Corrosion Resistance

Passivation accounts for most of the corrosion resistance encountered in practice. Aluminium and titanium are both thermodynamically reactive metals, yet each forms a thin, adherent and self-repairing oxide that isolates the metal beneath from its environment, and stainless steel achieves the same result through the chromium oxide layer that forms once chromium content exceeds roughly eleven percent. Alloying, coating and cathodic protection are the principal engineering responses where a native oxide is inadequate: galvanising covers steel with zinc, which corrodes preferentially as a sacrificial anode, and impressed-current systems apply the same principle to buried pipelines. Resistance is specific to the environment rather than absolute, since chloride ions break down passive films locally and produce pitting in alloys that perform well in other media.