1. Introduction 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 atom “Crystal” radius (Å)[2] % size difference from Mg Electronegati vity[3] Mg0.86-1.31 Fe0.6919.71.83 As0.7216.32.18 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 o Corrosion at anode Causes 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.CRC Handbook of Chemistry and PhysicsCRC PressISBN0-8493-0487-3 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 [4] [5]