The first stage in cavitation damage—indentation where cavitation bubbles collapse near the metal surface—was observed and measured by Knapp [1]. The author suggested that the high temperature of indentation points, caused by the absorption of work in their formation, should create conditions favorable to energetic local chemical action between water and metal. The brittle oxide films thus produced would be ruptured by later blows, resulting in erosion. Photomicrographs taken in monochrome and color show the form and texture of indentations with associated corrosive oxidation. Parallel experiments were made with toluene whose inertness permitted the elimination of chemical factors. The special features of the microscope technique employed to get the photographs are explained.

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