Scavenging scrap piles is Amon's unique free world ability.
There are 20 scrap piles in each free world area, which are:
- The Northwest
- The Desert
- The Plains
- Nadir
The scrap piles are not hidden, so you'll notice them by just wandering around these areas. The only exception would be Nadir, which is darker and requires a little more attention to the surroundings. Although I'd like to add a picture or video guide showing the scraps locations, I wasn't able to find or create one. But as I said, the scrap piles are hard to miss.
Before going after the scraps though, talk to Ifridus near Zaber arena. He will give you side quests for scavenging the scrap piles in each area, starting with Northwest. After cleaning one area, talk to Ifridus again so you get your rewards and the quest for the next area. Bear in mind it is possible to collect the scraps prior to getting the quest from Ifridus and doing so won't void the quest or the achievement.
Upon talking to Ifridus after getting the last 20 scraps from Nadir, achievement should unlock.
If a side quest is active, you can track how many scrap piles you've scavenged by going to the side quests menu, which will show X/20, being X the amount of scrap piles you got in a given area.
Alternatively, you can use the achievement tracker, which works really well in this game.