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Vol.61, No.8, 553 ~ 561, 2023
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| Title |
| Localized Corrosion Behavior of UNS N07718 in a Solution Simulating a Diluted-sour Environment |
| 이예진 Ye-jin Lee , 이준섭 Jun-seob Lee , 권순일 Soon Il Kwon , 신정호 Jungho Shin , 조영태 Young Tae Cho , 김석 Seok Kim , 이재현 Je-hyun Lee |
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| Abstract |
| The localized corrosion behavior of precipitation-hardened UNS N07718 was investigated by immersion tests in 6 wt% FeCl3 + 1.0 wt% HCl and the use of electrochemical techniques in a simulating solution of a diluted-sour environment of 25 wt% NaCl + 0.5 wt% CH3COOH. The Ti carbides and Nb-Mo carbides with 1-10 μm size were distributed in the alloy. After immersion at a solution temperature higher than 45℃, localized corrosion with a depth of over 25 μm was identified, and the critical pitting temperature was determined to be 45℃. Potentiodynamic polarization showed that the surface of the UNS N07718 was immediately passivated in the experimental solution. The passivity-maintaining current density was gradually increased with increasing solution temperature, and finally, localized corrosion was initiated or propagated at 0.5 VSSE in 80℃. The localized corrosion was initiated or propagated at the interface between the Ti and Nb-Mo carbides and the alloy substrate. Scanning Kelvin probe microscopic images revealed that the contact-potential difference values were in the order of Ti carbide > Nb-Mo carbide > alloy substrate, indicating that the carbides and alloy substrate act as a cathode and an anode, respectively, forming a micro-galvanic couple. Therefore, it is concluded that localized corrosion is initiated at the interface between the carbides and substrates in UNS N07718.
(Received 15 September, 2022; Accepted 8 May, 2023) |
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| Key Words |
| localized corrosion, UNS N07718, carbide, scanning kelvin probe, acidic chloride, diluted-sour environment |
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