Formation kinetics and rupture strain of Ni-Cr-Fe alloy corrosion films formed in high-temperature water
- PDF / 200,427 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 106 Downloads / 139 Views
CTION
Vscc ⫽
THERE has been considerable work characterizing the stress corrosion cracking (SCC) of austenitic materials in nuclear plant service. Intergranular SCC of austenitic stainless steel alone has cost over a billion dollars in research, repair, and mitigation.[1] In systems where Ni-Cr-Fe alloys are used, the higher Cr content Alloy 690 has largely replaced Alloy 600 as the material of choice in SCC-prone applications.[2,3,4] The fundamental reasons why Cr has a beneficial effect on pure, high-temperature water SCC resistance are not well understood. There is not a universally accepted SCC mechanism for Ni alloys in high-temperature water. There are two classes into which most mechanistic descriptions fall: rupture dissolution[5,6,7] and hydrogen-assisted cracking.[8,9,10] The two processes are closely linked and experimental separation is difficult, e.g., conditions which decrease the crack tip corrosion rate also reduce the hydrogen generation rate. Subprocesses that are influenced by Cr content, such as creep rate, corrosion rate, and oxide film mechanical properties, are common to both rupture-dissolution and hydrogen-assisted cracking mechanisms. The Ford–Andresen model[7,11–12] has been used to describe the SCC of stainless steel in BWR environments and, in fact, forms part of the design basis for operating decisions in some United States nuclear plants. The concept is that structural damage of the oxide film at the crack tip results in rapid local corrosion of the underlying metal. This corrosion repairs the oxide, slowing down the corrosion rate until the film is again damaged. Qualitative descriptions of this model refer to slip emergence at the crack tip as the damage mechanism. Quantitatively,
P.M. ROSECRANS, is the Manager, Materials Development Operation, Lockheed Martin Corporation, Schenectady, NY 12301-1072. D.J. DUQUETTE, is the Chairman, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted September 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
M i0tn0 ˙ c ZF 1 ⫺ n f
n
冢冣
[1]
where Vscc ⫽ average crack tip extension rate, Z ⫽ number of electrons, F ⫽ Faraday’s constant, i0, t0, n ⫽ from repassivation profile, f ⫽ oxide rupture strain, ˙ c ⫽ crack tip strain rate, M ⫽ atomic weight, and ⫽ metal density. The individual material properties such as creep rate (which influences crack tip strain rate) and oxide rupture strain have not been determined explicitly. Through empirical correlations between stress state and crack tip strain rate and values taken from a repassivation current decay curve, this framework was developed to predict stainless steel SCC growth rates. Some subsequent work has shown that the same analytical approach is consistent with the SCC behavior of Alloy 600.[13,14] The three material properties associated with the rupturedissolution mechanism, and also important for any hydrogen-assisted mechanism, are creep, oxide rupture strain, and corrosion kinetics. Of the three, more data are
Data Loading...
