PURPOSE: To obtain a surface treating material providing corrosion resistance to a heat exchanger and having high heat conductivity by specifying the compositional range of an Al-Bi-Sn alloy.

CONSTITUTION: A surface treating material for a heat exchanger consisting of 0.01W40wt% Al, 0.1W40% Bi and the bakance Sn provides resistance to corrosion due to steam, CO2, CO, SO2, NO, NO2, etc. contained in combustion gas to a heat exchanger utilizing town gas, liquefied gas, kerosene or the like as a heat source. By increasing the Al content of the alloy, the corrosion resistance is enhanced, yet the m.p. of the alloy is also increased, so the Al content is regulated to ≤40% in consideration of the softening temp. of copper as the base material of the exchanger. Since an increased Bi content is effective in decreasing the m.p. of the alloy but deteriorates the corrosion resistance, ≤40% Bi content is desirable. The compositional range of the alloy suitable for use as the surface treating material is restricted as mentioned above.