Sacrificial anodes for protection of seawater pump caissons against galvanic corrosion

摘要

If unprotected, seawater lift pump caissons can suffer severe galvanic corrosion. Within 2-5years holes in the caisson can appear. The corrosion to the steel caissons is caused by thenoble pumps and risers, which normally are made of Ni-Al-bronze, and/or (super) duplexstainless steel. If the caisson is not protected by a coating, corrosion rates of 2-5 mm/yearare reported. For coated caissons, corrosion rates of more than 10 mm/year have beenseen. This extreme high rate is caused by galvanic current concentration in small coatingdefects.This article will describe how corrosion was stopped on coated caissons on a FPSO. Alsoinstallation of a cathodic protection system for relatively smaller pumps on two platforms inthe Northsea will be discussed. Intensive reference cell measurements confirmed that theposition of the sacrificial anodes is crucial. For this reason an effective way of hanging theanodes in the annulus of the pump and riser in the caisson was developed. Duplex andsuper duplex stainless steels should not be protected at levels more electronegative than -900 mV Ag/AgCl. This is achieved by a potential control unit on the sacrificial anodeassemblies.