Corrosion of Magnesium and Magnesium Alloys

Introduction

The corrosion resistance of magnesium and magnesium parts is influenced by factors similar to other metals, though their electrochemical activity significantly magnifies these effects. While unalloyed magnesium sees limited structural use, the corrosion resistance of magnesium alloys remains a primary concern for designers and engineers.

Major Magnesium Alloy Systems

Aluminum-Containing Alloys

These alloys contain 2-10% aluminum with minor zinc and manganese additions. They offer:

• Wide availability at moderate costs
• Good mechanical properties up to 95-120°C
• Rapid property deterioration above these temperatures

Zirconium-Based Alloys

These alloys combine various elements (rare earth, zinc, thorium, silver) with zirconium, offering:

• Improved mechanical properties through fine grain structure
• Superior elevated temperature performance
• Higher costs due to specialized manufacturing requirements

Chemical Composition and Corrosion Effects

Element Categories and Their Impact

Six elements (aluminum, manganese, sodium, silicon, tin, and lead) plus thorium, zirconium, beryllium, cerium, praseodymium, and yttrium show minimal effect on saltwater corrosion when present up to 5% or their solubility limits.

Four elements (cadmium, zinc, calcium, and silver) demonstrate mild-to-moderate accelerating effects on corrosion rates.

Iron, nickel, copper, and cobalt show extremely deleterious effects due to their:

• Low solid solubility limits
• Activity as cathodic sites
• Water reduction capabilities at magnesium's expense

Tolerance Limits and Interactions

The iron tolerance in magnesium-aluminum alloys depends on manganese content. For AZ91 with 0.15% manganese, iron tolerance is 0.0048% (0.032 x 0.15%). Nickel tolerance varies with cast form and grain size, with low-pressure cast alloys showing 10-ppm tolerance in F temper.

Heat Treatment and Processing Effects

Studies using controlled purity AZ91 alloy in both pressure forms have established:

• F temper (as cast)
• T4 temper (16h at 410°C and quenched)
• T6 temper (16h at 410°C, quenched, aged 4h at 215°C)

High-iron AZ91C showed consistently poor corrosion performance, while high-purity alloys with T5 and T6 tempers achieved corrosion rates under 0.25 mm/yr.

Environmental Factors

Atmospheric Effects

Clean, unprotected surfaces develop protective gray films in salt-free atmospheres. Corrosion resistance varies with humidity:

• 9.5% humidity: No corrosion after 18 months
• 30% humidity: Minor corrosion
• 80% humidity: Considerable corrosion

Water and Solution Effects

• Fresh Water: Forms protective films in stagnant conditions
• Salt Solutions: Severity depends on metal content and chloride presence
• Acids and Alkalis: Rapid attack except by HF and H₂CrO₄
• Organic Compounds: Generally non-corrosive except for anhydrous methanol
• Gases: Minimal corrosion from dry halogens; increased attack with water presence