"Anodize black" is one of the most common finish notes on aluminum drawings, and one of the least complete. It does not state the coating thickness, whether the coating is decorative or hard, how color will be judged, which surfaces must stay bare, or whether the coating is sealed. Each of these affects the function, the appearance and sometimes the fit of the part. This article explains the process and lists what the drawing should contain, with reference to the two ISO standards that cover most anodized parts: ISO 7599 for decorative and protective coatings and ISO 10074 for hard coatings.
What anodizing is
Anodizing is an electrolytic process that converts the aluminum surface into a layer of aluminum oxide. Because the oxide is formed from the base metal, it does not simply sit on top of the part. Part of the coating grows into the original surface and part of it grows outward. The European Aluminium Association's TALAT training material states that for conventional sulfuric acid anodizing, the overall increase in dimension is approximately one-third of the coating thickness, and for hard anodizing approximately one-half 1.
The oxide formed in sulfuric acid is porous. Those pores are what allow the coating to absorb dyes, and they are closed afterward by sealing 1. The TALAT lecture notes that "hundreds of commercial dyestuffs are available for colouring anodic films" and that "the colour intensity depends on the amount of dye taken up by the film" 1. Electrolytic coloring, the other common method, produces mostly bronze to black shades, depending on the electrolyte 1.
Two families of coating, two standards
Decorative and protective coatings: ISO 7599
ISO 7599:2018 sets out a method for specifying decorative and protective anodic oxidation coatings on aluminium 2. It excludes chromic and phosphoric acid anodizing, non-porous barrier-layer coatings and hard anodizing for engineering use, which is referred to ISO 10074 2.
ISO 7599 defines thickness classes by minimum average thickness 3. In summary, the number in the class designation is the minimum average thickness in micrometers, from AA 5 (5 µm) through AA 10, AA 15 and AA 20 to AA 25 (25 µm), and the minimum local thickness is not less than 80 percent of the minimum average thickness, for example 12 µm for AA 15 3. Two further points matter for drawing callouts. First, the standard notes that some dyestuffs require class AA 20 or higher 3; a thin coating may not take a deep, even color. Second, the standard states that the choice of thickness class depends on relevant national standards 3.
For comparison, the Aluminum Anodizers Council (AAC) describes the architectural classes used in North America in mils: Architectural Class II covers 0.4 to 0.7 mil and Class I covers 0.7 mil and thicker, with 25 µm equal to 1.0 mil 4. That places Class II at about 10 to 18 µm and Class I at about 18 µm and above.
Hard anodic coatings: ISO 10074
ISO 10074:2021 sets requirements and test methods for hard anodic oxidation coatings on aluminium and its alloys, and its Annex A lists the information the customer should supply to the anodizer 56. It does not cover plasma electrolytic oxidation, micro-arc oxidation or spark anodizing 6.
According to the standard, hard anodizing produces a hard and usually thick alumina coating, generally between 25 µm and 150 µm 5. Coatings up to 25 µm are sometimes used on splines and threads; 50 µm to 80 µm is the normal range for wear or electrical insulation; and thick coatings tend to be softer in their outer regions 5. ISO 10074 also notes that hard anodizing usually increases the dimension of each surface by about 50 % of the coating thickness 5.
Wear resistance is tested by the abrasive wheel method of ISO 8251 6. Alloy choice matters: the standard groups alloys into classes with different minimum hardness and abrasion requirements, and states that alloys with more than 5 percent copper and/or 8 percent silicon, and die-casting alloys, require special anodizing procedures 57.
Coating thickness and dimensional growth
Because the coating grows partly outward, an anodized part is larger than the machined part. A worked example using the ratios above:
- A conventional sulfuric coating of 15 µm (class AA 15) increases each surface by roughly one-third of 15 µm, about 5 µm 1. A shaft diameter grows by about 10 µm, because two opposite surfaces each gain about 5 µm.
- A hard coating of 50 µm increases each surface by about 25 µm 5. A shaft diameter grows by about 50 µm, and a bore diameter shrinks by about 50 µm.
For close-tolerance fits, this growth can consume most of the tolerance band. The drawing should state clearly whether the dimensions apply before or after anodizing. If they apply after anodizing, the machining dimensions have to be offset, and the factory needs to know the specified coating thickness to do so. Fine threads are particularly sensitive to build-up, and the most common remedy is to mask them 8.
Hard anodizing: what changes
Hard anodizing is chosen for wear resistance, surface hardness or electrical insulation, not for appearance. Three practical points belong on the drawing:
- Thickness, with a tolerance. Given the 25 to 150 µm range in ISO 10074 5, "hard anodize" without a thickness is not a complete requirement.
- Sealed or unsealed. ISO 10074 warns that sealing and/or dyeing can reduce abrasion resistance, and that hydrothermal sealing and dyeing can reduce it by more than 50 % 6. If wear resistance is the reason for the coating, consider whether sealing is required at all; the standard lists resistance to corrosion as a property of hard coatings when they are sealed 6. The two goals conflict, and the drawing should state which one governs.
- Alloy. Hardness and wear requirements depend on the alloy class 7. A hard-anodized 7075 part and a hard-anodized 6061 part do not have to meet the same minimum values.
Color and dye consistency
Color is the most frequent source of disputes on anodized parts, mainly because it is rarely defined.
The alloy sets the starting point. The AAC states that "each alloy or alloy series yields a different appearance, even if treated to identical anodizing processes" 9. It notes, for example, that 5052 "anodizes yellowish in thicker coatings" and that 7xxx alloys anodize gray, blue-gray and brown-black, depending on the alloy and the process 9. Parts in different alloys, or bar and sheet from different sources, may not match even when anodized in the same tank.
Dyed anodizing also has limits. One industry reference notes that dyed colors are "not as consistent as those obtained through painting or powder coat," that there is no Pantone® color matching in most commercial anodizing, and that organically dyed parts fade to some degree over time 8.
ISO 7599 gives the mechanism for controlling this. Among the additional information the customer may supply are the color of the anodized parts and the maximum limits of color variation, and the standard states that acceptable limits of color variation "are identified preferably by agreed limit samples" 3. The AAC similarly recommends producing sample coupons of different versions of the finish to select from 4.
On the drawing or purchase order, state:
- the color, and whether it is dyed or electrolytically colored, if known;
- that color is to be matched to an approved sample, or to light and dark limit samples;
- whether parts from different batches are assembled side by side, which makes batch-to-batch variation visible.
Masking and contact points
Some surfaces must not be anodized: electrical contact faces, threads with tight class tolerances, bearing bores, or faces that must stay conductive for grounding. These must be shown on the drawing. ISO 7599 states that significant surfaces should preferably be indicated on drawings or by suitably marked test specimens 3. The same applies to surfaces that must stay bare.
Masking is usually done with soft plastic plugs for round holes, or with tape or painted-on liquid plastics for flat areas 8. Where the exact boundary of a masked area matters, dimension it on the drawing.
Every part is held on a rack or jig during anodizing, and the contact points remain unanodized 8. ISO 10074 acknowledges these jigging marks and excludes the area near them from thickness measurement 7. If contact marks are not acceptable on certain faces, the drawing should state where racking is permitted 8.
Sealing
Sealing closes the pores of the anodic film. The TALAT lecture describes conventional water sealing as immersion in good-quality water at 95 to 100 °C for 2 to 3 minutes per micrometer of coating 1. Sealing is needed for corrosion resistance and to retain the initial appearance of the coating in service 13. ISO 7599 lists, among the essential information, whether the product is to be sealed or left unsealed and, if sealed, by which method 3.
ISO 7599 refers to test methods for the quality of sealing, including the dye-spot test with prior acid treatment (ISO 2143), loss of mass after acid immersion (ISO 3210) and admittance measurement (ISO 2931) 2. For coating thickness it refers to the eddy-current method (ISO 2360), the split-beam microscope (ISO 2128) and the microscopical cross-section method (ISO 1463) 2. If a specific test is required for acceptance, it should be named on the drawing or purchase order.
A complete anodizing callout
ISO 7599 lists the essential information as the intended service use, the thickness class, whether the finish is clear or colored, and whether the product is sealed and by which method 3. Combined with the points above, a complete callout covers:
| Item | Example entry |
|---|---|
| Alloy and temper | AL 6061-T6 |
| Coating type | Sulfuric acid anodic coating to ISO 7599, or hard anodic coating to ISO 10074 |
| Thickness | AA 15, or 50 ± 10 µm for hard coatings |
| Color | Black, dyed; match approved sample |
| Sealing | Sealed (hot water), or unsealed for wear surfaces |
| Dimensions | Apply after anodizing (or before) |
| Masking | Surfaces marked "M" to remain bare |
| Racking | Contact marks permitted on surfaces marked "R" only |
| Acceptance tests | Thickness check; sealing test if required |
How PAXIS handles anodized parts
Anodizing is one of the finishes PAXIS supplies on aluminum parts manufactured to customer drawings. The finish callout is reviewed with the drawing before quoting. Open points on coating type, thickness, color, masking or sealing are raised with the customer, and any change is approved in writing before production. Where a color must match, first-article samples are submitted for approval before series production. Each order is inspected by the manufacturer to the drawing and then verified independently by our own team in China at the factory before the goods are released. To discuss a part, request a quote.
