Introduction

1. Magnesium Alloy Corrosion By Group 15: Nathan Lam, Graham Tait, Zhanyi Zhou, Muhammad Ibrahim (www.mgalloycorrosion.wikispaces.com) Introduction Anodization • Observations • Deposition of SO2increased with addition of NO2 and O3 • Dissolution of CO2 causes formation of carbonates, eventually becomes supersaturated and precipitates. • What are Mg alloys? • Mixture of metals to form a stronger and more corrosion resistant metal • Mainly magnesium mixed with aluminum, zinc, manganese, silicon, copper, rare earth metals or zirconium • Why use Mg alloys? • High strength • Light weight • Environmentally • friendly • Cost effective • What is it used in? • Aerospace technologies • Medical applications • Automobile parts • 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 [4] • Conversion coating • Chromate conversion • Phosphate-permanganate conversion • Important considerations • oxidants • promoters • corrosion inhibitors • wetting agents • pH buffer regulators • 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: • low porosity providing corrosion protection. • Coating of complicated shaped surfaces • Inexpensive for mass production Stress Corrosion Cracking • Hydrogen Embrittlement • H atoms diffuse into metal • Slow physical cracks form • Can cause failures of the alloy even under safe loading. [5] Galvanic Corrosion Atmospheric Corrosion Prevention Methods • 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 • Arsenic protection • Reduction in loss of metal & evolution of H2 • Sterics block hydrogen from recombining and poisons the reaction • Electrochemical process • Corrosion at anode • Causes • Electrochemical potential differences • Uneven distribution of atoms Reference [4] 1- Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN0-8493-0487-3. 2- Shannon, R.D., Prewitt, C.T., Effective Ionic Radii in Oxides and Fluorides. (1969). Acta Crystallographica,B25:925-946. 3 - 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 4- 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. 5 - Blawert C., dietzel W., ghali E., Song G. (2006), Advanced engineering Materials8(7): 511-533 • Alloy = solid solution • Mg solvent & Fe solute • Solubility is key • Depends on Hume-Rothery Rules • Atom size • Electronegativity • Conductivity increases when NaCl or (NH4)2SO4 dissolve in the layer [4]