Ren-Kang Zhu

Semiconductivity of steam generator tubing alloys in simulated crevice chemistries containing lead and sulphur

Semiconductivity of passive films on steam generator (SG) tubing alloys, Alloys 690 and 800, in simulated crevice chemistries containing lead and sulphur was investigated using the Mott–Schottky analysis. The interaction of the above species with other ions on the properties of passive film and the correlation between the semiconductivity and the breakdown of passive film were discussed. Experimental results revealed that semiconductivity could be either n or p type semiconductor; both Pb and S can incorporate into the passive layer, depending on the solution pH and alloy compositions. For n type semiconductors formed on Alloy 800, they are easily broken down but more resistant to anodic dissolution; for p type passive films formed on Alloys 690 and 800, they are not easily broken down but more prone to anodic dissolution. It is concluded that the semiconductivity of passive film on SG tubing alloys was dependent on the alloy compositions, the solutions, temperature and film formation potentials.

Characterization of a Stressed Passive Film Using Scanning Electrochemical Microscope and Point Defect Model

The corrosion resistance of passive film is investigated by in situ scanning electrochemical microscopy (SECM), and is also diagnosed by modified point defect model (PDM). Thus the constant rate related to defect reactions present in the passive film, can be obtained. SECM results indicate that both compressed and tensile stress decreases the stability of passive film, leading to an increase in surface reactivity and dissolution rate—which is possibly due to the decreased binding force between atoms caused by the stress and therefore make them more easy to dissolve. PDM results indicate that the reason of the decreased stability is an increase in the rate constant of defect reactions present in the passive layer, including transportation rate of oxygen and cation dissolution rate.