Electronic structures of oxide films on hot-dipped Zn-0.2%Al, Zn-5%Al, and Zn-55%Al coated steel sheets have been investigated by means of the synchrotron radiation photoemission spectroscopy and compared with those of Zn and Al pure metal sheets. The oxide films were prepared by 85 hour exposure in air saturated with water vapor at 323K. The oxide film on Zn-0.2%Al had the same photoemission spectrum as the pure Zn metal, while Zn-5%Al and Zn-55%Al were similar to the pure Al in the form of photoemission spectrum. It was found that the electronic structure of the oxide films and the depth profile of the ratio of zinc oxide to aluminum oxide are varied with the ratio of Zn to Al in the alloys. The corrosion resistance of Zn-Al alloys is discussed being related to the electronic structure of the oxide films.
In chemical plant equipment, corrosion under insulation (CUI) is sometimes more accelerated than common atmospheric corrosion. In order to simulate this environment, a carbon steel specimen is maintained at 20∼100°C and placed on cotton subjected to repeated drying and wetting cycles ; its corrosion behavior is then examined by measuring the cathodic current density of Pt in relation to dissolved oxygen, in addition to the corrosion weight loss, under potentiostatic-controlled conditions. It is found that the corrosion rate in the drying and wetting cycles is maximum immediately before drying, promoted by an increasing number of drying and wetting cycles, and 8∼20 times that measured in an immersion test. In addition, the rate of corrosion under insulation in actual equipment with insulation is accelerated, particularly in the range of 40∼80°C, because of the relationship between the temperature and the time for drying.
In order to evaluate the effect of mill scale and magnetite on crevice corrosion resistance of carbon steel which is used as a regenerator vessel material in the absorption refrigeration cycle, immersion test and electrochemical measurements of carbon steel in concentrated lithium bromide solution were carried out. As a result, when there was mill scale on surface of carbon steel and there existed magnetite as impurity in concentrated lithium bromide solution, probability of crevice corrosion became higher. It was supposed that reason was as follows : the existence of mill scale on carbon steel mainly restrained anodic reaction of iron and the existence of magnetite promoted cathodic reaction on the surface of carbon steel. Therefore, spontaneous potential of carbon steel became nobler than its repassivation potential of crevice corrosion. It was found that adding reducing agent to the solution was very effective for crevice corrosion protection.
The authors previously reported that the corrosion fatigue strength of thermally sprayed coatings was enhanced by changing the powder composition. This effect was recognized at low stress area near endurance limit on S-N curve but not the high stress amplitude level. This study focused on the spraying pressure as well as the powder composition. The mechanical, corrosion, and corrosion fatigue fracture properties were investigated for three thermal spray materials fabricated by a spray gun with variable pressure. The corrosion fatigue strength of a specimen coated by WC/NiCrMo-A was the highest in the three type of coated specimen. It is thought that the mechanochemical response was controlled by a combination of : (1) The residual compressive stress generated by high spraying pressure for decreasing the effective stress, and (2) Enhanced corrosion resistance resulting from easy passivation of the coating components and the densification of the coating without any penetration of the solution into the base metal/coating-interface.
Magnesium (Mg) alloys have advantages in abundance of natural resources and in ability for recycling. On the other hand, Mg alloys have a disadvantage in poor corrosion resistance. In order to apply the Mg alloys to practical engineering parts, it is important to investigate the mechanical properties of Mg alloys under corrosive environments. In this study, the rotating bending fatigue test for ASTM-AZ31B specimens have been carried out under corrosive environments at room temperature. Silicone rubber was used for covering the circumference of the test specimen and NaCl solution was injected for corrosive environment. Additionally, air was poured into silicone cover to keep the amount of O2 in the solution. Because the fatigue tests resulted in scattered fatigue strength, the S-N curve was calculated by a statistical method. The fatigue strength in 3wt% NaCl solution was decreased in comparison with that in atmosphere and fatigue limit became unclear. Evolution of gas bubbles was dominantly observed during corrosion fatigue test. Measurements of anodic polarization curves showed the corrosion potential lays hydrogen evolution region. Therefore, it is considered that the mechanism of crack propagation has influenced by hydrogen and the decrease of fatigue strength is related to hydrogen embrittlement. The failure modes are classified into three groups from fractographical view point. That is the failure originated from single crack, from plural cracks and from corrosion pits. Among them, the coefficient of variance of fatigue strength distribution became larger in case of the failure originated from plural cracks and corrosion pit.
In recent years, cathodic protection has been installed to many concrete structures damaged by steel corrosion due to chloride attack. These conventional cathodic protection systems had a few defaults as mentioned bellow ; (1) It needs much man power to remove steel pieces on the concrete surface to prevent short circuit and (2) It needs skilled workers for the applications. In order to solve the problems, we developed a new anode system for cathodic protection. The anode system is installed by fixing tray-shaped anode which is composed of titanium sheet, titanium anodes and insulating frame, followed by mortar injection into the tray. The system was installed on a beam of an actual pier. As a result, it has been confirmed that the easy installation and high durability.
In order to evaluate the strain rate dependence of the dynamic flow stress of 6061-T6 aluminum alloy, strain rate reduction tests are conducted in the strain rate range from about 2×104 to 4×104s−1, which is the strain rate range before reduction, and the reduction in strain rate is 58.6%. To eliminate an effect of elastic wave dispersion produced in a bar on the measured flow stress, a output bar being 2mm in diameter is employed. A steep increase in the flow stress was observed at the strain rate of about 2×104s−1. A simplified model for dislocation kinetics under dynamic plastic deformation is used which can represent a transition in the rate controlling mechanism of dislocation motion from a thermally activated process to a viscous drag. It is comfirmed that the steep increase in the flow stress of 6061-T6 observed at the strain rate of about 2×104s−1 is attributed to the rate dependence of the viscous drag on the dislocation motion and furthermore, the increase in the mobile dislocation density lowers a velocity of moving dislocations and shifts the transition region, or the strain rates in which the steep increase in the flow stress becomes to appear, to the higher strain rate side.
In order to understand the initiation behavior of microstructurally small cracks in a stress corrosion cracking condition, it is important to know the tensile normal stress acting on the grain boundary (normal G. B. stress). The local stress in a polycrystalline body is greatly influenced by deformation constraint which is caused by anisotropic and/or inhomogeneous property of each grain. In present study, the local normal G. B. stress on bi- and tri-crystal bodies and a three-dimensional polycrystalline body consisting of 100 grains were evaluated by the finite element method under a remote uniform tensile stress condition. The polycrystalline body was generated by using a Monte Carlo procedure and random orientations were assigned to each grain. It was revealed that the local normal G. B. stress on the polycrystalline body is inhomogeneous under uniform applied stress. The stress tends to be large near the triple points due to the deformation constraint caused by adjacent grains, even though the grain boundary inclination to the load axis has large influence. It was also shown that particular high stress was not observed at corners of the polycrystalline body.
In stress corrosion cracking, many surface cracks are initiated and the interaction between the cracks affects on crack growth behavior. Therefore, influence of the interaction on crack growth has to be taken into account in fitness-for-service evaluation. However, quantitative evaluation of magnitude of the interaction is difficult because many factors, such as the relative position, size and geometry, have influences on the stress intensity factor (SIF), which is driving force of the crack growth. In this study, in order to investigate the intensity of interaction, the SIF of interacting two semi-elliptical surface cracks was evaluated by the finite element method and finite element alternating method. These methods enable us to evaluate the SIF of interacting cracks for various conditions. The analyses were preformed under uniform tensile and bending load for various aspect ratios and relative crack sizes condition. The analysis results reveal that the change in the averaged SIF along the crack front caused by coalescence of two cracks can be estimated from the change in the area size. The maximum interaction can be estimated by simple addition of the area size of two cracks regardless of the loading condition and relative crack size. This was confirmed by the growth simulation of interacting cracks according to the power law of SIF.
In residual stress fields, the so-called partial elastic contact of crack surfaces sometimes occurs, where a fatigue crack is closed at the crack mouth while it is open at crack tip. The partial elastic contact has significant effect on the lives of fatigue cracks. However there are few studies on the partial elastic contact of crack surfaces for three-dimensional cracks. In this paper, the propagation paths and lives of slant corner fatigue cracks are predicted by using three-dimensional fatigue crack propagation simulations considering the partial elastic contact of crack surfaces. The finite element method was employed for the simulations. The crack propagation rates obtained by simulations considering the partial elastic contact of crack surfaces differed from those without considering the contact, demonstrating the necessity of simulation considering the partial elastic contact of crack surfaces. However the propagation paths and crack shapes estimated considering the partial elastic contact hardly differed from those without the consideration.
Post weld heating was applied to the simulated samples of the bond region of super duplex stainless steel in order to improve the toughness of weld bond region. The heating temperatures used were 1173K, 1273K and 1373K. The morphology of the secondary austenite changed from small intragranular type to massive austenite type as the heating temperature increased. The rate of austenite formation was fastest when the heating temperature was 1273K due to the C-curve kinetics. When the heating time exceeded 120s, the austenite content reached 50% in any heating temperatures. Charpy impact tests were performed using the specimens in which austenite content reached 50%. In the case of the heating temperature of 1173K, the toughness was hardly improved probably because of the precipitation of chromium nitrides or small amount of sigma phase. When the heating temperature was 1273K, good toughness comparable to the base metal could be obtained. It was found that the massive intragranular austenite arrested the cleavage crack and formed small steps and tear ridges among the cleavage facets from the fractographic observation based on three-dimensional morphology and observation of the cross sectional microstructure of cleavage fracture surface.