Aluminum Workshop: Macroetching and cleaning aluminum

Someone sent me an email recently asking how to macroetch samples of aluminum welds. I gave the standard—and correct—answer to use 10 percent hot (160 degrees F) ammonium hydroxide solution. However, the emailer replied that his shop rules prohibited him from using ammonium hydroxide and asked me if there were any alternatives. That made me think about common substances that can be used to macroetch samples and clean aluminum before welding takes place.

Macroetching

Let’s discuss macroetching first. You do not need to handle pure ammonium hydroxide, which is very corrosive. You should be able to buy a 10 percent solution, which is not dangerous to handle. If this still is not acceptable, you can choose among several alternatives.

Every industrial chemical company offers strong aluminum degreasers. If these are mixed according to the manufacturer’s recommendation and used hot (again, 150 to 160 degrees F), they will do a fine job macroetching aluminum samples. But there is an even simpler solution. Any of the spray oven cleaners that you can buy at the supermarket are also alkaline materials and will do a fine job macroetching aluminum weld samples, even if used right out of the can at room temperature. This is the easiest solution and probably is acceptable to use in almost any shop. You would be surprised at how effective this is.

Cleaning Aluminum Preweld

Let’s move on to preweld cleaning. Again, the standard and best recommendation is to wipe the area you are going to weld with a strong degreaser such as acetone or toluene before you weld. But use caution. If you do not allow these materials enough time to evaporate, they can evolve into poisonous phosgene gas when heated by an arc. Again, let these materials evaporate before you weld.

If you are not allowed to use these chemicals in your shop, you can choose between a couple of alternatives. Carburetor cleaner and liquid-penetrant cleaner make very effective preweld degreasers and should be allowed by your shop rules.

As a last resort, I have seen common cleaner/degreasers, such as Simple Green, used as effective preweld cleaners. I am a bit reluctant to recommend them because they do not evaporate and can leave a residue behind. If you do choose to use them, be sure to wipe them off completely before you weld.