A severe corrosion “Metal Dusting” occurs on metals in carbonaceous gas environments. Thermodynamic considerations have been reviewed to shed light on the corrosion phenomenon. Solutions to prevent metal dusting have also been presented in terms of protective oxide scale formation and atomistic interaction of carbon-bearing gas with a metal surface.
Stress corrosion cracking (SCC) growth tests were conducted on type 316L stainless steels (SS) with different cold rolling reductions of 10% to 40% under a simulated boiling water reactor coolant condition. In these tests, constant loads were applied to each compact tension(1 T-CT) specimen to maintain the initial stress intensity factors(K) to 20 MPa√m or 30 MPa√m. The crack growth rate increased with increasing the micro-Vickers hardness(Hv) and was larger than the tentative criterion of SCC growth rate for work hardened L-grade SS proposed by Kumagai, et al. at Hv>250. However, the crack morphology of specimens with high cold rolling reductions(Hv>250) was quite different form that observed in the actual components. Only a 10% cold rolled specimen with Hv230 showed intergranular cracking, the same cracking morphology as that in the heat affected zone of the damaged shrouds. This result would suggest that the heavily cold rolled specimens with Hv>250 should not be used to obtain the crack growth data for prediction of SCC propagation behavior in the actual components because of the different crack morphology.
The large amounts of localized corrosion were often observed at the part under low water level (L.W.L) of the steel structures built at the marine environment. But the reason why such corrosion maximum was observed at this part has not been well understood. Therefore, the corrosion phenomena at this part were investigated using the model samples to measure both amounts of corrosion and macro-cell currents. It was confirmed that the corrosion at the part under L.W.L. was influenced by not only the tidal zone but also the immerged part in sea water. When the tidal zone was painted, the maximum corrosion at this part was not observed. The large amount of corrosion at this part was observed in the case of the longer immersed part in the sea water than the shorter one. Macro-cell current measurement shows that the large current was observed at the time to exchange from low tide to high tide. This current flows from tidal zone as a cathode to the part under L.W.L. as an anode, and the deeper part of immersed zone acts as anode, too. And the time changes to low tide, the part under L.W.L. also stayed as an anode but the deeper part of immerged zone changed to cathode. The reason of the phenomenon is estimated that the strongly anodic polarized rust layer at the part under L.W.L. during the time of high tide is gradually return to the rest potential taking long time. Then, the part under L.W.L. stayed as anode and created macro-cell with deeper part of immerged part as cathode till the next coming high tide continuously.
The pitting behavior of SUS 304 stainless steel was investigated first in FeCl3 solutions of various concentrations to follow the variation of corrosion potential. Typical fluctuations of corrosion potential were observed in relatively dilute solutions because of large polarization of the cathodic reaction. Secondly, the internal current was evaluated by coupling the anode (SUS 304 stainless steel in a Fe3+ free chloride solution) and the cathode (SUS 316 stainless steel in a FeCl3 solution). Then cathodic polarization characteristic on SUS 316 stainless steel was evaluated in FeCl3 solutions. The data was inputted into a computer and a program was made to control a potentiostat in order that the measured anodic current be always same as the inputted cathodic one at each potential in magnitude. Finally, SUS 304 stainless steel in a Fe3+ free chloride solution was anodically polarized by the potentiostat. All three kinds of tests gave common potential fluctuation patterns indicating the validity of the idea of “artificial cathode”.