Exfoliation
What does Exfoliation have to do with aluminum? The answer is - for some alloys and tempers... plenty.
A Different Kind of Attack
We've explored elsewhere the effects of Stress Corrosion Cracking (SCC) — corrosion caused by a sustained tensile stress across the grains in a corrosive environment. This SCC caused unexpected failures of critical aircraft parts and prompted an industry-wide investigation to control it. But SCC isn't the only affliction the engineering community has had to guard against in aluminum structural material.
Exfoliation corrosion is first noticed as a visual disturbance, or subsurface corrosive attack (leafing or flaking) parallel to the exposed surface layers of the aluminum part. This attack is usually along the grain boundaries, predominantly in products with highly worked, elongated grain structures — thinner extrusions, plate, or forged material.
What It Looks Like
Layers of corrosion products forming in the grain boundaries cause the metal to swell and flake off, resulting in an obvious bulging condition on the surface.
The rusty holes you'd find in the lower sections of an old car are basically exfoliation corrosion of the steel sheet. Aluminum doesn't rust — but that doesn't mean it's immune.
Where It Starts
Exfoliation corrosion usually starts and is found on an edge of the end grain, or a machined edge of the part. Bolt or rivet holes are a prevalent source of problems. This type of corrosion has been present since the introduction of high-strength aircraft alloys, and in some high-strength 5XXX boat hull plates. Since the condition is visible, and can be somewhat controlled with surface protective treatments, its importance as a quality problem took a back seat behind stress corrosion cracking.
Most aging airplanes were retired because of excessive exfoliation corrosion in the structure — especially in inaccessible areas. Surface treatments such as anodize and paint helped, but any small break in the barrier, combined with water or another electrolyte, subjected the material to corrosion.
Two Competing Design Goals
Aircraft manufacturers are always caught between two competing design concepts: cost of possession and cost of operation. Neither is free.
Cost of Possession
Bought cheap, and enjoying the economics of low price — perhaps not the highest quality, but it does the immediate job, deferring the problem of longevity to someone else.
Cost of Operation
Designed with features that favor low maintenance costs and/or low fuel consumption over the life of the aircraft.
These two concepts are usually mutually exclusive — there's no free lunch. -T6 material "did the job," but did have its corrosion problems.
Solving It: A Timeline
High-strength hull plate material used on high-performance assault boats in Vietnam began corroding at an alarming rate, prompting an industry investigation.
Corrosion engineers modified the standard salt spray corrosion cabinet to enhance the conditions that promote exfoliation, so it could be created, duplicated, and studied — though results still took about a week. All major aluminum producers adopted these test facilities, and industry cooperation led to several ASTM specifications for exfoliation corrosion testing. The original boat-hull problem was traced and fixed through processing changes.
Lockheed set out to market the L-1011 wide-body as a lower-maintenance aircraft, pioneering the use of exfoliation-resistant 7XXX-series material. They found 7075-T6 material had just two exfoliation outcomes: great, or awful — with no way to predict in advance what an individual lot of -T6 material would do.
Engineers discovered that -T73 material never showed a hint of exfoliation corrosion — but applying that lower-strength treatment wasn't a satisfactory solution, given the weight penalty it imposed on the cost-of-operation goal.
It was discovered that if 7075 material was slightly overaged from the -T6 peak-strength condition, exfoliation corrosion stopped being a problem. There was a small strength penalty, but in the equation of possession versus performance, it was a winner. The temper assigned to this condition was eventually called -T76.
Having solved the boat-hull problem, the task group turned to establishing an industry testing standard for the new -T76 material — eventually replacing the week-long salt spray cabinet method with the faster, more efficient EXCO G-34 exfoliation test. Additional work later correlated electrical conductivity indicator tests to predict exfoliation performance.
When 7050 was introduced, its corrosion resistance was explored and found to behave very similarly to 7075 for both SCC and exfoliation resistance under overaging. The -T76 temper was assigned to extrusions and thin plate that could benefit from the treatment.
What This Means for Forgings
With forgings, the -T76 temper has never been found necessary or been employed. Exfoliation corrosion has not been a problem with forgings, due to the use of the -T73 temper and the fact that the grain structure formed in a forging is not normally susceptible to exfoliation corrosion.