Determination of the Thickness of Thin Porous Oxide Films on Aluminum

A method is described for measuring the thickness of thin porous oxide films on aluminum, employing the grazing angle interference colors character-istic of thin transparent films. In conjunction with oxide film step gauges formed under precisely controlled conditions, measurements to about 7A are possible. Application of the method for measuring film growth in various solu-tions and in the evaluation of the characteristics of barr ier-type lectrolytes is also demonstrated. Pract ical ly all oxide films on a luminum are con-sidered to consist of two layers, regardless of whether they are formed natura l ly or by thermal or anodic treatments. One of these, the barr ier layer, is almost a lways extremely thin, seldom exceeding a few mi l l ionths of an inch in thickness. The other, general ly described as the porous layer, may hav