November 4, 2026
Aluminum Anodizing for Industrial Parts: Coating Classes, Test Methods and a Buyer's Specification Checklist
Table of Contents
Table of Contents
What Aluminum Anodizing Actually Does to the Surface
The AA Thickness Classes Defined in ISO 7599
Matching a Class to the Service Environment
Why Alloy Choice Changes Both Colour and Protection
The Tests That Prove the Coating Is Right
When Hard Anodizing Makes More Sense
Cost and Lead-Time Drivers on the Line
A Specification Checklist for Your Next Order
About This Guide
FAQ
What Aluminum Anodizing Actually Does to the Surface
Aluminum anodizing is an electrochemical process, not a paint job. The part goes into an acid electrolyte bath as the anode, and the outer layer of the metal itself is converted into aluminum oxide. Because the film grows out of the substrate, there is no interface where it can chip or flake off the way plating or powder coating can.
The oxide has a highly ordered porous structure. Those pores are what make dyeing possible, and they are also the reason the process cannot stop at oxidation. If the pores stay open, moisture and chemicals travel down them. Closing them is a separate step called sealing.
The Aluminum Anodizers Council describes the resulting finish as decorative, durable and corrosion resistant, and lists a military grade specification, MIL-A-8625, covering anodic coatings for aluminum and aluminum alloys. That standard separates the common types: Type I chromic acid, Type II sulfuric acid and Type III hardcoat. Source: Aluminum Anodizers Council, https://www.anodizing.org/.
The AA Thickness Classes Defined in ISO 7599
Writing "anodized" on a drawing leaves almost everything to the supplier. The useful language is the ISO 7599 class, which is defined by minimum average coating thickness in micrometres.
ISO 7599:2018, "Anodizing of aluminium and its alloys — Method for specifying decorative and protective anodic oxidation coatings on aluminium", grades anodic oxidation coatings according to the minimum allowed average thickness and designates them with the letters AA followed by the grade.
Thickness class | Minimum average thickness (µm) | Minimum local thickness (µm) | Practical reading for a buyer |
AA 5 | 5.0 | 4 | Light decorative work, dry indoor use |
AA 10 | 10.0 | 8 | General interior parts and electronics enclosures |
AA 15 | 15.0 | 12 | Standard exterior profiles for buildings and equipment |
AA 20 | 20.0 | 16 | General outdoor exposure in urban or industrial air |
AA 25 | 25.0 | 20 | Coastal, marine or aggressive industrial atmospheres |
Source: ISO 7599, Anodizing of aluminium and its alloys, thickness classification table published by ISO (ISO/TC 79/SC 2), catalogue entry and preview at https://www.iso.org/standard/70156.html and https://cdn.standards.iteh.ai/samples/46061/b170a73405fb4bad9fd10eacfdec8d89/ISO-7599-2010.pdf. Note that the standard also states that the minimum local thickness must not fall below 80% of the minimum average thickness.
One detail is easy to miss. Some dyestuffs need class 20 or higher to reach adequate dye absorption and light fastness. If you specify AA 10 black and expect deep outdoor-stable colour, the two requirements contradict each other.
Matching a Class to the Service Environment
Thickness should follow the environment, not habit. Interior dry assemblies rarely need more than AA 10, and asking for AA 25 there only adds cost and processing time.
Outdoor products see a different picture. Sun, rain, salt in coastal air and industrial pollution all attack seal quality first, then the oxide itself. Higher classes help, but sealing method helps just as much.
A useful split for industrial equipment:
l AA 5 to AA 10 for decorative indoor parts where handling marks rather than corrosion drive complaints.
l AA 15 for standard exterior machine frames, guard rails and general covers.
l AA 20 to AA 25 for new energy equipment, marine hardware and anything washed with alkaline cleaners.
Whatever you pick, name the significant surfaces on the drawing. Coating thickness is measured on significant surfaces, and ISO 7599 excludes the first 5 mm next to contact points and the immediate area around sharp edges.
Why Alloy Choice Changes Both Colour and Protection
Because aluminium anodizing grows from the metal, the metal's chemistry shows through the film. That is why alloy selection belongs in the drawing and not in a later discussion.
l 6063 is the classic architectural alloy. Low copper content and a clean extruded surface give a bright, uniform clear finish with little preparation.
l 6061 carries more silicon, iron, copper and magnesium. Clear coatings typically come out darker and greyer, and corrosion resistance is somewhat lower than on low-copper alloys. Fine for functional equipment parts, less good for a bright decorative finish.
l The 2xxx and 7xxx series contain high copper. They anodize poorly, produce softer films and lower corrosion resistance, so they are avoided for decorative and architectural work.
There is one more sourcing point here. Even the same nominal alloy from a different heat can anodize to a visibly different shade. Keeping alloy, temper and source stable across a project is the cheapest way to avoid colour complaints on repeat orders, and it applies to every anodized aluminum part you buy, including custom aluminum components that are machined before finishing.
The Tests That Prove the Coating Is Right
On daily production, sealing is the step that suppliers quietly simplify. Hot sealing closes the pores properly and gives durable exterior performance, while cold sealing is faster and cheaper but generally regarded as lower durability for exposed work. Put the method in writing for outdoor parts.
ISO 7599 also lists how each property should be checked. Referring to these methods removes the usual argument about whose gauge is correct.
Property | Method referenced in ISO 7599 | Common acceptance reference |
Coating thickness | Eddy current per ISO 2360, cross-section microscopy per ISO 1463, split-beam microscope per ISO 2128 | Cross-section is the referee method for coatings of 5 µm and thicker |
Quality of hydrothermal sealing | ISO 3210 as referee test, admittance per ISO 2931 for production control | Maximum accepted loss of mass commonly taken as 30 mg/dm² |
Abrasion resistance | Defined in ISO 7599 through its listed test methods | Agree the method and the limit before the first article |
Source: test methods and acceptance references in ISO 7599, https://www.iso.org/standard/70156.html. Salt spray testing against ISO 9227 or ASTM B117 is usually added as a separate commercial requirement; agree duration and acceptance criteria up front, because "passed salt spray" means nothing without hours and a rating.
When Hard Anodizing Makes More Sense
Wear rather than looks may be the real problem. Sliding parts, hydraulic bodies and fixtures often need a much thicker and harder film. Those engineering hard anodic oxidation coatings fall outside ISO 7599 and are covered by ISO 10074 instead.
Practically speaking, hardcoating changes the part dimensions. The oxide grows partly outwards, so bores, threads and fits need allowance in the drawing. Raising this question late is one of the classic ways to scrap a first batch.
The MIL-A-8625 framework used in North American drawings covers the same split, with Type III as the hard anodizing category, according to the Aluminum Anodizers Council at https://www.anodizing.org/.
Cost and Lead-Time Drivers on the Line
Aluminum anodizing cost follows surface area, class, colour and handling far more than part weight.
l Class step-up: thicker films mean longer tank time and more electricity, so AA 20 costs noticeably more than AA 10.
l Racking and jigging: awkward shapes need custom racks, and rack marks must sit on non-visible faces.
l Colour matching: an agreed reference panel with a stated tolerance keeps repeat orders consistent, and electrolytic colouring holds up better in sunlight than simple dyeing.
l Lot size: very small parts suit a dedicated parts line, while long profiles need a large line. We run a 4,000 t/month oxidation line and a 1,500 t/month line for profiles, plus a parts anodizing line and a small anodizing line for precision pieces where coating thickness and colour consistency need closer control.
l Preparation: shot blasting before anodizing hides extrusion die lines and handling marks, and it changes the final appearance, so specify it when appearance matters.
On verification, our own laboratory runs material analysis with a spectrometer, tensile and hardness testing, salt spray testing, QUV weathering and full-dimension scanning or CMM inspection, so coating thickness and substrate properties can be checked against the same order.
A Specification Checklist for Your Next Order
Put these eight lines into the enquiry and most downstream problems disappear.
l Alloy and temper, for example 6063-T5 or 6061-T6.
l Anodizing class per ISO 7599, for example AA 15 or AA 20.
l Significant surfaces marked on the drawing.
l Colour with a physical reference sample and an agreed tolerance.
l Colouring method: dye or electrolytic, decided by how much sun the part will see.
l Sealing method: hot sealing stated in writing for exterior work.
l Test methods and acceptance criteria: thickness per ISO 2360, sealing per ISO 3210, plus salt spray hours if needed.
l Packing: separate wrapping for coated surfaces, since transit damage on an anodized face is expensive to fix.
About This Guide
This guide was written by Suzhou Boding Metal Technology Co., Ltd., an aluminum processing manufacturer founded in 2019 and based in Changshu, Suzhou, operating more than 5,000 sqm of production floor for aluminum extrusion, deep processing, anodizing, welding, CNC machining and testing. Equipment referenced above is the equipment we actually operate; nothing else should be read as a guarantee of the coating result.
Coating thickness, colour match, salt spray hours, lead time and price always depend on the specific part, quantity and drawing, so every figure discussed here must be confirmed in writing with the manufacturer before release of an order.
FAQ
How thick should industrial aluminum anodizing be?
Most exterior industrial parts fall between AA 15 and AA 25 per ISO 7599. Interior electronics and dry assemblies often sit at AA 5 to AA 10. Choose based on environment, not on habit.
Why does the same alloy sometimes arrive in two shades?
The film grows out of the metal, so batch chemistry, temper and pretreatment all shift the final colour. Keep alloy, temper and source fixed, keep a reference panel, and agree a colour tolerance for repeat orders.
Is anodizing enough for coastal use?
Thicker classes help, but seal quality matters just as much. Combine AA 20 or AA 25 with specified sealing, then verify with the agreed ISO test methods and a salt spray requirement.
Can anodized parts still be machined afterwards?
Normally no. The oxide is hard and brittle at the edges and machining would cut through it, so drilling, tapping and fitting are done before the part goes into the tank.
What is the difference between decorative and hard anodizing?
Decorative and protective coatings are specified under ISO 7599 by thickness class. Hard anodic oxidation coatings intended for wear resistance fall under ISO 10074, which is why they need separate drawing allowances on threads and bores.
Do purchase orders need all of this detail?
The critical lines are alloy and temper, class, significant surfaces, colour reference and sealing method. Those five remove most disputes; the rest of the checklist is for projects that need documented verification.