Pyro-hydrolysis

The proposed mechanism includes the production of HCl from the pyro-hydrolysis of the metal chlorides. Similar reactions are likely for bromides and iodides. Fluorides however are relatively stable and would not be expected to hydrolyse. It was considered that this might account for the inability of fluorides to cause cracking. Hydrogen absorption by titanium alloys exposed to chloride salts at elevated temperatures has been detected and found to be proportional to the amount of moisture participating in the reaction. 

Combustion pyro-hydrolysis is based on the chemical sequence ... 

Fig. 2. TG curves for pyro-hydrolysis of rare-earth fluorides in an ordinary atmosphere. TG measurement was performed under the water vapour pressure of 1.8 x 103 Pa at the heating rate of 5 C min-1.

Change in properties of Nd2Eu203F6 due to pyro-hydrolysis in an ordinary atmosphere... 

Fig. 19. Transformation of the crystal structure of NdiEuiCLFg due to pyro-hydrolysis.

The fusion of individual metal halides and of their mixtures is accompanied, as a rule, by high-temperature hydrolysis (recently, the term, pyro-hydrolysis has been used), which occurs due to the presence of traces of water vapour in the atmosphere over the liquid salts. These traces arise by evaporation of water from hygroscopic salts or are present in the inert gas, which has been insufficiently dried before use. 

---