Method and agent for passivating iron and steel surfaces

Metal treatment – Compositions – Heat treating

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Details

106 1412, 106 1415, 252389R, 252390, 252396, C23F 708

Patent

active

044378980

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method and agent for passivating iron and steel surfaces chemically with the aid of aqueous solutions which give a weak alkaline reaction and contain a combination of certain corrosion inhibiting substances.
It is often necessary to passivate iron and steel surfaces in order to avoid undesirable corrosion phenomena. This is done for example in or after cleaning operations, in metal fabricating or intermediate storage before further fabricating of the metals. Also, passivating agents are used as additives to mold release baths, quenching water, as for example in induction hardening, in cooling cycles, as in motor test stands, and also as hydraulic fluids.
A number of methods are known for passivating. Thus it can be effected for example with oils or greases or also with corresponding synthetic emulsions, although then often undesirable films will form. It is therefore often expedient to passivate the metal surfaces with aqueous solutions which contain appropriate chemical additives. Such corrosion-inhibiting passivation agents for iron surfaces are for example alkali metal nitrites, alkali metal chromates, soaps, benzoates and alkanolamines. Also maleic acid mono-isoalkylamides have been employed for the passivation of iron and steel surfaces.
It has now been found, surprisingly, that with respect to corrosion protection a strong synergistic action occurs when use is made of the method described below for chemically passivating iron and steel surfaces with the aid of aqueous solutions which give an alkaline reaction and contain a combination of certain corrosion inhibiting substances.
The new method is characterized in that the metal surfaces are treated with 0.5 to 5 weight-percent solutions, whose pH value is between 7.5 and 10.5 and which contain a combination, giving a clear solution in water, of 1:10 and the weight ratio maleic acid monoalkylamide to phosphonic acid, 1:0.01 to 1:0.5.
As maleic acid monoalkylamides may be used amides with straightchain as well as branched alkyl radicals with 6 to 14 carbon atoms, preferably with 8 to 10 carbon atoms.
As suitable alkanolamines may be used short-chain compounds such as mono-, di- and tri-isopropanolamine, n-propanolamine, N,N,N',N'-tetrakis-(2-hydroxyethyl) ethylene diamine, and preferably mono-, di- and tri-ethanolamine, in particular as a mixture.
As suitable complexing phosphonic acid may be named: 1-hydroxyalkyl-1,1-di-phosphonic acid, 1-amino-alkyl-1,1-diphosphonic acid, phosphonocarboxylic acid, as in particular 2-phosphono-1,2,4-tricarboxylic acid, and/or phosphonic acids of the general formula ##STR2##
Preferably 1-hydroxyethane-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid and nitrilo-trimethylene phosphonic acid have proved successful. The phosphonic acids can be employed also, incidentally, as alkali metal salts or alkanolamine salts. Alternatively the free phosphonic acids may be added to the respective solutions. In such a case, using an excess of alkanolamine in the solution, the latter can be adjusted to the pH value of 7.5 to 10.5.
An essential feature of the combination according to the invention is the weight ratio of maleic acid monoalkylamide to alkanolamine, which should be 1:0.3 to 1:10 and preferably 1:1 to 1:4. The weight ratio of maleic acid monoalkylamide to phosphonic acid is in the method of the invention 1:0.01 to 1:0.5 and preferably 1:0.1 to 1:0.5. While it is possible to increase the phosphonic acid addition further, it gives no additional synergistic effects.
If the pH value of the solution of 7.5 to 10.5 suitable for the practice of the method is not already achieved by the alkalinity of the substances used, it can be adjusted by a slight addition of alkali.
The treatment of the metal surfaces of iron or steel can be effected at elevated temperatures between 30.degree. and 100.degree. C., but preferably at room temperature.
Besides the components already mentioned, the agents may contain other substances as well. For this enter consideration in particular surfactants, prefera

REFERENCES:
patent: 2790779 (1957-04-01), Spivack et al.
patent: 3368913 (1968-02-01), Ziehr et al.
patent: 4045253 (1977-08-01), Banks et al.
patent: 4276089 (1981-06-01), Moran

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