| dc.description.abstract | Mg alloys find widespread applications in transportation industries especially in cars and trucks because of their edges in light-weight design, which can greatly help improve the fuel efficiency and decrease the greenhouse gas emissions of vehicles. However, Mg alloys’ high susceptibility to corrosion limit their penetration in automotive applications. Surface coating is one of the most effective and economic ways to protect Mg alloys from corrosion and thus one major aim of this thesis comes to identify a robust and cost-effective surface coating system for Mg alloys. As most Mg automotive components are subjected to corrosion and cyclic load simultaneously, another focus of this work is the development of corrosion fatigue model suitable for Mg alloys.
First, after reviewing various surface treatment methods available for Mg alloys in literature Mn-P conversion coating was chosen to treat cast Mg alloy AZ31B for corrosion protection and surface protection. The conversion coating process was optimized by studying the effects of processing parameters such as solution temperature, pH value, processing time, and chemical composition of the coating bath on the surface morphology, thickness, microstructure, and corrosion behavior of the Mn-P coating using SEM/EDS and electrochemical testing methods. Based on the results of optimization experiments, a new two-stage conversion coating process was developed to obtain thick and crack-free Mn-P conversion coating on Mg alloys with excellent coating quality. Results of salt spray corrosion test and electrochemical test showed that the conversion coating deposited from this two-stage process had a better corrosion performance than that produced from a single coating process. To attain further corrosion protectiveness, a top polymer painting was applied on Mg alloy pretreated via a variety of surface treatment techniques (i.e. Mn-P conversion coating, chromate conversion coating (CCC), and micro-arc oxide coating (MAO)). In this study, extrusion/forged AZ80 and ZK60 alloys were used as the substrates for evaluating the efficacy of various coating systems. Corrosion tests of the Mg alloys with and without scribes in the salt spray chamber (ASTM B117) were used to characterize the coating properties. The results indicated that the MAO-powder coating system could provide the best corrosion performance for the ZK60 alloy without a scribe while CCC-powder coating system could provide the best corrosion performance for the ZK60 alloy with a scribe.
Next, more attention was placed to the work on modeling of corrosion fatigue behavior of Mg alloys. A process interaction model is proposed to be used to describe the corrosion fatigue crack growth life, assuming that corrosion fatigue is an interaction process between pure fatigue and stress corrosion cracking. In this model, corrosion fatigue crack propagation is divided into three parts: i) when stress intensity factor K is smaller than KISCC, no stress corrosion cracking occurs, and corrosion fatigue crack propagation rates are only contributed by fatigue described by modified Kujawski’s model, in which two correlating parameters were introduced to explain the interactive effects of corrosion and cyclic loading on the material’s properties and the driving force ; ii) When K exceeds KISCC, stress corrosion cracking starts to kick in and join with modified fatigue to enhance corrosion fatigue crack growth rates; iii) With stress intensity factor K further increasing to a certain value where stress corrosion cracking is being independent to K, the effect of corrosion on the driving force of fatigue crack propagation can be neglected while the influence of corrosion on material properties still exists. Data of fatigue in vacuum and stress corrosion cracking for materials such as 4340 steel, 7075-T651, and Titanium alloy Ti-6Al-4V are used to validate this model and good agreements are obtained between the predictive corrosion fatigue crack growth rates and practical experiment results. | en |
