Effect of marine biofilm on fatigue resistance of an austenitic stainless steel

The marine environment plays a dominant role in the deterioration of marine structures. Offshore structures are susceptible to severe damage when two effects - the corrosive degradation by seawater and the periodic loading from wave action - are combined. Marine microorganisms are associated with the localized attack on the surface of engineering structures. Microbiologically induced corrosion pits behave as stress concentration areas where cracks nucleate and propagate. Therefore, the synergistic effect between wave loading, corrosive environment and microorganism activity increases the risk of structure failure. This work presents the effect of a marine biofilm on the mechanical performance of a stainless steel (SS) undergoing cyclic loading in seawater. A biofilm on the surface of the steel samples was formed during the first days of immersion, which promoted localized attack after two days of experiments. The pitting density increased drastically after two weeks, reducing the mechanical resistance of SS. A larger biofilm colonization on the specimens' surface was observed during the test performed under cyclic loading at a maximum stress (σ_MAX) of 140 MPa when compared with that formed on samples subjected to a σ_MAX = 180 MPa. This indicates that a lower σ_MAX increases the risk of pitting and cracking damage. After 15 days of testing, the area of maximum load showed damage caused by pitting corrosion and small intergranular cracks.