AC impedance technique has been applied to the monitoring of corrosion rates of a carbon steel in wet/dry cycle condition. The wet/dry cycles were conducted by exposure to alternate conditions of 1h-immersion in 3% NaCl solution and 5h-drying at 25°C and 60% RH. The corrosion rate of the carbon steel is greatly accelerated by the wet/dry cycles. Because the active FeOOH species, which are produced by the oxidation of Fe3O4 in air during drying, act as very strong oxidants to the corrosion of carbon steel in wet condition. Shortly before the surface dried out, a large increase in the corrosion rate and a small shift of the corrosion potential to the positive values were observed. This can be explained by an acceleration of the rates of oxygen transport through the thin electrolyte layers. When the carbon steel is immersed again in the solution after the surface almost dried out, it exhibited a high corrosion rate and a noble potential, followed by a gradual decrease in the corrosion rate and a shift of the corrosion potential in negative direction. This decline of corrosion rate is caused by a decrease of the amount of active FeOOH species. Based on the results obtained by the AC impedance and the corrosion potential measurements, the corrosion mechanisms of the carbon steel in wet/dry cycle conditions has been discussed.
An experimental study was done on the relation between the positions of specimen and erosion-corrosion behavior of aluminum alloy exposed to the two phase jet flow with high temperature of 388K and high speed of 177m/s as the variable of the volume flowrate ratio β of liquid to the sum of liquid and steam. The damage rate, pit diameter and pit depth of aluminum alloy were maximum at β=2×10-3, in the case that the jet impinged perpendicularly to the specimen surface on which the liquid film was thin. When the specimen was set parallel to the flow, the corrosion rate was increased due to local turbulent flows on the surface, but the corrosion rate without such a flow was a little at the same rate as that in the still liquid.
Pitting potential and repassivation potential were measured for copper tubes in 60°C of hot-water by using a test equipment simulating a central hot-water supply. The feed tap water with SO42-/HCO3->1.0 was classified as soft water. Prior to the measurements, copper tubes were subjected to 545d of hot-water circulation at 60°C so as to form copper-orthosilicate films on their inner surfaces. Pitting potential could not be determined from an anodic polarization curve because it did not give any current rise due to pitting. Accordingly, the pitting potential of 210mV (vs. Ag-AgCl) was determined by 100d of potentiostatic corrosion tests at a variety of potentials, wherein pitting corrosion was evaluated by visual examination by means of an optical microscope. The repassivation potential of 190mV was determined by 75d of potentiostatic tests. Prior to the tests, copper tubes with copper-orthosilicate films were held at 60°C, at the potential 250mV for 65d so as to introduce pits having depths of approximately 1/2 of wall thickness. Pit propagation was evaluated by means of the extreme value statistics. The structure and morphology of Type II pits observed in the field were reproduced exactly in the present simulated corrosion tests.
Relating to the absorption refrigeration and heat pump systems, corrosion of carbon steel and its inhibition by hydroxide and molybdate were studied in various LiCl solutions of 1-15m (mol/kg-H2O) at 100-160°C. The increase in LiCl concentration suppressed corrosion of carbon steel slightly at 100-140°C, whereas it promoted corrosion slightly at 160°C. Corrosion morphology was general corrosion, and magnetite particles adhered on the surface. The complicated features of corrosion are described by the several natures of the solution and adhered corrosion products. Both hydroxide and molybdate were effective to reduce corrosion, but pitting was likely to occur in the LiCl solutions of low and intermediate concentrations. In the highly concentrated solutions, on the other hand, only simultaneous use of hydroxide and molybdate was effective for passivation; Otherwise heavy general corrosion was observed. The results are discussed in terms of competitive actions between chloride and hydroxide ions and other natures of the solution.
High power lasers are being used increasingly in many industries for welding, cutting and drilling, but there are few reports on application of laser surface modification. This paper reviews improvement of corrosion properties by surface heat treatment, surface melting, surface alloying, cladding and plating using high power laser beam.