PURPOSE: To increase the surface area of a thin Al alloy plate and to improve the corrosion resistance and the water breaking effect by subjecting the Al alloy plate contg. a specified amount of Sn to soln. heat treatment and by immersing it in an acidic aqueous soln. to make the surfaces finely uneven by chemical etching.

CONSTITUTION: A thin Al alloy plate contg. at least 0.05W0.3wt% Sn is subjected to soln. heat treatment at 500W630°C, and it is immersed in an acidic aqueous soln. of ≤3.5pH for 1W60min to make the surfaces finely uneven by chemical etching. The soln. remaining on the surfaces is neutralized and removed by washing. The surface area of the thin Al alloy plate is increased, and a thin Al alloy plate for a fin material having superior corrosion resistance and a significant water breaking effect is obtd.