Over the past few years, under deposit corrosion (UDC) has been frequently cited by operators, corrosion engineers, failure forensic analysts, asset integrity authorities and Root Cause Analysis specialists as being a major player in different failures within the oil and gas operations. It has also been identified sometimes as a stand alone corrosion mechanism, separate from CO2, H2S, microbiological influenced corrosion, crevice or galvanic corrosion. This paper presents an overview of UDC definitions, commonly used laboratory testing methods as they relate to the prevalent UDC mechanism and finally proven mitigation measures. It highlights the applications and limitations of each testing method and sets the framework for the standard and the novel mitigation routes. The paper finally addresses the question whether or not UDC is a true standalone corrosion mechanism.
Under deposit corrosion usually represents an important threat in the integrity matrix of oil and gas production and transportation facilities, and has frequently been cited as responsible for high corrosion rates and loss of containment during operations.1-5 In order to control this threat, development and implementation of effective prediction, monitoring and mitigation methods are required. Under deposit corrosion is a general term that refers to localized corrosion that develops beneath or around deposits present on a metal surface. There may not be one single corrosion mechanism suitable for explaining UDC that occurs in different processes or conditions. For example, UDC mechanism occurring under salts exposed to the atmosphere may be different to that occurring under iron sulfide deposits in a sour system. It is important to note that root cause analyses of failures in the oil and gas industry that have occurred under deposits have often been related to a microbiologically influenced corrosion (MIC) mechanism and usually the MIC and UDC threats are combined.
