ASK THE EXPERT: Nickel Smut After Sealing? Don’t Replace the Bath Yet

Jeff Boehmer, Director of Business Development at Columbia Chemical, and Marcello Rossi, International Technical Sales Manager with Alufinish, explain what causes nickel smut on anodized parts following the nickel sealing process and expand on how to troubleshoot.

Products Finishing

Expert Clinic/Plating

Jeff Boehmer, Director of Business Development ~ Columbia Chemical

Marcello Rossi, International Technical Sales Manager ~ Alufinish

Q. We operate an aluminum anodizing line, and we have been noticing nickel smut or powder on the surface of the parts after the nickel seal process. What causes this and what can we do about it?

A. Nickel sealing is an important step in the aluminum anodizing process, as it helps to close the pores in the anodic oxide layer and improve the overall performance of the finish. However, when nickel smut appears on the surface after sealing, identifying the root cause can be a challenge.

What you will see
Nickel smut will often appear as a powdery or hazy, loose residue on the surface of anodized parts following the nickel sealing process. It is a result of nickel salts precipitating out of the bath and onto the surface of the part.

What causes it?
There is not necessarily one simple cause of nickel smut. While the inclination may be to look at the nickel seal as the source of the problem, it’s important to remember that sealing is only one step in a sequence of chemical processes, and conditions established during anodizing and coloring can influence how the part reacts during sealing.

A systematic approach to troubleshooting can help identify the source rather than simply replacing the sealing bath. Begin by confirming that the anodizing and coloring processes are operating within their expected parameters. If both are performing properly, you can then narrow the investigation to the nickel sealing step.

Anodizing conditions can play a major role

An important consideration is the quality of the anodic layer. If the anodic coating is soft, or otherwise below the required quality, the high temperature of the nickel seal — about 205°F — can dissolve aluminum from the coating more readily. The dissolved aluminum then reacts with the sealing solution and contributes to deposits on the part surface.

What causes this softer layer? A softer anodic layer may form when the desired thickness takes too long to build. From an operational perspective, it is important to note when the required oxide thickness is being achieved, but the process is taking longer than expected. For example, reaching 15 microns in 45 minutes instead of 35 minutes exposes the coating to the acid longer, allowing more corrosion. Low current density or higher bath temperature can slow coating growth and increase this effect, producing the same thickness but a softer, lower-quality anodic layer. In turn, because a softer anodic layer dissolves more easily, it increases the amount of aluminum available to form deposits of hydroxides on the surface as powdery smut.

If you discover your anodic layer is too soft, closely monitor your current density. Some shops regulate anodizing primarily by voltage rather than measuring current density
directly. However, be aware that changes in solution conductivity or temperature may alter current density even when the voltage remains unchanged. The objective is to consistently produce the required oxide thickness under comparable process conditions, rather than simply achieving the target thickness by extending the cycle time.

Importance of coloring and rinsing phases
Material from previous process steps such as coloring can enter the sealing tank through carryover or inadequate rinsing. For example, if tin-based e-coloring is used, tin can enter the
sealing solution. Nickel bearing coloring may also result in residue on the surface. A high-quality rinse phase is an important step to ensure contaminants are not carried into the seal.

Nickel seal conditions
Once the anodizing and coloring processes are confirmed to be operating correctly, you should focus on evaluating the sealing process itself. The primary parameters to review include:

Nickel or additive concentration. Commercial nickel seal formulations may contain nickel salts, surfactants and other additives that help support the sealing reaction, keep the
surface clean and improve process performance. The balance of these components affects the quality of the seal and the potential for surface defects.

Not all nickel seal chemistry is created equal. There are nickel sealants that have been developed with specialized wetting agents designed to enhance alkali resistance, bleed prevention or provide faster sealing times, and each chemistry has its own proprietary balance. The concentration of these levels can go out of range, especially when additions are done manually, which may result in rejected parts. It is important to pay attention to the recommended concentrations provided by your specific chemistry supplier.

pH. High pH levels will precipitate nickel hydroxide at a faster rate onto the surface; lower pH levels will impact pore
closure and corrosion resistance.

Temperature. It is important to maintain the correct temperature in your tank, or you run the risk of corrosion resistance issues, leaching of dye, incomplete sealing and potential instability in the bath.

Dip time. Shorter dip time won’t allow for proper pore closure and can lead to bleeding of dyes and failed corrosion resistance, whereas longer dip time can cause chalking or buildup on the surface and increase the risk of surface spotting or staining. Examine your dip time factor and tighten your operating window.

When it comes to nickel smut or residue on the surface of a part, in some cases, the problem occurs immediately. In others, a newly made-up nickel seal tank may produce good results for several days before smut begins to appear. The timing of the problem can provide an important troubleshooting clue. If the sealing solution works properly for several days and then begins producing smut, something in the process or solution is changing. This could be contamination from previous process steps, consumption of an additive or a component reaching critical concentration.

Once the cause has been identified, corrective action should focus on the specific process condition, rather than simply replacing the nickel sealing bath. It’s easy to just change the sealing bath and put new chemistry in, but you must consider that it is not just the cost of chemistry, it is also the energy costs, staffing costs, waste treatment, wear and tear on the air handling equipment and more. Relying on a trusted chemical supplier who understands the process and will direct their energy toward helping you diagnose the root problem and implement solutions to extend the life of your chemistry — that is the valuable difference between a partner and a vendor.

Troubleshoot the process, not just the bath
To recap, anodizing is a sequence of chemical processes which can impact one another. So when nickel smut appears on the surface of your parts, first ensure your anodizing and coloring steps are operating within the proper ranges. Monitor your process time — if your current density is changing because of variations in conductivity or temperature, those conditions need investigation. Once that has been addressed, look at the nickel sealing step and evaluate factors such as concentration, pH, working temperature, and dip time.

Remember, the goal is not simply to correct the immediate defect, but to understand why it occurred and improve the process so the chemistry can perform consistently over time. Process stability issues can primarily be avoided through proper system setup, consistent attention to operating parameters, preventive maintenance and testing, and a strong working relationship between the client and chemistry supplier. This approach will result in efficient operation and consistent part quality.

This article was published in the October 2026 issue of Products Finishing magazine.