Specific adsorption of anions on surfaces of metals and oxidized metals is reviewed in relation to the hard and soft acids and bases principle. Anions classified as soft bases are readily chemisorbed on the bare metal surface equivalent to a soft acid, resulting in suppression and stimulation of metallic corrosion in an acid solution with inhibitive and aggressive anions, respectively. Anions acting as hard bases are adsorbed on the oxidized surface, a hard acid without difficulty. Adsorption of aggressive anions on a passive surface of iron in a borate buffer is associated with passive film breakdown. The formation of protective films with metallic cations on the oxidized surface in a neutral solution is also related to the principle.
Legally mandated inspections for fuel strage tanks resulted in detecting many cases of corrosion both inside and outside surface on the bottom plates of tank. Superior coating materials were developed to prevent corrosion on the inside surface, however there were no corrosion prevention technologies for the outside surface. It is required to develop a technology to measure outside surface corrosion rate and the authors developed a new type two-frequency corrosion monitor. It was used to conduct long term monitoring of two in-service fuel storage tank bottom plates. One was submitted to zinc plates on the circumference of the outside surface for cathodic protection, and the second was not. Nine-years and two-months monitoring for the non-protected tank indicated that the maximum value of the corrosion rate on the circumference was two orders of magnitude larger than that at the center. The estimated corrosion rate led to considering tank reconditioning within four to five years. Four-years and six-months monitoring of the protected tank showed few indications for the necessity to recondition the tank due to the corrosion prevention effect.
Corrosion current densities were measured by the polarization resistance method for some metals and alloys in sodium chloride solutions with air or argon gas flowing. The data were analyzed to get regression equations in which logarithms of measured corrosion current densities were expressed as functions of overpotentials, polarization speeds, concentrations of sodium chloride in test solutions, gas flow modes, test solution temperatures, and some interactions of these variables. Those equations showed that measured values of corrosion current densities decreased as overpotentials increased and increased as polarization speed increased. Estimations of ideal corrosion current densities were expected by extrapolating the regression equations to null overpotential as well as to null polarization speed. The effect of test solution temperatures was not negligible.
Corrosion behavior of various stainless steels in boiling oxalic acid solutions was investigated and their corrosion resistance was compared. Various stainless steels corroded in boiling oxalic acid solutions and the corrosion rate decreased in the order of SUS 430>>SUS 444>SUS 304>SUS 316, DIN 1.4462>SUS 329J1>SUS 329J4. The dissolution contents of Cr in the stainless steels except SUS 329J4steel increased with increasing concentration of acid: This result corresponded to the result of weight loss test. From the results of X-ray photoelectron spectroscopy, two layers composed of hydroxy- and oxy (hydroxy) -compounds in the outer layer and oxide compounds in the inner layer as corrosion products on stainless steels existed. It was found that the difference of corrosion resistance of various stainless steels was due to component and composition such as Cr, Ni and Mo and the phase structure of steel.
Copolymers composed of the acrylic acid (AA) and acrylamide (AAm) derivatives were synthesized and these inhibition effects on corrosion of steels were investigated in cooling water systems. In particular, the relationship between the component and composition, and the inhibition ability of the copolymers was discussed. In a high concentrated solution, the copolymers at AA/AAm molar ratio of 5/1 showed the most effective as inhibitors. In the substituent effect of amido group (-CONR2), the inhibition ability of the copolymer increased in the order of (R=)H (acrylamide: AAm)<CH3 (dimethyl acrylamide: DMAAm)<C2H5 (diethylacrylamide: DEAAm). In low and high concentrated solutions, the copolymer at number average molecular weight of ca. 104 depressed strongly the corrosion of steel.
In order to make a practical application of boundary element method (BEM) to quantative predictions of catholic protection and macro-cell corrosion, a BEM analysis system based on the programs for 2D, 3D and axi-symmetric regions was developed. The system consists of a polarization curve database and programs to perform element discretization, input file set up, BE analysis and graphic display of input and output data. Even an engineer with no knowledge of corrosion will be able to easily operate this system and perform effective analyses for optional 3D regions. The usefulness of the system is demonstrated by an application example for the cathodic protection design of a seawater pump. In the system, the effect of flow-rate and time on the polarization curve can be taken into account for defining the boundary conditions.
Voltammogram and impedance measurements were applied for analysing the change in electronic property of the anodic oxide film formed on titanium in neutral borate solution with the electrolytic breakdown at anodic potentials higher than the breakdown potential of 7.5V. Anodic current histerisys was observed between the potential sweeps toward negative and positive directions for the film after the breakdown. The histerisys disappeared when the electrode was held at potentials lower than the breakdown potential. From analysis of the current histrisys it is suggested that high valency oxide such as TiO3 or high valency state such as Ti5+ or Ti6+ are formed in the film at the breakdown. From the change of the Mott-Schottky plot with oxidation potential and period, it is estimated that the donor density in the oxide film increases after breakdown.