How Sulphuric acid anodising Is Applied
The sulphuric acid anodising process is used to create a hard porous oxide layer on an aluminium surface. Understanding each stage of the process will enable you to specify your components correctly to avoid any potential problems that may occur when processing.
Cleaning and Degreasing
All parts arrive with machining oils on the surface, handling marks, and natural surface oxide. It is very important to remove all of these contaminants as completely as possible prior to processing. A good start is made by first cleaning the parts in an alkaline degreaser and then thoroughly rinsing to remove all traces of the degreaser.
Etching or Brightening the Surface
In addition to degreasing, most anodizers offer either an etch or a bright in a number of solutions depending on the aluminium alloy being anodized and the specific finish required. An etch in a solution of sodium hydroxide (NaOH) for example will remove a thin layer of the surface to produce a dull matte finish. Alternatively an acid bright in a solution such as phosphoric or chromic/sulphuric can be used to produce a very smooth surface with a highly reflective finish.
The Anodising Bath
Sulphuric acid anodising is generally carried out in a bath of sulphuric acid solution which is maintained at a temperature of around 18°C to 22°C. A direct current supply is used and the part is connected as the anode. The anodic oxide grows into the surface of the part and the thickness is generally related to the current density and the dwell time. The average thickness of a decorative anodized coating is around 5 to 25 microns.
Sealing the Oxide Layer
Sealing the Anodic Film. The anodic film is porous and therefore must be sealed to ensure that it will provide corrosion resistance. The part is put into a hot deionised water tank at about 96°C to 100°C or in a nickel acetate solution. The pores of the film are forced open by the solution and then instantly close up again. This has the effect of increasing the corrosion resistance of the coated part significantly.
Racking and Masking
Parts are held on the processing racks with contact points left bare. If any area of a part is to remain uncoated (e.g. a threaded hole, a mating surface) this must be masked prior to processing. This should be discussed at the quoting stage.
Post-Process Checks
The thickness of the coating is checked using an eddy-current gauge. All other visual defects such as pitting, streaks, etc. are recorded as well. For close-tolerance parts, such as those required for the aerospace industry, the specifications for the coating will usually be given as a range between a minimum and maximum thickness.
Getting the process sequence right in the first place will achieve consistent results and avoid rework.