US2271375A - Process of coating metal surfaces - Google Patents

Description

Patented Jan. 27, 1942 UNITED STATES PATENTiOFFICE' 2,211,375- cl'fiiiilififiififim l signonto Rust Limited, Montreal, (7

Canada This invention relates broadly to the treatment of metal articles for the preparation of the metal surfaces to resist rusting or corrosion. and to receive surface coatings of various sorts and in some cases for the improvement of characteristics of the metal articles, and the present application is a continuation in part of my prior application,

Ser. No. 36,217, filed August 14, 1935, which issued on June 3, 1941, as U. S. Patent No. 2,244,526.

The primary object of the invention is to provide a process of treating surfaces of iron, steel,

ferrous alloys, copper and cuprous alloys, to render them resistant to rusting, oxidation and corimparting to metal articles a finish which does not interfere with electric or acetylene welding or brazing and which serves to diminish or inhibit electrolytic action between difierent metals.

panyofcanada, aeorporationof- Ne Drawing 0 Application .m 14,'1oss,,seruu In oaaadad ns 3 19:5 2 Claims. (01.148-0) The treatment is The molten bath is composed of more than 50% of an alkali metal hydroxide, or mixture of alkali metal hydroxides, and less than 50% of'one or more of the alkali metal nitrates, phosphates and carbonates with or without bath fluxing or stabllizing agents or both. The addition of a small proportion of manganese dioxide or crystalline I copper sulphate as a flux improves the fluidity Various other objects and the advantages of the invention may be ascertained from the following description. In its broadest aspect, the invention comprises,

I firstl'y, subjecting the articles to be treated to the action of a molten coating agent comprising a major portion of an alkali metal hydroxide, or

finishing solution. The articles may be washed with water as may be desired or necessary between the several treatments.

More particularly, the invention comprises the features and combinations of features herein disclosed together with all such modifications thereof and substitutions of equivalents therefor as are within the scope of the appended claims.

, ticles immersed will have some influence.

equal surface areas. the casting will require somewhat longer immersion on account of heat loss 1 The invention is primarily intended to provide a durable and attractive surface finish on ferrous and cuprous metallic articles, independently of 1 any other surface coating and which finish is-resistant to rusting, oxidation and corrosion of the metal and forms an improved foundation for othersurfacecoatingk 3 of the bath and thus reduces the loss through dragout on the articles treated. The amount of either manganese dioxide or copper sulphate should be between 0.5% and 10% of the bath and preferably is about 1% or 2%. Copper sulphate appears also to have a stabilizing effect on the bath, especially at higher temperatures.

If it is desired to impart themost effective rust andcorrosion retarding finish to the surface of the metal, the bath temperature should be between 500 and 900 F. and preferably between 700 and 900 F. The actual temperature used depends upon the composition of the bath, the metal treated and the duration of the treatment,

which last depends upon the necessity, or otherwise. of first removing an existing coating.

As the duration of treatment depends to some extent on the temperature, the nature of the arcomparing a compact casting and a thin plate of from the bath to the casting.

tardant of rust and corrodon. This coating is so thin it does not cause any significant increase in the dimensions of the articles treatedn Screws or bolts with very fine threads are not appreciably tighter in the cooperating tapped holes after treatment than before treatment. Thin steel effected most conveniently by immersing the articles to be-treated in baths ofl Thus,

' The molten bath reacts with the metal to pro- I duce a very strongly adherent coating practically .integral with the metal, which is an excellent reand such surface coating applied to articles treated according to this invention are more adherent than when applied to bare metal.

Articles to be treated in the molten bath are merely immersed therein and no agitation of the articles or of the bath is necessary. The material of the bath is very penetrating and goes into the most minute crevices. Metal plates closely. packed together receive as good a finish on the contact surfaces as on exposed surfaces and do not show-any traces of the contactsuch as would occur in electroplating or after dipping in paint or enamel. Small articles such as bolts, nuts, washers and the like may be treated 'in bulk in baskets.

After immersion in the molten bath, the articles are lifted out, drained for a few seconds and then washed with water either by immersion therein or by spraying. The washing should be very thorough on account of the large percentage of alkali metal hydroxide in the molten bath.

The washed articles are next immersed in a neutralizing bath being a weak aqueous solution of hydrochloric, sulphuric, oxalic or other suit-v able acid or an aqueous solution of iron sulphate, maintained preferably at a temperature of about 160 F., but the temperature may be lower or higher, for example, from 100 to 200 F. A- l% to 2% solution of hydrochloric acid will serve while sulphuric or oxalic acids may be used up to 5%. The preferred bath is iron sulphate, about 4 ounces to each imperial gallon of water. This solution not only neutralizes any alkali adhering to the metal but darkens and improves the appearance of the finish imparted in the molten bath. i

After treatment in the neutralizing bath, the articles may be discharged and used as they are or may be painted, enamelled or metal plated,

but it is preferred that they be transferred to a finishing bath, preferably after washing to remove any free acid from the surface'thereof.

This finishing bath is composed of iron sulphate, crystalline haematoxylin, water soluble nigrosine, tannic acid or other suitable sealing or colouring may be used. The amounts of reagents used may be either more or less than above stated. This bath is maintained preferably at a temperature of approximately 180 F., but the temperature may be lower or higher, for example between 100 and 200 F., and the articles are immersed therein for two to ten minutes and then removed andwashed with water. The resulting finish is more resistant to rust and corrosion than that produced in the first bath and is a better base for paint, enamel and metal plating. The finish is of' very good appearance, ranging from gun metal to dark blue or black. If given a coat of oil or clear lacquer, the appearance is similar to good black enamel, In addition to colouring the finish The coating- Instead of crystallineimparted in the first bath, the materials used enter the pores or intermolecular spaces of the finish and seal the same against entrance of rust or corrosion producing agents.

The neutralizing bath is very important if a paint, lacquer or enamel is to be applied subsequently as every trace of alkali must be removed to avoid'the appearance of blemishes in the final coating.

The condition of the second and third baths I Fmsr Barn Example I Parts Sodium hydroxide 65 Potassium nitrate 45 I Example If 7 Sodium hydroxide to 80 Trisodium phosphate 40 to 20 Example HI Sodium hydroxide Sodium carbonate 35 Example IV. Sodium hydroxide 35 Potassium hydroxide 20 Sodium nitrate 45 v Example V Sodium hydroxide 35 Potassium hydroxide 35 Trisodium phosphate 30 Example VI Sodium hydroxide 60 Potassium nitrate 30 Trisodium phosphate 10 Example'VII I Sodium hydroxide 65 Potassium nitrate 30 Potassium carbonate 5 Example VIII Sodium hydroxide 30 Potassium hydroxide 35 Sodium nitrate-.. -1 25 Trisodium phosphate 10 Example IX Sodium hydroxide 30 Potassium hydroxide 35 Potassium nitrate 20 Sodium carbonate l5 Example X Potassium hydroxide "10 Sodium nitrate 30 To any of the foregoing mixtures, there may be added, if desired, from 0.5% to 10% of manganese dioxide or crystalline copper sulphate as a fluxing and stabilizing agent, the

m any of the foregoingexamples, the alkali metal compounds named may be replaced by the similar compound of the other alkali metal.

' The surface finish imparted according tothis invention, and especially that resulting from immersion in all three baths,'is very durable and tenacious, so that metal sheets thus treated may be formed in dies without injuring the finish.

The finishing treatment, especially when temperatures between 600 and 900 F. areused, has

- an annealing effect which improves the strength 0! the metal. In certain cases where annealing is required between forming operations, the parts may be treated according to thisinvention between the forming operations and annealing thus efiected simultaneously with surface finishing;

It is possible also to heat the bath above 900 F.,

say to 1200 or even 1500 F., to heat treat articles for removal of internal stresses therefrom or for otherpurposes.

If no subsequent surface coating is to be applied, it is preferable to use iron sulphate in the second or neutralizing bath and the thirdbath may be omitted, but a better appearing ilnish'and one more resistant to rust and corrosion is obtained if the articles are treated in the third bath also; It is usually preferableto use iron sulphate inone only or the baths, that is, 11 11-. is used in the second bath Lone oi the other agents isused in the third bath, but if the sulphate lito be used in thethird bath. the second bath is preferably made with hydrochloric or other acid.

Having thus described my invention. what I claim is:

- l. A process of treating articles 01 iron. copper 2. A process of treating articles of iron, copper and alloys of each of them, which process comprises fusing tosether manganese dioxideand a mixture of alkali metal compounds including aikali metal hydroxide, alkali metal carbonate and alkali metalnitrate, the resulting mass being in the molten state reactive with. the metal treated to produce a surface coating thereon, and subjecting the articles to.the action of the molten