On the corrosion protection of carbon steel by sol-gel derived TiO2 coating under the illumination of light, the photoelectrochemical behavior of TiO2/steel system is investigated. The diffusion of Fe ions into TiO2 in the heat treatment of the coating process greatly degrades the photo-effect of TiO2 coating. An Fe oxide layer at the TiO2-steel interface, formed by oxidizing the steel above 500°C in air before coating, has been shown to retard the diffusion. The importance of α-Fe2O3 in the photo-response of TiO2/steel and the effect of its changing properties with oxidation temperature are discussed. The results on the effect of heat treatment condition in the coating process show that the TiO2/steel specimens exhibit remarkably less noble photopotentials when the heating temperature is increased above 300°C, which is related to the crystallization of TiO2 above this temperature. The ennoblement of photopotential caused by the further increase in heating temperature above 500°C is a result of Fe diffusion through the oxide layer at the interface. Under the optimum conditions for steel oxidation and TiO2 coating, the feasibility of localized corrosion prevention by TiO2 coating under illumination is clearly demonstrated by the fact that the photopotentials of TiO2/steel in deaerated solution are much less noble than the repassivation potential of carbon steel.
In order to evaluate the corrosion rate of reinforcing steel (rebar) in coated concrete, the polarization resistance (Rp) of rebar was measured on the surface of concrete covered with and without various coatings by means of the coulostatic method. It was shown that Rp of rebar could accurately be measured on the bare surface of concrete by a combined sensor composed of reference and auxiliary electrodes and the values of Rp depended on the measuring time of decay curve of overpotential. Although the erroneous Rp values were obtained on the surfaces of epoxy and lacquer coatings, the normal values were measured in concrete under the coatings and on the epoxy primer paint with a low resistance. Based on comparison of decay curves, obtained on the coatings together with those measured under the coatings, it was revealed that residual voltage through the coating unavoidably measured by the combined sensor resulted in wrong Rp values. By separating a reference electrode from an auxiliary electrode, the decay curves being not affected by the residual voltage could be obtained and accurate Rp values of rebar could be evaluated even in the case of the coated concrete.
The quantitative evaluation of tarnish (stain and rust) of type 304 stainless steel was carried out using Computer Image Analysis. The shapes and degree of tarnish could be evaluated by several parameters such as total area of tarnish, ratio of red rust to stain one, and average size of rust particles. Effect of wet and dry test condition after salt spray at cyclic corrosion test on tarnish has been studied by these parameters, and main results are as follows. Total area of tarnish and average size of rust particles depend mainly on the relative humidity (RH) in the constant humidity process after salt spray, and they indicate maximum value at 70% RH. Ratio of red rust to stain one increases with the duration time in wet cycles. Red rust and stain one are also FeO(OH) in structure, and Fe-Cr-Ni ratio is nearly same as the stainless steel. To control the condition, cyclic corrosion tests were able to simulate tarnish (area and ratio of red rust) of one year seaside exposure tests.
Chelating agents are being used to remove radioactive contaminants in the corrosion products of cooling systems of nuclear power plants. To elucidate the reaction mechanism the dissolution rate of sintered magnetite pellets in solutions with and without EDTA was measured as a function of potential (0.6- -1.2V vs. saturated Ag/AgCl electrode) and pH (1.1-4.6) at 80°C. It was found that two kinds of reaction can occur: (a) reductive dissolution and (b) chemical dissolution due to chelate formation. Without EDTA only reductive dissolution occurs in the potential range 0.6- -1.2V. At pH1.1 the dissolution rate shows a maximum at 0V, the dissolution peak shifts to more negative values, and the peak height decreases as pH is increased. In the presence of EDTA only reductive dissolution occurs at pH1.1, but at pH2.5 chemical dissolution proceeds as well as the reductive dissolution, showing a maximum dissolution rate at 0V. These results were explained by a model considering the effect of potential, pH, and EDTA on the two coupled processes: (1) the transfer of Fe ions from solid to aqueous phase to form aqua ions or EDTA chelates and (2) the transfer of lattice OH- or O2- ions by reaction with H+ ions to form H2O.
Duplex stainless steel SUS 329J2L is degraded in mechanical properties and corrosion resistance when aged in the temperature range between 400 and 500°C. This degradation is caused by the decomposition of ferritic-phase into Cr-rich and Fe-rich phases. The decomposition during aging is characterized by Mossbauer spectroscopy and electrochemical potentiokinetic reactivation (EPR) measurement. Mossbauer spectroscopy measurement shows that the spinodal decomposition occurs in the ferritic-phase at the early stage of aging at 450°C. Clusters of Cr-rich and Fe-rich regions are formed when aged for 1-3 hours and begin to aggregate after 300 hours. The activation peak potential shifts to more noble direction and the activation peak current density increases with aging time after aging for 30 hours. Reactivation peak appear after aging for 300 hours. This leads us to the conclusion that the activation peak potential and peak current density are more sensitive to the microscopic fluctuation of Cr content even in the early stage of phase decomposition compared with the passivation and reactivation behavior. Because this may be that the dissolution behavior more precisely reflects the degree of Cr-depletion than the passivation and reactivation behavior, which is affected by the size of the Cr-depleted region as well as the degree of the Cr-depletion, as suggested by one of the present authors.
Critical issues to be solved were considered and a basic concept was proposed to establish effective corrosion protection of infrastructures that require increasingly long service life with a minimum of maintenance to conform to the aging of society and the avoidance of 3D (dirty, dangerous and demanding) jobs. An appropriate service life for each type of infrastructure must be set by the government from the standpoint of economy and the welfare of society rather than that of technological capability as is the case of 100 years life for long-span highway bridges in the bay areas. Future long-term corrosion engineering to meet the requirement must be the one most appropriate from the inclusive standpoint of total-life economy, labor saving and friendliness to the earth. Formulas to evaluate the total score are not available yet due primarily to the difficulty of judging relative size and importance of impact upon protection of the environment and conservation of energy and resources, and must be developed by the cooperation of technical societies of various areas including corrosioin engineering. Reliable life prediction of specific corrodible structures is a critical item in evaluating a corrosion protection system, which can basically be made by using compiled practical corrosion data with proper modifications with respect to structural, environmental and time factors.