US1971761A - Protection of metals - Google Patents

Description

Patented Aug. 28, 1934 1,971,761 a PROTECTION OF METALS William J. Travers, Buffalo, N. Y.

No Drawing. Application September 5, 1929,

, Serial No. 390.651

12 Claims. (01. 204-8) Y This invention relates to the protection of metals and more particularly to an improved method of plating aluminum, its alloys and similar metals or alloys.

Aluminum and its alloys have at the present time an extensive use in'the arts. Thesematerials possess physical characteristics such as light weight and relative cheapness which recommend their use in various fields. These materials, however, do possess some characteristics which restrict their utility. For example, there is a large demand for metals which combine the advantages of relatively low weight with high surface wearing qualities. This fact has been recognized before and many attempts have been made in the past to provide such a. composite structure by coating aluminum with a relatively non-oxidizable metal such, for example, as nickel. It is to this type ofprocess that the present invention re- 39 lates.

Within recent years the use of aluminum, through its incorporation in the high strength alloys of the duralumin type, has become greatly extended, particularly in the automotive and aeronautical fields. There is a continually in= creasing use of duralumin not only in the relatively massive forms but also in the sheet type. A disadvantage of the duralumin sheet when used for example upon airplanes, is that it is subjected not only to surface oxidation but to intercrystalline corrosion. Because of the greatly increasing use of sheet duralumin, the change in tensile properties resulting from intercrystalline corrosion becomes a matter of great importance. Various means have been suggested for pre= venting the deterioration of duralumin resulting irom intercrystalline corrosion. These may be considered under two groups. The first group of treatment comprises a special heat treatment of duralumln to increase its intercrystalline corrosion resistance. The second group comprises protecting the surface against the attack of cor rosive agents by a protective coating. These coatings generally consist of surfaces formed by anodic oxidation or by coating the sheet with an impervious resinous varnish or a bituminous paint. v

The present invention therefore contemplates the treatment not only of commercial aluminum but also of the high strength wrought alloys of the duralumin type, and other similar high strength light weight alloys.

As indicated above, attempts have been made' in the past to protect aluminum by electro-depositing a coat of another metal. The electro-deposition of a metallic coat upon aluminum presents peculiar difilculties. Deposited coats of nickel upon aluminum appear to be peculiarly susceptible to peeling and stripping. This would appear to be due largely to two main factors. The first is the formation of a film of oxide upon the aluminum base, and the second, the peculiar unctuous or greasy characteristic of aluminum metal.

Considered generally, the prior attempts at plating of similar metals upon aluminum consist in the utilization of an intermediate metallic layer. As a general rule, this intermediate deposit is of a metal of a relatively low fusion point. The rationale of such a use appears to be that the interposed coat of the third metal will either fuse at relatively low temperatures and thus serve as a bonding agent or will in fact form a true alloy with the aluminum. Another method comprises first subjecting the aluminum base metal to a. sort of corrosion action so that the surface to be plated is etched and thus forms a better adherent surface for the subsequent electroplated metal.

It is a major object of the invention to provide 39 a method of depositing a relatively non-corrodible metal as a firmly adherent coating upon an aluminous foundation metal.

Another object is to provide a method of plating light weight high strength alloys with a metallic surface of high wearing qualities.

A further object is to provide a method for plating nickel on aluminous foundation metals as a smooth and impervious surface.

A further object is to provide a structural material which comprises a light weight high strength foundation metal bonded or cemented to another metal of greater resistance to corrosion and greater hardness.

With these and other objects in view the invention comprehends a special treatment of aluminum, aluminum alloys and similar alloys followed by art electro-deposition of a metallic coat so as to provide a very firm, adherent, continuous and impervious surface.

In order to explain the underlying principles of the invention, several specific treatments will be described hereinafter. It will be understood, however, that these are given by way of example and as explanatory of the principles herein involved rather than as exclusive methods of carrying out the invention.

In carrying out the process, the object to be treated, whether commercially pure aluminum or a high strength alloy of the duralumin type,

, a marked white appearance.

the aluminum surface.

is first cleaned. If a very smooth deposit is required on the finished article, it is advisable first to polish the article to be plated. In any case the initial step in the process is degreasing. As to this part of the process, I have found that the ordinary alkaline solutions have a high corrosive action upon the aluminum base metal and should be avoided. For this reason I use a special type of cleaning or degreasing solution.

This special solution comprises an alkaline solution such as sodium carbonate, sodium phosphate or other alkaline degreasing solutions to which is added a soluble silicate such as water glass. I find that this solution very effectively removes the grease and has little, if any, dissolving action on the aluminum. While I do not intend to be restricted to any statement as to the mechanics of this process, I am of the opinion that water glass acts in the nature of an inhibitor, for I have found that with the addition of this adjuvant the evolution of gas bubbles is greatly decreased. This appears to be borne out by the fact that other substances having an inhibiting action, such for example as arsenic trioxide, may be employed in lieu of the sodium silicate. I have found a solution consisting of six to eight ounces of trisodium phosphate with two to four ounces of a water glass solution in a gallon of water to be very effective. In use this solution should be heated to 160 F. more or less. In using such a solution, it is advisable to replenish the chief detergent constituent, namely 'the trisodium phosphate, from time to time, to maintain the effective concentration.

While I prefer to use the degreasing solution described above, it is to be clearly understood that the invention is not limited thereto. Other means of removing grease, as for example utilizing mechanical detergents such as fine pumice or organic solvents such as carbon tetrachloride,

'may be employed. However, as indicated, the

degreasing solution described above I prefer because the final results are in some degree infiuenced by the efiicacy of the cleaning or degreasing treatment.

After the article has been degreased or mechanically cleaned in the manner described, it 'is then washed and cleaned chemically. This chemical cleaning preferably comprises immersing the article in a solution of hydrofluoric or nitric acid for a brief period. An acid strength should be chosen which will but slightly attack As an example of this acid dip, there may be employed a solution containing between one and two ounces of 50% hydrofluoric acid in a gallon of water. I have found that improved results are obtained if the chemical activity of the solution is restricted by the additions of small amounts of an inhibitor such, for example, as hydrogen peroxide. In a typical acid bath, such as that given above, ap-

proximately two ounces of strong hydrogen peroxide to the gallon is sufficient. The quantity of this required may readily be determined by noting the dimunition in the evolution of gas bubbles on the aluminum. The article is retained immersed in the solution until it assumes This surface appearance sets up in a few seconds and is readily discernible. After this treatment, the article 'is then removed and rinsed. During operation the hydrogen peroxide will be slowly reduced and hence it should be replenished from time to time when the activity of the acid becomes too great.

As intimated above, instead of I the hydrofiuoric acid, other acids such as nitric acid may be employed. A typical nitric acid pickle may consist of a 30% solution of commercial acid. The concentration of the acid employed, of course, should be carefully regulated so as to prevent accelerating action on the article undergoing treatment.

After the preliminary steps of degreasing and cleaning, the article is then subjected to a special and novel treatment. This consists of an, so to speak, anodic oxidation treatment of the surface of the metal to be plated. It has been found recently that the intercrystalline corrosion of high strength aluminum alloys may be greatly diminished or lessened by subjecting the material to anodic oxidation. However, the purpose of this is to form on the finished article an oxide coat. The aluminum oxide thereby formed presents a protective layer or coating for the subjacent material, This is to be carefully distinguished from the present invention wherein an electrolytic treatment is employed to prepare an aluminous or similar base alloy to receive a plate of metal. This present process is in sharp contradistinction to prior art processes, for in all of these every atempt was made to remove existing oxide coatings and toprevent the formation of any additional oxides on the surface to be plated.

While this treatment is described as anodic oxidation, it may not in fact be, strictly speaking, an oxidation process and it is to be clearly understood that I do not wish to be limited to any statement of reaction occurrence. It would seem, by analogy to anodic oxidation processes now employed on duralumin, that such an action did obtain. In any event the present process is based on the discovery that if an aluminum surface is subjected to treatment to be described hereinafter, the condition of the surface of the material is so modified that it becomes capable of receiving a smooth, highly adherent and practically impervious coating of an eleotro-plated metal.

Briefly stated, the electrolytic treatment comprises immersing the article to be plated, as the anode, in an electrolytic cell. In carrying out the step a number of electrolytes may be employed. These may be considered generically as comprehending two classes, namely electrolytes containing halogen compounds and those containing non-halogen compounds. Of these in the first group, I have found that excellent results may be obtained by employing electrolytes containing chlorine. A typical example comprises a solution containing ammonium chloride as the basic ingredient. In utilizing this electrolyte, improved results are obtained by acidifying a relatively weak acid such as oxalic or phosphoric acid. When the solution has been acidified, it has been found that the electrolyte maintains its activity and clarity over longer periods of time.

However, the acidification of the solution is not essential as satisfactory results have been obtained with alkaline solutions. To render the solution alkaline, ammonium hydroxide or a similar base may be added.

While ammonium chloride has been described as the prefered basic ingredient of the electrolyte, it is to be understood that this is merely typical; other halogen-containing compounds such as sodium chloride, sodium bromide, and so forth may be employed. This class I have characterized as electrolytes containing halogen derivatives.

Jib

1,971,761 As noted above, other non-halogen electrolytes may be employed. Typical examples of these are nitric acid, phosphoric acid, ammonium hydroxide and so forth. In using this type of electrolyte, it has been found desirable to utilize an alternating current rather than a direct current, which latter may be employed with the first type of electrolytes. In lieu of the alternating current a reversing direct current may be used.

The electrolytic cell, in which the treatment is carried out, may consist of a container provided with suitable cathodes, such as graphite electrodes. These electrodes are preferably disposed on opposite sides of the electrolytic bath and the article to be treated passed therebet'ween. As noted above, the aluminum article is connected to the positive side of the current supply. In operation the ordinary 6 to 12 volt plating current may generally be employed. In the ordinary treatment an electric current is passed through the electrolyte for several minutes at a current density of 25 amperes per sq. ft. more or less. It will be understood that the current density may be varied over a relatively wide range by allowing greater or less time for the treatment.

After such an electrolytic treatment, the aluminum article will usually assume a darkened appearance, the depth of color depending to a large extent upon metal treated. After such a treatment for a requisite period of time, the article may be removed and after rinsing may be plated.

However, I have found that improved results are obtained if, prior to the plating treatment, the treated article is subjected to a final dip. Preferably this consists in immersing the article in a relatively weak solution of nitric acid. The composition of this solution may be approximately thirty fluid ounces of 36 nitric acid to a gallon of water. This acid solution may be used either cold or heated. I have found also that a saturated solution of oxalic acid may be employed in lieu of the nitric acid, and that bane-- ficial results are likewise obtained.

I have found in numerous treatments, according to the above disclosure, that when the electrolyte consists of a solution of phosphoric acid or ammonium hydroxide, etc., it is preferable to use analternating current. If a low voltage direct current is employed, an oxidized film is rapidly formed upon the aluminum surface which retards or prevents further action. However, by using alternating currents the desired anodic film is formed. In using the alternating current method the A. C. amperage should be maintained between about 10 to 20 amperes per sq. ft. When the article is subjected to the action of an alternating current in the acid electrolyte, it has been found that it should not be placed directly in the plating bath, as may be done when electrolytes of the halogen type are employed. Articles treated in the non-halogen electrolyte should first be given a dip in the nitric or oxalic acid solution described above.

It is found that as a general rule a cold nitric or oxalic acid dip is preferable for the aluminum alloys. A final dip in the acid solution is of a brief period and the article should not be maintained immersed until gas bubbles are evolved,

for it is found that with prolonged immersion the proper surface condition, attained by the electrolytic treatment, is detrimentally altered.

It appears that when the electrolyte contains an element of the halogen group the electrolytic action, that is the action produced by the electhe nature and purity of thetrlc current, is probably modified somewhat by the simultaneous action of the halogen constituent. When an electrolyte of the non-halogen type, such as a phosphoric acid solution is employed, this modification factor is not present and hence it becomes necessary to subject the material to subsequent treatment, that is to the final acid dip in order to introduce this modification. This modification of the surface of the material, achieved either by the simultaneous action of the halogen compound or by a subsequent dip, is difficult of explanation. It appears. however, that this action is in effect a partial removal of the anodic film.

After the treatment hereinbefore described, the article may then be plated with the desired metal. As a typical example, the article may be introduced into one of the well known nickeliferous baths such as nickel ammonia sulphate acidulated with boric or acetic acid. However, it will be appreciated that other plating baths and other plated metals may be employed. Nickel is described as a typical example of metals of this group because it isone of the best metals for plating on aluminum.-;v

It is found that the coat of metal deposited on the aluminum object, after the described treatment, is highly adherent, smooth, continuous or substantially impervious. Improved adherence qualities of the plate are undoubtedly due to the preliminary treatment and more particularly to the electrolytic action discussed.

It will be appreciated, therefore, that this treatment may be employed when the aluminum article is to be submitted to other specific plating operations followed by mechanical working.

As an example, the following process is contemplated. It is known, of course, that the improved tensile characteristics of duralumin are as much a function of the heat treatment as of the constituents of the alloy itself. This heat treatment of duralumin usually consists in heating an alloy up to a certain definite temperature and then rapidly quenching. After the quench, the alloy may be aged at room temperature or subjected to accelerating aging conditions, that is to say aging at elevated temperatures. Furthermore it has been found that the aging of duralumin, that is the increase in tensile strength, does not begin until some time after the quenching. It is furthermore known that the tensile properties of this and similar alloys may be improved by working.

The advantages of the maximum tensile strength of duralumin and the lustrous and relatively non-corrodible surface of nickel may be obtained by utilizing the principles of the present invention. Instead of using the typical nickel plating bath, the nickel may be deposited on the aluminum base metal or alloy as an annealible and ductile metal. For example, the nickel may be deposited from a bath comprising nickel chloride, nickel sulphate and boric acid.

In lieu of the above, other plating baths from which nickel is deposited as a ductile metal may be employed. The aluminum alloy treated in this manner may be an alloyof the duralumin type which has-been heated and quenched but alloy may be heated up to the proper temperature plate and the foundation metal.

ceptible to materials, plated 4 in a fusednitrate bath and then quenched. It is to be noted here that heating should be done under reducing conditions because of the well known afiinity of nickel for oxygen. After I quenching, and before the foundation metal betively wide variety of specific treatments of a' plated aluminum base metal which may be carried out. It will be appreciated that any of the according to the descriptions, may be overplated with another metal, such as chromium, cadmium and so forth, by well known methods. The choice of the over-plated metal will, of course, depend upon the particular use for which the article is intended.

While several specific processes and compounds have been described as embodying the principle of the invention, it is to be understood that these are given merely as examples, for since these principles may be incorporated in other specific processes, utilizing different materials, I do not intend to be limited to those described except as such limitations are clearly imposed by the appended claims.

Iclaim:

1. A method of plating aluminumand aluminum-containing articles comprising passing an electrical current through an electrolyte in which the article is immersed as an electrode to, form an m anodic film on the surface of the article, subsequently subjecting the article to the action of a solution of an acid to partially remove the anodic film, and then electro-depositing a metal on the treated'surface of the article.

2. A method of plating aluminum and aluminum-containing articles comprising passing an electrical current through an electrolyte in which the article is immersed as an electrode to form an anodic film on the surface of the article, subsequently subjecting the article to the action of a solution of nitric acid to partially remove the anodic film, and then electro-depositing a metal on the treated surface of the article.

3. A method of plating aluminum and aluminum-containing articles comprising passing an electrical current through an electrolyte in which the article is immersed as an electrode to form an anodic film on the surface of the article, subsequently subjecting the article to the action of a solution of oxalic acid to partially remove the anodic film, and then electro-depositing a metal on the treated surface of the article.

4. A method of plating aluminum and aluminum-containing articles comprising passing an electrical current through an electrolyte in which 5 the article is immersed as an electrode to form an anodic film on the surface of the article, subsequently subjecting the article to the action of a solution of a reagent so selected and for a time sufficient to only partially remove the anodic film, and then electro-depositing a metal on the treated surface of the article. r

5. A method of plating aluminum and aluminum-containing articles comprising passing an electrical current alternately in opposite directions through an electrolyte in which the article is immersed as an electrode to form an anodic film on the surface of the article, subsequently subjecting the article to the action of a solution of a reagent so selectedand for a time sufficient to only partially remove the anodic film, and then electro-depositing a metal on the treated surface of the article.

6. A method of preparing aluminum and aluminum-containing articles for the reception of an electrically deposited metal comprising passing an electrical current through an electrolyte in which the article is immersed as an electrode to form an anodic film onthe surface of the article, subsequently subjecting the article to the action of a solution of a reagent so selected and for a time sufilcient to only partially remove the anodic film.

7. A method of electro-plating articles formed of aluminum alloys of the duralumin type comprisin'gpassing an electrical current through an electrolyte in which the article is immersed as an electrode to form an anodic film on the surface of the article, subsequently subjecting the article to the action of a solution of a reagent so selected and for a time sufiicient to only partially remove the anodic film, and then electro-depositing duc- =tile nickel on the surface thereof, and then subjecting the article to accelerated aging supplemented by cold working.

8. A metallic article formed of aluminous material having a metal electro-deposited on the surface thereof, which article prior to the elctrical deposition of metal thereon has been treated by passing an electric current through an electrolyte inwhich the article is immersed as an electrode to form an anodic film on the surface of the' article, and by subjecting the so treated article to the action of a solution of a reagent so selected and for a time sufficient to only partially remove the anodic film.

9. A method of plating aluminous material, comprising utilizing the material as an electrode, immersing'the electrode inan electrolyte, subjecting the electrode to an alternating current so as to form an anodic film on the surface of the material;"then subsequently subjecting the material to the action of a solution of an organic acid to alter and partially remove the formed film, and then electro-depositing a metal on the treated surface.

10. A method of plating aluminous articles, comprising passing an alternating current through an electrolyte containing oxalic acid, and in which the aluminum article is immersed as an electrode, forming a film on the aluminum surface, then subjecting the so-treated article to the action of a hot solution of tartaric acid to alter the film, and then electro-depositing the material on the so-prepared surface.

11. A method of preparing cast aluminum articles, which comprises utilizing the aluminum article as an anode in an electrolyte containing nitric acid, then subjecting the so-treated surface for a short period of time to the action of a hot solution of nitric acid, and then electro-depositing a differential metal on the surface.

12. A method of preparing aluminum articles of the casting type for electro-plating, which comprises subjecting the aluminum article to an anodic treatment in nitric acid, removing the article from the electrolyte and subjecting it to an acid dip and then electro-depositing another metal on the treated article.

, WILLIAM J TRAVERS.

an electrolyte containing I