In the 8625 Mil spec, the term “Class” is used to designate this: Class 1 is clear or undyed, and Class 2 is dyed. Anodizing layers are commonly dyed with organic dyes by dipping the part into a hot dye tank directly after anodizing. A wide variety of colors can be obtained through this process.
Type II anodizing, also know as sulfuric acid anodizing, typically with a thickness between 0.00010” and 0.0005” inches. Commonly derived from the military standard spec Mil-A-8625, Type II anodizing consists of artificially building an oxide coating in a sulfuric acid bath at around 70 degrees Fahrenheit.
However, with Type II it is much more complicated. Class 1, undyed anodic coatings, run anywhere from 0.0002 to 0.0005 inch thick with Class 2 dyed coatings running even thicker due to the fact that thicker layers are needed to adsorb a sufficient amount of dye to render the anodize a proper depth of coloration.
Light anodizing is the thinnest form and is used for decorative purposes. Medium anodizing is thicker and provides better corrosion resistance. Heavy anodizing is the thickest form and provides the highest level of corrosion protection.
Type I Chromic Acid Anodizing passes an electrical flow (anodizing) through a chromic acid solution. This creates a thin aluminum oxide film that is non-reactive, a good primer base, and corrosion-resistant. This is a favored coating for parts undergoing high-stress, such as in the aerospace industry.
In the 8625 Mil spec, the term “Class” is used to designate this: Class 1 is clear or undyed, and Class 2 is dyed. Anodizing layers are commonly dyed with organic dyes by dipping the part into a hot dye tank directly after anodizing. A wide variety of colors can be obtained through this process.
Type 2 is mainly for wear purposes: It protects the metal surface against the effects of wear. When untreated titanium parts rub against each other, they produce titanium dust – a result that is not desirable with orthopedic implants, for example.
The most widely used anodizing specification in the US is a U.S. military spec, MIL-A-8625, which defines three types of aluminium anodizing. Type I is chromic acid anodizing, Type II is sulphuric acid anodizing, and Type III is sulphuric acid hard anodizing.
The 720 rule is used to calculate the rate of that film growth (thickness per unit time) based on your chosen current density. Therefore, a selected current density will determine the time required to achieve a specific film thickness.
There is no “typical” hardness for Type III anodic coatings. Vickers Hardness 360 is roughly equivalent to Rockwell C of 37. The hardness of the coating depends on alloy and the anodizing conditions. Hardness can be compared to wear resistance only under like conditions.
We offer both clear anodizing and black anodizing, but there is a slight difference between the two surface finishes. It is all about the inclusion of a pigment during the anodizing process. While clear anodizing results in a silvery appearance, black anodizing presents an black attractive appearance.
Mil-A-8625 is much more simple; class I refers to non-dyed coatings, and class II means the coating is dyed / colored after anodizing and before sealing. PFI generally offers options for blue, red, yellow, green, and black coloring, though other colors may be available by request.
How can you tell if aluminum is anodized? Scrape a penny across an unobtrusive portion of the metal. Polished aluminum is very soft, as all natural aluminum remains. But, anodized aluminum has a surface strength harder than steel.
Type II chem-film, which uses trivalent chromium, is considered more environmentally friendly than Type I, which uses hexavalent chromium due to its lower toxicity.
Customers ask me, “Why do white spots happen on anodized parts?” The quick answer is those white spots are corrosive cells found beneath the anodized coating. The unfinished metal was probably not prepared properly and most likely, the coating applied was not thick enough.
The following aluminium alloys are best suited to the anodising process: 5XXX Series – Aluminium alloy containing a small amount of Magnesium. 6XXX Series – Aluminium alloy containing a small amount of Magnesium and Silicon. 7XXX Series – Aluminium alloy containing a small amount of Zinc.
Because of the need for electrical contact for anodising, any existing coating must be removed before items can be re-anodised (we do not offer 'brush anodising'). Paint or lacquer that has been applied over the original anodised coating must be mechanically or chemically removed before anodising can be stripped.
These natural oxide layers are generally 2 to 3 nanometers thick; Type II anodizing creates oxide layers that range in thickness from 1.8 μm* to 25 μm (0.00007″ to 0.001″). When applied properly, decorative anodic coatings are very regular and uniform across the surface of the treated object.
characterizes anodize by Type and Class. Type II refers to regular anodize while type III refers to hardcoat anodize. Class 1 is non-dyed or “clear” while class 2 is dyed either black or other colors. Below is a comparison of some of the primary characteristics of the 2 anodize types.
Hard anodising gives best results on 6061 or 6082 alloys, but 6000 and 7000 series aluminium is generally suitable for hard anodising. Hard anodising is possible on 5000 series alloys, but properties such as wear resistance will be compromised.
Sulfuric acid anodizing can be used on a wider range of materials. It can be used on aluminum, titanium, and magnesium parts. Chromic acid anodizing is for aluminum only.
Type II anodizing involves sulfuric acid electrolyte and typically uses higher temperatures and lower voltages. Type III, on the other hand, employs the same sulfuric acid electrolyte, but uses lower temperatures and higher voltages, resulting in a thicker oxide layer on the part.
Titanium Grade 2 or Titanium CP3 is a commercially pure alpha (α) alloy. Although not as strong as Ti 6Al-4V, Titanium grade 2 has outstanding corrosion resistance against variety of aggressive media.
Type I (Chromic Acid) Anodizing
Chromic Acid anodizing is a metal coating process used when the application calls for a thin coating and a high level of corrosion resistance. Type I coating is achieved by a chemical conversion processes involving immersion of an aluminum component in a chromic acid bath.