Silicon nitride ceramics were corroded in boiling 35wt% HCl and boiling 30wt% NaOH aqueous solutions. The corrosion resistance was evaluated by measuring weight loss and bending strength of the specimens before and after corrosion. The corroded and fractured surfaces of specimens were observed by SEM. When soaked in 35wt% HCl solutions, the corroded layer where the grain boundary was dissolved grew with soaking time. Bending strength dropped with soaking time. When soaked in 30wt% NaOH solutions, grain boundaries were dissolved slightly, and Si3N4 particles were dissolved and then dropped out. Close agreement was obtained between the measured bending strength and the bending strength predicted with the KIc of the silicon nitride ceramics and crack length. In this prediction the corroded layer thickness of the specimens soaked in 35wt% HCl solutions was considered to be the diameter of the semi-circular surface crack, and that of the 30 wt% NaOH solutions was considered to be the maximum depth of surface edge crack.
In order to obtain the data on the resistance to atmospheric corrosion of stainless steels, semi-sheltered outdoor exposure test (18 Sites) in marine atmosphere, where the amount of airborne salt particles had already been measured, were carried out for one year in 1987-1988. As the program was planned to find the effect of alloying element Mo, ferritic steel 18Cr-Mo(0-2%)-C, N(ultra-low), austenitic steel 18Cr-Ni-Mo(0-5%) and duplex steel 25Cr-Ni-Mo(1.5, 3%)-N were exposed. The effect of surface finishing was also investigated by using SUS 304. The resistance to atmospheric corrosion was evaluated by JIS D 0201 Rating Number Method. Rust stain appearance of stainless steels in the present test was different from that by an ordinary outdoor test. Rust stain generation related to the amount of airborne salt particles and the amount of deposited salt. Effective alloying elements in stainless steel were Cr, Mo and N.
Two-dimensional AE location has been carried out at delayed cracking tests of annealed Ti-6Al-4V alloys in air to determine crack growth along the thickness of CT specimens and unveil the delayed cracking behavior. AE events are measured during the tests with four-channel system. AE events are generated nearly at the surface of the specimens and then at the center of the specimens in the delayed cracking tests. In fracture toughness tests, AE events are generated mainly at the center of the specimens in the first stage. It is concluded that the delayed cracking occurs at the surface of the specimens mainly due to moisture adsorption in air.
An axi-symmetric boundary element method (BEM) for corrosion problems was developed, in order to simplify the analysis of axi-symmetric fields such as pipes. The validity and usefulness of the axi-symmetric BEM were confirmed through the experiment and analysis of a cast iron-stainless steel galvanic system in a seawater pipe. Being compared with the 3D BEM, the axi-symmetric BEM permitted large reductions in numbers of elements and nodes, greatly simplified preparations for analysis, and further reduced CPU time. Computational accuracy and CPU time were compared between the analyses using two approximation methods for polarization curves, i.e., logarithmic-approximation method and linear-approximation method. In the former a linear relation was assumed between the logarithm of current density and potential, and in the latter a linear relation between current density and potential. It was confirmed that the accuracy of the analyses using the both methods were sufficiently high at the same level, and the linear-approximation method was more advantageous for reduction in CPU time for complicated polarization curves than the logarithmic-approximation method.
Environmental compatibilities of ceramic-metal joints were reviewed on thermal cycles, wet and non-aqueous corrosions, and oxidation at elevated temperatures. The Al2O3/metal joints with a Ag-Cu-Ti solder were suffered from hot water and oxidation degradations, while the Si3N4/metal joints made by a Mn-metallized method were found to have relatively high anti-corrosion properties. An research trends and their significances were discussed for further improvement and applicabilities of the ceramic-metal joints.