The Effects of SO2 Scrubber Chemistry on Corrosion of Structural Materials

Abstract

The effects of 13 trace elements on the corrosion behavior of Ferralium 255, Hastelloy G3, stainless steel (SS) Type 317L, Monit and Ti Grade 2 immersed in an aerated baseline solution containing chlorides and sulfates at pH 1 or 4 in the presence of sulfur dioxide (SO2) have been studied using electrochemical techniques. In a first series of experiments, a Plackett-Burman test matrix was used to determine which of the trace elements have a significant accelerating or inhibiting effect. Polarization resistance measurements were carried out for electrodes that were totally immersed for the entire six-day test period and for electrodes that were exposed above the solution level, except for 1 h/day, when they were immersed in the test solution. A computerized system was used to perform simultaneous measurements for all five alloys and both types of electrodes using up to ten test electrodes. The results of the electrochemical measurements were confirmed by a correlation analysis using results from a solution analysis and visual observation of the number and dimensions of pits after the test. The concentration dependence of corrosion rates for the five alloys has been studied for those species for which a significant effect was observed in the first test series. For Ferralium 255, Hastelloy G3, SS 317L, and Monit, an accelerating effect of Cu and Fe was observed when a certain concentration threshold was exceeded. For Ti, 1000 ppm fluoride additions produced etching of the surface at pH 1. A comparison with the results of the first test series shows that addition of Al at comparable concentrations can prevent this corrosion damage by complexing the fluoride ions. The effects of single ions added to a modified baseline solution at pH 1 that contained all other trace elements at a median level were further studied by recording potentiodynamic polarization curves after exposure times of 1, 3, and 6 days. For SS 317L, polarization curves are obtained in the baseline solution that are typical for an alloy susceptible to pitting and crevice corrosion. In the presence of 1000 ppm Cu or 2000 ppm Fe, this corrosion damage is increased and the polarization curves are those of highly corroded surfaces. For Ferralium 255, Hastelloy G3, and Monit, which are more corrosion resistant, the anodic polarization curves did not show an active/passive transition, but dissolution at a more or less constant rate. For Ti, the anodic polarization curves showed passive behavior with very low corrosion rates. The susceptibility to localized corrosion was studied further in the same solutions using a multiple crevice device. For SS 317L, severe crevice corrosion was observed in the baseline solution with additions of 1000 ppm Cu or Fe additions exceeding 300 ppm. For the other alloys except Ti, severe damage also occurred for additions of 2000 ppm Fe, but not for 1000 ppm Cu.