US2379066A - Polishing stainless iron and steel - Google Patents

Description

Patented June 26, 1945 POLISHING STAINLESS IRON AND STEEL Irvine C. Clingan, Baltimore, Md., assignor to Rustless Iron and Steel Corporation, Baltimore, Md., a corporation of Delaware N Drawing. Application February 12, 1941,

Serial No. 378,576

2 Claims. (Cl. 204140) The present application is a continuation in part of my co-pending application, Serial No. 319,056, filed February 15, 1940.

The present invention relates generally to the art of manufacturing stainless iron and steel, and more particularly to the finishing of stainless iron and steel products such as wire, rods, plates, sheets, strip, rounds, bars and the like, and the articles fabricated therefrom.

Stainless iron and steel products have a surface which has a characteristic grayish appearance. This grayish appearance is possessed by such products even though subjected to cold rolling or drawing to size during the manufacture thereof, either with or without intermediate or final annealing. Furthermore, where articles are fabricated from such products by further rolling, drawing or forging, they still possess this grayish appearance. In most cases, this grayish appearance is quite objectionable and it has been necessary to resort to various means for providing a polished surface.

Prior to the advent of electrolytic polishing of stainless iron and steel products within the past few years, the only available method of obtaining a satisfactory high, mirror-like finish was by mechanical polishing. The mechanical methods of polishing have been found unsatisfactory in view of the high cost thereof and in view of the fact that considerable material is wasted due to the lack of care in the carrying out of the bumng and grinding operations. As a consequence, it has been proposed to polish stainless iron and steel products and articles fabricated therefrom by anodic treatment in various types of electrolytic baths.

It has been proposed heretofore to polish iron and steel products by anodic treatment in an electrolytic bath containing sulphuric acid. Such attempts have been unsuccessful for various reasons. In such prior attempts to utilize an electrolyte containing sulphuric acid, the acid concentration, temperature, current density and time of treatment have not been properly correlated in order to give a satisfactory bright, mirror-like polish to the material. I have found that careful control must be exercised over these various factors where an electrolyte containing sulphuric acid is used or the stainless iron or steel will be adversely rather than beneficially affected by the treatment. In addition, in some of the prior attempts to use sulphuric acid as the principal constituent of the electrolyte, other acids have been added which adversely affected the character of the material treated. For example, it has been proposed to use a mixture of sulphuric acid and hydrofluoric acid for the olishing of stainless steels. This bath is objectionable because of the fact that pitting of the material results if inappropriate current densities for polishing are used. Another of the difliculties which has been encountered in the prior attempts to use sulphuric acid as the principal constituent of the electrolyte is that its field of usefulness is limited. Prior attempts have been made .to use such an electrolyte for the polishing of various types of iron and steel other than chrome-nickel steels, but these attempts must have failed because I have found that it is impossible to obtain a polish except in the case of stainless steels of the chrome-nickel types. I have found that even the straight chrome stainless steels can not be satisfactorily polished with thi electrolyte.

In accordance with my invention I have found that the chrome-nickel stainless iron and steel products can be given a high mirror-like polish by anodic treatment in a bath consisting essentially of sulphuric acid and water. As stated above however, polishing can only be obtained where there is proper correlation of the acid concentration of the bath, the temperature of the bath, the current density employed and the time of treatment.

In carrying out my invention the electrolyte should contain from approximately to sulphuric acid, the balance being water. Where the acid concentration falls below approximately 60% a sufiicient polish cannot be obtainedeven in the case of the chrome-nickel stainless steels.

The appropriate operating temperature will vary somewhat with the acid concentration of the bath. I have found that where the acid concentration is between approximately 60% and 90%, satisfactory polishing can be obtained where the temperature of the electrolyte is between approximately 30 C. as a minimum and approximately 50 C. as a maximum. Of course some small amount of polishing can be obtained slightly beyond the temperature limitations just mentioned where slight variations in the acid concentration and the other factors involved are made. However, I have found that proper polishing withinreasonable time limits and without pitting cannot be obtained where there is any substantial variation from the operating ranges set fort herein.

The current density required in order to obtain the desired high, mirror-like finish will vary somewhat in accordance with the acid concentration and the temperature of the electrolyte.

' rent density slightly less than 6 amperes per square inch is employed, but in no case should a current density of less than approximately 3 to 4 amperes per square inch be used. Attempts to use low current densities result in a gray etching of the material.

The time required inorder to obtain a satisfactory polish'will vary'in accordance with the current densities, temperatures and acid concentrations used. I have found that under the conditions mentioned above, the time required to obtain a satisfactory polish will vary from 1 to 5 minutes. Ordinarily it will require approximate- 1y 3 to 5 minutes in order to obtain the desired polish.

In carrying out my nickel stainless iron or steel to be treated is immersed in the electrolyte and direct current of appropriate density passed through the material and then through the bath to the cathode. The steel being treated is the anode in the circuit and it may be supported in the bath in one position. or, if the article is of such character as to lend itself to a continuous operation, it may be passed continuously through the bath.

The tank and cathode will be selected in order to suit the bath requirements. The tank may be lead-lined or formed of some other metal which will resist the attack of the acid electrolyte. The cathode may be in the form of a plurality of lead plates of suitable size and configuration. The cathode area does not appear to be critical, but in order to obtain proper results, it is desirable to have the cathode of a size and configuration closely approximating that of the anode.

One of the particular advantages of my invention is that the acid employed is relatively cheap and non-volatile at the preferred operating temperatures. Another advantage is,v as indicated above, that lead-lined tanks may be employed. The use of a lead-lined tank is advantageous in that the equipment employed for polishing can be modified without any particular trouble or expense for use as an electroplating tank for chrome or nickel plating. This can be accominvention the chrome iron and steel products pickled in this way has not been obtained. Even after the pickling of such material it possesses a grayish or whitish appearance which renders the material undesirable for certain purposes, and where a high mirrorlike polish has been desired, mechanical polishing or buiiing has been utilized. As stated above, I am also aware of the fact that attempts have I been made to pickle and polish stainless iron and tures the hydrofluoric acid is rapidly lost. Where sulphuric acid alone is used no essential change takes place in the electrolyte during the continuation of the polishing process, except that it must be replenished periodically by the addition of water and sulphuric acid. As a consequence, where my invention is employed, it is relatively easy to maintain uniform operating conditions over a long period of time.

While I have described the ranges of temperatures, current densities and acid concentrations which may be employed in the polishing. of chrome-nickel stainless iron and steel, it should be understood that my invention is not limited to these specific conditions, as-some satisfactory polishing-may be obtained slightly beyond these expressed limitations without any departure from my invention. My invention may be Dracticed otherwise than as specified above within the scope of the following claims.

I claim:

1. The method of. polishing chrome-nickel stainless iron and steel by anodic treatment comprising immersing the metal to be treated in a bath containing 60% to 90% by weight of concentrated sulphuric acid and the remainder water, and passing direct current through the bath of a density of 6 to 12 amperes per square inch plished merely by reversing the polarity of the electrodes and by adjusting the distances between them. Sulphuric acid is the electrolyte commonly employed in such electroplating operations.

I am fully aware of the fact that a sulphuric acid bath has been used heretofore in the pickling of stainless iron and steel of the chromenickel variety for the removal of scale therefrom. Sulphuric acid baths have been used for both electrolytic and straight dip pickling of stainless iron and steel. However, polishing of stainless of the treated metal surface for such period of time as to prevent gray etching of the treated metal and to polish it while using the metal as the anode and while maintaining the bath at a temperature of approximately 30 C. to 50 C.

. 2. The method of polishing chrome-nickel stainless iron and steel by anodic treatment comprising immersing the metal to be treated in a bath consisting essentially of approximately 60% to by weight of sulphuric acid and water, and passing direct current through the bath of a density of approximately 6 to 12 amperes per square inch of the treated metal for a period of approximately 3 to 5 minutes to give a mirrorlike polish to the metal while at the same time preventing a gray etching of the surface, all while using the metal as the anode and while maintaining the bath at a temperature of approximately 30" C. to 50 C.

IRVINE C. CLINGAN.