Kyung-Hwan Na

Effect of Passivity of the Oxide Film on Low-pH Stress Corrosion Cracking of API 5L X-65 Pipeline Steel in Bicarbonate Solution

Abstract Effect of oxide film passivity on low-pH stress corrosion cracking (SCC) of API 5L X-65 pipeline steel in bicarbonate solution was investigated using potentiodynamic polarization, optical ...

Effects of Sulfate and Nitrate Ion Additives on Pit Growth of Pure Aluminum in 0.1 M Sodium Chloride Solution

Abstract Effects of sulfate (SO42−) and nitrate (NO3−) ion additives on pit growth of pure aluminum were investigated in 0.1 M sodium chloride (NaCl) solution as a function of anion concentration using a potentiodynamic polarization experiment, a potentiostatic current transient technique, and optical microscopy. The increase in SO42− and NO3− ion concentrations in NaCl solution raised the pitting potential (Epit) of pure aluminum and increased the steady-state current density at potentials above Epit. This means that SO42− and NO3− ion additives impede pit initiation on pure aluminum, but enhance the growth of preexisting pits. Potentiostatic current transients of artificial pits obtained from the moment just after adding SO42− and NO3− ions to NaCl solution revealed that the artificial pit grows more rapidly in the presence of SO42− and NO3− ions. From observation of the cross-sectional views of the artificial pit, it appeared that the pit grows preferentially in the lateral direction and in the downwar...

Evaluation of Pitting Susceptibility of Aluminum Anodized in Borate and Phosphate Solutions Using EN Analysis

The electrochemical noises (ENS) obtained from anodized aluminum in 0.1 M NaCl solution have been analyzed to evaluate the resistance to pitting corrosion as a function of anodizing solution and formation potential. The application of stochastic theory to the electrochemical noises provides more additional information on corrosion process than that of shot-noise theory alone. The occurrence of two linear regions on one Weibull probability plot indicated that the present analysis method allowed us to distinguish between pitting corrosion and uniform corrosion in an engineering sense. Moreover, from the lower pit embryo formation rate of aluminum anodized in boric-ammonium phosphate solution compared to the rate of aluminum anodized in boric-borate solution, it was concluded that the oxide film formed in boric-ammonium phosphate solution provides good pitting corrosion resistance, as compared with the film formed in boric-borate solution.

Characterization of Surface Roughness and Inhomogeneity of Hot-Rolled Carbon Steels by Using Image Analysis Method and Electrochemical Impedance Spectroscopy

The present work is concerned with characterization of surface roughness and inhomogeneity of four kinds of hot-rolled carbon steels in terms of the fractal dimension and the depression parameter by using image analysis method and electrochemical impedance spectroscopy, respectively. From the analysis of the 3D AFM image, it is realized that all the hot-rolled steel surfaces show the self-affine fractal property. The values of the fractal dimension of the hot-rolled steels were determined by the analyses of the AFM images on the basis of both the perimeter-area method and the triangulation method. In addition, the Nyquist plots were found to be depressed from a perfect semicircle form. From the experimental findings, the changes in the values of the fractal dimension and the depression parameter with chemical composition have been discussed in terms of the change in the value of hardness of base steel.

Effects of chloride ion addition on the pickling of hot-rolled steels in fluoboric acid solution

Abstract Effects of chloride ion addition to fluoboric acid (HBF4) solution on the pickling of a high-strength, hot-rolled steel were investigated by using open-circuit potential transient technique and alternating current (AC) impedance spectroscopy. The occurrence of an open-circuit potential plateau indicated dominant dissolution of one oxide phase in the scale. In contrast, the open-circuit potential decay was caused by the simultaneous dissolution of two oxide phases in the scale. Thus, the shape of the open-circuit potential transient was determined crucially by the oxide scale structure. Time to reach a steady-state value of open-circuit potential decreased with increasing chloride ion concentration, indicating accelerated descaling. The decrease in the electrical resistance of the oxide scale estimated from AC impedance spectra also suggested that dissolved chloride ions accelerate descaling. From the experimental findings, it was concluded that the addition of chloride ions to HBF4 solution accel...

Analysis of Electrochemical Noise of Pure Aluminium in Sulphate and Molybdate Ion-Containing 0.1 M NaCl Solution

Abstract In the present work, we investigated the effects of sulphate and molybdate ion addition on the pitting corrosion of pure aluminium as a function of anion concentration by using potentiodynamic polarisation experiment and electrochemical noise measurement. The experimentally obtained electrochemical noise was analysed based upon a stochastic theory. From the results, it was revealed that the localised corrosion process could be distinguished from the uniform corrosion process by the mean free time of events in the electrochemical noise. In addition, it was proposed that both sulphate and molybdate ions retards the metastable pitting or the pit initiation.