Elemental sulfur is often produced by sour gas wells, primarily due to the oxidation of hydrogen sulfide. This may occur as a result of oxygen contamination. Alternatively, with pressure and temperature changes in the line, sulfur may precipitate and deposit on the pipeline walls. In the presence of water, contact of solid sulfur with mild steel can result in the onset of catastrophic corrosion processes. The objective of this project is to study the effect of elemental sulfur on the corrosion of mild steel. In preliminary experiments, elemental sulfur hydrolysis and sulfur-mild steel corrosion were investigated. Sulfur was found to react with water at temperatures of greater than 80°C, resulting in significant acidification of the corrosion solution. It was also found that elemental sulfur can cause localized corrosion of mild steel when they are in direct contact. Iron sulfide films/scales readily formed on the surface of the steel; these were analyzed by infinite focus microscopy (IFM), scanning electron microscopy (SEM) and X-ray diffraction (XRD).
Elemental sulfur can readily form in aqueous systems via the oxidation of sulfide species.1 Possible reactions for the formation of elemental sulfur (S8) could involve high oxidation state metals (denoted Mn+) or oxygen:
(chemical equation available in full paper)
In addition to strictly chemical processes, sulfide oxidizing bacteria have been shown to form liquid sulfur droplets under ambient conditions.2 Consequently, the formation of elemental sulfur is almost inevitable in H2S corrosion environments in the field. Elemental sulfur is shown as S8 in the reaction equations below as that represents the stable allotrope of sulfur under standard conditions. Acidification in sulfur containing aqueous systems has been reported, with species such as H2S, H2SO2, H2SO3, H2SO4 and polysulfides being possible candidates as hydrolysis products.3 Maldonado- Zagal and Boden4 proposed that acid formation as a result of sulfur hydrolysis was the main factor governing corrosion in the presence of elemental sulfur:
(chemical equation available in full paper)
Alternatively, MacDonald et al. hypothesized that an electrochemical reaction between iron and polysulfide is the driving force for corrosion in systems where elemental sulfur is present.5,6
(chemical equation available in full paper)
The objective of this project is to study the effect of elemental sulfur on the corrosion of mild steel, with particular emphasis on detection of pH effects, identification of formed corrosion products, study of pitting phenomena and measurement of general/localized corrosion rates.
Experiments were conducted to determine temperature effects on the acidification of pure water with elemental sulfur. This involved adding elemental sulfur to deoxygenated deionized water purged with nitrogen and measuring the pH of each test system with time. The chosen temperature range was 25-150°C. Experiments were conducted in both glass cells and autoclaves. For the corrosion experiments, elemental sulfur (ACROS 99.999%) was deposited onto polished coupons by heating it slightly above its melting-point (115°C) then pouring it onto the coupon surface (C1018 steel). This gave uniform coverage of adherent sulfur to the coupon surface. Care was taken not to oxidize the sulfur as it was heated.
