Effects of dissolved oxygen, hydrogen ion concentration, temperature and velocity on corrosion

Corrosion of metals, particularly steel in sea water, is an electro-chemical process: at the anode metal goes into solution releasing electrons, and at the cathode the electrons combine with oxygen (oxygen absorption) or with hydrogen ions (acid) - the overall reaction is driven by dissolved oxygen in near-neutral sea water.

Dissolved oxygen is essential for the cathodic reaction in neutral/alkaline solutions. Increasing dissolved oxygen increases the rate of corrosion because it depolarises the cathode, allowing more anodic dissolution; water containing more oxygen (well-aerated, surface sea water) corrodes steel faster than deaerated water. At very high local oxygen, however, the metal may become passive (protective oxide film re-forms) and the rate falls. Differential aeration: parts of the metal with more access to oxygen become cathodic relative to lower-oxygen areas (e.g. waterline, under dirt/marine growth, at rivets/crevices), concentrating corrosion in the oxygen-depleted zones - the classic cause of pitting under deposits and at waterlines/fouling.

In acid solutions (low pH, excess H+) the cathodic reaction is hydrogen-ion reduction (hydrogen evolution), and as pH falls corrosion accelerates because more of the metal dissolves and the acid attacks the oxide film. In neutral solutions the rate is controlled by dissolved oxygen. In alkaline solutions (high pH) corrosion is reduced because less hydrogen is available and protective/insoluble films (e.g. carbonate/hydroxide) form - hence less corrosion at high pH. Iron/steel corrodes fastest in acid, slower in neutral (controlled by aeration), and slowest in alkaline conditions.

Raising temperature increases the rate of chemical reactions and diffusion (roughly doubling the rate for each 10 C rise), so corrosion increases with temperature because both the anodic and cathodic reactions and the diffusion of oxygen are faster. In closed systems loss of dissolved oxygen at higher temperature tends to offset this, but generally hot parts (hot water systems, heat exchangers, steam) corrode faster. High temperature also affects the protective film and can promote more aggressive attack.

A moderate water velocity increases corrosion by improving the supply of oxygen (cathodic reactant) and removing corrosion products. However, at high velocity, or with suspended solids/bubbles, erosion-corrosion and cavitation damage strip the protective film and accelerate attack, so the surface wears quickly (e.g. in pumps, valves, condensers and propeller and sea-chest regions). Low velocity or stagnant areas allow deposition and differential aeration, giving pitting underneath fouling/deposits. There is an optimum - too fast corrosion by oxygen supply/erosion, too slow corrosion by differential aeration/deposits.