Recent laboratory work to explore chlorine-induced high temperature corrosion

This paper summarizes some recent laboratory work at Åbo Akademi to explore the mechanisms of high temperature corrosion induced by metal chlorides. Chloride-induced high temperature corrosion was investigated by using two different methods: 1) Differential Thermal Analysis combined with Thermal Gravimetry (DTA/TG) for powder samples; and 2) one-week exposure tests in a tube furnace for steel plate samples. With the first method, the influence of eight different chlorides (BaCl₂, CaCl₂, KCl, LiCl, MgCl₂, NaCl, PbCl₂ and ZnCl₂) on the accelerated oxidation of pure, metallic chromium at elevated temperatures (400°C, 500°C, 550°C and 600°C) was studied. Selected samples were also studied and analyzed with a scanning electron microscope equipped with an energy dispersive x-ray analyzer (SEM/EDXA). With the second method, the corrosion tendencies of solid KCl and K₂CO₃ were compared by exposing an austenitic 304L steel to the salts for a week at three different temperatures (500°C, 550°C and 600°C). The exposures were carried out both in dried air and in moist air with a water content of 30%. Cross sections were prepared from all the plate samples and studied with SEM/EDXA. The extent of corrosion and the elemental distribution were determined, and the corrosion products were identified.

The DTA/TG-results indicated that KCl, LiCl, NaCl and PbCl₂ accelerated the oxidation of chromium. LiCl reacted only at 600°C, whereas the other three mentioned chlorides reacted at temperatures above 500°C. The presence of chlorine was not solely sufficient to cause accelerated corrosion, for with some chlorides no corrosion could be detected even at the highest temperature.

In the exposure tests, both salts were found to be corrosive. Generally, the formed oxide layer thickened as the temperature rose. In the case of KCl, the corrosion was more severe under dry conditions, whereas in the case of K₂CO₃ the corrosion was more severe under wet conditions. The structure of the oxide layer formed was similar for both salts: the outermost iron oxide layer was followed by an oxide layer containing chromium, iron, and nickel, followed by a nickel-rich region before the bulk metal.