Solution for anodic polishing of steel and iron articles

Patented Jan. 3, 1950

2,493,579

UNITED STATES PATENT OFFICE

2,493479

SOLUTION FOB ANODIC POLISHING OF STEEL AND BRON ARTICLES

Miltoll B. Hammond, Edgeworth, and Glade B. Bowman, Coraopolis, Pa., assignors to Standard Steel Spring Company, Coraopolis, Pa., a corporation of Pennsylvania

No Drawing. Application March 8,1947, Serial No. 733,426

Claims. (

This invention relates to a solution for use in the anodic electrolytic polishing of steel and iron articles. In such processes the steel or iron article is immersed in a solution and an electric current is passed from the article through the solution to a cathode. Various solutions have been employed in the anodic polishing of such articles but they are open to certain objections. Where a phosphoric acid solution is employed, it operates satisfactorily until a certain concentration of ₁₀iron phosphate in solution has been reached but after that time the solution becomes green and a red deposit is formed on the steel article. We have found that by including sulphate ion in proper amount in a solution in which phosphoric _lsacid is the principal constituent, we can cause the precipitation of iron sulphate from solution so as to maintain the concentration of iron in solution below a certain critical value such that the solution operates satisfactorily in polishing ₂₀the steel. We have found that the iron metal in solution should be maintained below one ounce per gallon for satisfactory results.

In accordance with our invention, we include in a bath which consists principally of phosphoric ₂₅acid, sodium sulphate or other alkali metal or ammonium sulphate in such amount as to react with the iron in solution and precipitate it and maintain the concentration of iron metal in solution below one ounce per gallon. It is pre- 3(. ferred that the solution also contain a small amount of sulphuric acid, i. e., up to 4% by weight of the solution, but the sulphuric acid can be omitted provided there is sufficient alkali metal or ammonium sulphate present to maintain 35 the iron in solution below the critical value.

A preferred solution according to our invention is as follows, all percentages being by weight and in the case of the acids, we mean per cent by weight of 100% acid. Of course the acid em- 40 ployed need not be of 100% strength but can be of the strength comercially available.

Example

Per cent by weight Phosphoric acid___________________________ 68 ⁴⁸

Sulphuric acid 4

Water 24

Tri-sodium phosphate 2

Sodium sulphate________________________ 2

Gelatin__________________________04 ⁸⁰

204—140.5)

The solution may contain the ingredients within ranges as follows:

<td></td><td> Range</td><td> Preferred Range</td>

<td></td><td> Percent</td><td> Percent</td>

<td> Phosphoric Acid._____________________</td><td> 55 to 85</td><td> 60 to 70</td>

<td> fitilphnrin Anid</td><td> 0.1 to 4</td><td> lto4</td>

<td> Water_________________________________</td><td> 7 to 30</td><td> 15 to 30</td>

<td> Tri-sodium Phosphate.______ ____</td><td> Ito 15</td><td rowspan="2"> lto5</td>

<td> Sodium Rnlphate</td><td rowspan="2"> at least 0. S</td>

<td></td><td></td>

As mentioned previously, the sulphuric acid may be omitted entirely provided there is sufficient sodium sulphate or its equivalent to maintain the iron in solution below the critical value. The amount of sodium sulphate or its equivalent depends upon the amount of iron in solution. The solution will operate satisfactorily at the start of the process without any sodium sulphate or sulphuric acid, but after a time it is necessary to provide the sulphate ion required to cause precipitation of the iron thereby keeping the iron in solution below the critical value. In general, the sodium sulphate should amount to at least 0.5% and it is preferred that the sodium sulphate be present in such amount as to saturate the bath and leave an excess of undissolved sodium sulphate in the solution. This excess acts as a reservoir for supplying the sulphate ion required for maintaining the iron in solution below the critical value. There is no upper limit to the amount of sodium sulphate which the bath may contain since it is desirable to have an excess in solid form.

While sulphuric acid is not essential, provided there is sufficient sodium sulphate or its equivalent present, the preferred solution contains some sulphuric acid but not over 4%.

In place of sodium sulphate, we may employ other alkali metal or ammonium sulphates, for example, potassium sulphate, lithium sulphate and ammonium sulphate.

The tri-sodium phosphate reduces the chemical attack of the solution on the article before current is applied and increases the polishing action after current is applied. In general, the amount of tri-sodlum phosphate should be increased as the percentage of water in the solution increases. In place of tri-sodium phosphate, we may use other tri-alkali phosphates, di-alkali

3,493,879 phosphate, mono-alkali phosphates or phosphorus pentoxide.

In order to aid in flocculating and causing the iron sulphate precipitate to settle, the solution preferably contains a small amount of gelatin or other suitable flocculating agent. The preferred bath contains about 0.5% of gelatin but amounts of flocculating agent within the range of 0.1% to 5% may be used.

In using the solution in the anodic polishing of steel, it is preferred to carry out the process at an anode current density of about 285 amps, per square foot and at a temperature of about 190° F. However, the solution may be used under widely varying conditions, the anode current density being as low as 25 or as high as 2000 amps, per square foot and the temperature can be any temperature up to about 250° F. The time for polishing the steel will vary depending upon the polish required, from about 1 minute to 20 minutes, the usual period being about 5 minutes.

The invention is not limited to the preferred ingredients or proportions but may be otherwise embodied or practiced within the scope of the following claims.