Abstract: Electrodes used for spot welding of aluminium workpieces are improved by a combination of steps:(a) A roughened surface is formed on the electrode tip, preferably to an average roughness depth of 12 to 30 microns.(b) also an artificial protective coating, e.g. a chromate-based coating or an anodic oxide film, is applied to the workpieces.(c) Optionally the nugget diameter in the initial welding set-up is arranged to be not more than 15% greater than the minimum acceptable.Features (a) and (b) act synergistically to increase the service life of the electrodes, typically to more than 2000 welds.

Abstract: The invention concerns a method of forming a joint between aluminium workpieces. These carry an artifically applied e.g. chromate or anodic oxide coating on their surface. A curable adhesive containing up to 40% by weight of particulate filler having a maximum particle size of 300 microns is provided between the workpieces to be joined. Resistance welding is performed using a welding electrode whose tip has a surface with an average roughness depth of at least 10 microns. This increases electrode life and makes it possible to use lower welding currents than previously recommended.

Abstract: An anodic oxide film is formed on aluminium strip by continuously passing the strip through a phosphoric acid electrolyte at from 25.degree. C. to 80.degree. C., the contact time between the strip and the electrolyte being not more than 15 seconds during which time the strip is anodized at a current density of at least 250 A/m.sup.2. In the resulting film, the pores are enlarged so that the effected surface area is increased. The film forms an excellent substrate for lacquer, paint, or adhesive.