US2393943A - Coating - Google Patents

Description

Patented Jan. 29, 1946 COATING Robert M. Thomas and Charles Baltimore, Md., assignors, ments, to Rheem Research W. Ostrander, by mesne assign- Products Incorporated, Baltimore, Md., a corporation of New York No Drawing. Application August 23, 1943, Serial No. 499,709

1 Claim.

This invention relates to the treatment of zinc and cadmium including castings thereof, and particularly galvanized metal, but may also be used in the treatment of brass and copper and other copper alloys including bronzes.

The primary object of the invention is to produce an insoluble, integral, corrosion resistant and visible coated metal surface by means of an aqueous dip containing a water-soluble chromium compound in predominant amount and a lesser amount of zinc chloride.

Another object of the invention is to produce a coating which is integral with the surface of the metal and therefore is more permanent and resistant to abrasion than coatings which are merely adherent.

A further object of the invention is to treat galvanized metal in such a manner that the dip acts upon the spelter surface or pure zinc surface, in the case of plated metal, as distinguished from the zinc-iron alloy beneath the same. We are aware that treatments have beenproposed-in which the dip compositions eat away the zinc surface of galvanized iron so as to act upon the zinc-iron alloy. This, in many cases, is both objectionable and unnecessary.

An additional Object of the invention is to provide a dip and process of coating which is free of complicated procedure, and; may be carried out at commercially acceptable costs to give uniformly satisfactory results. While various compositions and processes have been suggested which use inorganic acids and salts thereof with soluble chromium compounds, for causes which have so far defied explanation, consistently acceptable coatings are not produced commercially.

The present invention has met with widespread success, particularly where difilculties are found with previous dips and procedures. Although careful study has been made and is being continued, the reason for the consistently improved coatings obtained by the present invention has not yet been definitely ascertained.

In carrying out the invention, we find that certain proportions, temperature ranges and time periods are critical-and we will illustrate the invention in connection with galvanized iron or steel, either hot dipped or electroplated. While magnesium chloride, sodium chloride, cupric ammonium chloride and stannous chloride may be successfully used, we find that zinc chloride consistently gives a reliable result, and markedly so in cases where present available procedure and dips have been found wanting. The galvanized iron after being suitably cleaned is subjected, as

by immersion or spraying, to a solution containing chromic acid in the amount of 100 grams per liter and zinc chloride in amount of 11 grams per liter. These proportions may be varied from resistent coating, the hot the case of zinc or cadmium about grams of the chromium compound to about 100 grams per liter, and from about 5 grams of zinc chloride to about 15 grams per liter. 'To produce an integral bronze colored, corrosion dipped galvanized metal is 0treated for 15 seconds with the solution at 12 F.

Electroplated galvanized metal is treated with the same solution for a period of 15 seconds at F. I

The temperature range. at which the coating may be conducted, i. e., the temperature of the solution, varies from between 60 F. to about 212 F.,'the coating being more rapid at higher temperature.

The time of immersion or contact'with the spray may vary from about 30 to about 240 seconds for hot dipped galvanized articles and in plated articles, from about 1 to about 60 seconds.

While we have referred to chromic acid as the soluble chromium compound, it is' to be understood that chromates and dichromates as customarily employed may also be utilized, and the term soluble chromium compound is intended to embrace all of the same.

.The reaction between the dip and the galvanized metal takes place upon the surface layer of spelter or pure zinc to the end that the coating becomes integral with the surface layer of pure metal to form a molecular film, and does not act upon the underneath layer of zinc-iron alloy.

After this treatment, the coated metal having a bronze surface is rinsed in hot water and dried by hot or cold air whereupon it is ready for immediate use. By the foregoing treatment of the galvanized metal the rapid and destructive formation of zinc salts and oxides and hydroxides which form in the presence of moisture and moist air is prevented.

In explanation of the success of the present invention, although we are not yet sure of what takes place to give the improved result, we believe that the zinc chloride promotes oxidation-reduction reactions between the metal surface and the chromium compound, and assures the formation of a chromium compound with the base metal which is formed as an integral part of the metal surface and resists rubbing off or flaking and chip-ping. This formation of an integral surface compound is distinguished from mere adhesion.

' metal and the peratures as used on electroplated metal.

2 b The latter implies th gluing or sticking together of two 1 materials by attractive forces inherent in one or both materials. In the present instance the zinc chloride promotes the formation of this integral surface compound of the base chromium compounds. as disformation of a reduced tinguished from the tendency to chromium compound with some adhere to the base metal, but which can be rubbed oil to a greater or lesser extent.

The time period employed for immersion or spraying in some cases affects its color. -We ilnd that by altering the concentration of the ingredients of the bath or dip solution, and by varying the temperature of the bath and the time period of immersion or spraying, the composition of the formed coating may be controlled within limits to produce a variety of colors ranging from a golden yellow, through orange to green and black. 1. e., colors which reflect the greater or lesser extent to which the chromium compounds have been reduced from a valence of 6 to a lower valence. Precise shades of color are considered to be determined by a mixture of chromium ions of several valences in combination with the base metal, and to the presence of compounds of chromic dichromate, either alone or in combination with other ions.

In the treatment of brass and copper and other copper alloys, solutions, as described herein, are employed, preferably for time periods and at temiridescent bronze coating is obtained.

The lustre of the finished product is controlled by the preparation of the metal surface prior to treatment, 1. e., whether the plate be dull plate or bright dip plate.

In referring in the claims to zinc or cadmium, we intend to include pure zinc and pure cadmium, as well as mixtures thereof, articles'coated with zinc or cadmium, or mixtures thereof, socalled zinc and cadmium castings and, of course,

both hot dipped and electroplated galvanized metal.

In referring in the claim to copper, we intend to include copper and its alloys, of which brass is the most prominent example, as well as bronze alloys. y

We claim:

The method of producing an integral corrosion resisting coating on a surface selected from the group consisting of zinc, cadmium, and copper comprising subjecting the same to an aqueous dip consisting essentially of water, a water soluble chromium compound selected from the group consisting of chromic acid and salts thereof and zinc chloride, the chromium compound in the dip being in predominant amount with respect to the zinc chloride, and the zinc chloride'being in amount with relation to the amount of chromium com-pound that the dip reacts upon the surface only of the said metal and forms a coating which is integral with such surface. I

ROBERT M. Thomas. cams w. os-raarmna.