Aluminum Anodizing: The Basics

 

Terms and concepts

What is anodising

Anodizing - a method of improving the corrosion resistance of the metal product by forming an oxide layer on its surface. Product, is processed, is the anode electrolytic process. Anodizing product surface increases the resistance to corrosion and wear, and also provides better adhesion for paints and adhesives, than just a "naked" aluminum.

Anodic coatings can also be used as a decorative coating or a porous coating, which can absorb various dyes, or as clearcoats, which give interference effects in the reflection of light. Such interference coatings used, for example, biking or cycling clothes, so they can be nice to see at night.

How does the anodizing

The process of creating this protective oxide layer occurs electrolytically. The metal product, on which the anodic coating is to be obtained (usually aluminum) is immersed in an electrolytic solution bath. In the same bath set cathodes, usually along the sides of the bath. When an electric current is passed through the acid solution at the cathode hydrogen is released, and the anode - oxygen. This leads to, that at the anode - aluminum products - begins to grow an oxide film.

Depending on the application of the anode coating used and the anodization process can be produced anodic coating with different characteristics. anodic coating, which you can grow in the aluminum products, able to have a thickness 100 times more, than the oxide coating, which is formed naturally on aluminum.

Since the metallic article is "anode" in this electrolytic process, the whole process is called "anodizing".

metal anodizing

Although various metals, including titanium, hafnium, zinc and magnesium, They may also be formed anodic coating, usually meant by anodizing aluminum and its alloys anodized.

Why anodize aluminum?

aluminum popularity is largely due to its good natural corrosion resistance. It is achieved due to the high chemical affinity of aluminum to oxygen, i.e. their large mutual desire to join with each other in the reaction to form alumina. This very thin oxide film instantly covers any fresh aluminum surface immediately after contact with air. However, in some cases it is necessary to have a higher degree of protection (corrosive or chemical), modify the surface appearance (color, texture, etc.) or create specified physical properties of the surface (increased hardness, wear resistance or adhesion). In such cases resort to anodized aluminum and aluminum alloy.

Figure 1 – Scheme anodizing process

types of anodizing

QUALANOD organization divides aluminum anodizing into four main types with different requirements for their characteristics and properties:

• architectural (construction) anodizing
• decorative anodizing
• industrial anodizing
• hard anodizing.

Anodic coatings are divided into classes according to their thickness:

• minimum permissible thickness and average
• minimum allowable local thickness.

for instance, class AA20 means, that the average thickness of the coating should be at least 20 micrometers. Minimum local thickness of the coating usually should be at least 80 % Smaller average thickness. For the class of AA20 it is 16 m.

architectural anodizing

This anodizing for the production of architectural finishing products, that reside in external conditions and in steady state. The most important characteristics of anodized products is considered to be the appearance and long life.

For anodized aluminum, the degree of protection against pitting (pitting) corrosion of aluminum increases with an increase in the thickness of the anodic coating. Consequently, lifetime architectural or construction elements largely on the thickness of the anodic coating. However, to obtain thicker anodic coatings require considerably high cost of electric energy. Therefore, the so-called "pereanodirovanie" not recommended.

Architectural anodizing has the following classes:

• AA10
• AA15
• AA20
• AA25

Selection of anodic coating thickness for exterior aluminum structures depends on the aggressiveness of the atmosphere and is generally set to national standards. Moreover, use of certain coloring compositions required thickness class 20 um or above. This is necessary to achieve good filling of the pores and increased stain resistance of the colored coating to the sunlight.

Decorative

This type of anodizing aluminum for the production of decorative trim products. The main criterion of quality is uniform and aesthetically pleasing appearance.

Decorative anodizing is available in the following standard thicknesses:

• AA03
• AA05
• AA10
• AA15

Industrial and solid

Industrial anodizing of aluminum is used for the production of a functional product surface finish, when the appearance characteristic is minor. The aim is to obtain a solid anodizing coating with high durability or high microhardness.

Often, for example, in the automotive industry or medical equipment, appearance of the product does not matter, but the most important characteristic is the resistance to wear and / or ability to undergo efficient cleaning and having high hygiene requirements. In such cases these properties of the anodized aluminum are the principal.

If the main feature is the high wear resistance, used a special kind of anodizing - hard anodizing. It is produced at lower, often negative, electrolyte temperatures

The thickness of solid industrial and anodic coating is generally from 15 to 150 m. Threads and splines may have a coating to 25 m. thick anodic coatings are often required to obtain high electrical isolation from 15 to 80 m. coating thickness 150 microns are used to repair parts.

Technology

Electrochemistry

Aluminum anodizing refers to the electrochemical processes of the formation of stable oxide coatings (films) on the surface of metals. Anodizing of aluminum and aluminum alloy may occur with a variety of electrolytes with the use of direct or alternating current source, or combinations thereof. In this case, an aluminum product (hereinafter for the sake of clarity – profile) is always the anode, i.e. it is connected to the positive pole of the current source, and another suitable metal or alloy is the cathode and is connected to the negative pole (Figure 1).

Anode coatings are distinguished by the type of electrolyte, are used in their preparation. The coatings are porous, for example, in phosphate and sulfuric acid electrolytes, as well as the so-called "barrier" – completely without pores. Barrier anodic coatings possess high electrical resistivity and their use, for example, in the manufacture of electrical capacitors.

sulfate anodizing

ordinary, the most popular and widely used for aluminum profiles in the building structures is anodized aluminum sulfate. This type of anodizing is characterized by high manufacturability and enables a wide range of coating thicknesses. Sulphate is used as the anode coating without additional staining - it is called colorless, and with subsequent staining by one of several known methods – it is called the color anodizing. The final operation is usually always a pore filling (or sealing) operation..

Painting or anodising of aluminum

Sulfate anodic coating formed by the "reaction" with the aluminum ions of the sulfuric acid solution. It occupies a greater volume, aluminum than the original and therefore result in an increase anodization thickness of the product. When sulfuric acid anodizing is an increase of about one third of the total thickness of the coating. This is the fundamental difference between the anode coating by, for example, powder (figure 2):

• anodic coating is formed of the surface layer of aluminum,
• powder coating - on aluminum surface.

Figure 2 – Changing the thickness of the product and during anodization
powder coating

Methods of anodizing aluminum

A specific method of anodization depends on the type of product. for instance, small items or items, can anodize "in bulk" in drums or baskets. Profiles length 7 m, sometimes up 10 m, anodized on the special sample weights. These sample are typically several conductive rods, frameworks or scaffolds, to which the profiles are firmly and sufficiently rigidly attached (see. Figure 1). Strong fastening profiles necessary both to, that they, They do not fall from the test portions and passed all the cycles of "dipping" and "rinse" in the baths, including with intensive mixing of solutions and flushing water (bubbling) / In addition, more importantly, strong fastening of the articles to be weighed to ensure a steady and reliable electrical contact profile to the positive pole of the current source directly during anodization.

Preparation of the aluminum surface

A typical aluminum anodizing line profiles shown in Figure 3.

On line anodizing aluminum profiles served or directly after pressing, or after preliminary mechanical surface preparation (processing with steel brushes, processing fraction, polishing, grinding, etc.).

• A first step of anodizing process is hanging on the hinge profiles. Hinge with aluminum profiles generally first passes alkaline degreasing, and then alkaline etching for removing various contaminants from the surface of profiles: oils, particulate matter and oxide film.
• After alkaline etching, the sample is processed in a clarification bath (desmutting), most often – sulfate (80-100 g / l), Removal from the surface dark alkali etching products.
• Processing baths with the working solution followed by thorough washing articles in water, the final wash before anodizing – in demineralized. After that, the product, basically, ready for anodising.

Figure 3 – A typical line of baths for anodizing aluminum profiles [1]

matt anodized

In case of special requirements for the anodized surface, additional surface treatment of the profiles is carried out: matt etching, as well as chemical or electrochemical brightening. Frosted etching generally carried out in alkaline baths of a special chemical composition. When this surface a predetermined thickness aluminum layer is removed together with various surface defects, and the surface becomes matte (picture 4).

Figure 4- Brushed and shiny anodized aluminum surface [3]

Matte surface maximally scatters light and makes it "invisible" remaining surface defects. If the finished product must have a shiny or mirror-like surface, before the anodization product is subjected to chemical or electrochemical brightening. In this procedure, the surface is removed from the product aluminum, and forms a very smooth surface with very high reflectivity.

Filling anodizing

After anodizing profiles or sending down the line on painting, or immediately directed to the pores filling, if it is colorless anodized. The filling (or sealing) operation after colorless anodizing or color anodizing is then carried out, to close", "Clog" the pores of the anodizing coating. This operation is very important for long-term preservation of the appearance of the anodized product. After the filling operation the product is dried if necessary,, removed from the test portions and sent to the receipt and packaging.

Figure 5 – Hydrothermal filling of the anode coating [2]

Quality control

Control anodic coating thickness

Typically, for control of acceptance quality anodized aluminum profiles sufficiently control the appearance, the thickness of the anodic coating and the quality of the filling. The thickness of the coating is one of the most important parameters and there are many methods of measurement. Typically, the coating thickness was measured by, working on the principle of eddy currents. In case of dispute, the study used a cross-sectional metallographic products.

Control filling anodizing

drop method

One of the variants of so-called "drop method" Quality content is often used for the rapid control. As a control or arbitration test methods used mass loss of samples of products.

SUMMARY nondestructive "drop method" is to assess the degree of absorption of dyes anodized surface after, it was treated with a suitable chemical reagent. Various embodiments drop method with an acidic pre-treating the surface mounted ISO standards 2143: 2010 (aka – EN ISO 2143: 2010 and he – former EN 12373-4).

drop method to ISO 2143:2010

Qualonod standard [1] considers acceptable the degree (rating) of the intensity of the spot not lower than 2 (drawing 6). If the rating is 2, the standard requires to perform tests on weight loss or perform refilling.

Figure 6 – Quality criteria for filling method according to ISO drops 2143:2010

weight loss method

Test weight loss is based on the finding, that is not filled or under-filled anodic coating dissolves rapidly in an acidic environment, whereas a well filled with the coating can withstand prolonged immersion without significant exposure to. Variants of the method are described in ISO standards 3210: 2010 (aka – EN ISO 3210: 2010 and he - former EN 12373-7).

Sources:

1. Standard Qualanod (01/01/2018)
2. TALAT 5203.
3. Tom Hauge, Hydro Aluminium, IHAA Symposium, 2014, New York.