Scale removal, ferrous metal passivation and compositions therefor

Abstract

.Iadd.Removing iron oxide scale or a mixture of iron oxide scale and hardness scale from a metal surface by contacting said scale with an aqueous solution containing at least one of an ammonium, amine, or hydroxyalkylamine salt of nitrilotriacetic acid, N-2, -hydroxyethylaminodiacetic acid, or alkylenepolyamine polycarboxylic acid at a pH of 8 to 11. .Iaddend.

What is claimed is: 
     
       1. A process for removing (1) hardness scale and one or more iron oxide-containing deposits of the group of Fe 2  O 3  -containing and Fe 3  O 4  containing deposits or (2) one or more iron oxide-containing deposits of the group of Fe 2  O 3  -.[.cotaining.]. .Iadd.containing .Iaddend.and Fe 3  O 4  -containing deposits from a metal surface containing one or more of the aforesaid hardness scale and iron oxide-containing deposits by .[.conatcting.]. .Iadd.contacting .Iaddend.said metal surface with an aqueous alkaline saline solution having a pH of 8 to 11, the active ingredient of which consists of at least one salt of the group consisting of ammonium, amine and hydroxyalkylamine salts of nitrilotriacetic acid, N-2-hydroxyethyliminodiacetic acid, alkylenepolyamine polycarboxylic acids, said amine and hydroxyalkylamine having substitutent groups, each of which has up to 5 carbon atoms, for a time sufficient to dissolve said hardness scale and said iron oxide-containing deposits. 
     
     
       2. A process as claimed in claim 1 wherein the contacting treatment is carried out at a temperature up to the temperature at which the salt decomposes. 
     
     
       3. A process as claimed in claim 1 wherein the salt solution is rinsed from the cleaned metal surface with water. 
     
     
       4. A process as claimed in claim 1 wherein the salt is formed in situ by admixing the base and the acid which react to form said salt in the presence of said metal surface in a proportion to provide an alkaline solution. 
     
     
       5. A process as claimed in claim 1 wherein the alkaline saline solution employed contains from 0.1 to 28.6 weight percent of the salt. 
     
     
       6. A process as claimed in claim 1 wherein the alkylenepolyamine polycarboxylic acid salt of the alkaline saline solution employed is a salt of an acid of the formula   (HOOCCH.sub.2).sub.2 N[(CH.sub.2).sub.n NCH.sub.2 COOH].sub.m CH.sub.2 COOH     wherein n is 1 to 4, m is 1 to 4, up to m of the carboxymethyl groups may be replaced by β-hydroxyethyl groups and at least one of the carboxymethyl groups may be replaced by a carboxyethyl group. .[.7. A process for passivating a ferrous metal surface by contacting said metal surface with an aqueous alkaline saline solution wherein the solution employed contains as an essential constituent at least one salt of the group consisting of ammonium, amine and hydroxyalkylamine salts of nitrilotriacetic acid, N-2-hydroxyethyliminodiacetic acid and, alkylenepolyamine polycarboxylic acids, said amine and hydroxyalkylamine having substituent groups, each of which has up to 5 carbon atoms, for a time sufficient to form a thin, continuous, adherent film of magnetic iron oxide, whereby said ferrous metal surface is rendered resistant to further oxidation..]. .Iadd. 8. The process of claim 1 wherein the active ingredient consists of the ammonium salt of ethylenediamine tetraacetic acid. .Iaddend..Iadd. 9. The process of claim 8 wherein the pH is about 9. .Iaddend..Iadd. 10. The process of claim 1 wherein the temperature of the contacting solution ranges from about 20° C. to the boiling point of said solution. .Iaddend..Iadd. 11. The process of claim 1 wherein the process is carried out at superatmospheric pressure and at a temperature below the decomposition temperature of said salt, but above the atmospheric boiling temperature of said solution. .Iaddend..Iadd. 12. The process of claim 1 wherein the pH is about 9. .Iaddend. .Iadd. 13. The process of claim 1 for removing iron oxide scale from a metal surface wherein said metal surface is a heat transfer surface; and said deposit is formed because of water in contact with such surface wherein said scale is contacted with an aqueous alkaline saline solution having a pH ranging from 8 to 11 and wherein said active ingredient consists of an effective amount of an ammonium salt of ethylenediamine tetraacetic acid to remove said scale. .Iaddend. .Iadd. 14. The process of claim 13 wherein said solution also contains a corrosion inhibitor. .Iaddend..Iadd. 15. The process of claim 13 wherein an effective amount of said salt is employed to provide said pH. .Iaddend..Iadd. 16. The process of claim 13 wherein the process is carried out at superatmospheric pressure and at a temperature below the decomposition temperature of the salt, but above the atmospheric boiling temperature of said solution. .Iaddend..Iadd. 17. The process of claim 13 wherein the scale includes hardness scale in addition to said iron oxide scale. .Iaddend..Iadd. 18. The process of claim 13 wherein the iron oxide is Fe 3  O 4 . .Iaddend..Iadd. 19. The process of claim 13 wherein the pH is about 9. .Iaddend.