Compute the anodic oxide layer in microns from current density and time. Switch between Type II sulphuric and Type III hard anodizing presets, or back-solve the time you need to hit a target thickness.
Anodic oxide growth follows Faraday's law of electrolysis. For practical plant use, the industry simplifies it to:
Theoretical Faraday yield is never reached because some oxide re-dissolves into the acid bath, and some current is lost to side reactions and ohmic heating. A well-controlled Type II bath runs at 80-85% efficiency. If you are seeing under-thickness on parts that should be in spec, dropping efficiency to 0.65 in the calculator usually matches what is happening in your tank.
If your bath is consistently producing under-spec coatings even at the right current density and time, the problem is usually in the bath chemistry, temperature control, or rectifier ripple. We can diagnose it on a single plant visit.
Get a plant auditAnodic oxide thickness in microns is approximated as K × current density (A/dm²) × time (min) × efficiency, where K ≈ 0.30 for Type II sulphuric anodizing at 20 °C and K ≈ 0.50 for Type III hard anodizing at 0 °C. Practical efficiency is usually 70-85% due to ohmic losses and re-dissolution of oxide in the acid bath.
Type II sulphuric anodizing typically runs at 1.2-1.8 A/dm² in 18% sulphuric acid at 18-22 °C, producing roughly 1 micron per minute at the upper end of that range with bath efficiency around 80%.
Type III hard anodizing typically runs at 2.5-3.6 A/dm² in 15% sulphuric acid chilled to -2 to 2 °C, producing 25-50 micron coatings in 30-60 minutes. Higher current density requires aggressive bath cooling to prevent burning.
Three common causes: bath temperature too high (re-dissolves oxide), aluminium alloy with high copper or silicon (lower CE), and poor electrical contact at the jig (current shadow). Drop calculator efficiency to 0.65 to model a struggling bath.
MIL-A-8625 Type II Class 1 (clear) requires minimum 1.8 micron; Class 2 (dyed) requires 7-25 micron typical. Type III Class 1 requires 50 micron nominal; Class 2 requires 25-50 micron dyed. Use this calculator to back-solve the time you need at your chosen current density. See our MIL-A-8625 anodising guide for full classification details.
Related: Anodizing plant cost calculator · Bath chemistry reference · Type II vs Type III comparison