Improving the understanding of concrete sewer corrosion through investigations of the gaseous hydrogen sulfide uptake and transformation processes in the corrosion layer

Abstract

Sulfide-induced corrosion of sewer infrastructure costs multi-billion dollars globally. Sewer maintenance and management largely depends on the estimation of the sewer corrosion rate, which currently can be very rough due to poor understanding of details of the process. Hence improved understanding of the corrosion processes has a major beneficial impact globally through increased service life and reduced repair/replacement costs. The overall aim of this thesis is to improve the understanding of sewer corrosion processes and develop strategies for corrosion control. To rapidly and non-destructively monitor the corrosion process, a methodology was developed based on measuring the H2S (sulfide) uptake rates (SUR) of concrete. The SUR of concrete coupons was determined by monitoring the temporal gaseous H2S concentrations in a temperature- and humidity-controlled gas-tight reactor. The results from repeated tests showed good reliability of the method. A severely corroded coupon exhibited higher SUR than a less corroded coupon, suggesting that the former having a higher sulfide oxidation activity than the latter. Additionally, temperature changes had a stronger effect on the SUR of the heavily corroded coupon compared to the less corroded coupon. The corrosion rates estimated from the SUR agreed well with those observed in real sewers under similar conditions. The effect high H2S concentrations on the SUR of coupon were investigated. During the high load situation, the SUR increased significantly but then decreased (compared to the baseline SUR) by about 7 – 14% and 41 – 50% immediately after short- and long-term high H2S load periods, respectively. For both conditions, the SUR gradually (over several hours) recovered to approximately 90% of the baseline SUR. Further tests suggest multiple factors may contribute to the observed decrease of SUR directly after the high H2S load. This includes the temporary storage of elemental sulfur in the corrosion layer and inhibition of sulfide oxidizing bacteria due to high H2S level and temporary acid surge. The sensitivity of the coupon SUR towards high H2S loads was largely dependent on its historical H2S exposure levels. The deprivation of gaseous H2S for 1 h consistently caused temporary increase of the H2S uptake rate (SUR) immediately upon H2S re-supply whereas deprivation of both gaseous H2S and O2 for 1 h posed little increase of the SUR after re-supply. The results suggest that the H2S uptake process could be limited by the oxidation of reduced sulfur species. Furthermore, the SUR decreased by 1.2% after deprivation of H2S for a relative long-term (i.e. 12 h), suggesting a reduced biological activity after the extended “starvation”. The details of sulfide oxidation by comparing oxidation activity of a suspended solution of a corrosion layer with gaseous H2S uptake activity of a coupon were investigated. With sufficient dissolved oxygen (DO) and in the presence of sulfide, the mole ratio of consumed DO to consumed sulfide was 1.24 ± 0.35, indicating the formation of intermediate products during sulfide oxidation. These intermediate products were only fully oxidized to sulfate after complete depletion of sulfide. The microbial communities of the corrosion product prior to and after incubation in the solution were in high similarity, suggesting that the sulfide oxidizing activities detected in the solution could represent those in the corrosion layer. In addition, the sulfide oxidation activity strongly associated with the DO levels. Chemical oxidation of sulfide was found non-negligible, this was likely catalysed by metals existing in the corrosion layer. A strategy to reduce biological oxidation of sulfide through treating the corrosion biofilm with free nitrous acid (FNA, i.e. HNO2) was developed. The SUR of coupons sprayed by nitrite were reduced by 84% - 92% 15 days after the spray. The little recovery of the SUR during the experimental period indicates the long-term (up to 12 months) effectiveness of the spray in controlling the corrosion activity. The bactericidal effect of FNA on the microorganisms in the biofilms were demonstrated by the decrease of viable cells from a coupon by > 80% 39 h after the nitrite spray and the severe decrease of the biological activity (ATP level and ratio of viable bacterial cells) of a corrosion layer within a suspended solution after the treatment. The potential of mitigating sewer corrosion by surface washing was investigated systematically. Washing interrupted the corrosion activity of coupons by increasing the surface pH and decreasing the SUR. The SUR recovered to pre-washing level within 60-140 days. The slowest recovery rate was from the most severely corroded coupon. However, no significant difference was observed for concrete loss of the washed and unwashed coupons after 54 months. The results suggest that frequent washing at short intervals of a few months might be needed to control corrosion over a long term.