Influence of pitting corrosion on strength of steel ships and offshore structures

Abstract

The present paper deals with the influence of pitting corrosion on mechanical properties of mild and low alloy steels and strength of steel structures under static and quasi-static loads. Pitting corrosion is a very important phenomenon that influence local strength of ship hull members. The present paper is based mainly on Japanese publications. Analyzing the standard tensile diagrams one can see that that pitting corrosion reduces the load corresponding to yield stress (YS) and, even more markedly, tensile strength UTS, reduces almost to zero plastic flow strains at YS load level and dramatically reduces the total elongation to fracture. Nominal UTS load for uniformly corroded specimens is higher than that for pitted specimens at the same average thickness loss. The strength related to the true fracture surface area (i.e. reduced by pits, taking into account real path of the crack through the pits) is almost independent of the thickness loss and is almost the same as for uniform thickness loss. For wide specimens the tensile strength depends mainly on the local deformation ability, and the maximum loading ability for large members, predicted on the base of the total true fracture surface area, can be overestimated. Concept of equivalent thickness loss for plates under bending and compression and for more complex structural models as well as relation between the average and the equivalent thickness losses has been presented. Approach based on degree of pitting (ratio of pitted area to total plate surface area) has been shown as a real and more convenient than the approach based on the thickness loss.