Problem: Reworked chromate conversion coating highlighted material grain

A. You are looking at variable grain structure based of asymmetric cooling after "hot" rolling. It probably came from different lots of the same alloy, but you can see differences like this from the first feet of material to the last feet of a run on the same lot. The best answer is to not etch/clean it any more than is absolutely necessary, which will hold down the highlighting of the grain. Unfortunately, that does not hide scratches and makes reruns impossible.

A. Johan,

Alloy 6082 is a wrought alloy, not a die casting alloy, although it does contain some Si (0.7-1.3%). The grain size variation in the aluminium block is due to differences in recrystallization, which is affected by the thermomechanical process history (temperature, strain, strain-rate, etc.). Areas of the billet or slab that are more heavily deformed and/or see a higher temperature will tend to recrystallize, resulting in the "large grain" parts. Less working and temperature results in incomplete or no recrystallization. It is possible that these components are made from 3 different batches of material, or from material that underwent different processing (top of the slab vs. middle of the slab).

A. John, I believe what you are seeing is the grain structure of the aluminum, which is typically more prevalent in the middle of the stock. The part does not look like a casting to me at all. The way to avoid this "mottling", is to avoid caustic etching as a prep prior to chromate, but as you mentioned, these parts had to be reworked, most likely stripped in a caustic bath. They could be stripped again, and hand finished to remove the mottled look, and rechromated with very little, or no caustic etch.