What is Galvanizing steel?
Galvanizing is the the application of a zinc coating to a ferrous metal. It is an effective means for retarding the corrosion of steel. In the galvanizing process, a thin layer of zinc is applied to the steel by electrodeposition, metal spray, sherardizing, or hot-dipping.
The zinc used for galvanizing is nearly 100% pure. The thickness of the coating for galvanized sheet is expressed in ounces/ square foot, one oz/ sq-ft being equivalent to a layer of zinc 0.0017-in. thick. The protective layer on hot-dip galvanized steel & is pure zinc on the outer surface, under which are & iron-zinc alloys with a progressively increasing iron & content. After the pure zinc outer surface has corroded, the underlying iron-zinc alloys continue to protect the steel.
Welding of Galvanized steel
Galvanized sheet with a carbon content of less than 0.2% does not present any special welding problems. However, the lower maintenance costs of galvanized steel structures, such as bridges and storage tanks, have created a demand for galvanized materials in thicker sections and with higher tensile strengths. In general, galvanized plate over l/4 in.
thick has poorer weldability than the equivalent uncoated steel. Welds in galvanized plate have a tendency for porosity; small cracks are likely to develop in fillet welds. The porosity is usually within acceptable limits, but the cracking may pose a difficult problem.
Cracking issue with Galvanized steel
Investigations have established that weld-metal cracks in galvanized steel are intercrystallite with a zinc-rich layer lining the surface of the crack. Batch galvanized plates do not have a uniform coating of zinc, and the cracking may, thus, occur to a varying degree associated with the varying thickness of the coating.
Most, if not all, of the cracking can be eliminated by removing the zinc before welding. The following methods, some of which are slow and costly, are recommended:
- Shear or torch-cut the web members of fillet assemblies after galvanizing.
- Prepare the edges of butt joints after galvanizing.
- Burn off the zinc at the faying edges with a carbon arc or an acetylene torch while using an oxidizing flame.
- Remove the zinc by grinding with a silicon carbide wheel, by grit blasting, or by sandblasting.
Cracking can also be caused by restraint, especially in fillet welds. The tendency for this type of cracking increases as the plate thickness increases. To reduce restraint, fit the joint with a small gap. This practice not only reduces the restraint but also provides a means for the volatilized zinc to escape.
Stick Welding or SMAW Welding of Galvanized Steel
Use E6013 electrodes DCRP, with the same electrode size and current as recommended for uncoated steel . Class E6011 electrodes can be used if only AC power sources are available. Oscillate the arc parallel to the seam with a
whipping motion l/8 to 5/16 inch ahead of the molten crater. This procedure volatilizes the zinc.
The weld will contain porosity and possibly cracks if the zinc is not completely volatilized in the area where the weld metal is to be deposited.
On galvanized plate 1/4 in. or more in thickness, insufficient penetration may be a problem with fillet welds. Always use the largest electrode compatible with the plate thickness and welding position. With a whipping motion, concentrate on volatilizing the zinc ahead of the crater.
For complete penetration, butt, joints on galvanized plate require slightly more gap compared to those used with uncoated steel plate. Considerable spatter occurs when welding galvanized steel, but an anti-spatter compound applied in the area of the weld reduces the difficulty of removal.
MIG-MAG welding of galvanized steel
Gas metal-arc welding can be used on galvanized sheet and plate. Use ER70S-6 MIG-MAG wire for welding the galvanized steel. Short-circuiting transfer is used on sheet as it offer low heat input and avoid burn-though issues. Use 80- 110 ampere for 1/16 inch sheet thickness. Sheet with zinc coatings thicker than those covered in the table require higher currents, lower speeds, or both. A tendency for burn-through may require a slight side-to-side oscillation of the torch.
The same problems of porosity, cracking, and spatter encountered with shielded metal-arc welding are present with gas metal-arc welding, and the same corrective measures are used. With proper adjustment of the power source, the spatter can be reduced but not eliminated.
welding galvanized steel to mild steel
Welding of galvanized steel to mild steel is carried out using same method as explained above. When welding of galvanized steel to mild steel with stick welding, use E6013 type electrode. GMAW and TIG welding of galvanized steel to mild steel is carried out using ER70S-6 or ER70S-2 type filler wire. Preheating is not required as most of galvanized steel is only in sheet metal forms.
welding galvanized steel to stainless steel
Stick Welding or SMAW of galvanized steel to stainless steel is carried out using E309L type electrode. Before welding, clean the area by grinding. TIG-MIG welding of galvanized steel to stainless steel shall be made using ER309L type filler wire. Always clean the galvanized steel using grinder or emery paper in case grinder is not available before the welding operation. If you will weld directly on galvanized steel, the zinc will evaporate and can cause welding porosity. So it is recommended to prepare the surface well before start the weld. Clean the final weld using wire brush. Id required corrosion protection, zinc primer can be applied after welding or local galvanized is done on the welding area.
Health risk with Galvanized Steel Welding
Welding or cutting galvanized steel always produces zinc fumes. These fumes are toxic, and persons in the vicinity must be protected with proper ventilation or by some other means. Proper respiration PPE must be provided to the welder when welding galvanized steel. Zinc fumes can cause metal fever.
Safety when welding Galvanized steel
Always make sure to wear respiration mask when welding galvanized steel to prevent inhaling the zinc fumes which are toxic. When welding galvanized steel make sure to follow following safety rules:
- Always carry out the welding activities in properly ventilated areas.
- Use good quality respiration mask.
- Do not touch hot parts.
- Use respiration mask even while performing grinding activities.
- Use all welding PPE such as gloves, welding helmet, safety shoes, heat pad etc.