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 2-5 years holes in the caisson can appear. The corrosion to the steel caissons is caused by the noble pumps and risers, which normally are made of Ni-Al-bronze, and/or (super) duplex stainless steel. If the caisson is not protected by a coating, corrosion rates of 2-5 mm/year are reported. For coated caissons, corrosion rates of more than 10 mm/year have been seen. This extreme high rate is caused by galvanic current concentration in small coating defects. This article will describe how corrosion was stopped on coated caissons on a FPSO. Also installation of a cathodic protection system for relatively smaller pumps on two platforms in the Northsea will be discussed. Intensive reference cell measurements confirmed that the position of the sacrificial anodes is crucial. For this reason an effective way of hanging the anodes in the annulus of the pump and riser in the caisson was developed. Duplex and super 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 anode assemblies.