PURPOSE: To suppress reduction in a zinc phosphate processing characteristic in a ground part so as to prevent generation of a great difference between generation quantities of zinc phosphate coating between the ground part and an unground part by performing grinding under the existence of metals nobler than aluminum. ;CONSTITUTION: Al-Mg alloy material is ground under the existence of a metal nobler than aluminum, those kinds of metals, or oxides of those metals, so that a cathodic factor is given to a ground part, and an electrochemical surface condition equivalent to that of an unground part is formed in the ground part. In this way, reduction of a zinc phosphate processing characteristic of the ground part, in Al-Mg alloy containing not less than 0.02% by weight of Cu is suppressed, and coupling electric current flowing between the ground part and the unground part, is reduced even when partial grinding is carried out, so that no great gap between zinc phosphate processed coating generating quantities. Desirably, Al-Mg alloy material containing 2-8% by weight of Mg and not less than 0.10% by weight of Cu is used.;COPYRIGHT: (C)1996,JPO
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