|Publication number||USRE30796 E|
|Application number||US 05/942,212|
|Publication date||Nov 17, 1981|
|Filing date||Sep 14, 1978|
|Priority date||Jul 23, 1962|
|Publication number||05942212, 942212, US RE30796 E, US RE30796E, US-E-RE30796, USRE30796 E, USRE30796E|
|Inventors||Chester A. Lesinski|
|Original Assignee||The Dow Chemical Co.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (34), Non-Patent Citations (9), Referenced by (32), Classifications (19)|
|External Links: USPTO, USPTO Assignment, Espacenet|
(HOOCCH2)2 N[(CH2)n NCH2 COOH]m CH2 COOH
This application is a continuation-in-part of U.S. Patent application Serial No. 211,885, filed .[.June.]. .Iadd.July .Iaddend.23, 1962, now abandoned.
This invention concerns a method for removing scale from metal surfaces, for passivating ferrous metal surfaces and compositions useful therefor.
In almost any type of metal equipment in which water is evaporated or heat transfer occurs, insoluble salts deposit on the surfaces to form scale. Usually the deposits consist of calcium and magnesium salts and result from the use of hard water, although sometimes the scale is based on elements other than calcium or magnesium.
Common types of scale which deposit on metal surfaces include calcium sulfate (gypsum), calcium carbonate (limestone), complex calcium phosphate (hydroxy apatite) and calcium oxalate. In addition, scales containing a high iron content are also encountered.
It has previously been known to use inorganic or organic acids to remove scale deposits from ferrous metal heat exchange equipment. After descaling, the equipment is normally drained of the acidic solvent, flushed with water and residual acid, neutralized with an alkaline solution, drained again and finally flushed with water.
Alkali metal salts of alkylenepolyamine polyacetic acids (hereafter APAPA), prototype ethylenediamine tetraacetic acid (EDTA), have also been used to remove scale from ferrous metal surfaces. They are ineffective, however, for removing iron oxide deposits at a pH above 8.
When aqueous acids or aqueous solutions of alkali metal salts of APAPA having a pH below 8 are used, the resulting cleaned ferrous metal surfaces are prone to oxidation or rusting, and the build-up of scale is again under way.
In accordance with the process of the present invention, scale and particularly iron oxide containing scale is removed from metal surfaces by a contacting treatment with an aqueous, alkaline, saline solution. As an essential feature, the solution employed contains at least one ammonium, amine or hydroxy alkylamine salt of nitrilotriacetic acid (NTA), N-2-hydroxyethylamino diacetic acid (OHEtIDA), an alkylenepolyamine polycarboxylic acid or mixture thereof. The contacting treatment is preferably carried out at a temperature ranging between 20° C. up to the temperature at which the salt decomposes. It is generally desirable to rinse the salt solution from the cleaned metal surface with water.
In the process of the invention, the salt may conveniently be formed in place by admixing the base and the acid in the presence of the scale covered metal surfaces in a proportion to provide an alkaline solution which preferably has a pH value of at least 8. It is desirable for the alkaline, saline solution to contain from 0.1 to 28.6 weight percent of the salt.
The more preferred salts of the process of the invention are the salts of alkylenepolyamine polycarboxylic acids of the formula
(HOOCCH2)2 N[(CH2)n NCH2 COOH]m CH2 COOH
wherein n and m may each independently be 1, 2, 3 or 4, up to m of the carboxymethyl groups may be replaced with β-hydroxyethyl groups, and one or more of the carboxymethyl groups may be replaced by carboxyethyl groups.
The process of this invention, moreover, not only cleans metallic surfaces, but also passivates ferrous metal surfaces thereby due to the formation thereon of a fine, continuous, adherent film of magnetic iron oxide, Fe3 O4, so that subsequent oxidation is substantially reduced. This passivation, moreover, may be effected on substantially clean or new ferrous metal surfaces or those having at most a thin film of mill scale thereon.
As previously indicated, the effective salts of the process of the invention are ammonium, amine or hydroxyalkylamine salts of such chelant acids as nitrilotriacetic, N-2-hydroxyethylimino diacetic and alkylenepolyamine polycarboxylic acids. The preferred amine and hydroxyalkylamine salts of the invention are prepared from water-soluble amines having not more than 5 carbon atoms. These include methylamino-, dimethylamino-, ethylamino-, diethylamino-, methyl ethylamino-, propylamino-, isopropylamino-, trimethylamino-, the isomeric butylamino-, the isomeric amylamino- the corresponding mono(alkanol)-, di(alkanol)-, tri(alkanol)amino- particularly the mono(ethanol)amino-, di(ethanol)amino- and tri(ethanol)amino- salts.
Representative of the ammoniated or aminated chelants set forth broadly above are those of ethylenediamine tetraacetic acid, diethylenetriamine pentaacetic acid, N-2-hydroxyethyl ethylenediamine triacetic acid, propylene-1,2-diamine tetraacetic acid, propylene-1,3-diamine tetraacetic acid, the isomeric butylenediamine tetraacetic acids, etc. When the chelants set forth above are ammoniated or aminated to a pH of 8 to 11, an average of one of the --CH2 CO2 H groups remains in the acid form since the bases used are not basic enough to form a fully substituted ammonium or amino salt of such chelants. In the formula given for such chelants, n is preferably an integer ranging between 1 or 2, although, theoretically, it has no upper limit.
The aqueous solutions of such ammoniated or aminated chelants, when adjusted to a pH of 8 to 11 with ammonia or amine, are effective in weight concentrations of at least 0.1 percent, water basis, and up to their maximum solubility limit. The upper limit is essentially economic. The most generally useful upper limit is about 40 weight percent.
In use, the scale covered metal is contacted with such an aqueous solution for a time sufficient to remove the scale. The rate of scale removal is increased by increasing the temperature of the chelant salt solution from room temperature, ca. 20° C., up to its boiling point. The rate of solution of scale is further increased by heating under superatmospheric pressure up to a temperature just short of the decomposition temperature of said salt. It has been found advantageous to add one or more compatible metal corrosion inhibitors (e.g., mercaptobenzothiazole, polyhydric phenolates and non-reactive derivatives thereof such as gallic acid sodium and ammonium salts, etc.) to the above-described salt solutions in amount ranging between about 0.1 and 5 percent by weight, water basis.
Preparation of the descaling and passivating solutions is as follows. An APAPA, .[.AHEtIDA, NTA or CA,.]. .Iadd.OHEtIDA, or NTA, .Iaddend.e.g., EDTA, is slurried or mixed at room temperature or above with water and sufficient ammonia, amine, hydroxyalkylamine or combinations thereof to solubilize or dissolve the chelant and to adjust the pH of the resulting solution to between 8 and 11, preferably 9. The ammonia may be added as gaseous ammonia or as aqueous concentrated ammonium hydroxide.
In use the solution, as prepared above, in amount usually at least sufficient on a stoichiometric basis to dissolve hardness and iron oxide deposits is introduced into a fabricated metal apparatus to be cleaned and/or passivated and is allowed to stand with or without agitation until the surface is substantially clean. The amount of cleaning and/or passivating solution to be used is determined by an estimate of the amount of deposit to be removed. It is advantageous to use excess solution, the amount of excess being governed by economics. Advantageously, heating to boiling at atmospheric or superatmospheric pressure is used to accelerate the descaling process. An inhibitor in amount as specified above is advantageously present in solution.
The following examples describe completely representative embodiments and the best mode contemplated by the inventor of practicing the invention.
A solution of about 2 weight percent of ammoniated EDTA is prepared by adding 2 parts of EDTA to 100 parts of water and mixing with concentrated ammonium hydroxide in amount sufficient to solubilize the EDTA and to give a pH of about 9. The triammonium salt of EDTA is thereby formed. The solution so prepared readily dissolves hardness deposits and iron oxide from hot water boilers when the stoichiometric amount is allowed to stand therein for 3 to 4 hours at boiling temperature. The solution is then drained off and the boiler is washed free of solution with water. A thin passivating film of Fe3 O4 is left behind on the surfaces thereof.
A solution was prepared by mixing a slurry of 100 ml. of 5 weight percent EDTA in water with concentrated ammonium hydroxide in amount sufficient to give a solution having a pH of 9. When the solution was boiled for 3 hours with 1.00 g. of Fe3 O4, 0.98 g. or 98 percent of the Fe3 O4 was dissolved.
The preceding procedure when repeated with the same quantity of EDTA slurry but dissolved with NaOH in amount to give a pH of 9 dissolved only 0.05 g. (5 percent) of Fe3 O4 when boiled for 3 hours with 1.00 g. of Fe3 O4.
The procedure of Example 2 was repeated substituting 1.00 g. of Fe2 O3 in place of Fe3 O4. In the first instance, using ammoniated EDTA, 0.96 g. (96 percent) of the Fe2 O3 was dissolved as compared with 0.06 g. (6 percent) with Fe2 O3 when the sodium salt and NaOH were substituted for the ammoniated solution previously used.
Right-angled carbon steel coupons 1"×1"×3" (2.5×2.5×7.6 cm.) length having a thin film of magnetic iron oxide (mill scale) on the surface were exposed for 16 hours at 149° C. in aqueous 10 percent by weight solutions of the chelants shown below, the pH of which was adjusted to 9 with ammonium hydroxide prior to placing the oxide-coated specimens therein. After the exposure period, the coupons were washed with tap water and examined for effectiveness of mill scale removal as follows:
TABLE I______________________________________ Mill ScaleTest No. Chelant Removal, Percent______________________________________1 EDTA 1002 NTA 1003 OHEtIDA 854 (1) 1005 (2) 100______________________________________ (1) N2-Hydroxyethyl ethylenediamine triacetic acid. (2) Diethylenetriamine pentaacetic acid.
Angle iron strips 1"×1"×3" length (2.5×2.5×7.6 cm.) having mill scale (Fe3 O4) on the surface were used as test specimens. These were treated with various solutions to remove the mill scale, as shown below. They were further treated, as shown below, with other solutions, some for rinsing, some for neutralization and some for passivation, which in some cases, (1, 2, 3 and 4 below) conform to regularly accepted practice in chemical cleaning operations.
TABLE II__________________________________________________________________________ Water NeutralizeTest No.Mill Scale Removal in- Rinses 1% Soda Ash Passivating Solution__________________________________________________________________________1 Aqueous 5% HCl with 0.1% 2 Yes 0.5% sodium nitrite, 0.25% mono-commercial acid inhibitor. sodium phosphate, 0.25% disodium phosphate.2 Aqueous 5% HCl with 0.1% 1 No 1% diammonium citrate.commercial acid inhibitor.3 Aqueous 5% HCl with 0.1% 2 Yes 1% sodium nitrite.commercial acid inhibitor.4 3% citric acid, 0.2% commer- 2 Yes 0.25% monosodium phosphate, 0.25%cial acid inhibitor. disodium phosphate, 0.5% sodium nitrite.5 EDTA + NH4 OH, pH 9 0 No None6 EDTA + NH4 OH, pH 9 1 No none7 EDTA + NH4 OH, pH 9 2 No None__________________________________________________________________________
One set of the coupons prepared as above was placed outside on the roof of the laboratory. During a weekend exposure of 60 hours, it rained on the specimens once. The following visual evaluation was made of the specimens after exposure to an industrial atmosphere.
TABLE III______________________________________ Per- cent TotalSpecimen Sur-Prepared facePer Test AreaNo. Rusted Examination and Results______________________________________1 >95 Yellow rust on both sides.2 >95 Yellow rust on both sides less than 1.3 >95 Yellow rust on both sides.4 >95 "5 5 Little or no rust, metal dull gray.6 ca. 25 No rust on one side, half of other side rusted.7 ca. 25 "______________________________________
Another set of specimens was placed in a humidity cabinet at 140° F. (60° C.). The atmosphere within the cabinet was saturated with water vapor at this temperature. Exposure time was 24 hours, after which time the specimens were evaluated. All of the specimens showed after-rust formation with the exception of the coupon prepared per Test No. 5. This coupon did not rust and the metal surface was a dull gray.
Six specimens of cold rolled carbon steel strip 3"×1"×1/8" (7.6×2.5×0.3 cm.) were exposed for times up to 300 hours at 300° F. (149° C.) in 10 percent aqueous EDTA adjusted to a pH of 9 ammonium hydroxide, then rinsed off with tap water. After exposure, the specimens had a gun metal blue surface film. This film resisted staining and after-rusting from frequent handling. Fingerprints are known to be offensive in this respect, yet frequent handling by different persons did not result in after-rusting over a period of 60 days. The film was removed by scraping from one of the specimens. Its analysis by X-ray diffraction showed only Fe3 O4, a desirable film-forming constituent for industrial passivation.
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|U.S. Classification||134/2, 252/390, 252/180, 148/243, 510/247, 422/16, 252/394, 510/253, 134/19, 510/245|
|International Classification||C23G1/19, C02F5/12, F02B77/04|
|Cooperative Classification||C02F5/12, C23G1/19, F02B77/04|
|European Classification||C23G1/19, C02F5/12, F02B77/04|