Under-deposit corrosion: Investigating dissolution kinetics using artificial pits

Abstract

Sour (H2S-containing) environments present a major problem for oil and gas industries: of particular concern is 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 not fully understood and there is no consensus on the methods used to evaluate UDC in pipelines. In this work, both planar electrode and 1-dimensional (1D) artificial pit experiments were performed (where the artificial pit acts as an anode in a three-electrode electrochemical cell, simulating an actively dissolving pit). In the 1D case the dissolution kinetics can be measured directly as a function of system variables such as chloride ion concentration, pH values and the existence of different types of deposit at the pit cavity opening. The effects of deposit chemistry and morphology on the electrochemical dissolution behavior are discussed in terms of a transport-controlled model for pit propagation.