Cleaning a weld: Before and After

If you've welded very long at all, you've probably heard that a weld should be clean prior to welding, and should look clean (if properly done) after welding. But you have also heard talk about welding rods, or filler metals that help "deoxidize" the metal while welding or about the famous "rust rod" the E 6011. If you have been party to both, then you may also have been confused by the apparent double speak about dirty welding and the critical importance of a clean weld. Are these two issues in conflict with each other? Are there two points of view? Well, not really. Some of the talk refers to "Degrees" of dirty welds and some of it refers to critical standards in code welding. Most hobby users, famers, and general fab operators do not need to insure a 1" ground area around the weld. Welding rods like the E6011 are great for penetrating through the contamination of rust or mill scale and in the process floating contaminants out to the top of the weld, where they do little harm. But even the best welding rod will still have some contaminants left in the weld if the plate is not thoroughly cleaned. On average, in the real world on non-structural welds this level of contamination is acceptable.

Enter code welding. Many of your code welding requires near perfect welds with little tolerance for this type of contamination, and rightly so. Would you sleep well at night knowing the nuclear power plant 30 miles had welds that had particles of paint or mill scale trapped in the weld? No, probably not. In these circumstances the welds are highly scrutinized before, during and after the weld is made. Preparation goes the extra mile, and interpass cleaning of the weld is done with extreme care.

While that may not be the case for your average weld experience, some preparation is always helpful, even though it may not be necessary. Taking care to make sure that rust and paint are removed will make welding a bit easier as well, with reduced sticking, and more stable arc as an immediate result. A lot of people confuse making a surface "shiny" with a thorough cleaning. Mill scale is often confused with new welders with bare metal. The scale can be polished up, and rust knocked off, but the scale will still remain. When steel is truly clean, it will be a bright silvery finish. The quickest way to breach the mill scale and reach the true metal beneath is with a hard grinding disk. Softer flap disks, unless they are brand new, have a harder time getting down to the bare metal. Many times a flap disk will just continue "polishing" the mill scale for some time before reaching bare metal. Stainless and cold rolled steel don't have a mill scale per se, but they do benefit from grinding as well, because of the lubricating, and protecting oils that is used during manufacture. Of course, always remove as much as possible with acetone or special metal cleaner first. Never use chlorinated solvents though such as brake cleaner which can instantly kill you when heated by the welding arc. For TIG welding, you'll almost always want to clean metal thoroughly to prevent the ugly contamination that results. Particularly on aluminum, special prep work should be done, beginning with brushing the surface to be welded with a dedicated stainless brush, and then wiping down with aluminum cleaner or acetone.

After the weld is complete, many people neglect to finish it off by removing the slag, and allowing it to remain. Fluxes left over can be corrosive to a weld. If a part is to be painted, it leaves a potential spot for the paint to flake off later down the line as well. Removing flux properly can identify weak spots in the weld. Many times, a weld will look perfect if the flux is left in place, but be a horrible mess underneath. Removing flux can reveal holes, and incomplete fusion at the sides. Wire brushing usually is all that is needed if the weld is made properly. A light tap of the chipping hammer will also remove the bulk of the flux. However, in code work, usually only a needle scaler is allowed. Repeatedly having to tap with a chipping hammer to remove flux from iron powder based rods can indicate a poorly made weld as well.

If you are doubtful about when to clean, or when to just light it up and weld it, consider this: cleaning a weld properly has never caused a weld defect. This may be simplistic, but when doubt remains, play it safe. While you may think that it is not a structural weld, all welds to some degree are structural and in the right conditions can fail. You may never live to see the day it will, but having peace of mind is worth a few minutes of weld preparation.