In order to understand corrosion behavior, it is important to learn and understand electrochemically. Electrode potential, electrical potential and its variation which is caused by anodic and cathodic reaction balance were explained. Acidity progress in corrosion pits as a accelerator of anodic reactions, and size effect, velocity of sea water, metal salts, microbes as accelerators of cathodic reactions on the stainless steel in sea water are explained electrochemically.
Corrosion diagnostics of carbon steel in fresh water were carried out by Mahalanobis-Taguchi Method. Concentrations of sulfate ion, chloride ion and bicarbonate ion were used as items to judge the corrosion index. Artificial base areas were constructed by deciding standard deviations and the correlation coefficients arbitrarily. The items were classified into corrosion accelerator and corrosion inhibitor. “Closed” base area was constructed by using the axis of reciprocal concentration of corrosion inhibitor. The new method to construct on optimal base area artificially was developed. As the results to apply the present method to the experimental results of carbon steel in fresh water, the erroneous decision decreases remarkably comparing the conventional Mahalanobis-Taguchi Method.
The charge transfer resistance Rct and the electrode reaction impedance Z0 of a carbon-steel electrode were measured in ethanol with 0.0064 to 0.13 mass% HCl and 2 mass% H2O, in order to obtain basic data for applying on line corrosion-monitoring using electrochemical-impedance method in ethanol plants. The value of Rct was estimated from the mass-loss of the carbon steel and the value of Z0 was measured from the real-axis intersects of best-fitting curves to Nyquist plots. The ratio of Z0 to Rct was about 8 over the HCl contents examined in this study. The value of Z0 could be approximated 0.5 · (R0.1Hz-R1kHz), under the conditions employed in this study ; the value of R1kHz and R0.1Hz are the impedance measured by two-electrode method at 1 kHz and around 0.1 Hz, respectively. Thus, the corrosion rate icorr of carbon steel under similar conditions examined in this study would be given from the R1kHz and the R0.1Hz measured in the field as follows. icorr=2 · 160 mV/(R0.1Hz-R1kHz), where 160 mV is an apparent Stern-Geary constant under the examined conditions.
Air leakage is considered as a main factor of the troubles in absorption refrigerator. In this study, accelerated corrosion tests simulating air leakage have been conducted using 20 RT absorption refrigerator with air leak system. As a function of air leak speed, changes of dissolved oxygen levels and corrosion product concentrations (ion and oxide) of copper and carbon steel have been measured both in LiBr coolant and refrigerant. In addition, the effects of temperature and concentrations of dissolved oxygen and copper ion on corrosion potentials of carbon steel were investigated in LiBr solutions containing various inhibitors. From anodic polarization curves of carbon steel, pitting potentials were determined at various temperatures. From the relationship between corrosion potentials and pitting potentials, accelerating factors of corrosion damage of carbon steel were considered. From above investigations, a model of acceleration process of carbon steel corrosion by air leakage has been proposed as follows. (1) Copper ion dissolves into LiBr coolant by air leakage (2) Corrosion potentials of carbon steel shift noble near the pitting potentials by the combination effects of dissolved oxygen, copper ion and inhibitors. (3) Corrosion potentials of carbon steel exceed the pitting potentials only in special combinations of dissolved oxygen, copper ion and inhibitors.
Exposure tests of candidate materials were carried out up to 1000 hr in the sulfuric acid environments of thermochemical hydrogen production IS process, focusing on the corrosion of welded portion and of crevice area. In the gas phase sulfuric acid decomposition condition at 850°C, welded samples of Alloy 800 and of Alloy 600 showed the same good corrosion resistance as the base materials. In the boiling condition of 95 wt% sulfuric acid solution, test sample of SiC showed the same good corrosion resistance. Also negligible corrosion was observed in crevice corrosion.