• Steel and Composite Structures
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• v.46 no.3
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• pp.385-401
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• 2023

Steel is becoming increasingly popular due to its high strength, excellent ductility, great assembly performance, and recyclability. In reality, steel structures serving for a long time in atmospheric, industrial, and marine environments inevitably suffer from corrosion, which significantly decreases the durability and the service life with the exposure time. For the mechanical properties of corroded steel, experimental studies are mainly conducted. The existing numerical analyses only evaluate the mechanical properties based on corroded morphology at the isolated time-in-point, ignoring that this morphology varies continuously with corrosion time. To solve this problem, the relationships between pit depth expectation, standard deviation, and corrosion time are initially constructed based on a large amount of wet-dry cyclic accelerated test data. Successively, based on that, an in-situ pitting evolution method for evaluating the residual tensile strength of corroded steel is proposed. To verify the method, 20 repeated simulations of mass loss rates and mechanical properties are adopted against the test results. Then, numerical analyses are conducted on 135 models of corrosion pits with different aspect ratios and uneven corrosion degree on two corroded surfaces. Results show that the power function with exponents of 1.483 and 1.091 can well describe the increase in pit depth expectation and standard deviation with corrosion time, respectively. The effect of the commonly used pit aspect ratios of 0.10-0.25 on yield strength and ultimate strength is negligible. Besides, pit number ratio α equating to 0.6 is the critical value for the strength degradation. When α is less than 0.6, the pit number increases with α, accelerating the degradation of strength. Otherwise, the strength degradation is weakened. In addition, a power function model is adopted to characterize the degradation of yield strength and ultimate strength with corrosion time, which is revised by initial steel plate thickness.

• Corrosion Science and Technology
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• v.23 no.4
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• pp.315-323
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• 2024

This work examined pitting corrosion failure of a copper heat-return pipe used in a district heating system. The copper pipe was corroded with a 48% reduction in thickness due to localized corrosion on the inner surface exposed to heating water of 20 ~ 40 ℃. Fe and Si elements as corrosion products were found around pits. Cl element was also observed, which accelerated oxidation of copper inside pits. Cu₂O deposits on the pit's bottom surface decreased the pH inside the pit. X-ray diffraction analysis revealed hematite, cuprite, malachite and brochantite as corrosion products. Chemical analysis demonstrated that Fe and Si elements did not exist in the copper, supply water, or heating water, indicating that Fe and Si species might have entered into the pipe from the exterior. These results indicated that pits were initiated due to ion concentration gradient near Fe and Si species. Moreover, the interior of pits had lower pH due to Cl^- concentration and Cu₂O reactions, which accelerated the pit's growth and led to formation of pinholes. Additionally, we confirmed that the type of pitting corrosion was a complex combination of types I and II based on the HCO₃^-/SO₄^2- ratio, pH, temperature, and corrosion products.

• Journal of the Korean institute of surface engineering
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• v.37 no.2
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• pp.119-127
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• 2004

In order to develop the corrosion and wear resistance of stainless steels, effects of plasma-nitriding on the surface characteristics of stainless steels containing Nb were investigated by utilizing a potentiostat. It was found that plasma nitriding at $350^{\circ}C$, compared with $500^{\circ}C$, produced a good corrosion resistance as nitriding time increased, whereas stainless steel containing low Nb content showed that pitting potential and corrosion potential decreased.

• Corrosion Science and Technology
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• v.13 no.4
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• pp.152-156
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• 2014

This study was undertaken to examine the electrochemical corrosion properties of an $Co_{80}Nb_{10}B_{10}$ and $Co_{82}Nb_8B_{10}$ amorphous alloys prepared by melt-spinning method under various conditions. The potentiodynamic polarization responses at various levels of pH (pH 2, pH 7, pH 12) showed that the corrosion current rate of $Co_{80}Nb_{10}B_{10}$ alloy is lower than that of $Co_{82}Nb_8B_{10}$ alloy in all levels of pH, implying the general corrosion resistance of the alloy with higher Nb content is better than that with higher Co content. The pitting potential of $Co_{80}Nb_{10}B_{10}$ alloy was also better than that of $Co_{82}Nb_8B_{10}$, evidenced by the higher pitting potential. Nb is thought to be effective in increasing the protectiveness of the passive film and hence to improve the corrosion resistance of Co-Nb-B alloys.

• Journal of Advanced Marine Engineering and Technology
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• v.27 no.6
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• pp.704-713
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• 2003

The effect of partial substitution of tungsten for molybdenum on the microstructure and corrosion resistance in 22Cr-5Ni-3Mo duplex stainless steel(DSS) aging heat treated in a temperature range of 600~$1000^{\circ}C$ has been investigated. Electrochemical tests were carried out for the evaluation of corrosion resistance. Aging treatment had hardly influenced the general corrosion resistance. With the increase of aging time, the pitting corrosion resistance of the DSS had decreased, After aging for 2min at 700~$900^{\circ}C$, the pitting potential of the 3Mo steel decreased remarkably, while that of the W-substituted steel hardly changed. During aging. the intermetallic $\sigma$ and secondary austenite ($\gamma_2$) phases were precipitated. and the pitting corrosion and intergranular corrosion resistance were significantly decreased after aging at 700~$750^{\circ}C$ for 10 h, which could be caused by the $\gamma_2$ formation. The ${\gamma}$$_2$ phase could affect the depletion of molybdenum and chromium in the $\gamma_2/\alpha and \gamma_2/\sigma$ boundaries.

• Journal of the Korean Society for Heat Treatment
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• v.21 no.3
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• pp.150-156
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• 2008

The effects of austenitizing temperature in a range of $1000{\sim}1150^{\circ}C$ on the corrosion resistance in 420J2 stainless steel tempered at $150^{\circ}C$ were investigated by an electrochemical uniform corrosion test in a solution of 0.5M $H_2S0_4$. Pitting test and DL-EPR test for intergranular corrosion were carried out in a solution of 3.5% NaCl and 0.5M $H_2S0_4$ + 0.01 M KSCN respectively. In uniform corrosion test, specimens austenitized below $1100^{\circ}C$ showed similar corrosion current density and passive current density, whereas specimens austenitized at $1150^{\circ}C$ showed a little higher values. Pitting potential slightly increased with an increase of austenitizing temperature. The degree of sensitization, DOS, also slightly increased with an increase of austenitizing temperature, reaching the highest degree at $1150^{\circ}C$. It was expected that the increase of DOS was due to the larger grain size rather than the dissolved precipitates in the matrix.

• Corrosion Science and Technology
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• v.2 no.2
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• pp.75-81
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• 2003

Chromium, molybdenum. and nitrogen are very important alloying elements in stainless steels and its effect was approved in pitting resistance equivalent (PRE) equations and many experimental results. However, Cr can improve the corrosion resistance, but facilitate the formation of sigma phase. Also. Mo has the same effect in stainless steels. If Cr and Mo are added at high amount to increase the corrosion resistance of stainless steel, corrosion resistance in annealed alloys can be improved, but in case of welding or aging heat treatment. its resistance will be drastically decreased. In this work, increasing Cr and N contents but decreasing Mo than the commercial alloys made the experimental alloys. Typical alloys are 25Cr-4.5Mo-0.43N alloy, 27Cr-4.7Mo-0.4N alloy, 27Cr-5.3Mo-0.25N alloy, 32Cr-2.6Mo-0.36N alloy. After annealing and aging heat treatment, microstructures, anodic polarization test, and pitting corrosion test were performed. Annealed alloys showed $100^{\circ}C$ of CPT and aged alloys showed the different tendency depending upon Cr and Mo contents(SFI)

• Corrosion Science and Technology
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• v.23 no.4
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• pp.255-265
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• 2024

Duplex stainless steels (UNS S32205, UNS S32750) are used in various environments. The potentiodynamic polarization tests were conducted at 30 ℃ in order to study the electrochemical corrosion behaviors of duplex stainless steels under different seawater concentrations (fresh water, seawater, mixed water). The results of Tafel analysis in seawater showed that UNS S32205 and UNS S32750 had the highest corrosion current densities at 6.12 × 10^-4 mA/cm² and 5.41 × 10^-4 mA/cm², respectively. The pitting potentials of UNS S32205 and UNS S32750 were comparable to or higher than the oxygen evolution potential in fresh water, mixed water, and seawater. The maximum damage depths and surface damage ratio caused by pitting corrosion increased with chloride concentration. The synergy effect of molybdenum and nitrogen enhances the concentration of Mo, Ni, and Cr at the interface of the metal-electrolyte. In particular, in the case of nitrogen, NH₃ and NH₄⁺ are formed to compensate for the pH drop in the pitting region, thereby strengthening the repassivation of the film. The excellent corrosion resistance of UNS S32750 is attributed to the strengthening effect of the chromium oxide film due to the presence of molybdenum and nitrogen.

• Korean Journal of Metals and Materials
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• v.48 no.8
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• pp.741-748
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• 2010

We studied the corrosion and oxidation properties of Fe-18Cr-0.4Nb-(0.1~0.6)Ti-(1~3)Si-(0.5~2)Mo stainless steel. The resistance to general and pitting corrosion was evaluated and the results were analyzed by Response Surface Methodology (RSM) as a function of alloy composition. The effects of alloy composition and heat treatment on the oxidation resistance were also examined. Mo increased both general corrosion resistance and pitting corrosion resistance. Si improved the resistance of the alloys to pitting corrosion. Si was also beneficial for general corrosion resistance of the alloys containing Mo at more than 1 wt.%. However, Mo was detrimental when its content was lower. Effects of Ti on general corrosion properties appeared to be weak and a high concentration of Ti appeared to deteriorate pitting resistance. The thickness of the oxidation scale increased and adhesion of the scale worsened as the temperature increased from $800^{\circ}C$ to $900^{\circ}C$. Weight gain of the alloys due to oxidation at $900^{\circ}C$ clearly showed that the resistance to oxidation is improved by annealing at $860^{\circ}C$ and an increase of Si content.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.699-710
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• 2020

Pitting corrosion commonly shaped in hull structure due to marine corrosive environment seriously causes the deterioration of structural performance. This paper deals with the ultimate strength behaviors of stiffened ship panels damaged by the pits subjected to uniaxial compression. A series of no-linear finite element analyses are carried out for three stiffened panels using ABAQUS software. Influences of the investigated typical parameters of pit degree (DOP), depth, location and distribution on the ultimate strength strength are discussed in detail. It is found that the ultimate strength is significantly reduced with increasing the DOP and pit depth and severely affected by the distribution. In addition, the pits including their distributions on the web have a slight effect on the ultimate strength. Compared with regular distribution, random one on the panel result in a change of collapse mode. Finally, an empirical formula as a function of corrosion volume loss is proposed for predicting the ultimate strength of stiffened panel.