Matte aluminum anodizing

 

Frosted anodized aluminum profiles 6xxx series alloys, mainly 6060 and 6063, attracts many consumers. It gives a very nice look, in many ways to hide such extrusion defects such as "traces of the die" and has good repeatability from batch to batch.

Alkaline pickling of aluminum

Usually, to prepare the aluminum surface before anodizing, alkaline degreasing and alkaline etching - chemical reaction of aluminum with alkali, usually – with caustic soda (NaOH). Thus there is a cleaning of the surface and for bleeding the surface layer in order to increase surface haze and removal or smoothing of visible defects molded surface, such as "traces of the matrix". at aluminum anodizing its surface layer is converted into oxide coating semitransparent. The entire surface topography, which was formed by alkali etching, saved.

E6 alkaline pickling of aluminum

The present matte alkaline etching aluminum profiles obtained in the not conventional alkaline baths containing alkali 30-40 g / l and aluminum to 40 g / l, and in the so-called "eternal" or "long-lived" baths with a high alkali content (50-80 g/l) and a high aluminum content (120-180 g/l) (Figure 1). These baths are called, respectively, bath tub, E0 and E6 on the name of surface preparation mode before anodizing, which are set out in the German standard DIN 17611. The bath is usually handled E0 profiles 2-10 minutes at 40-50 oC, bath and E6 – 10-20 minutes at 60-70 oC.

Average bleeding in the bath is E6 70-100 g / m². After alkaline etching E6, the aluminum surface receives a special topography (Figure 2), which intensively disperses most of the incident light, which gives it a matte appearance and ability to mask minor surface defects.

Figure 1 – Viscous solution of E6 alkaline etching bath on aluminum profiles [1]

Figure 2 – Matt apperance of 6060 aluminium alloy after E6 etch treatment [2]

Iron in aluminum

It has long been known, that aluminum alloys 6xxx series with a high content of iron to get a good matte finish is much easier. This is due to the fact, that with increasing iron content increases etch pit density of not only the grain boundary, but within the grains. This is facilitated by an increased amount of intermetallic particles Al-Fe (Mn) -Si. These iron particles act as cathodes in aluminum dissolution reaction when an alkaline etching. The "pure" alloy, with a minimum iron, We occur common bleed aluminum matrix without the formation of deep etching pits, and that then provides brilliant, glossy anodized surface. With increasing iron content in the alloy thickness bleed, which is necessary for obtaining a matt surface, decreases.

Figure 3 – Effect of Fe content on gloss after anodising [2]

Copper and zinc in aluminum

Moreover, to achieve a good matte surface of the copper content in the aluminum alloy should be kept to a minimum - to 0,03 %. Elevated levels of copper in the alloy increases the gloss anodized surface. That is why alloys 6463 and 6463A, which is used to produce brilliant, reflective anodizing, the copper content increased - up 0,25 %, iron and low - no more than 0,15 %.

Figure 4 – Influence of Cu content on gloss [2]

Effect technology on "matteness" of aluminum surface

The manufacturing process of aluminum profiles also has an impact on the matteness of the surface. Fully homogenized microstructure of the pressing ingot (pillar) contributes to a matte finish. Good training on the press also has a positive effect on the matte surface profiles. The best combination of opacity and mechanical properties gives the typical state T6 - hardening and artificial aging at 185 ° C within 5 hours. Underaging of aluminum alloys (for example, condition T4) contributes to a higher gloss of the anodized surface. Severe over-aging (T7 state) - an increase in temperature and / or aging duration - can lead to an increase in surface mattness, but due to a significant reduction in the mechanical properties of the profiles (Fig.. 5 and rice. 6).

Figure 5 – Underaged, aged and overaged Tempers of 6000 aluminium alloys [3]

Figure 6 – The size and distribution of secretions in the underdrained, aged and overaged aluminum alloy

Sources:

1. Anodisation of Aluminum Surfaces / Architect Information Surface Finishes – Schuko International – 2014
2. Influence of Al Microstructure on Hard Anodising Quality – Profile Material / Tom Hauge, Hydro Aluminium, Norway – 2014
3. Basic Metallurgy 6000 series Extrusion Alloys, Comalco Publication