The corrosion of concrete is affected by a number of factors which are not fully understood. Therefore, when modelling its progress, an advantage is seen in a reduction of the number of the coefficients involved. Under certain limitations the conditions for a model are specified, and the second degree parabola is recommended. According to the type of corrosion, equation (4) or (7) has been accepted. The results of computations show a relatively good reliability of the proposed model.