• Corrosion Science and Technology
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• v.12 no.6
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• pp.265-273
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• 2013

Ti-stabilized 11 wt% Cr ferritic stainless steels (FSSs) for automotive exhaust systems have been experienced intergranular corrosion (IC) in some heat-affected zone (HAZ). The effects of sensitizing heat-treatment and silicon on IC were studied. Time-Temperature-Sensitization (TTS) curves showed that sensitization to IC was observed at the steels heat-treated at the temperature lower than $650^{\circ}C$ and that silicon improved IC resistance. The sensitization was explained by chromium depletion theory, where chromium is depleted by precipitation of chromium carbide during sensitizing heat-treatment. It was confirmed with the results from the analysis of precipitates as well as the thermodynamical prediction of stable phases. In addition, the role of silicon on IC was explained with the stabilization of grain boundary. In other words, silicon promoted the formation of the grain boundaries with low energy where precipitation was suppressed and consequently, the formation of Cr-depleted zone was retarded. The effect of silicon on the formation of grain boundaries with low energy was proved by the analysis of coincidence site lattice (CSL) grain boundary, which is a typical grain boundary with low energy.

• Korean Journal of Materials Research
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• v.27 no.8
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• pp.431-437
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• 2017

This study investigated the surface pit corrosion of SS420J2 stainless steel accompanied by intergranular crack. To reveal the causes of surface pits and cracks, OM, SEM, and TEM analyses of the microstructures of the utilized SS420J2 were performed, as was simulated heat treatment. The intergranular cracks were found to have been induced by a grain boundary carbide of $(Cr,Fe)_{23}C_6$, which was identified by SEM/EDS and TEM diffraction analyses. The mechanism of grain boundary sensitization occurred at the position of the carbide, followed by its occurrence at the Cr depleted zone. The grain boundary carbide of $(Cr,Fe)_{23}C_6$ type precipitated during air cooling condition after a $1038^{\circ}C$ solid solution treatment. The carbide precipitate formation also accelerated at the band structure formed by cold working. Therefore, using manufacturing processes of cooling and cold working, it is difficult to protect SS420J2 stainless steel against surface pit corrosion. Several counter plans to fight pit corrosion by sensitization were suggested, involving alloying and manufacturing processes.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.250-254
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• 2002

Intergranular corrosion of austenitic stainless steels is a conventional and momentous problem during welding and high temperature use. One of the major reasons for such intergranular corrosion is so-called sensitization, i.e., chromium depletion due to chromium carbide precipitation at grain boundaries. Conventional methods for preventing sensitization of austenitic stainless steels include reduction of carbon content in the material, stabilization of carbon atoms as non-chromium carbides by the addition of titanium, niobium or zirconium, local solution-heat-treatment by laser beam, etc. These methods, however, are not without drawbacks. Recent grain boundary structure studies have demonstrated that grain boundary phenomena strongly depend on the crystallographic nature and atomic structure of the grain boundary, and that grain boundaries with coincidence site lattices are immune to intergranular corrosion. The concept of "grain boundary design and control", which involves a desirable grain boundary character distribution, has been developed as grain boundary engineering. The feasibility of grain boundary engineering has been demonstrated mainly by thermomechanical treatments. In the present study, a thermomechanical treatment was tried to improve the resistance to the sensitization by grain boundary engineering. A type 304 austenitic stainless steel was pre-strained and heat-treated, and then sensitized, varying the parameters (pre-strain, temperature, time, etc.) during the thermomechanical treatment. The grain boundary character distribution was examined by orientation imaging microscopy. The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of coincidence-site-lattice boundaries indicated a maximum at a small strain. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanically-treated specimen than in the base material. An excellent intergranular corrosion resistance was obtained by a small strain annealing at a relatively low temperature for long time. The optimum parameters created a uniform distribution of a high frequency of coincidence site lattice boundaries in the specimen where corrosive random boundaries were isolated. The results suggest that the thermomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.6
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• pp.709-715
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• 2006

The effects of thermal cycle condition and applied stress on the intergranular corrosion in austenitic 316 type stainless steels were investigated. Specimens were solution-treated at 1100$^{\circ}C$ for one hour and then sensitized in the temperature range of $500{\sim}800^{\circ}C$ by holding $2{\sim}300s$ with a various applied stresses of $0{\sim}8kg/mm^2$. Degree of sensitization. DOS %, was measured through polarization curve by electrochemical DL-EPR test. Microstructural observations were also conducted DOS % increased with an increase of sensitization temperature and/or holding time. Increase of applied stress resulted in increase of DOS % and more corroded surface because of acceleration of intergranular corrosion and fine grain size due to the stress. Cr depleted zone near grain boundary was observed. The amount of depletion was profounded with an increase of sensitization temperature, holding time and applied stress. $M_{23}C_6$ carbides were precipitated discontinuously at grain boundary. However, its amount was relatively small in the thermal cycle condition of 800$^{\circ}C$, 300sec and 4kg/mm$^2$.

• Corrosion Science and Technology
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• v.14 no.6
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• pp.313-324
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• 2015

Austenitic stainless steels have been widely used in many engineering fields because of their high corrosion resistance and good mechanical properties. However, welding or aging treatment may induce intergranular corrosion, stress corrosion cracking, pitting, etc. Since these types of corrosion are closely related to the formation of chromium carbide in grain boundaries, the alloys are controlled using methods such as lowering the carbon content, solution heat treatment, alloying of stabilization elements, and grain boundary engineering. This work focused on the effects of aging and UNSM (Ultrasonic Nano-crystal Surface Modification) on the intergranular corrosion of commercial 316L stainless steel and the results are discussed on the basis of the sensitization by chromium carbide formation and carbon segregation, residual stress, grain refinement, and grain boundary engineering.

• Corrosion Science and Technology
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• v.17 no.4
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• pp.183-192
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• 2018

Stainless steel can be classified into three categories depending on the microstructure as austenitic stainless steel, ferritic stainless steel and martensitic stainless steel. Generally, stainless steel is extremely resistant to corrosion as the name implies. However, under specific environments, susceptibility to localized corrosion such as pitting, intergranular corrosion and stress corrosion cracking increases. This paper reviewed the state of arts on intergranular corrosion mechanisms, countermeasures on intergranular corrosion and intergranular corrosion test methods. Intergranular corrosion is mostly related with chromium depletion at the grain boundary and sometimes with segregation of electroactive elements in solution annealed stainless steel. Countermeasures on intergranular corrosion include avoiding chromium depletion by heat treatment and the addition of alloying elements. Sensitization evaluation of stainless steel was performed either through acid immersion test or electrochemical test. The methods were standardized in (Japanese Industrial Standards). Even though are useful in evaluating the degree of sensitization for industrial purpose but do not provide detailed information about sensitization mechanism, cause and chromium profile.

• Journal of the Korean Society of Safety
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• v.25 no.6
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• pp.28-33
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• 2010

Stainless steel is useful material for various industrial facilities such as the nuclear and steam power plant and the heavy chemical industry due to its good corrosion resistance and mechanical properties. However, it has also a large problem that is sensitized in the welding process and its corrosion resistance and mechanical properties decreases by sensitization. Thus, corrosion and corrosion fatigue characteristics of artificially sensitized austenitic STS304 were investigated through the EPR test and corrosion fatigue test. Obtained results are as follows: 1) According to the sensitizing period increase, Cr deficiency layer is linearly expanded. 2) Degree of sensitization(Ia/Ir) proportionally increased with sensitizing period. However, after 4hrs, it showed constant value. 3) Cr-carbide($Cr_{23}C_6$) in the grain boundary increased as sensitizing period increases until six hours. 4) corrosion fatigue strength of sensitized STS304 were remarkably reduced compare to non-sensitized ones.

• Journal of the Korean Society for Heat Treatment
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• v.23 no.2
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• pp.90-95
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• 2010

The effects of grain size on the corrosion resistance of 429EM ferritic stainless steels for automobile exhaust system were investigated. Using specimens held at maximum service temperature of $950^{\circ}C$ for 10~70 hours, electrochemical polarization tests were conducted. While corrosion current density, $I_{corr}$, was influenced little by grain size, pitting potential, $E_p$, was increased with an increase of grain size. Sensitizaton at grain boundary occurred when the specimen were held at $950^{\circ}C$ for above 50 hours because of the dissolution of precipitates and grain coarsening.

• Journal of Welding and Joining
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• v.8 no.4
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• pp.27-34
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• 1990

Integranular corrosion behaviors of KAL (Knife Line Attack) and mechanical properties such as tensile and creep rupture were investigated for the tube material used for nearly 20 years under the condition of 463.deg. C and 28 $kg/cm^2$. Based and weld metal were austenitic stainless steel AISI 321 containing Ti, AISI 347 containing Nb, respectively. KLA is a kind of the intergranular corrosion which often occurs just near the HAZ (heat affected zone) of AISI 321 and AISI 347 stainless steel due to the grain boundary sensitization. In KLA zone, intergranular corrosion crack has propagated outwards from the inner surface and carbides of white and narrow band type assuming as (Cr, Fe) carbide were confirmed. All the delta-ferrite formed in the weld metal during weld solidification has been transformed into sigma-phase since delta-ferrte was exposed for 20 years at 463.deg. C. Elongation was very low at the range from room temperature to 600.deg. C and it was confirmed that creep-rupture properties were not consideralbly affected.

• Journal of Advanced Marine Engineering and Technology
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• v.23 no.6
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• pp.761-769
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• 1999

In this paper the effect of aging heat treatment to the Corrosion behavior for the Nimonic 80A superalloy was studied by AC Impedance methods. Tested solution was 3.5% with tempera-ture $25^{\circ}C$ Electro-chemical corrosion test were carried out for the Nimonic 80A super-alloy which solution heat treated at $1080^{\circ}C$ for 8 hours followed by aging heat treated at $650^{\circ}C,\;700^{\circ}C,\;750^{\circ}C\;800^{\circ}C$ and $850^{\circ}C$ with 16hours under vacuum environment. The obtained results were as follows; 1. Base metal and solution-treated materials were exhibited similar corrosion tendency as Ran-dle equivalent cell. The value of passive film resistance was 579 ohms for the base metal and 124,770 ohms for the solutionized metal such a difference was arose by the ${{\gamma}_^'}$ precipitate on the metal surface during heat treatment. 2. The measure value of $R_p$ for heat-treated at $650^{\circ}C,\;700^{\circ}C,\;800^{\circ}C$and $850^{\circ}C$ were 97,943, 93, 111, 26,961, 15,798 and 11,780ohm respectively. Which indicated that the passive film resistance Rp was reduced as aging temperature increased due to the growth of grain size and sensitization at the grain boundary. 3. The similar tendency was exhibited for corrosion behavior of the electro-chemical corrosion polarization method and AC impedance method and confirmed that AC impedance method was useful tool for corrosion research.