Specific features of the corrosion inhibition of an aluminum alloy by a nonchromate pigment mixture
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SPECIFIC FEATURES OF THE CORROSION INHIBITION OF AN ALUMINUM ALLOY BY A NONCHROMATE PIGMENT MIXTURE I. M. Zin’,1 S. B. Lyon,2 L. M. Bilyi,1 and M. B. Tymus’ 1
UDC 621.193:669.71
The combination of zinc phosphate/molybdate and calcium-containing ion-exchange pigment enables one to inhibit efficiently the local corrosion of an aluminum-copper alloy in a slightly acid medium. Here, the charge-transfer resistance of the metal under the action of extract of this mixture of nonchromate pigments is at the same level as in a medium with the extract of strontium chromate. Using the methods of surface analysis, we have established that this protective effect is caused by the deposition of a protective film on the aluminum matrix (anode) and intermetallic phase (cathode). Under the corrosion potential, a protective film, consisting of a mixture of zinc and calcium phosphates with admixture of zinc hydroxide, is formed on a specimen of the alloy in a medium inhibited by a nonchromate mixture. Mainly zinc phosphate and zinc hydroxide with admixture of calcium phosphate deposit on the cathode polarized surface of the alloy under study. Under anodic potentials, the film consists of zinc and aluminum phosphates.
Aluminum-copper alloys, owing to their low density and high strength, are widely used as structural materials in the aircraft industry and transport facilities. However, they are susceptible to local corrosion, which begins near intermetallic phases, e.g., Al2 Cu and Al2 CuMg [1, 2]. Inclusions enriched with copper are cathodes, whereas the anodic reaction is localized mainly on the aluminum matrix of these alloys. The present-day technology of corrosion protection of aluminum alloys consists of the application of a protective paint-and-lacquer coating on the surface prepared beforehand. To a considerable degree, the efficiency of such a coating depends on its ground coat, containing inorganic corrosion inhibitors. At present, efficient ground coats for aluminum alloys contain anticorrosive pigments based on the compounds of hexavalent chromium. Among them, one often applies poorly soluble strontium chromate, from which chromate ions are slowly liberated. They are accumulated at the sites of through defects of the paint-and-lacquer coating and inhibit corrosion of the aluminum-copper substrate. Chromates represent very efficient inhibitors of the local corrosion of aluminum alloys but are toxic, and it is quite difficult to utilize them. During the last decades, researchers have tried to find substitutes for chromates in organic coatings. New approaches consist of the introduction of inorganic inhibitors, in particular, cerium (IV), molybdenum (VI), manganese (VII), silicates, molybdates, phosphates, and ion-exchange pigments, to ground coats [3]. The search for synergistic compositions of inhibitors has become more and more urgent. In particular, as is known [4], the combination of phosphate and calcium-containing pigment works efficiently in the case of corrosion of bimetallic systems with pronounced localization of t
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