In flue gas desulfurization plants, which remove sulfur dioxide from the flue gas of oil and coal, chloride coming from the flue gas and makeup fresh water is accumulated in circulating absorbent. Whenstainless steels are employed as construction materials, pitting corrosion due to Cl~cannot be disregarded. The Cl~concentration is usually controlled below a certain value to avoid pitting corrosion by discharging part of the absorbent (i.e., plant fluids) from the system. Inhibiting effects of NO3~, which also comes from flue gas, co-existing with Cl~in the absorbent were systematically investigated. As a result, operating instructions for actual plants are successfully established to protect stainless steel from pitting corrosion. Intro duction In flue gas desulfurization plants where sulfur dioxide in flue gas is removedby an absorbent, chloride coming from the fuel is absorbed by the absorbent, and makeupfresh water simultaneously carries chloride into the absorbent. The chloride is accumulated in the circulating absorbent. When stainless steels are employed as construction materials, their corrosion by localized attack, called pitting corrosion, due to highly concentrated Cl~cannot be disregarded. Indeed, pitting corrosion occurred at one of the early commercial plants. To find the sources of such pitting corrosion and to avoid it, investigations were performed under conditions simulated to resemble those of actual plants.

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