The Three musts in pretreatment of aluminum for coating:
Clean
Etch
Seal (Passivate) (Note: Followed by Dry-Off Oven is recommended).
Disclosure: Consult with a Metal Finishing Industry-Experienced and Successful Pretreatment Technical Service Representative for your specific options on pretreating your aluminum processing requirements.
1) Cleaning and removal of all surface contamination that may inhibit adhesion or application of pretreatment or conversation coating is critical and normally accomplished with surface active agents (surfactants) built into a complex cleaner.
2) Aluminum, like many non-ferrous and plastic substrates, requires a mild microscopic surface etch prior to coating to promote improved adhesion. This can be accomplished simply and safely, with the right chemistry additives in your pretreatment chemistry. The industry standard chemistries used to microscopically etch aluminum have been primarily acids such as Phosphoric, Sulfuric or Chromic acids, many times used in combination, somewhere in the pretreatment process with a fluoride containing additive to promote the etch. Hydrofluoric acid was used in the past to etch and brighten aluminum. Hydrofluoric acid should be or literally could be your last choice to utilize. This is due to the extremely hazardous and potentially deadly handling concerns and extremely low human threshold limits. Consult with your Pretreatment Representative for the best etchant and or conversion chemistry available for your specific process requirements.
3) The Seal is many times the last process applied prior to a low conductivity, Deionized, Distilled or Reverse Osmosis conditioned water rinse. The purpose of the Seal process is to completely passivate and seal any microscopic voids on the surface from the environment prior to coating. This helps to additionally improve adhesion and inhibit corrosion. The Seal process can be accomplished in many ways with today’s technological advancements. In some of the latest pretreatment technologies, the seal and passivation steps are accomplished in a single step during the conversion coating application process. The chemistry can be comprised of Hexavalent Chrome (which is a known carcinogen in the hexavalent state), Tri-valent chrome (which is thought to be a safe alternative to the hexavalent chrome), non-chromic seal such as molybdate and other organic, inorganic and nano-polymer products are commercially available.
Note: Most Finishing Job Shops and Coaters have a curing oven in-house, if you do not have the resources to purchase a dry-off oven to immediately follow the pretreatment process, I recommend the use of your Powder Coating Oven to quickly (10 minutes or less) dry any moisture from the pretreatment process prior to coating, that may create potential coating defects and coating rework.
Writer: Samuel Miles, Owner of www.MilesChemicalSolutions.com A company focused on providing the Finishing Industry with eco-friendly and affordable, concentrated Coating Removal Solutions to re-claim coating rework.
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