Preparation of surfaces before painting
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
The performance of a painted part is not only guaranteed by the choice of paint and the mastery of the painting process. The vast majority of painted parts receive a preparation or a surface treatment before painting. It is the TS / paint couple that ensures the performance of the part.
The purpose of this sheet is to review the most commonly used treatments on steel and aluminum alloys before painting. More information on these treatments is available in the literature and they will not be detailed in this sheet.
PREPAINT PREPARATION TREATMENTS ON ALLOY AND NON-ALLOY STEEL
1.1 MECHANICAL TREATMENTS
The most basic treatment is a mechanical preparation by sandblasting (not to be confused with microblasting).
Angular particles are projected on the surface and by abrasion clean it and give it a roughness favorable to the mechanical adhesion of the paint.
This technique implies a great control of the surface cleanliness and a very short delay between sandblasting and painting. It is not recommended for parts with savings, and also requires the use of a very good primer (first layer of paint in contact with the steel).
1.2 CONVERSION TREATMENTS (PHOSPHATATIONS)
The most generic bath treatment is Zinc or Zinc Calcium Phosphatization. This treatment leads to the crystallization of mixed iron and zinc phosphates on the surface of the steel, which creates a micro-roughness conducive to the mechanical bonding of the paint. It is reserved for fully painted parts.
Amorphous phosphating, also known as iron phosphating, phosphating impregnation is a treatment generally carried out by spraying. It is widely used in automatic lines, including phosphating, painting and drying. It is particularly well adapted to the use of powder paints.1.3 ELECTROLYTIC PAINT TREATMENTS
Cataphoresis or anaphoresis are also preparations before painting. These treatments, widely used in the automotive industry, consist of applying an integral primary paint to the part. Generally based on epoxy, it is compatible with a large majority of primers (anti-corrosion paint layer applied before the top coat). It should be noted, however, that these treatments are preceded by a tri-cation phosphating or a phosphating impregnation.1.4 CASE OF PARTIALLY PAINTED PARTS
All the previous treatments imply that the parts are completely painted because they have no capacity to resist corrosion. In the mechanical industry, it is frequent that parts are partially painted. In this case, either another treatment (Chemical Nickel, Hard Chrome, Zinc Nickel...) ensures the functional uses of the unpainted area, or Manganese phosphating is used.
Manganese phosphating is a bath treatment that allows the crystallization of mixed iron and manganese phosphates on the surface of the steel, which creates a roughness conducive to mechanical paint adhesion, but also to the retention of grease and oil. This solution is used in the mechanical industry for partially painted parts with a friction zone for example. The unpainted area must be "greased" in order to have a good corrosion resistance.
In other cases, the parts can be partially painted with specific treatments for the functional areas. It comes thus the choice either to treat the part in two times with a saving, or to realize the surface treatment completely and to come to apply the paint on this one for the painted zones.
In the latter case, not all surface treatments are favorable to good paint adhesion. Generically, we can consider the following classification:Favourable: Electrolytic Zinc, Electrolytic Nickel Zinc, Zinc shuppage
Less favorable: electroless nickel, silver plating, tinning
Unfavorable: Chrome plating, cold immersion (zinc lamellar deposits)
PREPARATION TREATMENTS BEFORE PAINTING ON STAINLESS STEEL
Generally, stainless steels are not painted and do not receive surface treatments (except passivation). The painting of these steels is therefore quite rare. However, if this operation has to be performed, the most common solution is a mechanical preparation by sandblasting and the use of a specific primer.PREPARATION TREATMENTS BEFORE PAINTING ON ALUMINUM ALLOYS
Unlike steel, mechanical preparation by sandblasting does not generally allow good adhesion of paints. In general, a surface treatment is imperative before painting on aluminum alloys. We distinguish two major categories of treatments: conversions and anodizing.
3.1 CONVERSIONS
A conversion treatment is a bath or spray treatment that will modify the nature of the surface of the alloy by chemical reaction between the metal and the conversion product. Multiple conversions on fully painted parts are used in the construction industry. For other sectors, the most well-known and used conversion treatment is Alodine 1200™. This conversion provides very good paint adhesion if its coating weight is suitable for this functionality.
However, Alodine 1200 is a chromic conversion and is therefore impacted by REACh regulations. Other Cr VI-free conversions are currently marketed and used. They also offer very good performance for painting.
3.2 ANODISATIONS
Before going into the details of the different anodisations, it should be remembered that anodisations are treatments that generate a porous oxide layer on the surface of the alloy in its upper part. These porosities may or may not be filled depending on the functional use of the surface. We then speak of filled (filled pores) or unfilled (free pores) anodizing.
In general, filled anodizations are not very favorable to paint adhesion. Indeed, the pores being blocked, the surface does not have any more sites favouring the mechanical adhesion of the paint film.
The most common anodizations for painted parts are sulfuric anodization (OAS) and chromic anodization (OAC). Hard Sulfuric Anodizing (HSA) does not fall into this category because it is chosen for its anti-wear and anti-friction properties, thus without subsequent painting. We can also mention the OAC with optimized clogging which is a compromise between a very good corrosion resistance of the unpainted areas and a good paint adhesion.
3.3 CASE OF PARTIALLY PAINTED PARTS
Unlike phosphating on steel, conversion or anodizing treatments have good corrosion resistance. The problem of partially painted parts is therefore simpler than on steel.
Generally, the part is completely treated (conversion or anodization) and then painted on the areas that have been determined as such.MIXED PREPARATION TREATMENTS ON STEEL AND ALUMINUM ALLOYS
Within the framework of rationalized installations for multi-material treatments (steel/aluminum), phosphatization-conversions capable of both families of metals have been developed. These processes are mainly used in the automotive industry before cataphoresis.
Implementation
Main equipment (furnace, reactor, line, machine...)
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