In order to ensure a safety margin for crack initiation and growth of LWR internal components, irradiation assisted stress corrosion cracking (IASCC) of austenitic stainless steel has been receiving a particular concern. IASCC is believed to be attributable to radiation induced segregation and/or depletion (RIS) of elements at grain boundaries. From this view point, materials characterization, namely RIS detection and quantitative evaluation of them, can be considered as an essential issue both for an understanding of IASCC mechanism and for evaluation of IASCC susceptibility of internal components. In this study, electrochemical methods for RIS evaluation have been investigated by use of simulated materials in which segregation and/or very narrow chromium depletion was reproduced by controlled thermal treatments. Slow strain rate tests at 288°C were also performed in the simulated LWR environments to evaluate IGSCC (intergranular stress corrosion cracking) susceptibility of those materials directly. A Modified EPR (electrochemical potentiodynamic reactivation) method that can evaluate very narrow chromium depletion with high sensitivity and a potentiostatic electrolysis method that can detect impurities segregation at grain boundaries have been developed. IGSCC susceptibility of the materials seemed to depend both on chromium depletion and on impurities segregation and was able to be evaluated by the above electrochemical methods.

View full abstract

To obtain an effective metal material using valve, the erosion-corrosion behavior of various cast irons in flowing water was investigated. The erosion-corrosion of cast irons was affected by environmental factors, i.e., flowing velocity, temperature and dissolved oxygen content. The erosion-corrosion rate increased with increasing influence of environmental factors. The rate (V_e·c) exponentially increased against flowing velocity (v). The relation of V_e·c with v was described by the equation of Lotz & Heitz, i.e., V_e·c=bv^a, where, a and b are flowing velocity index and constant, respectively, and the a values are 0.8 in closed system and 0.4 in open system. In comparison with gray and austenitic cast irons, the austenitic cast iron has the more effective resistance of erosion-corrosion. The austenitic cast iron depressed local anodic reaction.

View full abstract

Effect of NaF concentration, temperature, strain rate and dissolved oxygen on intergranular stress corrosion cracking (IGSCC) of sensitized type 304 stainless steel has been examined by using a slow strain rate technique (SSRT). Comparing with polarization curves, electrochemical conditions of IGSCC in this system were discussed and an IGSCC susceptibility-temperature-NaF concentration diagram was made. It was found that oxygen dissolved in the solution was necessary for occurrence of IGSCC, and corrosion potential during IGSCC was in the active potential region at which the anodic current peak was observed. The highest IGSCC susceptibility was found at 10³ppm, and the susceptibility increased with temperature. The critical concentration of NaF to cause IGSCC was more than 8.4×10³ppm at 303K, 5-10ppm at 323K, and less than 1ppm at 353K, respectively.

View full abstract

Premature leakage of hot water was found for Type 304 steel tube. Most cracks started from the inner surface of the tube in contact with 60°C water containing 2 to 4ppm Cl^-. A 304 tube, which had been used for several years and had thick film, showed markedly noble spontaneous potential, E_SP, of 156mV vs. SCE at one day and later of 236mV, which could be atributed to residual chlorine injected into the water originally before heating. A specimen cut from the failured tube showed a pitting potential, V′_C, less noble than 200mV. Such values of V′_C lower than E_SP were reproduced with 304 steel specimen which had been cold-rolled up to 66% and had martensite introduced. It was concluded that anodic dissolution started in a form of pitting at the martensite phase in the cold-rolled 304 tube in the highly oxidizing water and then developed into stress corrosion cracking under residual stress and the temperature higher than 50°C, critical one for Type 304 steel. Presented countermeaures are ensurement of solution-treatment for 304 steel tubing or use of Type 316 in place of Type 304.

View full abstract

Application of X-ray photoelectron spectroscopy (XPS) to corrosion science was introduced in both qualitative and semiquantitative aspects. In particular, abilities and limitations as a surface analytical technique were explained with practical examples.

View full abstract

This paper explains bases of molecular orbital (MO) theories and reviews of applications of the MO theories to new developments of materials and inhibitions of corrosions of iron, steel and aluminum. Shapes and symmetries of atomic orbitals (AOs) and MOs are explained. Schrödinger equation is introduced understandably and physical meaning of each term of the equation is described. Application of the MO theories (X_α method) has been very succeeded to developments of super heat-stable Ti alloys. The applications are based on the bond order (B₀) between a mother metallic atom and alloy atoms and are based on the energy level of d-AO of the alloy atom (M_d). Electron densities (Q) of the C and N atoms in p-substituted anilines have been calculated by CNDO/2 method and the good correlations between Q and corrosion rate (K) and between Q and corrosion current (i_corr) of a steel are obtained. Carboxylic acids inhibit the corrosion of Al and the O-H bound energy (ΔH) has theoretically been obtained by 6-31G calculations. The strong acid has small ΔH value and the acid dissolves easily in an equeous solution and the acid shows large corrosion inhibition effect.

View full abstract