INTRODUCTION

ABSTRACT

A steel-hulled vessel was found to have suffered serious corrosion damage near one of its impressed current cathodic protection anodes. The damage was observed at breaks in the anode shield near a weld and in the cofferdam housing the anode. Underwater inspections revealed that all of the vessels in this class were experiencing similar problems. Several potential causes, such as the reversal of the power supply leads, were ruled out, as the cathodic protection system appeared to be functioning as designed. As a result, a theory was developed that involved an unusual corrosion mechanism that was dependent on the accumulation of alkaline by-products of the cathodic reaction, in conjunction with an overly “protective” potential. Laboratory simulations using potentiostatic techniques were performed in order to test this theory. Corrosion of the steel was observed within three days at a nominally “protective” potential.

In 1824, Sir Humphry Davy reported on his development of a cathodic protection system to prevent corrosion of the copper sheathing on the wooden hulls of ships of the Royal Navy1. Unfortunately, his design was so successful that it reduced the corrosion of the copper to a level low enough to eliminate its biocidal properties. As a result, the underwater hull became heavily fouled with marine organisms, to the extent that cathodic protection of ocean-going vessels fell out of favor and its potential value was ignored for over 100 years. In 1951, K. N. Barnard demonstrated the value of cathodic protection in preventing corrosion of steel-hulled vessels of the Royal Canadian Navy2. Variations of this technique have since been utilized by naval and commercial fleets throughout the world.

After some initial teething problems, such as the need to establish strict limits on the iron content of zinc anodes3, the use of galvanic anode or impressed current cathodic protection systems has substantially reduced or eliminated corrosion problems on the underwater hulls of ships operated by the Canadian Forces. However, a steel-hulled vessel was recently found to have suffered serious corrosion damage near one of its impressed current anodes after 27 months of immersion in seawater. The damage was observed at breaks in the epoxy-based anode shield near a weld (Figure 1) and in the cofferdam housing the anode (Figure 2). An underwater inspection of the hull indicated that the anode shield was soft and chalky in the vicinity of the anode. Underwater inspections of other vessels in the class indicated that all of them were experiencing similar corrosion problems at the same location near the impressed current anode (Figure 3).

FAILURE ANALYSIS

The most obvious explanation for the observed corrosion would involve the cathodic protection system having been hooked up backwards. In this situation, the “anode” would, in fact, be the cathode and the steel hull would actively corrode as the true anode at breaks in the coating system. Electrical inspection and hull potential measurements indicated that the cathodic protection system was properly wired.

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