Go Okamoto

Properties of silica in water

Abstract In view of the discordant theories for the mechanism of sedimentation of silica in natural waters, chemical properties of silica have been studied further. The solubility of amorphous silica has been measured over a wide range of pH and temperature. The study on the depolymerization of colloidal silica indicated that sea-water is not saturated with respect to molecularly dispersed silica. The presence of a small amount of aluminium greatly influences the state of silica in water. Thus colloidal silica can be precipitated even by a trace amount of aluminium, while a larger amount of the latter is required for the precipitation of molecularly dispersed silica. The optimum pH for the precipitation of colloidal silica is about 4·5 in contrast to that of about 8 for molecularly dispersed silica.

Anodic Passivation of Nickel in Sulfuric Acid Solutions

The potential decay curve of a passive nickel electrode on open circuit was followed in solutions of pH ranging from 0.4 to 1.4. The three arrest potentials observed in the decay curve correspond to the equilibrium potentials for the , , and the electrodes, respectively; the second potential is called the Flade potential, and the passivation occurs beyond this potential. A process for formation of the film is proposed that can be represented by the successive reactions, , . Then the critical potential beyond which the oxide formation takes place is calculated, under conditions when the solution in the vicinity of the surface becomes saturated with corresponding to the solubility product of , and is found to be equal to the Flade potential. The proposed mechanism can also explain the ability of the passive film to protect the metal against active dissolution due to breakdown of the film.The mechanisms of dissolution of nickel in the passive and overpassive potential regions are discussed from measurements of the Tafel constant and the pH dependence of the dissolution rate .The results are summarized as follows: passive region; .

The inhibitive action of bound water in the passive film of stainless steel against chloride corrosion

Abstract The stability of passivated 304 stainless steel specimens in the presence of chloride ions was investigated in relation to the amount of bound water in the passive film. As a measure of the stability the induction period for pit initiation and number of the anodic fluctuation prior to pit initiation were used. The amount of bound water was controlled by varying of passivation potentials, passivation times and passivation temperatures. Two kinds of bound water with different inhibitive action against corrosion by chloride ions were found in the passive film.

Ellipsometric and radiotracer measurements of the passive oxide film on Fe in neutral solution

Abstract Ellipsometric and coulometric measurements were made of the thickness of the anodic oxde film on Fe in sodium borate butler solution. The ellipsometric thickness was always larger by 15 per cent than the one estimated coulometrically, assuming the film to be γ-Fe2O3. The amount of H2O in the anodic oxide film was measured radiometrically using tritiated water and the total content of water of the film was estimated to be equivalent to Fe2O3. 1·99H2O.

Effect of organic inhibitors on the polarization characteristics of mild steel in acid solution

Abstract The effect of inhibitors on the corrosion of iron in hydrochloric and sulphuric acids has been studied by a technique involving rapid polarization of the corroding metal. Tafel lines for the anodic and cathodic process have been obtained which can be used to interpret the mechanism of inhibitor action.

The anodic behaviour of passive iron in chromic acid-chromate solutions

Abstract The anodic polarization curves and the dissolution rates of passive iron at static potentials were measured in chromic acid-chromate solutions of various pH between 1·24 and 8·74. The polarization curves for pH values lower than 3 were very similar to those of iron-chromium alloys obtained in acid solutions not including chromate ion. They are characterized by the existence of a first potential break E b at 0·9V (vs. S.C.E., a second break E o due to oxygen evolution and a region of constant current extending between them. The dissolution rate of passive iron observed at static potentials (below E b ) is much smaller than that obtained above E b . These facts are explained by the concept that chromic oxide in the passive film begins to dissolve as chrome ion when the potential approaches E b and it is almost completely replaced by γ — Fe 2 O 3 in the potential region above E b . Change of the anodic polarization characteristics with the rise of pH is interpreted as being due to the decrease in the Cr-content of the film and to the lowering of the potential E o .