Soft anodic films that dyed pale and faded in weeks traced back to a bath running 6°C hot at 40% over the current density target. Hardness and dye shade live inside the current and temperature window.
This intermediate guide sets the current density, temperature and time windows that build hard oxide and stable dye, with the verification that proves them.
Common Mistakes and How to Avoid Them
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Temperature above 22°C | Soft, powdery oxide | Cool the bath |
| 2. Current density below 1.2 | Thin, weak film | Raise CD per area |
| 3. Loading area guessed | CD wrong per part | Calculate the rack area |
| 4. Cooling lag in summer | Afternoon soft film | Pre-cool before the load |
| 5. Aeration off | Hot spots in the bath | Keep the agitation |
| 6. Rinse between anodize and dye | Drag-in contaminates dye | Hold the rinse quality |
| 7. Dye temperature drifted | Pale shade | Check the dye temperature |
| 8. Seal too short | Fading later | Hold the seal time |
| 9. No hardness check | Soft film shipped | Test per batch |
Best Practices
- Calculate current per rack area, not per guess
- Hold bath temperature inside 18-22°C
- Keep aeration on during anodize
- Verify film hardness per batch
- Check dye temperature and pH per schedule
- Log CD, temperature and shade result
Implementation Roadmap
| Step | Frequency |
|---|---|
| Calculate rack area | Per load |
| Set current density | Per load |
| Check bath temperature | Per batch |
| Hardness test | Per batch |
| Dye and seal check | Per shift |
Hard film and dye stability flow
- Calculate - Rack area first. (Wrong area, wrong CD.)
- Control - 18-22°C, 1.2-1.8 A/dm². (Heat softens the oxide.)
- Verify - Hardness and shade test. (Pale dye hides weak film.)
- Log - CD, temperature, result. (Unlogged runs repeat.)
Film hardness follows current density and temperature. | Dye stability follows pore quality and sealing.
Anodizing process window data
Reference values for sulfuric acid anodizing film and dye control.
| Parameter | Reference | Why It Matters |
|---|---|---|
| Sulfuric acid | 160-200 g/L | Check bath strength |
| Bath temperature | 18-22°C | High temperature softens film |
| Current density | 1.2-1.8 A/dm² | Drives oxide growth |
| Anodize time | Per film target | 8-15 µm typical |
| Film thickness | 8-15 µm | Per customer spec |
| Seal temperature | 96-100°C DI water | Nickel salt or hot water |
| Seal time | 2 min per µm typical | Short seal fades dye |
| Dye temperature | Per dye spec | Drift changes the shade |
Process window card
| Parameter | Window | Fault outside |
|---|---|---|
| Sulfuric acid | 160-200 g/L | Slow growth |
| Temperature | 18-22°C | Soft film |
| Current density | 1.2-1.8 A/dm² | Weak or burned |
| Seal | 96-100°C | Fading dye |
Case 1 - soft film and fading dye fixed in the temperature window
Scenario. A line's black anodized parts came out soft, scratching under the fingernail test, and dye faded after two weeks in the field.
Action. The bath was running at 28°C during afternoon loads with current density near 1.0 A/dm²; cooling was repaired, loads were recalculated, and a per-batch hardness test was added.
Result. Film passed the hardness test and the dye shade held for months; the afternoon soft-film pattern disappeared.
Case 2 - architectural profiles fading after sealing
Scenario. An architectural supplier saw dye fade on extruded profiles while the film thickness passed every check.
Action. The seal tank was running at 88°C with a short seal time; temperature was raised to 96-100°C and seal time was set to the thickness rule.
Result. Fading stopped; the seal temperature is now charted every shift.
Frequently Asked Questions
Why is the film soft?
Usually bath temperature above 22°C or current density below target; heat dissolves the oxide as it grows.
What current density works?
1.2-1.8 A/dm² for sulfuric anodizing, set per rack area; calculate, never guess.
Why does the shade fade?
Weak or poorly sealed pores release dye; sealing at 96-100°C locks it in.
How long to seal?
About 2 minutes per micron of film, in deionized water or nickel salt at 96-100°C.
Why recalculate per load?
A different rack area with the same amps changes the actual current density.
How do I verify hardness?
Fingernail and scratch tests as a gate, with instrument checks on the schedule.
What about summer heat?
Cooling lag raises bath temperature; pre-cool and check the temperature before the load, not after.
Does dye temperature matter?
Yes - drift changes the shade batch to batch; hold the dye spec and log it.
What is the fastest check order?
Temperature, current density, then seal; one variable at a time.
What Would You Like to Solve?
Describe your bath temperature, current density, film target and the shade or hardness failure. We can help you set the window and the verification for your line.