Surface pre-treatment
Surface preparation and metal pre-treatment referred to in the surface technology , the chemical or physical processing of metals before they painted , glued or enamelled be. Common processes include mechanical pretreatment processes (e.g. blasting ), cleaning and degreasing , rinsing and the build-up of inorganic conversion or passivation layers . The latter two include phosphating , chromating and modern chromate-free systems.
Pre-treatment process
Mechanical pre-treatment
Mechanical pre-treatment methods are used to remove coarse impurities such as rust , scale or similar solid impurities. The mechanical pretreatment can also be used for paint stripping . The methods described here mostly also enlarge the active surface, since the roughness is increased. Typical processes are grinding , brushing and blasting (with metallic or mineral abrasives).
cleaning
The workpieces must be degreased before further processing. Depending on the material, existing impurities, the required degree of purity and legal requirements for the chemicals used, degreasing can be carried out with solvents or aqueous cleaners. When cleaning with aqueous cleaners, cleaning the surface is often combined with pickling and the creation of a new, targeted surface structure, such as a phosphate layer.
Aqueous cleaners usually consist of builders , complexing agents and surfactants . Builders are used to adjust the pH value and at the same time remove oxide layers and impurities. Complex images lower the hardness of the water and prevent the precipitation of poorly soluble compounds. Surfactants improve the wetting of the surface by the cleaner and bind oils and fats . The cleaning with solvents is mostly done with chlorinated hydrocarbons (strongly declining), hydrocarbons and oxygenated hydrocarbons.
In terms of process technology, cleaning is usually carried out by spraying or dipping . The cleaning result can be improved with immersion cleaning using ultrasound or electrolysis .
Application of conversion layers
The degreasing is followed by the application of a conversion layer . Pure degreasing is usually sufficient for a clean surface, but conversion layers also increase the active surface due to their roughness. This means that the connection of the paint or adhesive already works better mechanically. It can be further improved by the applied substance reacting with functional groups on the surface of the conversion layer. This is decisive in the area of corrosion protection .
Typical processes for applying conversion layers are phosphating on steel, pickling on galvanized steel, chromating and anodizing on aluminum. Due to the increasing legal restrictions for layers containing chromium (applies in particular to yellow chromating), chromium-free pretreatment methods are becoming increasingly important.
Additional procedural steps
Flushing
After each chemical pre-treatment step, the rinsing process is of crucial importance. It is usually necessary to carry out one or more rinsing steps to avoid buildup. The last rinsing process before a new process step is therefore carried out with deionized water .
Intermediate drying
Before applying the paint or adhesive, the surface of the workpiece must be absolutely dry. The workpieces are dried in the adhesive water dryer. Depending on the quality requirements, blowing off with compressed air may also be sufficient.
Individual evidence
- ↑ Karl H. Adams; Surface pretreatment , Wiley-VCH; 1999; ISBN 3527297499
- ↑ a b c d e f Judith Pietschmann; Industrial powder coating. Basics, applications, procedures. Vieweg Verlag, Wiesbaden; 2003 ;, ISBN 3-528-03380-0
- ↑ a b c Hansgeorg Kollek; Cleaning and pretreatment ; 1996; Vincentz Publishing House; Hanover; ISBN 3878704348
- ↑ a b c ITW Gema; electrostatic powder coating; Query on December 21, 2008 ( Memento of the original from December 27, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
