Anodizing defects: heterogeneity from graphite, obezzhirivaniya, etching
Below are visible inhomogeneity defects anodized surface, connected with the violation of manufacturing technology of aluminum profiles at any stage. These violations are the source of future defects anodizing. They are all caused by the irregular appearance of the anodized surface.
Defect of this type anodizing (or its source) may occur:
- at the end of the pressing operation,
- in arc welding,
- degreasing,
- the chemical surface preparation,
- anodizing or
- anode surface staining.
dark spots
English term
Вlack spotting (Ноt spotting, Soft spotting)
reason for a defect
Pressing
The manifestation of the defect
Travlenye - Anodyrovanye
Details of the defect
Also known as "hot spots" or "soft spots". Gray or Black, rough spots, arising after anodizing of aluminum sulfate, sometimes orderly, at regular intervals, but also randomly. These dark spots, usually, softer base metal.
This defect is clearly visible after etching aluminum profiles in the form of a dense dark coating on their surface, that is removed during clarification operation, but reappears after anodizing in the form of gray or dark spots (figure 1). These spots occur when selective etching separation places coarse particles of magnesium silicide, that arose due to different cooling rates of different parts of the aluminum profile.
These defects occur in the press, when aluminum profile adjudged from the matrix directly on graphite blocks output section, immediately cooled and then is heated again by the heat of the other parts of the profile.
Figure 1
Preventing defect
- When the pressing need to reduce to a minimum the time of contact between the hot aluminum profile and the output table until they reach a temperature 250 oC.
White pickling raid
English term
White etch bloom
reason for a defect
Pressing
The manifestation of the defect
Travlenye
Details of the defect
White plaque etch manifests as patchy white layer on the aluminum profiles from a 6xxx series alloys after alkali etching. The white layer is the result of inhomogeneous aluminum etching in solutions of caustic soda because of the presence of the film of MgO / Al2O3 on aluminum alloy profiles 6063 quenching them in the air.
Magnesium oxide is difficult to dissolve in caustic soda solutions, and so on the surface of an aluminum profile having increased roughness portions, are well scatter visible light and give the surface white.
The figure 2 shows such a defect on the profile of an aluminum alloy 6063. Dark spots are places, wherein the magnesium oxide film was removed by intrafactory profile displacements, and in which, after a certain time has formed and "toe" corrosion.
Figure 2
Defect Prevention:
- Processing aluminum profiles in a hot, neutral degreasing (non-etching) solution or
- Processing of aluminum in an acidic solution was clarified before alkali etching. In both cases, the defect formation is prevented by removing the layer of magnesium oxide.
Traces weld
English term
Weld staining
reason for a defect
arc welding
The manifestation of the defect
Travlenye - Anodyrovanye
Details of the defect
Dark or bright spots of irregular shape, arising after anodizing aluminum profiles in the heat affected zones of the compounds, made by arc welding (figure 3).
defect size depends on the geometry of the weld. This defect is typical for aluminum profiles subjected to welding of alloys 6xxx series. The cause of the defect is abundant, uneven allocation of magnesium silicide in the form of relatively large particles in the areas, reaches a temperature 400-450 oC, at which release occurs is very. These areas with decreased strength is prepared by etching a significant roughness due to higher sensitivity to be etched compared with adjacent areas, as in the case of defect “dark spots” (cm. higher).
Figure 3
Preventing defect
Aluminum profiles with such defects can not be restored reprocessing. Manifestation defect "traces welding" can be reduced considerably by replacing arc welding on contact. This defect can be completely avoided, if applied mechanical compound aluminum profiles.
poor degreasing
English term
Inadequate degreasing
reason for a defect
Degreasing
The manifestation of the defect
Travlenye
Details of the defect
The nonuniform etching as shown in Figure 4. The cause of this defect is an ineffective degreasing solution or a violation of technology (for example, low processing temperature). As a result, on the aluminum profile are traces of oils and fats. When cleaning occurs selectively etching the surface of the aluminum profile, that is difficult to alter reprocessing.
Figure 4
Preventing defect
- To avoid this defect, should be used degreasing solution such concentration and at such a temperature, adequately to degrease the surface of the aluminum profile.
pickling stains
English term
Etch staining
reason for a defect
Travlenye
The manifestation of the defect
Etching - Rinsing - Anodizing - Coloring
Details of the defect
Stains in the form of vertical "drip", as shown in Figure 5.
This defect can be observed after the etching, but more clearly it manifests itself after anodizing. These spots may also appear as interference colors after surgery clarification, sometimes disappear after anodizing.
The cause of these spots is a selective etching with an alkaline solution of aluminum profile, which for some reason, "he paused" in the cage after surgery etching or washing.
Figure 5
Preventing defect
- Spreyernaya processing cages made of aluminum profiles at the exit of the pickling bath and / or acceleration of its transfer from the pickling bath to the washing bath can prevent this defect.
Source: Barry R. Ellard, Aluminum Extrusion Technology, 2000
See. GOST 9.303-84 an anodic oxide coatings on aluminum and aluminum alloys
