On the optimum soil moisture for underground corrosion in different soil types

Corrosion rates in sand, silt, and clay were investigated at various degrees of saturation (S-r) revealing that all three soil types have a distinct optimum S-r at which the corrosion rate is maximised. Sand, silt, and clay showed maximum corrosion currents at S-r approximate to 0.5, S-r approximate to 0.7, and S-r approximate to 0.8 respectively. We show that the optimum moisture level at which the corrosion rate becomes a maximum for a given soil type corresponds to the air transition point related to its water retention curve. We provide an explanation for this phenomenon elucidating previous observations, and highlight implications to current engineering practice.