Corrosion Mechanism And Development Of The Zn-containing 5xxx Series Al Alloy For Shipbuilding Industry

5xxx series Al alloys are widely used in transport and vehicle industries due to their reasonable mechanical properties and good corrosion resistance. As now, mechanical properties of these alloys are required to be further improved to broaden the application of there alloys in other fields. It is known that 5xxx series Al alloys derive its strength from magnesium solution strengthening and strain hardening. However, when the content of Mg is over 3.5%, the alloys become susceptible to intergranular corrosion if exposed to temperatures above 50℃. Therefore, the key issue is how to simultaneously increase the strength as well as the corrosion resistance of the alloy. In this dissertation, we systematically investigate both mechanical properties and corrosion resistance of the 5xxx series Al alloys by the addition of a reasonable amount of Zn. The effect of processing parameters on the mechanical properties and corrosion resistance of the alloys are also investigated. The intergranular corrosion and exfoliation corrosion of stabilized and sensitized alloys are tested by ASTM G66 and G67 respectively. The microstructure of novel alloys are analyzed and characterized by using scanning electron microscope, electron backscattered scanning diffraction, transmission electron microscope and high resolution electron microscope. The conclusions are summarized as follows:A Zn containing 5xxx series Al alloy having the composition of (5.5-6.0)Mg-(0.9-1.2)Zn-(0.6-1.0)Mn-(0.05-0.25)Zr-0.15Ti-(0.1-0.2)Cu-0.1Cr-0.25Fe-0.2Si (wt.%) is developed. By optimizing the casting, rolling and heat treatment process, both mechanical properties and corrosion resistance of the new alloy are dramatically increased compared to traditional AA5059-H321 alloy. Both the composition of the novel alloy and processing technique are provided to Aluminum Corporation of China for industrialized production.During the fabrication of 5xxx series Al alloy plates, a stabilizing heat treatment is often used after rolling process to prevent age-softening as a result of the precipitation of the second phase particles and reduce the susceptibility to corrosion. During stability at a low temperature of 180℃, the relatively higher dislocation density provides enough diffusion paths for Mg and Zn atoms to reach the grain boundary. While the strength and hardness decrease sharply at a stabilizing temperature of 250℃, which indicates the beginning of recrystallization. At this temperature a sharp decrease of the dislocation density corresponds to a slow diffusion rate of Mg and leads to a homogeneous precipitation of the precipitates in the matrix.The resistance to intergraular corrosion and stress corrosion resistance increase with an increase of the Zn content in the range of 0-1.0wt.%. The morphology and distribution of grain boundary phase modified by Zn additions are observed by transmission electron microscope and the scanning transmission electron microscope. The Mg32(Al Zn)49 precipitates along the grain boundary rather than Al3Mg2 phase by the addition of Zn to Al-Mg alloy. The amount of low angle grain boundaries and small grains increase with an increase in the Zn content. The continuity of grain boundary is interrupted and leads to more "chean" grain boundaries. The stress corrosion crack propagation is resulted from both anodic dissolution and hydrogen cracking.A temper rolling process after hot rolling is developed to suit the novel Al-Mg alloys, in order to reduce the energy consumption and processing cycle, and simulataneously keep the mechanical properties and corrosion performance of the alloy equal or higher than the traditional processing technique. The optimized temperature and rolling reduction are 250℃ and 40-60% respectively. The mechanical properties and corrosion resistance of the alloy prepared by temper rolling are improved compared to traditional processing technique. However, the temper rolling plates are severely susceptible to intergranular corrosion after sensitizing treatment. During temper rolling at such low rolling temperature and large deformation resistance, the shear bands present in the thinner plates will lead to more precipitation of second plase particles after sensitizing treatment.