AE monitoring of the sensitized and non-sensitized SUS 304 steels exposed to the molten mixed chlorides (30mol%PbCl2+30%FeCl2+20%NaCl+20%KCl) at around 600°C was attempted utilizing the new heat-resistant AE monitoring system developed in the first paper. AE waves or out-plane displacements, indicating the fast Mode-I and -II crackings, were reproducibly monitored for both the sensitized and non-sensitized SUS 304 steels. Crack generation rate estimated by the forward signal processing with accounting the wave attenuation and the responce delay of the sensor was found to be faster for the sensitized steel than that for the non-sensitized steel. The maximum crack rate for the Mode-I and -II fracture, respectively, reaches 35 and 170m/s for the sensitized SUS 304 steel at 600°C under the applied stress of 157MPa. These fast cracks are assumed to occur isolately in time and space. Possible mechanisms for such fast crackings are discussed in relation to the grain boundary decohesion.
The SCC behavior of austenitic stainless steels (type 304 and type 316) was investigated in acidic chloride and sulfate solutions using constant load, constant strain and slow strain testing methods. Particular attention has been paid here to whether these testing methods contain parameters which allow accurate prediction of the time to failure. It was found that the parameters were steady state elongation rate for constant load, stress relaxation rate for constant strain, and maximum stress ratio for slow strain rate testing, respectively. The comparison between the characteristics of these parameters led to the conclusion that the best parameter for the prediction of time to failure was the steady state elongation rate, by which time to failure could be predicted at an early period of SCC experiment.
Laser surface melting (LSM) treatment using continuous wave CO2 laser was applied for improvement of general corrosion of nodular graphite cast iron and grey cast iron. Corrosion behavior of as-melted surface and reheated suface were evaluated by immersion and potentiodynamic tests. A chill surface layer was formed due to rapid solidification in applying LSM treatment, and corrosion rate in 0.05kmol/m3-H2SO4 solution decreased except heat affected zone. By LSM treatment, current density decreased in passive range in anodic polarization curves, which coincided with immersion test. This improvement of corrosion resistance by LSM treatment is caused by the disappearance of graphite due to rapid solidification. Resistance to general corrosion of LSM specimen was maintained by reheating treatment, which coincided with anodic polarization curves.
To improve the corrosion resistance of stainless steels, corrosion behavior of stainless steel with various sputtering-deposited oxide films, in acidic solutions, was studied. The effect of pinholes, inevitable structural defect in sputter coated oxides, on the corrosion protection properties of these films were examined. With a single, RF magnetron sputter-deposited, layer of oxide coating, the corrosion resistance was greatly improved compared to the non-coated stainless steel. Polarization experiments show the passive current density to be influenced by the pHpzc (pH of zero charge of the oxide). The defect probability was determined using both electrochemical measurements and optical method using nematic liquid crystals. The good correlation of the results between these two methods indicates that either method is available to determine the defect density of thin films. For double-layer coated oxide films, the effect of pHpzc was clearly observed. In cases where the inner oxide layer possesed a higher value of pHpzc, the corrosion resistance was proven to be greater. The effect is explained by the difference in ion migration through the coated films.
Pitting corrosion of SUS 316L stainless steel pipe was investigated on its application to high purity gas supply equipments in semiconductor manufacturing. Two types of 316L were used: a conventional one and a purified one by vacuum induction melting and vacuum arc remelting. The pipes were seamless pipe of 6.35mm outer diameter and 1.0mm wall thickness and their inner surfaces were finished with bright annealing (BA) and electro-polishing (EP). Those pipes were exposed to moist Cl2 at 30 degree C for 24h. After the test the dissolved Fe from the pipes was measured by dissolving the corrosion product with 30%HNO3. The dissolved Fe amount gave following ranking as the pitting resistance: purified EP>conventional EP=purified BA>conventional BA. The ranking suggested that the dissolved Fe, namely corrosion amount, was dependent on the amount of defects such as inclusions and scratches on the inner surface of the pipes.
This review describes current trend in development of corrosion protective plating for applications in the automotive, buildings, aircraft and others industries. Although zinc is widely used for providing sacrificial (cathode) protection to steel in a severe corrosion environment, zinc is consumed rapidly. Recent demands for higher quality finishes, and, more specifically, longer lasting finishes, have prompted a move to alloy zinc plating. The alloying elements successfully used with zinc have been iron, nickel, tin and cobalt. In Addition, newly devised corrosion protective plating such as double layer coating, dispersion composite coating, gradient composite coating and hybride coating have been developed for improving corroson protection, and among others, polymer adhesion, low-hydrogen embrittlement and low-coefficient of friction characteristics.