Influence of extracellular polymeric substances on electrochemical behaviours of stainless steels in circulating cooling water

Stainless steel alloys, often used in circulating cooling systems of power plants using reclaimed water as coolant, can be affected by biocorrosion induced by biofilm formation. Extracellular polymeric substances (EPS) produced by bacteria play an important role in the different stages of biofilm formation, maturation and maintenance. The influence of loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), isolated from Citrobacter freundii, on the adsorption properties and electrochemical behaviours of stainless steels 304, 316 L and 317 (SS304, SS316 L and SS317) in circulating cooling water, were investigated by evaluating the adhesion forces of SSs (AFM), surface analysis (SEM, XPS) and the effect of EPS on SSs (electrochemical measurements, ICP-MS). The results show that the LB-EPS and TB-EPS of the solutions influence the surface adsorption of stainless steels (SSs). The adhesion forces of SSs in TB-EPS are higher than those of LB-EPS. A statistical difference in the adhesion forces of SSs was detected, with higher adhesion force of SS317, followed by SS316L and finally by SS304. The adhesion forces to SSs are influenced by both the electrostatic force and metal surface hydrophobicity. LB-EPS and TB-EPS can inhibit the dissolution of metal elements on SS surfaces in circulating cooling water. The dissolution rate of each metal element on the surface of SS304, SS316 L and SS317 is different, and their dissolution trends are basically the same: Fe > Ni > Cr > Mn. In the presence of TB-EPS or LB-EPS, the dissolution rate of Fe on SS304, SS316L and SS317 decreases by 30.61%, 24.20% and 27.43% or 77.95%, 86.70% and 83.96%, respectively. Electrochemistry, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) indicate that the formation of a protective film composed of organic complexes such as C-(Fe, Ni), C–O-(Fe, Ni), N-(Fe, Ni), N–O-(Ni, Fe) and FeOOH on the SS surfaces can slow down the corrosion rate of stainless steel to some extent. Moreover, EPS films formed on SS317 and SS316 L have higher resistance and lower conductivity than those formed on SS304. [Display omitted] •The surface adsorption of SSs increases in the presence of LB-EPS/TB-EPS.•Metal dissolution rate on SS surfaces is basically the same: Fe > Ni > Cr > Mn.•The adhesion forces of SS317 and SS316L are greater than that of SS304.•LB-EPS had more significant inhibition on the dissolution of metal surface elements.•EPS films formed on SS317 and SS316L have higher