PALLET SHEET MATERIALS - 7075T6 vs 7021-T6
European suppliers regularly supply air cargo pallets with alloy 7021-T6 with the representation that 7021-T6 is "equal" to 7075-T6. Satco uses the stronger 7075-T6 aluminum alloy on its cargo pallets and container bases. This report shows salient properties of each.
Tensile strength Ftu 7075-T6 = 78,000 psi
Ftu 7021-T6 = 56,565 psi (72.5% of 7075-T6)
In terms of sheet thickness .160" gage 7075-T6 aluminum alloy has the same strength as .220" gage in 7021-T6 alloy.
The principal advantage of 7021-T6 aluminum alloy is that it can be produced at low cost, as much as @75% of cost of 7075-T6. 7021-T6 is produced primarily in Europe as one of their principal low cost heat-treated alloys, like 6061-T6 in the United States.
Although 7021-T6 alloy may be adequate for safety of flight considerations it is not equal from a service point of view. 7021-T6 pallets "bow" more readily than 7075-T6 pallets, and are subject to "exfoliate corrosion", a corrosion of the aluminum extending from cold worked bottom surfaces into the sheet along grain boundaries at an angle to the surface.
On the bowing, the stronger and harder 7075-T6 alloy resists "cold work" of the pallet bottom surface more than the weaker and softer 7021-T6 alloy, which is more subject to the deformation which causes bowing, while the harder surfaces resist such deformation. What appears as "cold work" is actually a multitude of very small groves forced into the bottom aluminum surface by repeated conveying while loaded. The greater the load and the softer the material the deeper are the groves, and the deeper the groves the greater is the bowing. The causes and effects are in direct proportion one to the other.
"Exfoliate corrosion" is instituted from the same "cold work" to the bottom surface. Some alloys have this defect, 7021-T6 does, 7075-T6 does not. It shows itself as an "orange peeling" of areas of the bottom surface, with failure occurring along grain boundaries. As pieces break away they tend to get caught in, and cause damage to, the rollers and balls in the aircraft floor systems. Sometimes larger pieces, as large as a dinner plate or so, partially break away and get caught in the rollers and are hung up there.