Study On Corrosion Behavior And Protection Technology Of 3C Magnesium Alloys In Simulated Sweat Solution

Study on corrosion and protection of metal materials has been the considerable issues and the toughest questions for a lot of material scholars. Corrosion and protection of modern society is not only a simply question of technique problem, but also a great social and economic problem relate to environmental protection, save energy, save material, ensure normal production and personal safety, developing new technology, and so on. Magnesium alloys have been widely used in 3C （computer, communication and consumer electronic）, aerospace, automotive and industry and other fields because of a series of excellent performances. It is considered to be the most potential for the development and application of "green engineering materials in the 21st century”. However, due to high chemical reactivity of magnesium and loose corrosion product layer, magnesium alloys have poor corrosion performance. It has been a major obstacle to their further applications.In this thesis, three casting magnesium alloy AZ31、AZ61、AZ80, which were widely used in 3C industrial fields, were selected and their corrosion behaviors in simulated sweat solution have been investigated. Potentiodynamic polarization curves showed that three magnesium alloys revealed poor corrosion resistance in simulated sweat solution and localized corrosion would spontaneously appeared on the surface of three alloys. Three main components of sweat played different roles in corrosion progress: Lactic acid was a kind of strong erosive medium for magnesium alloys, and NaCl could induce pitting corrosion on alloys surface, while urea acted as corrosion inhibitor. This thesis first proposed that the corrosion resistances of magnesium alloy increased with the increased of Al content by studying the corrosion resistances of three magnesium alloys with different Al content by electrochemical impedance spectroscopy （EIS）, surface analyses and weight loss experiment. XRD pattern showed that the corrosion product of three magnesium alloys in simulated sweat was Mg （OH）₂.In order to obtain further information of corrosion behavior of 3C magnesium alloy surface touched by simulated sweat, spectroscopic ellipsometry was employed to simulate the corrosion case of 3C products outer shell contact with sweaty hand. Analysis of experimental data revealed that corrosion reaction would stop after 2.04 minute when magnesium alloy surface was touched by simulated sweat. The liquid film thickness of magnesium alloy surface was 3.5nm at this time. Surface structure of corrosion layer was described with a three-layer structural model （substrate - EMA- Cauchy） and thickness of each layer for different soaking time was obtained by fitting experimental data with an appropriate model. Fitting results showed that the thicknesses of EMA and Cauchy increased with immersion time, but the increasing rate went down. The corrosion product films with a refractive index of 1.45-1.62, loose corrosion product layer could only provide limited protection to the substrate when magnesium alloy surface corroded by sweat again. Special surface treatments were necessary for magnesium alloy before coating. The thesis put forward a completely new research method of site monitoring the micro-corrosion system in a short time using ellipsometry and giving the reasonable explanation for the physics meaning of ellipsometry data.Green calcium series conversion was used treating surface of 3C magnesium alloys and an ideal chemical conversion film was obtained. Process of chemical conversion was optimized by orthogonal experiments: Monocalcium phosphate: 35g/L, ammonium metavanadate: 1.5g/L, phytic acid: 1 g/L, treatment time: 5 minute. Analysis showed that treatment time was the most important influence factor on thickness and corrosion resistance of chemical conversion film, the next was concentration of monocalcium phosphate and ammonium metavanadate, and the last was concentration of phytic acid. Monocalcium phosphate was main forming agent and calcium salts was the main components of conversion film. Ammonium metavanadate was the assistant of film forming, and phytic acid was the additive. Potentiodynamic polarization curves indicated the corrosion current densities was reduced one order of magnitude after treatment and corrosion current densities platform appeared on anodic polarization curves. The EIS data as a function of immersion time reflected the evolution process of chemical conversion film including three stages: the growth state （0-18h）, the steady state （18-30h） and the deterioration of film （after 30h）. Tested by experiment, the corrosion resistance and the impact resistance of the coating were good and the adhesive force was excellent. The surface film was good water affinity, highly conductive. It was an ideal bottom layer.Study of magnesium alloy inhibitors can widen application fields for magnesium alloy on one hand and it can play an active role for chemical conversion of magnesium alloys on the other. Inorganic/organic composite inhibitor for magnesium alloy was first reported here. The inhibitive effect and mechanism were also studied thoroughly and systematically. Strong evidences showed that although individual sodium silicate or sodium dodecyl benzene sulfonate （SDBS） played limited roles on protecting magnesium alloy from the corrosive medium, the combination of sodium silicate and SDBS exhibited much more effective effect on the controlling the corrosion of magnesium alloy in 1% NaCl solution. The optimum formulation consisting of 1mmol/L sodium silicate and 2mmol/L SDBS had an IE of 95%. Corrosion inhibition mechanism proposed in this thesis considered that sodium silicate could promote the formation of corrosion product and SDBS could enhance adhesive force between the film and the substrate. It was the synergistic effect of NaSiO₃ and SDBS that offered effective protection for magnesium alloy substrate.