1998 Fiscal Year Final Research Report Summary
STUDY ON GENERATION,GROWTH AND REPASSIVATION OF PIT UNDER UNSTEADY CONDITION
| Project/Area Number |
09650775
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Material processing/treatments
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| Research Institution | IWATE UNIVERSITY |
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Principal Investigator |
YASHIRO Hitoshi IWATE UNIVERSITY,FACULTY OF ENGINEERING,ASSOCIATE PROFESSOR, 工学部, 助教授 (60174497)
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| Project Period (FY) |
1997 – 1998
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| Keywords | pitting / stainless steel / repassivation / iron(III) chloride / cathodic polarization |
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| Research Abstract |
The relation between variation of pit depth and that of corrosion potential with time has been clarified for type 304 stainless steel immersed into iron (III) chloride solutions. The pit deepened proportionally to the square root of time when the concentration of iron (III) chloride is high. When the concentration of iron (III) chloride is not high enough to depolarize on growth of pits, corrosion potential fell sharply on initiation of pits and turned nobler after some duration, suggesting repassivation of pits. The number of cycles of potential fluctuation corresponded to that of pits. A new method to evaluate repassivation behavior was propose potential response after a given period of potential control well reflected if pits survived during that period. The deeper the pits formed before the potential control, the harder they repassivatated both in terms of potential and time.
Tow stainless steel specimen, one in iron (III) ion free chloride solution and the other in iron (III) chloride containing solution, were coupled to follow the potential fluctuation and galvanic current. The relation between potential and current was found to correspond to the cathodicpolarization curve for iron (III) ion on stainless steel. The total amount of coulomb well coincided to the total pit volume.
The cathodic polarization behavior of iron (III) ion on stainless steel (i-E) was input to a computer. A potentiostatic was programed to follow the i-E relation during the potentiostatic operation. Thus, the potential fluctuation observed in FeCl_3 solution was reproduced using the programed cathode in a NaCI solution without iron (111) ion.
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