Stress Corrosion Cracking of Aluminum Alloys

Aluminum alloys that contain appreciable amounts of soluble alloying elements, primarily copper, magnesium, silicon, and zinc, are susceptible to stress-corrosion cracking (SCC).

An extensive failure analysis shows how many service failures occurred in the industry and what kind of alloys and stresses led to initiation and propagation of stress corrosion cracks which caused these service failures. Alloys 7079-T6, 7075 -T6 and 2024 - T3 contributed to more than 90% of the service failures of all high-strength aluminum alloys.

Aluminum and its alloys can fail by cracking along grain boundaries when simultaneously exposed to specific environments and stresses of sufficient magnitude. Well-known specific environments include water vapor, aqueous solutions, organic liquids and liquid metals. Stresses sufficient for crack initiation and crack growth can be far below the stresses required for gross yielding, especially in those alloy/environment combinations that are of practical importance, e.g., high strength aluminum alloys in air. This phenomenon of environment-induced intergranular cracking is often called stress-corrosion cracking.

With most service failures specific causes for initiation or propagation of stress corrosion cracks have been observed. The various causes usually belong to one of the following three classes:

1. Metallurgical
2. Environmental
3. Mechanical

This follows quite naturally from the old observation that for stress corrosion cracking to occur, three conditions have to be fulfilled:

1. The alloy must be "susceptible" to SCC,
2. The environment must be "damaging" and
3. The stress (intensity) must be "sufficient".

The electrochemical theory of stress corrosion, developed about 1940, describes certain conditions required for SCC of aluminum alloys. Further research showed inadequacies in this theory, and the complex interactions among factors that lead to SCC of aluminum alloys are not yet fully understood. However, there is a general agreement that for aluminum the electrochemical factor predominates and the electrochemical theory continues to be the basis for developing aluminum alloys and tempers resistant to SCC.

Stress-corrosion cracking in aluminum alloys is characteristically intergranular. According to the electrochemical theory, this requires a condition along grain boundaries that makes them anodic to the rest of the microstructure so that corrosion propagates selectively along them. Intergranular (intercrystalline) corrosion is selective attack of grain boundaries or closely adjacent regions without appreciable attack of the grains themselves.

Intergranular corrosion is a generic term that includes several variations associated with different metallic structures and thermo mechanical treatments. Intergranular corrosion is caused by potential differences between the grain-boundary region and the adjacent grain bodies. The location of the anodic path varies with the different alloy systems.

2xxx Alloys. Thick-section products of 2xxx alloys in the naturally aged T3 and T4 tempers have low ratings of resistance to SCC in the short-transverse direction. Ratings of such products in other directions are higher, as are ratings of thin-section products in all directions.

These differences are related to the effects of quenching rate on the amount of precipitation that occurs during quenching. If 2xxx alloys in T3 and T4 tempers are heated for short periods in the temperature range used for artificial aging, selective precipitation along grain or sub grain boundaries may further impair their resistance. Longer heating, as specified for T6 and T8 tempers, produces more general precipitation and significant improvements in resistance to SCC.

5xxx alloys are not considered heat treatable and do not develop their strength through heat treatment. However, these alloys are processed to H3 tempers, which require a final thermal stabilizing treatment to eliminate age softening, or to H2 tempers, which require a final partial annealing. The H116 or H117 tempers are also used for high-magnesium 5xxx alloys and involve special temperature control during fabrication to achieve a microstructual pattern of pattern of precipitate that increases the resistance of the alloy to intergranular corrosion and SCC.

The alloys of the 5xxx series span a wide range of magnesium contents, and the tempers that are standard for each alloy are primarily established by the magnesium content and the desirability of microstructures highly resistant to SCC and the other forms of corrosion.

Alloys with relatively low magnesium contents, such as 5052 and 5454 (2,5 and 2,75% Mg, respectively), are only mildly supersaturated. Consequently, their resistance to SCC is not affected by exposure to the elevated temperatures. In contrast, alloys with the magnesium contents exceeding about 3%, when in strain hardened tempers, may develop susceptible structures as a result of heating or even after very long times at room temperature. For example the microstructure of alloy 5083-O is not susceptible to SCC.

6xxx Alloys. The service record of 6xxx alloys shows no reported cases of SCC. In laboratory tests, however, at high stresses and in aggressive solutions, cracking has been demonstrated in 6xxx alloys of particularly high alloy content, containing silicon in excess of the Mg2Si ratio and/or high percentages of copper.

7xxx Alloys Containing Copper. The 7xxx series alloy, which has been used most extensively and for the longest period of time is 7075, an aluminum-zinc-magnesium-copper-chromium alloy. When 7075 was used in products of greater size and thickness, however, it became apparent that properties of the products heat treated to T6 tempers were often unsatisfactory. Parts that were extensively machined from the large forgings, extrusions, or plate were frequently subjected to continuous stresses, arising from the interference misfit during assembly or from service loading, that were tensile at exposed surfaces and aligned in unfavorable orientations. Under such conditions, SCC was encountered in service with significant frequency.

The precipitation treatment used to develop these tempers requires two-stage artificial aging, the second stage of which is done at a higher temperature than that used to produce T6 tempers. During the preliminary stage, fine high-density precipitation dispersion is nucleated, producing high strength. The second stage is then used to develop resistance to SCC and exfoliation.

The additional aging treatment required to produce 7075 in T73 tempers, which have high resistance to SCC, reduces strength to levels below those of 7075 in T6 tempers. Alloy 7175, a variant of 7075, was developed for forgings. In the T736 temper, 7175 has strength nearly comparable to that of 7075-T6 and has better resistance to SCC. Other newer alloys - such as 7049 and 7475, which are used in the T73 temper, and 7050, which is used in the T736 temper - couple high strength with very high resistance and improved fracture toughness.

The micro-structural differences among the T6, T73 and T76 tempers of these alloys are differences in size and type of precipitate, which changes from predominantly Guinier-Preston (GP) zones in T6 tempers to h, the metastable transition form of h(MgZn2) in T73 and T76 tempers.

For quality assurance, copper-containing 7xxx alloys in T73 and T76 tempers are required to have specified minimum values of electrical conductivity and, in some cases, tensile yield strengths that fall within specified ranges. The validity of these properties as measures of resistance to SCC is based on many correlation studies involving these measurements, laboratory and field stress-corrosion tests, and service experience.

Copper-free 7xxx Alloys. Wrought alloys of the 7xxx series that do not contain copper are of considerable interest because of their good resistance to general corrosion, moderate-to-high strength, and good fracture toughness and formability. Alloys 7004 and 7005 have been used in extruded form and, to a lesser extent, in sheet form for structural applications. More recently introduced composition, including 7016,7021,7029, and 7146, have been used in automobile bumpers formed from extrusions or sheet.

As a group, copper-free 7xxx alloys are less resistant to SCC than other types of aluminum alloys when tensile stresses are developed in the short-transverse direction at exposed surfaces. Resistance in the other directions may be good, particularly if the product has an unrecrystallized microstructure and has been properly heat-treated. When the cold forming is required, subsequent solution heat treatment or precipitation heat treatment is recommended. Applications of these alloys must be carefully engineered, and consultation among designers, application engineers and product producers, or suppliers is advised in all cases.

Casting alloys. The resistance of the most aluminum of the casting alloys to SCC is sufficiently high and cracking rarely occurs in service. The microstructures of these alloys are usually nearly isotropic, consequently, resistance to SCC is unaffected by the orientation of tensile stresses.

It has been indicated by accelerated and natural-environment testing and verified by service experience that alloys of the aluminum-silicon 4xx.x series, 3xx.x alloys containing only silicon and magnesium as alloying additions, and 5xx.x alloys with magnesium contents of 8% or lower have virtually no susceptibility to SCC. Alloys of the 3xx.x group that contain copper are rated as less resistant, although the numbers of castings of these alloys that have failed by SCC have not been significant.

Significant SCC of aluminum alloy castings in service has occurred only in the highest-strength aluminum-zinc-magnesium 7xx.x alloys and in the aluminum-magnesium alloy 520.0 in the T4 temper. For such alloys, factors that require careful consideration include casting design, assembly and service stresses, and anticipated environmental exposure.