The Impact of Sulfate-Reducing Bacteria on Welded Copper-Nickel Seawater Piping Systems

Abstract

Abstract Field and laboratory experiments were designed to evaluate corrosion in welds and heat-affected zones in Cu/Ni seawater piping systems exposed to estuarine water and seawater. The localized corrosion was shown to be due to the following sequence of events: (1) metal segregation during heating processes; (2) settlement of sulfide-producing bacteria in welds; (3) sulfide derivatization; (4) disruption of the surface films by turbulence and (4) formation of adjacent cathodic and anodic areas. Metallic segregation in welded areas was detected by EDAX before the welds were exposed to estuarine water. Within the crevices, occlusions and surface irregularities of butt and socket welds, there were unique environments of microbial colonization. The sulfate-reducing bacteria (SRB) population in the weld areas was 1 × 104 cm-2. In the presence of in situ production of sulfides, copper and nickel-enriched areas were susceptible to sulfide derivatization; however, surface deposits on the nickel-enriched areas were easily removed by turbulence. The sulfide-coated, copper-enriched areas were cathodic relative to the adjacent nickel-enriched areas, resulting in selective attack. FeSO4 and Na2SO3 were evaluated as inhibitors of the localized attack. Fe++ (50 ppb) did not alter the attack of sulfides, but SO3-- (10 ppm) did lessen the observed attack.