Innovations in Corrosion and Materials Science (Discontinued) - Volume 8, Issue 2, 2018

FAC, Flow Accelerated Corrosion, arises in the inside wall surface of carbon steel pipes which carry boiler supply water. Metal loss rates due to this sort of corrosion is relatively high; 1mm/y usually, over 4mm/y some cases. Damage areas are comparatively wide: the axis-directional extent of the hole created in pipe wall is several times of the pipe diameter. The EPRI, Electric Power Research Institute USA, presented the corrosion rate-predicting equation to estimate the corrosion rate from a lot of parameters: properties of boiler supply water such as temperature and pH, dissolved oxygen concentration, etc., as well as flow conditions such as flow velocity, etc. The accuracy of estimation, however, was very low. This author interprets this failure as follows: FAC is not uniform corrosion but localized corrosion, whereas the metal loss rate of uniform corrosion is dependent on the physical properties and the flow conditions of water as well. The metal loss rate of localized corrosion is dependent only on the difference in “anodic dissolution rates”. Anodic dissolution rates are dependent on the temperature because it concerns chemical reactions. The degree of the temperature dependency of the rate in ordinary temperature range is, however, small. So that, usual temperature gradient cannot cause any difference in anodic dissolution rates to initiate any localized corrosion. Nevertheless, in case of carbon steels, there is singular temperature range where the temperature dependency of anodic dissolution rate is reversed and decreased. It is the range called passivation. In this temperature range anodic dissolution rate decreases rapidly with temperature because the composition of oxide which deposits in the corroding surface approaches that of passive state film. As a result, even a small temperature gradient brings about great difference in anodic dissolution rate, and localized corrosion occurs. The concentration of dissolved oxygen in the surface of carbon steel submerged in water is dependent on oxygen supply rate from bulk. Anodic dissolution rate is lower at the surface where the oxygen supply is higher because iron oxides of higher grade are generated there which suppress metal dissolution. Anodic dissolution rate is higher at the surface where the oxygen supply is lower because lowgrade oxides arise. Thus, differences in the rate of dissolved oxygen supply bring about differences in anodic dissolution rates, and this causes localized corrosion. In stagnant water areas in pipe flow, oxygen supply to the pipe wall surface is apt to be retarded. Accordingly, differences arise in anodic dissolution rate between the stagnant water area and the surrounding. Thus, the localized corrosion is easy to be generated at those locations. In short, temperature gradient, passivation and stagnant water are three main factors which bring about the localized corrosion which was named “flow accelerated corrosion” by the EPRI. Based on the generation mechanism described above, technique to detect and mitigate the localized corrosion was presented. Technique to protect pipes from rupture, even if the detection of the corrosion failed, was also given.

Background: The corrosion rate of biomedical magnesium alloys must be controlled to avoid excessive degradation in the physiological medium. Conversion coatings produced in fluoride-based electrolytes can be employed as a suitable corrosion protection route for these materials. The use of concentrated HF solutions is the most common procedure in the current literature. The aim of the present work was to employ a less concentrated HF solution to produce protective fluoride-based films on the AZ31B alloy and to investigate the effect of the treatment time on its corrosion resistance. Methods: The fluoride-based films were obtained in 10 wt.% HF solution at room temperature for 3 h, 6 h and 12 h. The top surfaces and cross-sections of the fluoride films were observed by scanning electron microscopy. The surface chemistry was assessed by X-ray photoelectron spectroscopy. The corrosion resistance was evaluated by potentiodynamic polarization. Results: Scanning electron microscopy and confocal laser scanning microscopy micrographs revealed that the fluoride-based layer became more cracked and rougher as the treatment time increased from 3 h to 12 h. X-ray photoelectron spectroscopy (XPS) analyses indicated that the surface layers were composed of Mg(OH)xF2-x or Mg(OH)2-xFx compounds, independently of the treatment time. The fluoridebased films enhanced the corrosion resistance of the AZ31B alloy. Conclusion: Surface morphology plays a core role in the corrosion behavior of the coated AZ31B alloy and can be controlled by the treatment time. The smooth surface of the 3 h-film presented the best corrosion resistance.

Background: The electrochemical corrosion properties of bulk nanocrystalline aluminum (BNC-Al) produced by severe rolling technique and its traditional polycrystalline aluminum (TPC-Al) counterpart in 0.022, 0.044, 0.066, 0.11, 0.22, 0.44 mol/L HCOOH solutions were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) at ambient temperature. Method: The corrosion current densities and capacitances of double electrode layer of BNC-Al were less than those of TPC-Al respectively, and the polarization resistances of BNC-Al were larger than those of TPC-Al in 0.022, 0.044 mol/L HCOOH solutions. However, the corrosion current densities and capacitances of double electrode layer of BNC-Al were larger than those of TPC-Al respectively, and the polarization resistances of BNC-Al were less than those of TPC-Al in 0.066, 0.11, 0.22, 0.44 mol/L HCOOH solutions. The passivation of BNC-Al was enhanced in 0.066, 0.11, 0.22, 0.44 mol/L HCOOH solutions. Results and Conclusion: These results demonstrated that the corrosion resistances of BNC-Al were enhanced in 0.022 and 0.044 mol/L formic acid solutions at a low potential and in 0.066, 0.11, 0.22, 0.44 mol/L at a high potential, compared to those of TPC-Al. The possible reasons for these electrochemical results for BNC-Al and TPC-Al were proposed in this work.

Background: Two techniques of laser optical interferometry have been reviewed for detection of crevice corrosion of low carbon steel samples in aqueous solutions. The first technique used an optical corrosion-meter to detect localized corrosion such as crevice corrosion, of the low carbon steel samples in seawater. The concept of the optical corrosion-meter was based on incorporating holographic interferometry and the cyclic polarization test. The second technique used a modified electrochemical noise technique based on holographic interferometry to detect crevice corrosion of the low carbon steel samples in 1 M of sodium hydroxide (NaOH) solution. Objective: Emphasis on the significant role of the optical techniques of detecting crevice corrosion of carbon steel in aqueous solutions alongside to the DC methods. Method: The optical corrosion-meter and the modified electrochemical noise technique were used in the present investigation. Results: Observations of crevice corrosion of the carbon steel samples in seawater by the optical corrosion-meter were found to be in agreement with the cyclic polarization test. In addition, the modified electrochemical noise technique was found to be very useful for quantitatively distinguishing of localized corrosion, passivation, and uniform corrosion of the carbon steel samples in 1 M NaOH solution. Conclusions: The optical techniques of detecting crevice corrosion of carbon steel in aqueous solutions were found efficient opto-electrochemical instruments for substantiating results of those DC methods.

Background and Method: The growth mechanism of sulphide films formed on copper in anaerobic 0.1 M NaCl + 5 x 10-4 M Na2S solution has been investigated under natural corrosion conditions for exposure periods up to 1691 hours using scanning electron microscopy, focused ion beam cross-sectioning, and a Au marker procedure. Results and Conclusions: The film formed by a chemical deposition process via an outward growth mechanism. This process was controlled by cuprous ion transport in the film combined with sulphide diffusion in solution.

Background: In this paper, a comparative study of the correctness of the results of registration of corrosion kinetics with the methods of current-voltage curves (V / a) and Coulometric Registration of Corrosion Products (CDCP method) was carried out. Methods: It is shown that the action of the electric field under polarization changes the character of the corrosion process and, in particular, stimulates the formation of the oxide layer, leading to a distortion of the results of the loss of the mass of the metal. Results and Conclusion: The convergence of the results of coulometric and "tafel" measurements of mass loss is established only on "pure" metal. After the formation of the oxide layer, the "tafel" results show a protective effect, which does not exist in a real corrosion process.

Background: Surface wrinkle responding to external stimuli has potential applications in information storage, sensors, construction of special surfaces and so on. Ferrocene and azobenzenecontaining polymer Poly[2,2-Bis(4-glycidyloxyphenyl)propane-co-4-(4-ferrocenylimidephenylazo) benzenamine] (PAZO-Fc) and azobenzene-containing polymer Poly[2,2-Bis(4-glycidyloxyphenyl) propane-co-4- (4-nitrophenylazo)benzenamine] (PAZO) which have redox- and/or photo-responsive properties were synthesized by ring opening polymerization method. Methods: The structures were characterized by 1H NMR, FT-IR and GPC. TGA and DSC curves were recorded to study their thermal properties. Cyclic Voltammetry (CV) and Ultraviolet-visible (UV-vis) absorption studies showed redox- and photo-responsive properties of PAZO-Fc. Results and Conclusion: The polymers were dissolved in THF and then spin-coated on a plasma-treated PDMS substrate to induce surface wrinkling by heating. The effect of light irradiation and redox reagents on the regulation of surface wrinkles was investigated. This kind of regulation has potential application in information storage.