Why Do Aluminum Welded Joints Corrode?

Corrosion of Welded Joints

Corrosion is damage or destruction caused by chemical or electrochemical interaction between a metal and its environment. It is classified as chemical or electrochemical corrosion, and most commonly observed corrosion is electrochemical.

Welded joints can undergo general corrosion, localized corrosion, or stress-assisted corrosion. General corrosion affects all or most of the surface; when its rate is approximately equal everywhere, it is called uniform corrosion. Localized corrosion develops preferentially at specific locations and much faster than elsewhere, as in intergranular corrosion or pitting. Stress intensifies attack and can produce stress corrosion and stress-corrosion cracking.

General corrosion is predictable and comparatively less dangerous. Localized attack is difficult to predict and may cause sudden failure without a clear warning.

Corrosion Characteristics of Aluminum Welds

Aluminum generally resists corrosion well, but behavior depends on alloy type, composition, and environment. Non-heat-treatable aluminum, Al-Mn, and Al-Mg alloys resist corrosion better than heat-treatable Al-Cu-Mg and Al-Zn-Mg alloys. Al-Cu-Mg alloys and their welds tend toward intergranular corrosion, while Al-Zn-Mg alloys and their welds tend toward stress corrosion and stress-corrosion cracking. Aluminum alloys generally resist atmospheric corrosion but perform less well in seawater and marine climates.

Intergranular corrosion progresses along grain boundaries or adjacent regions and often precedes stress corrosion.

The surface may show only slight attack while an internal network of intergranular cracks substantially reduces strength. A principal cause is grain-boundary precipitation of a second phase, which depletes a more noble constituent nearby. In Al-Cu-Mg duralumin, for example, CuAl₂ precipitation creates a copper-depleted zone. In a shared medium, a metal at more negative potential becomes the galvanic anode and corrodes more readily, whereas a more positive metal tends to act as the cathode. Microstructural change thus drives electrochemical intergranular attack. Stress corrosion is the most important corrosion consideration in fracture control of welded aluminum structures.

Stress Corrosion of Aluminum Welded Joints

Stress-Corrosion Cracking

Stress-corrosion cracking (SCC) is fracture of a joint under static stress in a corrosive medium. It results from the combined action of stress, usually tensile, and a specific environment. A joint or structure can crack spontaneously at stresses far below yield strength and in even a mildly corrosive environment. It is therefore a particularly dangerous form of low-stress brittle fracture.

The driving stress is the sum of fabrication and welding residual stress and service stress. SCC does not occur when the applied stress, stress-intensity factor K, or J-integral is below the corresponding threshold—critical stress σ_SCC, threshold stress-intensity factor K_ISCC, or threshold integral J_ISCC. An existing stress-corrosion crack will not propagate below the threshold. When the crack-tip stress-intensity factor exceeds K_ISCC, the crack initiates or grows; fracture occurs when growth reaches the material’s critical condition.

The fracture process has three stages: crack initiation, stable subcritical growth, and unstable growth. Pitting and intergranular attack often become crack origins and shorten initiation time. An initiated crack branches like a tree root; one branch may become the rapidly growing main crack while others stop or grow slowly. Once established, the crack grows at a nearly steady rate until mechanical instability.

The crack-growth rate da/dt versus crack-tip stress-intensity factor K commonly has three regions. In Region I, da/dt falls sharply as K decreases, and no growth occurs below K_ISCC. Region II is stable growth at da/dt_II. When K reaches fracture toughness K_IC, Region III begins and unstable growth continues to fracture.

From an engineering perspective, K_ISCC and da/dt_II are useful resistance indices for materials and welded joints.

Macroscopically, the fracture is usually perpendicular to the principal stress and the crack branches. Corrosion-product accumulation removes metallic luster from the origin and subcritical-growth region. The origin may contain pitting, intergranular corrosion, or a welding defect. The final unstable region may show radial or chevron patterns. Microscopically, cracks may be intergranular, transgranular, or mixed according to environment and stress. Fracture surfaces often retain plastic-deformation traces with rock-candy, shell-like, or feather-like patterns.

Factors Affecting SCC

1. Material Type and Composition

Heat-treatable high-strength alloys, especially ultra-high-strength Al-Zn-Mg and Al-Zn-Mg-Cu alloys, are highly susceptible to SCC in welds. Even corrosion-resistant non-heat-treatable Al-Mg alloys become susceptible at high magnesium content.

2. Matching Filler and Base Metal

Filler selection often emphasizes weldability, composition, strength, ductility, and toughness. This can create large compositional and microstructural differences between weld and base metal and corresponding electrochemical differences that may increase SCC susceptibility.

3. Microstructural Changes

Welding can create grain coarsening, grain-boundary melting, and grain-boundary embrittlement in the weld, fusion zone, and heat-affected zone. Such nonuniform or nonequilibrium structures may increase SCC susceptibility. A thermodynamically equilibrated lattice structure generally offers the highest resistance, whereas a structure far from equilibrium is more susceptible.

4. Welding Residual Stress

Residual stress has a major effect. Even without external load, residual tensile stress may be sufficient to cause SCC.

5. Welding Defects

Planar defects, particularly solidification or liquation cracks, create severe stress concentration. Their interaction with residual tensile stress readily promotes SCC.

Because composition, microstructure, and mechanical properties vary across a weld, its SCC resistance is often below that of the base metal even with correct material selection and process control. In a liquid medium containing 3% NaCl, for example, σ_SCC/R_eL is 0.6 for the aluminum base metal and 0.5 for the welded joint.

Preventing SCC in Welded Joints

1. Select structural materials comprehensively. Consider service characteristics, physicochemical and processing properties, weldability, and corrosion resistance. Test base-metal K_IC and K_ISCC when necessary.
2. Select welding consumables comprehensively. In addition to defect prevention and mechanical requirements, assess environmental compatibility, corrosion resistance, and SCC susceptibility of the weld and joint. Test K_IC and K_ISCC in each joint zone when necessary.
3. Reduce residual stress. Any process measure that reduces or eliminates welding residual stress helps prevent SCC.
4. Apply post-weld treatment. Welds in alloys such as Al-Zn-Mg and Al-Zn-Mg-Cu may require heat treatment to improve microstructure and reduce susceptibility. Some environments also require surface protection of the structure and joint.