Visual inspection
By eye, from 50–70 cm: colour uniformity, and the absence of stains, streaks, burn marks and mechanical damage. For architectural profiles, only minor shade variation within a batch is acceptable.
Quality
Anodizing quality control and defects
Anodizing does not hide defects in the metal - it makes them more visible. Which is why inspection starts at the bare surface of the part and finishes with tests on the completed coating.
70–80%
Between 70 and 80% of anodic coating quality is decided by the condition of the part before the process starts. Anodizing does not hide defects in the metal - the transparency and gloss of the oxide film make them more obvious, not less.
Surface requirements for incoming parts
01
No heat tint
Areas overheated during welding or cutting - the yellow-to-blue tint colours - have an altered oxide structure. The film over them comes out loose and dark, or does not form at all.
02
Edge and corner condition
Sharp edges concentrate the electric field: current density at a point rises by an order of magnitude, causing local overheating, a burnt film and wasted energy. Every external corner needs an edge break or chamfer of at least 0.2–0.5 mm radius.
03
Consistent machining direction
Grinding direction must be uniform across the whole cosmetic surface. Scratch marks of differing depth refract light differently, producing a clouded, blotchy appearance.
04
No contact marks on show faces
The metal deforms where the jig makes electrical contact, and a black mark is left there after anodizing. Contact points belong on non-cosmetic surfaces, or in areas with machining stock still to be removed.
Inspection methods
Inspection is governed by the applicable standards and by the process and design documentation. It divides into visual, instrumented and laboratory testing.
Coating thickness measurement
Non-destructive testing by magnetic induction or eddy current.
Hardness testing
For hard anodizing; measured on the Vickers scale.
Adhesion test
Tape applied and pulled: the film must not lift.
Seal quality (dye spot test)
A drop of methyl orange or acid red solution: if the pores are not sealed, the solution is absorbed and leaves a vivid stain.
Salt spray chamber
Accelerated corrosion testing: a sound architectural coating survives at least 1,000 hours without white corrosion products.
Defects and what causes them
Eight defects that recur on anodic coatings: how each one looks, and which process failure produces it.
Burn marks
01How it looks
Dark brown or black patches with sharply defined edges.
Principal causes
Excessive current density, poor jig contact, insufficient electrolyte cooling.
Bare spots
02How it looks
Uncoated aluminium, often following the outline of liquid runs.
Principal causes
Poor degreasing, silicone contamination, a surface that passivated before loading.
Colour variation and mottling
03How it looks
Uneven colour across parts from the same order.
Principal causes
Mixed alloy grades in one batch, inconsistent etching, copper contamination of the electrolyte.
Over-etching
04How it looks
A matt, spongy coating structure that rubs off under a finger.
Principal causes
Electrolyte above 22–25 °C on the sulphuric route, or excess dissolved aluminium in the bath.
White bloom
05How it looks
Matt whitish patches after withdrawal from the tank, or after sealing.
Principal causes
Hard rinse water - calcium blocks the pores - or pH drift during hot sealing.
Orange peel
06How it looks
A fine dimpled texture in place of gloss.
Principal causes
Machining that was too coarse, or the natural structure of a high-alloy material.
Crazing
07How it looks
Fine cracks in coatings thicker than 50 µm after drying.
Principal causes
A heavy part cooled too quickly; internal stresses in a thick oxide layer.
Smut retention
08How it looks
Darkening of a clear coating, particularly on copper-bearing alloys.
Principal causes
Inadequate desmutting after the alkaline etch: residual copper and manganese compounds.
Most defects are designed out, not inspected out
Cooling capacity, current density control and rinse quality are all settled when the line is engineered.