Preparation of aluminum for powder coating
Chemical surface preparation
Surface preparation aluminum profiles before powder coating It is typically performed by dipping sections - on hinges or in baskets - baths in line, which contain various chemical solutions, as well as pure water for washing profiles between various baths. Most manufacturers of aluminum profiles used in preparing the surface for painting chromate - yellow or green, so it is this chemical treatment of surface profiles will be considered below.
The sequence of chemical processing
Typical chemical surface treatment of aluminum profiles for powder - and the liquid too - coloring bath comprises the following sequence – bath line:
- alkaline etching;
- deoxidation (clarification);
- flushing (double flushing);
- flushing (double flushing);
- powder coating;
- curing the powder coating.
Figure – Changing the thickness of the aluminum with powder coating
In step surface degreasing it removes oil, fats, grease and other contaminants aluminum profile surface, to get a clean surface prior to the etching step. The degreasing solution is added various inhibitors to prevent excessive etching of the metal during this operation surface cleaning.
The solution temperature in the bath is usually from degreasing 50 to 70 oC, and treatment duration - 3-4 minutes or more, depending on the surface state profiles.
Typically, after each working baths - baths with a chemical solution - followed by one or two rinsing baths. Appointment washes - to remove the chemicals from the surface of profiles and prevent their transfer to the next working bath.
In the early stages of processing - degreasing and pickling - usually, it is sufficient to use ordinary tap water. In the final stages washing water must be particularly pure, to avoid problems with paint adhesion or later - corrosion problems.
Therefore, in the last stage of processing is used Deionized water, which does not exceed electroconductivity 30 microsiemens per centimeter (μS / cm). This rinsing in deionized water is extremely important, so salts (usually calcium), contained in hard water, when it dries on the profiles can be a cause of corrosion starts and subsequent formation of bubbles in the finished painted surface.
For the first washing baths there is no temperature limitations, however, after chromating bath water temperature washes should be no higher 50 oC. If the wash water is too hot, it may dissolve and rinse with fresh chromate coating. Duration of washing is usually several minutes.
Cyclic movements or batches of baskets with profiles, and air sparging rinsing baths provide better quality rinse.
A thin layer of native oxide on the aluminum surface prior to chromating to be removed. The alkaline solution is generally added suitable inhibitors, to prevent excessive bleeding aluminum during this operation. temperature between 50 and 70 ° C is normal. etching duration is usually from 3 to 4 minutes or more depending on the surface condition profiles.
The bath is removed with lightening dark plaque profiles, which was formed in the step of undissolved alkali etching compounds. This plaque consists of those not soluble in alkalis, and metal compounds, as magnesium and silicon oxides, in the case of aluminum alloys 6xxx series, usually alloys 6060 and 6063.
This, usually darkish, plaque is removed from the surface profile by dipping them in an acidic solution of sulfuric or nitric acids with additives.
The duration of treatment in the bath is from lightening 30 seconds to 5 minutes depending on the composition and thickness of the removed plaque on profiles. Bath lightening typically has a room temperature.
Chromate is the most commonly used surface preparation processes of aluminum profiles for powder coating.
Chromate coatings serve two purposes, corrosion protection and adhesion of the paint to the aluminum surface. For aluminum, two main types of chromate treatment are used:
- chromating (yellow chromating) and
- phosphochromating (green chromating).
See. more Aluminum chromating: yellow and green
The thickness of the chromate coating
Chromate coating on the profiles provides a reliable adhesion between them and the surface of the paint and, Moreover, improves the corrosion resistance of colored profiles. If the chromate coating is very thick, it gives a very good resistance to corrosion, but its adhesive properties will be worse, thinner than the chromate coating. On the other hand, too thin chromate coating gives very good adhesion, but weaker corrosion protection.
Prior to applying the powder coating aluminum profiles must be perfectly dry. Moisture on the surface will prevent the powder spraying process. The temperature profile in the drying oven should be not too high. For yellow chromate coating maximum temperature should not exceed 65 oC, and for green chromate coating - 80 oC. If the drying temperature exceeds these limits, this leads to cracking of the chromate coating and reduce its resistance to corrosion.
See. also The quality control of coating powders: indicators, methods, standards - Guide