The corrosion rates of AISI 316L and AISI 321H austenitic stainless steel, immersed in a stagnant isothermal mixture of 60% NaNO 3 and 40% KNO 3 molten salt at 550°C in atmospheric air are 8.6 and 9.0 µm/yr, respectively. The corrosion mechanism was proposed by recording the weight changes of the steel coupons at different time intervals up to 3000 h, and by the characterization of multilayer oxide scales formed on the steel surface. Multilayers made of different oxides, mainly Fe 2 O 3 and Fe 3 O 4 , are the principal scale products. At 3000 h, the thickness of the scale layer formed on AISI 321H (7.5 ± 2.9 µm) is slightly higher than the one formed at the AISI 316L (6.9 ± 2.1 µm). This small difference might reflect the partial spallation of the corrosion layer on AISI 321H, which is seen for times longer than 1000 h. A minimal change of the composition of the molten nitrate salt is observed in time and is predominantly due to the appearance of soluble chromate products and nitrite compounds (0.004 wt% and 1.4 wt% at 3000 h, respectively). The observed corrosion behaviour of these alloys shows that they are good candidate for usage as containers of molten nitrate salts in the thermal energy storage (TES) system for a CSP plant.
The demand for new protection systems for aluminium alloys that might inhibit corrosion to the performance level of chromate, has driven the search for replacement candidates. Lanthanide compounds represent a promising alternative incorporating an environmental issue of significant impact, since chromate treatments are highly toxic and still used on both civil and military aircraft. A Ce based treatment was developed for use on structural alloy Al 2024-T3 implemented as a dipping process, without the use of external polarisation. The formation time was shortened by means of addition of a catalytic agent, hydrogen peroxide. When prior to the conversion coating formation, a chemical pre-treatment and a desmutting procedure was performed, no significant improvement was found in the susceptibility to pitting. This is thought to be probably due to the formation of a copper rich layer, as a result of etching, and the attack suffered by intermetallics during the chemical pre-treatment.
Al 6061 powder and F800 grade SiC particles (SiCp) were used as raw materials for the preparation of metal matrix composites (MMC), via a powder metallurgy route (PM), with 0% to 20 wt % of SiCp, the reinforcement material. Corrosion studies revealed a linear dependency of the pitting potential (Ep) with the logarithm of the concentration of NaCl with a slope of ~ -90mV/decade, but there was no significant difference in the Ep values of the composite materials as a function of the concentration of reinforcement. Corrosion protection of PM Al SiC alloys was achieved by favouring the formation of a Ce/Mo-based conversion coating on the surface without the use of external polarisation. Pitting potentials were assessed in a 0.1 M NaCl solution after treatment. No increase in current density was evident for samples treated at pH 4.4 followed by post-treatment at 60°C for one hour, when polarised up to 0 mV (SCE). The role of Ce and Mo as inhibitors is analysed and discussed in terms of the protective character of the produced cerium-molybdenum conversion coatings.
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