Hard Anodising Inspection and Quality Checks

Hard Anodising is a surface treatment that is frequently specified for engineering parts. It has been used for many years in industries such as aerospace, engineering, marine and construction. It is one of the hardest surface treatments that exist and is often specified when other coatings will not provide sufficient wear resistance.

Measure Coating Thickness

Use eddy current measurement with a gauge such as a Fischer or Elcometer instrument. Typical engineering specifications for hard anodising on aluminium are 25-75 microns plus or minus the figure shown on drawing tolerance. Measure thickness at several points around the part including edges and recesses as these can have quite different thicknesses.

Verify Hardness If It Matters

Coating hardness is typically quoted in HV (e.g. 350/500 HV for hard anodised aluminium parts). The hardness can be measured and verified by the processor by means of Vickers microhardness tests on a cross-section witness sample produced in the same batch as the finished parts. This method is more reliable than surface hardness tests that take into account the substrate material beneath the coating.

Check for Surface Defects

For Hard Anodising surface defects such as pitting, streaks, bare areas, and powdery or other surface defects must be inspected. Defects caused by overheating of the coating such as powdering must be particularly checked. The surface must be compared against a reference sample, or against the specific requirements for surface defects for aerospace parts as laid down in BS EN 2536.

Confirm Dimensional Tolerances

The anodizing process typically grows the coating about half its thickness into the metal and the other half above it. Hence a coating of 50 micron thickness would result in the metal being thickened by about 25 microns on each face, hence it is important to check what stock was left on the part prior to processing in order to ensure the part will meet close dimensional tolerances.

Request Process Documentation

There is more on Hard Anodising at www.poeton.co.uk/surface-treatments/anodising/hard-anodising/. This information should enable specific verification or, where traceability exists for safety-critical products, linkage to process runs during which component(s) in question were manufactured.

A few minutes checking these points on receipt of the anodized parts will prove to be far cheaper than subsequent failure in service.