Influence on Phosphate Pretreatments and Organic Coatings on the Sport Welding Characteristics of Zinc Coated Steels

This paper describes the influence of phosphate pretreatments and organic coatings on the resistance spot weldability of electrozinc coated steels. Measurement of the surface contact resistance of the pretreated layers, the determination of weld current ranges and electrode life tests were carried out. A very high degree of scatter in the contact resistance values was noted with both phosphated and organic coated steels. Resistance spot welding became more difficult as the phosphate and organic coating thickness was increased. When weldable, phosphated and organic coated steels possessed an acceptable welding range, although the electrode tip life was substantially reduced with increasing phosphate and organic coating thickness. In terms of ease of welding, the phosphate pretreatments and organic coatings studied may be rated in the following order; tricationic phosphate 2.5g/m² (1.92μm), zinc-nickel/organic coating (Durazec) - 1μm, electrozinc/organic coating (Bonazinc 3000) - 3μm, and electrozinc/organic coating -7μm.