US 3725217 A
Description (OCR text may contain errors)
United States Patent 6 US. Cl. 204-32 R 19 Claims ABSTRACT OF THE DISCLOSURE A method of plating upon a workpiece of titanium, zirconium, or their alloys with heavy metals which comprises cleaning and lightly pickling without excessive etching or roughening of the surface, which will maintain close tolerance dimensions and surface smoothness in the area of the workpiece to be plated, deoxidizing and coating said area with a conversion coating by immersion in a solution of fluorosilicate or fluoroborate, of which the alkali metal, ammonium, alkaline earth, aluminum and magnesium fluoroborates and fluorosilicates are notable, substantially or wholly removing non-adherent surface layer of conversion film by immersion in an acid, and plating the coated area electrolytically as the cathode in a heavy metal electrolytic plating solution to form a strongly adherent metallic bond. A fiuoro film of unknown constitution, which is sometimes so thin as to be invisible even under the electron microscope, joins the chromium or nickel plate firmly to the titanium or zirconium body.
This is a continuation-in-part of my pending application, Ser. No. 843,185, now abandoned. Priority of that application is asserted.
This invention relates to the plating of titanium and zirconium and their alloys with other metals, with heavy metals, especially chromium and nickel. These metals are most difiicult to apply successfully to titanium and zirconium and may be regarded as a preferred species, to the plating of which upon titanium and zirconium the specification will be largely addressed. Titanium will be used as the exemplar in the description. Nonetheless the same process has had success in the application of gold, silver, iron, and copper plate to titanium and zirconium so that its use may be regarded as not limited to the more difficult metals; these metals may be applied by this method as a base for other plate.
Titanium alloys, because of their excellent strengthweight ratio, superior corrosion resistance and ease of fabrication are finding increased application in the aircraft and aerospace industries. However, these applications are limited to uses which do not involve moving components such as bearing surfaces, sliding members and piston assemblies because of the inherent tendency of titanium and its alloys to seize or gall. Titanium metals are the most difficult of all metals to lubricate because of the nature of the adsorbed gas film on the metals surface. Another disadvantage is the tendency to adsorb nitrogen, oxygen and hydrogen at elevated temperatures, this adversely effecting their physical properties.
Many efforts have been made to deposit nonporous plate of chromium or nickel on these metals but with limited success, the plate being poorly adherent, porous, not resistant to high temperature, or of high coeflicient of friction. Other efforts have been made to plate after long and difiicult cleaning, or with special pre-plating baths involving noble metals, or post-treatments, or special chromium plating solutions, or with multiple steps. Some prior processes depend upon an immersion deposit to preice cede electrodeposits, but the adhesion is unreliable and not reproducible. Other methods require a strongly acidic etch which affords only a mechanical bond and defeats the purpose of plating to close tolerance. Still other methods require a subsequent heat treating operation to diffuse the deposit into the base metal, but the high tern- I peratures required adversely affect the original properties of the titanium and warp or distort the parts. So poorly have such prior attempts satisfied the demand that no such process of plating titanium or zirconium with chromium or nickel is currently in successful commercial operation.
A primary object of the invention is to plate titanium or zirconium with chromium or nickel so that the plate will not peel, will have low friction, be dense and nonporous, resistant to high temperatures, be strongly adherent, and which will by standard test (Timken, so called) in the preferred mode be classified as of infinite life. Secondary objects of the inventions are to plate with less difficult metals to achieve superior adhesion.
The objects are accomplished by a process which includes the following steps:
Clean the titanium.
Put it in an alkaline type cleaner at elevated temperature. Use current to make the titanium cathodic if desired. If it be made anodic it oxidizes and plating to close tolerance is defeated by removal of the outer surface of the piece. The cleaners used are standard in heavy duty cleaning of metals, e.g., aqueous solutions of caustic soda plus sodium ortho or meta silicate, or plus sodium phosphate, or plus sodium carbonate, as a buffer. Ordinary cleaning takes 30 seconds to 15 minutes, the end point being freedom from oil, grease, and soil.
Rinse with clean water.
Use an acid pickle to remove oxide from the surface of the titanium. A typical pickle is 50% HNO 25% H 25% H O plus 1 lb./gal. of Actane (Actane 70 a fluoride type pickle additive). Some HP is formed by hydrolysis and bites into the titanium. This removes oxide, and some of the titanium if the treatment is prolonged. Assuming no scale, a dip of 1 minute or less is adequate. It takes longer if some scale exists. If the piece is badly scaled a sandblast should be used before the cleaning or before the pickle.
Rinse after the pickle. Oxide film forms again.
Deoxidize the oxide formed during the rinse and form a fluoroborate coating. An aqueous mixture of 6 oz./gal. of sodium fluoroborate plus 6 oz./-gal. of NaNO, or KNO is suitable. This forms a conversion coating by displacing the oxide film formed after the pickle and replaces it with an impervious layer the nature of which is difficult to determine but which could be of titanium fluoroborate. This prevents reoxidization. Immersion time of 5 minutes at F. is usually adequate. Room temperature can be used but is too slow. Boiling temperature introduces unnecessary problems.
, Gentle gassing occurs during the formation of the conversion coat; When it stops, the coating has been formed. The pH of the sodium fluoroborate mixture is usually around 2. Agitation can be used.
Rince again. The fluoroborate film is impervious and prevents reoxidation.
The fluoroborate coat is now to be attacked with a weak acid and the attack may proceed until the fluoroborate is invisible at magnification of 250x. Sometimes the electron microscope shows no film. For the attack any aqueous acid solution can be used, examples of which are NaHSO citric acid; muriatic acid; and H 80 A concentration of 5% of mineral acid is adequate. 1 oz./ gal. of powdered acid or acid salts is often adequate. Any pH below 5 is usually adequate.
Rinse. No oxide forms. This is astonishing. If theelectron microscope shows no film on the surface, yet there must be a protected surface although nothing is visible. The object may now be plated in any good plating solution of nickel or chromium. Bright nickel solution, and sulfamate nickel solution work well.
The process produces what is believed to be the first objects of titanium or zirconium, plated with nickel or chromium, in which firm adhesive is characteristic and reproducible, withstanding all tests including infinite life, the strength of the bond being such that when pieces are brazed or welded together, plate to plate, the pieces failed under tension before the plate.
The product is characterized by titanium or zirconium bodies electroplated with chromium or nickel in which a visible or invisible barrier of borofiuoride or silicofluoride (also called fluoroborates or fluorosilicates as salts derived from fiuoroboric or fluorosilicic acid) is interposed between body and plate. The impervious barrier is applied as a conversion coating and the plate is applied electrolytically. The use of silver fluoroborate has been proposed in the past but the manner of application was not the same and the results were not equivalent.
The invention forms an adherent titanium or zirconium conversion film on the metal by displacing the oxide layer on the surface and replacing it with a fluoroborate film which is largely or wholly removed by acid. In the preferred form of the invention the fluoroborate film is made extremely thin, by a dip in acid, in some cases being invisible even under the electron microscope, while in other cases it constitutes a layer of visible thickness suggesting an intermediate coat between the plate and the titanium object. Chromium, nickel and other heavy metals, are then readily deposited by catalytic reduction, or by electroplating, on the titanium or zirconium from any of their conventional electroplating solutions. Whether complete reduction of the oxide is achieved or not is questionable, but the adhesion obtained is excellent and is the greater as the boroor silico-fluorate is the thinner. The film of fluoroborate can be reduced in thickness in solutions of low pH, when maximum adhesion is desired. Acids and acidic salt solutions of any type seem to be effective, both mineral and organic. Twenty or more acids and many acid salts have been tried, at various concentrations and pH, and all have been successful at acidities below neutral. Hydrochloric and acetic acids are exemplary. Acidity around pH 2-3 is typical of preferred practice.
The following description constitutes the preferred mode. It is accompanied by a description of some valuable modifications, but it is to be understood that the particulars of the preferred mode and the modifications do not limit the generalities elsewhere herein expressed.
A preferred fluoroborate solution consists of 4 to 6 oz./gal. of sodium fluoroborate in water containing 25% methyl carbitol. The temperature of application of the fluoroborate should be between about 150 F. and about 200 F. in the preferred mode. Outside this range efiiciency is lowered and imperfections such as smut, use of excessive time, and areas of poor adhesion appear. Lower temperatures require longer time. Temperatures from F. to 210 F. have been used but the preferred range is recommended. Optimum temperature and time for the application of sodium fluoroborate solution to titanium articles is about 175 F. for about 4 to 6 minutes. In applying the fluoroborate coat to zirconium from the same solution the optimum temperature is 140 F., the time required being about the same. Varying the time or temperature changes the thickness of the film of fluoroborate on the object. Thin films have advantages over thick films. In ordinary cases the fluoroborate solution is applied by immersion of the whole workpiece, but it can be applied to local areas when total coverage is not essential. Areas in which plating is not to be built up can be masked in accordance with hard chrome plating technique. Its application should be preceded by cleaning and pickling.
Any standard metal cleaner can be used to prepare the metal for pickling but it is preferred to clean cathodically in an alkaline cleaner such as Enbond (Enthone), which may be used, as an illustration, at 8 to 12 oz./gal. at 180 F. for 2 minutes at 4 to 12 volts D.C. This treatment is followed by rinsing, and the cleaned object, after rinsing, should be immersed at once in the pickling bath. When such electrolytic cleaning is used the over-all process requires two electrolytic baths.
The preferred pickling used in this invention is an acid treatment to remove oxide and scale. Sand or vapor blasting can be used to assist in this process in difiicult cases to loosen the oxide and scale. A superior pickling bath is 25% H 80 50% HNO 25% H O (by volume) with one pound per gallon of Actane 70 (Enthone), which is a water soluble pickling additive. The pickling bath should be uncontaminated by metals, such as Fe and Cu, which will deposit on the titanium or zirconium body. A oneminute pickle at room temperature is adequate to eliminate surface etching unless scale or oxide is present, in which case a longer pickling is indicated. Some contaminated baths can be made safe by adding chelate such as EDTA.
Pickling is followed by a water rinse.
The titanous or zirconous object is transferred promptly to the fiuoroboron bath and rests there with or without agitation from about 4 to 6 minutes at about F., assuming about 4 oz./ gal. of sodium fluoroborate in water containing 25% methyl carbitol. It is withdrawn, rinsed in 8 02/ gal. NaHSO at room temperature for 5 minutes, rinsed in water, immersed in acid, and immersed in clean electrolyte of the metal being plated for 5 minutes at plating temperature. In plating with hard chromium the plating current need not be turned on until the workpiece is at bath temperature.
Any standard plating bath for the electrolytic deposition of chromium, nickel or the other heavy metals can be used. Their number is great and their compositions are described in the electroplating literature. The object, with its conversion film of fluoroborate, is transferred quickly to the plating bath, and connected as cathode; it can be connected with the current on at plating density, or it can be connected before the current is turned on. Plating continues until the desired thickness of plate has been deposited, whereupon the plated object is removed, cold rinsed, hot rinsed and dried.
The fluoroborate film is incredibly homogeneous. It is apparently a conversion coating. Its constitution is unknown but may correspond to a reaction between water, the fluoroborate ion, and titanium to produce a film of titanous salt. Difliculties of analysis and metallographic techniques have prevented verification of this theory, but electronmicrographs demonstrate an intimate bond.
Sodium fluoroborate is a preferred mode because of cost, availability, solubility, and consistent performance. The barrier film should be formed at elevated temperature to secure maximum adherence and density. At the highest temperatures a dark smut may appear. The smut may be prevented by reducing the ionization of the fluoroborate solution with inert additives. It was found that potassium or sodium nitrate, in concentrations of 1 to 16 oz./gal. for sodium nitrate, effectively slow down the reaction to form a more adherent conversion film with a minimum of smut. An excellent solution contained 6 oz./gal. NaNO and 6 oz./gal. NaBF Further investigation proved that the addition of a water-soluble organic solvent to the fluoroborate-sodium nitrate solution would eliminate the smut. After trying a number of materials, methyl carbitol at a concentration of about 25% was selected. Other water-soluble organic solvents such as methyl Cellosolve, 'butyl carbitol, Cellosolve, or the alcohols are equally useful. The use of boric acid in similar concentrations will repress ionization somewhat, and reduce the formation of a heavy or porous film, both of which tend to decrease adherence of the plating.
The object would be racked as in the art of hard chromium plating, with the exception that any area or protrusion normally requiring a thief or mask to prevent excessive buildup would be shielded with titanium rather than copper or steel, because steel or copper, when immersed in the pickling solution, will dissolve and redeposit as a smut on the titanium, thus impairing the adhesion of the subsequent electroplate, and because steel or copper in contact with the titanium will set up a galvanic couple at the interface and prevent the formation of an adherent fiuoroborate film.
The preliminary method used for testing the adhesion of the electroplate was to process 1 x 4" x .040" strips and if the deposit was acceptable to the eye the strip was tightened in a vise and bent or broken with a hammer. This method is rapid and effective in obtaining comparative visual indications of adhesion. If a strip failed around the edges, or if peeling was observed around the bend and on the sides, the test was deemed a failure. When no peeling was observed in the area of the bend or break, the plating was considered so perfected as to require a more sophisticated method of testing; one widely used in the United States for evaluating solid film lubricants. It measures and records coefiicient of friction, speed, load in pounds per square inch and the time until failure. It is commonly called the Dow Corning LFW test (load, fatigue, wear). Cylindrical titanium rings, 1.378" OD, and a rectangular, hardened-steel block 0.250" x 0.400" x 0.620, were used. The rings tested were chromium plated by the present process to a thickness of approximately .010", then ground down to about 0.005". The ring was attached to the machine and revolved at a speed of 72 rpm. in an SAE oil. Weights were added until the maximum load was equivalent to about 35,000 psi. The machine was supposed to be run until failure of the plating. The first ring was tested in the laboratory of a metals corporation and the machine shut down after 1,249,326 cycles without failure. The plating had exceeded the infinite life test. Other samples have been tested by an aircraft manufacturer, exceeding the cycles of an infinite life test without failure. These methods of testing the adhesion of chromium on titanium are particularly valuable because they are reproducible and indicate values in numerical terms.
EXAMPLE 1 The titanium rings were cleaned cathodically from 1 to minutes in Enbond cleaner 160 (Enthone) in an 8-12 oz./gal. aqueous solution at 180 F., cold rinsed in water, pickled for 2 minutes at room temperature in an aqueous solution containing 25% H 80 25% water, and 75% HNO and cold rinsed. The rings were immersed for 4 minutes at 175 F. in an aqueous conditioning solution containing 6 oz./gal. of sodium nitrate and 6 oz./gal. of sodium fluoroborate. They were then rinsed in cold water and immediately immersed for 5 minutes in a 5% solution of HCl at 80 F. They were then cold rinsed and put into a standard nickel sulfamate plating bath as the cathode. The current was turned on at a density of 72 amps/ft.
The fluoroborate film is apparently removed except for a very thin adherent coating and the adhesion of the plate is much improved. This can follow the sodium fiuoroborate treatment immediately or after the cold rinse. Electronmicrographs prove that this operation improves the adhesion materially and the tests indicate an improvement in adhesion as much as fourfold.
Copper and iron, if present unavoidably, can often be deactivated in the baths by including a chelate such as EDTA.
The anodes in the plating bath can be inert such as carbon and platinum.
The deoxidizing solution used prior to plating may contain alkali metal or ammonium fluorosilicate, e.g. Na SiF in total or partial replacement of the fluoroborate. The concentration range of the fluorosilicate is from A to 5 oz./gal. The current density employed will be different for different metals. For example, gold can be plated on titanium or zirconium by this process at current densities as low as 1 amp/ft. and chromium can be plated on titanium or zirconium at current densities as high as 500 amps/ftl Other alkali metal nitrates are useful in place of sodium nitrate but have the disadvantages of higher costs and lesser solubility.
As many apparently widely different embodiments of the present invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments.
What is claimed is:
1. A method of plating upon a workpiece of titanium, zirconium, or their alloys with heavy metals which comprises cleaning and pickling the area of the workpiece to be plated, coating said area with a conversion coating by immersion in a solution of one of the group of compounds consisting of the alkali metal, ammonium, .alkaline earth, aluminum and magnesium flnoroborates and fiuorosilicates, and plating the coated area electrolytically as the cathode in a heavy metal electrolytic plating solution.
2. The method according to claim 1 in which the fluoroborate or fluorosilicate solution is of an alkali metal fluoroborate or fluorosilicate.
3. The method according to claim 2 in which the fluoroborate or fluorosilicate solution contains a smut preventive.
4. The method of claim 3 in which the smut preventive is an alkali metal nitrate.
5. The method of claim 3 in which the fluoro solution contains a water soluble organic solvent of the class consisting of the alkyl carbitols, the Cellosolves, the alcohols.
6. The method of claim 1 in which the plating current density is from about 1 to 500 amps/ftP, according to the metal plate being applied.
7. The method of claim 1 in which the baths and apparatus of the process are free of active iron and copper.
8. In the method of claim 1 the steps of energizing the electrolytic bath before the introduction of the metal with its conversion coating of fluoroborate, and immersing the said metal in the said bath as the cathode of the electrolytic system.
9. A method of plating a workpiece of titanium, zirconium, or their alloys with other metals which comprises cleaning and pickling the area to be plated, displacing the oxide layer on the metal and forming a metal fluoroborate or fluorosilicate conversion film in its place, thinning the fiuoro film in an acid bath, and plating the workpiece from a heavy metal plating bath.
10. The method according to claim 2 in which the fiuoro coat on the titanium or zirconium workpiece is formed from ammonium or alkali metal fiuoroborate or fluorosilicate solution by immersion.
11. The method of claim 9 in which the fluoro solution contains the equivalent of about 4-6 oz./gal. sodium fluoroborate, in water containing 25-40% of its volume of a water soluble organic solvent, and the fluoro plating is by immersion for several minutes at about 175 F. for titanium metals, or at about 140 F. for zirconium metals.
12. The method of claim 9 in which the pickling bath comprises about 25% H 50% HNO 25% H O by volume.
13. The method of claim 9 in which the cleaning is electrolytic, cathodic, alkaline and at about 210 F.
14. The method of claim 2 in which the metal with the thinned fluoroborate plate is immersed in the plating bath before the current is turned on.
15. The method of preparing a titanium or zirconium object for plating with heavy metals which comprises cleaning and pickling the object, reacting the surface thereof with an aqueous alkali metal, or ammonium, fluoroborate solution, and thinning the fluoro film deposit in an acid bath.
16. A method of plating titanium, zirconium, and their alloys which comprises cleaning, pickling, forming an alkali metal or ammonium fluoroborate or fluorosilicate conversion coating, immersing the coated metal in acid, and plating electrolytically in a heavy metal plating bath.
17. Chromium plated titanium or zirconium metal having a thin conversion bonding layer of fluoroborate or fiuorosilicate between and attached to the titanium or zirconium metal and the chromium plate.
18. Nickel plated titanium or zirconium metal having References Cited UNITED STATES PATENTS 3,540,943 11/1970 Grogan 148--6.14R 3,472,742 10/1969 Webb 2O438 B 3,065,154 11/1962 Wiesner 204-51 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R.