Stress Corrosion Cracking of Aluminum Alloys

---

Understanding Stress Corrosion Cracking in Aluminum

Aluminum alloys containing significant amounts of soluble alloying elements—primarily copper, magnesium, silicon, and zinc—are susceptible to stress-corrosion cracking (SCC). Industry failure analyses reveal that alloys 7079-T6, 7075-T6, and 2024-T3 account for more than 90% of service failures among high-strength aluminum alloys.

When exposed to specific environments and sufficient stress levels, aluminum and its alloys can fail through cracking along grain boundaries. Common environmental factors include water vapor, aqueous solutions, organic liquids, and liquid metals. Notably, the stress levels required for crack initiation and growth can be significantly lower than those needed for gross yielding, particularly in practically important alloy/environment combinations.

Fundamental Mechanisms and Contributing Factors

Three essential conditions must exist for SCC to occur:

1. Alloy susceptibility to SCC
2. Presence of a damaging environment
3. Sufficient stress intensity

These conditions align with three primary contributing factor categories:

• Metallurgical factors
• Environmental influences
• Mechanical stress conditions

Electrochemical Mechanisms and Microstructural Effects

The electrochemical theory of stress corrosion, developed around 1940, remains fundamental to understanding SCC in aluminum alloys. While research has revealed some limitations in this theory, the electrochemical aspect continues to dominate aluminum SCC behavior and guides the development of resistant alloys and tempers.

SCC in aluminum alloys typically manifests as intergranular cracking, where grain boundaries become anodic relative to the surrounding microstructure. This selective attack occurs along grain boundaries or adjacent regions while leaving the grains themselves relatively unaffected.

Behavior of Major Aluminum Alloy Series

2xxx Series Alloys

Thick-section 2xxx alloys in T3 and T4 tempers show low SCC resistance in the short-transverse direction, though resistance improves in other orientations and in thin sections. Quenching rate significantly affects precipitation patterns and subsequent SCC susceptibility. T6 and T8 tempers, achieved through longer heating periods, demonstrate improved SCC resistance through more uniform precipitation.

5xxx Series Alloys

While not heat-treatable, 5xxx alloys require careful processing to achieve optimal corrosion resistance. Magnesium content significantly influences SCC susceptibility:

• Low-magnesium alloys (≤3% Mg): Generally resistant to SCC
• High-magnesium alloys (>3% Mg): May develop susceptible structures over time
• Special tempers (H116/H117): Engineered for enhanced corrosion resistance

6xxx Series Alloys

These alloys demonstrate exceptional SCC resistance, with no reported service failures. Laboratory testing shows potential susceptibility only under extreme conditions in high-alloy-content variants.

7xxx Series Alloys

Copper-Containing 7xxx Alloys: The widely-used 7075 alloy highlighted limitations in T6 temper applications, leading to the development of improved T73 and T76 tempers. Newer alloys like 7049, 7475, and 7050 combine high strength with superior SCC resistance through optimized precipitation treatments.

Copper-Free 7xxx Alloys

These alloys offer good general corrosion resistance and formability but require careful engineering for SCC prevention, particularly regarding short-transverse direction stresses.

Casting Alloys

Most aluminum casting alloys exhibit high SCC resistance due to their nearly isotropic microstructures. Specific characteristics include:

• 4xx.x and 3xx.x series (Si/Mg only): Virtually immune to SCC
• 3xx.x series (with Cu): Moderate resistance
• 7xx.x series and 520.0-T4: Require careful consideration of design and service conditions

Prevention and Quality Assurance

Quality control measures for 7xxx series alloys in T73 and T76 tempers include:

• Monitoring electrical conductivity
• Verifying tensile yield strength within specified ranges
• Correlation with laboratory and field stress-corrosion tests
• Validation through service experience

Conclusion

Understanding and controlling SCC in aluminum alloys requires careful consideration of metallurgical, environmental, and mechanical factors. Proper material selection, processing, and application design can significantly minimize SCC risks in service conditions.