Corrosion and Aluminum – Painting or Anodizing?

Does aluminum rust? No. Rust consists of iron oxides and hydroxides, so it forms on steel and cast iron, not on aluminum. Aluminum does corrode too, but differently: in air it almost instantly forms a thin, tight layer of aluminum oxide that protects the metal from further oxidation. Thanks to this layer, bare aluminum withstands ordinary atmospheric conditions well.

This natural protection has its limits, however. Road salt, marine air, contact with steel or copper in moisture, or strongly acidic and alkaline environments can break it down. That is why aluminum parts used outdoors or in aggressive surroundings are given additional protection – most often powder coating or anodizing. We explain how aluminum corrodes, what its resistance depends on and how to choose a protection method.

In brief

• Aluminum does not rust – rust applies only to iron and its alloys.
• It is protected from corrosion by an aluminum oxide layer a few nanometers thick, which rebuilds itself on contact with air after being scratched.
• The most common types of aluminum corrosion are pitting (chlorides), galvanic (contact with another metal) and crevice corrosion.
• Resistance depends on the alloy: alloys containing copper, e.g. EN AC-46200, corrode more easily than Al-Si-Mg and Al-Mg alloys.
• Castings with a high silicon content are usually better protected by powder coating than by anodizing.

Why Aluminum Does Not Rust

Aluminum is a highly chemically active metal, but that is precisely why it passivates quickly. A thin layer of aluminum oxide (Al₂O₃) a few nanometers thick forms on a fresh surface. It is tight and adheres well to the metal, so it cuts the metal off from oxygen and moisture. If the layer is scratched, it forms again on contact with air.

Aluminum oxide is stable roughly in the pH range of 4 to 9. In a strongly acidic or alkaline environment – e.g. in contact with fresh cement mortar, strong cleaning agents or certain chemicals – the layer dissolves and the metal begins to corrode uniformly across the entire surface.

How Aluminum Corrodes – the Most Common Types of Corrosion

Type of corrosion	When it occurs	Appearance
Pitting	in the presence of chlorides: road salt, marine air, water containing chlorides	white, powdery deposit and small, localized pits
Galvanic	when aluminum is in contact with steel, copper or brass and moisture is present between them	material loss in the aluminum around the joint, e.g., at bolts
Crevice corrosion	in crevices, under gaskets and washers, where moisture is trapped	localized material loss hidden in the crevice
In acids and alkalis	at pH outside the range of approx. 4–9	dulling and uniform dissolution of the surface
Filiform (under the coating)	under a damaged or poorly prepared paint coating	thread-like blisters under the paint, starting from an edge or a scratch

White deposit on aluminum consists of corrosion products – mainly aluminum hydroxides and oxides. It generally does not weaken the part, but it shows that the protective layer has been breached at that spot and that the part operates in an environment where an additional coating would be beneficial.

Corrosion Resistance Depends on the Alloy

Pure aluminum is more corrosion resistant than most of its alloys, but alloys are used for castings because they fill the mold better and are stronger. Their corrosion resistance differs markedly:

• Al-Mg alloys, e.g., EN AC-51300 – very good corrosion resistance, including in marine atmospheres.
• Al-Si-Mg alloys, e.g., EN AC-42000 – good resistance to atmospheric corrosion.
• Al-Si-Cu alloys, e.g., EN AC-46200 and EN AC-46500 – copper improves castability and strength but reduces corrosion resistance. Castings made from these alloys and used outdoors should be protected with a coating.

The composition and properties of the alloys we cast are listed on the page aluminum casting alloys.

How to Protect Aluminum from Corrosion

Powder coating

Powder coating is applied electrostatically to a grounded part and then cured in an oven, usually at approx. 180–200 °C. This produces a coating that is most often 60–120 µm thick and separates the aluminum from moisture and salts. Coating durability depends on surface preparation: degreasing, chemical surface treatment (passivation or a conversion coating) and a primer. Without them, the paint adheres poorly to aluminum, and filiform corrosion can develop under a damaged coating.

The advantage of powder coating is the full RAL color palette and a range of finishes – matte, gloss, textured. The coating also hides surface irregularities of the casting well. Its disadvantage is lower resistance to impact and abrasion than anodizing.

Anodizing

Anodizing is an electrochemical process that thickens the natural oxide layer – usually to 5–25 µm. The anodic layer is hard and abrasion resistant because it is part of the metal rather than a coating applied to it. However, the color range is limited to the natural silver shade and a few dyes.

For castings, it is important that the anodizing result depends on the alloy. Silicon, which is present in large amounts in casting alloys (from approx. 7 to 13% in typical alloys), does not anodize, so the layer on such castings usually turns out gray and uneven. Anodizing works better on profiles and sheets made of wrought alloys, while castings are more often protected by powder coating.

Design and assembly

Many corrosion problems are solved by the design itself: isolating aluminum from steel and copper with plastic washers or sleeves, avoiding crevices where water collects, and providing drainage where water could otherwise stand. When joining aluminum to other metals, insulated stainless steel fasteners or aluminum fasteners work best.

Powder Coating vs. Anodizing – Comparison

Feature	Powder coating	Anodizing
Corrosion protection	good with proper surface preparation	good; the layer is bonded to the metal
Abrasion and scratch resistance	medium – the coating may chip on impact	high – hard oxide layer
Colors and finish	full RAL palette, matte, gloss, textured	natural shade and a limited number of dyes
Castings with high silicon content	suitable	result usually gray and uneven
Typical layer thickness	approx. 60–120 µm	approx. 5–25 µm
Damage repair	possible locally	difficult – usually the entire part has to be re-anodized

How to Choose a Protection Method

• Operating environment – with road salt, in coastal zones and in the chemical industry, careful pretreatment and a coating matched to that corrosivity class are required.
• Alloy – castings made of alloys containing copper or with a high silicon content are usually powder coated.
• Appearance – if a specific RAL color or texture matters, the choice is simple: powder coating.
• Mechanical loads – under heavy abrasion, an anodic layer has the advantage.
• Joints with other metals – the coating alone is no substitute for isolation from steel and copper.

We write more about how coatings perform in outdoor weathering conditions in the article on powder coating of parts used outdoors, and about surface preparation – in the guide sandblasting or glass bead blasting before coating.

Frequently asked questions

Powder coating of castings in our foundry

We can powder coat the aluminum castings we produce right away – at the same facility, together with casting and CNC machining. Before coating, we degrease, passivate and prime the castings, and then powder coat them in the RAL color of your choice. We describe the details on the page powder coating of aluminum castings, and color selection – in the article on RAL colors for aluminum. Request a quote.