Identification of defects generated during cathodic charging in pure iron by thermal analysis technique
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Identification of Defects Generated during Cathodic Charging in Pure Iron by Thermal Analysis Technique JONG-LAM LEE and JAI-YOUNG LEE Although the mechanism of hydrogen embrittlement has not been understood yet fully, it has been thought to be related to the lattice defects such as microvoid or microcrack and dislocation, which can be generated by hydrogen during cathodic charging and/or rapid cooling from high temperature after the gaseous hydrogen charging. Therefore the study of the damages caused by hydrogen is important to interpret the phenomenon of hydrogen embrittlement because it may act as variables of hydrogen embrittlement such as composition, microstructure, process history, and strength level, which result in the above phenomenon. 1-4 Also, the study of hydrogen-induced damages is necessary to explain the hydrogen trapping phenomena because it can be generated during electro-permeation experiments. Although it has been reported5'6 that the lattice defects, like microvoid and dislocation, would be produced as a result of precipitation of hydrogen molecules in some site when the iron specimen was charged cathodically with hydrogen at a high applied current density, 10 mA/cm 2, such defects have not been investigated in the light of trapping site of hydrogen, and dominant type of defect among them has not been determined. Therefore, the present work is undertaken to determine the types and the amount of lattice defects by the thermal analysis technique and to propose a major trapping site of hydrogen among such defects, created in pure iron during the cathodic charging of hydrogen. JONG-LAM LEE and JAI-YOUNG LEE are a Graduate Student and a Professor, respectively, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea. Manuscript submitted July 13, 1984. 468
VOLUME16A, MARCH 1985
Table I.
Chemical Composition of Specimen (Wt ppm)
C
N
S
P
50
35
40
--
Ni 511
Cr --
Si --
Mn --
In order to charge the hydrogen cathodically, electrolytic pure iron specimen with rectangular shape (1 x 6 x 14 mm) was chemically polished after surface grinding to emery paper # 1200. The chemical composition of pure iron used in this experiment is given in Table I. Cathodic charging was performed for 12 hours at room temperature in the range of current density, 0.5 to 20 mA/cm 2 in a 10 pct sulfuric acid solution where a recombination poison of about 10 mg As203/1 was added. No external stress was applied during cathodic charging. After cathodic charging, the specimen was thermally analyzed 5 to 623 K to reduce the possibility of the annihilation of dislocations during heating (first thermal analysis). It is confirmed that the hydrogen dissolved in iron specimen by cathodic charging is escaped from specimen completely during first thermal analysis. So as to determine the types and amounts of damages caused by hydrogen during the cathodic charging, a second thermal analysis was carried out. After the first thermal analysis, the s
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