Steel surface coating is designed to corrosion or decorative, or both. The seam welding, not only to obtain a sufficient joint strength,but also required to maintain the function of the coating. By the strength requirements, seam welding process and uncoated steel is similar, taking into account the requirements of the coating, welding process parameters shall make the necessary adjustments, then pay attention to the impact of contact resistance coating, allowing the electrode indentation, coating and mother wood and metal alloys tendency to form adhesions occur with electrodes.

Galvanized steel pipe is to increase the corrosion resistance of steel, but after galvanized mild steel is more difficult than not galvanized steel plate welding. Mainly due to the low melting point of the zinc coating (about 419C), during welding galvanized layer is first melted and the roller member and the welding electrode and the welding member and the contact surface of the welding current distribution member, the contact area increases, the current density decreases and the contact surface of the electrode and weldments galvanized layer melts, and the electrode face bonding, the copper electrode alloying (CuZn), its electrical conductivity, thermal performance deterioration. The boiling point of zinc 906C, when the temperature exceeds this temperature, the evaporation of zinc. The formation of pores or cracks in the nugget, the heat affected zone is diffused into the connector causes embrittlement cracks may be under pressure. Tests showed that the smaller the penetration rate (10 – 26%), fewer crack defects; welding speed is high, the cooling conditions are poor, hot surface and deep penetration, is easy to crack. Therefore, ensuring the nugget diameter and joint strength conditions, should try to use a small current, low welding speed and a strong external cooling. Electrodes should be used for embossing roller drum drive, in order to keep the tube round trim and clean the surface electrode size.