Aluminum anodizing: filling

Fresh anodic coating of aluminum products like hard brittle blotter. It easily absorbs the dye in its adsorption staining, and possible contamination. Contaminants trapped in the pores reduce its future functional properties: corrosion resistance, wear resistance or resistance to staining by frequent contact with the hands. Therefore, immediately after the anodization or anodic coating is subjected to dyeing filling.

Filling of the anode-oxide coating

official, “gostovskoe” the name of the process – filling the anode-oxide coating, for example, GOST 9.031 or GOST 9.301. Often use the term "long filler". Often, instead of "Gostovskaya" "filling" is used not standardized, the term "seal", which the, may be, and better reflects the essence of the matter. In general, the confusion in terms of the need to speak in a separate article. Separate conversation is needed and regulations, which regulate the anodizing process and its quality control - domestic, and more – overseas.

As before under aluminum anodizing we understand standard sulphate anodnoe oksidirovanie, the, that applies, in particular, aluminum profiles for building. Anodizing in other solutions, for example, chromic acid, apply, primarily, for machine parts, Vehicle, Military and aerospace.

To better visualize the processes, occurring during the filling of pores, it is useful to be aware of the ratio of the sizes of the anode pore (figure 1).


Figure 1 – The dimensions of a typical anode cell with sulfuric acid anodizing

If the pore diameter is somewhere 25 nm, and its height 25 m = 25000 nm, just imagine if the same aspect ratio of "time" in diameter 25 cm and a height 25000 cm = 250 m. This is - a pipe diameter of a quarter, and almost as high as the Eiffel Tower!

Filling of the anode coating by QUALANOD

European organization , which produces licensing manufacturers sulfate anodic coatings on articles for construction application, "Recognizes" only two standard types of filling: hydrothermal and partially so-called cold filling based on nickel fluoride.

All other methods in the preparation of QUALANOD license requires proof of their effectiveness.

Hot filling anodizing

This filling type is most widely used in industry, including, for construction products. It is also called "hydrothermal filling". Its essence consists in the treatment of anodic coatings well washed in saturated steam at atmospheric pressure or in a boiling water. The exact mechanism of the process is still not fully known, however, usually the process of hydrothermal filling is represented in the form of three overlapping stages (figure 2). Initially dehydrated coating gradually becomes saturated with water to 8-13 %. alumina at the pore walls is converted into alumina hydrate, generally considered, it - the so-called boehmite – Al2O3∙H2O. First, this is similar to the hydrate gel on the pore walls, then it gradually increases in volume, fills the whole time, sealed and permanently "locks up" time. It turns out dense coating, which is impermeable to liquids and resistant to most chemical environments.

mexanizm-napolneniya-anodnogo-pokrytiya

Figure 2 – The main stages of the mechanism of hydrothermal filling of the anode coating [1]

What affects the quality of hot filling?

The most important factors are the following hot-filling.

Water temperature

The water should be at a temperature of 99 to 100 ° C, but apart 96 to 99 ° C is considered acceptable.

The pH of the water

The pH of the water is very important. At a pH below 5,4 filling quality deteriorates, and at a pH in the alkaline range, that is above 7,0, there is a risk of chemical damage to the coating. The pH interval used in industrial practice by 5,5 to 6,5.

water quality

Used demineralized water. Water pollution phosphates, silicates and fluorides leads to slow filling process.

filling duration

The minimum duration of the filling - 2 minutes per micrometer coating. This requirement is, in particular, in technical terms the organization QUALANOD. At higher requirements for the quality of filling or, let us say, problems with the purity of water or a temperature, filling length increased to 3 or even 4 minutes per micron coating.

white fur

At hot filling may form white bloom. For small production volumes it's just wipe with a soft cloth. When large volumes of production to deal with this problem a special additive used in the filling of a bath.

Cold filling anodizing

Cold filling of so called, because, that is - the process, which occurs at much lower temperatures, than hot filling. Typical cold filling produce a solution of nickel fluoride at a temperature of from 25 to 30 ° C for about 1 minutes per micrometer coating. Unlike hot fill, based on the hydration process, cold filling "working" on a chemical reaction between aluminum oxide and nickel fluoride. For this reason, “clean” cold filling without additional hot working is often referred to as non-sealing., impregnation.

The two steps of cold filling QUALANOD

Since doubts remain purely cold filling efficiency of anodic coatings for external use it, QUALANOD prescribes cold filling in two stages: 1) cold "impregnation" in a solution based on nickel fluoride and thorough rinsing and 2) immersion in "hot" water at a temperature not lower than 96 ºC or "warm" water, with the addition of 5 to 10 g / l nickel sulfate at a temperature not lower than 60 oC. The first step is called "Impregnation", second - "aging treatment in hot water". The duration of both stages - from 0,8 to 1,2 minutes per micrometer of anodizing. QUALANOD calls this process "cold impregnation / cold sealing (CI-CS)" – Cold impregnation / cold filling. This process is widely used in many European countries, especially in Italy, but, As I know, It does not apply in the UK.

Other methods of filling the anodizing coating

There are other methods of filling the anodizing coating, but they are mainly used not for building profiles and articles, and for, for example, machine parts or vehicles. In most cases, filling is used in solutions of nickel salts or cobalt, dichromates (dichromates) sodium or potassium, ammonium acetate. Bixromatы, for example, used for anodizing parts in aerospace engineering.

See. also The choice of anode-oxide coating GOST 9.303-84

Sources:
1. TALAT 5203
2. QUALANOD SPECIFICATIONS, edition 01.01.2017