Anodized Aluminum Corrosion Resistance: A Buyer's Guide

Anodizing is the most common way to protect an aluminum profile and make it look good at the same time. It is also one of the most loosely specified finishes in the industry - plenty of purchase orders simply say "anodized" and leave everything that decides corrosion performance to the supplier. Here is what actually drives the result, and how to write it down.

Why Anodizing Resists Corrosion: a Layer That Grows Out of the Metal

Anodizing is not a coating. Paint and powder sit on top of the aluminum; anodizing converts the outer surface of the aluminum itself into aluminum oxide - a hard, ceramic-like layer that is grown from the metal rather than glued to it. Because there is no interface between metal and film, an anodized surface does not peel or flake the way a coating can.

That oxide layer grows with a structure of microscopic pores, which is what allows it to be colored. Those same pores are also the weak point: if they are left open, moisture and chemicals can travel down them and attack the metal underneath. Closing them is a separate production step called sealing.

So corrosion resistance is a function of three variables, not one: film thickness + sealing quality + alloy choice. Two parts both described as "anodized" can behave completely differently in the same environment.

Thickness Is the Number That Matters: the ISO 7599 Classes

Under ISO 7599, anodizing is specified by class, and each class is defined by a minimum average film thickness in microns. This is the single most useful line on an anodizing specification:

A practical way to choose: interior and dry service = AA10-15; general outdoor exposure = AA15-20; within a few kilometers of the coast or in an aggressive industrial atmosphere = AA20-25, and in that case insist on hot sealing. Those environment categories are defined in ISO 12944 (C1 to CX), the reference most European specifiers use.

Sealing and Coloring Decide How It Ages

Sealing is where suppliers save money quietly. Hot-water or nickel-acetate sealing closes the pores properly and gives durable exterior performance. "Cold" sealing is faster and cheaper, but it is generally considered a lower-durability option for exposed architectural work. If your parts will be outside, say "hot sealed" in writing.

Coloring method matters too. Dye coloring sits in the pores and can fade under UV. Electrolytic (two-step) coloring produces metallic tones - champagne, bronze, black - that are considerably more lightfast, so specify the method for sun-exposed work, plus a color tolerance (Delta E) on repeated orders.

If documented performance is required, name the test: thickness per ISO 2360 or ASTM B244, and agree the salt-spray method (ASTM B117 or ISO 9227) and acceptance criteria up front.

Alloy Choice Changes Both Appearance and Performance

The anodic film grows out of the metal, so the metal's chemistry shows through:

• 6063 - the architectural alloy. Low copper content, fine extruded surface, and it anodizes to a bright, uniform clear finish with minimal preparation. First choice when appearance and corrosion resistance matter.
• 6061 - higher in silicon, iron, copper, magnesium and chrome. Clear anodizing normally comes out darker and grayer, and the film is somewhat less corrosion resistant than on low-copper alloys. Perfectly acceptable for industrial and functional parts, less so for a bright architectural finish.
• 2xxx and 7xxx series - high copper, harder to anodize, softer film, lower corrosion resistance. Avoid them for decorative or architectural work.

Practical consequence: keep alloy, temper and supplier stable across the project - the same nominal alloy from a different heat can anodize to a visibly different shade, a leading cause of colour complaints on multi-batch orders.

What Actually Damages Anodized Aluminum in Service

Anodized aluminum outperforms most architectural metals, but it has real enemies:

• Alkaline materials. Mortar, cement, plaster and strong alkaline cleaners attack the film quickly. Wipe splashes off at once and flush with clean water - once dried-on mortar has marked the surface, the damage is usually permanent.
• Masonry cleaning acids. Muriatic (hydrochloric) acid on brickwork is a common cause of stained anodized window and facade profiles - keep it off, or mask the profiles.
• Abrasive tools and heat. Steel scrapers and abrasive pads scratch the film, and cleaning sun-heated aluminum accelerates any chemical reaction - clean early in the day or in shade.
• Cut and drilled ends. Anodizing covers the extruded surface, so every face cut or milled afterwards exposes raw aluminum. That is normal, but in coastal or chemical service design so bare cut faces are not sitting wet and salt-loaded, or plan a touch-up.
• Galvanic contact. In wet conditions, direct contact with stainless steel, copper or carbon steel can drive galvanic corrosion. Use isolating washers or a suitable barrier.

Day-to-day maintenance is simple: mild detergent, water and a soft cloth.

How to Write Anodizing on Your Purchase Order

A complete specification reads something like: "Alloy 6063-T5, anodized per ISO 7599, class AA20, natural (or black), electrolytic coloring, hot-water sealed, film thickness verified per ISO 2360." In addition, state:

1. Alloy and temper, kept stable across batches
2. Anodizing type - Type II (sulfuric) or Type III (hard) if you need wear resistance
3. Thickness class plus the acceptance range you will measure
4. Color, coloring method and a Delta E target for repeated orders
5. Sealing method
6. Cosmetic-critical faces, and where rack marks are acceptable (racking always leaves small contact marks)
7. Cut tolerance (EN 755-9 is the usual reference) and machining tolerances
8. Documentation: batch certificate, EN 10204 3.1 if required, RoHS / REACH declarations, first article report

How SYHARVEST Produces Anodized Profiles

SYHARVEST Aluminium runs extrusion and anodizing under one roof in Foshan, Guangdong. Eight extrusion presses from 660 t to 3600/4000 t cover billets of 90-254 mm and profiles up to roughly 320 mm in circumscribed circle, in 6063, 6061 and 6005. Our horizontal anodizing line handles clear, matte and colored finishes, with a vertical powder coating line and optional electrophoresis behind it, and eight CNC machines handle drilling, tapping and milling - so a profile can be extruded, anodized and machined in one controlled supply chain.

Extrusion tolerances follow EN 755-9, and batch certificates or first article reports are available on request. Typical lead times: mill-finish blanks in 5-7 days, profiles with surface treatment within about 12 days, and parts requiring machining in 15-18 days.

Getting the Right Finish First Time

Tell us where the parts will live - interior, exterior, coastal, industrial - and we will recommend the anodizing class, sealing and alloy that fit the environment instead of defaulting to a generic finish. Send your drawings (PDF, DXF or STEP) with alloy, color, quantity, cut length and packaging, and our engineering team will reply with pricing and a written anodizing specification you can sign off.