THE EFFECT OF PRECORROSION ON THE CORROSION RATE OF STEEL IN NEUTRAL SOLUTIONS CONTAINING SULFIDE - RELEVANCE TO MICROBIALLY INFLUENCED CORROSION

The effects of pre-existing sulphide or oxide corrosion products were examined in corrosion tests of low-carbon steel at various sulphide concentrations (0.6 to 15 mM) in de-oxygenated, neutral, buffered NaCl solutions. For fresh steel surfaces, the lowest polarization resistances of ca 1400 ohm cm2 were recorded at low sulphide concentration, with evidence for enhancement of the cathodic reaction by the porous FeS corrosion product. High sulphide concentrations produced protective FeS films and an essentially passive state (R(p) > 50,000 ohm cm2). However, if the surface was first corroded at a low sulphide concentration, then transferred to a high sulphide concentration, the polarization resistance did not increase to the passive value but continued to decrease, reaching values as low as 600 ohm cm2. Low R(p) values were also measured on specimens precorroded in oxygenated NaCl solution and transferred to high-sulphide solutions. The results are interpreted in terms of the surface concentration of sulphide, which is decreased (for a given metal dissolution rate) whenever the effective diffusion layer thickness is increased by the formation of a porous surface film. In order to interpret corrosion data on iron or steel in cultures of sulphate-reducing bacteria, it is necessary to take into account the time-varying concentration of free sulphide and the suppression of transport by iron sulphide deposits, bacterial cells or extracellular organic material.