Anodizing defects: rinsing corrosion and chloride

 

The cause of many defects in anodizing aluminum profiles anyway is corrosion.

Corrosion in the washing

English term

Rinse water corrosion

defect Source

washing

The manifestation of the defect

washing

Details of the defect

Shallow corrosion in the form of star-shaped pits (Figure 1) on aluminum profiles made of Al-Mg-Si alloys, which can occur in seconds in rinsing baths after the alkali etching bath or sulfuric acid bath lighten.

They tend to form a pit on the surface of aluminum profiles along the trace of the matrix or other surface defects. The exact cause of this type of defect is not known, although it is believed, that is a consequence of the combination of very clean, active surface of aluminum, pollution of the wash water and certain "optimal" pH.

it happens, that the two cages are aluminum extrusions of alloy 6063 are processed in the same washing, but one of them exhibits such defects, and the other – not.

korroziya-ot-promyvochnoy-vodyFigure 1 – Corrosion of washing water

Preventing defect

  • To prevent this defect use cleaner rinsing baths. Sometimes some oxidizing acid is added to the wash water, for example, hydrogen peroxide.

chloride corrosion

English term

Сhloride corrosion

defect Source

anodizing

The manifestation of the defect

anodizing

Details of the defect

This defect is easily recognized by relatively deep, black, radial pitting, which are randomly distributed over the entire surface of the aluminum profile (figure 3.9).

xloridnaya-corrosionFigure 2 – chloride corrosion

Along with black pitting sometimes have white flecks. Above the corroded sites offline anode layer and the thickness of the layer on the other profiles cages are always smaller than the predetermined.

This defect arises due to contamination in the anode electrolyte concentration of the sulfuric acid chlorides over 150 ppm. Corroded aluminum profiles can not be restored reprocessing.

Preventing anodizing defect

It is necessary to control the chloride content in the anode solution and the source of water used. If the chloride content in the original water exceeds 50 ppm, then only deionized (demineralized) water should be used to prepare the anode solution.

Source: Barry R. Ellard, Aluminum Extrusion Technology, 2000