Surface treatment · failure and acceptance

Aluminium Profile Finishes: Choose by Failure Mode, Not by Process Name

A list of process names tells you what a finish is called, not how it will fail in your application. Finishing should be selected the way any other engineering decision is: from the failure you have to avoid, and the checks that will confirm you avoided it.

Processes: Anodising, powder, E-coat, mechanical
Decisive factor: Pre-treatment quality
Checks: Thickness, adhesion, corrosion
Read: 10 minutes

Short answer

Choose a finish by naming the failure you cannot accept, then specify the process that avoids it along with a measurable acceptance check. Anodising is an integral oxide layer that resists abrasion and wear-through; powder coating is a thicker barrier film whose life depends almost entirely on pre-treatment and cure; electrophoretic coating gives a thin, very uniform film. Whichever is chosen, the three checks that matter are coating thickness, adhesion and corrosion resistance, and the largest single influence on all three is surface preparation before the coating is applied.

[01]

Why a process list is not a specification

A typical finishing article lists the available processes: anodising, powder coating, electrophoretic deposition, polishing, brushing, chromating. That is useful background and it is not a specification, because it does not connect any of them to a consequence.

Processes differ in what they are, not in what they prevent. Anodising grows an oxide layer out of the aluminium itself, so it cannot peel and it is hard, but it is thin by comparison with a paint film and it can be scratched through. Powder coating is a polymer film applied over the metal, so it is thick and available in any colour, but it is a layer sitting on top and it depends entirely on what is underneath it.

Once the difference is stated that way, the selection becomes straightforward. If the failure to avoid is wear and abrasion, the integral layer wins. If it is colour, coverage of a rough casting or a thick barrier over a difficult substrate, the applied film wins. If it is uniform coverage on a complex section with recesses, the immersion process wins.

The question to ask first

Not “which finish” but “what does this part have to survive”. Abrasion, UV, salt, cleaning chemicals and impact each point to a different answer, and a finish chosen without naming the exposure is a guess with a certificate.

[02]

What each finish does, and how it fails

Every finish has a characteristic failure, and recognising them is most of the inspection skill. The table below pairs each process with the mode it fails in and the exposure that tends to produce it.

Finish	What it is	Characteristic failure	Exposure that causes it
Anodising	Integral oxide grown from the metal	Abrasion through the layer; poor sealing shows as staining	Sliding wear; inadequate sealing
Powder coating	Polymer film fused over the metal	Undermfilm corrosion spreading from a chip; chalking	Poor pre-treatment; prolonged UV
Electrophoretic (E-coat)	Thin, uniform immersion-deposited film	Thin coverage gives limited barrier life alone	Outdoor exposure without a topcoat
Polishing	Mechanical brightening, no added layer	Oxidises and dulls; shows fingerprints	Handling, humid or outdoor air
Brushing	Directional mechanical texture	Wear removes texture; repairs show direction	Contact wear, rework
Chemical conversion	Thin conversion layer, usually a pre-treatment	Too thin to protect on its own	Used alone outdoors

Two rows deserve attention because they are routinely misread. Anodising is often specified for outdoor exposure on the assumption that it is the most robust option; it is excellent against wear but the layer must be thick and well sealed for weather, and the specification has to say so. Powder coating is often assumed to be maintenance-free; it is thick, but when it is chipped on a poorly prepared surface the corrosion travels underneath it and lifts the film well beyond the chip.

[03]

Three checks that can actually be performed

A finishing specification is only as good as what can be inspected on delivery. Three measurements cover it, and all three are quick enough to do on a sample before a batch is accepted.

Thickness is measured with a gauge, using the method appropriate to the substrate and coating type. It is the single most commonly specified and most easily verified number, and it is where substitutions usually show up first: a coating supplied to a lower thickness class looks identical and performs differently.

Adhesion is assessed by a cross-cut test, in which a lattice is cut through the film to the substrate and the amount removed is compared against a classification. It is the test that reveals pre-treatment problems, because a film that is the right thickness and correctly cured will still lift if the surface beneath it was not prepared.

Corrosion resistance is assessed by accelerated exposure, most commonly a neutral salt spray test run to a recognised standard. It is a comparative test rather than a prediction of service life, and that distinction matters: it is excellent for comparing two samples and poor for telling you how many years a part will last.

CHECK 1

Thickness. Gauge it on a sample. Confirms the specified class was applied.

CHECK 2

Adhesion. Cross-cut and classify. Reveals pre-treatment failure.

CHECK 3

Corrosion. Accelerated exposure to a standard. Comparative, not a life prediction.

CHECK 4

Appearance. Agree the standard for colour and gloss before production.

Standards exist for each of these — cross-cut adhesion, neutral salt spray and coating thickness measurement all have published methods. Name the standard in the specification, because “adhesion tested” without a method is not a requirement anyone has to meet.

[04]

Pre-treatment: the part that decides the outcome

Ask a coater what determines whether a coating lasts and the answer will not be the paint. It will be what happened to the surface before the coating went on.

An aluminium extrusion arrives with a surface that has been through a die, a run-out table and a stretch, and it carries rolling lubricant, die pick-up, handling marks and a native oxide of variable quality. A coating applied over that is bonded to whatever happens to be on the surface. Cleaning, etching or conversion coating removes the variable layer and replaces it with a controlled one, and the quality of that step dominates the result.

This is why two quotes for “powder coated” can perform completely differently at the same thickness, and why a thickness check alone does not protect a buyer. Adhesion testing is what distinguishes a properly prepared surface from a merely coated one.

What pre-treatment does

Removes lubricant, oxide and handling residue
Produces a consistent, repeatable surface
Provides a key for the coating to bond to
Improves resistance to underfilm corrosion

What happens without it

Film lifts from a chip far beyond the damage
Adhesion fails the cross-cut test at correct thickness
Batch-to-batch results become unpredictable
Failures appear months after delivery

[05]

How to write a finishing specification

Most finishing disputes trace back to a specification that named a process and nothing else. Six items turn it into something that can be quoted, produced and inspected.

RFQ checklist — surface finishing

Exposure — indoor, outdoor, coastal, chemical, or washdown. Name it; it drives everything else.
Process and class — not just “anodised” but the thickness class required.
Thickness and tolerance — a number in microns, with an acceptable range.
Adhesion method and acceptance — name the test standard and the classification you accept.
Corrosion test and duration — name the standard and the hours, understanding it is comparative.
Pre-treatment — state the requirement; do not leave it to the coater’s default.
Colour and gloss standard — agree a reference before production, not after.
Masked areas — threads, machined faces and slots; state what must stay uncoated.

Ask for a coated sample before the first production batch, and test it. A sample is the cheapest way to find out whether a specification means the same thing to both parties.

Four ways a finish fails after delivery

Process named, class omitted“Anodised” can mean a thin decorative layer or a thick architectural one. Without a thickness class the supplier chooses, and the choice is usually the cheapest.

Pre-treatment left unspecifiedThe largest influence on coating life is not the coating. State the pre-treatment requirement or the coater’s default applies, and defaults vary.

Salt spray hours treated as a life predictionAccelerated corrosion testing is comparative between samples. It does not tell you how many years a part will last outdoors.

Masked areas not statedSlots, threads and machined faces get coated unless the drawing says otherwise, and coating in a slot changes what fits in it.

Related on this site

Aluminium angle profiles— sections commonly supplied with a specified finish
PG40 40 x 40 four-slot heavy profile— where groove masking matters
Strut profiles category— the section range
Anodised vs powder coated profiles— the two options compared directly

Terms used in this article

Anodising: An electrolytic process that grows an oxide layer out of the aluminium; integral, so it cannot peel.
Sealing: Closing the pores of an anodic layer after anodising; poor sealing shows as staining.
Cross-cut test: A method of assessing adhesion by cutting a lattice through the film and classifying removal.
Undermfilm corrosion: Corrosion spreading beneath a coating from a point of damage, lifting the film.
Chalking: Surface degradation of a coating under UV, leaving a powdery residue.
Conversion coating: A thin chemical layer applied to improve bonding and corrosion resistance before painting.

Questions buyers ask

Anodising or powder coating for outdoor use?

Both are used outdoors, and the deciding factor is the specification rather than the process name. Anodising needs a sufficient thickness class and good sealing; powder needs proper pre-treatment and a suitable polymer. State the exposure and the class.

How do I check a finish on delivery?

Three checks: thickness with a gauge, adhesion by a cross-cut test to a named standard, and appearance against an agreed reference. Corrosion testing is usually done on samples rather than on delivered parts.

Does a thicker coating always last longer?

Up to a point, and only if it is properly bonded. A thick coating on a poorly prepared surface fails by underfilm corrosion, which is worse than a thin well-bonded one. Adhesion matters as much as thickness.

Will coating in the slot stop my hardware fitting?

It can. A coating builds on the groove faces and reduces clearance. State whether slots are masked, or specify the section so the fit after coating is correct.

Can salt spray hours tell me the service life?

No. Accelerated corrosion testing is a comparative tool: it tells you whether sample A resists better than sample B. It does not convert into years of outdoor life.

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