Newly developed surface treatment of conventional weathering steel and high atmospheric corrosion resistant Ni containing steels for the use of construction such as bridges in coastal environments were reviewed. The surface treatment have been developed to promote the formation of protective rust layer and to suppress the stain of concrete by rust-laden water at early stages for use. New Ni containing steels have been used under uncoated condition have higher tolerable limiting amount of air-borne sea salt particles compared that of conventional weathering steel. The evaluation method for the atmospheric corrosion resistance of these materials used in coastal area were also discussed from the standpoint of concentration of the thin chloride solution layer formed on the surface with an decrease in humidity.
An overview is given of the development of new stainless steels for architectural application that had been accomplished in Japan in 1990s, as well as the results of atmospheric corrosion research that had sustained the advances in corrosion control. Atmospheric exposure tests were widely performed to know the corrosion characteristic of stainless steel. The relationship between the amount of sea salt deposition and the critical alloy content to prevent the corrosion damage of steel was elucidated. Based on those results, 22Cr-Mo, 30Cr-2Mo, and austenitic 6Mo stainless steel were newly developed. Moreover, rust nucleation and growth behavior in wet/dry process was studied to make clear the mechanism of atmospheric corrosion. In contrast to the weathering process of carbon steel, it was found that the thickness of water layer formed on steel surface is not the critical factor affecting the atmospheric corrosion of stainless steel. Modeling methods of outdoor corrosion environment was also reviewed, this approach will become a significant technique in corrosion control in atmospheric environment in the future.
Precoated steel sheets are widely used as construction materials, such as roofing and sidings. In this report, organic coatings on Zn and Zn-Al alloy plated steel sheets are mainly described as precoated steel sheets. Trends, material profiles, properties (weather resistance, corrosion resistance), and evaluation methods for the precoated steel sheets are reviewed. In addition, thin organic coatings on Zn-Al alloy plated steel sheets also are described.
Recent development of high-strength concrete enabled the construction of high-rise buildings by concrete, which was only possible by using steel, A new concrete, that does not need vibration consolidation which was necessary in traditional construction, has also been developed. Contrasting such development of concrete technologies, quality problems of concrete has been pointed out intermittently in these 20 years; cracking of concrete, corrosion of steel reinforcements and alkali-aggregate reaction. In this paper outline of the above mentioned trends and problems of concrete engineering are presented along with the corrosion protection technology for steel reinforcement to prolong the life time of concrete structures.
The effect of Mo on the pitting potential, dissolution kinetics, and repassivation behavior of high purity ferritic, Fe-18%Cr-x%Mo, and austenitic, Fe-18%Cr-12-15%Ni-x%Mo, stainless steels was studied in bromide and chloride solutions. Large increases in pitting potential of Fe-18%Cr-x%Mo in chloride solution were found with increased Mo content, compared with distinctly smaller increases in bromide solution. The difference in pitting potentials of ferritic alloys between Fe-18%Cr-2% and 5% Mo in chloride and bromide solutions were in good agreement with the difference in the increased dissolution overvoltages in the solution saturated with dissolved products within artificial pits. Austenitic stainless steels also showed larger increases in pitting potential in chloride solution than in bromide solution with increased Mo content. In the case of the austenitic steels, the difference in the pitting potentials between in bromide and chloride solutions, however, was not attributable to the difference in dissolution rates or repassivation characteristic in the solution saturated with dissolved products. It is supposed that initiation processes may play an important role in the pitting of austenitic stainless steels.
In the view of the increasing need for applying water cooling to electronic equipment, the corrosion behavior of copper was evaluated in a test loop simulating a water cooling system. Pure water or water containing inhibitor was used as the cooling water in order to maintain reliability of the system. Three conditions of water quality were used in the tests: First, testing water controlled by ion-exchanger (pure water) second, 160ppm benzotriazole (BTA) inhibitor added to the testing water; third, testing water uncontrolled by ion-exchanger or BTA inhibitor. In pure water, the formation of fine CuO film inhibits a corrosion of copper when a flow rate is under 4m/s. The corrosion kinetics follows a parabolic rate law. In the testing water containing BTA inhibitor, the corrosion is inhibited by the formation of fine copper-BTA complex film at a flow rate under 4m/s. The corrosion kinetics follows a logarithmic rate law. In the uncontrolled water, the corrosion is not inhibited because copper dissolves through porous CuO film at a flow rate above 1m/s. Then, the corrosion kinetics follows a linear rate law. It is thus concluded that the corrosion rate increases when the ion exchange resin is degraded or concentration of BTA decreases. Consequently, maintenance of ion exchanger and concentration of BTA are required for a high reliability of the system.
Corrosion current distributions of the friction welding joints between commercially pure titanium (Ti) and type 5083 aluminum alloy (A 5083) were measured in a 3.5% NaCl solution by using the scanning vibrating electrode technique. A 200nm-thick intermetallic compound layer was formed at the welded interface at the friction time of 3.0s. This intermetallic compound layer reduces the tensile strength of the friction welding joint. It is suggested that the intermetallic compound layer were comprised of τ-Al (Ti2Mg3Al18), Mg2Al3, and Al3Ti phases. The friction welding joint with the intermetallic compound layer at the welded interface was less susceptible to galvanic corrosion, whereas pitting corrosion occurred in the friction welding joint having only the Mg concentrated area at the welded interface. Therefore, it is considered that the intermetallic compound layer can suppress the galvanic corrosion.
To clarify the acidic and alkaline Intergranular Stress Corrosion Cracking (IGSCC) mechanism of thermally treated alloy 690 (alloy 690 TT) and shot peened alloy 800 (alloy 800 SP), C-ring tests were conducted in deaerated HCl solutions and in deaerated NaOH solutions at 350°C, compared with the acidic and the alkaline IGSCC susceptibilities of mill-annealed alloy 600 (alloy 600 MA), full-sensitized one (alloy 600 FS) and thermally treated one (alloy 600 TT). Grain boundary characteristics, such as chromium depleted zone and chromium carbide precipitation, were examined using modified Huey test and Transmission Electron Microscopy. Potential-pH diagram for Ni, Cr, Fe-H2O system at 350°C was constructed and the solubilities of NiO, Cr2O3 and Fe3O4 were also calculated to evaluate the stability of oxide films which were formed on the surfaces of alloy 690, 800 and 600. Under the acidic condition, the IGSCC susceptibility of alloy 800 SP was high. The selective dissolution of Fe and Ni caused by the slip dissolution at the grain boundary could be a contribution factor of acidic IGSCC of alloy 800 SP. For alloy 690 TT, IGSCC was not observed after 15000hr in the C-ring test. Stable oxide film of Cr2O3, which is formed on the surface of alloy 690 TT, and the suppression of slip at the grain boundary due to the chromium carbide formation is responsible for prominent IGSCC resistance. On the other hand, under the alkaline condition, the IGSCC susceptibility of alloy 800 SP was very low. Stable oxide film formation of Fe3O4 on alloy 800 SP is responsible for prominent IGSCC resistance. For alloy 690 TT, IGSCC did not occur after 10000 hr in the C-ring test. Judging from the test results that the IGSCC susceptibility of alloy 600 MA is very high, and that the one of alloy 600 TT and alloy 690 TT is very low, the suppression of slip at the grain boundary due to the chromium carbide formation is responsible for prominent IGSCC resistance of alloy 690 TT.