TWI Industrial Member Report Summary 1043/2013
By Roger Barnett
Background
Molten chloride salts are highly corrosive, but find uses in multiple areas including metal processing and nuclear applications as well as by-products in other industries. Material performance is typically determined by immersion testing of coupons; this work is intended to obtain data on corrosion mechanisms while adapting existing test methods and allow simultaneous assessment/measurement of corrosion above and below the melt surface.
Industrial Challenge
Performance in high-temperature environments where chloride melts are present (either intentionally or as a by-product) is an issue for several industries (e.g. biomass burning, metal processing). Corrosion data in the literature are often limited and are determined by field trials or immersion testing of coupons. The 'splash' zone where the molten salt meets the surrounding atmosphere, is not considered, nor is attack above the surface of the molten salt. No standardised method exists to determine performance in these cases. This report extends existing test methods to include the region above the melt surface and the 'splash zone' while also providing corrosion data for common engineering alloys in the LiCl-KCl eutectic melt at 600°C under air
Key Findings
- All materials were severely attacked with overall corrosion rates of 5-10mm/year and formation of highly complex oxide scales both above and below the melt-air interface
- Corrosion was heaviest at the melt-air interface, but in the above-surface and below-surface regions the corrosion appeared similar
- The test method led to rapid ranking of material performance though further standardisation was needed to fix the melt-air interface position
- Alloy 625 performed the best in this environment, with pitting, intergranular attack and void formation
- 310 stainless steel suffered heavy intergranular and general attack, with massive chromium depletion
- P91 corroded by pitting and rapid scale spallation