US2450861A - Composition for descaling ferrous metal - Google Patents

Description

Patented Oct. 5, 1948 UNITED STATES Examine 7 PATENT OFFICE Harold A. Robinson, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application April 16, 1945, Serial No. 588,711

3 Claims. 1

The invention relates to a method of cleaning the surface of ferrous metal articles with aqueous acid solutions. It more particularly concerns an improved composition of aqueous acid solution possessing especial characteristics adapting it for the purpose in hand.

Among the various uses of aqueous acid solutions is that of descaling ferrous metals. As is well known, such metals normally carry a coating of scale of one or more of the iron oxides formed during the manufacture or use. In some uses, other substances may be deposited upon the surface forming an adherent coating or scale either alone or in admixture with the aforesaid oxides. In the operation of steam boilers, for example, a variety of scales are deposited on the metal from the water depending upon its nature and other factors. All such deposits generally are undesirable and may seriously interfere with efiicient use of the ferrous metal as a heat transfer medium. In the case of ferrous metal heat-exchange surfaces, the presence of oxidic coatings formed as oxidation products of the ferrous metal are a cause of more or less inefliclency of heat transfer. There are many other uses of ferrous metals, the efficiency of which is adversely affected by oxidic scale deposits.

Many of these scale deposits are soluble in aqueous hydrochloric acid solutions and usually can be readily removed merely by maintaining for a long enough time a sufficient quantity of the acid solution in contact with the deposit at an appropriate temperature. However, the acid solution also attacks the underlying metal as it becomes exposed to the acid solution during the removal of the scale. Because of this, an inhibitor of the corrosive action of the acid is usually added to the solution, thereby reducing the severity of this attack. I have found that in spite of having added a conventional corrosion inhibitor, the acid solution may and often does persist in severely corroding the underlying metal. In investigating the causes of this effect I have found that there are two more or less independent corrosive factors usually involved, one of these is the corrosive action of the acid itself, the other the corrosion due to reduction by the ferrous metal of the ferric ion of the compounds formed as the acid dissolves ferric constituents of the scale.. Such a reduction is expressed quantitatively by the following equation:

Fe 2Fe+++ 3Fe++ (solid metal) dissolved (disso' lved erric iron) ferrous iron) This equation shows that each 2x55.84 pounds of ferric iron constituent dissolved from the scale is a solvent for 55.84 pounds of solid metal underlying the scale. While the conventional inhibitors exert a retarding effect on the rate of corrosion of the solid ferrous metal by the acid of the descaling solution, they have little or no effect in retarding the corrosive action of ferric compounds dissolved by the acid solution as it attacks the scale. Attempts to descale ferrous metal surfaces having ferric compounds in the scale using conventionally inhibited hydrochloric acid generally result in undesirable corrosion. In addition, the corrosive effect of the dissolved ferric compounds in the descaling acid solution is accelerated by the heating usually necessary to facilitate the removal of the iron oxide-containing scale deposits. This heating has the further disadvantage of decreasing the effectiveness of the conventional inhibitors.

One of the objects of the invention is to provide a h droc ic i solution suitable for dissolving from ferrous metal surfaces scale containing a ferric ion-producing material.

Another object of the invention is to provide a method of removing ferric oxide containing scale deposits from a ferrous metal surface by treatment with an aqueous solution without significant attack upon the underlying surface.

Other objects and advantages will appear as the description of the invention proceeds.

My invention is predicated upon the discovery that by including an oxidizable soluble salt 01 a metal selected from tlieffciufibf'thdfibh-blating metals consisting of chromium and titanium in the aqueous hydrochloric acid, together with an acid corrosion inhibitor scale containing ferric ion (Fe+++) producing material, may be removed from a ferrous metal surface with greatly reduced attack compared to that obtained when the acid solution and acid corrosion inhibitor are used without the addition of the aforesaid soluble salt of a non-plating metal. I

The invention, then, consists in the scale removing composition and method of removal of scale therewith hereinafter more fully described and particularly pointed out in the claims.

In carrying out the invention, aqueous hydrochloric acid is used in a concentration suitable for decomposing, dissolving. or disintegrating the scale deposit to be removed from the ferrous metal surface, such as those of iron and steel. Complete solution of the scale is not always necessary. Some scale deposits contain both acid-soluble and -insoluble constituents and usually slough off when attacked by the acid sulphata acetate, etc.

without completely dissolving. A concentration between about and per cent is usually suitable, although other concentrations can be used. A preferred concentration is about 15 per cent of hydrochloric acid by weight. Some of the higher concentrations are more diflicult to inhibit against acid corrosion; hence, the lowest concentration that will effectively remove the scale is preferably used.

To minimize the attack of the acid ion (H+ ion) on the ferrous metal underlying the scale a suitable inhibitor of this action is added to the acid. A very large number of substances and products having the property of inhibiting the corrosive action of acid on iron or steel are known, examples of which are found among the organic nitrogen bases, such as pyridine, quin oline, orthotoluidine. In addition, many organic sulphur compounds possess this property, e. g. the mercaptans, thiourea, and thiocyanates. Some aldehydes, likewise, may be used. Soluble arsenic compounds are also effective. The concentration of the acid corrosion inhibitor to employ depends upon its effectiveness and the extent to which it is desired to suppress the corrosion due to the acid and the temperature at which the acid solution is to be used as is well understood in the art. Generally, the amount used is in the range of 0.1 to 1 per cent of the weight of the acid solution.

In order to suppress the added corrosiveness acquired by the acid solution as it dissolves ferric iron from the scale to form ferric ions in the acid solution I add to it, in addition to the H+ ion corrosion retardant or acid inhibitor, an inhibitor of ferric ion corrosion consisting of an oxidizable salt of a metal selected from the group consisting of the non-plating metals, chromium and titanium. Various soluble chromium salts [maybe used, such as the chloride, iodide, acetate sulphate, etc. Similarly, various soluble titanous salts may be used, such as the chloride,

Ih' amount used is preferably that calculated to be in excess of that approximately stoichiometrically equivalent to the amount of ferric iron to be dissolved by the acid in descaling the ferrous metal surface.

The stoichiometric proportions for the oxidizable divalent chromium ion and the oxldizable trivalent titanium ion are given, respectively, in the following equations:

An equal amount or as much as 10 to 50 per cent in excess of these amounts may be used.

' To determine the amount of ferric ion which will be produced in a given scale removal operation one may resort to a conventional analysis of the scale giving the percentage of ferric iron therein. From such an analysis and a knowledge of the area of the ferrous metal surface covered by the scale to be removed, an estimate of the total weight of ferric ion-forming material that can be dissolved by the acid solution is computed on the assumption that during the acid treatment of the scale all the ferric ion-forming constituents therein will be dissolved by the acid solution. The stoichiometrically equivalent weight, or preferably 10 to 50 per cent in excess thereof, of oxidizable'chromium or titanium compound is then ascertained from the weight of ferric ionforming material in accordance with the weight ratios of the above equations.

Similar data as to the amount of acid-soluble material in the scale may be used to ascertain the quantity of acid solution needed, the amount used being preferably substantially in excess of actual needs. In some instances in order to reach all the scale, a larger volume may be needed as is usually the case in treating steam boilers. In such cases, suflicient solution is usually used to fill the vessel even though the amount of acid is more than enough to attack the scale.

The following example is illustrative of a mode of carrying out the invention:

A Water tube steam boiler having a volume of 5000 gallons is to be descaled. Examination shows the heat transfer surfaces are coated with scale containing ferric ion-forming constituents and other acid-soluble matter, the total amount of ferric iron in the scale over the area to be descaled being about 200 pounds. For treating such a vessel, the simplest procedure is to prepare enough treating acid to fill the boiler, i. e.- 5000 gallons. About 840 pounds of titanium trichloride (i. e. 50 per cent more than the stoichiometrical equivalent of the 200 pounds of ferric iron 0f the ferric iron content of the scale to be removed) is the amount used in the treating solution. For the acid corrosion inhibitor about 225 pounds of sodiumarsenite may be used, thereby giving a concentration of about 0.5 per cent in the acid solution. These two inhibitors are preferably added to the treating acid by first dissolving them in a small volume of 15 per cent hydrochloric acid so as to make concentrated solutions of these inhibitors and then diluting these strong solutions with enough 15 per cent hydrochloric acid to make 5000 gallons of treating acid solution. After these additions have been added to and uniformly mixed with 15 per cent hydrochloric acid to make 5000 gallons of mixed solution, the treating solution thereby obtained is ready for introduction into the boiler to remove the scale. The boiler is filled with the descaling solution so prepared and it is allowed to remain in contact with the scale therein for several hours or until the scale dissolving action ceases. This can be determined by making periodic titrations for acidity of the solution which declines as the acid decomposes the scale. When titration shows that the decline of the acid concentration has ceased, the scale removing action usually has ceased also. It is advantageous to heat the acid solution during its scale removal action as this accelerates the rate of attack, temperatures up to about 175 F. usually may be used without serious attack on the underlying metal.

The boiler is then drained and rinsed with water. If desired to insure removal of all traces of acid from the boiler, it may be filled with a dilute (e. g. 1 per cent) solution of sodium carbonate in water and boiled for a few minutes.

Further illustrations of suitable descaling acid solutions are: (1) those containing about 5-25 per cent of hydrochloric acid, 1.65 to 2.5 of titanium trichloride, and 0. o 1. per cent of tially water.

Descaling solutions containing divalent chmmium salts should not be unduly exposed to the? air as they are subject to atmospheric oxidation;

Examiner The descaling solutions containing soluble trivalent titanium salts are not so easily oxidized.

As illustrative of the effect on corrosion of employing a non-plating metal salt in the inhibited acid solution. the following series of tests conducted in a 15 per cent solution of hydrochloric acid are cited. In these tests pieces of mild steel 1" x 2" x 54;" were submerged in 300 grams of the acid solution for 16 hours while the solution was continuously agitated and maintained at 150 F. and the loss in weight due to attack on the pieces was noted. The weight loss data was converted into pounds of metal dissolved per square foot per day giving the corrosion rates reported in the table below. To ascertain the effect of ferric ion-forming material being present in the acid, 1.33 per cent of F8304 by weight was dissolved in the acid solution giving a concentration of ferric iron of 1 per cent. In two series of tests, runs 1 and 2, an acid corrosion inhibitor consisting of 1.3 per cent of sodium arsenite was present in each test solution of 300 grams of 15 per cent hydrochloric acid. In the other, run 3, the acid corrosion inhibitor consisted of about 1 per cent of crude quinoline bases known in the trade as Barrett Inhibitor No. 93.

Table Corrosion rate in lbs. of metal dissolved per sq. ft. per day at 150 F. in 15% H01 without and with added Fe and inhibitors Initial ferric iron concentration Acid Corrosion 1111119 acid solution Inhibitor Run No.

1 per cent Ferric ion corrosion inhibitor 2.77 2.97 NW 'rioi, 0rd,

1 1.3% of As as sodium ar- 0. 182 0. 534 0. 094

senite. 2 1.3% otl As as sodium ar- 0. 236 0. 630 0. 242

seni e. 3 1% of crude quinoline.-- 0. 055 0. 185 0. 126

Reference to the table shows that the nominal rate of corrosion of mild steel by inhibited 15 per cent hydrochloric acid (0.182 pound per square foot per day in run 1, 0.236 in run 2, and 0.055 in run 3) is greatly increased when even a small amount (1%) of dissolved ferric iron is present in the acid solution. The increases for each of the two tests with sodium arsemte as the acid inhibitor but without the ferric iron inhibitor are 193 per cent and 167 per cent, respectively, and with the crude quinoline 236 per cent. when a ferric iron corrosion inhibitor is also present in the acid along with the acid corrosion inhibitor, there is a large reduction in corrosiveness in spite of the presence of the added ferric iron. In the case of run 1, the reduction in corrosiveness was 82.4 per cent, in run 2 the reduction was 61.6 per cent, in run 3, 46.8 per cent.

By the term non-plating metal used herein is meant a metal which does not deposit as a metallic film on an iron or steel object brought into contact with an aqueous solution of one of its salts.

I claim:

1. A composition for descaling a ferrous metal surface comprising by weight 5 to 25 per cent of .hydrogmchlorigle, 0.1 to 1 per cent of an organic nitrogen base selected from the group consisting of p l n quinoline, methyl quinoline, and otoluxdi'riEind 2.2 to 3.3 per cent of a water-soluble salt selected from the group consisting of the water-soluble divalent chromium salts and the water-soluble trivalent titanium salts, the balance being water.

2. A composition for descaling a ferrous metal surface comprising by weight 5 to 25 per cent of hydrogen chloride, 0.4 to 1.0 per cent of crude quinoline, and 2.2 to 3.3 per cent of chromous chloride, the balance being water.

3. A composition for descaling a ferrous metal surface comprising by weight 5 to 25 per cent of hydrogen chloride, 0.1 to 1.0 per cent of crude quinoline, and from 1.65 to 2.5 per cent of titanium trichloride, the balance being water.

HAROLD A. ROBINSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,371,584 Urruty Mar. 15, 1921 1,778,634 I-Iartmenn et al. Oct. 14. 1930 1,785,513 Calcott et al. Dec. 16, 1930 1,877,504 Grebe et al. Sept. 13, 1932 2,357,991 Ayers Sept. 12, 1944 2,384,467 Hill Sept. 11, 1945 FOREIGN PATENTS Number Country Date 442,052 Great Britain J an. 31, 1936