How does the presence of sodium chloride and acidic substances increase the speed of rusting?
Will alkaline substances increase the speed of rusting?
In any electrolytic (salt or acid ) solution, electrons are transferred from Fe to O2 in a series of steps. Here is the overall reaction:
2Fe + O2 + 2H2O ---> 2Fe(OH)2
Acid reduces the concentration of OH- ions which drives this reaction faster.
The Fe^+2 ions in Fe(OH)2 are oxidized further to Fe^+3, and the hydroxides are dehydrated to Fe2O3 (rust):
Fe^+3 + 3H2O ⇌ Fe(OH)3 + 3 H+
Fe(OH)3 ⇌ FeO(OH) + H2O
2 FeO(OH) ⇌ Fe2O3 + H2O
Some of the above steps will go faster in the presence of a basic solution. However the loss of electrons by metallic iron which starts the entire sequence will be slower in a basic solution.