(516a) Experimental Corrosion Tests of Metals, Metal Alloys, and Ceramic Materials, in Boiling and High Temperatures Vapours I2-HI-H2O Mixtures for Sulphur-Iodine Thermochemical Cycle

The Sulphur-Iodine thermochemical cycle is one of the most promising way to produce hydrogen by splitting water. This cycle has been investigated by ENEA over the last five years, focusing on chemical aspects, reactions, total efficiency, technical feasibility and cost analysis. The majors goals for futures applications are:

- providing the energy demand with free carbon energy;

- rising as much as possible the total efficiency;

- solving some technical matter.

In order to realize this cycle in a massive way it is very important to find the materials for the equipments able to withstand the corrosive environment, which involves strong acids and oxidants in extreme conditions such as high temperatures and pressures. Only a few and incomplete data are available in literature on materials corrosion resistance in hydriodic acid (HI) environment. To overcome this lack, at the ENEA laboratories some corrosion tests are currently under investigation. Corrosion tests were set up in different conditions, in particular: boiling azeotropic HI (127 °C, 1 bar) and vaporized azeotropic HI at 500 °C, 1 bar, with the aim to simulate some critical components of a designed future plant, especially regarding the distillation columns and the HI decomposer.

The first apparatus consists in a heated 250 mL pyrex flask, where the samples have been dipped for 100 hours in a bath of boiling azeotropic HI-H2O and I2, above which a serpentine condenser is installed to avoid the vapour loss and to maintain the boiling equilibrium condition. The second device basically consists in a quartz tube (6 cm as diameter and 90 cm as length), placed in a tubular furnace at 500 °C. Inside the quartz tube a multi sample quartz holder is positioned. The azeotropic solution of HI has been pumped and injected inside the tube by a metering pump. Some quartz wool has been inserted in the initial part of the reactor in order to ensure the homogeneous vaporization and heating of the feed. Also, a little flow of Argon is sent inside the tube to guarantee a continuous and constant renewal of the vaporized mixture. Since the flows of reagents and carrier gas have been set to very low values, the test conditions can be considered quasi-static.

The vapour stream that comes out from the tube is forwarded in a flask, where it has been condensed and analysed by titration.

The tests concerned materials, such as:

Ti, Ta, Nb;

Ti-6Al-4V, Nb/Mo alloy and a large number of Ni-base Superalloys, provided by different companies;

Al2O3 coatings and Ceramic ;

Peek Carbon Fiber (Polyetherketone) and Torlon® Polyamide-imide (PAI).

After 100 hours of exposing time, the samples have been analyzed by the following techniques:

? Optical Microscopy (O. M.);

? Scanning Electron Microscopy (SEM);

? Energy Dispersive X-ray Spectroscopy (EDS);

Interesting conclusions have been found about the behavior of the materials, in these extreme conditions, because it has been possible to correlate their performances to the composition.

Moreover, the same materials showed a very dissimilar resistance at the variation of the environmental conditions. Generally, the higher was the temperature, the more aggressive resulted the environment, considering also that at high temperature some hydrogen has been produced inside the tube by the HI decomposition.