Magnesium Alloy Corrosion By Group 15: Nathan Lam, Graham Tait, Zhanyi Zhou, Muhammad Ibrahim Reduction in loss of metal & evolution of H2 Introduction Prevention Methods Sterics block hydrogen for recombining & poison the reaction What are Mg alloys? Mixture of metals to form a stronger and more corrosion resistant metal Mainly magnesium mixed with other metals, such as: aluminium, zinc, manganese, silicon, copper, rare earth metals and zirconium Why use Mg alloys? High strength Light weight Environmentally friendly Cost effective What is it used in? Aerospace technologies Medical applications Automobile parts Conversion coating solution oxidants promoters corrosion inhibitors wetting agents pH buffer regulators Chromate conversion coating & Phosphate and phosphate-permanganate conversion coating Chromate conversion treatment is a very fast process The chromate conversion treatment process is acidic, which causes the dissolution of magnesium The process of phosphate conversion coating typically involves magnesium to be placed in the phosphoric acid, which reduces the H+ ions, thus raising the pH Electrophoretic coating (E- COATING) Core idea: colloidal particles are suspended in a liquid medium, mitigated under the influence of an electric field and then are deposited onto an electrode. Advantages: o low porosity providing corrosion protection. o Coating of complicated shaped surfaces o Inexpensive for mass production. Reduced corrosion in the presence of As Conductivity increases when NaCl or (NH4)2SO4 dissolve in the layer Anodization Observations Deposition of SO2 increased with addition of NO2 and O3 Dissolution of CO2 causes formation of carbonates, eventually becomes supersaturated and precipitates. Passivation layer Formation of a oxide layer that prevents, and slows down further oxidation Layer formed by anodization includes: magnesium oxide, magnesium hydroxide, and magnesium silicate Quality and thickness Pilling-Bedworth Ratio Voltage, current density, concentration Surface treatment Environment Classes Rural Urban Industrial Marine Galvanic Corrosion Element of interest: reduction reaction Electrochemical potential E0 (V) [A] Magnesium (Mg): Mg2+ + 2e-  Mg(s) -2.372 Iron (Fe): Fe2+ + 2e-  Fe(s) -0.44 Stress Corrosion Cracking Electrochemical process Corrosion at anode Causes electrochemical potential differences Uneven distribution of atoms Hydrogen Embrittlement H atoms diffuse into metal and recombines to H2 molecules Causes amplified stress levels Slow physical cracks form Can cause failures of the alloy even under safe loading. Atmospheric Corrosion Potential corrosion pollutants Sulfur Dioxide (SO2) Nitrogen Dioxide (NO2) Ozone (O3) Carbon Dioxide (CO2) Nitric Acid (HNO3) Sea salt (NaCl) Ammonium sulfate ((NH4)2SO4) Formation of electrolyte layer Occurs by adsorption on the hydroxylated oxide Alloy = solid solution Mg solvent & Fe solute Solubility is key Depends on Hume-Rothery Rules Atom size Electronegativity Reference A - Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3. X - Shannon, R.D., Prewitt, C.T., Effective Ionic Radii in Oxides and Fluorides. (1969). Acta Crystallographica, B25:925-946. Y - Elect neg : J.E. Huheey, E.A. Keiter, and R.L. Keiter in Inorganic Chemistry : Principles of Structure and Reactivity, 4th edition, HarperCollins, New York, USA, 1993 B - Birbilis, N., Williams, G., Gusieva, K., Samaniego, A., Gibson, M.A., McMurray, H.N. Poisoning the Corrosion of Magnesium. (2013). Electrochemical communications. 34: 295-298. atom “Crystal” radius (Å)[X] % size difference from Mg Electronegativity[Y] Mg 0.86 - 1.31 Fe 0.69 19.7 1.83 As 0.72 16.3 2.18