Sometimes the cleaning operations is done with an Acidic bath instead of Alkaline bath. This is true for Aluminium processing prior to Phosphating, Chromating or Chrome free treatments.
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.
Any rinses following an Acidic step will be Acidic as a result of the Acid being carried over.
In a metal Finishing line, Activation is required prior to Zinc Phosphating operations in order to create germination centers for a good Phosphate crystals growth.
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.
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.
A very popular treatment process is Amorphous Phoshate (called also Iron Phosphate). It is a very thin layer, usually on steel surfaces, by using NaH solution at pH 4-4.5, to improve bonding and addhesion of paint and provided protection against corrosion. Amorphous Phosphate is used ONLY as pretreatment of Paint (powder, liquid or electrophoretic).
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.
Chemical colouring of metals is the process of changing the colour of metal surfaces with different chemical solutions. For Anodizing lines this can be an alternative to Electrocolouring.
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 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.
For superior rinse quality, and particulary for final rinses, Deionized water is necessary in order to prevent corrosion or paint damaging done by salts usually present in raw or tap water. A good Demi Rinse should be feed with water that has a conductivity of less than 10 microSiemens.
Deoiling is an operation to remove oils and fats from the surface before the Alkaline or Acidic Cleaning. This is used in Aluminium Cans, Aluminium and Steel Coil (before pickling or before cleaning) operations.
Die Cleaning is the operation to remove Al from dies used for extrusions of aluminium once they become unusable due to build-up of Al within the die. This operation is done in NaOH or KOH solution at 25-30%. Once the aluminium concentration reaches 6-9% the cleaning solution is discarded and a new bath is formulated.
Etching is a process for aluminium with alkaline chemicals such NaOH or KOH. This operation is the first step of an anodizing line that removes all surface oxides in order to prepare the product for its anodized layer. There is now a recent trend to use acidic etches for lighter etching requirements.
Is the electrolytic application of a layer of paint as protection coat against corrosion. There are two system: Anaphoretic or Cataphoretic. The Cataphoretic process is more widely used thanks to its superior corrosion resistance.
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.
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.
Is a finish that provides reduced friction when applied as Zinc, Calcium or Manganese Phosphate layers.
Manganese phosphates are used both for corrosion resistance and lubricity and are applied only by immersion. Manganese phosphating is used to reduce the friction properties of engine, gear, and power transmission systems. Some examples include screws, nuts and bolts, motor vehicle components in brake and clutch assemblies, casting interiors.
Is a new technology developed as an alternative to Phosphating where the coating is done by a very thin layer of oxides and organic products that give good performances in terms of bonding and corrosion resistances.
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.
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.
Treatment processes rinses
Some Nanotechnolgy treatment process require final rinses and some do not.
Is a particular version of Zinc Phosphate process where the addition of Manganese and Nickel improve bonding and corrosion resistance performances. This process is used in Automotive industry where strong corrosion resistance is required.
It is used in most operations where the corrosion resistance of finished product must be especially high, conversion coatings are applied using zinc phosphate . This process is widely used in the automotive industry and in certain sectors of the appliance and electronics industries.
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