The Point Defect Model (PDM), proposed and developed by Macdonald and coworkers, is one of excellent models for growth and breakdown of passive films on metals. This review introduces a concent of passivity breakdown based on the PDM, and explains how the model deterministically derives passivity breakdown potential, induction time to passivity breakdown in detail. The review also introduces the recent two findings of dependence of passivity breakdown potentiasl on potential scan rate and probability distribution in induction time to passivity breakdown derived from the PDM. These theoretical expressions for passivity breakdown are reasonably verified in comparison with the experimental data for passivated Ni in borate buffer solutions with chloride ions.
To investigate the corrosion behavior of materials of boiler tubes in simulated waste incineration environments, A. C. impedance measurements have been carried out for Alloy 625, SUS347H and STBA24 embedded in NaCl-KCl-K2SO4-Na2SO4-Al2O3 mixed synthetic ash at 823K. The evaluation of corrosion resistance was conducted by using 1/ΔR (the reciprocal of the difference of resistance at 10mHz and 20kHz). The HCl addition in the atmosphere accelerated the corrosion of all materials. On the other hand, the H2O addition did not affect the corrosion of Alloy 625 significantly, but it accelerated the corrosion of SUS347H and STBA24. In all cases a good relation was obtained between the integrated value of 1/ΔR with time to tested period and real corrosion losses of materials after tests. This result reveals that A. C. impedance measurements can be a useful technique for monitoring the corrosion rate of materials in a waste incineration environment.
Activity can not be approximated with concentration in concentrated solutions, because an activity coefficient is very different from unity. Activities of OH-and H2O were calculated in NaOH solutions of 1M to 15M and at the temperature of 298.15K to 343.15K. The activity coefficients obtained experimentally by G. Åkerlöf and G. Kegeles were employed for the calculation. The potential-molarity equilibrium diagrams for Fe-NaOH solution system were established. These diagrams are more useful than the potential-pH ones for the purpose of thermodynamic consideration where the theory of dilute solutions can not be applied.
Pitting corrosion behaviors for carbon steels in an aqueous solution of 60%LiBr+0.2% LiOH+0.2%LiNO3 at 150°C were investigated. Similar to the carbon steel in an aqueous solution of 60%LiBr+0.2% LiOH+0.2%Li2CrO4 which was described in the previous paper, the possibility for the initiation of a growing macro-pit was detected by applying the critical depth concept. The pitting potential, Vc, pit the film breakdown potential, Ez, and the pit repassivation potential, ER, pit were measured to be -480, -490 and-540mV, respectively. And the critical depth, r*, was determined to be 10μm. When a carbon steel was merely immersed in the solution, it reached the steady state in a few hours; micro-pits which were shallower than r* generated intermittently and the initiation of growing macro-pits was inhibited. However, when the carbon steel which was held at-800mV for longer than 10h in the solution prior to the open-circuit immersion as a pretreatment, pits that grew beyond r* during the initial period of the immersion were found to continue growing with a rate of 7mm/y. The pretreatment strengthened the resistance to film breakdown and activated the cathodic reaction on the steel surface. These effects of the pretreatment were found to be more significant for steels in the LiNO3 added solution than those in the Li2CrO4 added solution.
Effects of the composition of surface film on the corrosion resistance of bright annealed stainless steels were investigated. Bright annealing and immersing in nitric acid solution were carried out in order to make specimens having various surface film. The composition of the surface film was determined by SIMS (secondary ion mass spectrometry) and the corrosion resistance was examined by pitting potential measurements, crevice corrosion potential measurements, a ferroxyl test, an atmospheric exposure test at coastal-industrial sites and immersing test in sulfuric acid solution. It was found that the corrosion resistance was much influenced by the composition of the surface film of bright annealed stainless steel. Surface film having higher Si or Cr content, and lower Fe content which is made by the bright annealing in 75vol%H2-25vol%N2 atmosphere with lower dew point than -40°C accompanied by sufficiently long soaking is desirable for raising the pitting or crevice corrosion resistance of SUS304 and SUS430LX. Reduction of Fe content in the surface film by immersing in nitric acid solution also raised the corrosion resistance by the atmospheric exposure test and the ferroxyl test.
A corrosion inspection for bottom plate of fuel storage tanks using ultrasonic thickness gauge (UT) must be performed regularly as provided by law. In order to investigate reliability of UT, corrosion depth of samples cut from bottom plates measured by laser displacement meter (LDM) was compared with results of UT. As a result, it was indicated that UT was enough to know mean corrosion depth, however maximum corrosion depth estimated statistically from corrosion depth measured by LDM was 1-1.5times larger than that from UT. It became clear that the estimated maximum corrosion depth would be smaller than the actual value when UT is used in its estimation.
The corrosion characteristics of metals (Al, Ti, Cr, Fe, Co, Ni, Cu, Mo, Ta, and W), stainless steels (SUS304 and 316), and a Ni-based alloy (Alloy 600) in a downstream reactor of CF4-11%O2 plasma have been studied. The mass change and penetration depth of the specimens were measured after corrosion tests at temperatures in the range of 298 to 573K. The surfaces of the specimens were examined using Auger electron spectroscopy (AES) with Ar+ion sputtering. It was found that metals such as Ti, Cr, Mo, Ta, and W suffered from corrosion leading to a decrease in mass, and their corrosion rates increased with rising test temperature. At 573K, Fe and the stainless steels showed an increase in mass, while Al, Co, Ni, Cu, and Alloy 600 no change in mass. The results of AES analysis exhibited that the corroded surface of Cr, Mo and Ta was covered with a thin film of parent metal oxyfluorides, and that of W with a thin oxide film. Thin films of metal fluorides were formed on Co, Ni and Cu. Thicker films consisting of metal fluorides and oxyfluorides existed on Fe and stainless steels. In the downstream environment of CF4/O2 plasmas, metal fluorides are thermodynamically favored for all the metals examined. The vapor pressures of MoF6, TaF5 and WF6 in the temperature range of 298 to 573K are high enough to evaporate when they are formed. Therefore, the corrosion of metals and alloys in the downstream environment of CF4/O2 plasmas is primarily controlled by the rates of reactions with fluorine atoms, which are generated in the plasmas, and the vapor pressures of metal fluorides formed by the reactions.