• Journal of the Korean Society for Precision Engineering
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• v.23 no.4 s.181
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• pp.146-152
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• 2006

In micro electrochemical machining (ECM), electrodes should be prevented from unfavorable oxide and Passive layer formation on the machined surface or overall corrosion of the entire surface. Generally, metal electrodes corrode, passivate or dissolve in the electrochemical cell according to the electrode potential. Therefore, each electrode must maintain its stable potential. Tn this paper, the stable electrode potentials of tool and workpiece were determined using the potentiodynamic polarization test and verified experimentally considering machining stability and surface quality. Stable workpiece electrode potentials of two different passive materials of 304 stainless steel and nickel were determined in the 0.1 M sulfuric acid. Experimental results show good machined surface and fast machining rate using the determined electrode potentials.

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.8
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• pp.828-832
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• 2015

This study is conducted for the evaluation of corrosion and lifetime prediction of fire extinguishing pipelines in residential buildings. The fire extinguishing pipeline is made of carbon steel. Twenty-four samples were selected among all the fire extinguishing pipelines in a building; the selection was based on specimenspositions, pipeline diameters, and pipeline thickness. Analysis was conducted by using the results of visual inspection, electrochemical potentiodynamic anodic polarization test, pitting depth measurements, and extreme value statistics with the Gumbel distribution. The maximum pitting depth and remaining life were statistically predicted using extreme value statistics. During visual inspection, pitting corrosion was observed in several samples. In addition, extreme value statistics demonstrated that there were several pipelines that were very sensitive to pitting corrosion. However, the pitting corrosion was not critical in all the pipelines; thus, it was necessary to change only those pipelines that were severely corroded.

• Journal of Welding and Joining
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• v.15 no.5
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• pp.124-133
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• 1997

Corrosion behavior of welded SWS400 steel used for bridges was studied in a range of the acid-rain environment using immersion, potentiodynamic polartization, polarization resistance, and galvanic corrosion tests. The SWS400 steel exhibited active corrosion behavior in the range of acid-rain environment, i.e. no passivation. As the results of immersion corrosion test, Tafel extrapolation method, and polarization resistance measurement, the average corrosion rats of the steels were 0.31-0.72 mm/year in the pH of 4-5, and 0.17 mm/yera in the pH 6, respectively. The steel showed a resistance to corrosion in the pH 6. The observed active behavior of SWS400 steel in chloride-containing environment indicated that the chloride ions exerts a detrimental influence on the formation of passive films. Galvanic corrosion was observed between the weld and the base metals because the weld is anodic to the base metal.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.3
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• pp.576-584
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• 2020

The extruder screw material is mainly SKD11, but the recent development of synthetic resins have increased the occurrence of chemical corrosion and wear. To solve this issue, high chromium cast iron is needed because of its good abrasion resistance and corrosion resistance, but its use is avoided because of its poor machinability. In this study, to improve the machinability of high chrome cast iron, 0, 0.5, 1.0, 1.5% of nickel, which has excellent workability, was added to high chromium cast iron with a composition of Fe-23Cr-2.5C-1.2Si-1.08Mn-0.48Mo-0.3V, and annealed after casting. Subsequently, the effect of nickel on the machinability and corrosion resistance was analyzed using a turning test and coin polarization test, and compared with SKD11. After casting using a high-frequency vacuum induction furnace, the annealing treatment was performed at 750 ℃ for five hours and then reheated at 1100 ℃ for five hours. A turning test after annealing at 750 ℃ showed that the machinability was improved remarkably when the nickel content was over 1.0%. In the potentiodynamic polarization test in a 5% NaCl solution, the corrosion resistance decreased with increasing nickel content in the as-cast and annealing treatment. On the other hand, after reheating, the corrosion resistance was best with a 1.5% nickel content.

• Corrosion Science and Technology
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• v.20 no.4
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• pp.175-182
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• 2021

Magnesium alloy is limited in the industrial field because its standard electrode potential is -2.363 V vs. NHE (Normal Hydrogen Electrode) at 25 ℃. This high electrochemical activity causes magnesium to quickly corrode with oxygen in air; chemical conversion coating prevents corrosion but causes surface defects like cracks and pores. We have examined the anti-corrosion effect of sol-gel coating sealed on the defected conversion coating layer. Sol-gel coatings produced higher voltage current and smaller pore than the chemical conversion coating layer. The conversion coating on magnesium alloy AZ31 was prepared using phosphate-permanganate solution. The sol-gel coating was designed using trimethoxymethylsilane (MTMS) and (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) as precursors, and aluminum acetylacetonate as a ring-opening agent. The thermal shock resistance was tested by exposing specimens at 140 ℃ in a convection oven; the results showed changes in the magnesium alloy AZ31 surface, such as oxidization and cracking. Scanning electron microscope (FE-SEM) analysis confirmed a sealed sol-gel coating layer on magnesium alloy AZ31. Electrochemical impedance spectroscopy (EIS) measured the differences in corrosion protection properties by sol-gel and conversion coatings in 0.35 wt% NaCl solution, and the potentiodynamic polarization test and confirmed conversion coating with the sol-gel coating show significantly improved resistance by crack sealing.

• Journal of Welding and Joining
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• v.18 no.6
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• pp.48-54
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• 2000

The corrosion properties of Ni-Cr-W-Mo-B alloy sprayed by the high velocity oxy-fuel spraying (HVOF) was studied as a function of heat treatment by using both potentiodynamic polarization and immersion tests in the H₂SO₄ solution. The mechanical property was also evaluated by a microhardness tester. Microstructural characteristics of te as-sprayed and annealed coatings at 550, 750 and 950℃ have been analyzed by means of OM, XRD, SEM and TEM. The results showed that the corrosion resistance was improved by increasing the annealing temperature. As-sprayed coating had metastable and heterogeneous phases such as amorphous, nanocrystalline and very refined grain and precipitates, which induced a localized corrosion. The localized corrosion occurred preferentially at the unmelted particles which were composed of Ni matrix and Cr, W and Mo riched phase segregated in the boundaries. As annealing temperature was increased, the microstructure had shown some changes - reduction of porosity and s[plat boundary decomposition and crystallization of amorphous/nanocrystalline phases, grain coarsening,, formation and growth of precipitates such as {TEX}$M_{23}C_{6}${/TEX} and {TEX}$M_{7}C_{3}${/TEX}. In addition, the compositional difference between matrix and boundary phases gradually disappeared, which changed the corrosion type from localized corrosion to general corrosion and thus enhanced corrosion resistance.

• Journal of Applied Reliability
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• v.17 no.2
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• pp.122-128
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• 2017

Purpose: To investigate the corrosion inhibition effect of the natural spearmint oil extracted from Mentha Spicata plants on 304 stainless steel in different concentrations of hydrochloric acid. Method: The anti-corrosive effect has been investigated in 0.5m, 1m and 2m HCl using weight loss test and electrochemical polarization method as a function of inhibitor concentration and immersion time in strong chloride environment. The surface morphology was analysed by scanning electron microscopy (SEM). Results: The corrosion rate of steel decreased and inhibition efficiency increased with the increase in inhibitor concentration. Microscopic evaluation revealed significant corrosion in the specimens immersed in uninhibited conditions. Potentiodynamic polarization test results showed an increase in corrosion potential (Ecorr) and decrease in corrosion current (icorr) value with increasing concentration of inhibitor. Conclusions: Immersion of steel in higher concentration of inhibitor resulted in greater surface coverage value and hence lesser number of surface corrosion sites/pores were formed; thus lowering the corrosion rate.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.5
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• pp.551-556
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• 2018

The material normally used in hot dip galvanizing facilities is SM45C (carbon steel for mechanical structure, KS standard), mainly because of its price. During this process, the oxidation of the plating facility occurs due to the heat of the Zn fumes coming from the molten zinc. Since the cycle time of the current facilities is 6 months, much time and money are wasted. In this study, the corrosive properties of various materials (Inconel625, STS304, SM45C) were investigated by oxidation in a high temperature and Zn fumes environment. The possibility of applying the hot-dip galvanizing equipment was investigated for each material. The Zn fumes were generated by directly bubbling Ar gas into Zn molten metal in a 650 degree furnace. High-temperature, Zn fumes corrosion was conducted for 30 days. The sample was removed after 30 days and the oxidation of the surface was confirmed with EDS and SEM, and the corrosion properties were examined using potentiodynamic polarization tests.

• Corrosion Science and Technology
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• v.17 no.3
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• pp.101-108
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• 2018

AA7010 is an Al-Zn-Mg-Cu alloy containing Zr, developed as an alternate to traditional AA7075 alloy owing to their high strength combined with better fracture toughness. It is necessary to improve the corrosion resistance and surface properties of the alloy by incorporating plasma electrolytic oxidation (PEO) method. AA7010-T7452 aluminum alloy has been processed through the forging route with multi-stage working operations, and was coated with $10{\mu}m$ thick $Al_2O_3$ ceramic aluminina coating using the plasma electrolytic oxidation (PEO) method. The corrosion, stress corrosion cracking (SCC) and nano-mechanical behaviours were examined by means of potentiodynamic polarization, slow strain rate test (SSRT) and nano-indentation tests. The results indicated that the additional thermomechanical treatment during the forging process caused a fully recrystallized microstructure, which lead to the poor environmental cracking resistance of the alloy in 3.5% NaCl solution, despite the overaging treatment. Although the fabricated PEO coating improved general corrosion resistance, the brittle nature of the coating did not provide any improvement in SCC resistance of the alloy. However, the hardness and elastic modulus of the coating were significantly higher than the base alloy.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.6
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• pp.98-103
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• 2018

Many coating technologies have been developed so far to improve the corrosion resistance, strength, abrasion resistance and other surface properties of materials and equipment. Among them, the formation of CCO (CaCoO, then CCO) thin films has been studied and used in the electronic material field. One of the characteristics of CCO thin films is that it is resistant to high temperature heat. Particularly, the method of forming the CCO thin film is relatively simple, and it was judged that it could be introduced into the existing equipment. Therefore, in this study, an experiment and analysis were carried out to determine whether the coating of CCO thin films can be applied to hot dip galvanizing facilities. A CCO thin film was formed on the surface of STS304 base material and oxidized in a Zn fume atmosphere in a $650^{\circ}C$ furnace with an air atmosphere. Oxidation was carried out for 30 days, after which the shape of the CCO thin film was confirmed by SEM and its corrosivity was analyzed through a potentiodynamic polarization experiment.