Protective film strengthening

Active anticorrosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and the primer. Water that has diffused into the binder dissolves soluble anticorrosive components (e.g., phosphate, borate, or organic anions) out of the pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on the iron is thereby strengthened and sometimes chemically modified. Any damaged areas are repaired with the aid of the active substance. Inhibition by formation of a protective film is the most important mode of action of the commoner anticorrosive pigments. 

The oxidation of most mixlera alloys is dependent on the foimuiiun of a compact protective film of a slow growing chemically stable oxide such as chromium Ill) oxide. U Oi. alumina. AljOi. or silica. SiO>. The oxidation behavior of multicomponent y -strengthened alloys can be estimated hy considering the Ni. C . and AI. content of the alloy... 

In contrast, hydrolyzed silane compounds, presumably adsorbed as oligomeric films, confer corrosion resistance in both hydrating and Cl environments. These inhibitors can also couple with applied epoxy primer or adhesive formulations to further protect the metal against corrosion by strengthening the metal-epoxide bond. The organosilanes do not appear to affect the curing process, e.g., % crosslinking, of the polymeric epoxy systems. 

Where the corrosion resistance of a coating depends upon its passivity, it is common to follow plating with a conversion coating process to strengthen the passive film. Zinc, cadmium and tin in particular are treated with chromate solutions which thicken their protective oxides and also incorporate in it complex chromates (see Section 1S.3). There are many proprietary processes, especially for zinc and cadmium. Simple immersion processes are used for all three coatings, while electrolytic passivation is us on tinplate lines. Chromate immersion processes are known to benefit copper, brass and silver electrodeposits, and electrolytic chromate treatments improve the performance of nickel and chromium coatings, but they are not used to the extent common for the three first named. 

Certain elements will strengthen the protective properties of the oxide film by forming mixed oxides, if their stmctures are compatible. This is the case of magnesium. For this reason, alloys of the 5000 series have excellent corrosion resistance. On the other hand, certain elements such as copper will weaken these protective properties. This explains the poor corrosion resistance of copper-containing alloys of the 2000 and 7(X)0 series. 

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