Q5Engineering Chemistry
Question
Q.5 Why do pure metals show more resistance to corrosion than impure metals?
Answer
Absolutely pure metals are vastly more resistant to electrochemical corrosion strictly because they entirely lack internal chemical impurities, physically preventing the spontaneous formation of destructive microscopic internal galvanic corrosion cells.
The absolute fundamental mechanism of wet electrochemical corrosion mathematically requires the simultaneous physical existence of distinctly different anodic and cathodic sites completely connected by an electrically conducting electrolyte.
In chemically impure metals or multi-phase alloys, the distinct microscopic impurities or separate metallurgical phases inherently possess radically different electrochemical potentials. When exposed directly to atmospheric moisture (the electrolyte), these distinct sites systematically form millions of tiny, highly destructive 'local galvanic cells'. The strictly more electrochemically active metallic region becomes the anode and suffers extremely rapid, violent localized dissolution (corrosion). Because absolutely pure metals are chemically and physically completely homogeneous, no distinct internal potential differences exist, totally eliminating the fundamental driving force for electrochemical corrosion.