Considerations about the characteristics of the superficial films formed on some components of the heavy water production installations

It is known that during the Girdler Sulfide (GS) process of heavy water production some ferrous components of the installations are covered with films of variable thickness constituted by various sulfide types. The protective character of this superficial films depends, mainly on two factors, the S2- ion concentration and the pH-value of the aqueous medium in the installation. Heaving in view that the working agent in the installation of heavy water production is H2S, it is expected that on the ferrous alloys various stoichiometries sulfide films form, namely on pyrites, machinawites, troilites, pyrothites, etc. Having in view the actual conditions of operation in the installation of heavy water production we have used in our tests the following types of samples: pyritized samples, pyritized samples exposed to water vapors, pyritized samples exposed to water vapors and then neutralized, control samples. Since the installation in the laboratory did not allow the operation in H2S medium, the presence of S2- ions was simulated by using Na2S solutions and S2- ion concentrations within the range 1 - 1000 mg/l and pH-values in the range 4 - 13, adjusted by acid or base addition. The passive character of the films formed on the G52/28 carbon steel utilized in the experiments was assessed by making use of the following methods: X-ray diffraction; potentiodynamic method; electrochemical impedance spectroscopy. The main electrochemical characteristics obtained from the results of the experiments performed on the above mentioned samples, which are relevant for the corrosion behavior of the samples in H2S medium, were the following: variation of the corrosion potential in open circuit; the values of the corrosion rates computed from Tafel slopes and polarization resistances. On the basis of the above mentioned techniques we established the variation of the passive character of the sulfide films as a function of the pH-value and concentration of sulfide ions in the solution in which the tests were performed, hence the factors which contributed to the superficial film formation. We also evaluated the effects of applying neutralization treatment to the pyritized samples and their influence upon the superficial film characteristics