chemical blackening of stainless steel troubleshooting
This blog is specifically focused on Chemical blackening problems and solutions for stainless steel.
Activation
Stainless steel often requires a more powerful activator. Use Verzinkshop Metal Activator or 32% hydrochloric acid diluted 1 on 1 with water.
- Verzinkshop metal activator for stainless steel from the 400 series.
- dilute hydrochloric acid for stainless steel from the 300 series.
Dip at room temperature for 2 to 5 minutes. This removes the oxide repellent layer so that the black stainless steel oxide liquid can react. Immersion too much or for too long can lead to over-etching and pitting corrosion. Wear PPE and always mix acid with water – never water with acid!
Home Settings
black stainless steel oxide: for stainless steel 300 series you can start around 50 percent v v, for stainless steel 400 series often 25 to 33 percent v v. Dipping time 2 to 5 minutes at room temperature. Very passive surfaces can be tested with higher concentration if necessary.
Starting the reaction with very passive parts: let contact with the stainless steel object and ordinary steel take place in the black stainless steel bath, for example with a steel hook or a piece of steel wire in the basket. This can help the reaction.
Bath materials: do not use stainless steel baskets or stainless steel tanks in the stainless steel black bath. Choose plastic such as PP, PE or PVC for baskets and bins, and regular steel hooks for hanging. Stainless steel is suitable for cleaning, rinsing and sealing, but not in the black bath itself.
Rinse and neutralize
after acid or black stainless steel oxide, acid can still remain in threads, bores and cracks. Neutralize briefly and rinse again to prevent later brown corrosion.
- Preferably: 1 to 2 percent sodium bicarbonate baking soda, 10 to 20 grams per liter, 30 to 60 seconds dipping.
- Alternative: 0.5 to 1 percent sodium carbonate crystal soda, 5 to 10 grams per liter, 15 to 30 seconds dipping.
Rinse thoroughly again immediately afterwards. Do not allow to dry between blackening and applying the sealer, seal immediately.
Solution Compatibility
For stainless steel, only use Verzinkshop Black Stainless Steel Oxide. Verzinkshop Black Oxide and Black Oxide Ultra do not work on stainless steel.
Stainless steel passivation
the passive layer on stainless steel grows back within 24 to 48 hours. This can be accelerated with citric acid or nitric acid.
Problems with the chemical blackening of stainless steel
| Problem | Cause | Solution |
| No black staining or light gray | Surface not active or very passive | Provide thorough cleaning and:
|
| Spotty finish | Insufficient pre-treatment or carry over between baths | Extend activation, rinse thoroughly in a flow-through or overflow sink, move lightly during blackening. |
| Coating rubs off | Insufficient activation or too strong too long blackening | Activate again or longer, adjust concentration and time of the blackbath, work on clean parts. |
| Parts rust after treatment | Residual acid or water remained, water in the sealer | Neutralize briefly with baking soda, rinse well again, do not let dry before sealing, drain collected water at the bottom of the sealer tank. |
| White haze or stains | Hard water or mineral deposits | For the final rinse, use demi or distilled water and remove water droplets before sealing. |