US3470072A - Process for the electro-deposition of paint coating onto article having predeposited porous zinc layer - Google Patents

Description

United States Patent 3,470,072 PROCESS FOR THE ELECTRO-DEPOSITION 0F PAINT COATING ONTO ARTICLE HAVING PREDEPOSITED POROUS ZINC LAYER John Gooch Ransome, Abingdon, England, assignor to Pressed Steel Fisher Limited, Oxford, England, a British company No Drawing. Filed Mar. 13, 1967, Ser. No. 622,439 Int. Cl. C23b 13/00; B01k 5/02; C23f 17/00 US Cl. 204-481 5 Claims ABSTRACT OF THE DISCLOSURE This invention involves a process for imparting a coating to an article by electrodeposition and includes electroplating the surface of an article with a thin porous layer of zinc in an alkaline plating bath and subsequently painting the article by electrophoresis.

This invention relates to a process for imparting a protective coating on metal articles particularly ferrous articles.

At the present time it is usual for metallic articles to be painted by the conventional techniques of spraying or dipping, or, sometimes, by the so called electrophoretic process in which the paint is deposited on an article immersed in the coating medium by the passage of an electric current through the coating medium between the article and an electrode.

Moreover, particularly in the case of ferrous articles, it is usual for the adhesion of the paint and the corrosion protection afforded thereby to be improved by pretreating the surface of the article; in the case of ferrous articles, this pretreating comprising treating the surface with various solutions which form a variety of complex zinc-iron phosphate layers before painting.

It is, of course, well known that ferrous articles may be galvanised; galvanising is usually effected by a hot dip process, but when the physical properties of the article may be adversely affected by the heat, an electroplating process may be required. It is well known that the corrosion resistance of a galvanised article may be improved by subsequently painting with conventional paints, but the zinc slowly reacts with the resins of the paint or the solubilising agents of conventional paint to form soluble zinc soaps which ultimately destroy the adhesion, and, therefore such a method is considered unsuitable for articles that will suffer abrasions or frequent handling.

The object of this invention is to provide a process for imparting a coating on metal articles, and more particularly ferrous articles, which coating not only protects the article from corrosion but also does not suffer from deterioration in its adhesion to the article.

According to the invention, a process for imparting a coating to an article includes electro-plating the surface of the article with a thin porous layer of Zinc in an alkaline plating bath, and subsequently painting the article by an electrophoretic process.

An alkaline plating bath is essential because an acid bath produces a coating which is subject to rapid corrosion and loss of adhesion as a result of the absorption of anions.

In referring to a thin layer of zinc, we intend to distinguish between the layer of zinc which heretofore has been considered suitable for protecting electro-galvanised articles and which is customarily .001"-.0O02" thick, (16 mg.-3.2 mg. per square centimeter) and the layer necessary for fulfilling the objects of this invention which could be only about 0.03 milligram per square centi- "ice meter, although generally it is preferred that the layer is between 0.15 and 2.5 milligrams per square centimeter and the upper limit is only important from the aspect of the economics of the process.

In contradistinction to the known electroplating galvanising processes, the plating operation should be performed using plating bath compositions and current densities that yield a porous film to give improved adhesion to the subsequent paint film.

During the electrophoretic painting process the anodic reaction inhibits the subsequent formation of zinc soaps and the consequent loss of adhesion; and preferably a waterborne paint is deposited whilst the Zinc film is still wet from a washing operation immediately following the plating.

It has been found that the invention provides other advantages over electrophoretic painting alone: firstly, the resins that are at present used for electrophoretic painting appear to accept a wider range of contaminants when painting onto a zinc film than when painting directly onto steel or onto a phosphated steel; and, secondly, during electrophoretic painting onto steel, the iron ions are retained in the paint causing discolouration during stoving, but when electrophoretic painting is performed on a zinc film there is no discolouration because the compounds resulting from reactions of the zinc ions are colourless.

In the following examples, reference is made to I.C.I. Electrocoat M972 with a numerical suffix. M972 is a proprietary water soluble alkyd phenolic resin and the suffix is used to denote colour e.g. 2003 is black, 2005 is red and 2006 is fawn.

EXAMPLE 1 A zinc plating solution was made by dissolving 20 grams of zinc dust in 10 litres of 1 N sodium hydroxide solution. This solution was electrolysed using a mild steel anode and cathode (the workpiece) at a current density of 20 amps per square foot of cathode surface area (potential difference across the electrodes of 2.5 to 3.0 volts) at a solution temperature of 240 C. for four minutes. When deposition was completed, the workpiece was removed and it was found that the average weight of zinc deposited over the workpiece surface was equivalent to 450 milligrams per square foot of surface area. The workpiece was then washed for three minutes using a demineralised water spray and then transferred to an aqueous bath containing paint, available as I.C.I. Electrocoat M972-2003, diluted to 10% solids N.V. (non-volatile) with the pH adjusted to 7.6 by adding ammonia. The workpiece was made the anode in the above bath and a potential difference of volts maintained between the workpiece and the bath cathode for three minutes with a bath temperature of 23 C. The paint film so applied was stoved for 30 minutes at 160 C. when it was found that the paint film was 0.8 thousandths of an inch thick.

The corrosion resistance of the electrodeposited paint film on the Zinc deposit was compared with the corrosion resistance of an electrodeposited paint film from the same paint on a zinc phosphate layer (300 milligram per square foot of surface area). The corrosion resistance of the former is in all cases at least 50% better than the latter using A.S.T.M. salt spray test.

EXAMPLE 2 grams of zinc oxide was dissolved in 40 litres of 1.25 N sodium hydroxide solution. This solution was electrolysed using a zinc anode and a mild steel cathode at a current density equivolent to 10 amps per square foot of surface area for five minutes. The temperature of the solution was maintained at 35 C. When deposition was complete, the workpiece (the cathode) was removed, spray rinsed for half a minute using demineralised water and transferred to a bath containing demineralised water, wherein it was immersed for four minutes. On removal from the water bath the workpiece was examined and found to be coated with zinc equivalent to 500 milligrams per square foot of surface area.

The workpiece was then made, the anode of an aqueous paint solution formulated from I.C.I. Electrocoat M972- 2006 diluted to 11% solids N.V. with a pH of 7.5 and a potential difference of 100 volts maintained across the anode and cathode immersed in the paint solution for two minutes. The workpiece was then removed from the paint solution, rinsed with demineralised water and the paint film baked at 160 C. for 25 minutes. It was found that the stoved paint film thickness was 0.75 thousandths of an inch thick.

The corrosion resistance of this combination of treatments was compared with that of a similarly electrodeposited film in zinc phosphate (270 milligrams per square foot of surface area) by exposing complex parts on a south facing roof at Cowley, Oxford for one year. It was observed that the paint film based on zinc phosphate had deteriorated much more than that based on the zinc deposit.

EXAMPLE 3 A zinc plating solution was made by adding 20 grams of zinc dust to 1 litre of N potassium hydroxide and then diluting the mixture with demineralised water in the ratio 1 part solution to 3 parts demineralised water. This solution was electrolysed using a compacted carbon anode and a mild steel cathode at a current density of 15 amps per square foot for two minutes maintaining the eletrolyte temperature at C. The workpiece (cathode) was then removed from the solution and washed with demineralised water for two minutes, when it was found that the zinc coating corresponded to an average weight of 525 milligrams per square foot of surface area.

The workpiece was then painted by electrodeposition using an aqueous solution of I.C.I. Electrocoat M972- 2005, making the workpiece the anode and maintaining a potential difference between the anode and immersed cathode of 80 volts for a time of three minutes. The workpiece was then removed, rinsed with demineralised water and the paint film stoved for 20 minutes at 70 C. The stoved paint film was found to be 0.7 thousandths of an inch thick.

The corrosion resistance was tested as in Example 1, with similar results.

EXAMPLE 4 An experiment was conducted as for Example 2, save that auxilliary internal electrode (anode) were inserted into recessed and screened areas of a workpiece comprising a side sill sub assembly of an automotive vehicle which it would normally be difficult to coat. The average deposited zinc film weight was found to be 700 milligrams per square foot of surface area due to the increased cathode efl'lciency caused by the more uniform current distribution, and the corrosion protection was found to be even better than the performance of the sample having the paint film based on the zinc deposit of Example 2.

What I claim is:

1. A process for coating a metallic article which includes the steps of first electro-plating in an alkaline plating bath the surfaces of the article with a thin porous layer of zinc and then painting the said surfaces by the electrophoretic deposition of a paint.

2. A process for coating a metallic article which includes the steps of first electro-plating the surfaces of the article in an alkaline plating hath made up of zinc dust dissolved in sodium hydroxide solution to produce a thin porous layer of zinc using a mild steel anode with the article as cathode at a current density of 20 amps per square foot of cathode surface and at an alkaline solution temperature of 24 C. for four minutes, second washing the article and third coating the workpiece in an aqueous bath containing paint by electrophoresis.

3. A process as claimed in claim 2 in which the alkaline solution is made up by dissolving 20 grams of zinc dust in 10 litres of 1 N hydroxide solution.

4. A process as claimed in claim 2 in which after washing the article for three minutes in a demineralised water spray, the article is transferred to a bath containing paint diluted to 10% solids (non-volatile) with the pH value adjusted to 7.6.

5. A process for coating metallic articles which includes the steps of first electroplating the surfaces of said article in an alkaline plating bath using as the anode one of the following, mild steel, zinc or compacted carbon to produce a thin porous layer of zinc, then washing the article with demineralised water and finally painting said surfaces by electrophoresis and stoving the paint film.

References Cited UNITED STATES PATENTS 1,268,987 6/1918 McMullen 204--38.42 1,856,261 5/ 1932 Phillips et a1. 20438.42 2,056,399 10/1936 Hochwalt et a1. 204-3842 2,919,233 12/1959 Cox 204- 3,039,943 6/1962 Cox et al. 20455 3,127,331 3/ 1964 Neher 204-38.42 3,317,412 5/1967 Dahlmann 20455 JOHN H. MACK, Primary Examiner E. ZAGARELLA, J 11., Assistant Examiner U.S. Cl. X.R. 20438