The corrosion and passivation behaviour of molybdenum thin films obtained by Physical Vapor Deposition (PVD) was investigated in aerated chloride and sulfate solutions at different pH values. Open circuit potential (ocp) measurements, polarisation experiments and electrochemical impedance spectroscopy (EIS) were employed. The experimental results suggest that the metal surface is covered by a passive film; however, corrosion still occurs. For the samples assessed during the current research, the acidic electrolytes tended to be less corrosive; however, a limited passive region was associated with the most basic sulfate or chloride solution. The effect of the pH was found to be more pronounced than the effect of the ion (chloride or sulfate).
There is a need to destroy both military and civilian hazardous waste and an urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). By capitalizing on the properties of water above its critical point (374 °C and 22.4 MPa for pure water), this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern. While Ni and Ni-based alloys are generally considered important for severe service applications, results from laboratory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and localized effects, including stress corrosion cracking and dealloying, are seen in some environments. Although exotic liners such as platinum are currently promoted as a solution to aggressive conditions, some evidence suggests the potential for corrosion control by judicious feed modification. Various alloys were exposed in a SCWO system at 600 °C for 66.2 h. After exposure, samples were coated with a thick outer salt layer and an inner oxide layer. It is considered likely that, at the high supercritical temperature employed during this test, the salt was molten and contained a substantial quantity of gas. The inner oxide layer revealed the presence of numerous defects and a thickness that is proportional to the corrosion rate determined by mass loss, suggesting the oxide layer is nonprotective. Of the alloys tested, G-30 exhibited the highest corrosion resistance. Experiments in which a C-276 tube was instrumented with thermocouples and exposed to a HCl feed indicate for this simple non-salt-forming influent that there is a strong correlation between temperature and the extent and form of corrosion, with the most pronounced degradation being at high subcritical temperatures. These experiments corroborate previous results from a failure analysis for C-276, suggesting a corrosion maximum in the subcritical region.
Supercritical water oxidation (SCWO) is a technology which can effectively destroy civilian and military wastes by oxidation in water at high temperature and pressure (T 550 -650°C, 250 bar). Surmounting the problems of severe degradation of the materials of construction is, however, a pivotal issue in the development of scaled-up systems. High-nickel alloys are important for severe service; however, results indicate they will not survive certain SCWO environments. Although exotic liners such as platinum are currently promoted as a solution to aggressive conditions, this paper provides evidence suggesting it may ultimately be possible to use more conventional materials.
This paper presents a mathematical model describing the change in the capacitance of both single and multilayer organic coatings exposed to a wet environment. The indication is that the capacitance technique can successfully be employed to monitor, in situ, the absorption of water through single-layer organic coatings for values of equilibrium water uptake to a maximum of 5% by volume. In addition, this model indicates a relationship for dUn /d5/ t ,where C is the coating capacitance and t is the time of exposure to the environment, that can be used to rank different single-layer coatings. Conversely, no quantitative correlation between mass uptake and coating capacitance was found for a multilayer coating system, even for low values of water uptake. Although, at face value, these findings suggest that this technique may be inapprepriate for in situ monitoring of water absorption in such complex systems, a comparison between gravimetric and capacitance data indicates that useful qualitative information may still be generated by the use of the latter technique.
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