The condition and cleanliness of the metal surfaces can affect weldability, particularly if the contaminant affects the electrical resistance at the electrode contact or sheet interface. Depending on the material type and the quality standard required (eg aerospace quality), some special cleaning may be recommended.
Materials should be free from rust, scale, paint, dirt and other insulating materials. These can cause variability of weld quality, increase the wear of electrodes, and may also cause weld splash and surface splash.
Light coating with press lubricant or corrosion protecting oil does not normally affect weld quality.
Loose contamination may be wiped from the sheet surface, whereas paint may need to be removed by solvent or brushing. Scale and rust on steel can be removed by pickling, abrasive cleaning or shot peening. Methods likely to embed grit in the material surface are undesirable. The surface condition of a pickled or shot peened material may have a lower surface resistance than the as-rolled material and need a higher current setting. Confirmation test samples should be made.
Surfaces are normally degreased for aerospace quality welds and a mechanical or chemical treatment may be used to achieve improved consistency. This is particularly true for aluminium alloys where a consistent surface resistance is essential.
Where specific surface treatments are provided by the material supplier (for use with resistance welding and adhesive bonding, for example), these should not be removed, otherwise joint performance could be seriously affected.
Resistance welding of sheet metals - a guide to best practice