Aluminum anodizing 

Sealing of Anodic Films

 

Why is it called “sealing”?

Fresh anodizing on an aluminum product is like hard filter paper. 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 anodizing or painting, the anodic coating is subjected to compaction..

The sealing operation is the last step in the anodizing process.. It is necessary to maintain the original appearance of the anodized product during operation.. Compaction causes an increase in the volume of the oxide film (increase in water content by 4-10 percent). As a result, there is a complete “zakuporivanie” anodic pores due to the formation of various forms of hydrated alumina [1, 2].

Structure of Anodic Sell

Under aluminum anodizing we understand standard sulfate anodizing, 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).


Fig. 1 – Structure of the Anodic Cell [1]

Fig. 2 – Typical Dimensions of the Anodic Cell (Sulphuric Acid Anodizing) [1]

Accept, for example, what is the pore diameter 18 nm, and its height 25 m = 25000 nm. Imagine, with the same size ratios, a "pore" with a diameter 18 cm cm and height 25000 cm = 250 m. The height of the Eiffel Tower!

Sealing of anode film according to QUALANOD

European organization , which produces licensing manufacturers sulfate anodic coatings on articles for construction application, recognizes only two standard seal types:

  • hydrothermal and
  • partially, so-called cold seal based on nickel fluoride.

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

Hot sealing

This type of seal is the most widely used in the 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. Typically, the process of hydrothermal filling is presented in the form of three overlapping stages (Fig. 3):

  • 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.

 


Figure 3 – The main stages of the mechanism of hydrothermal filling of the anode coating [1]

On pic. 4 shown similar, more detailed, revision version, occurring in the anodic coating during compaction. Anode oxide layer, formed on aluminum after anodizing, consists of two layers; thicker and more porous outer layer (measured in µm) and much thinner, but compact inner layer, barrier layer (measured in nanometers). On the surface of anodized aluminum before compaction, tubular pores can be seen, more or less perpendicular to the interface [4].


Fig. 4 – Structure of the anodized layer on aluminum prior to sealing and changes that occur during sealing;
(a) anodized aluminum prior to sealing,
(b) pore mouths filled with hydrated products,
(c) dissolution of pore walls,
(d) precipitation of hydrated alumina gel in pores and emergence of surface layer of acicular boehmite,
and (e) crystallization of crystalline alumina from alumina gel [4].

What affects the quality of hot sealing?

The most important factors are the following hot-filling.

Water temperature

  • Ideally, the water should be at a temperature of 99 to 100 ° C
  • Interval of 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 a slowdown in the compaction process.

Compaction duration

  • Minimum compaction time - 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 for every micron of coating.

White bloom

During hot compaction, a white coating may form:

  • For small production volumes it's just wipe with a soft cloth.
  • For large production volumes, special seal bath additives are used to combat this problem.

Cold seal anodizing

Cold seal is called “cold”, because, that is - the process, which occurs at much lower temperatures, than hot seal. Typical cold sealing is done in a nickel fluoride solution at temperatures ranging from 25 to 30 ° C for about 1 minutes per micrometer coating. Unlike hot seal, based on the hydration process, cold filling "working" on a chemical reaction between aluminum oxide and nickel fluoride. For this reason, "clean" cold sealing without additional hot working is often referred to as not “sealing”, a “impregnation”.

Two stage cold sealing by QUALANOD

Since doubts remain about the effectiveness of purely cold sealing of anodized coatings for outdoor applications, then QUALANOD prescribes to apply cold compaction in two stages:

  • cold "impregnation" in solution based on nickel fluoride and thorough rinsing and
  • immersing at a temperature in the "hot" water not lower 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)". This process is widely used in many European countries, especially in Italy, but, as far as is known, It does not apply in the UK.

Other Anodizing Sealing Methods

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

 

Sources:
1. TALAT 5203
2. The Surface Treatment and Finishing of Aluminium and Aluminium Alloys – Volume 2, Chapter 11 /Sheasby P.G and Pinner R. – 6th edition
3. QUALANOD SPECIFICATIONS
4. The Sealing Step in Aluminum Anodizing / S.The. Ofoegbu, F.A.O. Fernandes and A.B. Pereira – Coatings 2020, 10(3), 226