Influence of Marine Microfouling on the Corrosion Behaviour of Passive Materials and Copper Alloys

摘要

The influence of marine biofilms on corrosion varies with alloy composition. At temperatures below 60 C, resistance to crevice corrosion is the limiting factor for selecting alloys for seawater service and is the most problematic issue affecting the performance of passive alloys in seawater. Several investigators have documented the tendency for biofilms to cause a noble shift, or an ennoblement, in open-circuit potential (OCP) of passive alloys exposed in marine environments. Ennoblement in marine waters has been ascribed to depolarization of the oxygen reduction reaction due to organometallic catalysis, acidification of the electrode surface, the combined effects of elevated H2O2 and decreased pH and the production of passivating siderophores. The alloys tested include, but are not limited to: UNS S30400, S30403, S31600, S31603, S31703, S31803, N08904, N08367, S44660, S20910, S44735, N10276, N06625, platinum, gold, palladium, chromium, titanium, and nickel. Theoretically, potential ennoblement should increase the probability for pitting and crevice corrosion initiation and propagation, especially for alloys with pitting potentials within 300 mV of the OCP. The relationship between passive alloy composition and ennoblement will be discussed. The well-known toxicity of cuprous ions toward living organisms does not mean that the copper- based alloys are immune to microbial colonization and microbiologically influenced corrosion (MIC). It does mean, however, that only those organisms with a high tolerance for copper are likely to have a substantial effect. Most of the reported cases of MIC of copper alloys in marine environments are not related to ennoblement of OCP, but are caused by the reduction of sulfate (concentration > 2 gm/L) to hydrogen sulfide.