Wear/corrosion sensing in flowing seawater

Abstract

Pipework for cooling and firefighting purposes on naval vessels is typically, subject to corrosion by pumped seawater. Close to land, erosion by ingested sand also occurs. The expensive alloys presently used to resist corrosion do not resist erosion very well. There are requirements both to understand conjoint erosion and corrosion better and to develop improved materials. This Thesis examines the possibilities for using metal-on-steel claddings for this duty, and also the possibilities for the use of Electrochemical Noise techniques to evaluate their performance.

Erosion failure of the metal claddings was found to be entirely ductile, though the angular dependency of this was not classical; this was probably associated with the relatively dense slurries used. Corrosion behaviour under static conditions was heavily influenced by the deposition of solid corrosion deposits, especially in pores in the cladding, and by the aeration of the corroding solution.

Flow corrosion and erosion-corrosion were examined in a newly built test rig, specially adapted for electrochemical measurements. Zinc claddings were found to suffer severely from flow corrosion; aluminium-based claddings do not. Neither metal performs very adequately under erosion-corrosion conditions; hardening by alloying or composite formation will be required in practical systems, as will be protection against pitting.

Simple erosion is non-Faradaic, so direct prediction of mass loss by electrochemical means is not possible where this process dominates. However, in brine pitting occurs and impacts cause depassivation/repassivation events. These can be monitored using Electrochemical Noise techniques. A new application of the wavelet transform allows separation of these effects, and a preliminary quantitative understanding of how the chloride inhibition of impact scar repassivation can result in increased corrosion.

Proposals are made as to how this work may be further developed, in terms of more realistic materials, improved experimental technique and better data analysis.

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