CA2071188C - Metal product with composite coating - Google Patents

Abstract

A procedure for improving the adherence, in the presence of moisture, of (a) a top coat of paint or polymer to (b) a metal substrate coated with another metal. The metal substrate is coated with a composite coating of particles of oxide dispersed in the coating metal. The surface energy of the composite coating is controlled. The article comprising the metal substrate plus composite coating need not be subjected to any phosphating and chromate sealing treatments before application of the top coat.

Description

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METAL PRODUCT WITH COMPOSITE COATING
Background of the Invention The present invention relates generally to metal products with composite coatings and more particularly to a metal product of that type and which enables improved adherence thereto of a top coat of paint or polymer.
A metal product with a composite coating typically comprises a metal substrate comprising a first metal, such as steel, and a composite coating on at least one surface of the metal substrate. The composite coating typically comprises a second metal, different from the first metal, for enhancing a surface property of the first metal (e. g. corrosion resistance). In addition, the composite coating comprises non-metallic particles on at least the surface of the second metal, but usually also dispersed within the bulk of the matrix formed by the second metal, for enhancing the properties due to the presence of the second metal or for producing entirely different properties not present with the second metal alone (e. g. lubricity, abrasion resistance, etc.).
Typical examples of the second metal are zinc, aluminum, nickel, copper, chromium and alloys of each.
Typical examples of non-metal particles, for improving the corrosion resistance of the coating, include alumina and silica. Typical examples of non-metallic particles for improving the lubricity of the product's surface comprise molybdenum disulfide and graphite, as well as polymers such as polytetrafluoroethylene (PTFE) better known as Teflon TM. Typical examples of non-metallic particles used for abrasion resistance and for dispersion hardening of the composite coating include alumina, titania or other refractory powders.
Composite coatings are typically applied to a metal substrate by a procedure known as co-deposition which embeds small particles of the non-metallic material into a metallic matrix, defined by the second metal, by intentionally adding the non-metallic particles to a plating bath utilizing electrolytic or electroless deposition of the second metal.
Additional details regarding the concepts and the fundamental principles underlying composite coatings and co-deposition are contained in an article by Roos, et al. entitled "The Development of Composite Plating for Advanced Materials" in the Journal of Metals (JOM), Nov., 1990, pages 60-63.
Metal products comprising a metal substrate composed of a first metal coated with a second metal, such as steel coated with zinc or aluminum to improve the corrosion resistance of the steel, have many uses, particularly in producing parts for automotive vehicles.
It is desirable to apply a top coat consisting of a paint or polymer to the metallic product described in the preceding sentence, but there are problems with maintaining the adherence of the paint or polymer top coat to the underlying metal product when the metal product has a substrate coated with another metal such as zinc or aluminum. As used herein, the term "paint or polymer" refers to materials conventionally used as either primers or finish coatings applied to metal surfaces for protective or aesthetic purposes and to materials conventionally applied as an adhesive to adhere a metal surface to the surface of another article, and the term "top coat" applies to them all.
When a top coated product of the type described above is subjected to moisture, the moisture can have an adverse effect on the adherence of the top coat to the metal product. This adverse effect is particularly 20'1188 noticeable when the top coated metal product is scored or otherwise scratched.
Procedures have been developed in an attempt to offset the adverse effect of moisture on the adherence of the top coat to the metal product. Such procedures include subjecting the zinc or aluminum plated metal product to a pretreatment such as phosphating or phosphating plus chromate seal coating. These are conventional treatments which need not be described herein in detail. These pre-treatments, however, entail additional time, effort and expense.
Summary of the Invention The present invention produces a product composed of a metal substrate, such as steel, coated with a second metal, such as zinc or aluminum, and which can be top-coated with a paint or a polymer, and which is substantially resistant to the adverse effect of moisture without pre-treating the coated metal substrate with a phosphate treatment or a phosphate treatment plus chromate sealer treatment.
The coating on the metal substrate is a composite coating comprising, in addition to the second metal (e. g.
zinc), non-metallic particles on at least the surface of the second metal (but usually also embedded within the bulk of the matrix defined by the second metal).
The composite coating, comprising the second metal and the non-metallic particles (dispersoid), has a surface energy, expressed as ergs/cm2, within a limited range which substantially improves the adherence to that product of a top coat of paint or polymer when exposed to the influence of moisture, compared to the same product without the non-metallic particles. Surface energy has two components, polar and dispersive.

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The dispersoid is typically alumina or silica.
Other non-metallic particles which may be employed in the composite coating comprise titanium oxide, chromium oxide and iron oxide. Alumina-coated silica may be employed.
Other second metals which may be employed in the composite coating, in addition to zinc and aluminum, comprise copper, nickel, chromium and alloys of each of the second metals previously described.
Other features are inherent in the subject matter claimed and disclosed or will become apparent to those skilled in the artfrom the following detailed description.
Detailed Description In accordance with the present invention, the surface energy of the composite coating is such that the work of adhesion, WA, and the work of adhesion in the presence of a liquid environment (here, water), WAL, are each greater than zero. The adhesion in question is the adhesion between (A) the paint or polymer and (S) the surface to which (A) is applied: in this case, the surface of the composite coating. When a top coat comprises a finish coat atop a primer, the primer is (A).
If both the work of adhesion, WA, and the work of adhesion in the presence of a liquid environment, WAL, are less than zero, the top coat will debond from the surface to which it is applied. If W,, is greater than zero, and WAL is less than zero, the top coat will adhere in a dry 2~~11~8 environment but debond when exposed to the liquid environment.
The work of adhesion, WA, and the work of adhesion in the presence of a liquid environment, WAL, are expressed as ergs/cm2 and are respectively defined by the two equations set forth below.

WA=2 ( Ya Ys + YA Ys ) WAL=2 ~YL- YS YA Ys YA
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wherein:
y - surface energy, ergs/cmz P - (superscript), polar component of surface energy D - (superscript), dispersive component of surface energy L - (subscript), liquid environment (here, water ) A - (subscript), adhesive (typically epoxy, used as a primer) S - (subscript), material to which the top coat or primer is applied.
For a material (S) to which a top coat is to be applied, the polar component of the surface energy, ysP, should be below about 25 ergs/cm2, and the dispersive component of the surface energy, ySD, should be above about 50 ergs/cm2. Both of these criteria can be satisfied by a 20?1188 _ 7 _ composite coating in which either silica particles or alumina particles are the dispersoid.
A typical epoxy used as a primer has surface energy components as follows, for example:
YA = 5 ergs/ cm 2 ; YA 5 0 ergs/ cm 2 .
Top coats, generally, including both paints and polymers, have surface energy components as follows:
YA = 5 - 3 0 ergs/ cm 2 ; YA = 2 0 - 5 0 ergs/ cm 2 .
Water, the liquid environment here, has typical surface energy components as follows:
Yi = 51 ergs/cm2; Yi = 22 ergs/cm2.
A range of surface energy components for a composite coating in accordance with the present invention are as follows:
~ySD = about 50 to about 200 ergs/cm2; ~ysP = about 1 to about 25 ergs/cm2.
The total surface energy for a composite coating in accordance with the present invention should be from about 75 to about 201 ergs/cm2. (Total surface energy is the sum of the polar and dispersive surface energy components.) A composite coating comprising silica or alumina as the dispersoid particles in a matrix of zinc can satisfy these criteria. Surface energy may be determined by ~~~1~~~
_8_ employing conventional procedures such as those described in the following publications: (1) Owens, D.K., et al., Journal of Applied Polymer Science, Vol. 13, p. 1741 (1969); (2) Carre, A. et al., Journal of Adhesion, Vol.
15, pp. 151-162 (1983). Generally, polar materials (i.e.
those materials with higher surface energy polar components) adhere better to other polar materials.
Examples of products in accordance with the present invention utilize steel as the metallic substrate and a composite coating utilizing zinc as the metallic component thereof. The non-metallic particles employed in the composite coating may be silica or alumina. The composite coating may be deposited on the metallic substrate employing an electrolytic deposition procedure of a conventional nature unless otherwise specified below.
The steel substrate may be of a type heretofore conventionally utilized as a substrate for the electrolytic deposition of zinc to produce an electrogalvanized steel product. The electrolytic bath, and the conditions employed during the electrolytic deposition process, may be those conventionally employed in the electrolytic deposition of zinc without non-metallic particles dispersed therein, unless otherwise specified below.

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_ g _ The steel substrate may be subjected to conventional preliminary cleaning and other pre-treatment procedures conventionally preceding the electrolytic deposition of zinc on steel when producing an electrogalvanized steel product.
Samples of products in accordance with the present invention were prepared. These samples utilized a steel substrate and a composite coating comprising zinc and non-metallic particles dispersed within the bulk of the matrix defined by the zinc and on the surface of the zinc. The non-metallic.particles were silica in some samples and alumina-coated silica in other samples.
Alumina-coated silica particles will reflect how alumina particles per se will work.
The composite coatings were electrolytically deposited on the steel substrate in a procedure employing plating conditions set forth below in Table I. For comparison purposes, there were also samples employing steel substrates having electrolytically deposited zinc thereon, but without non-metallic particles; and these comparative samples were either produced in the laboratory or obtained from a commercially produced galvanized steel product employing the electrolytic deposition of zinc. The plating conditions for the comparative samples are also shown in Table I.

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The samples produced in the laboratory employed steel coupons having dimensions of 2.125 x 9 x 0.03 in.
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Those samples comprising a composite coating were produced in three plating runs of 50 specimens each, while the laboratory-produced samples containing only zinc in the coating were produced in two runs of 50 specimens each. The plating run sequences were randomized so that a plating run producing a coating of one given composition was typically followed by a plating run producing a coating of another composition.
After the samples had been produced, they were subjected to one or more of the following treatments, as summarized in Table II set forth below: oiling, cleaning, phosphating, sealing and priming plus finish coating.

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The oil applied to the samples was a conventional shipping oil applied to cold-rolled steel sheet in preparation for shipment thereof; it was identified as Nalco 6295 and obtained from Nalco Chemical Company, Oakbrook, Illinois. The cleaner was an alkaline cleaner identified as Parco 2331, the phosphate was a zinc phosphate identified as Bonderite B952 and the sealer was a chromate sealer identified as.Parcolene P80, all of which were obtained from Parker-Amchem (now Henckel).
The primer was an electro-deposited primer coat identified as DuPont M64J28, and the top coat was a white enamel top coat identified as DuPont M33J100A 5920A, both obtained from E.I. DuPont DeNemours Company, Wilmington, Delaware.
All of the materials described in the preceding paragraph are conventional materials utilized in connection with (a) the preparation of conventional electrogalvanized steel for the application of a top coat of paint and (b) the top coating of the electrogalvanized steel. Other materials of a similar nature, conventionally utilized for performing the same function as one or more of the materials described in the preceding paragraph, may be employed in lieu thereof.
After the samples were painted, many of the specimens in each sample were subjected to testing to determine the resistance thereof to the adverse effects of moisture on the adherence of the top coat. The specimens undergoing testing were initially scribed to simulate scratching. The length of the scribe line was 2.5 in. (6.35 cm). After scribing, the specimens were tested by exposure in a salt spray box (ASTM test B117) for 14 days (336 hours). The order in which the specimens were scribed, and the positions of the specimens in the salt spray box were randomized.
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were withdrawn and prepared for removal of loosely adhering paint. In order to maintain paint softness for consistent removal of loosely adhering paint, the specimens were put into plastic bags immediately upon removal from the salt spray box, then transported in the plastic bags to a measurement lab and placed in tepid water while awaiting the loose paint removal step. Loose paint was removed by blasting the area around the scribe line with an air jet and simultaneously abrading that area with the air nozzle. All scribe areas were blasted within 3 hours of removal of the specimens from the salt spray box.
The resistance of a specimen to the adverse effect of moisture was estimated by (a) "average" width of creepage from the scribe line and (b) maximum width of creepage from the scribe line. The "average" width was estimated by aligning the center axis of a transparent ruler along the scribe line and measuring the total width of creepage at six locations spaced one centimeter apart.
The first measurement was taken 0.5 cm from that end of the scribe line which was at the top of the specimen when the specimen was vertically disposed in the salt spray box. The average of the six measurements define the "average" width of creepage. The maximum width of creepage was the largest creepage occurring anywhere along the scribe line, and this was not necessarily the greatest of the six measurements described in the preceding sentences in this paragraph.
The creepage data are summarized in Table III.

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The conclusions set forth below were based on the results of the salt spray testing described above.
Samples in which the primer and finish coat (cumulatively, the top coat) were applied directly atop the composite coating (samplesC, D, E, F) had a resistance to the adverse effect of moisture on the adherence of the top coat equivalent to that of electrogalvanized samples in which the topcoat was applied over an intermediate layer resulting from phosphate treatment (sample I) or phosphate treatment plus chromate sealer treatment (samples G and H).
Samples A and B employed the same dispersoid (alumina-coated silica) and the same composite coating as samples C and D (see Table I), but samples A and B did not perform well. The disparity in performance is attributable to the difference in plating conditions for the samples (see Table I), which can have an effect on the surface energy of the composite coating.
Other conclusions derived from the results reflected in Table III are described below. Oiling and alkaline cleaning a sample having a composite coating has no substantial effect on the resistance of that sample to the adverse effect of moisture on the adherence of the top coat to the sample (compare samples D and F with samples C and E). The chromate sealer does not increase the resistance of zinc coatings to the adverse effect of moisture on the adherence of the top coat (compare samples H and I). The samples employing silica as the non-metallic particle in the composite coating (samples E
and F) had better properties, from the standpoint of resistance to the adverse effect of moisture, than any of the other samples. Samples C and D, employing alumina-coated silica particles in the composite coating, were next best. Samples C-F, employing silica and alumina-A

coated silica composite coatings, were at least as good or better than any of the samples employing zinc coatings alone without non-metallic particles (samples G, H and I), even though the latter were subjected to a treatment employing phosphating or phosphating plus chromate sealing prior to the application of the top coat.
The composite coatings of samples C-F, which employed either silica or alumina-coated silica, had a total surface energy and surface energy components within the ranges described above, i.e., ~ySD of about 50 to about 2 0 0 ergs / cm2, ~ysP o f about 1 to about 2 5 ergs / cm2, and a total surface energy, ~ys, of about 75 to about 201 ergs/cmz. As noted above, the performance of the alumina-coated silica particles reflect how alumina particles will perform; and as used herein, reference to alumina particles includes both alumina particles per se and alumina-coated silica particles unless otherwise indicated.
When employing either electrolytic or electroless deposition, the surface energy of the composite coating can be controlled by adjusting one or more of the following parameters: the number of non-metallic particles per unit area of coated substrate surface, and the size of the non-metallic particles. When employing electrolytic deposition utilizing an electrolyte bath for depositing the composite material on the metal substrate, the surface energy of the composite coating can also be controlled by adjusting the current density per unit area of the substrate surface, the pH of the electrolyte bath, and the extent to which there is circulation of liquid flow currents in the electrolyte bath (i.e. turbulence).
The effects of changes in plating conditions are non-linear and interactive. In general, an increase in pH produces an increase in ~yP and ~yD while an increase in current density produces a significant increase in yD but not in ~yP. A change in dispersoid composition significantly affects the degree of polarity (i.e. the polar component of surface energy, ~yP). Current density and pH interact, dispersoid composition and pH interact, and dispersoid composition and current density interact.
Flow conditions in the plating both will affect the extent to which the non-metallic dispersoid particles will be deposited. It is expected that the greater the degree of turbulence, the greater will be the amount of dispersoid particles deposited.
The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

Claims (17)

1. A metal product composed of a metal substrate and a composite coating, which enables improved adherence of a top coat of paint or polymer to said metal product without subjecting the composite coating to any treatment to offset the adverse effect of moisture on said adherence prior to application of said top coat, said product comprising:
a substrate comprising a first metal;
an exposed composite coating on at least one surface of said metal substrate;
said composite coating consisting essentially of a second metal different than said first metal, for enhancing the corrosion resistance of the first metal, and non-metallic particles on at least the surface of said second metal;
said composite coating having a surface energy, expressed as ergs/cm2, which substantially improves the adherence to said product of a top coat of paint or polymer, when the product is exposed to the influence of moisture, compared to the same product without said non-metallic particles, said improved adherence being due to said surface energy of the composite coating;
said surface energy of the composite coating being such that (i) the work of adhesion (W A) between the surface of said composite coating and a top coat of paint or polymer deposited thereon and (ii) the work of adhesion in the presence of a liquid water environment (W AL) are each greater than zero;
said composite coating having a polar surface energy component of 1 to 25 ergs/cm2 and a dispersive surface energy component of 50 to 200 ergs/cm2.

2. A product as recited in claim 1 and which, when provided with (a) a top coat of paint or polymer but without (b) an intermediate layer which would result from a phosphate treatment plus a chromate sealer treatment, has a resistance to the adverse effect of moisture on the adherence of said top coat at least about equal to the resistance of the same product without said non-metallic particles but which does have said intermediate layer (b).

3. A metal product as recited in claim 2, wherein:
said composite coating has a total surface energy of 75 to 201 ergs/cm2.

4. A product as recited in any of claims 1-3 wherein:
said substrate is steel;
said second metal is selected from the group comprising zinc, aluminum, copper, nickel, chromium and alloys of each;
and said non-metallic particles have a composition selected from the group comprising silica, alumina, titanium oxide, chromium oxide and iron oxide.

5. A product as recited in claim 4 wherein:
said second metal is zinc;
and said non-metallic particles are silica or alumina.

6. A product top coated with paint and substantially resistant to the adverse effect of moisture on the adherence of the top coat, said product comprising:
a substrate comprising a first metal;
a composite coating on at least one surface of said metal substrate;
said composite coating consisting essentially of a second metal different than said first metal for enhancing the corrosion resistance of the first metal, and non-metallic particles on at least the surface of said second metal;
said composite coating having a surface energy, expressed as ergs/cm2, which substantially improves the adherence to said product of a top coat of paint when the product is subjectged to the influence of moisture, compared to the same product without said non-metallic particles;
and a top coat of paint applied directly atop said composite coating prior to any treatment to the composite coating to resist the adverse effect of moisture on adherence of said top coat to said composite coating;
said top coated product being devoid of an intermediate layer between said composite coating and said top coat;
said top coated product having a resistance to the adverse effect of moisture on the adherence of said top coat, said resistance being due to the surface energy of the composite coating, and being at least about equal to the resistance of the same product without said non-metallic particles but which does have an intermediate layer which would result from a phosphate treatment or a phosphate treatment plus a chromate sealer treatment;
said surface energy of the composite coating being such that (i) the work of adhesion (W A) between said top coat and the surface of said composite coating and (ii) the work of adhesion in the presence of a liquid water environment (W AL) are each greater than zero;
and said composite coating having a polar surface energy component of 1 to 25 ergs/cm2 and a dispersive surface energy component of 50 to 200 ergs/cm2.

7. A top coated product as recited in claim 6 wherein:
said composite coating has a total surface energy of 75 to 201 ergs/cm2.

8. A top coated product as recited in any of claims 5 to 7 wherein:

said substrate is steel;
said second metal is selected from the group comprising zinc, aluminum, copper, nickel, chromium and alloys of each;
and said non-metallic particles have a composition selected from the group comprising silica, alumina, titanium oxide, chromium oxide and iron oxide.

9. A top coated product as recited in claim 8 wherein:
said second metal is zinc;
and said non-metallic particles are silica or alumina.

10. A top coated product as recited in claim 9 wherein:
alumina-coated silica is excluded from said composite coating.

11. A method for enabling improved adherence of a top coat of paint or polymer to a composite coating on a metal substrate, without subjecting the composite coating to a treatment to offset the adverse effect of moisture on said adherence prior to application of the top coat, said method comprising the steps of:
providing a metal substrate comprising a first metal;
introducing said metal substrate into a plating bath consisting essentially of a second metal different than said first metal for enhancing the corrosion resistance of said first metal, and non-metallic particles;
co-depositing onto at least one surface of said metal substrate a composite coating consisting essentially of said second metal and non-metallic particles, whereby said non-metallic particles are on at least the surface of said second metal;
producing a coated metal product as a result of performing said previously recited steps;

and employing plating conditions that provide said composite coating with a surface energy, expressed as ergs/cm2, which substantially improves the adherence to said product of a top coat of paint or polymer when the product is subjected to the influence of moisture, compared to the same product without said non-metallic particles;
said surface energy of the composite coating being such that (i) the work of adhesion (W A) between the surface of said composite coating and a top coat of paint or polymer deposited thereon and (ii) the work of adhesion in the presence of a liquid water environment (W AL) are each greater than zero;
said composite coating having a polar surface energy component of 1 to 25 ergs/cm2 and a dispersive surface energy component of 50 to 200 ergs/cm2.

12. A method as recited in claim 11 wherein:
said composite coating has a total surface energy of 75 to 201 ergs/cm2.

13. A method as recited in any of claims 10 to 12 wherein:
said substrate is steel;
said second metal is selected from the group comprising zinc, aluminum, copper, nickel, chromium and alloys of each;
and said non-metallic particles have a composition selected from the group comprising silica, alumina, titanium oxide, chromium oxide and iron oxide.

14. A method as recited in claim 13 wherein:
said second metal is zinc;
and said non-metallic particles are silica or alumina.

15. A method as recited in claim 14 wherein:

alumina-coated silica is excluded from said composite coating.

16. A method as recited in claim 11 wherein:
said coating step employs electrolytic deposition utilizing an electrolyte bath.

17. A method as recited in claim 11 or 16 and comprising:
controlling the surface energy components of said composite coating by adjusting at least one of the following parameters: (a) the number of non-metallic particles per unit area of coated substrate surface and (b) when employing the method of claim 19, (i) the current density per unit area of substrate surface, (ii) the pH of the electrolyte bath, and (iii) the turbulence in the electrolyte bath.