HeeJin Jang

Effects of Cr on the Structure of the Passive Films on Ni-(15, 30)Cr

The structure of the passive films formed on Ni-(15, 30)Cr in pH 8.5 buffer solution and in pH 2.0 sulfuric acid solution was investigated by examining the chemical composition and the electronic properties of the films. The concentration of Cr in the passive film formed on Ni-(15, 30)Cr was increased with increase in Cr content in the alloy and decrease of solution pH from the XPS depth profiles. The photocurrent spectra for the passive films formed on Ni-(15, 30)Cr alloys in pH 8.5 buffer solution were found to be generated by NiO, Ni(OH)2, and Cr(OH)3. The chemical composition and the capacitance behavior suggested that the base structure and electronic properties of the Ni-rich or Ni-Cr mixed oxide films on Ni-(15, 30)Cr alloys are dominated by Ni oxide, whereas those for the highly Cr-enriched film on Ni-30Cr is determined by Cr oxide.

High temperature oxidation resistance of Ni-(5∼13)Co-(10∼16)Cr-(5∼9)W-5Al-(1∼1.5)Ti-(3∼6)Ta alloys

The oxidation behavior of Ni-based superalloys was examined by cyclic oxidation tests at 850 and 1000 °C. The present study focused on the investigation of two newly developed and three commercial alloys (GTD-111, IN738LC, and CM247LC). The oxidation resistance of the superalloys were evaluated by the oxidation rate constants and the mass gains. The oxidation scales mainly consisted of outer Cr2O3 and inner Al2O3 layers at both temperatures, as well as oxides of minor elements such as TiO2, NiCr2O4, CrTaO4, HfO2, and TaO. The oxidation resistance of the alloys containing larger amounts of Al, W, and Ta and lower Mo and Ti concentrations was higher than those of the other alloys. The ranking of oxidation resistance for the alloys corresponded to the statistical prediction drawn from a response surface analysis. Furthermore, these alloys contained more Ta oxides, such as CrTaO4 and TaO, suggesting that Ta oxides had an active role in improving the oxidation resistance.

Photoelectrochemical analysis of passive films formed on Ni and its alloys and its application to their corrosion behaviors

A review is presented of the photoelectrochemical and Mott–Schottky analysis for the passive film formed on Ni and its alloys, and also of its application to clarifying the role of chloride ion and solution temperature in their corrosion behaviors. Photocurrent spectra for the passive film of Ni and its alloys, measured by the continuous illumination technique, can be resolved into spectral components, each of which originated from each compound comprising the passive film such as inner NiO and outer Ni(OH)2. The composition, structure, and electronic band structure of the passive films were characterized by comparing the band gap energy and the shape and peak position in the photocurrent spectra for the films with those for the thermally grown oxide. Mott–Schottky analysis revealed that the concentration of cation vacancy in p-type passive film of Ni is significantly increased either with addition of chloride ion or with raising solution temperature The significant increase in the concentration of cation vacancy is found to be main reason for the Cl− inducing passivity breakdown and also for the increase in corrosion rate with solution temperature. These experimental results are well corresponded to the role of Cl− in the passivity breakdown proposed by the point defect model.

Statistical study of the effects of the composition on the oxidation resistance of Ni-based superalloys

The effects of alloying elements (Co, Cr, Mo, W, Al, Ti, and Ta) on the oxidation resistance of Ni-based superalloys are studied using the Response Surface Methodology (RSM). The statistical analysis showed that Al and Ta generally improve the oxidation resistance of the alloy, whereas Ti and Mo degrade the oxidation resistance. Co, Cr, and W did not alter oxidation rate significantly when examined by the mass gain averaged for all model alloys. However, it is remarkable that the degree of the effects of alloying elements varied with the concentration of other elements. Further, the effect of each element was sometimes found to be reversed for alloy groups specified by the concentration of another element.

Study on corrosion resistance of gas cylinder materials in HF, HCl and HBr environments

Abstract Corrosion properties of Mn steel, Cr–Mo steel and 316L stainless steel and effects of surface treatments including Cr or Ni coating, nitriding and oxynitriding on the corrosion resistance of steels were examined in halogen acid gas and solutions. Corrosion behaviours were evaluated by the potentiodynamic polarisation tests in 0·001M HF, HCl and HBr solutions and the exposure test in HBr gas with 1·13 ppm H2O for 24 h. The surface morphology of the tested samples was examined by optical microscope and SEM and the composition of the corrosion product was analysed by EDS after exposure of the tested alloys to HBr gas. The corrosion resistance of Mn steel and Cr–Mo steel were very poor but was considerably improved by Cr or Ni coating. Oxynitriding also reduced the corrosion rate while nitriding was not effective. However the corrosion resistance of steels with Cr, Ni plating or oxynitriding was still much inferior to electropolished 316L stainless steel, which was practically not damaged by corrosion tests in aqueous solutions nor in gas.

Determination of the Diffusivity of Cation Vacancy in a Passive Film of Ni Using Mott-Schottky Analysis and in-situ Ellipsometry

The diffusivity of cation vacancies in a passive film formed on Ni in a pH 8.5 buffer solution was estimated based on the Point Defect Model with high field migration equations. The parameters required to calculate the diffusivity of the cation vacancies include the flux of cation vacancies, the acceptor density, and the thickness of the passive film as functions of the film formation potential and they were measured from potentiostatic polarization, a Mott-Schottky analysis, and in-situ ellipsometry, respectively. The passive current density, proportional to the flux of the point defects, linearly increased with an increase of the passivation potential, while the acceptor density was reduced with the potential. The thickness of the passive film appeared to increase very rapidly and the growth became stable in thousands of seconds, and increased linearly with an increase of the potential. The dependence of all parameters investigated in this study on the passivation potential corresponded well to the theory. The diffusivity of the cation vacancies in the passive film of Ni was determined to be 2.7×10−18 cm2s−1.

Effects of Mo on the Passive Films Formed on Ni-(15, 30)Cr-5Mo Alloys in pH 8.5 Buffer Solution

The composition and semiconducting properties of the passive films formed on Ni- (15, 30)Cr-5Mo alloys in pH 8.5 buffer solution were examined. The depth concentration profile of passive films formed on Ni-(15, 30)Cr-5Mo in pH 8.5 buffer solution showed that Mo enhances the enrichment of Cr. The Mott-Schottky plot for the passive film on Ni-(15, 30)Cr- 5Mo closely resembled that for the film on Cr, whereas those for the less Cr-enriched film on Mo-free alloys showed similar behavior to that for the film on Ni. The acceptor density was reduced by increasing Cr content in Ni-(15, 30)Cr-(0, 5)Mo alloys, but addition of Mo considerably increased the acceptor density.

Statistics of oxidation resistance of Ni–(0–15)Co–(8–15)Cr–(0–5)Mo–(0–10)W–(3–8)Al–(0–5)Ti–(0–10)Ta–0.1C–0.01B superalloys at 1000 °C by compositional variations

The effects of alloying elements (Co, Cr, Mo, W, Al, Ti, and Ta) on the oxidation resistance of Ni–(0–15)Co–(8–15)Cr–(0–5)Mo–(0–10)W–(3–8)Al–(0–5)Ti–(0–10)Ta–0.1C–0.01B alloys were studied. The sample compositions were designed by the Box–Behnken method of design of experiments (DOE). The alloying elements show complicated effects on the mass gain due to oxidation, depending on the alloy composition. Al reduces the mass gain largely. The other elements except Al do not appear to exert a strong effect on the oxidation rate on average, but their influences are shown clearly in the alloys with a low Al content. Co, W, and Ta reduce the oxidation rate, while Cr, Mo, and Ti promote oxidation. Ta is the most effective element in reducing the oxidation rate of the alloy with a low Al concentration. It is confirmed that a continuous Al 2 O 3 layer is essentially required for high oxidation resistance. The oxide scale of easily oxidized alloys has various oxides such as NiCr 2 O 4 , NiAl 2 O 4 , NiO, Cr 2 O 3 , CrTaO 4 , and TiO 2 .

Effects of Crystallization on the Corrosion and Passivity of Amorphous Pd-Fe-Co-Si-B Alloys

Corrosion behavior of Pd48.2Fe17Co16.7Si13.4B4.7 and Pd51.4Fe18Co18Si11.1B1.5 alloys was examined by potentiodynamic polarization tests in pH 8.5 buffer solution. The amorphous alloy ribbons showed passivity with the passive current densities of 2 × 10−6 A/cm2~4 × 10−6 A/cm2. The crystalline alloys showed a higher corrosion rate in pH 8.5 buffer solution with the degree of variation depending on the alloy composition. It is suggested from the Mott-Schottky analysis that the donor density was lower for the amorphous alloy than the crystalline alloy.

Study on Corrosion of Automotive Coil Spring Steel by Electrochemical Impedance Spectroscopy, 전기화학 임피던스 분석을 통한 자동차용 코일스프링 강의 부식 평가

Coil spring steels from the automobile suspension part after field exposure for 10 years and those after anti-corrosion validation test in proving ground of 5,000 ~ 10,000 km were examined for corrosion damages. Partial loss of paint, accumulation of corrosion product, and cracking of paint and superficial material were observed. The surface and subsurface region of spring steels had compressive residual stress and high hardness by shot peening. The surface hardness values of the specimens were 620 ~ 670 Hv. They were 60 ~ 80 Hv higher than those of the samples taken from the middle part of the spring. The maximum compressive stress was -916 ~ -1208 MPa measured at depth of about 100 μm. Electrochemical impedance spectroscopy showed that the resistances of charge transfer and the paint layer of the spring steels ranged from several tens to millions Ωㆍcm 2 . The resistance of the field samples was much higher than that of the proving ground samples used in this study, implying that the proving ground test condition would be more corrosive than the field environment.