Evolution over time of mackinawite generated on carbon steel by the SRB metabolic activity: an in-operando Raman study

Abstract

Mackinawite was biologically synthetized by immersing carbon steel coupons in artificial seawater containing Sulphate-Reducing Bacteria (SRB) for 6 months. These coupons were removed from the culture medium solution and some were stored in air while others were placed in SRB-free culture medium solution. In operando Raman spectroscopy was used to analyse all these coupons immediately after extraction from the incubation medium and nanocrystalline, well-crystallized and partially oxidized mackinawite was detected. The evolution of these corrosion products was also monitored as a function of ageing time with this technique. Nanocrystalline and well-crystallised mackinawite transformed into partially oxidised mackinawite, greigite and sulphate green rust for an ageing time between 4 and 72 h. After 120 h, maghemite, magnetite, lepidocrocite, goethite appeared on the coupons placed in SRB-free culture medium solution as opposed to those stored in air atmosphere. Greigite and sulphate green rust were not observed for Raman measurements performed in air.