The corrosion rate of structural steels in the hostile environments of the coastal, harbour or ocean zones effects the economic interest of offshore structures since both the loss of steel and pitting may have significant impacts on structural safety and performance. With the increasing emphasis to maintain existing structures in service for longer periods of time and hence to defer replacement costs, there is increasing interest in predicting corrosion rate at a given location for a given period of exposure once the protection (coating or cathodic protection) is lost. A more accurate approach for engineering purposes is to develop predictive corrosion allowance based on corrosion science, marine microbiology and good quality field data. The immersion depth, salinity, steel composition and water pollution will be taken into account to obtain useful data. The underlying approach is probabilistic because the input is not known precisely and may be natural phenomena such as weather conditions. © 2011 IEEE.