Effects of magnetite-seawater slurry electrolyte on sacrificial anode performance, acidification, and corrosion rate (C2026-00215)

Matthew Taylor

Some offshore structures use seawater-magnetite slurries for ballast, which may result in long-term corrosion risk. A laboratory study simulating the interior environment such ballast tanks. pH evolution, anode performance, electrochemical potential, cathode/anode current exchange, and carbon steel corrosion rate were observed. Cases include 100% seawater electrolyte, 50% seawater/50%slurry and 100% seawater-magnetite fill. For each electrolyte case, Al-Zn-In or Zinc anodes were included, either fully immersed, or protruding from the magnetite into the seawater phase for the 50% case. All measurements were carried out at ambient air-conditioned temperature and pressure. A check valve was included to prevent for corrosion-caused hydrogen gas. Multiple carbon steel corrosion coupons (welded, unwelded, CP isolated or continuous) were included in each test fixture. pH measurements were carried out at three elevations and a Silver/Silver Chloride (seawater) reference electrode was mounted at the top of the chamber.
The study found that the slurry impeded H+ diffusion away from the anodes, causing lower pH than previously observed from aluminum acidification. Performance of zinc anodes was better than aluminum . No evidence of corrosion differences were observed between welded and unwelded samples, with the major driver of corrosion rate identified as pH and isolation from cathodic protection.