Selecting the Best Anodizing Seal Process
Sealing of anodized aluminum is considered to be the most important of all the various steps in the anodizing process, and Coventya’s Michael McGinty explains how you seal ultimately determines the performance of the anodic oxide in its end-use environment.
Q: We’re looking to begin processing aluminum and are going to add an anodizing line in our shop. With all the different types of sealing out there, what is the best kind of seal process we should consider?
A: That depends on the types of parts you are planning to process, whether they require color or not, and the layout of the anodizing line you’re looking to build. Sealing of anodized aluminum is considered the most important of the various steps in the anodizing process. How you seal determines the performance of the anodic oxide in its end-use environment. Depending on the projects you expect to undertake in your new line (and of course availability of space), you could consider having multiple sealing bath types in your line.
After the anodic oxide layer is formed, there are a number of sealing options that exist. In some applications, an unsealed or partially sealed coating is specified for hard anodized coatings to improve bonding of adhesives or organic coatings. This method is often used for internal components to enhance lubrication, or in the case of specifications for hard anodizing with the belief that an unsealed hard anodic oxide layer has higher hardness than one that is sealed. Contaminants are easily absorbed into the unsealed oxide layer, therefore if working with unsealed coatings, limited storage time and special packaging must be considered.
If the work you are looking to perform with your anodizing line requires full sealing, the three most common methods of sealing are high temperature, mid-temperature, and room-temperature or cold sealing. High temperature sealing traditionally involves hot deionized water, with or without additive, or a high temperature bath containing metal salts. This bath achieves its seal through hydration of the anodic coating, causing the porous layer to close off through swelling. It is simple to maintain and is an environmentally friendly option as it contains no metallic ions. Hot sealing will yield results for uncolored anodized aluminum and parts that have been electrolytically colored. If you’re working with parts requiring high reflectivity, such as those that are bright dipped or electropolished, a hot water seal is typically used.
Hot water sealing can be more expensive than other sealing methods as maintaining the bath temperature above 200ºF can be costly. Parts require longer sealing times and high quality deionized or distilled water. Formation of sealing smut or bloom is also more likely when using this method. To help with this, some hot sealing bath additives allow the use of hard water or less than deionized water quality, and also helps prevent sealing smut.
Sealing dyed parts in a hot water solution causes dye-bleed or leaching, increasing costs and reproducibility of color shades can become difficult. When sealing dye colored parts, consider using a sealing solution containing metal salts such as nickel, magnesium or cobalt. Sealing in a mid-temperature metal salt solution prevents leaching of dyes, requires less time in the sealing solution and can be operated at temperatures that are around 20-30ºF lower than a hot water seal. This creates an advantage over hot water sealing with energy and cost savings for your shop. While mid-temperature sealing baths can be difficult to analytically control than hot water, the prevention of sealing smut and the consistent color appearance make mid-temperature seals a well-balanced choice.
Room-temperature or cold sealing offers an advantage over the previous seals because it operates at 75-85ºF. While this reduces the energy cost for sealing, cold sealing differs from high- and mid-temperature seals in other ways. Typical formulations of cold sealing chemistries are nickel-fluoride-based that functions to plug the pores, while simultaneously etching the surface of the anodic coating. This acts as a cleaning method for improving bonding and adhesion, while reducing the tendency for forming seal smut. Cold sealing is more challenging to control than hot water seals and sometimes requires a hot water post-seal rinse to help cure the sealing and allow for immediate quality testing. If you are looking for the highest quality seal, you should consider including a cold seal in your process. Some suppliers offer cold seals that produce no greenish tint, even at thicker anodized aluminum layers greater than 25-30 micrometers. Similarly, some cold seals are available without anionic surfactants that you need to consider if you are sealing for Mil-Spec work (MIL-8625-F). Cold sealing processes are being improved to meet the automotive industry standard for high alkaline resistance sealing at a 13.5 pH, which has always been a concern of anodized surfaces exposure to car wash fluids.
There are other sealing methods out there, such as Teflon sealing to improve lubricity and two-step sealing for corrosion resistance. Again, your choice in seal depends on the type of parts you plan to anodize and the color choices. Beyond that, whether you are targeting high quality spec work or if you are looking specifically for energy savings, there are many sealing options available for anyone processing aluminum parts with anodize today which should be helpful in planning for your new line.
Michael McGinty is the product manager aluminum surface treatment at Coventya.
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