In this work, six common stainless steel grades are compared with respect to corrosion performance and tendency to biofouling in a brackish sea water in a pilot-scale cooling water circuit. Two pilot-scale tests were performed, each employing three test materials, until differences between the stainless steel grades were detected. Open circuit potential (OCP) measurements were carried out in-situ. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) measurements were conducted before and after the pilot-scale tests, and the exposed surfaces were further subjected to quantitative polymerase chain reaction (qPCR), microbial community analysis with Next generation sequencing (NGS) and examinations by scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS).