introduction atmospheric corrosion prevention methods what are mg alloys? mixture of metals to form...

Introduction

Magnesium Alloy CorrosionBy Group 15: Nathan Lam, Graham Tait, Zhanyi Zhou, Muhammad Ibrahim

Magnesium Alloy CorrosionBy Group 15: Nathan Lam, Graham Tait, Zhanyi Zhou, Muhammad Ibrahim

Atmospheric CorrosionPrevention 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 (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

• Conductivity increases when NaCl or (NH4)2SO4 dissolve in the layer

Observations • Deposition of SO2 increased with addition

of NO2 and O3

• Dissolution of CO2 causes formation of carbonates, eventually becomes supersaturated and precipitates.

Stress Corrosion CrackingHydrogen 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

Electronegativity[3]

Mg 0.86 - 1.31Fe 0.69 19.7 1.83As 0.72 16.3 2.18

Element of interest: reduction reaction

Electrochemical potential E0 (V) [1]

Magnesium (Mg): Mg2+ + 2e- Mg(s)

-2.372

Iron (Fe): Fe2+ + 2e- Fe(s)

-0.44

• Electrochemical processo Corrosion at anode

Causes• Electrochemical potential differences• Uneven distribution of atoms• Alloy = solid solutiono Mg solvent & Fe solute

• Solubility is keyo Depends on Hume-Rothery Rules

1. Atom size2. 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 Materials8(7): 511-533

Arsenic protection• Reduction in loss of

metal & evolution of H2 • Sterics block hydrogen

from recombining and poisons the reaction

(www.mgalloycorrosion.wikispaces.com)

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