Study On The Atmospheric Corrosion And Chemical Conversion Coatings For Pure Magnesium And Magnesium Alloys

Magnesium alloys are of great importance at present and future fields of engineering for their attractive combination of low density and high strength/weight ratio. However, since magnesium is intrinsically active in the environment, the corrosion resistance of magnesium alloys is generally inadequate, which limits their application. In order to improve the corrosion resistance of magnesium alloys, surface treatment technologies, like chemical conversion, are commonly applied. Nowadays, the methods applied introduce chromate ions in the solutions, which is progressively restricted due to its high toxicity to the environment. The trend in the field is oriented to investigate the corrosion mechanism of magnesium alloy and develop environmental friendly chemical treatment technologies.In this paper, the atmospheric corrosion behavior of extruded AZ31 magnesium alloy and pure magnesium samples is studied by weight loss test, electrochemical measurements and physical detection technologies, such as SEM, EDX, EPMA and XPS. After 400d atmospheric exposure station in Dalian city, the samples are covered with a plumbeous corrosion production film. Compared with pure magnesium sample, the film on AZ31 magnesium alloy is more compact. Moreover, after the corrosion production is wiped off, some insulated "island", which formed on the corrosion slightly area, are observed on the surface of the samples. The proportions of the corrosion region to the general area are 65.0% and 42.3% on AZ31 and pure magnesium, respectively. The compositiones of corrosion production on the two samples are almost similar. They are MgO, Mg(OH)₂, Al(OH)₃, Al₂O₃ and carbonate, sulfate and chloride of Mg and Al. The corrosion kinetics calculated by image methods accord with exponential equation and the dynamics equations for AZ31 and pure magnesium are H_AZ31=0.403×t^0.653 and H_PMg=0.549×t^0.665, respectively.The effect of NaCl, Na₂SO₄ and NaNO₃ on the atmospheric corrosion of AZ31 magnesium alloy and pure magnesium samples was studied by the indoor simulated exposure experiment and electrochemical test under thin electrolyte layers. After 40d indoor simulated exposure station with the addition of NaCl, Na₂SO₄ and NaNO₃ on the surface of AZ31 and pure magnesium, the samples occur local corrosion. The corrosion rate of the samples increases with the salt concentratiom increasing. The aggressiveness of the salt is NaCl > Na₂SO₄> NaNO₃ Electrochemical tests under thin electrolyte layers show that decreasing the thickness of electrolyte layers results in accelerating corrosion rate. The thinner the layer is, the easier the occurrence of local corrosion on the samples is. The difference of the corrosion behaviors between in solution and under thin layer is mainly due to the dissolve of O₂ and CO₂ in the electrolyte. In addition, the NO₃^- is more sensitive to the thickness of the layer than Cl^- and SO₄^-, which is caused by the participation of NO₃^- in the cathodic depolarization.In order to avoid high toxicity like Cr treatment to the environment and improve the corrosion resistance of magnesium alloys, KMnO₄-REMS chemical conversion treatment technique is developed in this paper. The conversion coatings obtained with this method show better corrosion resistance than previously used phosphate-based conversion treatment for AZ91, AM60, AZ31 and pure magnesium samples, and reach the levels of chrome-based treatment. Furthermore, the electrochemical polarization tests show that compared with the samples treated by chrome-based method, the anodic current density of the alloy coated in permanganate-REMS bath, at the same potential, decreases evidently in 0.05mol·L^-1NaCl solution. Simultaneously, the open-circuit potential of magnesium alloy increases 0.06V compared with blank samples. The thickness of the coating formed by the permanganate-REMS bath on AZ91 magnesium alloy is about 10μm and net-like cracks exists on the coatings. The conversion coating mainly composes of CeO₂, MnO, MnO₂, and some oxides of Mg and Al.Phytic acid, as an environmental- friendly agent, has been growing applied in the application of metal surface treatments. In this paper, the corrosion protects afforded by phytic acid conversion coatings on magnesium and magnesium alloy has been developed. The results show that optimal pH range for the treatment is 9-10. In 0.05mol L^-1NaCl solutions, Magnesium alloy coated by phytic acid solution don't corrode until 8 hours. Simultaneously, compared with blank samples, the open-circuit potential of magnesium alloy treated by phytic acid solutin is increased and the anodic current density is decreased at the same potential. A compact film is found on the surface of magnesium alloy with the combination of phytic acids and the metal. The film is mainly composed of phytate and some oxides of Mg and Al. This film may insulate the metal from the deleterious anion, thus protect the magnesium alloy against corrosion. Moreover, by reacting to the paint, the phytic acid coating can improve the resin binding power.