Corrosion resistance of TiN-coated stainless steel by drying coating process is extremely influenced by the existence of various defects in the thin films, such as pinhole defects. Recently, the critical passivation current density (CPCD) method have been frequently used for evaluating the defect ratio (pinhole-ratio) from among various tests employing electrochemical methods. However, CPCD method showed much difficulties when applying to plasma CVD film, because relatively large scale spalling-off of the coated film was recognized after the electrochemical test. In this study, a new method, in which potential and current density changes were measured under coupling condition between specimen and counter electrode of Pt, was proposed for conducting less destructive electrochemical testing. Testing method under coupling condition was less destructive than CPCD method, for causing little peeling off in coated film. Therefore, good correspondence between the defect region in coated film and the penetrated area of it to cause substrate corrosion by electrochemical test was recognized. Also in this coupling test method, the pinhole-ratio Ra of coating can be determined quantitatively from the quantity of electricity Qa through evaluating corrosion pit morphology.
Effects of P, Si, Mn, Cu and Al on the dissolution behavior of crevice corrosion were studied by using high purity 18Cr-14Ni and 14Cr-16Ni steels with systematic variations in alloying elements in 3% NaCl solution at 80°C via the moire method. It was found that while no major alloying effects were found in the initial stage of crevice corrosion, alloying does influence the behavior of crevice dissolution in the stationary growth stage of corrosion development. The dissolution rate of steady growth stage, VII, increases with increasing content of phosphorous at a fixed electrode potential, while it is found to remain constant at the repassivation potential for crevice corrosion which becomes increasingly noble with increasing phosphorus content.
Aminoethanol is generally used for carbon dioxide recovery as a solvent at purification of natural gas. Steels are severely corroded in aqueous solution of aminoethanol absorbing carbon dioxide at high temperatures above 60°C. In this study, corrosion and inhibition of iron were investigated in an aqueous solution containing aminoethanol at high concentration, 8.5M (52% in volume), by weight loss measurements, spectroscopic analysis of dissolved ferrous ion, and a potentiostatic polarization curve technique. Aminoal kanes containing carbon numbers from 8 to 18 were used as corrosion inhibitors against iron corrosion in the aqueous solution. 1-Aminododecane and 1-aminohexadecane were found to inhibit corrosion effectively in the aqueous solution at 80°C, but to decrease remarkably corrosion inhibitor efficiency at temperatures higher than 80°C. It is considered that these amines are chemisorbed on the iron surface and inhibit the anodic reaction of iron. The critical temperature for chemisorption of 1-aminohexadecane is concluded to be between 80 and 90°C.
Corrosion behavior of Zn (EG) and Zn-Fe alloy (Zn-Fe) electroplated steel sheets in atmospheric exposure test was investigated by observation of the appearance, polarization measurement, anodic dissolution analysis, and corrosion products analysis. Corrosion resistance of EG and Zn-Fe changed as follows with increasing Fe content in deposit; nearly constant in 0-10 mass%Fe, decreasing in 10-20 mass%Fe, nearly constant in 20-30 mass%Fe, decreasing over 30 mass%Fe. Corrosion rate of the coating was 12.3g/m2·year and 22.9g/m2·year for 0-10 mass% and 20-30 mass%Fe, respectively. The mechanism of the corrosion behavior was considered as follows; Uniformly formed zinc corrosion products such as 4ZnO⋅CO2⋅4H2O, etc. suppress diffusion of oxygen through the corrosion product layer in the range of 0-10 mass%Fe, whereas α-FeOOH formed in the corrosion products promotes the oxygen diffusion in the range over 10 mass%Fe, thus resulting in decrease in corrosion resistance.
In this thesis, the present condition and problems about painting of steel bridges, which consist of construction technology and maintenance technology, are described. In order to extend the life cycle of painting, our Institute has been carrying out various researches, such as the develoment of “the diagnosis system for the deterioration of paint films”, the developments of new paint systems by conducting the exposure test of painted specimens, the development of accelerated corrosion test method, the pilot work of field painting work, the development of the evaluation method of workmanship of the cleaning and so on. The outlines of these researches are also described.