What is anodized aluminum?

Aluminum alloys are a very broad group of engineering materials whose compositions are defined by standards. They are grouped into series, due to the main alloying additives. For example, the 1XXX alloy series means a minimum of 99% pure aluminum, and the 7XXX series means alloys where the main alloying component is zinc. Users of these materials usually use the classification by series, so no trade names for aluminum alloys have been widely adopted. One exception may be the name Duralumin, which designates an aluminum-copper alloy, the 2XXX series, but this name is also overused for other alloys.

Post-anodized alloys as materials are not covered by the standard, but the anodizing process itself is standardized. The oxide coating achieves different characteristics depending on the selected process parameters. Examples of standards are PN-EN 2101:1997, describing the anodizing in chromic acid of aluminum and its wrought alloys, PN-EN 2536:1999 describing the anodizing of hard aluminum, and PN-EN ISO 7599:2011 conditioning the general specifications of anodic oxide coatings.

In essence, the anodizing process is similar across variants and is divided into several stages. The first is the preparation of the metal surface by thorough cleaning and degreasing. In situations where a high gloss is required, grinding and mechanical polishing are also used. The next step is the anodic oxidation process. In an appropriately selected electrolyte, the anode is the treated aluminum surface, while the cathode is an electrolyte-resistant metal, such as lead or aluminum. The electrolyte, meanwhile, is most often sulfuric, chromic or oxalic acid. The buildup of the oxide coating also proceeds in stages. At the beginning of the process, a 0.01-0.1 µm-thin barrier layer grows, which then remodels into a porous layer, in which, under the influence of increasing oxide volume, stresses appear, leading to microcracks that are the base for the growing pores. The porous layer grows by simultaneously building up the oxide and deepening the pores deep into the metal.

The next step is surface staining. This can be carried out by immersing the workpieces in a bath of organic dyes and inorganic salts, or by electrolytic coloring. The final stage of anodizing is sealing the coating. It is carried out most often in a hot water bath at a temperature of about 96ºC for a time of about 2 minutes per 1 µm layer thickness. During the bath, sealing of the porous coating takes place, creating a smooth glassy surface to protect the metal from corrosion.

Aluminum owes its versatile use in structural engineering primarily to its low density and good mechanical properties. It is easy to cast and has good thermal and electrical conductivity. Pure aluminum is soft and has low mechanical strength, but its alloys significantly improve these parameters while slightly degrading thermal and electrical properties.

Aluminum oxide Al2O3 is an engineered ceramic with mediocre performance compared to current ceramics, but it works very well as a surface protection. It has ten times the hardness of aluminum alloys, which perfectly protects the metal from scratching, acidic agents and corrosion. The mechanical properties largely depend on the size of the grains in the structure. For example, a coarse-grained structure with a grain size of 50-150 µm achieves a bending strength value of 70 MPa, but a structure with a grain size of 2 µm increases this parameter to a value as high as 450 MPa. Oxide glazing enhances the mechanical strength of aluminum workpieces, but this is highly dependent on the amount of coating in the cross-section of the workpiece. For example, the increase in stiffness of a thick-walled workpiece will not be as noticeable as in the case of sheet metal or embossed thin-walled profiles. Thermal and electrical conductivity, which is many times lower than that of aluminum, can be both a disadvantage and an advantage, depending on the intended use of the anodized workpiece. Aluminum workpieces are most often anodized to protect the surface, so the object of interest in the research is primarily the oxide coating.

The most important parameters are its thickness and microhardness, which depend on the selected electroplating parameters. Tests are carried out on transverse grinds of samples. Such prepared specimens can be subjected to microscopic observation at x1000 magnification. Thickness measurement is carried out with the help of microscope indicators, while hardness measurement is carried out with the help of a microhardness gauge integrated with the microscope. Loading on the order of a few tenths of Newton is done on a similar basis to the Vickers method. Pressure from a few micrometers away from the surface of the deposit is collected from several locations across the width of the coating layer.

Aluminum and its alloys, are important materials in structural and electrical engineering. Due to its low density, it is readily used for load-bearing structures, engines and bodies and housings. It has satisfactory mechanical properties, which can be further improved by precipitation strengthening or by crushing, so it is particularly used in the aerospace and transportation industries. Post-anodized aluminum is used where parts are exposed to atmospheric factors or aggressive acid and moisture environments that cause corrosion. These can be elements of building structures (e.g. doors, windows, hardware) and machinery (e.g. housings, structural profiles). Due to the high hardness of the enamel, the details are also used for parts exposed to abrasive agents.

The oxide coating has poor thermal and electrical conductivity, so anodized aluminum is also used as an insulator in areas where the presence of aluminum structural components in the vicinity of electrical wiring is required. The insulating functions of the coating on 1XXX series alloys are also used in the food industry for containers and packaging. A distinctive possibility offered by anodizing technology is the coloring of details. This gives a very aesthetically pleasing appearance to the details with the possibility of personalization 8when needed. Dyed parts are used as parts of machine and equipment housings, furniture, as control panels and everyday gadgets.The possibility of dyeing the oxide coating gives great potential for future applications of anodized aluminum. One technology that takes advantage of this feature is digital printing, which IMAGO Printer uses in its equipment.

Post-anodized aluminum with a flat surface is printed with any graphic using the digital printing method and then subjected to a sealing bath. The technology makes it possible to obtain indestructible printing resistant to abrasion and chemicals. It competes with engraving and other printing methods, which do not provide such durability and precision. The special ink is deposited in the pores of the oxide coating, which makes it possible to obtain precise printing, the accuracy of which can be seen even with a magnifying glass. The technology is most often used for machine nameplates, information boards exposed to harmful conditions, measuring instruments and interior decoration.

The anodic coating is resistant to most chemicals, so the amount of degradation factors is low. The biggest threats to aluminum oxide are strongly alkaline substances (above pH 9), which dissolve the coating and, downstream, the core material. The oxide coating is hard and brittle, so it is necessary to avoid heavily stressing the workpieces to prevent cracks in the layer, which can allow corrosive agents to pass into the aluminum.

When using anodized aluminum details in production, it is worth keeping in mind their effects on the environment. The material itself is not harmful, while the process of its production 9must be conditioned by environmental standards. Electrolytic and water baths must take place in special tanks, and the electrolytes and rinses used in them must be disposed of. In addition, a large amount of heat is generated during the anodizing process, which must be dissipated.

Anodized aluminum, like unprotected aluminum, can be re-melted, so the anodizing process does not hinder the material’s recycling activities.