The Essentials of Anodizing:
When Aluminum is immersed in a sulphuric acid eletrolyte there a always a thin oxide film present. When the current source is switched on and a potential applied, an anodic film begins to grow whose thinkness is initially proportional to the applied voltage. However, in many electrlytes this would only result in a film of a few hundred angstrom units in thinkness. For a think film to be formed it must have a porous structure. Pores are formed if the electrolyte can dissolve anodically formed oxide. Initially a thin "barrier layer" is formed. The electric current passed is converted to heat and this forms cracks in the oxide film and pores are formed at the cracks. This leads to growth of the hexagonal oxide cells with a central pore. The volume of pores decreases with the increase in the applied voltage. Part of the reason "hard" anodic coatings appear "hard" is that there is more oxide and fewer pores per square inch. Under conditions of porous growth, theoretically, it could be argued that it is possible to keep the film growing indefinitely, but there is no practical limit.
Limits to Film Growth:
There are two main reasons why the coating will not continue to grow indefinitely. As film impedance increases an equilibrium can occur such that the rate of film formation = The rate of film dissolution. The development of polarization in the anodic coatings prduces a build-up os resistance which is converted to heat. This heating can lead to "burning" (catastrophic dissolution of the coating). Two further reasons are, alloying elements affecting film structure & dissolution, and the resistance of the electrolyte in the pores.
Anodizing depends on aluminum ions moving through the oxide combining with oxygen ions derived from dissociation of the electrolyte. As the film thickens it becomes increasingly difficult for the numberof oxygen ions available to equal the number of aluminum ions passing through the oxide with oxygen ions derived from dissolution of the electrolyte. With build-up of the coating an excess of oxygen ions is produced at the base of the pores and polarization occurs. Polarization produces a rapid rise in the resistance leading to heat build-up and ultimately "burning".
Growth Verses Penetration:
A Unique feature of anodic coating formation is that it does not involve depositing a coating on to the surface of the aluminum. Instead the anodic coating grows by converting the metal into anodic oxide. Because the oxide formed is a greater volume than the metal from which it is formed there is a net increase in dimensions. The problem can further appreiated from the fact that because of lack of space only two out of three aluminum atoms can be converted into oxide. The other third of the aluminum atoms dissolves directly into the electrolyte. The net result of the process is that part of the metal below the original surface is converted to oxide (referred to as "penetration") and the rest of the coating increases the net dimensions of the component being anodized (referred to as "growth"). Generally speaking it is assumed that half the coating thickness is the growth that takes place. If a bar is anodized to produce a 0.002" (50 microns) coating the net growth is 0.001" per side, 0.002". However, this is an approximation since on the more readily anodized materials as as 5052 and 1095 the net increases in dimentions can be 0.00125" per side.
What is Hard Anodizing?:
By definition a "hard" anodized coating implies the production of a film which is hard than normal decorative or protective coatings and meets specifcation requirements in terms of hardness and/or wear test. Its demands the production of an anodized coating substantially harder than the normal protective coating.
What Governs Film Hardness?:
Sufphuric acid electrolytes: The higher the concentration of sulphuric acid the softer the film.
Current Density: Provided agitation and cooling is adequate, the higher the current dendity the harder the film.
Temperature: The lower the temperature the harder the films produced, except when around 32 degrees F. is reached.
Agitation: High rates of agitation ensure that the maximum film hardness is obtained by rapidily removing electrical heat generated in the coating.
Alloy Used: Components in 2000 seris give lower hardness than any other alloy group. Although giving higher hardness values 5000 and 7000 series alloys give rise to lower values than those from the 6000 series. Casting alloys generally are difficult to hard anodize.
Colors and sealing:
Because of its porous structure an anodic coating may be colored by immersion in suitable organic dye stuffs which combine with the oxide to give a strong chemical bond. More recently, processes have been developed for coloring by electrolytic deposition of metalic particals at the bottom of the pores. The first processes produced only bronzed shades, but now almost any color can be obtained.
The final process, sealing is immersion of the component in boiling deionized water. This prcess firmly fixes the dye into the coating and greatly increases its corrosion resistance and dioelectic properties. For many applications hard anodic coatings are left unsealed.