Stress Corrosion
Corrosion is one of the problems with any metal you have to deal with, except for gold.
A Different Kind of Corrosion
Corrosion comes in many forms. We're familiar with the corrosion that appears on our cars and on the aluminum screens of our homes. These effects are visible — rust and white powder-pitting — and there are ways to prevent this type of corrosion: galvanizing for steel, or anodizing and painting for aluminum.
With stress corrosion, it's different. You can't see it beginning — only the effect after it has already done its internal damage. Stress Corrosion Cracking (SCC) is a special category of corrosion, and its greatest impact has been within the airframe industry.
A Costly Discovery
In the 1950s and '60s, aircraft maintenance crews began to notice cracking in large aluminum forgings made from 7075 alloy after they had been in service for a while. Examinations showed no obvious external corrosion, but a closer look sometimes revealed many cracks starting from the surface — and when they got too big, the part would fail completely.
There were embarrassing failures of airplanes sitting on the ramp when the landing gear suddenly collapsed without warning. A lot of work went into finding the cause and devising a fix.
What was discovered was that the "new" alloys — like 7075 and the German import that was morphed into 7079 — seemed to exhibit this behavior in the -T6 (full strength) heat-treat condition. The cracks also always seemed to follow a path in the severe short-transverse grain direction, meaning for die forgings, the parting line was most susceptible.
Two other factors stood out: the length of time a part was exposed to corrosive elements helped predict failure, and the level of sustained (constant) tensile stress at the surface of the part affected the threshold time before cracking began. The normal protective treatments for aluminum — anodizing and painting — did not prevent this type of corrosion from occurring.
Three Conditions, All at Once
After much research, it was determined that stress corrosion cracking was causing the failures — and that three things must exist at the same time for SCC to occur. Eliminate any one of the three, and SCC will not occur.
A Susceptible Material
The 7000-alloy series material is quite susceptible in the -T6 condition.
A Sustained Tensile Stress
A residual tensile stress, often resulting from fabrication or service loads.
A Corrosive Environment
Any outdoor exposure is generally enough to provide this third factor.
This may occur when the short-transverse plane is exposed to the surface and there is a residual tensile stress resulting from fabrication or service loads. Since residual stresses are part of the manufacturing process and operating environment, it isn't practical to try to eliminate them entirely — hot or warm water quenches reduce residual stresses, but come with a strength penalty. Alloys such as 7079 were the most susceptible to SCC and have, for the most part, been eliminated from aircraft use — especially for new designs. Some applications, such as pressure vessels in hydraulic systems and landing gear, will always have load-induced residual stresses that can't be fully designed out. As for the corrosive environment, any exposure to the outdoors is generally enough to provide that third factor.
The Fix: Overaging
The solution was to develop a material less susceptible to SCC. Investigations showed that heating -T6 7075 aluminum in an oven for several hours, at temperatures around 350°F, provided a measure of resistance to SCC.
This resistance didn't come without a penalty. The treatment is known as aging beyond the peak (-T6) strength, or "overaging." Designers had to account for the reduced strength of the part, accepting a weight penalty over the peak-strength tempers. This condition is called -T7, and the first material application was assigned the -T73 temper. The overage thermal treatment spread rapidly for 7075 applications; some alloys, like 7079, would not respond to the technique and have since been phased out of production and design.
A Faster Way to Check
For quality control purposes, a need existed to find a quick, inexpensive method to determine whether the overaging treatment was effective in reducing SCC susceptibility. At the time, production couldn't wait 30 days for laboratory testing to confirm SCC behavior — but that was the only way to be sure.
It turns out a physical characteristic of aluminum — electrical conductivity (EC) — is a good indicator of relative resistance to SCC. Thousands of laboratory SCC tests, run alongside EC measurements, showed that if the EC was at or above a defined threshold, almost no test bars would fail after 30 days of exposure to a salt-water solution while stressed above a defined tensile threshold in the short-transverse direction. This easy-to-obtain EC determination became a routine test, performed and reported on all -T7-type treatments — which is why a conductivity value is almost always included in test reports for alloys processed for SCC resistance.
Where Things Stand Today
All newer alloys, such as 7050, were developed with SCC resistance in mind and benefit from the overaging treatment. Today, airframe designers working with these newer materials generally don't have to worry about SCC, because years of history have shown there are no sustained residual stresses above the 40,000 PSI threshold in current designs. Stress Corrosion Cracking is not considered a problem today — though within the airframe industry's available alloy and temper choices, work continues to find better solutions that reduce the weight penalty imposed by the cure.