John:
A writer stated that, in his plane, the corrosion was from INSIDE the spar; something to do with copper content in the aluminum? And so was not visible until it became advanced? What is this about?
Editor:
What he has experienced is intergranular corrosion that led to exfoliation corrosion. It is a risk with all heavily alloyed metals, especially high-strength aluminum. The root cause is due to metal grains that have not properly blended with other metals in the alloy. While it can be caused by a bad alloy operation, in which the various alloying elements are not completely blended, by far the most common cause is faulty heat treatment of a forged part. The part temperature is raised to the target point during heat treatment, but the part is then allowed to cool down too fast. That causes critical alloying elements to crystallize out as unique grains inside the part, rather than remaining blended with all the other metals during a very slow cool-down. As a result, there is a galvanic junction deep inside the part (dissimilar metals in immediate contact), and ion transfer begins; intergranular corrosion is now underway.
The first visible indication is actual swelling of the part. Once it progresses to the point that surface fissures appear, they will often look like the layers or flakes of crust in a croissant or other layered pastry. What began as intergranular corrosion now becomes classified as exfoliation corrosion. And once the surface is fractured, the corrosion process really accelerates due to the available oxygen. Regardless of how or when it appears, both intergranular and exfoliation corrosion are generally regarded as fatal to the part in question. I know of no approved treatment once these forms of corrosion have been identified; part replacement is the only solution.
There have occasionally been ADs issued for certain serial number ranges of certain parts, usually forged parts, because a high rate of intergranular/exfoliation corrosion identified a problem with heat treatment in a certain batch of parts. There have even been part recalls because a heat treatment failure was successfully identified before it ever led to corrosion. If the automated process monitoring is that good, and the parts are quickly recovered, they can often be re-treated successfully, to prevent future problems.
Here is a clip from one article on the subject of corrosion. There is a lot on it in AC43, an FAA manual that should be owned and read by anyone who performs some of their own aircraft maintenance.
Intergranular Corrosion
This type of corrosion is most insidious and can represent a real hazard to aircraft aluminums. Intergranular attack originates along the grain boundaries of the material. This is chemically different from the metal within the grain center. Many alloying constituents migrate toward the grain boundaries during the metals solidification process. The grain boundary and grain center can react with one another as cathode and anode when in the presence of an electrolyte. As the grain boundaries break down, delamination and exfoliation can occur (Figure 2). Left unchecked, this could lead to catastrophic structural failure. High strength aluminum alloys, such as 2014 and 7075, have a higher susceptibility to this form of corrosion if improperly heat treated.
Many stainless steels are prone to this kind of attack where chromium carbides precipitate to the grain boundaries. This lessens the chromium content adjacent to the grain boundaries, creating galvanic potential. Rapidly cooled austenitic stainless steels are especially prone to this kind of corrosion.
Exfoliation Corrosion
Exfoliation corrosion is an advanced stage of intergranular attack. The surface grains of the material are lifted up by the coercive force of expanding oxidation products at grain boundaries located just beneath the surface. This blistering effect is quite noticeable in aircraft aluminums and is most prevalent in wrought products such as plate, thick sheet and extrusions, where the grain structure of metal tends to be elongated (Figure 3).