Stability criteria for buckling of thin anodic films on aluminum

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Striae are defects which are sometimes observed in anodic films. A special arrangement of the anodization cell was built in order to obtain a symmetric stress distribution resulting in a regular arrangement of striae along the outer perimeter of the oxide layer. This mechanical instability was analysed using Euler's method applied to thin cylindrical shells under compressive forces. By comparing the Euler critical stress value with the stress present owing to metal-metallic oxide volume expansion it is possible to obtain a measurement of the coating plastification energy or its ability to withstand or relieve the stresses which may arise during growth. The ratio between the aluminum oxide-aluminum work Δγ of adhesion per unit length and the bulk modulus E of the film-forming aluminum (hydrous) oxide was determined for coatings formed under various current densities; the ratio Δγ E was shown to increase with deposition current density. Adhesion is shown to be an effective factor in preventing the formation of coating striae.