Tomonori Namito

On Electrochemical Polarization Curve and Corrosion Fatigue Resistance of the AZ31 Magnesium Alloy

Recently, due to increasing popularity of the magnesium alloys, many studies on corrosion resistance of the magnesium alloys have been reported to improve their corrosion resistance. However, it is not clarified yet whether an electrochemical polarization curve or a corrosion rate is a good measure for the corrosion fatigue resistance or not. In the present study, corrosion fatigue tests, the weight loss test, and measurement of the electrochemical polarization curve were performed in 3% sodium chloride solution using both the extruded and the rolled Mg alloys. It was clarified that there are no differences in the corrosion fatigue lives between the extruded and the rolled Mg alloys, though they have different corrosion resistance. So, it was concluded that the corrosion fatigue characteristic does not correspond well with the trend in corrosion resistance.

Improvement in Corrosion Fatigue Resistance of Mg Alloy Due to Plating

Magnesium alloys are very attractive as structural materials, because they are extremely light, possessing excellent specific tensile strength, good stiffness, good cutting performance, and good vibrational absorption [1]. In addition, the recycling energy requirement of these alloys is only 4–5% of the energy required to obtain magnesium from the ore [1]. Due to their energy and weight saving characteristics, magnesium alloy are considered to be good candidates for material in auto parts, portable personal computers, and telephones. However, magnesium alloys have not been extensively used until recently, because of their vulnerability to corrosion. When considering the use of magnesium alloys as structural materials, a thorough understanding of the corrosion-fatigue characteristics is necessary to reflect the results in machine design. Further, improvement in the corrosion fatigue performance of magnesium alloys is highly desirable to realize their potential as structural materials. Surface treatments, such as coating or plating of the material surface have been examined for enhancement of the corrosion fatigue resistance of magnesium alloys, and in general they have not been successful. In this paper, two types of plating treatments, i.e., electroless-Ni-plating [2] and electrolytic Ni-plating [3] were applied to the Mg alloy AZ31 to improve the corrosion fatigue resistance of the alloy. Corrosion fatigue lives and fatigue behavior of the specimens were investigated in detail and their mechanisms will be also discussed. Specific contents in this chapter are summarized as follows. In section 2 [2], fatigue tests were performed on electroless nickel-plated magnesium alloy specimens in laboratory air and 3% sodium chloride solution. In laboratory air, the effect of surface treatments (plating, blasting and polishing) on the fatigue lives of specimens was found to be minimal. However, in 3% sodium chloride solution, the electroless Ni-plated specimens were found to have shorter fatigue lives than those of the polished and blasted specimens. In order to study the fatigue mechanisms, successive observations of the specimen surfaces were conducted during the fatigue process in both laboratory air and sodium chloride solution. Observations of the fracture surfaces were also conducted to clarify the fatigue mechanism. In section 3 [3], the fatigue behavior of electrolytically nickel-plated Mg alloy both in laboratory air and in a corrosive environment (3% NaCl) were described. The fatigue

Distribution of Fatigue Lives for the Diecast Magnesium Alloy

. In this study, fatigue tests were carried out using a diecast Mg alloy AZ91 to study the distribution of fatigue lives under constant stress amplitudes. During the fatigue process of the diecast Mg alloy, cracks initiated from the casting defect within inside of the specimen, and then propagated prior to final failure of the specimen. Distributions of fatigue lives at the constant stress amplitudes can be represented by the Weibull distributions. The scatter in the distribution of fatigue lives becomes larger at the lower stress amplitude of 80 MPa as compared with those at the higher stress amplitudes. There is a common relationship regardless of the stress amplitudes between the initial maximum stress intensity factors Kimax and fatigue lives Nf. The lower Kimax, the longer Nf becomes. Integrating the fatigue crack propagation law from the initial maximum stress intensity factor Kimax to the fatigue fracture toughness Kfc, the relation Kimax vs. Nf can be successfully evaluated.