1. 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 
X - Shannon, R.D., Prewitt, C.T., Effective Ionic Radii in Oxides and Fluorides. (1969). Acta Crystallographica, B25:
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:
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