The corrosion rate of a metal or alloy in an aqueous electrolyte is often determined by the kinetics of the cathodic reactions occurring at the surface. The primary cathodic reactions in aqueous corrosion are listed below.

Proton reduction reaction (acids)

$$2{H^ + } + 2{e^ – } \leftrightarrow {H_2}(g) \tag{1} \label{eq:1}$$ 

Water reduction reaction (bases)

$$2{H_2}O + 2{e^ – } \leftrightarrow {H_2}(g) + 2O{H^ – } \tag{2} \label{eq:2}$$  

Oxygen reduction reaction (acids)

$$ {O_2}(sol.) + 4{H^ + } + 4{e^ – } \leftrightarrow 2{H_2}O \tag{3} \label{eq:3}$$  

Oxygen reduction reaction (bases)

$$ {O_2}(sol.) + 2{H_2}O + 4{e^ – } \leftrightarrow 4O{H^ – } \tag{4} \label{eq:4}$$  

Equivalnet reactions

Equations (\eqref{eq:2}) is equivalent to (\eqref{eq:1}) and (\eqref{eq:4}) to (\eqref{eq:3}), assuming dissociation of water to (H^{+}) and (OH^{-}) and subtracting (OH^{-}) from both sides.[ref]D.A. Jones, “The Technology and evaluation of corrosion,” in Principles and Prevention of Corrosion, 2^nd ed., (Upper Saddle River, NJ: Prentice Hall, 1996), p.5-9. ISBN: 0-13-359993-0. Buy at Amazon[/ref]

LaTeX code

## Proton reduction reaction (acids)

2{H^ + } + 2{e^ - } \leftrightarrow {H_2}(g) 

## Water reduction reaction (bases)

2{H_2}O + 2{e^ - } \leftrightarrow {H_2}(g) + 2O{H^ - } 

## Oxygen reduction reaction (acids)

{O_2}(sol.) + 4{H^ + } + 4{e^ - } \leftrightarrow 2{H_2}O

## Oxygen reduction reaction (bases)

{O_2}(sol.) + 2{H_2}O + 4{e^ - } \leftrightarrow 4O{H^ - }