Context

Finishes are not cosmetic on precision parts. They change dimensions, fatigue life, corrosion risk, and surface friction. The base alloy decides which finishes are safe and how much you can trust the resulting surface.

Finish compatibility is a function of base alloy, hardness, and oxide growth. That means anodize thickness and coating adhesion should be planned with tolerance impact in mind.

The Trap

The trap is treating anodize, hardcoat, or nickel as a universal add-on. 6061 anodizes cleanly. 7075 often produces a more porous layer, with higher risk of micro-cracks and fatigue loss. Hardcoat can cut endurance by 30-50% on 7075 if you do not compensate with prep.

The Geppetto Take

We plan finish before tolerances. If the part needs tight fits, we size the bore for the finish growth. If it needs fatigue life, we avoid heavy hardcoat unless shot peening or alternative coatings are in the plan.

Evidence / Data

  • Type II anodize is typically 5-25 um thick and causes moderate fatigue reduction.
  • Type III hardcoat is typically 25-100 um and can reduce fatigue strength by 30-50% in 7075.
  • 6061 produces a more uniform anodic layer; 7075 is more porous due to higher Zn/Cu.

Control Actions

  • Specify anodize type and thickness in the drawing.
  • Apply the 50/50 rule: half the coating grows outward, half inward.
  • Shot peen before hardcoat when fatigue is critical.
  • Use 6061 for parts where surface finish consistency matters more than strength.

Checklist

  • Define finish type and thickness.
  • Reserve dimensional allowance for coating growth.
  • Choose alloy based on finish compatibility.
  • Confirm post-finish inspection plan.

What to Send

Send base alloy, finish type, thickness target, and all fit-critical dimensions.

FAQ

Why does 7075 anodize worse than 6061?

Its alloy chemistry leads to a less uniform oxide layer and higher crack risk.

Does hardcoat always improve durability?

It improves wear resistance but can reduce fatigue life.

How much growth should I expect from anodize?

Use the 50/50 rule unless your finisher specifies otherwise.

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