US1859734A - Method of removing oxide coatings from metals - Google Patents

Description

Patented May 24, 1932 UNITED STATES PATENT OFFICE HARRY S. GEORGE, OF MASSAPEQUA, NEW YORK, ASSIGNOR TO ELECTED METALLURGICAL COMPANY, A. CORPORATION OF WEST VIRGINIA METHOD OF REMOVING OXIDE COATINGS FROM METALS Ho Drawing. Application filed December 21, 1928. Serial No. 827,756.

My invention relates to methods of removing oxide coatings from metalarticles and especially to methods of removing such oxides without attacking the metal.

The method consists broadly of treating certain conditions of surface oxides in an alkaline bath and then removing the oxide or scale in an acid bath. The alkaline bath may be a solution of an alkali, such as sodium, potassium or calcium hydroxide and it preferably contains an oxidizing agent, such as chlorine or its oxygenated derivatives, sodium hypochlorite, hydrogen peroxide, or bromine. A satisfactory treating bath may consist of a sodium hydroxide solution through which chlorine has been bubbled. This solution may be made by heating a 20% caustlc soda solution to about 70 C. and then passing chlorine into the solution until it is clouded with liberated oxygen. A 20% solution of sodium hydroxide at 7 0 to 100 C. is preferred but more concentrated or more dilute alkali solutions may be used. Adilute acid bath consisting of a mixture of nitric acid and hydrochloric acid is a suitable acid bath but more concentrated baths and other pickling acids of various concentrations may be used. A suitable pickling bath consists of a solution containing 10 to 100 grams per litre of concentrated hydrochloric acid and about 50 grams per litre of concentrated nitric acid. This bath is used at about 70 C. but other concentrations and mixtures of theae acids and other temperatures may be use The particular condition of surface oxidation that is suited to my process is believed to be a suboxidized condition. The special scale may be-developed in box annealing and it is referred to herein as box annealed scale and as suboxidized scale but the precise structure and composition of the scale is unknown to me. The desired scale may be developed to some extent on all ferrous metals by heating the metals in reducing or non-oxidizing atmospheres, and at times scales amenable to my process are developed in the usual manufacturing operations without heating in special atmospheres. The scales on iron and chromium alloys known as temper colors are readily amenable to my process and are included within the meaning of the terms box annealed scales or suboxidized scales. ome of the oxide coatings are almost insoluble in acid and others are soluble to a considerable extent. The more insoluble scales have the property of being bodily loosened or sloughed from the surface of the metal by acids after the coatings have been subjected to the alkaline bath referred to herein. This sloughing action may be eflected by acids which do not attack the metal.

Although my process is useful for cleaning all ferrous metals, it is especially useful for removing acid resistant scales such as those occurring on iron alloys containing 8% or more of chromiuin. These alloys contain about 8% to 40% chromium and may contain substantial amounts of other alloying elements such as manganese, tungsten and nickel.

For example, the surface oxide on hot rolled rustless iron may be removed with acid but when the article is box annealed, the scale is practically immune to the action of acid. This scale and some of the scales on stainless steel are so difficult to remove with acid that methods involving sand blasting and grinding which are so laborious and costly that the cleaning often amounts to 10% of the manufacturing cost of the metal, have heretofore been used. With the processes of my invention the acid resistant scales on these materials may be removed so as to expose bri ht clean metal with a minimum cost of 1a or and materials.

These scales are by no means a product of special processes as they occur to some extent on portions of heat treated sheets which are not accessible to air. WVhere sheets are box annealed under such conditions that oxidation proceeds to a considerable extent, the scale on the exposed edges of the sheets are apt to contain only a small amount of suboxide constituents or none at all and the unexposed portions are generally suboxidized. Such a variation in the scale is undesirable in my process since thealkali bath acts on the suboxidized scale and gives it its properties of sloughing and quick reaction when it is treated with acid. Therefore, a uniform quality of scale of suboxidized character gives the best results.

When a carefully box annealed scale is treated with the alkali bath and then with the acid bath, a material which occurs next to the metal appears to be dissolved almost immediately and the main body will slough off of itself. However, the rapidity of the sloughing action varies according to the composition of the metal and according to the condiltioning to which the scale has been sub- Jecte Suboxidized or box annealed scales are not always developed in the ordinary methods of manufacture but they occur to some extent, on many metals. For the purpose of developing the oxide in some instances and of making I a uniform coating in other instances, it is desirable with some oxidized surfaces and it is necessary with other oxidized surfaces to provide a conditioning. The conditioning may be done for the sole purpose of changing the nature of the scale or it may be accomplished in the course of other methods of manufacture.

The best method of conditioning scale is to subject it to a high temperature in the presence of a reducing or a non-oxidizing atmosphere. This treatment produces a suboxidized scale of uniform quality on all parts of the article. A uniformly conditioned scale permits of a uniform reaction when the scale is treated with the alkali and subjected to the action of the acid. It is convenient to condition the oxide during box annealing. For example, I place the article in an annealing box with a few pieces of charcoal which are out of contact with the metal and then heat the metal to an annealing temperature, for a short time. Instead of placing charcoal in the box, I may displace the air in the box with a reducing atmosphere. For chromium alloys such as rustless iron and stainless steel the reducing gas should be free from carbon-containing gases which breakdown and deposit an adherent carbon-containing material on the metal at these temperatures. 1

As illustrative of my invention, a satisfactory method of cleaning hot rolled sheets of rustless iron containing 12% to 18% chromium, follows. The scale is conditioned by box annealing the sheets at- 600 to 1000 C. in a box in which a small amount of wood charcoal is placed out of contact with the metal inorder to produce a neutral or reducing atmosphere. The scale is then treated by immersing the sheets in a 20% sodium hydroxide solution into which chlorine has been bubbled until a cloud of liberated oxygen appears. The oxidizing condition in the alkali solution may be produced by adding a chloride to the bath, electrolyzing the bath with insoluble anodes to produce chlorine or by other well known methods. A treatment of three or four minutes in this solution atabout 70 C. is usually sufficient but some scales require a longer treatment. Scales that have not been completely conditioned have required several hours treatment in the alkaline bath in order to remove them satisfactorily when they were subjected to acid.

When the scale is properly conditioned and treated, it sloughs off in an acid bath as if a layer of oxide next to the metal were dissolved and loosened by the acid. The overlaying layers or admixed portions of scale drop or peel off. The scale which drops iron-chromium alloys and on the major por:

tion of the scale is negligible when the acid is dilute. An acid bath containing grams per litre of nitric acid and about 10 to 100 grams per litre of hydrochloric 'acid' works satisfactorily at C. with iron chromium alloys but other concentrations and temperatures may be used. In general, it is preferable to vary the concentration of hydrochloric acid according to the chromium content of the alloy. The more dilute solutions are used with those alloys which are lower in chromium. A box annealed scale will ordinarily peel or slough off of itself after an exposure of one or two minutes in the acid bath with- I-ACUHHNI out materially attacking the metal or the scale. If the scale does not fall off of itself it may be washed off in water or brushed off.

The acid bath may be and preferably is adjusted so that it will not attack the metal or dissolve the major portion of the scale. It is evident that the use of a bath which does not dissolve the metal or the major portion of the scale is advantageous since it will be exhausted less rapidly than a corrosive bath would be and the metal will not be destroyed. When the scale has been properly condi-. tioned and treated and an acid bath of this character is used, the metal is left bright, thus obviating the necessity of giving the metal a final brightening treatment.

With some scales the alkaline treatment may be advantageously preceded by subjecting the scale to the action of acid, but for most purposes the process may consist of providing a scale of the sub-oxidized character which may occur as the result of other processes of manufacture or as the result of a conditioning as hereinbefore described, treat- 7 ing in an alkaline bath which may contain an oxidizing agent and then subjecting to an acid bath. Since variations of the process other than those which have been specifically described may be made without departing from my invention, I do not wish to be understood as limiting my invention to the preferre 1 steps or to these modifications. Therefore, I desire to be limited only by the prior art and by the invention as defined in the annexed claims.

I claim as my invention:

1. The method of removing surface oxides from ferrous articles which comprises subjecting the same to an alkaline bath containing an oxidizing agent and to an acid bath.

2. The method of removing surface oxides from ferrous articles which comprises subjecting' the same to a chlorinated alkaline bath and to an acid bath.

3. The method of cleaning a surface oxidized article containing iron which comprises providing a box annealed scale thereon. subjecting the article to an oxidizing alkaline bath containing chlorine and then to an acid bath.

4. The method of removing surface oxides from articles containing iron and at least 8% of chromium which comprises subjecting the same to an alkaline bath containing an oxidizing agent and to an acid bath.

5. The method of removing surface oxides from articles containing iron and at least 8% of chromium which comprises subjecting the same to a chlorinated alkaline bath and to an acid bath.

box annealed scale thereon, subjecting the ar-- ticle toa chlorinated oxidizing alkaline bath and to an acid bath containing nitric and hydrochloric acids.

8. The method of cleaning a surface-oxidized article containing iron and at least 8% of chromium which comprises providing a box annealed scale thereon, treating the article to a hot bath containing about 20% of sodium hydroxide in which chlorine has been bubbled until oxygen is liberated and subjecting to a hot acid bath containing about 50 grams per liter of nitric acid and 10 to 100 grams per liter of hydrochloric acid.

9. The method of cleaning a surface-oxidized article containing a substantial amount of iron which comprises heating in a closed box in an atmosphere which has access to a reducing agent, treating in an alkaline solution containing an oxidizing agent and then subjecting to an acid bath.

10. The method of cleaning a surface oxidized article containing a substantial amount of iron which comprises heating in a closed box in an atmosphere which has access to wood charcoal, treating in an alkaline solution containing an oxidizing agent and then subjecting to an acid bath.

11. The method of cleaning an oxide coated article containing a substantial amount of iron which comprises heating in'an atmosphere under conditions which will produce partial reduction of the oxide material, treating in an alkaline bath containing an oxidizing agent and then subjecting to a dilute acid bath so as to slough the oxide coating from the article.

12. The method of removing surface oxides 4 from ferrous articles which comprises provlding a box annealed scale thereon, sub ectmg the same to an alkaline bath containing an oxidizing agent and to an acid bath in which the metal of the article is not materially soluble.

13. The method of removing oxide from ferrous metal articles consisting in first immersing the articles in a hot bath consisting of a solution of caustic soda and an oxidizing agent to break down the resistance of the oxide to acid, and then immersing the articles so treated in a hot acid pickling bath.

14. The method of removing oxide from stainless steel articles consisting in first immersing the articles in a hot bath consisting of a solution of caustic soda and an oxidizing agent to break down the resistance of the oxide to acid, and then immersing the articles so treated in a hot acid pickling bath.

In testimony whereof, I aflix my signature.

HARRY S. GEORGE.