Conventional anodizing decorates; hard anodizing protects. The two processes share a name and a tank of sulfuric acid, but they are aimed at different problems. A standard decorative coat is a thin, translucent film that accepts dye and resists mild corrosion. A hard coat is a thick, dense, deliberately hard oxide layer whose job is to take wear, and the way you get from one to the other — cold electrolyte, higher current density, and a longer dwell time — changes everything about the part that comes out. This guide walks through what the Type III process actually is, how the thickness builds, what it costs, and what the machinist has to do before and after the tank so the coating comes out on size instead of on scrap.
What makes Type III different from a decorative coat
All anodizing builds an oxide layer on aluminum by passing current through the part while it sits as the anode in an acidic bath. What changes between a conventional (Type II) coat and a hard (Type III) coat is how aggressively the oxide is grown. Hard anodizing runs the sulfuric acid bath cold — near or just above freezing — and pushes a much higher current density through it. The low temperature keeps the oxide from dissolving back into the acid as fast as it forms, so the layer can grow thick and stay dense, while the high current drives the growth quickly.
The result is a film that is structurally different, not just thicker. A hard coat is denser and harder than a decorative one, and it penetrates the substrate as well as building upward: roughly half the total thickness grows into the aluminum and half stands proud of the original surface. That split is the single most important fact for a machinist, because it means a part grows by about half the coating thickness on every exposed face, and any surface that is supposed to keep a precise size has to be machined undersize before it goes in the tank.
The tradeoff is color and cost. Hard anodizing produces a dark gray to bronze to near-black finish that is difficult to dye in bright colors, and the process is more expensive to run because of the refrigeration, the longer cycle, and the tighter control the bath needs. Where a decorative coat might be a few microns and a few minutes, a hard coat is tens of microns and can run for an hour or more. You choose it when the part will see sliding contact, abrasion, or a corrosive environment — not when you need a specific Pantone.
How thickness builds, and what it costs
Hard coat thickness is specified as a range, and that range is the main cost driver. Typical engineering callouts land between 25 and 75 microns of total oxide, with the workhorse being roughly 50 microns. Because roughly half of that total grows into the part and half grows out, a 50 micron total coat adds about 25 microns to every exposed surface — enough to swallow a normal press-fit allowance if nobody accounts for it.
The relationship between thickness and time is not linear, and it is capped by the alloy. The coating grows roughly in proportion to the ampere-hours passed, but as the film thickens it becomes more resistive, growth slows, and the bath's ability to keep dissolving the oxide at the surface sets a practical ceiling. High-silicon and high-copper alloys do not hard anodize well: the silicon and copper phases do not oxidize into a continuous film, so the coating comes out porous, soft, or patchy. A 6061-T6 part hard anodizes cleanly; a 7075 or a casting with heavy silicon is more trouble, and a foundry alloy may be a poor candidate altogether.
| Factor | Type II decorative | Type III hard coat |
|---|---|---|
| Electrolyte temperature | Room temperature, roughly 18–22 °C | Cold, near 0–5 °C |
| Typical thickness | 5–25 microns | 25–75 microns, up to 100+ |
| Surface growth | Thin, mostly additive | Roughly half in, half out of the substrate |
| Hardness and wear | Moderate, primarily cosmetic | High, engineered for sliding and abrasion |
| Color | Clear, dyed easily | Dark gray to near-black, hard to dye |
| Cycle time and cost | Short, low cost | Longer, more control, higher cost |
Machining to the coating — the undersize problem
The coating does not care about your tolerance; it adds its thickness to whatever you machined. The discipline is to decide, feature by feature, whether a surface is functional after coating or cosmetic after coating, and to machine accordingly. A bore that will hold a bearing, a thread that will take a screw, a press-fit diameter that must stay put — all of these need to be cut undersize by the growth allowance, or oversize in the case of an internal feature, so that the finished coated size lands inside tolerance.
The growth allowance is not a single number you look up. It depends on the coating thickness, on the alloy, and on whether the surface is sharp or shielded, because corners and edges grow at a different rate than flat faces. Edges build up faster and more brittle, which is why hard-anodized parts should have broken edges and radii rather than knife edges that will crack the coating as soon as they are touched. Internal corners, small holes, and deep features anodize unevenly because the current density drops where the geometry crowds the electrolyte — a blind hole will coat thinner at the bottom than at the mouth unless the shop takes care with cathode placement and agitation.
Threads, holes, and the features that bite back
Threads are the classic hard-anodizing failure point. A threaded hole machined to its final size will come out of the tank with a smaller minor diameter and a rougher flank, and a screw that went in before coating will bind or cross-thread after. The fix has to be planned before the part goes to the anodizer: either cut the thread oversize by the growth allowance, or machine the part, hard anodize, and then chase or cut the threads after coating, leaving a bare thread in an otherwise coated part. The second route is common for functional fasteners because it gives a thread that assembles cleanly, at the cost of leaving the thread uncoated and therefore less protected.
Holes and bores carry the same issue, and the correction depends on what the hole is for. A through-hole that is only a passage can simply be machined oversize by the allowance. A precision bore or a dowel-pin location that must hold position and size is usually masked, machined after coating, or left deliberately undersize and honed to finish. Masking is the blunt instrument: a plug or a stop-off keeps the coating off a surface entirely, which is cheaper than post-machining a hard coat but leaves an unprotected edge where the masked and unmasked zones meet. Each of these choices is a cost and a durability decision, and none of them is free.
Sealing, and what the finish is good for
The as-grown hard coat is porous at its surface, and those pores are both a weakness and an opportunity. Left open, the pores let moisture and contaminants wick into the film and can hold onto them, which is why hard-anodized parts are normally sealed. Sealing closes the pores — either by hydration in hot water or steam, or by impregnating the film with a sealant — and it trades a little hardness and wear resistance for a big gain in corrosion resistance and stain resistance. The decision to seal or not depends on the application: a sliding bushing that wants maximum hardness may be left unsealed or sealed lightly, while a part going into a marine or chemical environment gets a full seal.
The finished hard coat earns its keep in specific places. It resists abrasion from sliding contact and grit, it holds up against corrosion in salt spray and industrial atmospheres, and it provides electrical insulation that a bare aluminum part does not. What it does not do is restore a part that was machined wrong: the coating is a skin, not a repair, and it will reproduce the surface it was grown on, tool marks and all. If the machined surface is rough or torn, the hard coat will be a hard, rough, torn surface. The finish underneath is still the machinist's job, and it is the last reason a hard-anodized part succeeds or fails.
- Decide functional vs. cosmetic per feature — only surfaces that must stay on size need an undersize allowance.
- Machine undersize by half the coat thickness — external faces grow up, bores close in, and both move.
- Break edges and radii — knife edges grow brittle and crack the coating.
- Plan threads before the tank — cut oversize or chase them after coating, never leave a final thread to be coated.
- Mask or post-machine precision bores — a dowel-pin location should not be left to the electrolyte.
- Match the alloy to the process — high-silicon and high-copper alloys hard anodize poorly.
- Specify seal for the environment — seal for corrosion resistance, hold back for maximum hardness.
"Hard anodizing is a machining decision wearing a finishing decision's clothes. Half the coating grows into the part, which means the part you put in the tank is not the part you get out, and by the time you discover that, the part is already coated and expensive. The machinist who thinks about the growth allowance before the first cut — who machines the bore undersize, breaks the edge, and decides what the thread will do — gets a part that comes out on size and stays there. The one who treats it like paint gets a part that looks right and does not fit."
Hard anodizing earns its place when a part has to survive sliding contact, abrasion, or a corrosive environment on top of everything else aluminum is good at. It is not a coating you bolt on at the end; it is a growth allowance you plan from the first drawing, a set of decisions about threads, holes, and edges, and a seal choice matched to where the part will live. Get the machining right underneath and the coating becomes a durable skin that protects the part for years. Get it wrong and the tank will faithfully deliver a hard, thick, precisely wrong layer over a part that no longer fits.
If you have an aluminum part that needs a hard coat — a wear surface, a sliding component, or a corrosion-critical housing — send the drawing over and we will plan the growth allowance, thread strategy, and edge breaks around the coating, from the machining approach to the finishing that puts the hard coat where it needs to be.