Specific features of the corrosion inhibition of an aluminum alloy by a nonchromate pigment mixture

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.