Metal corrosion in partially saturated sands: pore fluid conductivity and water saturation

Sediments can facilitate the electrochemical processes that drive the corrosion of buried metallic components. Common analyses and guidelines emphasize the effect of pore fluid conductivity on soil corrosivity and overlook the effect of partial saturation; yet, most buried metals are situated within the vadose zone. The detailed experimental study reported herein used mass loss measurements from passive corrosion tests, X-ray microtomography, and image analyses to examine the evolution of corrosion in C-steel coupons embedded in sand specimens mixed with de-ionized water and brine at various degrees of saturation. Experimental observations show that corroding cells preferentially form at contacting grains, and that the evolution of corrosion is biased by the variability in packing density and saturation, while fluid conductivity plays a lesser role. Above all, results highlight the critical importance of percolating gas and water phases, and show that water–grain–metal interfaces restrict the actively corroding area to a fraction of the entire metal surface. A complementary macroscale analysis anticipates asymptotic conditions based on mass and charge conservation, and the transport of corroding agents and residuals. Together, the measurements and model results highlight the significant impact of the degree of saturation on corrosion rates and mass loss.