BIZOMAT MAKEOS · Manufacturing
Anodising Time and Coating Thickness
How long must this part anodise to reach the coating thickness, and how much current is needed?
Anodic coating thickness from current density and time by the common “720 rule” for sulfuric acid anodising of aluminium, the time for a target thickness, the build-up and penetration split, and the rectifier current.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Current density | A/dm² | Anodising current density (typically 1.2–2 A/dm² for sulfuric acid, higher for hard anodising). |
| Target coating thickness | µm | Required total coating thickness. |
| Area on the rack | dm² | Total area being anodised. |
| Process efficiency factor | % | Correction from your own process records (100% = textbook rule; alloys, temperature and bath age change it). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Current density
- What it is
- The anodising current divided by the surface area being treated, as set on the rectifier for this load. It sets how fast the oxide coating grows.
- Units and format
- Enter a number in A/dm²: amperes per square decimetre. The form accepts 0.000001 A/dm² to 20 A/dm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 A/dm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Anodising time is inversely proportional to current density; rectifier current rises in proportion.
- Get it right
- Enter amperes per square decimetre: 1 dm² = 100 cm². Values in A/ft² must be converted first.
What this means: Target coating thickness
- What it is
- The total anodic coating thickness you need, as called up on the drawing or finish specification. About a third builds up on the surface and the rest grows into the metal.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0.000001 µm to 200 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Time rises in direct proportion to the target thickness.
- Get it right
- Specifications usually give total coating thickness; the dimensional growth per surface is only part of it.
What this means: Area on the rack
- What it is
- The total surface area of all parts on the rack that is exposed to the bath, including inside surfaces the solution reaches. Used only for the rectifier current.
- Units and format
- Enter a number in dm²: dm². The form accepts 0.000001 dm² to 1000000 dm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 dm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rectifier current rises in direct proportion to area; time is unaffected.
- Get it right
- Count every face the solution wets, including the rack contact area if it is not masked.
What this means: Process efficiency factor
- What it is
- A correction to the textbook growth rate, from your own process records. It reflects alloy, bath temperature, concentration and age; 100 % means the plain rule.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% to 150%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Time is inversely proportional to this factor.
- Get it right
- Set it by comparing measured coating thickness with the time and current used on past loads.
Method and formulas
- 720 rule: thickness (mil) = current density (A/ft²) × time (min) ÷ 720, which in metric is about 0.328 µm per A/dm² per minute.
- Time = thickness ÷ (0.328 × J × efficiency).
- About one third of the coating builds up on the surface and two thirds penetrates into the metal (sulfuric acid process).
- Example: 20 µm at 1.5 A/dm² takes about 40.682 min.
Limitations
- Rule of thumb for Type II sulfuric acid anodising; hard anodising and other electrolytes differ.
- High-silicon and high-copper alloys grow coatings more slowly.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set anodising time for a specified coating
- Size rectifier current for a rack
- Allow for dimensional build-up on toleranced parts
Who it is for
- Finishing supervisors
- Platers
- Design engineers
What you get
- Time to the target thickness
- Build-up per surface and penetration
- Rectifier current
Interpreting the result
- Check thickness with an eddy-current gauge and calibrate the efficiency factor.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1500–1516. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Coating thickness by lift-off
In Eddy-Current Inspection by the Numbers
What it covers: Measuring anodising on aluminium as lift-off, why coating thickness is not dimensional growth, calibrating with foils, a wrong zero that drove the process off target, and tolerance intervals.
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