Soil properties affecting corrosion and cathodic protection of steel in Texas soils

Abstract

Corrosion in the United States is a major economic problem with losses in 1984 estimated at $125 billion (Siebert, 1984). A larger portion of this corrosion can be attributed to degradation of underground metallic structures. The objective of this study was to characterize the interactions which occur between metals and their underground environment. Dropstructures with corrugated, galvanized steel spillways were selected for study of the effects of runoff water quality on corrosion. Failure of dropstructures was attributed to corrosion of the galvanized steel. The life expectancy of dropstructures was modeled using amount of CaCO3 precipitated and Cl- concentration resulting from simulated evaporation cycles. Precipitation of CaCO3 tended to increase structure life, whereas Cl- concentration tended to decrease life expectancy. Corrosivity of anions increased in the order of HCO3-, SO4= and Cl-. In the second investigation, corrosion of steel and aluminum was monitored for one year in 16 Texas soils at depths of 0.6 and 1.5 m. A system of classification was devised to delineate soil horizons into low, medium or high corrosion hazard classes. The resulting system more accurately reflected field corrosion measurements than did the existing USDA-SCS system. The two classification schemes were tested on independent data collected by the National Bureau of Standards and the new system proved superior. Cathodic protection of steel in 4 soils was achieved using sacrifical Mg anodes. The electrical parameters of the systems and water content of the soils were monitored with time. Cathodic production systems in well-drained soils were controlled primarily by soil resistivity and the dominant cathodic reaction was O2 reduction. Cathodic protection systems in soils with aquic moisture regimes, or belonging to aquic subgroups, showed seasonal changes in cathodic mechanisms. During dry periods, O2 reduction was the dominant mechanism and cathodic protection current was greatest. Wet periods decreased O2 diffusion, however, and the dominant mechanism became H2 reduction. Gradients of pH, CO2 and O2 resulted in a suite of three Mg minerals (brucite, nesquehonite, and hydromagnesite) near the soil/metal interface.