• Korean Journal of Metals and Materials
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• v.47 no.10
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• pp.644-651
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• 2009

The passivation (or deactivation) of a metal surface during oxide film formation has been quantitatively explored for a ferritic stainless steel by using dynamic electrochemical impedance spectroscopy (DEIS). For this purpose, the electrochemical impedance spectra were carefully examined as a function of applied potential in the active nose region of the potentiodynamic polarization curve, to separate the charge transfer resistance and oxide film resistance. From the discrepancy in the potential dependence between the experimental charge transfer resistance and the semi-empirically expected one, the degree of passivation could be quantitatively estimated. The sensitivity of passivation of the steel surface to anodic potential, which might be the measure of the quality of the oxide film formed under unit driving force or over-potential, decreased by 31% when 3.5 wt% NaCl was added to a 5 wt% $H_2SO_4$ solution.

• Corrosion Science and Technology
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• v.20 no.3
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• pp.129-142
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• 2021

Effects of corrosion inhibitors (i.e., sodium nitrite, sodium hexametaphosphate, trimethylamine (TEA), sugar, and urea) on the corrosion resistance of carbon steel in CaCl₂ solution were investigated. The test solution was designed with response surface methodology of design of experiments (DOE) in the range of 0 ~ 50 ppm for NaNO₂, 0 ~ 200 ppm for (NaPO₃)₆, 0 ~ 2000 ppm for TEA, 0 ~ 3000 ppm for sugar, 0 ~ 200 ppm for urea with 3 wt% CaCl₂. The corrosion potential and the corrosion rate were measured with potentiodynamic polarization tests and analyzed statistically to find main effects of inhibitor concentrations and interactions between them. As a result, hexametaphosphate was the most effective compound in reducing the corrosion rate. Sugar also reduced the corrosion rate significantly possibly because it covered the surface effectively with a high molecular weight. The inhibiting action of sugar was found to be enhanced by adding trimethylamine into the solution. Nevertheless, trimethylamine did not appear to be effective in inhibiting corrosion by itself. However, urea and sodium nitrite showed almost no inhibition on corrosion resistance of steel.

• Corrosion Science and Technology
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• v.19 no.6
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• pp.318-325
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• 2020

Generally, Al alloys of 5000 and 6000 series show excellent weldability, workability, and specific strength, and are widely used in ship building. A combined experiment via cavitation erosion and corrosion damage involving 6061-T6 Al alloy was performed using potentiodynamic polarization under cavitation erosion (hybrid experiments) with amplitude (cavitation strength). The corrosion current density was approximately 52-fold higher at 30 μm than under static conditions, suggesting that the amplitude greatly affected the damage. The degree of damage increased with increasing cavitation amplitude. After the hybrid experiment, the corrosion rate was compared according to the weight loss and damage depth, and the relationship between the two values was expressed as alpha value. The alpha (α) values at amplitudes of 5 μm, 10 μm and 30 μm were 5.11, 12.81 and 8.74, respectively, suggesting that the α value at 10 μm was greater than at 5 μm, and indicating local corrosion damage. However, the α value at 30 μm was smaller than that of 10 μm, which is attributed to higher damage via uniform corrosion than damage induced by local corrosion.

• Corrosion Science and Technology
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• v.20 no.6
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• pp.347-360
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• 2021

Currently, the demand for eco-friendly energy sources is high, which has prompted research on polymer electrolyte membrane fuel cells. Both aluminum alloys and nickel alloys, which are commonly considered as materials of bipolar plates in fuel cells, oxide layers formed on the metal surface have excellent corrosion resistance. In this research, the electrochemical characteristics of 6061-T6 aluminum alloy and Inconel 600 were investigated with chloride concentrations in an acid environment that simulated the cathode condition of the PEMFC. After potentiodynamic polarization experiments, Tafel analysis and surface analysis were performed. Inconel 600 presented remarkably good corrosion resistance under all test conditions. The corrosion current density of 6061-T6 aluminum alloy was significantly higher than that of Inconel 600 under all test conditions. Also, 6061-T6 aluminum alloy and Inconel 600 presented uniform corrosion and intergranular corrosion, respectively. The Ni, Cr, and Fe, which are the main chemical compositions of Inconel 600, are higher than Al in the electromotive force series. And a double oxide film of NiO-Cr₂O₃, which is more stable than Al₂O₃, is formed. Thus, the corrosion resistance of Inconel 600 is better.

• Corrosion Science and Technology
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• v.20 no.6
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• pp.466-474
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• 2021

A wet type scrubber for merchant vessel uses super austenitic stainless steels with pitting resistance equivalent number (PREN) of 40 or higher for operation in a harsh corrosive environment. However, it is expensive due to a high nickel content. Thus, electrochemical behavior and cavitation erosion characteristics of UNS S32750 as an alternative material were investigated. Microstructure analysis revealed fractions of ferritic and austenitic phases of 48% and 52%, respectively, confirming the existence of ferritic matrix and austenitic island. Potentiodynamic polarization test revealed damage at the interface of the two phases because of galvanic corrosion due to different chemical compositions of ferritic and austenitic phases. After a cavitation test, a compressive residual stress was formed on the material surface due to impact pressure of cavity. Surface hardness was improved by water cavitation peening effect. Hardness value was the highest at 30 ㎛ amplitude. Scanning electron microscopy revealed wave patterns due to plastic deformation caused by impact pressure of the cavity. The depth of surface damage increased with amplitude. Cavitation test revealed larger damage caused by erosion in the ferritic phase due to brittle fracture derived from different strain rate sensitivity index of FCC and BCC structures.

• Corrosion Science and Technology
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• v.21 no.3
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• pp.221-229
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• 2022

To understand effects of cooling rates of coating layer on microstructures and corrosion behaviors of hot-dip alloy coated steel sheets (Zn-5%Al-2%Mg) in a neutral aqueous condition with chloride ion, a range of experimental and analytical methods were used in this study. Results showed that a faster cooling rate during solidification decreased the fraction of primary Zn, and increased the fraction of Zn-Al phase. In addition, interlamellar spacing became refined under a faster cooling rate. These modifications of the coating structure had higher open circuit potentials (OCP) with smaller anodic and cathodic current densities in the electrochemical potentiodynamic polarization. Surface analyses after a salt spray test showed that the increase in the Zn-Al phase in the coating formed under a faster cooling rate might have contributed to the formation of simonkolleite (Zn₅(OH)₈Cl₂·H₂O) and hydrotalcite (ZnAl₂(OH)₆Cl₂·H₂O) with a protective nature on the corroded outer surface, thus delaying the formation of red rust.

• Corrosion Science and Technology
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• v.21 no.5
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• pp.340-347
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• 2022

In this research, mixed metal oxide (TiO₂, RuO₂) coating was applied to grade 1 titanium as a bipolar plate for polymer electrolyte membrane fuel cell (PEMFC). Electrochemical experiments were carried out in an aqueous solution of pH 3 (H₂SO₄ + 0.1 ppm HF, 80 ℃) determined by DoE. The air was bubbled to simulate a cathode environment. Potentiodynamic polarization test revealed that corrosion current densities of the titanium substrate and MMO-coated specimen were 0.180 µA/cm² and 4.381 µA/cm², respectively. There was no active peak. After potentiostatic experiment, current densities of the titanium substrate and the MMO-coated specimen were 0.19 µA/cm² and 1.05 µA/cm², respectively. As a result of observing the surface before and after the potentiostatic experiment, cracked dried clay structures were observed without corrosion damage. Both the titanium substrate and the MMO-coated specimen could not satisfy the interfacial contact resistance suggested by the DoE. Thus, further research is needed before they could be applied as bipolar plates.

• Journal of Electrochemical Science and Technology
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• v.12 no.2
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• pp.188-203
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• 2021

The specific benefits of the modified films formed on preliminary anodized aluminum, including the versatility of their potential applications impose the need for evaluation of the exploitation reliability of these films. In this aspect, the durability of Cu and Ni modified anodized aluminum oxide (AAO) films on the low-doped AA1050 alloy was assessed through extended exposure to a 3.5% NaCl model corrosive medium. The electrochemical measurements by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) after 24 and 720 hours of exposure have revealed that the obtained films do not change their obvious barrier properties. In addition, supplemental analyses of the coatings were performed, in order to elucidate the impact of the AC-deposition of Cu and Ni inside the pores. The scanning electron microscopy (SEM) images have shown that the surface topology is not affected and resembles the typical surface of an etched metal. The subsequent energy dispersive X-ray spectroscopy (EDX) tests have revealed a predominance of Cu in the combined AAO-Cu/Ni layers, whereas additional X-ray photoelectron (XPS) analyses showed that both metals form oxides with different oxidation states due to alterations in the deposition conditions, promoted by the application of AC-polarization of the samples.

• Corrosion Science and Technology
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• v.21 no.6
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• pp.423-433
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• 2022

The objective of this study was to determine whether Cymbopogon citratus extract as a corrosion inhibitor from natural tropical resources could prevent corrosion of carbon steel in sulfuric acid solution. Inhibitory action of this extract was investigated using electrochemical methods such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Those methods revealed corrosion rate, efficiency of inhibition, and adsorptions isotherm values when the extract was added to the sulfuric acid solution at concentration up to 500 ppm with various immersion time at ambient temperature. Results revealed that higher concentration of the extract and longer immersion time decreased the corrosion rate of carbon steel whereas the inhibition efficiency of the extract was increased up to 97.25%. The value of charge transfer resistance was increased significantly by adding the extract at concentration up to 500 ppm with an immersion time of 60 minutes. The type of the extract was a mixed inhibitor. It could inhibit the corrosion process in both anodic and cathodic sides electrochemically. Results of this study suggest that the mechanism of adsorption on the surface of carbon steel is related to Langmuir adsorption isotherm.

• Corrosion Science and Technology
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• v.21 no.6
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• pp.454-465
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• 2022

The objective of this study was to determine effects of temperatures and pH of sodium chloride solution with MgCl₂ ions on corrosion resistance of duplex stainless-steel X2CrNiMoN22-5-3 (DSS) and Ti 6Al 2Nb1Ta1Mo (Ti). Effects of sodium chloride concentration on corrosion resistance were also studied. Corrosion behavior and pitting morphology of duplex stainless steel (DSS) and Ti alloys were evaluated through potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). It was found that a decrease in pH significantly reduced the corrosion resistance of both alloys. Changes in chloride concentration and temperature had more substantial impact on corrosion behavior of DSS than on Ti alloys. Pitting corrosion was formed on DSS samples under all conditions, whereas crevice corrosion was developed on Ti samples with the presence of magnesium chloride at 90 ℃. In conclusion, magnesium chloride ions in an exceedingly strong acidity solution appear to interact with re-passivation process at the surface of these alloys and influence the resulting surface topography.