Suppressing the oxygen ingress into Ti-alloys by a one-step Al- plus F-treatment

Oxygen ingress into technical Ti-alloys during high temperature exposure in oxygen containing environments leads to an enriched surface zone due to the high oxygen solubility in titanium. This affected zone is known as alpha case and is embrittled compared to the substrate material. Therefore, the operating temperature of these alloys is limited. An enrichment of Al in the surface zone of Ti-alloys leads to an improvement by the formation of intermetallic TixAly-phases with decreased oxygen solubility. This one-step Al-enrichment plus a F-treatment is proposed for the formation of a continuous stable alumina scale which prevents oxygen inward diffusion. In this paper the results of high temperature exposure tests in air of technical Ti-alloys are presented without any treatment and treated with Al-enrichment including fluorination. Post experimental investigations such as SEM reveal the formation of thin alumina layers on treated samples. Hardness measurements confirm that no embrittlement was observed for the treated samples while untreated specimens show increased hardness values in the subsurface zone corresponding with the oxygen inward diffusion profile

Improving the corrosion resistance of ferritic-martensitic steels at 600 °C in molten solar salt via diffusion coatings

Molten salt corrosion in the receiver’s piping system and storage tanks is still one of the major drawbacks of concentrated solar power (CSP) plants and is currently covered by using expensive high-alloyed steels or Ni-base alloys. The employment of cheaper structural materials combined with protective coatings is an attractive alternative to increase cost-efficiency. The present study investigates the corrosion resistance of three different coatings (a pure Ni, a Cr and a combined Ni+Cr coating) deposited on ferritic-martensitic X20CrMoV12-1 steel during isothermal immersion in molten solar salt in comparison to uncoated Ni-base alloy Haynes 230. Exposure tests were conducted at 600°C for up to 1000 h. To identify the individual role of the main elements, exposure tests of pure Fe, Ni and Cr were undertaken as well. In addition to the crosssectional investigations via light-optical microscopy and EPMA, X-ray diffraction measurements were conducted to identify the corrosion products. These were complemented by weight change measurements and chemical analysis of the salts after the exposure tests. The study reveals differences between the corrosion behaviour of the different coating approaches, which is discussed as a function of coating composition and degradation mechanisms. Whereas pure Cr or pure Ni coatings offer hardly any improved protection of the uncoated X20CrMoV12-1 substrate, the combined Ni+Cr coating performs very well and improves the scaling behaviour of X20CrMoV12-1 significantly. The outcome highlights the correlation between the solubility of the pure metals Fe, Ni and Cr in molten solar salt and the corrosion resistance of the investigated coating