The results of exposure of 304 stainless steel and grade 2 titanium to Tuticorin harbour waters are presented. With coupons immersed under low‐light conditions, microfouling caused the polarisation of free corrosion potentials to nobler values (ennoblement) during the early stages and toward more active values after about 10 d. Under natural conditions, on the other hand, the ennoblement of corrosion potentials occurred more consistently in the presence of sunlight. This effect was particularly marked during the first 10 to 12 d but became much reduced upon further exposure. Potential excursion data are complemented by test results on biofilm succession and characterization. It is suggested that the ennoblement process may not be an effect of low pH, but rather linked to neutral biofilm conditions.