Alloy coated workpieces

Abstract

A process for providing a corrosion resistant coating on uncoated ferrous components, involving mechanical plating, using as a coating medium a zinc metal-containing powder, such as zinc or zinc alloy powder, or a powder mixture of zinc or zinc alloy and at least one other metal, so as to build up a firmly adherent coating of the coating medium over exposed surfaces of the components, heating the components with the firmly adherent coating to produce solid-solid diffusion to form an Fe/Zn intermetallic over the surfaces, at least in a base layer of the coating built up by the plating, and cooling the components.

Claims

1. A process for providing a corrosion resistant coating on ferrous components, the process comprising: mechanical plating uncoated ferrous components using as a coating medium a zinc metal-containing powder, so as to build up a firmly adherent cold welded coating of the coating medium over exposed surfaces of the components, the mechanical plating conducted without heating, the mechanical plating including (i) charging the uncoated ferrous components into an inclined rotatable housing along with glass beads, a quantity of the coating medium, and an aqueous solution, and (ii) rotating the housing for a period of time to achieve a cold welded coating thickness by particles in the coating medium being cold welded to the components under a hammering action of the glass beads against the components, the coating medium corresponding to at least one of elemental metal powder or alloy metal powder, the coating medium substantially free of iron and containing: from 6 to 25 wt % of tin, from 0 to 15 wt % of aluminium, from 0 to 6 wt % of magnesium, from 0 to 0.8 wt % of silicon, from 0 to 0.8 wt % of copper, from 0 to 0.1 wt % of manganese, not more than 0.5 wt % each and 2.5 wt % in aggregate of metals other than the zinc, the tin, the aluminium, the magnesium, the silicon, the copper, and the manganese, and a balance of the zinc; heating the components with the firmly adherent cold welded coating so as to produce solid-solid diffusion, with diffusion of iron from the components to the cold welded coating, to form a substantially pore-free layer of an Fe/Zn intermetallic over the surfaces of the components in at least a base layer of the firmly adherent cold welded coating built up by the mechanical plating; and cooling the components.

2. The process of claim 1, wherein the exposed surfaces of the components, on which the cold welded coating is to be produced by the mechanical plating, are bare metal surfaces, to enable formation of the Fe/Zn intermetallic, with the components free of any film or layer prior to the mechanical plating.

3. The process of claim 1, wherein prior to the mechanical plating, the components are at least one of degreased or treated to remove surface rust.

4. The process of claim 1, wherein the components produced by the process have a level of corrosion protection that is significantly improved over the mechanical plating without the heating of the components.

5. The process of claim 4, wherein the level of corrosion protection is at least comparable to that attained by Sherardising.

6. The process of claim 1, wherein the cold welded coating thickness is 2 to 150 m.

7. The process of claim 1, wherein the period of time for the mechanical plating is from about 0.5 to 4 hours.

8. The process of claim 1, wherein after the mechanical plating, the cold welded coated components are heated in the housing while the housing either is open to the atmosphere or maintains an atmosphere having a reduced oxygen content, below about 100 ppm, at a positive over-pressure.

9. The process of claim 1, wherein the heating to produce the solid-solid diffusion is conducted at a temperature ranging from 315 to 415 C.

10. The process of claim 1, wherein a duration of heating is from about 0.4 to 3 hours.

11. The process of claim 1, further including cooling the components either in an atmosphere in which the components were heated or in an ambient atmosphere.

12. The process of claim 11, wherein at least one of forced cooling is used to cool the components or the components are allowed to cool naturally.

13. The process of claim 12, wherein the forced cooling is implemented by a water-quench.

14. The process of claim 12, wherein the components are allowed to cool naturally by air-cooling.

15. The process of claim 1, wherein prior to the mechanical plating, the components are at least one of degreased, treated by acid pickling, or treated with an engineered abrasive.

16. The process of claim 1, wherein the zinc metal-containing powder is a powder mixture of zinc with about 6 to 20 wt % tin.

17. The process of claim 1, wherein the zinc metal-containing powder is a zinc alloy powder with about 6 to 20 wt % tin.

18. The process of claim 1, wherein the cold welded coating thickness is from about 10 to 75 m.

19. The process of claim 1, wherein the period of time for the mechanical plating is from 0.5 to 3.5 hours.

20. The process of claim 1, wherein the period of time for the mechanical plating is from 1.5 to 2.5 hours.

21. The process of claim 1, wherein the heating to produce the solid-solid diffusion is conducted at a temperature ranging from 360 to 380 C.

22. The process of claim 1, wherein a duration of heating is from 1.5 to 2.5 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a photomicrograph of an example zinc coated component prepared in accordance with teachings disclosed herein.

(2) FIG. 2 is a photomicrograph of an example zinc coated component prepared in accordance with teachings disclosed herein.

(3) FIG. 3 is a photomicrograph of an example zinc coated component prepared in accordance with teachings disclosed herein.

(4) FIG. 4 is a photomicrograph of an example zinc coated component prepared in accordance with teachings disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

(5) A number of batches of components were provided with corrosion protection coatings by the process of the invention. In each case, step (a) of the process was conducted by using a standard mechanical plating procedure utilising an aqueous solution with the addition of inhibited acid for cleaning and pH control and a cleaning process to remove all oxides and produce a surface suitable for further processing. The processing steps after cleaning comprised: (1) Copper immersion coating; (2) Tin immersion coating; (3 Addition of plating promoter and zinc flash; (4) Metal powder additions at regular intervals to achieve desired thickness; (5) Flush out solution at completion of plating cycle, with additional rinses; and (6) Separation of parts from impact media.

Example 1Zinc Coating

(6) A quantity of components comprising 1.3 Kg of 1250 Hexagon head T17 steel roofing screws was processed in 2 liters of impact media (40% 5 mm, 40% 3 mm and 20% 0.7 mm) using the above standard procedure. 90 grams of zinc powder with a nominal particle size 4.5 m was used to achieve a desired plating thickness. The zinc powder was added in 615 gram increments at intervals of 3 minutes. A period of 10 to 12 minutes was allowed after last addition of zinc for plating completion and polishing. The components then were rinsed and separated without any additional treatments. The coating thickness achieved was approximately 55 m.

Example 2Zinc/Tin Coating

(7) A quantity of components comprising 1.2 Kg of 1250 Hexagon head T17 steel roofing screws and 200 grams of 5 mm10 mm long flat head semi tubular steel rivets were processed in 2 liters of impact media (40% 5 mm, 40% 3 mm & 20% 0.7 mm) using the above standard procedure. 60 grams of blended zinc and tin powders were used to achieve desired plating thickness. The zinc powder had a nominal particle size 4.5 m while the tin powder grade was 325 mesh. The composition of the blended powder was Zn-80% and Sn-20%. The blended powder was added in 610 gram increments at intervals of 3 minutes. About 10-12 minutes was allowed after last addition of powder for plating completion and polishing. The components then were rinsed and separated with no additional treatment. The coating thickness achieved was approximately 35 m.

Example 3Temperature

(8) Ten samples of zinc coated components produced by Example 1 were placed in a 1 m diameter fan-forced oven that was preheated to a temperature of 320 C. The components were supported in a steel mesh cage. The parts were held for 120 minutes and then removed with the cage and allowed to cool in air. The screws were cross-sectioned, polished to 1 m abrasive and etched in a mild caustic solution. There was a clear intermetallic layer formed, as illustrated in FIG. 1 of the accompanying drawings.

Example 4Time Comparison

(9) Example 3 was repeated, with ten other zinc coated components produced by Example 1, except that the oven temperature was 380 C. and the components were held at that temperature for 30 minutes. Again the screws were cross-sectioned, polished to 1 m abrasive and etched in mild caustic solution. There was a clear intermetallic layer formed, as seen in FIG. 2.

Example 5Atmosphere

(10) Ten samples of zinc-coated screw components produced by Example 1 were placed in a glass tube that then was flushed with argon. The glass tube was closed at one end and, after insertion of the components, the other end was closed, sealing the screws in an argon atmosphere. The glass-encased screws were placed in a wire mesh basket and placed in a 1 m diameter fan-forced oven that was preheated to a temperature of 380 C. The parts were held for 120 minutes and then removed with the cage and allowed to cool in air. The glass capsules were then broken and the screws released.

(11) The screws were cross-sectioned, polished to 1 m abrasive and etched in a mild caustic solution. There was a clear intermetallic layer formed, as shown in FIG. 3.

Example 6Alloy

(12) Ten Zn/Sn coated screw components from Example 2, all comprising screws, were placed in a 1 m diameter fan-forced oven that was preheated to a temperature of 380 C. The screws were supported in a steel mesh cage. The parts were held for 120 minutes and then removed with the cage and quenched into water. The screws were cross-sectioned, polished to 1 m abrasive and etched in mild caustic solution. There was a clear intermetallic layer formed, as seen in FIG. 4.

(13) Extensive standard salt spray testing already has been conducted on components produced in the manner detailed in the preceding Examples, and such testing is continuing. To date the testing has established that the process of the present invention provides excellent corrosion protection for ferrous components. The level of protection is superior to that obtained by mechanical plating alone, and at least comparable to the level of corrosion protection obtainable with Sherardising. The invention provides a level of corrosion that is well suited to a wide range of uses and environments for ferrous components.