Corrosion Randy Beals Of All The Issues That Influence Mg Use - - PDF document

Corrosion

Randy Beals

Of All The Issues That Influence Mg Use Corrosion, And Bimetallic Corrosion Associated With Fastening, Are The Least Friendly

General Corrosion is Not the Problem

• ASTM B117 salt spray tests
• Magnesium alloys: AZ91D, AM60B,

AM50A, AE42 and AS21 showed lower corrosion rates than aluminum alloy A380

• Magnesium alloy AM20 showed higher

corrosion rates than A380.

Galvanic Corrosion is the Issue Galvanic Corrosion is the Issue

Re = Electrolyte Resistivity Rm = Metal Circuit Resistivity I = 0, when Re & Rm I = Epk- Epa Re + Rm

α

Ref: Hydro Magnesium

Galvanic Corrosion Crevice Corrosion

O2 O2

Dirt + H2O

+

_ _

+ Magnesium

Nonmetallic Cupped Washer Bolt Mg Potential Crevice Site

O2

Epk Epa

H2 H + H2 (g) 2H + + 2e H + OH - Electrolyte H + OH - OH - OH - Me Cathode Mg Anode

I RM RE e -

+

• W

W

Cathode reaction Anodic reaction Mg Mg 2+ + 2e-

V

There are few low-cost green fasteners...chromate coated steel bolts are no longer allowable. “Friendly” designs to isolate moisture and Fe ions from interfaces are expensive and clumsy.... but possible Fastening Mg parts to a vehicle’s steel structure requires high loads (>90 Nm) in the US and Al fasteners are not strong

• enough. In EU lower loads

(~ 40 N) are allowed and thus Al bolts can be commonly used

Good designs can eliminate corrosion

Eliminate electrolyte Break electric circuit

• Plastic washers
• Non-conductive coatings

Reduce electropotential

• 5000/6000 series Al washers

Mg Mg Steel Bolt Steel Bolt Al Al Al Al Steel Steel

Coatings

Delamination/deadhesion Delamination/deadhesion

Better mechanical property databases are required for FEA deformation/crash modeling. CAE filling models for HPDC do not describe how to modify gate designs and filling profiles (shot speed & pressure) and how to control porosity throughout all cross-sections in all locations.... unlike LP/G DC

Test bar databases are widely available, but are not statistically related to HPDC component processing conditions, nor actual component mechanical properties at different locations Component mechanical property databases (such as for FEA) are closely held within individual companies and not widely available.

Magnesium HPDC structures are Magnesium HPDC structures are

• inhomogeneous. Are their
• inhomogeneous. Are their

properties inhomogeneous too? properties inhomogeneous too?

Gate End Overflow End