Aerospace MIL-A-8625 Aluminum Anodizing, Type III Hardcoat & CNC Pre-Plate Allowance
Aluminum Sulfuric Anodizing "720 Rule" & Growth Formula
Calculate Target Coating Thickness in Mils (0.001 in), Sulfuric-Bath Anodizing Tank Time via the 720 Rule (min), Total Titanium-Rack DC Rectifier Load (A), and Outside-Diameter Dimensional Growth (um).
Anodizing converts aluminum INTO Al2O3—50% penetrates inward and 50% grows outward, shrinking bores and swelling ODs by 1x total coating thickness!
Unlike electroplating (which deposits 100% of metal ON TOP of the part surface), MIL-A-8625 Type II decorative and Type III hardcoat sulfuric-acid anodizing electrochemically oxidizes the aluminum substrate itself into porous hexagonal `Al2O3`! Because `Al2O3` occupies roughly twice the molar volume of consumed metallic `Al`, `~50%` of Total Coating Thickness `t_ox` (um) penetrates INWARD into the parent metal while `~50%` builds OUTWARD per surface—meaning an external shaft OD grows by `2 * (0.50 * t_ox) = +1.0 * t_ox` (and a precision bearing bore SHRINKS by `-1.0 * t_ox`), forcing CNC machinists to pre-machine bores oversize and shafts undersize before hardcoating! Across aerospace finishing lines, tank cycle time follows the classic **720 Amp-Minute Rule**: `720` amp-minutes per square foot (`ASF * min`) grows `1.0` mil (`25.4` um) of anodic oxide, so `Time_min = (K_720 * (t_ox_um / 25.4)) / Current_Density_ASF`!
Verify on a MIL-A-8625 Type III Hardcoat Aluminum Load—Target_Total_Oxide_Thickness `t_ox` = 50.0 um (`1.969` mils in E2, B2), Refrigerated_Bath_Current_Density = 24.0 ASF (`2.58` A/dm², B3), Wetted_Titanium_Rack_Area = 14.5 ft² (B4), and Anodizing_Constant `K_720` = 720.0 ASF·min/mil (B5): Required Bath Cycle Time equals `59.06` minutes (E3), Total DC Rectifier Load equals `348.0` A (E4), and Shaft/Bore Diameter Shift (`2 * 50%` build) equals `50.0` um (`0.050` mm!, E5)!
| MIL-A-8625 Anodizing KPI (50 um Type III @ 24 ASF on 14.5 ft², K=720) | Formula | Verified Output |
|---|---|---|
| Target Oxide Thickness (mils) | Target_Oxide_um / 25.4 | 1.969 mils (0.709 mils @ 18 um Type II) |
| 720-Rule Tank Time (min) | (K_720 * (Target_um / 25.4)) / ASF | 59.06 min (34.02 min @ 18 um, 15 ASF) |
| Total Rack Rectifier Current (A) | Current_Density_ASF * Rack_Area_ft2 | 348.0 A (217.5 A @ 15 ASF Type II) |
| Shaft OD Growth / Bore Shrink (um) | 2 * (0.50 * Target_Oxide_um) | 50.0 um (18.0 um @ 18 um Type II) |
MIL-A-8625 Sulfuric Anodizing 720-Rule Cycle Time & Diameter Shift Formula
Validate `Target_Oxide` in `(1, 125]` um, `Current_Density` in `[8, 48]` ASF (`12–18` ASF ambient Type II; `24–36` ASF chilled `0 °C` Type III hardcoat!), `Rack_Area > 0` ft², and `Rule_Constant` in `[650, 820]` ASF·min/mil (`720` standard 6061-T6; `~760–780` high-copper 2024-T3 where acid dissolution lowers net efficiency)!
=LET(tx_u, B2, cd_a, B3, ar_f, B4, kr_c, B5, IF(OR(tx_u <= 1, tx_u > 125, cd_a < 8, cd_a > 48, ar_f <= 0, kr_c < 650, kr_c > 820), "Invalid Anodize Input", ROUND(2 * (0.5 * tx_u), 1)))Clear Type II Decorative Anodize Schedule Check (Set Oxide = 18.0 um & Current Density = 15.0 ASF)
Switch to an `18.0` um (`0.709` mil, E2) MIL-A-8625 Type II clear coat at `15.0` ASF (`B2 = 18, B3 = 15`): Bath Cycle Time E3 drops to `34.02` min, Rectifier Load E4 eases to `217.5` A, and Bore Shrinkage / OD Growth E5 drops to `18.0` um!
=IF(ISTEXT(E5), E5, ROUND((B5 * (B2 / 25.4)) / B3, 2))Try the example
Use the original values or change them to test your understanding.