This paper seeks to determine the causes and mechanism of failure of stainless steel piping in a sulphur recovery unit of a gas‐processing plant and to recommend suitable measures to avoid recurrences.
The integrity of the material of construction was verified using various laboratory and analytical techniques. Standard metallographic techniques were used to prepare representative samples obtained from failed stainless steel piping for metallurgical evaluation. Microstructural characterization was carried out in an inverted metallurgical microscope equipped with an imaging facility. Elemental analysis and hardness were used to confirm the identification of the material. Corrosion product/deposits were analyzed using wet chemistry supported by X‐ray diffraction analysis. Microbes were enumerated through standard methods.
The piping failed due to severe pitting corrosion, which resulted in the formation of holes. Microbial‐induced corrosion (MIC) and under‐deposit corrosion were chief contributing factors that caused the failure in stainless steel piping. In addition, the HAZ near welds at some places was found to be sensitized causing accelerated pitting corrosion at these sites.
The expected service life of stainless steel piping could not be realized as the piping failed due to the combined effect of MIC and under‐deposit corrosion. Periodic monitoring of sulphate‐reducing bacteria and sulphur‐oxidizing bacteria coupled with implementation of an effective biocide treatment programme in process fluid was recommended, together with the introduction of a procedure for frequent cleaning of the pipe walls to minimize under‐deposit attack.
The paper – a technical case study of process industry – provides an account of failure investigation. It identifies the causes and mechanism of failure and suggests suitable preventive and corrective measures. This is useful industrial experience that provides valuable information for process and plant corrosion engineers involved in the operation of this type of equipment.
