Statistical Assessment of the Risk of the Accelerated Low-Water Corrosion in the Marine Environment

Abstract With increasing awareness of accelerated corrosion at or just below the low-water mark of steel piling structures in European harbors a multinational project was sponsored by the European Coal and Steel Community to determine the involvement of biological and physico-chemical factors in this process and to establish possible protective measures against this form of attack. Site assessments of European harbors were performed combining screening for biological, environmental, chemical and physical parameters. The data collected were statistically analysed and significant parameters, differentiating between accelerated and normal low-water corrosion sites, were identified and carefully screened for their possible role in the accelerated low-water corrosion process. Bacterial groups, known to be involved in microbiologically-influenced corrosion were detected in corrosion products recovered from steel piling in harbours across Europe. Most probable number enumeration of culturable sulfate-reducing bacteria, sulfur-oxidising bacteria and chemoorganotrophic bacteria were conducted but could not reveal difference between accelerated and normal low-water corrosion sites, with the exception of sulfur-oxidizing bacteria, which were detected in higher levels at the affected sites. Other parameters identified to vary between accelerated and normal low-water corrosion harbours were the mean tidal range, the thickness, homogeneity and organic content of corrosion products, the pH underneath corrosion products, the redox potential of seawater, the presence of algae and invertebrates, and the total organic carbon content of seawater. Based on these results, a statistical preliminary risk assessment model was developed. Combining the results of field and laboratory studies, the cause of accelerated low-water corrosion could be identified as microbiologically-influenced. The results presented in this paper are implemented in the final report to the European Commission.