US 2419231 A
Description (OCR text may contain errors)
Patented Apr. 22, 1947 ,ELEC'I'ROPLATED CORROSION PROOF METAL ARTICLES AND METHOD OF v MAKING THE SAME Donald H. Schantz, Coraopolis, Pa., assignor Standard Steel Spring Company, Coraopolis, Pa., a corporation of Pennsylvania No Drawing. Application December 21, 1940, Serial No. 371,100
9 Claims. ((129-1915) of metallic coatings to'the surfaces of the articles which are to be protected. Its main object is to provide effective coatings having many advantages over any prior coatings known to the applicant. v
The protection afforded by applied metallic coatings arises mainly from the degree to which they are continuous or free from defects. Until recently, at least, .it has been generally assumed by workers in the art that the protective value of metallic coatings also depends upon the position of the coating materials in the electro-chemical scale relative to the base metal. Lately, doubts have arisen whether that theory is a sound one, but it is used in this specification only as a convenient basis for explaining the invention. Nickel and copper, for example, are,
according to the theory, electro-negative to iron may actually accelerate corrosion of the underlying metal.
Zinc is electro-positive to iron and a zinc coating on ironor steel will give substantially complete protection if the coating is free from breaks or defects exposing the underlying metal. If there are breaks or defects in the zinc coating the underlying or base metal will not corrode Or rust so long as there is zinc available on the surface of the article in the vicinity of a break or defect. It is difficult, if not impossible, to get a zinc coating in comrr'ierically practical thicknesses having perfect continuity, and, in addition, such coatings are subject to accidental damage. The electrolytic potential of zinc relative to iron is such that the consumption or dissolution of zinc when it starts to give way under corrosion is more rapid than is necessary for good protection, so that the effective life of the coating is shorter than it should be.
An object of the invention is to provide a method of applying a protective coating, which is singularly free from pores, pinholes and defects of a similar nature, and which is also of such a composition that it is not easily damaged.
Another object is to employ zinc for protective coating purposes, but in such a way that the difference in electrolytic potential between the coating and the metal base is reduced to a point intermediate that of zinc and iron. This result is accomplished by coating th surface of the article with an alloy of nickel and zinc which is less anodic to iron or steel than zinc, thereby slowing down the decomposition of the coating in the case of a break or defect through which harmful agents might penetrate to set up electrolytic action. 1
Still another objectis to make it possible to electroplate alloys of nickel and zinc on a commercial basis. This has not previously been done successfully. This is done by employing a new buffered electrolyte, high in metal content, and using quite high'current densities. The preferred range of density is from 300 to 700 amperes per square foot of cathode area, although densities below and above the ones specifically mentioned may be used. The invention further includes new correlations of temperature, pH or hydrogen ion concentration, and buffer concentration, which may be varied to control the percentages of nickel and zinc in the alloy deposited. Microscopic examination of a typica'Fnickelzinc alloy, formed by heating the two metals while in contact, shows that the alloy has a 25 stratifled appearance and analysis shows that the nickel and zinc, in what are for convenience termed the strata or layers of alloy, conforms in structure to the nickel-zinc alloy system as follows:
Alpha Up to 28%, Zn Beta (sub-one) -44% Ni Gamma prime 24-20% Ni Gamma 20-13% Ni Delta. 12-10% Ni gamma alloy is immersed in a corrodingsolution its potential with respect to the solution is only slightly less electro-negative than that of zinc immersed in the same solution. when, however,
gamma is coupled to iron and the potential of 50 a the couple is measured, the resulting value is the same as that'obtained' for iron alone; when zinc is coupled to iron, the potential of the combination is that of zinc. It may b statedfrom this that the gamma-iron couple is under "anode con- 58 tro "-1, e., the area of the anode (gamma) consuccessively stripping ofi the difi'erent strata or trols the rate of corrosion. For the zinc-iron combination, the couple is under "cathode control"- i. e., the area of the cathode (steel) controls the corrosion rate. Thus for zinc coated steel, as any pore in the zinc coat is enlarged by corrosion, the area of the cathode increases considerably, and the corrosion rate correspondingly increases. As
. a pore in gamma-coated steel is enlarged, the anode area is not changed much, and therefore the corrosion rate remains constant. This may in part be due to the physical structure of the gamma alloy, or it may be due in part to the fact that the gamma alloy has greater continuity, or it may be almost entirely due to the fact that the gamma alloy is only slightly electro-positive to iron or steel. While, as stated, the gamma alloy seems to be the best from the standpoint of protection, the gamma prime and delta a1- loys are effective, but to a lesser extent than the gamma alloy. In fact, nickel-zinc alloys having an amount of nickel less' than the amount found in th delta alloy give worthwhile results. Apparently, however, pronounced benefit from the nickel-zinc alloys is found only when the percentage of nickel is somewhere between about 3 percent and about 24 percent, with about 11 I to 18 percent giving best results.
sulphate calculated as ZnSO4.7HzO or about 16.7
ounces of zinc chloride ZnClz. The zinc metal content should be from about seven to about nine ounces. Nickel is introduced as a chloride. About eight ounces of nickel metal should be introduced by adding to the electrolyte about thirty-two ounces of NiCI2.6H2O. The electrolyte also contains a buffer which may be acetic acid, formic acid or citric acid at a concentration of about .5 normal. The salts of some of these acids may be used. This buffered electrolyte may have,
in operation, a pH of from about 1 to about 3, but a pH of from 1.5 to 2.5 is preferred. The pH may be adjusted by using hydrochloric acid to lower it, and either nickel or Zinc carbonate or zinc oxide to raise it. The electrolyte should be kept in circulation in a well known manner when the articles being plated are simply suspended in the electrolyte, but in plating continuously upon wire or strip moving through the electrolyte such circulation may not be necessary.
It is preferred to use separate nickel and zinc anodes having their surface areas relatively proportioned on the same ratio as the nickel and zinc in the, desired alloy. In order to get the right distribution of the current to the cathode, the article which is receiving the deposit, it is preferred to use a plurality of zinc anodes and a plurality of nickel anodes and so position them on the anode support as to get the current distribution desired.
Instead of using separate zinc and nickel anodes, cast or rolled alloy anodes having the metals in about the same proportions as in the alloy to replace the zinc plated out; or zinc anodes may be used and the nickel replenished by adding nickel salts. Insoluble anodes may also be used, but this requires constantly adding both the zinc and the nickel salts to replace the metals which have been plated out.
The operating temperature for an electrolyte made up as above directed may be from 70 to 150 F., a temperature of 125 F. giving good results in plating an alloy containing 11 percent to 18 percent nickel.
The current generators employed are of the usual type, the voltages running from 6 to 24 volts, depending both flipon the distance from the anode to the cathode and'the amperage desired. For coating sheets and strip about 300 amperes to the square foot of cathode surface appears to be best, regardless of whether the material is suspended in the electrolyte or is fed continuously through the electrolyte. Wire can be plated at about the same amperage per square foot as is mentioned for sheets and strip, but better results commercially are obtained in the continuous plating of wire if the current density is 500 amperes orhigher per square foot of cathode surface. It is important in all instances that the current density selected fOr a particular alloy plating operation shall be kept uniform,
The electrolyte constituted as above specified, and the preferred arrangement of zinc anodes and nickel anodes receiving a current adjusted to about 300 amperes per square foot of cathode surface, will simultaneously deposit on the cathode a nickel-zinc alloy in which the zinc and nickel will be present in percentages averaging approximately 85 percent zinc and 15 percent nickel.
The percentage of-zinc in the alloy may be controlled by raising and lowering the pH of the electrolyte, by raising or lowering the amperage within definite limits, by changing the temperature of the electrolyte, by the use of zinc sulphate instead of zinc chloride when the lower percentages of nickel are desired, by regulating the degree of agitation or rate of circulation of the electrolyte, and by changes in the relative amounts of zinc and nickel in the electrolyte. Such changes are a matter of experience with particular products. The total metal content to the gallon of electrolyte should, however, generally be at least fourteen ounces for the best results with high current densities.
The following are some examples illustrating changes in the procedure to get a desired percentage of nickel and zinc in the alloy.
For continuous plating of the alloy on a wire, a solution has been employed which contained ,8 counces of nickel metal (about 35 ounces NiClaBI-IzO), 10 ounces of zinc metal (about 21. ounces ZnClz), and acetic acid 3 percent by volume. 2.2 and the temperature of the solution was raised to 124 F. with the wire traveling at seven and one-half feet per minute through the solution, a current of 515 amperes per square foot of cathode area deposited on the wire a coating of alloy containing 14 percent nickel and the remainder zinc. Sheets suspended in an electrolyte in which the nickel metal was 7.2 ounces to the gallon and the zinc 8.5 ounces to the gallon and which had a temperature of 115 F. and a pH of 2.3 deposited 10 percent'nickel and percent zinc when a current density of amperes per square foot of cathode surfacewas employed. The zinc was introduced is sulphate form.
The pH of the solution was adjusted at form) to the gallon with a pH of 2.4 deposited aeiaasi 1. The method of plating articles with a nickelzinc alloy which includes making the articles cathode in an electrolyte, having a pH between about 1 and about 3 in which nickel chloride and zinc salt of the group consisting of zinc sulphate What is claimed is:
. and zinc chloride have been dissolved in sufilcient 20 percent nickel when the current density of i 400 amperes per square foot cathode surface was employed and the temperature of the electrolyte raised to 145 F.
In all of the examples just mentioned the nickel was introduced as a chloride and the bufier was acetic acid.
The alloy may be plated directly upon the metal article after the surface thereof has been properly cleaned, and the alloy coating so deposited is adherent, continuous and protective to pronunced degrees. It is preferred, however, to first deposit upon the surface of the article a primary coating of copper or nickel (nickel is preferred) from 0.000025 to 0.000100" thick. Nickel is in itself a protection against corrosion if continuous and,
'in addition, being electro-negative to steel, it
probably at least slows down the electrolytic action between the anodic alloy and the base metal where the latter is exposed.
The nickel plated article is next rinsed and then given a coat of alloy plating from the nickelzinc electrolyte. The thickness of the alloy coating may vary within a wide range, being dependent upon the environment in which the article is to be used. Usually a thickness of 0.000300" is ample for protection against all ex cept very severe conditions.
Also in the foregoing, reference has been made according to this invention have shown no signs oi'corrosion of the base metal after having been subjected to the standard salt spray tests for hundreds of hours. The process is a simple one to follow and the resulting product is better from a corrosion-resistant standpoint then anything found in the prior art, with many other advantages present which are not found in products resulting from any prior practice. The equipment necessary is, of course, more or less standard in the plating art.
In explaining the invention, various examples have been given, but it is apparent that they were illustrative only. It is obvious that the invention can be employed in other ways to produce various products, all coming within the claims which follow.
I formic acid, citric acid, and salts thereof, the
amounts to give, to each gallon of the electrolyte,
a metal zinc content of about 7 to 9 ounces and a nickel metal content of about 7 to 8 ounces, and passing a current having a density of to 700 amperes to the square foot of cathode surface through the electrolyte to the articles.
2. The method of plating articles with a nickel-'- zinc. alloy which includes making the articles cathode in an electrolyte consisting of, to the gallon of water, about 17 ounces of zinc chloride, about 32 ounces of nickel chloride and about 3 percent by weight of a buffer selected from the group consisting of acetic, citric and formic acid and passing through the electrolyte to the oathodes a current of from about 100 to about 700 amperes per square foot of cathode area, the amperage and the time during which the article is in the electrolyte depending upon the percentage of nickel and zinc and thickness of deposit desired.
3. A ferrous metal article having an adherent protective coating consisting of a layer of electrodeposited nickel, and an outermost exposed alloy layer at least as thick as the nickel layer consisting of nickel and zinc simultaneously deposited on the nickel layer from a single electrolyte and containing an average of about 15 percent nickel.
4. A ferrous metal article having'an adherent protective coating consisting of a layer of electrodeposited nickel, and an outermost exposed layer at least as thick as the nickel layer consisting of nickel and zinc-simultaneously deposited from a single electrolyte and containing from about 10 percent to about 24 percent nickel and the rest 2111C.
5. A plating solution having a pH of over about 1.0 and under about 3.0 containing, to the gallon of solution, about thirty-five ounces of zinc sulphate, about thirty-two ounces of nickel chloride, and about 3 percent by volume of a buffer of the group consisting of acetic acid, formic'acid, citric acid, and salts thereof, the rest of the solution being water.
6. A plating solution having a pH between about 1.0 and about 3.0 containing, to the gallon; about l6.7 ounces of zinc chloride, about thirtytwo ounces of nickel chloride, and about 3 percent by volume of a buffer of the group consisting of acetic acid, formic acid, citric acid, and salts thereof, the rest of the solution being water.
7. A plating solution containing to the gallon of solution, about thirty-five ounces of zinc sulphate, about thirty-two ounces of nickel chloride,
about 3 percent by volume of a buffer of the group consisting of acetic acid, formic acid, citric acid, and salts thereof, and addition agents adjusting the pH to about 2.5, the rest of the solution being water.
8. A plating solution having a pH of over about 1.0 and under about 3.0 containing, to the gallon of solution, a salt of the group consisting of zinc sulphate and zinc chloride in amount to give about '7 to 9 ounces of zinc, nickel chloride to give about '1 to 9 ounces of nickel, and a bufferingagent of the group consisting of acetic acid,
rest of the solution being water.
give about 7 to 9 ounces of zinc metal to the gal- 10 ion, and nickel chloride in amount to give about 7 to 8 ounces of nickel metal to the gallon, and plating nickel and Zinc simultaneously on the cathode by passing a current of over 100 amperes and under 1,000 ampere to the square foot of 15 cathode through the electrolyte to the cathode. DONALD H. SCHANTZ.
v 8 REFERENCES crrEn The following references are of record in the 5 file of this patent:
UNITED STATES PATENTS Number Name Date 'Schoch et al 1907 1,564,581 King Dec. 8, 1925 OTHER REFERENCES Transactions of the Electrochemical Society, vol. XI (1907), Schock et al., pp. 136-139. (Copy in Div. 36.) l
Trans. Electrochem. Soc; vol. 73 (1938), pp. 417-433. (Copy in Div. 56.)