The extraction of oil and gas from relatively abundant sour fields was triggered by the continuous growing need for fossil fuels. Sour (H₂S –containing) gas environments present a major problem of underdeposit corrosion (UDC) which is a type of accelerated localized corrosion observed under deposits (in this case typically iron-sulfides), creating a risk to asset integrity. The mechanism of UDC is still not fully understood and there is no consensus on the methods used to evaluate UDC in pipelines. In addition, the efficacy of inhibitors may vary in such a scenario and so the amounts and efficiencies required cannot be a priori decided. There is, therefore, an urgent need for a study that addresses the characteristics of deposits representative to the different conditions present in the real environment, and the use of such a protocol to provide kinetic and mechanistic information on UDC phenomena. 1-dimensional artificial pit experiments were performed where the artificial pit acts as an anode in a three-electrode electrochemical cell, simulating an actively dissolving pit. The dissolution kinetics were obtained as a function of system variables such as chloride concentration, pH and existence of deposit at the pit cavity opening. Data will be presented on the role of deposit chemistry and morphology on the electrochemical dissolution behaviour and discussed in terms of a transport-controlled model for pit propagation.

Keywords: Pitting corrosion; artificial pits; underdeposit corrosion; sour gas