Electrolytic Process For Cleaning High-carbon Steels

Abstract

A single-step process for uniformly etching a high-carbon-steel surface, including its crystal boundaries, and for continuously removing from the surface the carbon exposed by etching. The steel surface is made anodic in an aqueous solution of orthophosphoric acid containing fluoride ion and a wetting agent. The applied voltage is selected to provide an overvoltage condition at the anode so that copious amounts of oxygen are liberated at the anodic surface to sweep exposed carbon therefrom. High-carbon-steel surfaces so processed form a chemical bond with subsequently deposited metal coatings, such as nickel platings.

Description

BACKGROUND OF THE INVENTION

This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

Our invention relates to a method for cleaning metal surfaces, and more specifically to a method for cleaning high-carbon-steel surfaces. The term high-carbon steel is used herein to refer to steels containing about 0.3 percent carbon or above.

To obtain a strong, tenacious bond between high-carbon-steel surfaces and metal coatings deposited thereon, it is necessary to uniformly etch the steel surfaces and then remove the carbon smut exposed but not removed by the etching operation. Various treatments, both electrolytic and nonelectrolytic, have been employed to etch and clean such surfaces before plating, but these treatments share the disadvantage of including a final mechanical scouring step for removal of the carbon smut. The scouring often is done manually and in any event is both tedious and time-consuming. More important, such scouring is largely ineffective in removing smut from small bores, inside corners, dimples, and the like.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a method for the uniform etching of surfaces composed of high-carbon steel.

It is another object of this invention to provide a method for effectively removing carbon smut from etched high-carbon-steel surfaces, including surfaces not amenable to mechanical scouring.

It is another object to provide a single-step method for uniformly etching carbon-containing steel surfaces and removing carbon smut therefrom during the etching operation.

Other objects will become apparent from an examination of the following description.

DESCRIPTION OF THE INVENTION

In carrying out our invention, the high-carbon-steel surface to be coated with metal is made anodic in a special acidic solution under conditions designed to provide a carbon-free etched surface. The treatment is designed to produce a steel surface which consists predominantly of undistorted, unbroken crystals. Electropolishing does not produce a suitably active surface, nor does simple chemical etching. To provide the desired surface, one which will form a chemical bond with the metal coating to be deposited, we etch the entire steel surface, including its crystal boundaries. In addition, we continually remove from the surface the carbon exposed during etching. Removal of the carbon promotes uniform etching and provides a clean etched surface amenable to bonding.

EXAMPLE I

A machined rectangular workpiece composed of SAE-4340 steel (carbon content, 0.43 percent) and formed with a bore having a diameter of 0.25 inch and a length of 2.5 inches was degreased by immersion in technical-grade chromic acid solution for 1 minute. After degreasing, the workpiece was immersed for 5 minutes in a 20 percent-HCl solution to remove rust and scale. The workpiece then was rinsed thoroughly in tap water.

The rinsed workpiece was made anodic in a solution formed of equal parts by volume of water and 85 percent orthophosphoric acid, containing 0.1 gram/liter ammonium bifluoride and 1 milliliter/liter anionic wetting agent. A nickel block having an area approximately equal to that of the workpiece was immersed in the solution as the cathode. A DC voltage of approximately 6 volts was impressed across the electrodes to establish a current density of approximately 50 amperes per square inch for 2 minutes. During this treatment period the solution was maintained at a temperature of 27.degree. C. .+-.5 and was agitated by sparging with air.

Throughout the treatment copious bubbling was observed at the surface of the steel anode. This was the result of oxygen being liberated at the anode, the voltage impressed across the electrodes having been preselected to ensure an overvoltage condition. The observable surfaces of the anode remained clean throughout the treatment.

Examination of the workpiece after removal from the electrolyte established that its surfaces-- including the wall of the above-mentioned bore--were free of smut. The steel surfaces and their crystal boundaries were smoothly etched but not polished (brightened).

The etched workpiece was chemically plated with an amorphous nickel-phosphorous alloy by immersion in a standard bath for the autocatalytic reduction of nickel cations by means of hypophosphite anions in aqueous solution. Examination established that the resulting plating (thickness, 0.25 mil.) was uniformly adherent.

Referring to the electrolyte used in our process, we have found that if a selected proportion of fluoride ion is maintained in the phosphoric acid solution, the usual polishing action of this acid is modified so that the entire surface of the metal is etched uniformly, including its crystal boundaries. As indicated above, we have also found that it is necessary to maintain an overvoltage condition at the anode to ensure copious liberation of oxygen so that exposed carbon is continually swept away from the surface thereof.

We prefer to use an aqueous etching solution comprising about 32 to 52 percent phosphoric acid, by volume, and containing a water-soluble fluoride in the amount of about 0.06 to 0.2 g./l. and an anionic wetting agent in the amount of about 0.5 to 4 ml./l. Such electrolytes not only provide the desired kind of etching (as opposed to polishing) but also have a desirably high electrical conductivity, thus reducing the time required for etching. If high electrical conductivity is not considered essential, etching can be accomplished with electrolytes having a broader range of concentrations--for example, electrolytes containing from about 20 to 70 percent phosphoric acid and about 0.03 to 10 g./l. water-soluble fluoride.

In our opinion orthophosphoric acid is the only acid which will accomplish the purposes of this invention. For convenience, we prefer to use commercial-grade phosphoric acid. If desired, of course, the orthophosphoric acid can be prepared by hydrolyzing metaphosphoric or pyrophosphoric acid in hot water.

The inclusion of fluoride in our electrolyte is essential because, for reasons not well understood, the fluoride ion promotes preferential attack of the crystal boundaries of the metal. Any water-soluble fluoride may be incorporated in the electrolyte to provide the fluoride ion. The following are examples of a few such fluorides: NH.sub.4 HF.sub.2, AgF, LiF, NiF, FeF.sub.2, NaF, HF, BeF.sub.2, and Ag.sub.2 SiF.sub.6 .sup.. 4H.sub.2 0. During use of the electrolyte, replenishment of the fluoride may be necessary. The need for additional fluoride can be determined by observation of the steel surface being treated, additional fluoride being needed if there is a trend toward polishing (brightening) of the anode.

The incorporation of an anionic wetting agent in the electrolyte promotes wetting of the steel surface, as well as fast escape of liberated gases. Any of a variety of commercially available wetting agents can be used, and the concentration thereof is not highly critical.

As mentioned, it is essential to maintain an overvoltage at the anode to ensure removal of carbon smut during etching and thus to provide a clean and uniformly etched surface for the deposition of metal. In example I, above, an overvoltage condition was provided by impressing a DC voltage of 6 volts across the electrodes. An overvoltage condition is characterized by abundant bubbling at the anode. Loss of the overvoltage condition is evidenced by loss of bubbling, darkening of the anode, and, eventually, by the formation of a velvetlike smut on the anode.

The composition of the cathode is not critical. Various inert conductors, such as carbon, can be used. Preferably, the surface area of the cathode approximates that of the surface being treated. For highly uniform treatment of the anode surfaces it may be preferred to dispose several cathodes about the anode. The time of treatment does not vary greatly with the size of the part. Preferably, the electrolyte is agitated continually in the vicinity of the anode.

Preferably, our treatment is conducted at a temperature in the range of about 20.degree. to 35.degree. C., but this is not highly critical.

High-carbon steels tend to absorb hydrogen, and hydrogen embrittlement impairs bonding of the steel to a metal coating. In our process, such embrittlement is minimized or eliminated, since oxygen is liberated at the anode throughout the treatment.

We have found our invention to be effective for the cleaning of various types of high-carbon steels, such as those commonly used in the fabrication of cutting tools, threaded fasteners, drill bits, and the like. Although our invention has been illustrated above in terms of preparing a high-carbon-steel surface suitable for forming a chemical bond with a chemically deposited nickel-phosphorous alloy, the surfaces so treated will form chemical bonds with other metal coatings deposited by other techniques, as by electroplating.

The foregoing description is intended to be illustrative of this invention, and it is to be understood that this invention is not to be limited except as indicated in the appended claims.

Claims

What is claimed is:

1. The process of uniformly etching a high-carbon-steel surface and concurrently removing therefrom carbon exposed by said etching, comprising making a high-carbon-steel surface anodic in an electrolyte consisting essentially of an aqueous solution of orthophosphoric acid containing a small but effective amount, sufficient to effect etching rather than polishing of said surface, of a water-soluble fluoride, and establishing an overvoltage condition at the anodic surface to liberate oxygen at said surface during etching.

2. The process of claim 1 wherein said aqueous solution is from about 20 to 70 percent by volume orthophosphoric acid and contains from about 0.03 to 10 g./l. of said fluoride.

3. The process of claim 1 wherein said aqueous solution is from about 32 to 52 percent by volume orthophosphoric acid and contains about 0.06 to 0.2 g./l. of said fluoride.

4. The process of claim 1 wherein said electrolyte contains an anionic wetting agent.

5. The process of claim 1 wherein said water-soluble fluoride is a metal fluoride.

6. The process of claim 1 wherein said water-soluble fluoride is hydrogen fluoride.

7. The process of claim 1 wherein said water-soluble fluoride is ammonium hydrogen fluoride.

8. The process of claim 1 wherein said electrolyte is maintained at a temperature of about 20.degree.-35.degree. C.

9. The process of claim 1 wherein said electrolyte is agitated during etching.