Anodising guide - 3. Industrial practice

In the following we will describe (in broad terms) the industrial practice of anodising to give an insight into what is possible. Which of these stages you can do by yourself we will explain below.

3.1  Surface preparation

First the surface of the aluminum is thoroughly cleaned and possibly prepared for the later appearance. Depending on the desired surface different mechanical and chemical processes are used, for example:

• Grinding uniformly rough surface
• Brushing rough surface with a preferential direction
• Polishing very smooth surface
• Sandblasting surface rather rough
• Bating very slight roughening
• Matting slight roughening
• Pressing slight roughening

By special brushing and dyeing it is possible to create a surface that looks just like stainless steel - with the great advantage that fingerprints do not left traces on anodized layers. This makes them suitable especially for easy to clean objects.

3.2  Cleaning

An essential prerequisite for successful anodizing is a absolutely dust- and greaseless piece. Therefore the parts have to be thoroughly chemically cleaned before anodizing. This mostly happens with special cleaners developed for aluminium.

3.3  Flushing

To remove all remnants of the cleaning, the parts are rinsed thoroughly with water.

3.4  Anodising

Es existieren eine Vielzahl an Elektrolyten für die verschiedensten Spezialanwendungen. Das mit Abstand am weitesten verbreitete und auch einfachste Verfahren zur Herstellung der Eloxalschicht ist das sogenannte Gleichstrom-Schwefelsäure-Verfahren (GS-Verfahren). Hierbei werden die Werkstücke als Anode (Pluspol) in ein wässriges Schwefelsäurebad getaucht, während die Kathode (Minuspol) z.B. durch Blei-, Aluminium-, oder Titanplatten dargestellt wird. Bei ca. 12-18 Volt Gleichspannung und Stromdichten von 1,5 A/dm² erfolgt das Anodisieren über 20-60 Minuten. There are a variety of electrolytes for special applications. The by far most widespread and simplest method for producing the anodic coating is the so-called DC-sulfuric acid process. The parts as the anode (positive terminal) are immersed in sulfuric acid, while the cathode (negative terminal) is built e.g. by lead, aluminum, or titanium plates. At about 12-18 volts DC and current densities of 1.5A/dm² anodizing duration is about 30-60 minutes.
Further processes are chromic acid and oxalic acid methods.
During hard anodising the layer thickness is increased by very high bath voltage and low bath temperatures. These layers are used in industry for the highly demanding operating conditions (plain bearing, hydraulic components, sea water area).

3.5  Flushing

To remove all remnants of acid and protect the dye bath, the parts are again rinsed thoroughly with water, sometimes remaining acid may be neutralized specifically.

3.6  Dye bath

In the dye bath the coloring of the anodized layer is processed at about 50 degrees Celsius. Depending on the concentration, the desired shade and alloy of the parts they stay here for up to 20 minutes.

3.7  Sealing

Immediately after the dye the pieces are dipped into boiling water, where the anodized layer is compressed. Included dye are also sealed and it finally forms the desired scratch-resistant surface.

3.8  Notes

The entire course is subjected to continuous industrial process control, such as determination of the acid or color concentration, measuring the layer thickness and visual inspections. In part elaborate procedures are needed, which are beyond the scope of the beginner. However, with little care these disadvantages are compensated because in small batches it is possible to check every part carefully.