Creep and the corrosion characteristics of Incoloy Alloy 800 in molten nitrate salts

The effects of deformation on the corrosion resistance of Incoloy Alloy 800 in sodium nitrate and potassium nitrate salt mixtures have been studied. Hollow tube specimens filled with the salt mixture (60 pct NaNO340 pct KNO3) were tested in constant load tension creep at elevated temperatures (550 °C≤ T≤ 670 °C). Depending on the temperature and initial stress, fracture times (and therefore salt exposure times) ranged between 300 and 1000 h. While the fracture strain of specimens tested to failure was only slightly reduced when exposed to the salt environment, metallographic observations of polished crosssections revealed severe surface oxidation. In order to characterize the effect of total imposed strain on oxide morphology a number of creep tests were terminated prior to fracture. Increasing deformation resulted in a more extensively damaged surface oxide as well as a more rapid rate of corrosion. EDX analysis revealed that the oxide was multiphase, with a near surface, iron rich oxide above a chromiumrich oxide layer. Beiow 630 °C the oxidemetal interface was more diffuse with fine oxide intrusions growing into the base metal and small particles of the alloy visible in the oxide near the base metal interface.