US2120276A - Metal pickling - Google Patents

Description

Patented June 14, 1938 UNITED STATES PATENT OFFICE No Drawing. Application September 28, 1934, Serial No. 145,945

6 Claims. ((51. 148-8) This invention relates to the pickling of metals, e. g., the cleaning or conditioning procedure for iron or steel, which includes an acid treatment and subsequent washing of the metal. As commonly practiced, this pickling procedure includes subjecting the metal to the action of a hot solution of a strong acid, usually sulfuric acid in a diluted state, for removal of surface ir-- regularities and impurities such as scale and rust. 10 Along with the acid various other substances are frequently employed as inhibitors, to prevent or retard attack of the actual metal while permitting the pickling bath to remove scale, rust, and the like; these inhibitors usually comprise one or more organic compounds, such as sugars, proteins, thioureas, etc. Following treatment in the pickling bath and preparatory to subsequent working and/or use of the metal, the latter is customarily washed or rinsed in a bath of hot water to; carry off remnants of the pickling bath, i. e., acid and other substances, and 'in many cases the metal is dipped into a tank of limewater intermediate the pickling and the hot water rinsing steps, in a further effort to free the metal :5 from the acid. 0n removing the metal from the hot water bath, it quickly dries but very often,

' in spite of these washing treatments it is immediately found to have a badly discolored surface, usually of a reddish or yellowish tinge.

This surface impairment, generally recognized by an uneven coloration of the character stated, reduces the marketability of the metal, particularly where it is being shipped to customers without further working or fabrication, and more particularly where it is subsequently to be coated,

or in other instances where a clean, bright metal is desired. The described surface condition is also physically disadvantageous when subsequent rolling, machining or other working of the metal is 40 involved and it in fact seems to promote rusting and like impairment of the utility and life of the ultimate product. The exact causes and nature of this surface condition have been difiicult to ascertain, and although there are indi- 41 cations that it is caused by remnants of the pickling acid and the inhibitor, and particularly by impurities in the water used for the washing or rinsing step, different batches of steel or iron have varied so much in surface appearance under what are apparently identical conditions, that no satisfactory solution of the problem has hitherto been attained. I

It is accordingly animportant object of the present invention to aflord a pickling procedure which includes provisions for correcting or obviating surface impairments of the sort described, and at the same time to carry out such procedure without additional steps in the pickling and washing treatment. The invention is thus designed' to provide a simple and inexpensive way of improving the appearance and surface character of the iron or steel, with an actualreduction of labor and expense in most cases. Still other objects of the invention, including the provision of a new product for use in correcting surface impairment of metals in pickling, will be apparent from the following description.

It has now been found that many difliculties are overcome and the surface discoloration and like impairment of the metal corrected or obviated by pickling procedure in which the acid treatment is followed by a rinse bath which comprises, instead of the ordinary water and/or lime water, a dilute solution of an inexpensive material, to wit, tribasic sodium phosphate. This compound .is particularly effective; when a relatively small quantity of it is dissolved in the hot rinse bath (say, one pound per gallons), and the previously pickled steel or iron is dipped and washed in such a bath, the metal dries with a remarkable absence of surface discoloration or impairment. At the same time other distinct advantages have been found. For example, it has been common commercial practice of steel plants to change the ordinary rinse water frequently, often three or more times a day,--both because of the amount of dirt, scale and other impurities which accumulate in the bath, and in an unsatisfactory attempt to avoid surface discoloration and impairment. In operating with large sheets or other shapes of steel, the rinse tank capacity is upwards of 3,000 or 4,000 gallons, so that thecost of fresh water alone becomes high, while the labor and time element involved in changing water add measurably to production expense. Furthermore slime and scale are frequently formed or deposited on the walls and bottom of the tanks, and necessitate frequent cleaning and/or replacement. By using tri-basic sodium phosphate in the rinse bath, in accordance with the present invention, these difficulties are greatly obviated;

the rinse bath itself may be used a great deal longer than heretofore customary (in many instances three or' more times as long), andjthe interior of the tank or vat is kept in an unusually clean and clear condition. In fact, in one vealed.

It has been found convenient, particularly for handling and marketing purposes, to make up the phosphate material in briquettes or blocks of' a 1 standard size and weight. To this end, the phosphate, usually in a friable or pulverulent form, can-be solidified with sodium silicate as a binder. A relatively small quantity of the silicate in conveniently concentrated solution, can be readily added to the dry tri-basic sodium phosphate (N33P04.12H2O), and the mixture then thoroughly melted by heat and run into molds, where it solidifies. A product of this character, thus cast into briquettes or the like for convenience in handling, storing and shipping, is a commercial product of considerable importance for correcting and improving the surface character of pickled iron and steel. A requisite number of such briquettes or blocks may simply be dissolved in the hot rinse water, and the rinsing step. carried out without further attention or regulation.

As explained above, the use of this material, particularly tri-basic sodium phosphate, has been found to provide a metal of clean, bright, grey surface, of superior utility and marketability, and in many instances a metal much less subject to rust and corrosion. Undesirable persisting effects of the pickling bath, due .to the acid and/or the inhibitor, are overcome; and difilculty arising from the nature of the rinse water is corrected by reaction of the phosphate or its equivalent, often with the formation of precipitates, although it has not been found necessary to remove the latter from the bath during rinsing of the metal. In many cases, another substance or substances may be used in the product and procedure of my invention, to effect the described results. For example, the tri-basic sodium phosphate has been successfully replaced with d ibasic sodium phosphate, or the corresponding ammonium salts.

These materials are inexpensive, non-poisonous,

and non-fuming. The sodium and ammonium phosphates can be embraced in the term alkali phosphates. They are used in dilute solution (less than 2 pounds per 100 gallons being customarily suilicient) and preferably have a mildly alkaline reaction; some acicic reaction, however, is not always disadvantageous-for example, a dilute solution of monobasic sodium phosphate, although acid to the litmus test and less effective than the alkaline phosphates, is efficient as a rinse bath after pickling, in contradistinction to certain strongly alkaline compounds e. g. lime solution, which 'are unsuitable. As stated above,

. however, tri-basic sodium phosphate is at present preferred, because of its unexpectedly superior action. in obviating discoloration and other surface impairment of the freshly pickled steel or iron, while at the same time it is cheap and easy to use.

As a specific example of the invention, a product of. tri-basic sodium phosphate bound with sodium silicate was made up as follows: dry crystalline tri-basic sodium phosphate was mixed with sodium silicate solution (i. e., water glass) of specific gravity 41 Baum (the sodium silicate containing 1 part sodium oxide to 3.22 parts silicon dioxide, or silica- -solution containing about 62.2% water by weig pounds of phosphate to 3 gallons of the silicate solution. The mixture was heated until complete fusion of the entire mass took place, and thereafter poured into molds, where it cooled and solidified in the form of blocks. This product was then used in washing metal after actual pickling operations, a number of large steel sheets first being pickled for about 15 minutes in customary sulfuric acid (say, a- 5% to 10% solution) containing an inhibitor. The rinse tank was filled with hot water, and the described phosphate blocks were added in the ratio of about 1% pounds of the solid per gallons of water, and completely dissolved. The steel sheets, .freshly removed and wet from the pickling bath, were immediately dipped in the hot dilute solution in the rinse tank. Taken from the latter, the sheets dried quickly and had a clean, bright, grey surface of satisfactory character and surprising permanence, whereas comparison sheets similarily pickled and washed in ordinary water from the same source (but lacking the phosphate product) dried with a bad discoloration of uneven yellowish cast, over their entiresurfaces.

The rinsing step of the invention is preferably carried out in hot water, with the materials described, e. g., tri-basic sodium phosphate and/or others, in dilute solution therein,i. e., a solution of the order of 2 or 3 pounds, or often less, per hundred gallons, varying in accordance with operating conditions. Generally, dipping the metal, such as steel sheets, in the rinse bath for a period of about 30 seconds to two minutes is adequate for the purpose. It will particularly be' noted that no additional steps in the pickling process are needed; the metal is pickled as usual with a solution of a strong acid (stron'g acids being those such as sulfuric or hydrochloric,- while the salts of the rinse bath are those of relatively weak acids, such as phosphoric), and the described rinsing step supplants the customary waterrinse. The surface of the steel or iron is thus greatly improved, without additional apparatus or steps, and without, it is believed, forming on the metal any appreciable coating such as results from certain expensive rust-proofing processes. No other wash or rinse is usually necessary in pickling according to the present invention. As

stated above, lime water has been hitherto employed (with little effect-particularly where the lime forms a deposit on the metal or sets up rustenhancing compounds) or a dilute acid dip has been tried, followed by treatment of the metal in cumbersome scrubbing and drying machinery, but with the pickling procedure here described, such supplementary treatments are in general entirely obviated.

It will be understood that the invention is not limited to the specific embodiments hereinabove described, but may be carried out in other forms without departure from its spirit as defined by the following claims.

I claim:

1. The process of treating a ferrous metal article to remove scale, rust and the like, which consists in treating the said metal article with only a single mineral acid solution for a time sufficient to effect pickling, and thereafter removin said pickled metal directly into a dilute alkali phosphate solution as a washing treatment, removing the washed metal article from said solution and drying the same.

2. A process as in claim 1, in which the phosphate solution is tri-sodium phosphate.

in the ratio of 56 3. A process as in claim 1, in which the acid pickling solution also contains an inhibitor.

4. The process of treating a ferrous metal article to remove scale, rust and the like, which consists in treating the said metal article with only a single mineral acid solution for a time sufficient to effect pickling, and thereafter removing said pickled metal directly into a hot alkali phosphate solution of below 1% concentration as a washing treatment, removing the washed metal article from said solution and drying the same.

5. The process of -treating a ferrous metal article to remove scale, rust and the like, which consists in treating the said metal article with only a single mineral acid solution for a time sufflcient to efiect pickling, and thereafter removing said pickled metal directly into an alkali phosphate solution of below 1% concentration, as a washing treatment, removing the washed metal article from said solution and drying the same.

6. The process of treating a ferrous metal article to remove scale, rust and the like, which consists in treating the said metal article with only a single mineral acid solution for a time sufficient to effect pickling,,and thereafter removing said pickled metal directly into ahot alkali phosphate solution as a washing treatment, removing the washed metal article from said solution and drying the same.

CHARLES H. GRANT.