Preweld cleaning requires two operations: oil and grease removal and oxide removal. Before you do anything else, remove any oil, grease, or water vapor from the surface. Some people try to do this with a wire brush, but this practice is wrong because a wire brush won’t remove these materials—it only spreads them around and creates small scratches and other nooks and crannies on the surface that hold oil and grease, making them impossible to remove later. Never wire-brush first. Always remove oil, grease, and other contaminants first. So how do you remove such contaminants? Two classes of liquids make effective degreasers: organic solvents such as acetone and mild alkaline solutions such as strong soaps. Alcohol isn’t a good degreaser and shouldn’t be used to clean aluminum. In the past vapor degreasers often were used to clean aluminum, but they emit volatile compounds into the air. Most of them have been prohibited over the years for environmental reasons. For the same reason, many shops have stopped using many organic solvents. Because of that, the question sometimes changes from “What solvent should I use?” to “What solvent can I use?” The choice of degreasing solvent often can vary from facility to facility. Acetone, methyl ethyl ketone, lacquer thinner, and toluene all make good solvents. Once you choose a solvent, wipe the parts to be welded with a clean, lint-free rag dipped in the solvent. The solvent will evaporate rapidly from the part. Make sure the solvent has evaporated completely before fitting the parts together, and never pour solvent into a weld joint that already is fit together. Allowing a welding arc to impinge on any of these solvents can release poisonous gases. Mild alkaline solutions, sold under different brand names, can be effective degreasers when used as an alternative to organic solvents. They are applied by immersing the part, pressure-washing it, or by wetting a rag and wiping the weld area, depending on the application. In all cases, wash off the alkaline with water and dry the part before you weld on it. These chemical solutions often are used hot (at 160 degrees F) for maximum cleaning action. Other effective degreasers are citrus-based cleaners and degreasers. if you use these degreasers, make sure to rinse the part off and dry it before welding. After you have degreased the pieces, it’s time to remove surface oxides. The most common removal technique employs a stainless steel wire brush. The brush either can be powered or manual, but it must be clean and used only for aluminum. Choose a brush with fairly soft, fine bristles. Avoid power brushes with heavy, twisted bristles; such a brush often isn’t as effective at removing oxides as a softer brush. The heavy brush will tend to burnish the oxide rather than remove it. Chemical removal also can eradicate oxide. To remove oxides, you must physically remove material from the aluminum surface. Strong alkaline solutions can be good for this. Parts usually are immersed in such solutions and then rinsed in hot water and dried. Many welding suppliers also sell oxide-removal solutions in spray or squeeze bottles for local application. Most of these are phosphoric acid-based. Many of them also contain hydrofluoric acid. They usually are effective at removing oxides, but exercise extreme care when handling these solutions. Again, rinse the parts in hot water and dry them before welding on them. So now that you have clean parts, you want to know how quickly you should weld them. This depends on the application. Many people say one or two hours because the oxide that you carefully removed with the wire brush will re-form quickly. The facts are different. Immediately after wire brushing—within seconds—a thin oxide coating 20 to 50 angstroms thick forms on the aluminum. If the aluminum is kept dry and at room temperature, although the oxide thickness increases, it does so slowly. In any application other than a critical aerospace application, you have weeks or even months before the oxide thickness increases to the point where it would affect welding. Long before that, enough dirt and grit from the air would settle on the joint so that welding would be affected. However, if the aluminum is allowed to get wet—from condensation, for example—be sure to clean it again before welding. To summarize, take the following steps before welding aluminum: Remove oil, grease, and water vapor using a solvent or mild alkaline. Remove surface oxides with a wire brush or strong alkaline or acid. Assemble the joint. If it won’t be welded immediately, cover the joint with brown kraft paper to prevent dirt and grit in the air from getting into the joint. Keep the joint dry. Weld within a few days. Clean the joint again if it isn’t welded within that time.