Galvanic interactions during erosion corrosion

The impinging flow of an aqueous fluid, with or without the presence of suspended solids, can result in the deterioration process of erosion corrosion. This occurs by extremely complex mechanisms, which include pure mechanical erosion, pure corrosion and synergistic interactions between the mechanical and electrochemical components. These contributors to erosion corrosion are generally recognized, measured and discussed in the literature. This paper considers an additional complexity that has received very little attention hitherto; namely galvanic interactions between the different regions of a single component subject to an impinging-liquid flow. These galvanic processes arise from different environmental conditions experienced by a particular material. The basis of the paper is a series of experimental studies that have been undertaken on the erosion corrosion of a range of engineering materials: stainless steels, copper-nickel alloys, titanium, and cermet coatings. The erosion-corrosion behaviour of these materials has been studied in high-velocity saline solutions, with and without suspended sand particles. The range of materials and environmental conditions investigated represents a significant spectrum of performance and this has facilitated a comprehensive examination of galvanic interactions between the directly-impinged and surrounding zones of samples. It has been observed that the magnitude and type of galvanic interaction varies widely and this has facilitated an assessment of the circumstances in which the galvanic processes can represent a significant contribution to the overall degradation process together with other instances when such interactions are of a minor nature. The paper contains a detailed discussion of the galvanic mechanisms and the methods of interpretation, in terms of the overall erosion-corrosion material loss, of the corrosion rate data from different types of experimental set-up