The stress corrosion behavior of fine grained, low alloy steels has been investigated using constant strain rate tensile tests. Studied materials were a A508 Class II type KS05 (German grade: 22 NiMoCr 3 7) and a A533 B Class I (German grade: 22 MnMoNi 5 5). The susceptibility to stress corrosion cracking was determined as a function of the amount of dissolved oxygen and temperature using cylindrical smooth specimen (DIN 50125) in purified water (conductivity below 0.2 µS/cm). The environment was controlled and conditioned using a closed loop refreshing apparatus. The strain rate was varied between 2-10-2 and 5-10-9 1/s. Most of the experiments have been carried out until fracture of the specimens. In addition some experiments were stopped after various exposure times and the specimens were broken in liquid nitrogen in order to observe initial stages of crack formation and crack propagation. The fracture surface of broken specimens has been examine by means of light and scanning electron microscopy. The fraction of brittle fracture mode on the rupture surface has been quantified in relation to exposure time to estimate a crack growth velocity. The results of the experiments indicate a lowest oxygen concentration in the water necessary for the appearance of stress corrosion cracking which is estimated to be 10ppb. Brittle cracking generated by stress corrosion during exposure to high pressure high temperature conditions only appears beyond a preliminary plastic deformation. The value of this critical plastic deformation was found to be 3% but a lower critical value cannot be excluded. Additional stress corrosion is significant only at strain rates below 10-4 1/s. Values of true crack propagation rates can be determined only after correcting for the slow straining time necessary to reach the critical elongation. At this stage crack initiation always starts at sulphide inclusions in or beneath the specimen surface.
The following paper is a summary of previous and current work in the authors? laboratory on the stress corrosion susceptibility of fine grained low-alloy steel typically used for high-temperature pressure vessels, with a particular view on nuclear power plants . In these plants, any corrosion failure would, in the first place, require a proceeding failure of the thick lining with austenitic stainless steel, and such failures are exceedingly improbable. This aspect of the matter however is not considered further presently. For the issue under consideration, a number of conclusions obtained by many authors appear to go, by now, uncontested. Stress corrosion cracking (SCC) in fine grained, low-alloy steel in high temperature, high-pressure water apparently is favored by, and possible only with
- high mechanical load,
- sulfur content of the steel producing manganese sulfide inclusions ,
- stagnant feed water ,
- increased water conductivity ,
- dissolved oxygen raising the corrosion potential beyond a critical value.
Details of the modeling of SCC kinetics arc adequately described elsewhere . Since the steel surface certainly is passivated by an oxide or else hydroxide layer and the onset of SCC also certainly will require the failure of this layer, a film rupture - anodic dissolution mechanism is generally favored. This model closely follows the slipstcp - anodic dissolution model of ordinary SCC of, e.g. stainless steel in hot chloride solutions, where the oxide film is very thin. Slip steps going through the much thicker oxide films in the present case cannot well b expected, so a somewhat less well defined film rupture- anodic dissolution model is more likely. The model will then still predict the dependence of the crack propa