Effect of Amplitude on Erosion-Corrosion of Metals by Vibratory Cavitation in Relation to pH of the Cavitation Media

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Abstract
Erosion-corrosion tests of 0.35% carbon steel, 99.86% aluminum, and 99.84% copper were conducted in aqueous solutions with controlled pH values in the range of 2 to 12 at a frequency of 6.5 kHz with a broad range of double amplitudes. Amplitudes used were in the range of 5 to 85 µm for 0.35% carbon steel; 5 to 40 µm for 99.86% aluminum; and 5 to 55 µm for 99.84% copper. Plots of weight loss by erosion-corrosion against pH indicated, as a whole, that the corrosion factor plays an important role in the mechanism of erosion-corrosion in all metals. Relationships between the logs of the weight loss and the amplitude for all metals revealed few regions in which the nth power law between the weight loss and the amplitude are satisfied. Furthermore, an upper threshold amplitude, above which the corrosion factor becomes ineffective, as well as a lower threshold amplitude, below which the erosion factor becomes ineffective, were observed in all cases irrespective of the cavitation medias pH. Between the two threshold amplitudes, an interplay of erosion and corrosion factors took place in such a manner as to increase the erosive action with an increase in amplitude. The lower threshold amplitude will be a true breakaway amplitude, wherein some areas of the protective films are eroded through. The effect of the corrosive factor is summarized in this order: carbon steel > aluminum > copper.

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