: 150
Corrosion is electrochemical oxidation of metals. Iron rusting: anodic areas (Fe -> Fe2+ + 2e-) and cathodic areas (O2 + 2H2O + 4e- -> 4OH-) form on the iron surface. Fe2+ + 2OH- -> Fe(OH)2, oxidised to Fe2O3.xH2O (rust). Requirements: moisture + oxygen + electrolyte. Prevention: (1) barrier coatings (paint, oil, grease), (2) galvanisation (Zn coating — sacrificial anode), (3) cathodic protection (attaching Mg or Zn blocks), (4) alloying (stainless steel — Cr forms protective Cr2O3), (5) electroplating (Ni, Cr plating). Tin-plated iron corrodes faster than bare iron if scratched (Sn is less reactive, making Fe the anode), while zinc-plated iron is protected even when scratched.