US1904732A - Alloy plated iron and steel and process of making the same - Google Patents

Description

Patented Apr. 18, 1933 UNITED STATES PATENT. OFFICE BA'IIB'I. B. HAUEISEN AND JAMES 0.. I'A'I'TEN, OI INDIANAPOLIS, INDIANA; SAID HAUEISEN ASSIGNOR TO SAID PATTEN; MARGARET LUCILE IPATEN'I', EXEOUTRIX OI SAID JAMES C. PATTEN, DECEASED ALLOY PLATED IRON AND STEEL AND PROCESS OF MAKING THE SAME No Drawing.

This invention relates to alloy plated iron and steel ;and it comprisesiron or steel coated.

and protected with an elect-rodeposited layer of zinc-tin alloy; and it further comprlses a method of producing such articles wherein iron or steel articles are plated with a ZlIlC- tin alloy from acompound bat-h containing zinc and tin salts and also containing cyacomposition of said bath being advantageously adjusted to give zinc and tin about the same deposition potential, replenishment of the bath being advantageously by electric solution of double anodes-of zinc and tin or by anodes of zinc-tin alloy; all as more fully hereinafter set forth and as claimed. The protection against corrosion afforded to iron and steel by anelectroplated coating depends partly upon its continuity and partly n on the position of the coatingmetal 1n the e ectrochemical scale. Copper coatings, for example, owe .their entirevalue to their continuity, since copper is electronegative to iron and, with a break or pore in the coating, corrosion is accelerated. Zinc, being highly electropositive to iron, rotects not only because of its shielding va ue, but because corrosion is restricted to zinc where both zinc and iron are exposed. However, this also means that the zinc corrodes more quickly because of the presence of iron; any kind of a pore or break in the surface tending to enlarge. Cadmium coatings have lately come into extensive use, since, in addition toi their decorative value, they are found in practice to have great protective value even with very thin films; a fact which is prob- I ably due, in part, to the position of cadmium relative to iron in the electrochemical scale. It does not tend to accelerate corrosion of the iron. Cadmium coatings have a much greater protective value than zinc coatings. Standard salt spray tests on electrodeposited zinc coatings on iron or steel sheets indicate an average life of about 200 hours with a coating of about 0.0012 inches, while much thinner cadmium coatings, say, only 0.0002 inches will give a life of 500 to 1,000 hours. A cadmium coating on iron of 0.0003 inches is sometimes regarded as equivalent, for pronides, pyrophosphates and alkali, the saline Application filed March 5, 1980. Serial No. 438,500.

sive metal than zinc. It is an object of the present invention toproduce a coating practically as nexpensive as one of zinc,of greater protective value and without its undesirable characteristics; this coating being even more advantageous than one of cadmium, while cheaper. To this end, we use as a protective coating a zinc-tin alloy; zinc containing some alloying tin. We find that in practice the amount of tin in the. coating should not be less than about 5 per cent and we often use as high as 35 per cent. Much depends upon the particular urpose for which the coating is intended. enerally, however, we do not go below 7 per cent or above 25 per cent. A 25 per cent alloy is particularly suitable for our purposes. The appearance of these alloys, after buffing and polishing, is extremely decorative. Coatings of zinc-t 1n alloys made under the present invention have a protective value commensurate with that of cad mium of the same thickness. The new coatings buff and polish well and give an excellent base for the deposition of chromium coatings tee Patents 1,614,303 and 1,615,585).

In ma ing baths for depositing tin-zinc alloys, any ordinary soluble salts of zinc and of tin may be used; but it is usually desirable to ad'ust the saline composition of the bath to re uce the difference of potential between zinc and tin electrodes immersed therein. An equipotential condition is not essential but the difference in voltage should be small; a condition which may be readily determined by immersing a plate of each metal desirable to enhance conductivity. Presence p otffldextr ine as an addition agent is desira e.

A typical 10 liter bath under the resent invention may be made as follows: no solution is prepared by dissolving 22.5 grams of commercial stannous chlorid and 180 grams of sodium pyrophosphate in 3 liters of water. Dextrine to the amount of 22.5 grams may be added. Another solution is made by dissolving 472.5 grams of zinc cyanide, 420 grams sodium cyanide 52.5 grams caustic soda and 157.5 grams of sodium chlorid in water enough to make 7 liters. The two solutions are then mixed. This bath is adapted to deposit a 75 :25 alloy of zinc and tin.

It is found in practice that baths of this type have very good throwing power; the throwing power being superior to that of the usual zinc solutions employed in zinc plating.

This solution is used in an ordinary way, being contained in a steel or wood tank of an ordinary type fitted with suitable conductingrod or rods connected to the positive side of a direct current generator. A voltage of about 6 is correct. From these rods are hung rolled or cast anodes of zinctin alloy. With the bath just mentioned, the alloy may have a zinc-tin ratio of 75:25. Instead of using a zinc-tin alloy, which is sometimes not convenient, two anodes may be used, one of zinc and one of tin, In this particular case, the relative surface areas should be in the ratio of 75 :25. The work to be plated is hung from cathode rods in the usual way. A cathode denslty of about 1 ampere per square decimeter is suitable.

With the ordinary thickness of coating, 8. plating time of about 30 minutes is required.

In the example just given the electroplated coating has a zinc-tin ratio of 75:25. This ratio may be readily varied to suit-the requirements of special uses to which the plated articles may be put. Probably the simplest method of-varying this ratio is to change the metal content of the bath. In order that the metal content of the bath should be replenished in the desired ratio it is usually necessary to adjust the active anode areas of zinc and tin to approximately the same tential condition of the bath is' of less importance. In a mixed method, soluble anodes of one of the metals may be used in connection with insoluble anodes; the relatlve ratios of the two being adjusted and re lenishment of the bath being accomplished b the addition of a salt of the other metal. huszinc anodes in addition to insoluble anodes may be used; the zinc content of the bath being replenished as the zinc anode dissolves while the tin is sup lied to the bath in the form of one of its sa ts.

What we claim is 1. An iron or steel article carrying an electrodeposited coating of an alloy consisting of zinc and tin, the zinc and tin being in the ratio of about 75:25.

2. An iron or steel article carrying an electrodeposited coatin of an alloy consistm of zinc and tin;- t e percentage of tin being from 7 to 35 per cent of the total.

3. In the protection of iron and steel articles, the process which comprises making such articles cathodes in a bath containing zinc salts and tin salts and also containing sufliclent cyanide and pyrophosphate to make the solution approximately equipotential as regards tin and zinc.

4. In the electroplating of iron and steel articles with alloys conslstingof zinc and tm, the process which comprises making such articles cathodes in a bath containin stannous chlorid, zinc cyanide, sodium 0 lorid, caustic soda and sufficient sodium pyrophosphate and sodium cyanide to make the bath approximately equipotential as regards tin' and zinc.

In testimony whereof, we have hereunto afiixed our signatures.

BATIST R. HAUEISEN. JAMES C. PATTEN.

ratio; replenishment being by solution of the metal from the anodes.

Some variation in the zinc-tin ratio of the coating may be accomplished by change in 1 the current density. We have noted that,

other conditions remaining the same, there is a tendency for a larger proportion of tin to plate out at the lower current densities and vice versa.

While soluble anodes are usually employed in our invention, it is sometimes convenient to use insoluble anodes and to replenish the metal content of the bath by the addition of zinc and tin salts.-. In this case the equi-