as hereinbefore described.

20. 21. A process according to Claim 6 for depositing a protective coating on galvanized steel parts substantially as hereinbefore described.

21. 22. A process according to Claim 12 for increasing· the zinc and phosphate content of a protective solution substantially as hereinbefore described.

22.

23. A composition according to Claim 17 for producing an aqueous solution for rapidly depositing a protective coating on the surface of galvanized parts substantially as hereinbefore described.