Electrochemical conditions for stress corrosion cracking of Type 316L stainless steel have been investigated in the chloride solutions containing thiosulfate ion using polarization curves for Pt and Type 316L. And the following results were obtained; 1. At lower pH or at higher concentration of thiosulfate, chloride solution containing thiosulfate ion became turbid because of sulfur produced by disproportionation of thiosulfate ion. 2. In the region of potential at which stress corrosion cracking occurred, thiosulfate ion was reduced to sulfide in addition to main reduction reactions of dissolved oxygen and hydrogen ion. 3. Decrease in pH within pits and cracks of Type 316L leads to depassivation. Dissolved metal ion reacts with hydrogen sulfide to produce metal sulfide on the surface of pits and cracks. Therefore, this promotes local anodic dissolution.
Effects of variations of temperature and stress on stress corrosion cracking (SCC) of JIS STPT 42 carbon steel were investigated in high temperature water. SCC tests were conducted on small tubular specimens under individual or combined variations of water temperature and stress with a triangular wave form. The bi-axial stresses were applied to the tubular specimens by combining the external, axial load and internal pressure with testing water. Tubular specimens were axially loaded in the range of 98 to 392MPa in water containing 8ppm oxygen under variation of temperatures of room temperature to 523K. The results obtained are summarized as follows. (1) SCC susceptibilities were remarkably accelerated by combined variations of temperature and stress. (2) SCC failures occurred in circumferential direction of tube inner surface. Cracks initiated from corrosion pits and propagated transgranularly. (3) Crack propagation could be detected by an acoustic emission technique. The crack propagation rates estimated in this study were in the range of 4.0×10-10 to 1.58×10-9m/s and generally lower than those for sensitized Type 304 stainless steel.
Recently, it has been reported that sensitization in welded joints of austenitic stainlness steels could be promoted by post weld heat treatment at lower temperature region, where sensitization would be hardly induced for solution heat treated materials. This phenomenon is known as LTS (Low Temperature Sensitization), which becomes a serious problem for operating all sort of reactors and pressure vessels used in high temperature conditions. This LTS would be caused by the formation of nuclei of fine carbides which was induced by heat cycles during weldings. In this paper, effects of welding methods on the susceptibility to LTS were discussed. The following properties became evident through the experimental results. Type 304 steel was senistized in welding process in the case of large heat input welding. On the other hand, in the case of low heat input welding such as EBW, LTS was detected, even if it was not sensitized under as weld condition. These results suggested that it was difficult to prevent LTS only by controlling the weld heat inputs. In addition to that, the susceptibility of several type of austenitic stainless steels for LTS was studied. As the results, stabilized stainless steels were found to be almost immune for LTS.
The effects of fluid flow on the corrosion resistance of oil-well materials were studied with two experimental techniques. One method employed a large flow-loop which can attain a maximum gas flow rate of 100m/sec. The other consisted of electrochemical measurements with a rotating cylinder electrode. The corrosion rate of carbon steel measured in the flow loop increased with the increase of flow rate. Corrosion rates were obtained from the electrochemical data using the Stern-Geary equation, as a function of the speed of rotation. The results obtained by the two independent experimental methods were compared on the basis of hydrodynamic analysis. Using the similarity solutions obtained for mass transfer with pipe flow and the rotating electrode, the rotating velocity was converted to the equivalent velocity in the pipe. On that basis the corrosion rate of the pipe was equal to the corrosion rate of the rotaing electrode. The corrosion rate of rotating electrode obtained by the electrochemical method was used to predict the corrosion rate of the pipe at the equivalent velocity. The predicted corrosion rate is in good agreement with the measured corrosion rate.
The aspects of surface analyses are summarized to afford a birds-eye-view of typical surface analytical techniques to corrosion scientists and engineers in order to choose the proper analytical methods for acquiring surface information on appearance, chemical composition and lateral distribution of a specific element, elemental distribution in depth, defects and electrical characters. The comparison of typical surface analytical terms, such as lateral and depth resolution, sensitivities is presented. Several exercises are given to help make the readers familiar with solving surface analytical problems.