Effects of the addition of Fe, Ni, or Cr to Zr-Sn binary alloy and material processings on corrosion resistance in high temperature, high pressure steam were investigated in order to make clear the correlation between corrosion resistance of Zr alloy and electrical characteristics of oxide film. The increase of Fe or Ni content in Zr-Sn alloy improved the corrosion resistance, whereas with the increase of Cr content, improvement of corrosion resistance was not remarkable. Corrosion resistance was also improved by the heat treatment which depresses the precipitation and the. growth of intermetallic compounds containing Fe, Ni, or Cr. From the facts that the increase of Fe or Ni content and the heat treatment to improve the corrosion resistance decrease the electrical conductivity of the oxide film formed on Zr alloys, electrical conductivity of oxide film is considered to be the rate-determining factor in oxidation of Zr alloys in high temperature, high pressure steam.
Stainless steel pipes whose inner surface is electro-polished have become applied as the pipings for purified gases used in the semiconductor processing. In order to maintain the purity of the gases, the piping is required not to release particles and impure gases from its surface and to have excellent resistance to corrosive gases such as chlorine, hydrogen chloride and so on. The corrosion behavior of the electro-polished SUS 316L pipe was investigated in a moist chlorine environment. The heat affected zone (HAZ) of its weldment preferentially suffered pitting corrosion. The results of the pitting corrosion test using ferric chloride solution revealed that the pitting resistance of the HAZ was deteriorated as compared with the base metal. The chemical compositions at the inner surface of the pipe were analyzed by the secondary ion mass spectrometer (SIMS). The surface film was composed of Cr rich oxide on the base metal, but the Cr concentration in the film on the HAZ was decreased because of the enrichments of Si and Mn. The relationship between the film formation and the corrosion resistance on the HAZ was discussed in relation with the previous work on the pitting corrosion of the tinted HAZ of stainless steels.
High temperature corrosion of the water-wall tubes for the low grades oil firing boilers was investigated during operation under low-oxygen combustion condition. Adjacent to the corroded area near the burner, combution gas contained 0.3-0.6% of oxygen, 0.4-1.0% of hydrogen, hydrogen sulfide and other reducing compounds. The scale deposited on the corroded surface of the water-wall tubes was mainly composed of the metal oxide at outer layer and the metal sulfide at inner layer. Corrosion in these cases was caused by sulfidation reaction of the metal surface with sulfate compounds such as sodium sulfate and unburned carbon in the low oxygen concentration combustion gas. The sulfidation of water-wall tubes in actual boilers can be prevented by Plasma spray coating of 50%Ni-50% Cr alloy.
In order to verify the mechanism of the formation process of hemimorphite (Zn4Si2O7(OH)2· H2O) in galvanized steel pipe for water service, the formation of hemimorphite in the corrosion process of zinc was confirmed through a procedure by which anodic oxidation was united with aging, which was named the anodic oxidation method. Zinc coupons were anodically oxidized in sodium metasilicate (Na2SiO3·9H2O) solution using a galvanostat and then were aged in aqueous solution. The corrosion products on the surface of the zinc coupons aged for the prescribed periods were analyzed using Fourier-transform infrared micro spectroscopy and energy dispersive X-ray analysis. The surface morphologies of the aged coupons were observed using a scanning electron microscope and an optical microscope. Samples of the corrosion products and the solution in which the coupons were aged were analyzed using inductively coupled plasma emission spectroscopy. In combining the results of the co-precipitation method with the results of the present study, it was proved that hemimorphite formation in the zinc-silicate-water corrosion system proceeded in the following sequence: 1) precipitation of Zn(OH)2 as a corrosion product of zinc 2) adsorption of dissolved silicate onto the precipitate of Zn(OH)2 3) breaking of the structure of Zn(OH)2 by the adsorbed silicate 4) formation of an amorphous compound as a precursor of hemimorphite 5) rearrangement of atoms into the framework of the hemimorphite crystal In addition, two types of morphology of the hemimorphite crystal were found.
Principle, apparatus and measurement method of glow discharge optical emission spectrometry (GDS) have been described. Applications of GDS include surface analysis of electroplated film, oxide film, hydrogen absorption layer of titanium, implanted layer of steel, and insulation films.
The request of heat utilization such as power generation of refuse incinerators has been rising. And even advanced heat utilization is being asked. However corrosion of boiler tube has become problems recently. Studies of corrosion eovironment are required to cope with these problems. Then the outline of actual state of corrosion of boiler tube was described and the present state of studies on specific corrosion environment (temperature and components of combustion gas, properties of ash adhered to tube) was described.