Corrosion behavior of thermal sprayed coating of SUS 304, 316, and Hastelloy C was studied in 3.5% NaCl solution by anodic and cathodic polarization measurements, and by SEM and EPMA. It was found that anodic dissolution occurs along the crevices between the deposited particles. The anodic process is thought to be essentially the same as that of crevice corrosion, It was also found that the corrosion resistance of the sprayed alloys is considerably less than that of their solid wire counterparts. This was attributed not only to the accelerated anodic process but also to the increase of cathodic reactivity. The latter was suggested by the existence of diffusion controlled cathodic current observed from the onset of cathodic polarization measurements. Such behavior is not observed usually for the stainless steels in neutral solutions. Sealing of the micro-crevices in the spray deposited alloy layers is essential to improve their corrosion resistance in chloride solutions. The thermal sprayed Hastelloy C, with a lower crevice corrosion susceptibility, coated with the epoxy resin exhibited the highest corrosion resistance.
A simple and compact wetness meter has been developed. The instrument uses an Fe-Fe couple or humidity sensor, and a mercury coulometer which records the wetness time. It is driven by a SUM-3 type 1.5V dry cell which lasts for over one month on continuous use. Time-of-wetness was 31.2% of total exposure time when the sensor was exposed to outdoor atmosphere in Tokyo. Time-of-wetness was longer and critical relative humidity was lower when the sensor surface was contaminated by NaCl than by Na2SO4. When Fe-Fe couple was immersed in a solution, wetness time was 92%. It is also demonstrated that the wetness meter can be applied successfully for the measurement of permeation rate of electrolytes through paint film, when coated sensor is used.
A study has been made on the relation between concentration of dissolved oxygen in a beverage solution in Japanese market and pitting attack of aluminum alloy 5182. The anodic polarization resistance of the alloy was measured by the area polarization technique in a drink solution environment, where dissolved oxygen concentration was continuously changed from 10ppb to 600ppb. It was found that more than 40ppb of 5182 aluminum alloy is subjected to pitting corrosion in the solutions containing more than 40ppb of dissolved oxygen. The value of dissolved oxygen concentration agrees well with the realistic can-rupture condition of beverage solution in a market.
A new system for analyzing the three dimensional radiation pattern of elastic waves was developed and applied to evaluate the mechanical characteristics of the TiN film deposited on the stainless steel by the hollow cathode discharge method. The system consists of four highly sensitive small sensors to measure both the arrival time and the amplitude of the P-waves, and a computer algorithm to fit the measured data to the two-dimensional profile locus of the P-wave amplitude for an assumed fracture model, The fracture mode estimated by the developed system was compared with the results obtained by both the source wave analysis and the microscopic observation. It was found that the TiN film (4μm thickness, double layered) under the compressive loading suffered various damages according to the following sequence. (1) At the elastic stress level, vertical cracks along the periphery of the splashed particles parallel to the loading direction were induced by the Mode-I fracture. This cracking is limited in the outer TiN layer, and therefore the substrate metal is isolated from the external environment. (2) At plastic strain of around 0.8%, Mode-I fracture along the boundary between the substrate and the TiN film was observed. This fracture was induced by emergence of slip steps in the substrate which was still isolated from the external environment. (3) At large plastic strain of 1.25%, the TiN film itself suffered the Mode-I fracture which was also associated with the slip step. The developed system was useful to elucidate the various fracture modes in or under the coated film.
Effect of curing temperature on corrosion resistance of aluminum sheet coated with epoxy resin using electrodeposition process has been investigated. Corrosion test revealed that weight loss of aluminum sheet in 1wt% NaCl solution (pH3.0, 301K) decreased with an increase of curing temperature. It was found that water absorption of electrodeposition film was independent of an increase of curing temperature. Water permeability and adhesion between film and aluminum sheet, however, increased with an increase of curing temperature. (Water permeability)×(adhesion)-1 was correlated well with corrosion resistance. Corrosion behavior of aluminum sheet coated with epoxy resin is discussed in terms of physical properties of electrodeposition film against water and related with adhesion.