Study Explores Effects of H2S Scavenger Byproducts on Carbon Steel
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Triazines are commonly used as hydrogen sulfide (H2S) scavengers. However, stress corrosion cracking (SCC) has been reported recently when using certain triazines, making it necessary to understand the failure mechanism to control this phenomenon. In the complete paper, the authors seek to understand the effect of triazine on the electrochemical response of a carbon-steel surface in a mixed-gas system.
One of the more frequently used H2S scavenger triazines is hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine, or monoethanol amine (MEA)-triazine. It is a yellow, viscous liquid that can be injected into gas streams or used in a contactor tower. It removes H2S from the gas stream. It is relatively inexpensive, easy to use, and effective. The MEA byproducts that form during the scavenging process, however, may cause SCC of steel pipes, as reflected in the literature.
On the basis of these findings, it is suggested that SCC is governed by corrosion processes and specifically by the transition from a passive to an active surface. This transition is a complicated function of acid gas concentration [carbon dioxide (CO2) and H2S] and amine adsorption, which are likely different for different systems. Several possible mechanisms exist by which cracking can be induced. First, a reduction in H2S content can reduce the semiprotective iron-sulfide film, thus changing the nature of the active/passive transition and making cracking more likely. Second, specific adsorption of amines onto the surface of the steel will result in a change in the active/passive transition. Therefore, characterization of the MEA/H2S reaction process, the adsorption of amines onto steel, and the electrochemical behavior of the steel under these circumstances are important factors that require study. In this paper, the electrochemistry of API X601 steel is assessed under a variety of test solutions containing H2S, CO2, and MEA triazine.
The specimens were 0.25 in. in diameter and 4 in. long. Approximately 3 in. of the specimen length was exposed to the solution. All electrochemical tests were conducted at room temperature and atmospheric pressure. Three gas mixtures were employed: 20-ppm H2S balanced with dinitrogen (N2), 20-ppm H2S/2 mol% CO2, and 100% N2. In addition, three triazine-inhibitor (35% triazine concentration solution) volumetric additions were used. The actual inhibitor concentrations in solution were not determined; instead, the H2S concentration in the gas phase was measured with a sensor after the addition of the inhibitor. The supporting electrolyte [0.1-M potassium chloride (KCl)] solution was deaerated N2 gas.
Before transfer of the KCl solution to the test cell, the solution was saturated with the target gas for 48 hours following standard operating practices. Specimens were placed into the cell, and the cell was sealed (Fig. 1). Before the start of the test, the specimens in the test cell were kept oxygen-free by flowing N2 gas. Triazine inhibitor was added within 10 minutes after the specimens were exposed to the sour gas. Open circuit potential (OCP) was recorded throughout a 24-hour exposure period. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) testing were performed at 24 hours of exposure. EIS tests were conducted at the OCP, using 10-mV AC amplitude with a frequency range of 100 kHz to 10 mHz. After the conclusion of the EIS tests, all CV tests were performed with a potential sweep rate of 100 mV/sec, which provided distinction between the different anodic and cathodic peaks associated with the different speciation reactions. For all CV scans, the potential was swept from OCP to –1 V (vs. the reference potential) and to 0.2 V (vs. the reference potential) in multiple cycles, returning to the OCP. The CV scans were conducted multiple times over a 48-hour period following immersion.
EIS and CV scans were performed to identify key differences in the surface electrochemistry in systems with and without triazine. Pourbaix diagrams were generated to help explain electrochemical data.
During the experiments described in this work, the specimens were first exposed to the H2S and H2S+CO2 environments for at least 3 hours before the triazine injection. Under these conditions, film formation is likely to have occurred before the introduction of triazine; therefore, experiments did not specifically measure electrochemical changes resulting from formation processes in the presence of triazine. However, given the cathodic sweep of the CV scan, some degree of surface film reduction likely occurs such that, upon sweeping the potential in the anodic direction, some indications of anodic formation processes occur. Further, because the sweep rate is rapid, the measured current does not represent steady-state conditions.
The following categories of results were obtained and are described in detail in several pages of the complete paper, including supporting figures and tables:
- Cyclic voltammetry results
- Effect of CO2 addition
- Effect of triazine addition with no CO2
- Effect of triazine additions with CO2 (Fig. 2)
- Electrochemical impedance analysis
- OCP analysis
Electrochemical testing, including EIS and CV, was performed to provide insight related to the mechanisms involved with SCC observed in slightly sour pipelines where triazine scavengers are used. The following conclusions can be made:
- Cyclic voltammetry was used to identify specific oxidation events associated with iron sulfide formation. However, extreme care must be used in the interpretation because of apparent double layer charging that confounds a critical interpretation of the data.
- Cyclic voltammetry was used to show that increases in triazine additions resulted in a reduction in the formation kinetics of FeO(OH). For the highest triazine additions, the kinetics were reduced enough to provide evidence of the presence of Fe3O4.
- EIS showed that, for all levels of triazine, solutions containing 2% CO2 had lower charge transfer resistance (Rct) values and that triazine additions had a much stronger effect of Rct for these solutions. While pH plays a strong role in affecting Rct for systems with 20-ppm H2S, other factors contribute to Rct for systems to which CO2 is added.
- Several potential mechanisms by which triazine may affect SCC are presented. However, no conclusive data from this study exists to validate them and more work is needed.
Study Explores Effects of H2S Scavenger Byproducts on Carbon Steel
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