RPI Masters Project Final ReportSiobhan Fleming

8/6/12

Evaluation of Coating Methods for Corrosion Protection of Magnesium

Castings

AgendaProject ReviewResearch ReviewMagnesium Magnesium AlloysMagnesium CoatingsRecommendations for alloys and coatings

Project ReviewExtensive review of technical journals

completedMagnesium AlloysCoating for corrosion protection

Review highlighted new technology in coatings that may be very successful in corrosion protection with more research.

Good alloys for certain application were identified throughout the document and segregated in a final table.

Research ReviewTechnical journals reviewed through

electronic villageOffered excellent details for specific usesProvided insight into new technologies

Specific texts reviewed for background on magnesium, magnesium alloys and coatings for corrosion protection

MagnesiumLightest of light metal alloysLow DensityGood Heat DissipationGood DampingGood Electro-magnetic shieldUseful in applications were weight is a

critical design elementSusceptible to corrosion

Magnesium AlloysAlloys designed to improve corrosion

resistanceSpecific elements offer different benefits

when combined into alloy formBenefits are laid out in table format by

element in the paperEach alloy has its own benefits

Selected alloys are documented in paper for benefits

Magnesium CoatingsNumerous different coatings provide

coating protectionExample of coating protection for

aerospace applications is provided in paperCold spray appears to be the future of

coatingsMore research is required on specific alloys in

order to eliminate the concern of galvanic corrosion.

Recommendations for AlloysAlloy Application Notes

AZ91 Cast Helicopter Transmission

Housings

Less costly, good tensile strength,

very susceptible to creep above

120ºC. Good corrosion resistance.

AM60/AM50/AM20 Automotive (wheels, seat frames,

steering wheels)

Greater ductility and fracture

toughness.

AS41 Automotive (crank case,

transmission housing)

Easier than AS21 to cast with

better fluidity

QE22 Aerospace (landing wheels,

gearbox housings, helicopter rotor

fittings)

Superior tensile properties,

Expensive due to silver

Coatings Advantages and DisadvantagesCoating Type Advantages Disadvantages

Chemical Conversion Most effective and mature process Cr6+ in chromate bath is a highly toxic carcinogen

facing restrictions with use.

Phosphate Treatments Chromate Free Coarse grains can cause cracks; heavy metal ions

in solution can cause environmental pollution.

Anodizing Can improve paint adhesion to metals, Coatings are brittle and prone to cracking or

shedding after collision.

Plating (electro and electroless) Can improve corrosion resistance depending on

what material is plated on surface.

Plating films have a weak adhesion to magnesium

alloys. Issues with galvanic corrosion depending

on type of metal used.

Sol-Gel Can achieve better layering than electroless

plating. Some testing has proved very successful

using TiO2

Potential issues with galvanic corrosion depending

on material used.

TAGNITE Chromate free, better abrasion resistance, wet

paint adhesion, impact resistance and fatigue

properties.

Has not been proven as a complete protection.

Useful more in touch up conditions where fine

tolerances are required.

Paint Provides a final protective coating when used in

combination of other coating techniques.

Only provides additional protection when built up

correctly with other coatings. Does not provide

protection when chipped or cracked.

Cold Spray Adheres well with fewer issues than plasma spray.

Testing has proved improved corrosion resistance

when high purity aluminum is used as sprayed

material.

Still a new technology, not widely tested.

Concerns with galvanic corrosion depending on

coating material used.