Brown Patches Near Welds

Q. We are having an issue with our bus seat frames—when they come back from the electrocoater, there are brown patches in and around the welds. Our electrocoater says it is an age-old industry problem caused by silicon in the weld wire. They are suggesting we wire brush the welds to get the silicon off the surface. This would be costly and would drive us to powder coating. Do you have any suggestions? E.W.

A. The short answer to your question is—your electrocoater is right. “Slag,” either from stick welding or wire welding, is non-conductive. Therefore, it will not e-coat because the electrocoat process is a plating process. This is one of the few disadvantages when of e-coat compared with liquid or powder coating.

While all powders and most liquids are electrostatically applied, the applied coating normally “bridges” over the slag. While the weld is totally covered, the adhesion of the liquid or powder to the slag is much poorer than the rest of the weld because the slag will not accept the coating pretreatment and is very hard due to its chemical composition. Therefore, while the liquid or powder may coat the part completely, the coating is just “hiding” the weld slag. Weld are still points of potential loss of adhesion and corrosion. While e-coat will not cover the weld slag at all, powder or liquid only hides potential defect points. Therefore, product quality (both aesthetic and corrosion resistance) is compromised if the slag is not eliminated.

While stick welding normally produces more slag than wire welding, all of it needs to be removed or eliminated to produce high-quality coated parts. There are several options to solve this problem. The remainder of my answer will address wire welding, because the stick welding process quite often includes removing the slag after the weld is complete.

The slag produced during wire welding is at the beginning and/or end of the welds. The number of slag locations depends on weather the welding is robotic or manual and on the experience of the manual welder or robot programmer. I believe the welding equipment and process can be altered to reduce the amount of slag but not eliminate it completely. Removal options vary, each with varying labor, material, equipment costs and time delay in getting the parts to the e-coat process. Basically, the removal options fall into two categories: mechanical or chemical.

Mechanical removal can be done by wire brush (manual or powered), blasting (steel grit or shot, sand or other media) or media vibration (commonly used for deburring). Each of these methods has varied labor and material costs. Manual removal is also very operator-dependent as to whether all slag is removed. While automatic removal is more controllable, surface appearance can be altered with blasting or deburring and would need to be approved by the customer and/or end-user.

Another potential problem with blasting the slag off is trapping of blast media (shot, grit, or sand) in the part and carrying that media into the e-coat pretreatment and/or coating systems.

Robotic wire brushing is the only automatic removal method that will not create a change in surface appearance. Regardless of method, slag removal may be less than 100% because the slag is difficult to see—it looks similar to the weld itself.

Chemical removal will usually result in 100% slag removal provided the chemical concentration, temperature and time are adequate. Chemical removal of weld slag is usually the same process as chemical de-rusting of parts known as pickling.

Pickling usually uses an inorganic acid such as hydrochloric (muriatic), sulfuric or phosphoric at an elevated temperature. Just like rust removal, weld slag removal will depend on the amount of slag to be removed. The pickling process does not chemically dissolve the slag but creeps under it and pops it off the weld.

Once a part is pickled, it needs to be protected from exposure to air; flash-rust can result quickly. Options for protection after pickling include oiling or phosphating within minutes of removal from the pickle bath. If the parts are sent to a sub-contractor for pickling, they usually have light water-soluble oil available but seldom have phosphating as an option. Parts that are “pickled and oiled” (P & O) will coat 100% as if there were no welds since all slag is removed and the parts are clean and rust-free. (Pickling as part of the e-coat process will be addressed in the next question; see below.)

A third alternative to mechanical or chemical removal of the weld slag is not to remove it at all. While this might be a “hard sell” to the customer and/or end user, the slag is not metallic and will not corrode. Salt spray testing can show this.

Unfortunately, the slag is not black (it’s usually brown and glassy). If the part is not seen by the consumer and/or aesthetics are not critical, not removing the “slag” is the most cost-effective way of handling weld slag on e-coated parts.

From a cost only standpoint, my recommendations for dealing with weld slag would be: 1) not remove it, 2) pickle and oil prior to e-coat or 3) automatic mechanical removal.

From a quality (aesthetic and corrosion resistance) standpoint, my recommendations would be: 1) Pickle and oil prior to e-coat, 2) automatic mechanical removal (if parts will not collect media) or 3) manual (wire brush) removal.