| The corrosion experiments of iron were performed in boiling 55% Ca(NO₃)₂solution (115℃) to elucidate kinetics of intergranular corrosion (IGC) of iron. Four kinds of iron specimens were used in this study. Three kinds of specimens with the following different chemical compositions were prepared from a starting material with 20 to 40 ppm C, 17 ppm N, and 20 ppm P. One batch (A) of this material was normalized only, a second batch (B) was also decarburized up to under 10 ppm C prior to normalizing, and a third batch (C) was decarburizbd, and then nitrided up to 140 to 220 ppm N prior to normalizing. The fourth specimen (O) contained 200 ppm C, 60 ppm N, and 110 ppm P (Armco iron). All normalized specimens (A, B, C, O) were annealed (A₁,B₁,C₁,O₁) and tempered (A₂,B₂,C₂,O₂). After the specimen was exposed for 15 min to 24 h to the IG Ctest solution at 1000 mV_SHE (passive region) and the time-current curve was recorded, the specimen was then examined for IGC by optical microscopy. The time-current curves and optical micrographic observations suggested that IGC occurred after passivation of specimen and passivation is also necessary for occurrence of IGC of iron. All normalized (A, B, C, O), the annealed (A₁,B₁,C₁,O₁) and the tempered (A₂,B₂,C₂) specimens show no intergranular attack. Only the annealed (O₁) and tempered (O₂) Armco iron specimens were markedly susceptible to IGC. The excess dissolved nitrogen or nitride at the grain boundaries (gbs) does not lead to a definite IGC. The presence of fine carbides at the gbs may cause IGC, but a critical amount of carbon at the gbs may be necessary for occurrence of IGC. |
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