PURPOSE: To improve corrosion resistance to sodium polysulfide by forming a high concentration chromium-aluminum-iron alloy layer on the surface of a battery cell or sodium-sulfur battery.

CONSTITUTION: A chromium calorizing treating agent is obtained by adding yttrium or a mixture of yttrium with lanthanum as an accelerator for accelerating forming the chromium-aluminum-iron alloy layer on the surface of a battery cell base material into a mixed agent for chromium calorizing working composed of 30-60% powdery chromium, 5-10% powdery aluminum, 0.5-2.0% powdery ammonium chloride and 20-50X powdery alumina. The battery cell base material composed of steel or iron low in carbon content is chromium calorized by using the treating agent.