Anodizing Types Explained: Type II, Type III, and Why Your Aluminum Parts Need the Right One

Last month a customer sent us a batch of 2,000 aluminum brackets that they'd had anodized at another shop. The brackets were black anodized (Type II) and they looked great - uniform color, no streaks, no bare spots. But the brackets were for a food processing conveyor, and within 6 weeks of installation, the black coating was wearing through at the contact points. Turns out the customer specified "black anodize" on the drawing without specifying the type. They got decorative Type II (5-15 microns) when they needed hard anodize Type III (25-75 microns). The rework cost them $12,000.

This happens constantly. Engineers write "anodize per MIL-A-8625" on the drawing and stop thinking about it. But that spec covers five different types of anodize, and each one produces a completely different coating. Using the wrong type is like specifying "paint" without saying whether you want house paint or epoxy tank lining.

What Anodizing Actually Does

Anodizing is an electrochemical process that converts the aluminum surface into aluminum oxide (Al2O3). It's not a coating applied on top of the aluminum - it's the aluminum itself, transformed. The part is submerged in an acid electrolyte (sulfuric acid for Type II and III, chromic acid for Type I) and connected as the anode in an electrical circuit. Current flows, oxygen ions migrate to the aluminum surface, and a porous oxide layer grows.

The key word is "porous." The as-formed oxide layer has millions of microscopic pores per square centimeter. These pores are what allow dye to penetrate (for colored anodize) and what give the coating its wear characteristics. Type II (sulfuric acid anodize) produces a relatively thin, porous layer that takes dye well but has moderate wear resistance. Type III (hard anodize) uses the same sulfuric acid but at lower temperature and higher current density, producing a thicker, denser layer that's extremely wear-resistant but doesn't take dye as uniformly.

The coating grows both outward (above the original surface) and inward (into the aluminum). Roughly 50% of the coating thickness is above the original surface and 50% is below. This means a 25-micron Type III coating removes about 12.5 microns of the original aluminum surface and adds 12.5 microns above it. If your part has a critical dimension, you need to account for this.

Type II: Sulfuric Acid Anodize (The Standard)

Type II is what most people mean when they say "anodized aluminum." It's the default for decorative and general-purpose corrosion protection.

Coating thickness: 5-25 microns (typical 8-15 microns for most applications). MIL-A-8625 Type II, Class 1 (non-dyed) or Class 2 (dyed).

What it's good for: Corrosion protection (outdoor and indoor), decorative coloring (black, blue, red, gold, clear, etc.), maintaining aluminum's natural metallic appearance, and providing a decent paint base. Type II anodized aluminum survives 200+ hours in ASTM B117 salt spray without visible corrosion.

What it's NOT good for: High-wear applications. The coating is relatively soft (about 200-300 HV) and will wear through at contact points, sliding surfaces, and threads. If your bracket slides against another part, or if it's handled frequently, Type II will wear through and expose bare aluminum.

Machining implications: The coating grows about 50% above the original surface. For a 12-micron coating, your part grows by about 6 microns (0.006mm) on each surface. This is within normal machining tolerance for most parts, so you don't usually need to adjust dimensions for Type II. Thread dimensions may need adjustment if the threads are tight-fitting.

Cost: Lowest of all anodize types. About $0.50-2.00 per kg of parts (batch pricing), depending on color and quantity. Clear (natural) is cheapest. Black is slightly more. Custom colors (Pantone matching) add cost.

Type III: Hard Anodize (The One You Actually Need for Moving Parts)

Type III is a completely different animal. Same basic chemistry (sulfuric acid electrolyte) but processed at lower temperature (-1C to +5C vs 18-22C for Type II), higher current density (2.5-4.0 A/dm2 vs 1.0-1.5 A/dm2), and longer process time. The result is a thick, dense, hard coating.

Coating thickness: 25-100 microns (typical 25-50 microns for most applications). MIL-A-8625 Type III.

Hardness: 400-600 HV (about 50-60 HRC equivalent). This is hard enough to resist wear from most mechanical contact. Hard anodized aluminum surfaces can run against steel without significant wear.

What it's good for: High-wear applications (sliding surfaces, pivot points, bearing bores), abrasion resistance, electrical insulation (the thick oxide layer has high dielectric strength), and applications where maximum corrosion resistance is needed (Type III's thicker coating provides significantly better salt spray performance than Type II).

Color: Natural Type III is dark gray to bronze, depending on the aluminum alloy and coating thickness. It can be dyed black, but achieving bright colors (blue, red) is difficult because the thick, dense coating doesn't absorb dye as readily as Type II. Most hard anodized parts are either natural gray/bronze or dyed black.

Machining implications: This is where people get into trouble. A 50-micron Type III coating removes 25 microns of the original surface and adds 25 microns above it. That's a net dimensional change of 25 microns (0.025mm) per surface. If you have a 50mm bore that needs to be 50.000mm after hard anodize, you need to machine it to 50.025mm before anodize. We account for this in our machining process when we know the part will be hard anodized.

Thread dimensions absolutely must be adjusted. A 50-micron coating on an M6 thread will make the thread too tight to assemble. We either pre-size the threads for coating allowance, mask the threads during anodize, or chase the threads after anodize with a tap.

Cost: 2-3x more than Type II. About $1.50-5.00 per kg of parts. The higher cost comes from longer process time, lower batch throughput (due to lower temperature and higher current), and tighter quality control.

Type I: Chromic Acid Anodize (The Aerospace One)

Type I uses chromic acid instead of sulfuric acid. The coating is thin (2-10 microns), soft, and provides moderate corrosion protection. Its main advantage is that the chromic acid electrolyte doesn't get trapped in crevices and joints (unlike sulfuric acid), making it suitable for assemblies that can't be fully rinsed.

This is primarily an aerospace process. If you're making parts for aircraft, you'll see Type I specified. For general industrial applications, Type II is almost always a better choice (better corrosion resistance, lower cost, available in colors).

The environmental concern with Type I is significant - chromic acid contains hexavalent chromium (Cr6+), which is a known carcinogen and is heavily regulated. Many manufacturers are moving away from Type I toward thin-film sulfuric acid or other alternatives.

Quick Decision Guide

The Dimensional Thing That Everyone Gets Wrong

Here's the mistake we see most often: the engineer designs a part with a 30.000mm bore, sends it out for hard anodize, and then finds out the bore measures 29.950mm after coating. The coating grew inward by 25 microns per side and the bore shrank.

Rule of thumb: Type II (12 micron coating) changes dimensions by about 6 microns per surface. Usually negligible. Type III (50 micron coating) changes dimensions by about 25 microns per surface. Definitely not negligible. You MUST pre-size critical features to account for coating growth.

We machine parts that will be hard anodized to a pre-anodize dimension that accounts for the specified coating thickness. The customer tells us the target coating thickness (or we calculate it based on the spec), and we adjust all critical dimensions accordingly. This is part of our DFM review - if you tell us the part will be anodized, we'll flag the dimensional implications before machining.

What About Non-Aluminum Parts?

Anodizing only works on aluminum. Period. There's no "stainless steel anodizing" or "titanium anodizing" in the industrial sense. Titanium can be anodized (it produces a thin oxide layer with interference colors), but it's primarily decorative and doesn't provide significant wear or corrosion protection. Stainless steel gets passivated, electropolished, or plated - not anodized.

If you need a hard, wear-resistant surface on a non-aluminum part, the options are: hard chrome plating (steel, brass), electroless nickel plating (steel, brass, copper), physical vapor deposition (PVD) coating, or thermal spray coating. Each has its own thickness, hardness, and dimensional characteristics.