Corrosion testing of carbon steel-determination of H2S partial pressure from dissolved concentration of corrosion products

摘要

When carbon steel is exposed to CO_2 containing small amounts of H_2S, both lab and field experiences show that a multi layered mixed corrosion product film forms on the steel surface. The fraction of FeS vs. FeCO_3 is determined by the balance between the corrosion rate and the availability of H_2S, i.e. how fast H_2S is replenished when it is consumed due to corrosion and FeS precipitation. In experiments where the test liquid is bubbled with CO_2 containing a predefined partial pressure of H_2S, the actual amount of dissolved H_2S can be substantially lower (> one order of magnitude) than that corresponding to equilibrium with a predefined gas mixture. The precipitation rate is generally low for FeCO_3 and a high SR_(FeCO3) is necessary in order to form FeCO_3 particles and/or deposits on the steel surface or other surfaces. The precipitation rate of FeS is much higher and experiments have indicated that precipitation of FeS is essentially instantaneous. Assuming that the SR of FeS is close to 1 gives a direct relationship between the dissolved amount of corrosion products and the partial pressure of H_2S. Based on the measurements of dissolved Fe~(2+) it is possible to calculate the actual H_2S partial pressure in the corrosive liquid. The paper presents and discusses the relationship between Fe2+ concentration and the partial pressure of H2S, how this realationship can be applied to monitor/estimate small amounts of H2S and results from experiments that verify the Fe~(2+)-H_2S relationship.