Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Patent
1995-07-06
1997-11-18
Green, Anthony
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
106 1415, 106 1434, 106 1444, 106 1405, 252392, 534682, 534684, 534722, C23F 1100, C23F 1114
Patent
active
056883102
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method and a material for protecting a non-ferrous metallic material against corrosion, in particular a light aluminium alloy, for example an aluminium/magnesium alloy (type 5154), an aluminium/magnesium/copper alloy (type 2024) or an aluminium/magnesium/zinc alloy (type 7075).
BACKGROUND AND OBJECTS OF THE INVENTION
Non-ferrous metallic alloys, such as light aluminum alloys (notably alloys containing magnesium), are very much used in aeronautics and, increasingly, in other sectors, in particular in motor vehicle manufacture. The corrosion inhibiters currently used in aeronautics for this type of alloy are all based on strontium or zinc chromate, since these are the only ones that have sufficient efficacy of protection to comply with aeronautical standards (salt water tests). However, the chromate-based hexavalent compounds used are toxic and carcinogenic and are in the process of being prohibited in the majority of countries: because of this, there is currently a serious problem, which has not been resolved, in the protection of light alloys in the aeronautical industry. Moreover, it has been found that these chromate-based compounds are more or less rapidly destroyed in the presence of oxidisable products such as the glycols which are used as antifreeze liquids; thus these compounds are not very suitable for protecting internal parts (which are impossible to repaint) on which these oxidisable products may spread.
Other types of corrosion inhibiters have been proposed for light aluminium alloys, but these have very mediocre efficacy and cannot be used in the aeronautical industry because of the standards currently in force. The best products at present appear to be the following: a product based on modified aluminium triphosphate (trade name "K WHITE 84" manufactured by the TEIKOKU KAKO Co Ltd) products based on zinc phosphate (trade name "DELAPHOS 2" manufactured by "CHIMILAB ESSOR"; trade name "HEUCOPHOS ZPO" manufactured by "HEUBACH"; trade name "SICOR NOP" manufactured by "BASF"), and products based on barium and calcium phosphosilicate (trade name "HALOX BW 111" manufactured by "HALOX PIGMENTS") ; tests carried out for accelerated corrosion in a saline solution (the method described by P MAURET and P LACAZE, Corrosion Science, Vol 22, 4, 321, 1982) showed that these products provide full inhibition periods of around 1% to 10% compared with the inhibition periods provided by the chromate-based inhibiters.
In addition, the following publication should be noted: DATABASE WPI, Derwent Publications, London (GB) AN 79-07066B & JP-B-53047980 & JP-A-50076531 (HITACHI MAXELL), which describes the use, in a zinc battery, of a corrosion inhibiter mixed with the electrolyte; this inhibiter consists of an azonaphthalene comprising two nitrogen atoms, one carrying a hydroxyl group and/or a sulphonic group, the other a benzene nucleus.
DESCRIPTION OF THE INVENTION
The present invention sets out to provide a novel process for protecting non-ferrous metallic materials against corrosion, in particular light aluminium alloys.
The objective of the invention is to use a corrosion inhibiter totally free of chromium and having protection efficacy comparable to that of chromates (for an equal number of moles).
Another objective is to provide effective protection vis-a-vis oxidisable products such as glycols.
To this end, the process according to the invention for protecting a non-ferrous metallic material against corrosion consists of covering the said material with a coating containing at least one dihydroxyazoic salt with a solubility in water of between 0.01 g/l and 3 g/l, having the following structural unit: ##STR2## where .gamma. is a salifiable anion and M a metal.
It has been found that protection for light aluminium alloys produced with such a dihydroxyazoic salt (totally chromium-free) has an efficacy comparable to that of chromate-based products, enabling aeronautical standards to be met. Preferably, a dihydroxyazoic salt is used in which the .gamma. group is a sulphonate and the me
REFERENCES:
patent: 3706698 (1972-12-01), Buehler et al.
patent: 4110073 (1978-08-01), Mollet et al.
patent: 4527994 (1985-07-01), Lienhard et al.
Japanese Patents Gazette Week 8347, Issued Jan. 11, 1984.
Japanese Patents Report vol. 79, No. 01, Issued Feb. 2, 1979.
Corrosion Science vol. 22, No. 4, pp. 321-329 (19 Jun. 1981).
Gaches Louis
Mauret Pierre
Dutton, Jr. Harold H.
Green Anthony
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