US2928757A - Process of chemical nickel plating of amphoteric elements and their alloys - Google Patents

Description

March 15,

ROCESS OF CHEMICA w. G. LEE ETAL 8,757

L NI

2,92 L PLATING 0F AMPHOTERIC LOYS ELEMENTS THEIR AL Filed NOV. 2-7, 1957 77 Alloy Body INVENTOR. Warren 6'. Lee BY Emil/an Browar 4 JWZGW M 2,928,757 rnocnss or CHEMICAL NICKEL L TING 6F AMPHOTERIC ELEMENTS AND THEIR ALLOYS Warren G. Lee, East Chicago, and Emilian Browar, Gary, Ind, assignors to General American Transportation Corporation, Chicago, 11]., a corpo'ratioii of New York Application November 27, 1957, Seriztl ldo. 699,373 9 Claims. cl. 117-50) The present invention relates to chemical meter plating of the amphoteric metals of group Vl- B of the periodic systems of elements, and their alloys, and to processes of effecting such plating with aqueous chemical nickel plating baths of the nickel cation=hypophosphite anion type.

Heretofor'e the chemical nickel plating of workpieces having surfaces formed essentially of the amphoteric metals of group IV B and their alloys, has been of little practical utility, as such coatings as have been produced thereon readily flake-off, since the coatings are not intimately bonded thereto. This statement is particularly applicable to the important amphoteric element titanium, although it is applicable to the other elements of this group that consists of: titanium, zirconium and hafnium.

The principal reason for this lack of adhesion between the amphoteric metal surface and the coating is that such a surface is extremely sensitive to hydrolysis and iin' mediately films over upon contact with the aqueous chemical nickel plating bath; whereby there is a film layer at the interface between the amphoteric metal surface and the coating of nickel-phosphorus alloy that is inherently produced by a plating bath of this type. Of course, this prevents initiation of the plating and intimate bonding between the amphoteric metal surface and the coating,- With the result that the coating exhibits no substantial adhesion to the surface carrying the same;

Accordingly, it is a general object of the invention to provide a process of chemical nickel plating of a workpiece having an amphoteric metal surface, wherein the coating produced is intimately bonded to the surface and exhibits great adhesion thereto.

Another object of the invention is to provide a process of preparing the arnphoteric metal surface of a workpiece for plating with chemical nickel plating bath so that the subsequently produced coating carried thereby is intimately bonded to the prepared surface.

A further object of the invention is to provide a process of the character noted, that is expressly adapted for use in the chemical nickel plating of titanium and its alloys.

A further object of the invention is to provide a process of the character noted that involves a heat-treatment step following the nickel plating of the amphoteric metal surface, whereby a diffusion reaction is produced between the arnphoteric metal surface and the nickel-phosphorus alloy coating, whereby the interface is alloyed to produce an intimate bond therebetween.

Further features of the invention pertain to the particular arrangement of the steps of the method, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as toits organization and method of operation, together with further'objects and advantages thereof, will best be understood by referenceto the fol= lowing specification taken in connection with the accorri- CPI ice

Figure 2 is an enlarged fragmentary view, partially in section, of a portion of the compressor rotor, shown in Fig. 1', illustrating the manner in which one of the blades is detachably secured to the associated support and involving' a threaded connection therebet'ween;

Figure 3 is a greatly enlarged sectional View of the threaded connection or joint between the root of the blade and the support, illustrating the protective coatings respectively carried by these two parts and preventing galling the'rebetween; and

Figure t is a fragmentary vertical sectional view of another machine that is made in accordance with the process of the present invention:

Referring now to the drawing, there is illustrated in Fig. 1 a fragmentary portion of a compressor rotor 10" that may comprise a part of the air compressor that cooperates With an associated gas i'l'l'rb'iri if! a jet engine, or the like. More particularly, the rotor 10 comprises a support 11 of substantially drum-like configuration, and a plurality of annular rows of blades 12 disposed about the outer surface thereof; The rotor 1t) is housed in an associated casingor stator,- not shown, and mounted forrotation at high speed therein so that the annular rows of blades' 12 cooperate with other annular rows of blades carried internally of the stator, so as to provide a multi-v stage centrifugal air compressor. In the rotor 10, the support 11 and each of the blades 12 is formed of a suit able titanium alloy,- and each of the blades 12 is detach ably secured to the support 11 to accommodate replace- As best shown in Fig. 2, each of the blades 12 terminates at the base thereof in a threaded root 12a that is normally arranged in threaded relation with a cooperatin threaded spar ng 11a provided in the support 11'; and as illustiated in Fig. 3, the threads carried by the root 12d are designated 12f and the threads carried by the wall of the support 11' about the opening 11a therein are designated l lt. Also, as shown in Fig; 3, the threads 12! and the root I ZQa'Q'and the adjacent shoulder provided on the base of the blade 12 carry a unitary coating 12c, and the threads 11! andthe surface of the support 11 adja'cerit to the opening 11 therein carry a unitary coating 11; wherebythe coatings 12cland are in direct fricv herently produced by chemical nickel plating from an aqueous" chemical n'iek'el plating bath of the nickelcationhypophosp'hite anion type; and such alloy essentially comprises 89% to 97% nick eland 3% to 11% phosphorus, by weight.

The arrangement of the coatings 11c and 12c in direct engagement with each other is Very advantageous as these coatings'preve'nt galling that would otherwise take place between the parts if and 12 formed essentially of titanium, with the resulting freezing of these parts together so that subsequently itwouldb'e' impossible to remove the blade lz'fro'ni the support 11, a's'is the usual case where titanium" alloy parts are arran'ged'in pressure engagement with each other. However, the coatings 11c and do notexhibit g'a'llingttherebetween, whereby the titanium alloy: parts' li and li may. be readily. disassembled aii'd reassembled at will, without damage thereto.- I

This anti-galling characteristic thus imparted to the titanium alloy partsi1 audit. by the protective nickel- Percent Al 4 Mg 4 Ti Remainder These titanium alloys normally contain titanium in the general range 90% to 95% and other alloying elements in the range 5% to Another typical titanium alloy of this character contains the following principal components:

Al 3.18 Cr 4.68 Fe 0.22 Ti Remainder Still another typical titanium alloy of this character contains the following principal components:

Cr 2.84 Fe 1.43 Ti Remainder A further typical titanium alloy of this character contains the following principal components:

Mn Ti 7.8 Remainder The composition of the titanium alloy is in no way critical with respect to the present process. since any such alloy containing in excess of 90% titanium exhibits the previously mentioned amphoteric characteristic rendering it impossible to obtain an intimately bonded nickel-phosphorus alloy coating thereupon, without special treatment in accordance with the present process.

In the foregoing description of the rotor 10 it was explained that the elements 11 and 12 are provided with the respective coatings 11c and 120, and while this is the preferred construction, it will be appreciated that really only one of the coatings 110 or 120 is essential to prevent the titanium-upon-titanium contact and resulting galling therebetween. More particularly, there is no galling of a titanium alloy upon a nickel-phosphorus alloy of the character described.

Referring now to Fig. 4, a fragmentary portion of a machine or device 20 is illustrated that comprises a support or plate 21 having an opening 210 formed therein into which a shaft or bar 22 is mounted and arranged for relative movements with respect thereto, the shaft 22 being mounted for rotation about its longitudinal axis or longitudinal sliding movement, or both. The plate 21 and the shaft 22 are formed of suitable titanium alloys and respectively'carry chemically deposited nickel- phosphorus alloy coatings 21c and 22c arranged in engagement with each other, so as to prevent galling between the elements 21 and 22, as previously explained. This bearing arrangement is very advantageous as the nickelphosphorus coatings 21c and 22c resist wear and may be lubricated with any light oil, or the like.

In the foregoing description of the machine 20 it was explained that the elements 21 and 22 are provided with the respective coatings 21c and 220, and while this is the preferred construction, it will be appreciated that really only one of the coatings 210 or 220 is essential to prevent the titanium-upon-titanium contact and resulting galling therebetween.

Considering now the process of the present invention, the work-piece formed essentially of titanium, and nor- 4 mally comprising an alloy thereof, as previously noted, is first subjected to a conventional vapor degreasing step in order to effect substantial and ordinary cleaning thereof. Next the work-piece is subjected to alkaline cleaning (such as Enthone #160, or equivalent) to remove therefrom bufiing compounds, etc., that may have been employed in the machining thereof; and normally a hot aqueous solution of the alkaline cleaner is employed at a temperature of about 195 F. throughout a time interval of about 2 to 5 minutes. The work-piece is then rinsed in cold water.

Next the work-piece is pickled by immersion thereof for about 10 to seconds in a special pickling solution described hereinafter; and then the work-piece is again rinsed with cold water. The two last-mentioned steps are repeated so that the work-piece is again subjected to-the with cold water for about 10 to 15 seconds.

time, the surface of the work-piece has been conditioned pickling step for about 10 seconds and thereafter rinsed At this it essentially comprises an aqueous solution of hydrochloric acid, hydrofluoric acid and a mild oxidizing agent, both the hydrochloric acid concentration and the hydrofluoric acid concentration being high. Specifically, the pickling bath may be composed from 900 cc. :10 cc. hydrochloric acid (18 B.) and cc. :2 cc. hydrofluoric acid (70%), together with the mild oxidizing agent, such, for example, as FeCl .6I-I O in the approximate range 25 to 100 gms. This pickling bath thus comprises about 8 mole/liter of hydrochloric acid, about 4% mole/liter of hydrofluoric acid and about 0.1 to 0.4 mole/ liter of ferric chloride. Specifically, in formulating the plating bath, the following proportions are recommended:

900 cc. HCl solution (18 B.)

100 cc. HF solution (70%) 50 gm. FeCl .6I-l O In preparing the pickling bath the two acid solutions are mixed, and the required amount of ferric chloride is melted in a steam bath and then added to the acid mixture under agitation.

While ferric chloride is preferred as the mild oxidizing agent, because of the simplicity and economy thereof, other suitable oxidizing agents may be employed, such, for example, as the water soluble salts of iron: ferric sulfate, ferric acetate, etc.; and the water soluble salts of manganese: manganese chloride, manganese sulfate, manganese acetate, etc.

Apparently, the fundamental modulus of this pickling bath is dependent upon this high concentration of hydrochloric acid in combination with this high concentration of hydrofluoric acid, since with this composition of the pickling bath there is removed the amphoteric film that immediately forms upon the surface of the titanium work-piece incident to contact thereof with the aqueous chemical nickel plating bath, and as a result of hydrolysis. It is postulated that this film that normally forms upon the surface of the titanium work-piece involves the equilibrium:

In any case, the film that is present upon the surface of the titanium work-piece is effectively removed by this pickling bath, so that the subsequently applied coating of the nickel-phosphorus alloy in the chemical nickel plating bath is continuous and intimately bonded thereto.

A number of suitable plating baths are available for the presentpurpose, as disclosed in US. Patent No. 2,532,283, granted on December 5, 1950, to Abner Brenner and Grace E. Riddell, in US. Patent No. 2,658,841, granted on November 10, 1953, to Gregoire Gutzeit and Abraham Krieg, and in U.S. Patent No. 2,658,842, granted on November 10, 1953, to Gregoire Gutzeit and Ernest J. Ramirez, as well as in: the copending application of Gregoire Gutzeit, Serial. No, $376,917., filedt August 27., 1953, the copending application. of, Gregoire Gutzeit', Paul Talmey and: Warren G. Lee, Serial, No. 478, filed December 29, 1954, and in the, copendingapplica tion of Gregoire G-utzeit, Raul, T almey: and Warren, G. Lee, Serial No. 569;8 1 5, filed- March 6,, 195,6, now Batent No. 2,822,294, grantedt February 4, 1958.

A chemical nickel plating bath of; this type essentially comprises an aqueous solution of nickel: cations and hypophosphite anions, thenickel cationsbeing derived from nickel sulfate, nickel chloride, nickel;hypophosphite,etc., and the hypophosphite anions being derived from hypophosphorous acid, sodium hypophosphite, potassium hypophosphite, nickel hypophosphite, etc, Preferably, such a plating solution comprises an absolute concentration of hypophosphite anions within: the range 0.15 to 1.20 moles/liter, a ratio between nickel cations and hypophosphite anions expressed in: molar CQncentrat-ions within the range 0.25 to 1,60 and a, in; the; range 4.5V to IL. The plating bath disclosed. in; the Gutzcit, Talmey and Lee application, Serial NO. 569,815, is; particularly advantageous and comprises, in; addition to the nickel cations and the hypophosphite anions, lactic anions and propionic anions, and having a pH in the acid range 4.4 to 5.6; and specifically this; plating bath; comprises an absolute concentration of the hypophosphite anions within the range 0.15 to 1.20 moles/liter, a ratio. between nickel cations and hypophosphite anions expressed in molarconcentrations within the range 0.25 to 1.60, an absolute concentration of lactic anions. within the range 0.25. to 0.60 mole/liter, and an absolute concentration of propionic ions Within the range 0.025 to 0.060 mole/liter. A typical plating bath of this. type has the following composition:

Nickel ion (as nickel sulfate) ..m.p.l 0.08 Hypophosphite ion (as sodium hypophosphite)v m.p.l 0.225 Lactic ion (as lactic acid) m.p.l 0.30 Propionic ion (as propionic acid) m.p.l (1.03

pH (adjusted with H230 and NaOH) 4.7

In the plating step, the Work-piece is immersed in the chemical nickel plating bath, while the plating bath is at a relatively high temperature, normally: in the general range 94 C. to 98 C., throughout an, appropriate time interval depending upon the thickness of the nickel plating required. For example, the particular plating bath set forth above has a normal platingrateof about 1 mil/hour; and of course, as previously noted, the coating that is inherently produced; by; a plating bath of this type essentially comprises anickel-phosphorus alloy containing about 89% to 97% nickel and about 3%: to 11% phosphorus, by weight. In this step, the work-piece is suspended in the plating bath by an insulating line (Dacron etc.) and out of contact with any metallic part, so as to avoid any galvanic action, and the resulting surface blemish or discontinuity.

Following this chemical plating step, the work-piece is subjected to heat-treatment that merely constitutes elevating the temperature thereof to about 425 C., where by a difiusion reaction takes place at the interface between the surface of the titanium work-piece and the nickel-phosphorus alloy coating. The diffusion. reaction mentioned occurs at the temperature 421 C, (8003 F.); whereby there is probably formed in the interface a ternary alloy of titanium and nickel and phosphorus. This ternary alloy in the interface intimately bonds the nickel-phosphorus alloy coating to the titanium workpiece; and the resulting coating is highly corrosive-resistant, smooth and bright, and prevents the normal galling of titanium-upon-titanium, as previously explained.

In the foregoing description of the present process, the work-piece has been described as being formed essentially of titanium or a suitable titanium alloy. ,How-

ever, the. nrc esshas een. r e out s cessfully-ere cisely in. the. manner, described When the worlepicde has been r d: f. he. ot r. of h a ph ri elem t of. group, IV B including: zirconium alloys and hafni m alloys. For example, the heretofore experienced; difficulty in the chemical. nickel plating of the zirconium ailoy. surface of a work-piece has resided in the circumstance that this amphoteric metal (like titanium). is subject to the formation, of a film upon the outer surface thereof due tohydrolysis upon immersion in the aqueous chemical nickel plating solution. However, in accord ance with the present process, the film mentioned is re-. moved from the outer surface and the reformation of this film thereupon. isinhibited; whereby upon. contact with the aqueous. chemical nickel plating solution, the plating reaction, takesplace directly upon the, zirconium alloy surface of the work-piece, without the interfering film at the interface. Of course, in, this. example, and following the chemical nickel plating of the zirconium surface'of the work-piece, the heat-treatment step at the elevated temperature of about 425 C. is, employed, so as probably to produce a ternary alloy at the interface comprising; zirconium and nickel and phosphorus.

In view of the foregoing, it is apparent that there has been provided an improved process of chemical nickel plating of work-pieces having amphoteric metal surfaces, as wellas an improved subcombination in the process with respect to the preparation of the surfaces noted for the subsequent chemical nickel plating of the coatings thereupon. Also the process is productive of an improved. article of manufacture and particularly in an, assembly of two parts formed essentially of titanium and completely devoid of galling therebetween.

While there has been described what is at present considered tobe the preferred embodiment of the invention,

it willbe understood that various modifications may be,

made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spiritand scope of the invention,

What is claimed is:

1. The process of plating with nickel; the metal surface of a work-piece, wherein said metal surface essentially comprises an amphoteric element selected from the group consisting of titanium, zirconium and hafnium; said process comprising cleaning saidv metal surface by contact with a suitable cleaning bath, then conditioning said metal surface by'contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and hydrofluoric acid and a. mild oxidizing agent, the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/liter, the absolute concentration of hydrofluoric acid in said pickling bath beingabout 4 /2 mole/liter, then contacting said metal surface with a chemical plating bath of the nickel cation-hypophosphite anion type throughout a suflicient time interval to obtain a nickel plating upon said metal surface, and then heating said work-piece to a temperature at least as high as about 425 C. so as to effect a diffusion reaction at the interface between, said nickel plating and said metal surface.

2. The process of plating with nickel the metal surface of a work-piece, wherein said metal surface essentially comprises an amphoteric element selected from the group consisting of titanium, zirconium and hafnium; said process comprising cleaning said metal surface by contact with a suitable cleaning bath, then conditioning said metal surface by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and hydrofluoric acid and a soluble salt of iron, the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/ liter, the absolute concentration of hydrofluoric acid in said pickling bath being about 4 /2 mole/liter, then immersing said work-piece in a chemical plating bath of the nickel cation-hypophosphite anion type throughout 7 a sufficient' time interval to obtain a nickel plating upon said metal surface, and then heating said work-piece to a temperature at least as high as about 425 C. so as to effect a diffusion reaction at the interface between said nickel plating and said metal surface.

3. The process of plating with nickel the metal surface of a Work-piece, wherein said metal surface esssentially comprises a titanium alloy, said process comprising cleaning said metal surface by contact with a suitable cleaning bath, then conditioning said metal by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and hydrofluoric acid and a mild oxidizing agent, the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/liter, the absolute concentration of hydrofluoric acid in said pickling bath being about 4 /2 mole/liter, then contacting said metal surface with a chemical plating'bath of the nickel cation-hypophosphite anion type throughout a suificient time interval to obtain a nickel plating upon said metal surface, and then heating said work-piece to a temperature at least as high as about 425 C. so as to effect a diffusion reaction at the interface between said nickel plating and said metal surface.

4. The process of plating with nickel the metal surface of a work-piece, wherein said metal surface essentially comprises a titanium alloy, said process comprising cleaning said metal surface by contactwith a suitable cleaning bath, then conditioning said metal surface by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueoussolution of hydrochloric acid and hydrofluoric acid and a soluble salt of iron, the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/liter, the absolute concentration of hydrofluoric acid in said pickling bath being about 4 /2 mole/liter, then inunersing said workpiece in a chemical plating bath of the nickel cation-hypophosphite anion type throughout a sufficient time interval to obtain a nickel plating upon said metal surface, and then heating said work-piece to a temperature at least as high as about 425 C. so as to effect a diffusion reaction at the interface between said nickel plating and said metal surface.

5. The process of preparing the metal surface of a Work-piece for plating thereon by a .chemical plating bath of the nickel cation-hypophosphite anion type, wherein said metal surface essentially comprises an amphoteric element selected from the group consisting of titanium, zirconium and hafnium; said process comprising cleaning said metal surface by contact with a suitable cleaning bath, and then conditioning said metal surface by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and hydrofluoric acid and a mild oxidizing agent, the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/liter andthe absolute concentration of hydrofluoric acid in said pickling bath being about 4 /2 mole/ liter.

6. The process of preparing the metal surface of a work-piece for plating thereon by a chemical plating bath of the nickel cation-hypophosphite anion type, wherein said metal surface essentially comprises an amphoteric element selected from the group consisting of titanium, zirconium and hafnium; said process comprising cleaning said metal surface by contact with a suitable cleaning bath, and then conditioning said metal surface by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and hydrofluoric acidand a soluble salt of iron, wherein one liter of said pickling bath contains the substantial equivalent of 900:10 cc. hydrochloric acid (18 B.) and 100:2 cc. hydrofluoric acid 70%.

7. The process of preparing the metal surface of a work-piece for plating thereon by a chemical plating bath 10 of the nickel cation-hypophosphite anion type, wherein said metal surface essentially comprises an amphoteric element selected from the group consisting of titanium, zirconium and hafnium; said process comprising cleaning said metal surface by contact with a suitable cleaning bath, and then conditioning said metal surface by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and hydrofluoric acid and ferric chloride, the absolute concentration of ferric chloride in said pickling 20 bath being in'the general-range 0.09 to 0.36 mole/liter, the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/liter, and the absolute concentration of hydrofluoric acid in said pickling bath being about 4 /2 mole/liter.

8. The process of preparing the metal surface of a work-piece for plating thereon by a chemical plating bath of the nickel cation-hypophosphite anion type, wherein said metal surface essentially comprises a ti tanium alloy, said process comprising cleaning said metal surface contact with a suitable cleaning bath, and then conditioning said metal surface by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and hydrofluoric acid and a mild oxidizing agent, the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/liter and the absolute concentration of hydrofluoric acid in said pickling bath being about 4%. mole/liter.

9. The process of preparing the metal surface of a work-piece for plating thereon by a chemical plating bath of the nickel cation-hypophosphite anion type, wherein said metal surface essentially comprises a titanium alloy; said process comprising cleaning said metal surface by contact with a suitable cleaning bath, and

then conditioning said metal surface by contact with a pickling bath, wherein said pickling bath essentially comprises an aqueous solution of hydrochloric acid and bydrofluoric acid and ferric chloride, the absolute concentration of ferric chloride in said pickling bath being in 5 the general range 0.09 to 0.36 mole/liter the absolute concentration of hydrochloric acid in said pickling bath being about 8 mole/ liter, and the absolute concentration of hydrofluoric acid in said pickling bath being about 4% mole/liter.

References Cited in the file of this patent UNITED STATES PATENTS 2,717,218 Talmey et al. Sept. 6, 1955 2,774,688 Girard Dec. 18, 1956 2,825,682 Missel et al. Mar. 4, 1958 OTHER REFERENCES Brenner et al.: Part of Journal of Research of the Nation Bureau of Standards, Research Paper RH, 835, volume 39, pp. 385-395, November 1947.

r. MA.

UNITED STATES PATENT OFFICE I CERTIFICATE 0F CORRECTION Patent No, 2 928 757 March 15, 1960 Warren G, Lee et a1,

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8, line 3O after "surface" insert by =9 Signed and sealed this 30th day of August 1960.

(SEAL) Attest:

ERNEST Wu SWIDER ROBERT C. WATSON W Attesting Oflicer Commissioner of Patents

Claims (1)

1. THE PROCESS OF PLATING WITH NICKEL THE METAL SURFACE OF A WORK-PIECE, WHEREIN SAID METAL SURFACE ESSENTIALLY COMPRISES AN AMPHOTERIC ELEMENT SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND HAFNIUM, SAID PROCESS COMPRISING CLEANING SAID METAL SURFACE BY CONTACT WITH A SUITABLE CLEANING BATH, THEN CONDITIONING SAID METAL SURFACE BY CONTACT WITH A PICKLING BATH, WHEREIN SAID PICKLING BATH ESSENTIALLY COMPRISES AN AQUEOUS SOLUTION OF HYDROCHLORIC ACID AND HYDROFLUORIC ACID AND A MILD OXIDIZING AGENT, THE ABSOLUTE CONCENTRATION OF HYDROCHLORIC ACID IN SAID PICKLING BATH BEING ABOUT 8 MOLE/LITER, THE ABSOLUTE CONCENTRATION OF HYDROFLUORIC ACID IN SAID PICKLING BATH BEING ABOUT 4 1/2 MOLE/TIER, THEN CONTACTING SAID METAL SURFACE WITH A CHEMECIAL PLATING BATH OF THE NICKEL CATION-HYPOPHOSPHITE ANION TYPE THROUGHOUT A SUFFICIENT TIME INTERVAL TO OBTAIN A NICKEL PLATING UPON SAID METAL SURFACE, AND THEN HEATING SAID WORK-PIECE TO A TEMPERATURE AT LEAST AS HIGH AS ABOUT 425*C. SO AS TO EFFECT A DIFFUSION REACTION AT THE INTE

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