There is a strong demand for corrosion protection measures of steel structures against severe corrosion, due to exposure in an arctic ocean. Corrosion resistance of stainless steel, SUS 329J1, was studied by electrochemical measurement and immersion test in order to apply to the most corrosive part of steel structures, the water line area. It was clarified that crevice corrosion and corrosion at welds covered with oxide film occurred on the stainless steel, irrespective of low temperatures near the freezing point. Stainless steel also causes considerable galvanic corrosion to carbon steel, especially at the part adjacent to the weld metal. By applying catholic protection to the dissimilar joints, both crevice corrosion on stainless steel and galvanic corrosion on carbon steel can be prevented to occur. Thus, it was concluded that a combination of stainless steel and catholic protection was useful measures.
The effects of phosphorus content on the structure and corrosion resistance of electrodeposited Co-P alloys have been investigated. X-ray diffraction revealed that alloys containing less than 4% phosphorus are super-saturated solid solution, the deposits containing 5-15% phosphorus are amorphous, while more than 16% phosphorus alloys show the structures of Co2P crystal, intermetallic compound. The corrosion resistance of alloys with different P content was investigated by anodic polarization. The amorphous alloys containing more than 10% of P showed excellent corrosion resistance in 1 N HCl and 3.5% NaCl solutions. The AES profile revealed that P was condensed on the surface of alloys. However, it polarized to more anodic potential, the phosphate was formed and the corrosion resistance become poor. In alkaline medium, corrosion resistance was improved with increasing content of P. Thus, alloys containing more than 10% of P were self-passivated in 1 N NaOH solution.
A new technique “in situ HIC Measurement Method” has been developed for measuring the HIC susceptibility more exactly. This technique is based on the combination of the electrochemical hydrogen permeation method and the automatic ultrasonic wave inspection system. The threshold hydrogen permeability (Jth×l) in segregated portion of line pipe steels decreased with the hardness of segregated portion. The diffusion coefficient of hydrogen (D) also decreased with the hardness. The threshold hydrogen content (Cth), which was calculated by the formula; Cth=Jth×l/D, was constant regardless of the hardness. On the other hand, the hydrogen content in steel (C0), which was obtained by the formula; C0=J×L/D, increased with the hardness, because the hydrogen permeation rate in a corrosive environment (J×L) was dependent on both the severity of the environment and the chemical composition of steel. Therefore it can be thought that the high HIC susceptibility is not attributed to the decrease in the Cth, but the increase in the C0.
Recently, resulting from pitting corrosion of copper tube, leaks in air-conditioning coils for open heat-storage water tank systems are frequently happening. Water-side cause of the corrosion is the sediment on tube inner surface of either CaCO3 due to too high pH and saturation index or a compound containing PO43- or MoO42- supplied by some corrosion-inhibitor. Coil-side cause of the corrosion is the scattered thin oxide film on tube inner surface originated in oxidization by humid air at room temperature during the period from the creation of copper tube till the water pour at installation site. This oxide film shows nobler corrosion potential than ground copper by 35 to 103mV. Consequently, when the sedimentation has occurred, oxygen concentration differential cell will be formed and, increasing potential difference to the oxide film, corrosion of the neighbouring ground copper will be accelarated toward the pitting. In restraint of the oxide film formation, it may be important to keep the tube inside clean throughout the manufacturing, but, as a sure means, it is recommendable that the coil should be filled with dry nitrogen gas at the completion of manufacture, and be kept intact till the water pour at the site.
Chemical adsorption of anions on metals or a metallic oxide and their effects on corrosion and passivity of metals were discussed on the basis of the hard and soft acids and bases principle (HSAB principle). The surface coverages of various anions on iron and nickel in 1N HClO4 and borate buffer solutions and the corrosion and passive current densities of the metals in the solutions containing the anions were determined by polarization and impedance measurements. The coverages of anions on the metals were concluded as being closely related to the HSAB principle. Because a surface site around an adsorbed anion acts as a softer acid, a neutral molecule classified as a soft base is chemisorbed at the site without more difficulty. Since the passive current density of iron in the borate buffer with anions is closely related to the HSAB principle, the adsorption of anions on the oxidized iron surface is also associated with the principle. It was concluded that the pitting potential of an iron electrode in the borate buffer containing Cl-, Br-, I-, or S2-, being aggressive upon the passivity of iron, is also related with the HSAB principle. These anions were detected near the iron substrate in a passive film by XPS measurement. Other anions, F-, ClO4-, CH3CO2-, SO42-, SO32-, SCN-, CN-, and N3- scarcely induce the breakdown of passive film on the iron surface. Inhibition effects of a tetraalkylammonium ion in the presence of added anions on the iron corrosion and passivity are also shown.
Acidity change in rain in Japan, China, USA and Europe are overviewed and material damages to both wet and dry deposition of acid compounds is summarized. The major groups of materials discussed are metals, masonry, and painted surfaces. An emphasis is placed on the deposition process that relate air pollution to materials damage, as well as the roles of rain and rain acidity.