In the paper we discuss an experimental method of nondestructive diagnostics of metallic materials structures based on multi-frequency eddy current measurements. The measurements are performed with a consignment parametric sensor, and further digital signal processing using Walsh functions is carried out for error reduction. Hodograph plotting is described on a set of examples of aluminum- magnesium alloys. We investigate the influence of artificially created structural defects simulating subsurface defects (cavities, cracks) at different depths of a testing material on the hodograph forms. A quantitative description of hodograph locus shift for various defects in relation to hodograph line of defect- free region is provided. The dependence of the shift on the distance from the sensor to a defect allows to locate the defect and to estimate its depth. The obtained results demonstrate the prospects of the proposed method for diagnosis and monitoring of structural defects in aluminum alloys products.