Only the Fittings Corroded while the Straight Pipe did not Corrode. Why?

Only the fittings in the intermittent service steel piping developed pits, and the straight pipe sections were unblemished. Why? The fittings had lower “residual elements” than the pipe. Seemingly minimal compositional differences can result in the preferential attack of steel components in services with acids – including in somewhat acidified water, like the pitted line shown. In some lines, the parts with lower residual elements are attacked, while in other lines, the parts with higher residual elements suffer damage; this is process dependent.

These compositional differences can have weighty consequences, such as the Philadelphia 2019 refinery explosion, (Philadelphia Energy Solutions (PES) Refinery Fire and Explosions | CSB). The failure was initiated because some joined parts had unsuitable residual elements for their service conditions – by a fraction of a weight percent. For example, API RP 751 “Safe Operation of Hydrofluoric Acid Alkylation Units” can limit the addition of % of copper and % of nickel to no more than 0.15 weight %.

New and replacement equipment going into HF service is subjected to Optical Emission Spectroscopy (OES, see Field Spectroscopy – Spark Testing | IRISNDT) and Laser Induced Breakdown Spectroscopy (LIBS) to prevent preferential corrosion failures. IRISNDT performs these examinations.

On the left, pitted flange with lower residual elements than the adjoining pipe. On the right, pitted elbow with lower residual elements than adjoining pipe.

#PMI #NDT #NDE #LIBS #OES #alkylation #IRISNDT