Brief Specialist Information

During the anodizing process the oxide layer grows forming columns to about 2/3 within the bas material; 1/3 builds up externally which must be taken into account in (see fig. 1).

The achievable layer-thickness is up to 30 µm, layers that are thicker than this generally begin to chalk and are unusable. Higher layer thicknesses are possible with the hard-anodizing process. Information on this can be found in the corresponding chapter.

In order to make the anodized layer weatherproof and to close the absorbent pores it is necessary to carry out a so-called “compaction”.

This will be reached by a chemical conversion of the anodized layer (aluminium oxide) into a large voluminous aluminium hydroxide (see fig. 2). Simultaneously, thus an effloresce of dyed items will be prevented.

Transverse Seam/Two Wax

The two figured items (Figure 3.0) made of extruded round and rectangular aluminium sections show stripes of dark discolouration after anodizing, which were previously not visible on the rough part.

Such faults can typically occur during the manufacturing of profiles by extruding block-on-block in the inner and outer areas of the material. This effect (inhomogeneity) cannot be covered (see fig. 3.1) even by dying the anodized layer. For this purpose, the only remedy is to cut out this profile area in the extrusion die on request of the customer.

Contact Corrosion

(galvanic corrosion) with the typical pitting or the formation of indentations as shown in the adjacent figure, occurs particularly on components made of the highly alloyed AlCu-group. A frequent cause is the contact of the component in a damp condition with a metallic surface after it has been machined mechanically. In this way a voltage potential is formed which slowly begins to break down the unprotected surface electrolytically. This type of corrosion is difficult to see in the unanodized condition.

Packing Corrosion

due to wrapping paper containing sodium bicarbonate in a damp atmosphere, is similar to the chemical process that takes place when pickling (matting) in sodium hydroxide solution and likewise causes similar surface defects. Both types of corrosion emerge clearly after anodizing and can only be resolved by mechanical surface removal.

Web Marks

on the profile in the direction of extrusion after anodizing are a consequence of the connection bridges in the mould. There are many causes, ranging from the mould design and the temperature to the alloy composition and the speed of extrusion. Generally, it is not possible to carry out any rework on these.

Coarse Grains

can be seen in terms of various sized spottings after pickling. This effect can extend completely at thin wall thicknesses. On thick cross-sections it can be recognized in the form of individual grains. The edges of coarse grains can be found directly on the surface (see Fig.) or close underneath. Only such defects on the surface can be resolved by material removal and subsequent decorative anodizing. The causes lie both in the alloy composition and in the extrusion process.

Wavelike Samples

on blank sheets, which remind of sandy waves in tidelands, result by expanding oxides into the surface of the raw pellet. Minimum ovalities of the press rolls may cause similar effects in the structural condition. This surface fault will be clearly visible only after anodizing and can normally not be eliminated by grinding or heavy pickling. A reliability can only be provided by ordering blank sheets in anodizing quality.

Forgeable alloys DIN EN 573-3