An extensive inspection campaign was executed in a West European refinery on circuits potentially subjected to high temperature low silicon carbon steel sulfidation (<0.1wt.%). This effort came on the top of the existing refinery sulfidation inspection plan for carbon steel piping. Ultrasonic testing (UT) measurements showed no excessive corrosion on any inspected components.
However, after these inspections were completed, a pinhole leak occurred on a weld in the Crude Distillation Unit (CDU) Heavy Gasoil (HGO) circuit. This circuit operates at 350°C (660°F), the HGO sulfur concentration ranges between 1.5 wt. % & 2.5 wt.%, and the Total Acid Number (TAN) is below 0.15 mg KOH/g. The affected piping spool was sent for metallurgical investigation, results indicating severe weld preferential corrosion whereas the surrounding base metals were not corroded.
Additional inspections were performed and results showed that 60% of this circuit circumferential welds were affected, with 30% below the retirement thickness and 30% that would have reached the retirement thickness before the next Turn-Around. Very different corrosion behavior was observed between adjacent welds, and even sometimes on the same weld, changing from no corrosion to heavy corrosion.
This paper describes the investigation that was performed to identify the corrosion mechanism, specifically trying to understand the influence of filler metal composition and welding parameters used on the corroded parts.
Key words: sulfidation, weld preferential corrosion, oxygen, silicon, carbon steel, welding process