Chemical blackening of iron and steel troubleshooting
In general, most problems in the chemical blackening of iron and steel arise from
- Not rinsing properly and thoroughly, leaving acids behind. You can see this especially on the threads of bolts.
- Drying the object during the process. The object must remain wet from etching to sealing and must not be dried between steps. If this does happen, the black oxide coating or the metal will oxidize faster due to contact with oxygen.
- Sealer not applied sufficiently or rinse water not completely expelled.
It is very important that acids from any etching bath, or from the black oxide agent itself, are properly rinsed away. If acids remain, the metal will react to this during sealing or later and brown rust will occur.
Baking soda can be added to the final rinsing water, after the black oxide process and before the sealer, to neutralize residual acids. Then rinse the object well.
It is also important to make sure that, during sealing, the chosen sealer expels all the water and no moisture remains on the black oxide coating. Immerse the object in the sealer several times to expel the water.
Problems with the chemical blackening of steel and iron
| Problem | Cause | Solution |
| Irregular finish | Insufficient surface preparation | Provide thorough cleaning and degreasing. Mechanical preparation such as sanding may be required. With etching, you get a uniformly clean finish. |
| Black oxide finish too light |
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| Coating rubs off | Too thick black oxide coating | Shorten the process or dilute the liquid. |
| Spotty and non-uniform finish |
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| Bluish or grayish finish | Insufficient activation | Pre-treat the metal with an etching liquid such as Verzinkshop Metal Activator or another acid. |
| Parts rust after treatment |
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| Poor adhesion |
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| Thick Releasing Layer | Too long treatment | Shorten the immersion time in the black oxide solution or dilute the liquid. |
| Dark streaks | or markingsIrregular application | Make sure the parts are completely submerged and agitated. |
| Rapid discoloration | Insufficient rinsing | Rinse the object very well and do not let it dry between steps. |
| Residual oil after sealing | Excessive application of the sealer | Apply less sealer, shake the object well after sealing to remove excessive sealer. |
| White spots | Hard water or mineral deposits |
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| Corrosion stains during the process | Drying black oxide coating without sealer | Make sure that the object between the acid bath and the sealer is not dry; the object must remain wet from the acid bath onwards, otherwise it will oxidize due to oxygen. |
| Reaction stops prematurely | Solution Exhausted or Contaminated | Change the black oxide solution. |
| Spotty and not uniform black |
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| Bluish glow on parts or gray finish | Insufficient activation | Carbon steel, 4130 and parts that result in a grey finish may require more time in Galvanising Shop Metal Activator or the chosen acid bath. A longer time in the black oxide can also help. |
| Excessive abrasion | Improper blackening | Check the strength and duration of the blackening solution; adjust if too strong or too long. Make sure parts are clean before blackening. |
Tips for cold chemical blackening of metal
Checking the metal: Inspect the metal surface under adequate lighting to identify rust, shell, and contaminants that may interfere with the processes.
Degreasing the metal: use a solvent or alkaline detergent to remove grease and oil. Ultrasonic cleaning can be beneficial for complicated parts. Do not use an oil-based cleaner.
Remove rust: Apply rust removers or use mechanical methods such as sandblasting, wire brushing, or manual sanding for heavy rust.
Chemical activation and etching: treat the metal with an acid pickling solution to lightly etch the surface and improve adhesion. Pay attention to the concentration and time to avoid over-etching that can damage the metal. You can use Verzinkshop Metal Activator for this.
Solution preparation: ensure that the black oxide solution is mixed with distilled or demi water according to the specifications in the manual. Store it in a sealed container so you can use the liquid again later.
Chemical blackening applications
Immersion: For even coverage, fully immerse the component in the solution. Agitation can help to achieve an even layer.
Brush, sponge or cloth: for large parts or repairs, apply the solution with a brush for example, ensuring even coverage and minimal dripping.
Observe the color change and reaction progression. Adjust the immersion time and liquid strength based on the desired color depth.
Rinsing and neutralizing the parts
After an acid or black oxide treatment, acid can still remain in threads, holes and cracks. If you seal afterwards, it can cause brown corrosion later. Prevent this with a short neutralizing rinse.
Procedure
- Rinse thoroughly to remove residue.
- Neutralize:
- Preferred: 1 to 2 percent sodium bicarbonate (baking soda), 10 to 20 g per liter, 30 to 60 seconds dipping.
- Alternative: 0.5 to 1 percent sodium carbonate (crystal soda), 5 to 10 g per liter, dip for 15 to 30 seconds.
- Rinse well again immediately, preferably with demi water.
- Do not allow to dry between acid or black oxide bath and sealer; Seal immediately.
Do not rinse under the tap; Use a flow-through or overflow sink.
Sealing and finishing the conversion coating and metal
Oil finish: apply an oil for parts that require lubrication or rust protection. Provide full coverage. For example, use Verzinkshop DeepSeal Oil Metal Sealer, which dries dust-dry.
Wax finish: for decorative items, you can also use a clear wax to apply a protective layer.
Acrylic paint finish: Electrolyte shop Acrylic Sealer is a very thin satin to glossy acrylic paint that dries without structure and is almost invisible. Ideal for indoor applications.
2K paint finish: for items that are exposed to harsh conditions, a high-quality 2K paint offers durable protection.
Curing: Allow the seal to cure according to the product’s instructions. This step is crucial for durability and wear resistance.