Water Treatment Chemicals for Aluminum Coil Coating and Pickling - Metal - Aluminium
Coil coating makes use of a simple, but effective principle, to clean, pretreat and coat flat coils or sheets of steel or aluminium in a continuous operation, before other stages of industrial manufacture. Pickling is the cleaning part of this operation where acids are used to remove oxide layer and other impurities.
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Alkalin Cleaning
Cleaning is an essential part of the finishing process. The cleaning method can vary depending on the contaminant to be removed, amount of material to be removed, automation and overall effect on subsequent operations. Alkaline cleaning is needed for removal of oils, greases and particulates from fabricated items before they are electroplated, conversion coated or finished by other processes. Alkaline cleaning - Alkaline salts, usually caustic, silicates and phosphates, together with a balanced amount of surface active agents, constitute a very effective blend for metal cleaning.
Acid Pickling
The chemical pickling is an operation carried out to remove surface layers of a material by means of acid solutions. The surface is thus cleaned of rust residues formed from the hot-rolling operation such as surfactants (soaps that increase the wettability), corrosion inhibitors or other chemicals, mill scale and other contaminants, in order to make the surface of the metal suitable to be covered by another finishes. After the pickling, the treated surface becomes porous and allows the other materials to bond securely to the one below.
Phosphating
The process of phosphating aluminum and steel parts is typically listed as a conversion coating because the process involves metal removal as part of the reaction. The final surface is a layer of very fine phosphate crystals adhering to the surface of the metal.
Chromating
Chromating is applied on aluminium or its alloy, as a coating pre-treatment as it provides high-quality adhesion and excellent protection against corrosion. General method is to clean the aluminium surface and then apply an acidic chromium composition on that clean surface.
Chrome Free
Chrome free treatment is applied on aluminum and its alloys, surfaces instead of Chromating . The coat is based on titanium or zirconium oxides. While the bath contain also fluorides to form Al F3 complex
Passivating
Passivation is, usually, the final operation after coating (Phosphating, Chromating or Chrome free) and before to dry the coat. The scope of passivation is to improve both corrosion resistance and bonding capacity ( in case of final painting) . Passivation baths can be formulated with many chemicals such, Cr +3 , Cr+6 , Ti or Zr oxides , or organic products
Anodizing
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called anodizing because the part to be treated forms the anode electrode of an electrical circuit. Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than does bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference to reflected light.
Electrocolouring
After anodizing different colouring finish can be given by Electrocolouring and Immersion in organic dye solutions. This is possible because the anodize coating has microscopic pores that, depending on variables already described, allow aluminium to absorb dyes through immersion. Typically, the darker the color desired, the longer the anodize time is required.
Alkaline and acid rinses
After any chemical operation, water rinses are used to reduce the risk of introducing unwanted chemicals to subsequent baths. Rinses system may be optimized by using either a 2 or 3 cascading rinse tank configurations in order to maximize rinses quality and minimize consumprion of water.
Hot and Cold Sealing
In the Anodizing industry the final treatment is called ' SEALING' that seals the porous film created from Anodization . This operation is absolutely necessary for Anodizing ss the final step of the process . Sealing can be realized by Hot system or cold system depending on required quality and by Anodizing thickness. Sealing is not necessary if the Anodized Aluminium is sent immediately onto the Painting operation.
