September 2004

Piping/Reliability

Preferential corrosion of welds in HF service

A high content of residual elements was the suspected cause

Mayorga, A., Munsterman, T., Capstone Engineering Services, Inc

A failure was detected during the shutdown of an HF alkylation unit, when a leak was discovered after chemical cleaning of the pipe. The perforation occurred at a weld joining a 90° elbow with a straight pipe section. The failed section was removed for replacement. Internal visual inspection revealed preferential corrosion at the weld. Steel samples were taken from several places including the welds and the base metal on both sides of the weld and sent for chemical analyses. The objective was to determine the level of residual elements, in particular Cu, Ni and Cr, that are said to reduce the HF acid corrosion resistance of carbon steel. The weld that was corroded through thicknes

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