Type 2 anodize (sulfuric anodizing) is the standard finish for aluminum enclosures and machined parts. This spec explainer covers corrosion resistance, color options, and the drawing callouts—film thickness, masking, and contact points—that prevent rework.
What Type 2 Anodize Delivers on Aluminum Parts
When a drawing calls out Type 2 anodize, the buyer is specifying a sulfuric acid anodizing process that builds a controlled aluminum oxide layer on the part surface. That layer is integral to the substrate—it does not flake or peel like a coating—and it serves three practical functions on machined parts and enclosures: corrosion resistance, wear resistance on handling surfaces, and a stable base for dyeing.
Type 2 anodize is the default anodizing specification for a reason. It is widely available, accepts a range of dyes, and can be specified with a film thickness that suits the part's exposure and handling requirements. The complications rarely come from the process itself. They come from drawing details that are missing, ambiguous, or in conflict with the part geometry.
Key takeaways
- Type 2 sulfuric anodizing provides corrosion resistance and a dyeable surface on aluminum.
- Colors include natural (clear), black, and a range of dyed colors; color must be specified on the drawing.
- Film thickness is a drawing callout, not a shop default—state the target and tolerance.
- Masking and contact points must be defined to protect critical dimensions and mating surfaces.
- Inspection criteria for appearance and film thickness belong on the drawing to avoid rework.
Corrosion Resistance and Surface Performance
The anodic oxide layer formed in sulfuric acid is porous as grown. A sealing step—typically hot deionized water or a dichromate seal, depending on the specification—closes the pores and improves corrosion resistance. Type 2 anodize is not a hard anodize (that is Type 3, using a different electrolyte and typically thicker films), but for enclosures, brackets, and machined housings it provides a durable, electrically insulating surface that resists oxidation and handling marks.
Corrosion performance depends on film thickness, alloy, sealing quality, and the service environment. A thin film on a 6061-T6 enclosure will perform differently from a thicker film on a 7075 component in a marine atmosphere. If the part sees salt spray or repeated washdown, state the film thickness and any salt-spray test requirement on the drawing so the anodizer can plan the cycle.
Color Options: Natural, Black, and Dyed
The anodic layer accepts organic dyes before sealing. Common specifications include:
- Natural (clear): no dye added; the color comes from the alloy and film thickness, so it can vary between lots and between alloys.
- Black: a dyed black finish, widely used on enclosures and consumer-facing machined parts.
- Dyed colors: a broad range of colors can be matched to a sample or a color standard.
Color is not a cosmetic afterthought in anodizing. It must be specified on the drawing or purchase order, and a physical sample or color standard should be agreed before production. Natural anodize in particular is not a single uniform color—different heats of aluminum and different film thicknesses produce different tones.
Drawing Details That Prevent Rework
Most anodizing rework traces back to a drawing that did not answer a question the anodizer needed answered. The four areas below cover the majority of those gaps.
Film thickness
Film thickness is a drawing callout. A typical Type 2 anodize film might be specified in the 0.0002–0.0007 in (5–18 µm) range, but the correct value depends on the part's function and exposure. State the target thickness and tolerance, and note whether the thickness applies to all surfaces or only to functional areas. Remember that the oxide layer grows both outward and inward: roughly half the film thickness is below the original surface, which affects tight-tolerance features.
Masking
Masking protects areas that must not be anodized—threads that will receive a mating fastener, electrical contact pads, press-fit bores, and sealing surfaces. Masking requirements belong on the drawing, with the masked areas clearly identified. If masking is not called out, the anodizer may anodize the entire part, which can change critical dimensions and cause assembly problems.
Contact points
Parts must be electrically connected during anodizing, and the contact points do not receive a full anodic film. When contact points fall on a cosmetic or functional surface, the result can be a visible mark or a dimension that is out of tolerance. Identify acceptable contact point locations on the drawing—often a non-critical edge or an area that will be masked.
Appearance and film inspection
Appearance criteria should be defined, including acceptable color variation, scratch and handling limits, and whether the part is a Class 1 (decorative) or Class 2 (functional) finish. Film thickness inspection is typically done with eddy-current gauges, and the drawing should state the measurement locations and acceptance criteria. Sealing quality may also be checked per the applicable specification.
Specifying Type 2 Anodize on a Drawing
The table below summarizes the callouts that should appear on a drawing or purchase order for a Type 2 anodized part.
| Callout | What to specify |
|---|---|
| Process | Type 2 sulfuric anodize per the applicable standard |
| Film thickness | Target thickness and tolerance; functional vs. cosmetic areas |
| Color | Natural, black, or dye reference; sample approval required |
| Masking | Areas to be masked, with dimensions or notes |
| Contact points | Acceptable locations for electrical contact |
| Appearance | Class 1 or Class 2; scratch and handling limits |
| Inspection | Measurement locations, gauge type, acceptance criteria |
For machined parts where anodize is the final operation, tolerances may need to be adjusted to account for film growth. This is a conversation to have with both the machinist and the anodizer. If you are sourcing the machined part and the finish separately, the drawing should define the as-machined dimensions and the post-anodize requirements.
Suppliers operating under IATF 16949:2016 maintain documented process controls and traceability for automotive programs. If your part is destined for an automotive application, confirm that the anodizing supplier's quality system covers the scope of your program, and review their process control plan for the anodize line.
Related Services
For buyers specifying both machining and finish, it helps to source the machined blank and the anodizing from suppliers who can coordinate on the same drawing revision. Our Custom Aluminum Anodizing Services cover Type 2 sulfuric anodizing with color and masking options, and our Custom CNC Milling Services produce the aluminum parts that feed into the finishing line.
FAQ
What is the difference between Type 2 and Type 3 anodize?
Type 2 uses sulfuric acid and produces a thinner, dyeable film suited to decorative and corrosion-resistant applications. Type 3 (hard anodize) uses a different electrolyte and typically produces a thicker, harder film for wear applications. They are separate specifications and should not be used interchangeably.
Can Type 2 anodize be done in any color?
A wide range of colors is possible through dyeing, including natural, black, and custom colors. Color must be specified on the drawing, and a physical sample or color standard should be approved before production. Natural anodize varies with alloy and film thickness.
How does anodizing affect part dimensions?
The oxide layer grows both outward and inward, so roughly half the film thickness is below the original surface. Tight-tolerance features and threads may need masking or dimensional adjustment. State the film thickness and any critical dimensions on the drawing.
What should be included on a drawing for Type 2 anodize?
Include the process type, film thickness and tolerance, color, masking requirements, acceptable contact points, appearance class, and inspection criteria. Missing these details is a common cause of rework and assembly problems.
Next Step for Your Drawing
If you have a drawing that specifies Type 2 anodize, or you are still deciding on the finish, send the drawing and the application details. We can review the film thickness, masking, and contact point callouts and flag any gaps before production. For machined parts that will be anodized, include the as-machined tolerances and the post-anodize requirements so we can quote both steps together.