Sulfuric acid · anodic oxide
312 rule · solves any direction
Surface area
Count every wetted face — both sides, bores, and the jig below the waterline.
The sequence
Colour marks the kind of bath. Rinse in distilled water between all of them.
Even matte or satin finish, and it takes out machining marks. Anodising follows the surface exactly.
Media that has been through steel embeds iron, which shows up later as heavy smut — use the long etch if so.
Oil and fingerprints make the etch go patchy, and the times below assume a clean part. Gloves from here on.
Finish: rinse in distilled water.
75 g NaOH per 1 L water. Caustic into water, never the reverse.
Go by the surface, not the clock. Over-etching rounds off detail.
Finish: rinse in distilled water.
The etch leaves a black film of copper, silicon and iron that will not anodise.
Repeat until no grey shadow is left.
Finish: rinse thoroughly in distilled water.
12–18 % H2SO4 by weight. Supply negative to the cathodes — lead, aluminium or stainless — and positive to the part.
Run constant current and treat the voltage figure as a ceiling; it climbs on its own as the coating grows. Contact to the part must be aluminium or titanium, and tight.
Finish: rinse in distilled water.
Straight from the rinse into the dye, ideally 50–60 °C for 5–30 min. Longer time and thicker coating both deepen the colour.
Finish: rinse in distilled water.
Boil in distilled water for 20–30 min. The oxide swells shut, locking in the dye and the corrosion resistance.
Distilled only — tap water leaves a white bloom.
Before you start
Where the numbers come from
312 amp-minutes per square metre lays down one micron. Fix any two of thickness, current density and time; the third follows.
t = 3.12 × T ÷ J
T = t × J ÷ 3.12
J = 3.12 × T ÷ t
I = J × A
t minutes · T microns · J A/dm² · A dm² · I amps
Current efficiency near 85 % — the rest of the charge goes into oxide the acid dissolves again. A cold, clean bath runs slightly ahead of the number; a warm one falls behind. It is the metric twin of the imperial 720 rule, and the two agree within 2 %.
Below 0.7 A/dm² the acid dissolves the coating nearly as fast as it grows. Above 1.5 A/dm² thin edges burn — chalky, powdery patches. 1 A/dm² forgives a warm bath and a rough area estimate.
If the volts hit your limit early, the contact is poor or the area estimate is too low.
Hydrometer
Float a hydrometer in the bath at 20 °C and read the strength off here. Highlighted rows are the working range.
| H2SO4 % w/w | Specific gravity | °Baumé | g/L H2SO4 |
|---|---|---|---|
| 10 % | 1.066 | 9.0 | 107 |
| 11 % | 1.073 | 9.9 | 118 |
| 12 % | 1.081 | 10.8 | 130 |
| 13 % | 1.088 | 11.7 | 141 |
| 14 % | 1.095 | 12.6 | 153 |
| 15 % | 1.102 | 13.4 | 165 |
| 16 % | 1.110 | 14.3 | 178 |
| 17 % | 1.117 | 15.2 | 190 |
| 18 % | 1.124 | 16.0 | 202 |
| 19 % | 1.132 | 16.9 | 215 |
| 20 % | 1.139 | 17.7 | 228 |
Values at 20 °C. A warmer bath reads low — add roughly 0.0007 to the SG for every °C above 20.
Dissolved aluminium raises specific gravity too, so an old bath reads stronger than it is. Trust the hydrometer on fresh solution; once the bath has run a while, judge it by titration or replace it.