The history of studies on metallic corrosion inhibitors is reviewed. Most of fundamental adsorption and oxidation inhibitors were developed by the 1950s while precipitation inhibitors were mainly thereafter. It is important to be noted that novel environmentally acceptable (green) inhibitors were investigated mostly after 1970. The quality of inhibitor investigation was gradually changed from finding a new, effective inhibitor for metallic corrosion to elucidating inhibition mechanism by discussing data of electrochemical experiments and surface analyses and also using physical and chemical properties of inhibitors since the 1960s. The linear free energy relationship, the hard and soft acids and bases principle, and quantum chemical calculations were introduced for discussing mechanisms of corrosion inhibition. In future, effective green inhibitors will be developed by derivation from naturally occurring substances.
To examine the applicability of the cathodic protection, which is accompanied with calcareous deposition, to ferrous structures in estuary areas, a study has been made concerning the effects of seawater dilution on the amount, the composition and the structure of the deposit. The dilution resulted in few effects on the amount of the deposit, until the dilution decreased the salinity to 1/10 of the original value. On the other hand, the Mg/Ca ratio of the deposit was greatly influenced by the dilution. As for the structure, the deposit thickness was sharply increased when the dilution decreased the salinity less than 1/10 of the original value. These results were discussed on the basis of the difference in the crystallization rate and the hydrophilic character between CaCO3 and Mg(OH)2.
To estimate a proper anodes arrangement in the cathodic protection of a gigantic marine structure, quite a simple method was proposed, and the validity and applicability of the method was examined. The method was based on the assumption that the electric resistance between an anode and the structure mainly come from the part of seawater in catholic protection of a gigantic marine structure. To examine the validity of this assumption, the potentials of the anode and cathode under the catholic protection at a constant current density were measured using seawaters diluted to various salinities. The results showed that the dilution method is valid for the estimation of the anodes arrangement in the catholic protection of gigantic marine structures. To show how to apply the dilution method to a concrete design problem in protecting a gigantic marine structure cathodically, the electric current distributions on a structure during catholic protection were measured in seawater diluted to 1/10 with fresh water. A close relation was found between the distribution of current density and that of accompanying calcareous deposit amount. The current density data were treated statistically, and a dimensionless number was introduced to evaluate quantitatively the unevenness of the calcareous deposit distribution. It was found that the dimensionless number is very useful for designing the arrangement of electrodes for the catholic protection of a gigantic marine structure accompanied with calcareous deposition.
We have attempted to apply cathodic protection to steel in the atmosphere by employing a newly developed paint system comprising steel, a layer of insulating coating, a layer of conductive coating containing carbon powder, and a layer of insulating finish. Coastal industrial area exposure tests were conducted in Chiba, Japan. The percentage area of red rust in defects with applied potential was lower than in defects without applied potential. After two weeks the defects with applied potential were covered with a white film containing magnesium carbonate and the cathodic current had decreased. It is considered that the electro-deposited white film enhanced the anti-corrosive protection of the defects. It was demonstrated that measured cathodic current density through the defects conformed to the distributed parameter circuit model, assuming that electrolyte film or white film on a defect is a homogeneous and consecutive resistance film. From this model, we can deduce that a severely corrosive environment enhances the corrosion prevention effects of the newly developed paint system.
Titanium is thought to be one of candidate materials for an overpack container for the geological disposal of radioactive wastes in Japan. It is presumed that an overpack container will be corroded in bentonite-contact water under reductive conditions after longterm disposal. In this study, optical and compositional changes in air-formed oxide films on titanium was examined by ellipsometry and XPS analysis during and after catholic polarization in the simulated solution of bentonite-contact water, respectively. A decrease in a refractive index was first occurred with decreasing potential among a refractive index, an attenuation coefficient and a film thickness. This was due to the reduction of Ti4+ ions to Ti3+ ions in the films. Based on the initiation potential of change in a refractive index, an oxide film formed in air with 75% RH was thought to have the highest resistance to degradation by cathodic polarization among oxide films formed in air with 35-75% RH.