Introduction Magnesium Alloy Corrosion By Group 15: Nathan Lam, Graham Tait, Zhanyi Zhou, Muhammad Ibrahim Atmospheric Corrosion Prevention Methods 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 Galvanic Corrosion 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: 1. low porosity providing corrosion protection. 2. Coating of complicated shaped surfaces 3. Inexpensive for mass production Potential corrosion pollutants • Sulfur Dioxide (SO 2 ) • Nitrogen Dioxide (NO 2 ) • Ozone (O 3 ) • Carbon Dioxide (CO 2 ) • Nitric Acid (HNO 3 ) • Sea salt (NaCl) • Ammonium sulfate ((NH 4 ) 2 SO 4 ) Formation of electrolyte layer • Occurs by adsorption on the hydroxylated oxide • Conductivity increases when NaCl or (NH 4 ) 2 SO 4 dissolve in the layer Observations • Deposition of SO 2 increased with addition of NO 2 and O 3 • Dissolution of CO 2 causes formation of carbonates, eventually becomes supersaturated and precipitates. Stress Corrosion Cracking Hydrogen Embrittlement • H atoms diffuse into metal • Slow physical cracks form • Can cause failures of the alloy even under safe loading. Reference Anodization 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 ato m “Crysta l” radius (Å)[2] % size differen ce from Mg Electrone gativity[ 3] Mg 0.86 - 1.31 Fe 0.69 19.7 1.83 As 0.72 16.3 2.18 Element of interest: reduction reaction Electrochemical potential E 0 (V) [1] Magnesium (Mg): Mg 2+ + 2e - Mg (s) -2.372 Iron (Fe): Fe 2+ + 2e - Fe (s) -0.44 Electrochemical process Corrosion at anode uses Electrochemical potential differences Uneven distribution of atoms • Alloy = solid solution o Mg solvent & Fe solute • Solubility is key o Depends on Hume-Rothery Rules 1. Atom size 2. Electronegativity 1 - Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press . ISBN 0-8493-0487-3 . 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 Materials 8(7): 511-533 Arsenic protection •Reduction in loss of metal & evolution of H 2 •Sterics block hydrogen from recombining and poisons the reaction (www.mgalloycorrosion.wikispaces. com) [4 ] [4 ] [4 ] [5 ]