A new approach is proposed based on box-counting method to estimate the fractal dimensions, D2d and D3d of corrosion images. They denote two-dimensional (2D) and three-dimensional (3D) characteristics of corroded surface morphology. The results have been tested through dealing with classical fractal figures,pit morphology of moon surface and practical corroded samples etc. to show that this new method is computationally simple, convenient and curate. More specially, D2d and D3d have definite corrosion expressions, which respectively denote the fractal dimensions for pit diameter distribution and pit depth distribution on real corroded surface.