Innovations in Corrosion and Materials Science (Discontinued) - Volume 5, Issue 1, 2015

Recently, transition metal nitride (TMN) coatings have attracted numerous attentions. The superior corrosion-resistant properties, combined with high abrasion resistance, thermal stability and good biocompatibility, make TMN coatings very promising candidates for applications as corrosionprotective coatings in many fields. In this paper, the recent progress in research on corrosionprotective TMN coatings is reviewed. The relationship between deposition process, structure and corrosion-resistant properties of TMN coatings is revealed. The important patents and applications concerning corrosion-protective TMN coatings are presented, and the current status of the research and future challenges are discussed.

Localized corrosion, in the form of pitting is one of the most dangerous degradation processes of the metallic structures in Cl-containing aqueous environment. It is the major cause of unpredictable leakage of fluids, like petroleum products, causing accidents. Of the three parameters, pitting potential (Epit), pit density and pit depth, representing the severity of the damage, the maximum pit depth, which indicates the growth of a pit in vertically downwards, is the most important. It decides the time of onset of the damage and initiation process of a structural failure. Pit depth has been mathematically modeled. It is seen that the pit depth is strongly influenced by the electrochemical force ΔE=E-Epit and time of duration Δt, after incubation period, along with few other factors. Pit has been measured under scanning electron microscopy and optical microscopy with different conditions. The experimental data are found to be good match with the predicted ones from the model. The pictorial image of Chloride distribution inside and outside the pit, simulated by Finite Element Method (FEM), shows that the concentration of Cl is maximum at and near the mouth of the pit. It is in complete agreement with the mechanism of the pit growth.

Complex sol-gel process was applied to synthesize WO3-ZrO2 and WO3-TiO2 composites at different ratio of oxides. These composites are used to obtain radionuclide W-Re generators important due to applications in nuclear medicine. The structure of above composites depends on annealing temperature. At 800 °C specimens are fully crystalline containing separate oxides with crystallites of nanometric sizes. WO3 existing in monoclinic form stabilizes tetragonal ZrO2 form. Long time annealing at 1000 °C causes martensite type phase transition in ZrO2 from tetragonal to monoclinic form whereas in WO3-TiO2 composite in the same conditions phase transitions do not occur.

The atmospheric corrosiveness was studied by the installation of seven monitoring places throughout the five main regions of the Veracruz state and applying standard methodologies as a) determination of the corrosiveness indexes using the test method ¨wire on bolt¨ for the marine atmosphere, industry and rural indexes, b) corrosion levels of ¨flat plate¨ samples in accordance with the ISO 9223 for mild steel, galvanized steel, copper and aluminum. Additionally measurements were made for the chlorides, sulphur dioxide pollutants and suspended particles on the atmosphere surrounding the exposed metallic samples. Morphological studies were conducted using the scanning electron microscopy (SEM) for the exposed samples and energy dispersive X-rays analysis (EDX) for the identification of the aggressive elements and compounds responsible for the corrosivity of the tested materials. The outcome after one year of exposure showed the very high marine corrosiveness index for the coastal places and less notorious for the rest of the state. The industrial corrosiveness index is high only in oil producing and refining regions, but it was low in rest of the areas, therefore a rural corrosivity index threshold low level were observed. The standard ISO 9223 levels for mild steel were very high for the city of Boca del Rio (BR) and the port of Veracruz (V), while for galvanized steel and copper were: high for the industrial port of Coatzacoalcos (C) and the industrial city of Mundo Nuevo (MN), moderate for the industrial city of Poza Rica (PR) and Orizaba (O) and low for the city Xalapa (X) considered a rural-urban atmospheric classification.

The anti-corrosion behavior of expired Fluconazole (2-(2,4-Difluorophenyl)-1,3-di(1H-1,2,4- triazol-1-yl)-2-propanol) as an inhibitor on mild steel was studied in a neutral chloride/sulphate solution, at room temperature using electrochemical techniques including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. Fluconazole characterization was performed using FTIR. Polarization curves showed that both corrosion and film formed pseudo-passivation current density values were lowered by almost one order of magnitude and therefore fluconazole acts as an anodic type of inhibitor on the steel surface. Impedance measurements confirmed the decrease in the corrosion rate by almost an order of magnitude, as a function of inhibitor concentration. The electrochemical results showed that this compound has a corrosion inhibitor effect and the best results were achieved with 150 ppm of Fluconazole, remaining around 80% efficiency up to 160 hours of immersion.

Chernobyl NPP (ChNPP) lava fuel-containing masses (LFCM) chemical composition and properties are analysed with emphasis on corrosion resistance. It is shown that LFCM need additional conditioning because of changes occur and overall low corrosion resistance. The expedience to vitrify ChNPP LFCM is discussed with an optimal conditioning method which would combine joint vitrification of ChNPP LFCM and boron-containing liquid radioactive waste (salt cake) from NPP using cold crucible melters (CCM). Glass corrosion resistance is analysed with focus on alkali-borosilicate glasses typical for nuclear waste immobilisation. Radiation effects are discussed including self-heating which is found small at expected sizes of waste canisters. The impact of radiation-induced effects in glasses is also considered.

The electrochemical behaviour of different Pb-Ag (0.5, 0.6 and 0.7%) anodes during zinc electro winning has been studied in acid zinc sulphate electrolyte containing Mn2+ at 38 °C, and 50 mA/cm2 by electrochemical noise measurements “ENM” using the zero resistance ammeter set-up (ZRA). During 16 h decay period after 24 h of electrolysis, Pb-0.5% Ag anode had the highest corrosion rate among the three anodes, followed by Pb-0.6%Ag anode and Pb-0.7% Ag anode. It can be observed that at the end of 16 h decay, the 0.7% Ag lead anode had the lowest potential (the most active) followed by 0.6% and 0.5% Ag lead anodes. Also, 0.5% Ag lead anode had the longest first plateau as compared to 0.6% and 0.7% Ag lead anodes. This plateau had higher corrosion rate “1/Rn”, higher slope of ratio “Sr” than that in its following decay period. In addition, at the end of 16 h potential decay, the potential-time record shows that Pb-0.7% Ag anode had several high amplitude and high frequency oscillations with several peak transients; while Pb-0.5% Ag anode had more frequent repetition of fluctuations with smoother potential. Moreover, at the end of 16 h decay, it was also obtained that the absolute magnitudes of the slopes of potential of power spectral densities (PSD) of 0.7% Ag lead anode are also lower than that of 0.5% Ag lead anode. Finally, it can be admitted that 0.5% Ag-Pb anode has more corrosion rate during polarization and this is shown by important plateau at the average value of 1.61 V/SHE for the longest time (45 min); while 0.7% Ag lead anode showed more active galvanic cells at the end of decay (11-16 h) that can correspond to more localized corrosion because of more abundant silver containing phases.

A 316 stainless steel plate heat exchanger of a process water cooling system failed due to microbial corrosion during operation at a nuclear power station. A detailed failure analysis was done on the heat exchanger plate and described in this paper. The results established that inadequate chlorination has resulted in the infestation of the heat exchanger unit by corrosion and biofilm forming bacteria and caused the breakdown. Optical microscopy revealed concentric ring patterns on the SS- 316 plate, which is typical of sulphate reducing bacteria which induced microbial corrosion. SEM EDAX showed the presence of significant sulphur peak. XRD and Raman spectroscopy analysis showed the presence of FeS2 peak. Various observations recorded in the study were characteristic to typical microbial corrosion failure, particularly by SRB. The paper further highlights the remedial measures taken to prevent microbial corrosion by optimizing biocide dosing and water chemistry changes in the power plant.