Coated Article

Abstract

An article comprising a metallic substrate and a mechanically deformable corrosion-protective coating thereon comprising a layer on the substrate of ductile bright nickel containing less than about 0.005 percent sulfur and about 0.2 to 2 mils thick, and a layer thereon of chromium at least about 0.01 mil thick and having cracks substantially uniformly distributed therein at an average spacing of not more than about 3 mils, the coating having been maintained at about 350.degree. to 450.degree.F for at least about one minute. The bright nickel layer typically is electroplated from an aqueous solution comprising nickel sulfate, nickel chloride or fluroborate, boric acid, and coumarin or a derivative or analog thereof. A layer of ductile bright copper not more than about 1 mil thick may be provided between the substrate and the layer of bright nickel, and a layer of ductile semibright nickel not more than about 2 mils thick may be provided between the copper layer and the bright nickel layer.

Description

BACKGROUND OF THE INVENTION

This invention relates to a metallic article having a bright ductile chromium coating system where a bright nickel with ductility higher than conventional bright, sulfur-containing nickel is used with microcracked chromium. During bending the deformation takes place by expansion of the cracks already in the microcracked chromium system. Cracks do not penetrate a significant distance into the sulfur-free ductile nickel layer.

This system is useful for plating flat steel sheets which are later formed to the desired shape. By plating flat sheets, cost savings are realized because less metal is deposited, and smaller plating equipment is used to plate sheet materials.

SUMMARY OF THE INVENTION

A typical article according to the present invention comprises a metallic substrate and a mechanically deformable corrosion-protective coating thereon comprising a layer on the substrate consisting essentially of ductile bright nickel and a layer thereon consisting essentially of chromium having cracks substantially uniformly distributed therein at an average spacing of not more than about 3 mils, the protective coating having been maintained at about 350.degree. to 450.degree.F for at least about one minute. The bright nickel layer typically has a ductility factor of about 100 percent as measured by micrometer bend test, and a ductility equivalent to at least about 0.15 percent elongation for a thickness of 2 mils.

The bright nickel layer preferably is substantially free of sulfur. It typically contains less than about 0.005 percent sulfur and is about 0.2 to 2 mils thick. The chromium layer typically is at least about 0.01 mil thick. The bright nickel layer typically is electroplated from an aqueous solution comprising essentially nickel sulfate, nickel chloride or fluoroborate, boric acid, and coumarin or a derivative or analog thereof.

The article may comprise also a layer consisting essentially of ductile bright copper between the substrate and the layer of bright nickel. The copper layer typically is not more than about 1 mil thick, and the bright nickel layer typically is not more than 2 mils thick.

The article may comprise still another layer consisting essentially of ductile semibright nickel between the copper layer and the bright nickel layer. The bright nickel layer preferably is substantially free of sulfur and about 0.2 to 2 mils thick, and the semibright nickel layer typically is not more than about 2 mils thick.

A typical method of providing the article comprises electroplating on the substrate a layer of ductile bright nickel from an aqueous solution consisting essentially of nickel sulfate, nickel chloride or fluoborate, boric acid, and coumarin or a derivative or analog thereof, (typical solutions consist essentially of about 150 to 400 grams per liter of nickel sulfate, about 5 to 25 grams per liter of nickel fluoborate, about 20 to 40 grams per liter of boric acid, and about 1 to 3 grams per liter of coumarin or a derivative or analog thereof); typically with a pH of about 5 to 5.5, at a temperature of about 110.degree. to 140.degree.F and a current density of about 10 to 60 amperes per square foot, for about 2 to 10 minutes; electroplating on the bright nickel layer a layer of microcracked chromium, and maintaining the protective coating at about 350.degree. to 450.degree.F for about 1 to 60 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph at 150 power of the surface of an article comprising a steel substrate coated with a layer of conventional bright nickel (containing sulfur) and an outer layer of microcracked chromium, showing an area that was bent to an angle of about 90.degree. around a one-fourth inch radius and exposed to 16 hours of copper accelerated salt spray (CASS).

FIG. 2 is a photomicrograph at 500 power of a cross section of the area shown in FIG. 1 (two layers, prior art).

FIG. 3 is a photomicrograph as in FIG. 1 of a similar article except that the layer of bright nickel comprises ductile bright nickel (sulfur free) in accordance with the present invention.

FIG. 4 is a photomicrograph similar to FIG. 2 of a cross section of the area shown in FIG. 3 (two layers, this invention).

FIG. 5 is a photomicrograph as in FIG. 1 of a similar article except that the coating includes also a layer of conventional bright copper between the steel substrate and the bright nickel layer.

FIG. 6 is a photomicrograph similar to FIG. 2 of a cross section of the area shown in FIG. 5 (three layers, prior art).

FIG. 7 is a photomicrograph as in FIG. 5 of a similar article except that the layer of bright nickel comprises ductile bright nickel (sulfur free) in accordance with the present invention.

FIG. 8 is a photomicrograph similar to FIG. 6 of a cross section of the area shown in FIG. 7 (three layers, this invention).

FIG. 9 is a photomicrograph as in FIG. 5 of a similar article except that the coating includes also a layer of semibright nickel between the copper layer and the bright nickel layer.

FIG. 10 is a photomicrograph similar to FIG. 6 of a cross section of the area shown in FIG. 9 (four layers, prior art).

FIG. 11 is a photomicrograph as in FIG. 9 of a similar article except that the layer of bright nickel comprises ductile bright nickel (sulfur free) in accordance with the present invention.

FIG. 12 is a photomicrograph similar to FIG. 10 of a cross section of the area shown in FIG. 11 (four layers, this invention).

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred operating limits have been identified for depositing decorative ductile bright nickel-chromium composites on sheet steel for forming after plating. One of the best of several ductile composites consists of a four coat system as follows: leveling acid copper, ductile semi-bright nickel and bright nickel deposited without current interruption in the same sulfur-free nickel bath, and a microcracked chromium system applied by a post-nickel-strike (PNS) and a conventional chromium plate. The microcracks prevent the macrocrack formation that is widely encountered with prior procedures. Macrocracks commonly extend through the plated coatings to the basis metal. A short heat treatment of the plated sheet before forming improves ductility. Corrosion, metallographic, ductility, and percent elongation studies have confirmed the benefits of this invention over prior procedures of plating.

When steel panels 0.025-inch thick plated with the preferred ductile composite coating system were bent up to 180.degree. around a 1/8-inch-diameter mandrel, the cracks intentionally introduced in the microcracked chromium propagated less than one-fourth the thickness of the coating. In comparison, cracks propagated all the way through the coating system applied by prior techniques, on similar panels subjected to similar bends. Thus, the new system offers superior corrosion protection to the steel substrate. The decorative appearance (brightness) of the coating was retained on the bent areas. No orange peel was seen at the bends.

TYPICAL PROCEDURES FOR PLATING DUCTILE NICKEL-CHROMIUM COMPOSITES ON STEEL PANELS

A. preliminary (all systems)

1. Degrease if necessary

2. Electroclean

3. Acid dip in 10 percent hydrochloric acid for about 30 seconds.

4. Rinse in tap water

B. copper (3 and 4 coat systems only)

5. Copper strike in a cyanide copper bath, to protect the basis metal

6. Rinse in tap water

7. Acid dip in 5 percent sulfuric acid

8. Rinse in tap water

9. Copper plate in a conventional bright acid copper bath containing copper sulfate, sulfuric acid, and proprietary organic brighteners (Dayton Bright Copper Co., Cuflex Brighteners)

C. semibright Nickel (4 coat system only)

10. Immerse in a semibright watts type nickel bath containing coumarin addition agent within the ranges listed in U.S. Pat. No. 2,635,076, DuRose, and plate to a thickness of about 0.5 mil; or ((10) alternative) Immerse in a bath as in (11) below, and with the following operating conditions

Range Preferred pH 5.0 to 5.5 5.2 Current density 60 to 150 75 amp/sq ft Temperature, F 110 to 140 135 Agitation Mild air Anodes Sulfur de- polarized nickel (International Nickel Company) Filtration Continuous

10.5. plate to a thickness of about 0.5 mil

D. ductile Bright Nickel (all systems)

11. Immerse in a sulfur-free nickel bath with the following composition

Range Preferred Nickel sulfate, NiSo.sub.4.sup.. 6H.sub.2 O, 150 to 400 225 g/l Nickel fluoborate, NiBF.sub.4, 10 to 50 20 mil/l Boric acid, H.sub.3 BO.sub.3, g/l 20 to 40 30 Conventional anti-pit agent, 0 to 0.5 0.3 such as Harshaw AG4, per- cent by volume Brightener, coumarin, g/l 0.5 to 3.0 3.0

and the following operating conditions

Range Preferred pH 5.0 to 5.5 5.2 Current density 10 to 60 50 amp/sq ft Temperature, F 110 to 140 135 Agitation Vigorous air Anodes Sulfur depolarized nickel (International Nickel Company) Filtration COntinuous

12. Plate to a thickness of about 0.2 to 2 mils

13. Rinse in cold tap water E. Microcracked Chromium (all systems)

14. Immerse in a post nickel strike bath as in U.S. Pat. No. 3,563,864, DuRose (PNS- Harshaw Chemical Company) for 2.5 minutes at 80 amp/sq ft and a temperature of 80.degree.F

15. Rinse in cold water

16. Immerse in a conventional 33 oz/gal chromic acid bath containing fluosilicic and sulfuric acids as catalysts

17. Plate for 2.5 minutes

This outer layer develops about 800 to 1,000 continuous cracks per linear inch. or (E. alternative)

14 alt. Immerse in a conventional 33 oz/gal chromic

acid bath containing fluosilicic and sulfuric

acids as catalysts

15 alt. Plate for 3.5 minutes

16 alt. Without rinsing, transfer to, and immerse in, a bath as in (14 alt.) but containing also about 18 mg of sodium selenate

17 alt. Plate for 3.5 minutes

The outer layer deposited by this alternative procedure develops about 3,300 cracks per linear inch.

F. heating (all systems)

18. Heat at 400.degree.F for one to 30 minutes

EXAMPLES

A. two-Coat System

Panels were coated in accordance with Parts A, D, E, and F of the above Typical Procedures.

In this system 0.8 to 1.2 mil of ductile bright nickel was deposited directly on steel in the sulfur-free nickel bath. The nickel plated steel was then plated with 0.1-mil PNS nickel, in accordance with U.S. Pat. No. 3,563,864, DuRose, and 0.01 mil of conventional chromium, and heated at 400.degree.F for 1 to 30 minutes before bending.

FIGS. 3 and 4 illustrate the effectiveness of this system by comparison with FIGS. 1 and 2 of the prior art. A microcracked chromium system is essential. Microcracked chromium developed by the two different procedures (E and E alternative) produced the same good corrosion resistance. The microcracks must be not more than about 3 mils apart. When they are farther apart the cracks propagate deeper and are likely to penetrate through the bright nickel layer and into the substrate of steel. In corrosive environments this can result in rusting, especially in any areas of the coated article that are subjected to bending or other strain.

B. three-Coat System

Panels were coated in accordance with Parts A, B, D, E, and F of the above Typical Procedures.

In the three-coat system, (1) copper, (2) ductile bright nickel, and (3) microcracked chromium were applied. About 0.5 to 1.0 mil of leveling bright copper was applied to the steel after a cyanide copper strike. These steps involve conventional techniques. The nickel plate of 0.2 to 1.0 mil applied on the copper is the ductile, sulfur-free bright nickel. Over the ductile bright nickel, a microcracked chromium system was applied by either the two-layer chromium (E alternative) or the PNS nickel and conventional chromium system (E). The advantage of the three-coat system over the two-coat system is that more deformation can be tolerated because the copper uniformily distributes the bending forces in the ductile nickel coating.

FIGS. 7 and 8 illustrate the effectiveness of this system by comparison with FIGS. 5 and 6 of the prior art.

C. four-Coat System

Panels were coated in accordance with all parts of the above Typical Procedures.

In the four-coat system, (1) copper, (2) semibright nickel sulfur free, (3) bright nickel sulfur free, and (4) microcracked chromium were deposited. The advantage of the four-coat system over the three-coat system is that sharper bends can be sustained without causing cracks to penetrate to the steel substrate, Cracks which penetrate through the coating to the steel are corrosion sites.

FIGS. 11 and 12 illustrate the effectiveness of this system by comparison with FIGS. 9 and 10 of the prior art.

The sulfur-free, bright nickel plating process, which is an important part of the invention, deposits full bright metal according to accepted standards. However, maximum ductility is not achieved until the deposit is backed. Minimum time and temperature for obtaining 100 percent ductility as measured by the bend test are approximately:

Temperature Time, minutes 280.degree.F 15 340.degree.F 8 400.degree.F 1

ductility was determined by two methods. In the first one, 1-mil thick foils were electroformed, heated, cooled, and bent between the jaws of a micrometer. The jaws were closed until cracking of the foil occurred or the jaws were closed to give a reading equal to twice the thickness of the foil. When no cracking occurs, ductility equals 100 percent, which is the limit of this measuring procedure, ASTM Designation B-490-68. Ductility of the sulfur-free, bright nickel deposit after heating was consistantly 100 percent whereas the ductility of conventional bright nickel foils measured by the same foil test ranged from 2 to 22 percent. All the nickel foils containing sulfur decreased in ductility when heated. This change is typical of sulfur-containing nickel.

Elongation of nickel foils was measured by making tensile specimens from about 2.4 to 3.5 mil thick foils. Elongation values were 0.17 to 0.35 percent for the sulfur-free bright nickel. By contrast, the sulfur containing bright nickel had elongation values of 0.04 percent, only about 10 to 25 percent of the values for the sulfur-free bright nickel.

The article shown in FIGS. 11 and 12 was prepared in accordance with the Typical Procedures, comprising all of the Parts A-F for preparing the four-coat system according to the present invention. The article shown in FIGS. 9 and 10 was made in the same way except that the ductile bright nickel layer of the present invention (Part D) was replaced by a bright nickel layer as in conventional commercial practice (containing sulfur).

The article shown in FIGS. 7 and 8 was prepared in the same manner as that of FIGS. 11 and 12 except that the deposition of the semi-bright nickel layer (Part C) was omitted, and the thickness of the ductile bright nickel layer was increased by the thickness of the omitted layer. The article shown in FIGS. 5 and 6 was made in the same way as the article in FIGS. 9 and 10 except that the semi-bright nickel layer was omitted and the thickness of the conventional bright nickel layer increased by the thickness of the omitted layer.

The article shown in FIGS. 3 and 4 was made in the same way as the article in FIGS. 7 and 8 except that the layer of copper (Part B) was omitted. The article shown in FIGS. 1 and 2 was made in the same way as the article in FIGS. 5 and 6 except that the copper layer was omitted.

A typical procedure for depositing the conventional bright nickel layer in the examples of the prior art, FIGS. 1, 2, 5, 6, 9, and 10, (upper half of the drawings) comprises the preferred commercial practices developed over many years using a modified Watts bath composition containing organic brighteners, which contribute sulfur to the deposit. The procedure is described below in more detail.

G. typical Procedure for Plating a Conventional Bright Nickel Layer

19. Immerse in a Watts-type bath with the following composition:

Range Preferred Nickel sulfate 225-450 g/l 260 Nickel chloride 37.5-60 g/l 45 Boric acid 45-49 g/l 45 Addition agents P-1* 7-14 ml/l 10 Addition agents P-224* 0.7-1.25 ml/l 0.75 Addition agents P-4* 3-5 ml/l 4 *Harshaw Perglow Bright Nickel Plating Process The Harshaw Chemical Company

and the following operating conditions

Range Preferred pH 3.3-4.4 4.0 Current density, 20-100 50 amp/sq ft Temperature, F. 125-160 140 Agitation Vigorous air Filtration Continuous

20. Plate to a thickness of about 0.2 to 2 mils

21. Rinse in cold tap water

Referring back to the brief description of the drawings as well as to the upper half of the drawings themselves, it is apparent that with the articles made in accordance with the best available prior art, bending of the articles caused a relatively few of the microcracks in the outer chromium layer to expand and that some of these cracks propagated further so as to penetrate through the conventional bright nickel layer (containing sulfur) which is fairly brittle. In contrast, as shown in the lower half of the drawings, depicting articles made in accordance with the present invention, bending caused a substantially uniform expansion of most of the microcracks in the outer chromium layer, and thus the expansion of each crack was much less than in the articles made according to the best prior art. Moreover the more evenly distributed cracking of the chromium layer exposed more nickel from the contiguous layer of ductile bright nickel. This increased the ratio of nickel to chromium in the cracked regions, providing increased protection against electrolytic erosion (such as is commonly encountered on bumpers and other automotive hardware) and thus minimizing such corrosion.

It is apparent therefore that the present invention provides much greater protection against corrosion of the substrate, while providing also a bright, decorative, and pleasing appearance to the articles coated in accordance therewith.

While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all of the possible equivalent forms or ramifications of the invention. It is to be understood that the terms used herein are merely descriptive rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.

Claims

We claim:

1. An article comprising a metallic substrate and a mechanically deformable corrosion-protective coating thereon comprising a layer on the substrate consisting essentially of ductile bright nickel substantially free of sulfur and a layer thereon consisting essentially of chromium having cracks substantially uniformly distributed therein at an average spacing of not more than about 3 mils, the protective coating having been maintained at about 350.degree. to 450.degree.F for at least about 1 minute.

2. An article as in claim 1, wherein the bright nickel layer has a ductility factor of about 100 percent as measured by micrometer bend test.

3. An article as in claim 1, wherein the bright nickel layer has a ductility equivalent to at least about 0.15 percent elongation for a thickness of 2 mils.

4. An article as in claim 1, wherein the bright nickel layer contains less than about 0.005 percent sulfur.

5. An article as in claim 4, wherein the bright nickel layer is about 0.2 to 2 mils thick.

6. An article as in claim 1, wherein the chromium layer is at least about 0.01 mil thick.

7. An article as in claim 1, comprising also a layer consisting essentially of ductile bright copper between the substrate and the layer of bright nickel.

8. An article as in claim 7, wherein the copper layer is not more than about 1 mil thick.

9. An article as in claim 8, wherein the bright nickel layer is not more than about 2 mils thick.

10. An article as in claim 7, comprising also a layer consisting essentially of ductile semi-bright nickel between the copper layer and the bright nickel layer.

11. An article as in claim 10, wherein the bright nickel layer is substantially free of sulfur and about 0.2 to 2 mils thick.

12. An article as in claim 11, wherein the semi-bright nickel layer is not more than about 2 mils thick.

13. A method of providing an article as in claim 1 that comprises electroplating on the substrate a layer of ductile bright nickel from an aqueous solution consisting essentially of nickel sulfate, nickel chloride or fluoborate, boric acid, and coumarin or a derivative or analog thereof, electroplating, on the bright nickel layer a layer of microcracked chromium, and maintaining the protective coating at about 350 to 450.degree.F for about one to 60 minutes.

14. A method of providing an article as in claim 1 that comprises electroplating on the substrate a layer of ductile bright nickel from an aqueous solution consisting essentially of about 150 to 400 grams per liter of nickel sulfate, about 5 to 25 grams per liter of nickel fluoborate, about 20 to 40 grams per liter of boric acid, and about 1 to 3 grams per liter of coumarin or a derivative or analog thereof, with a pH of about 5 to 5.5, at a temperature of about 110.degree. to 140.degree.F and a current density of about 10 to 60 amperes per square foot, for about 2 to 10 minutes, electroplating on the bright nickel layer a layer of microcracked chromium, and maintaining the protective coating at about 350.degree. to 450.degree.F for about one to 60 minutes.