Cathodic Dip Painting

Further detailed information about CDC painting and CDC coating:

Scope and purpose

This technical specification applies to cathodic dip coating for components that are painted in a single-layer and heat-dried.

Definition of the terms

CDC coating refers to the application of coating materials to metal substrates made from water-based paints in the dipping process through electrochemical reactions. For the cathodic dip painting, the workpiece to be coated is switched as a cathode.

Brief description of the two main processes of the CDC

Pre-treatment

We use a zinc phosphating technology for the pre-treatment.
The layer has a dual function: it improves the adhesion of the coating to the parts and at the same time it acts as corrosion protection. The zinc phosphate coating can be applied to almost all types of metals, from steel to pre-galvanized parts all the way to aluminum and other light metals.

Cataphoretic coating process (CDC)

Cathodic dip coating or cataphoresis is a fully automatic dip coating process based on the principle of moving charged paint particles in an electric field to a pole with the opposite sign (the metal part in the paint). The main aim of the treatment is to protect the surface from corrosion in areas where this treatment is ideal for further use. This treatment allows painting in inaccessible areas such as cavities, pipes, etc. Before the paint is applied, the parts undergo surface preparation consisting of degreasing, zirconium thin-film passivation and various rinsing processes. After the coating has been applied, the parts are thermally polymerized. The industry is placing increasingly high demands on the choice of cataphoresis coating in terms of excellent surface quality, high productivity, reasonable costs and high efficiency. Cataphoresis can be used as a final coating or as an excellent base for other surface treatments. Very suitable for the assembly of original parts while maintaining dimensional tolerances.

Materials that can be coated:

Kokinetics’ pre-treatment and coating process is suitable for a wide range of materials. These include steel and sheet steel.
All types of galvanized sheet, steel and grey cast iron, extruded aluminum profiles, aluminum sheet and various aluminum and zinc alloys for high-pressure die casting are generally paintable, but a sampling at our facility is recommended.

Die-cast alloys

Due to the variety of alloys in die-cast materials, the suitability of the pre-treatment process must be checked from case to case.

Galvanized surfaces

In the case of galvanized sheet metal, it should be noted that the coating can be affected very negatively by pre-passivation, such as transparent or particularly yellow chromating. Such pre-passivation is often part of the galvanizing process and is used both for strip galvanizing of steel sheet (coil galvanizing) and for piece galvanizing (electroplating). It is used to prevent corrosion of the zinc on the sheet or the raw part. However, passivation often means that during pre-treatment no passive layer can form on the surface, which has a negative effect on adhesion and, in particular, on the behavior during corrosion tests.
Transparent chromate coatings are not visually recognizable on the blank workpiece and can only be detected analytically with special methods. Pre-passivation must therefore be explicitly excluded in the material specification, unless it is compatible with the pre-treatment and coating process.

Mixed construction

For workpieces made of different materials, the suitability of the product for CDC must be checked before coating.

General properties of cathodic dip coating

In the following, the basic, generally valid technical properties of cathodic dip coating are listed. We explicitly emphasize that these and other specified properties must always be checked and agreed on the original workpiece.

Surface appearance

Cathodic dip coating is essentially a corrosion protection coating, not a visible coating. If such requirements are demanded by the workpiece, e.g. the use as visible coating, special agreements must be made for this, particularly regarding the quality of the raw part (see point: “Materials that can be coated”) and the permitted error patterns on the workpiece. Ideally, this is done by agreeing on limit samples, with which, for example, the maximum quantity and size of foreign inclusions in the cathodic dip coating can be clearly defined.

Coating thickness

The thickness of the cathodic dip coating layer is usually 20.0 – 25.0 μm. The coating thickness tolerances for cathodic dip coating are in the range of approx. ± 5 μm.
As a general rule: the thicker the coating, the greater the corrosion load that the workpiece can withstand.

Color

The paint system we use corresponds to approx. RAL 9005, black.

UV stability

Epoxy resin based on binders is generally not suitable for direct exposure to UV light. UV rays destroy the surface of the binder, which appears on the part as a grayish haze, the so-called chalk. For this reason, cathodic dip coating cannot be used as a single-layer coating on the exterior of the vehicle. This chalking can also occur, for example, due to continuous exposure to artificial light sources with high UV radiation, such as halogen discharge lamps. If KTL workpieces are to be used outdoors, they must be provided with an additional protective layer against UV radiation (e.g. powder coating or wet painting).

Overcoatability

In principle, cathodic dip coating can be overcoated with conventional powder and wet paints. In any case, however, the adhesion of the entire layer structure and in particular the intermediate adhesion between the cathodic dip coating and the coating applied on top must be examined, as this is influenced by the chemical composition of the applied layer as well as by the baking or hardening conditions of the cathodic dip coating and the coating applied on top. The conditions for the application and hardening of the cathodic dip coating and the topcoat must be matched and specified for the respective workpiece. In particular, if the baking temperature of the topcoat is too high, this can lead to subsequent reactions of the cathodic coating, which can then lead to yellowing of the topcoat or loss of intermediate adhesion, among other things. When overcoating, it must always be ensured that any impurities such as dust, grease or hand sweat that may arise during the packaging of parts are removed before the topcoat is applied. Packaging materials are often the cause of this contamination. The black color of the CDC requires sufficient coverage of the coating applied on top, especially at the edges.

Gluing of sealants and adhesives

In a further manufacturing process, adhesives, sealants or foams are often applied to the cathodic dip coating. As with a new coating, the adhesion of the entire layer structure and in particular the intermediate adhesion between the cathodic dip coating and the applied sealant or adhesive etc. must be checked in each individual case. The adhesion is influenced, among other things, by the chemistry used, but also by the baking and hardening conditions of the cathodic dip coating and the sealant or adhesive applied. The respective application and hardening conditions must therefore be coordinated and determined from case to case. As with repainting, it is important to ensure that all contaminants such as dust, grease or hand sweat, which can arise during the packaging of parts, are removed before the application of sealants, adhesives, glues, etc. Packaging materials are often the cause of this contamination.

Corrosion protection

The basic binder for cathodic dip coating is epoxy resin, which achieves good corrosion protection values even with low coating thicknesses thanks to its high crosslinking density in combination with the pre-treatment. The paint and pre-treatment system we use have been approved in general by various OEMs and Tier 1 suppliers. However, as the achievable corrosion protection values are workpiece-specific, they must be tested on the original workpiece and agreed individually. At this point, explicit reference is made to the corresponding explanations in points “Materials that can be coated”. For the coverage a functional hanging hole is required. Burring may occur in the area of this suspension hole and the corrosion protection may be affected due to the lack of paint.

Chemical resistance

Binder epoxy resin is highly resistant to a wide range of chemicals. Similar to the corrosion protection test, a detailed test of the resistance profile of the original workpiece is also required here.

Machinability

A subsequent mechanical processing of the cathodic dip coating is generally possible, but is only suitable to a limited extent for subsequent processes such as edge or penetration joints due to the forming properties. This can lead to the formation of cracks in the field of vision (paint). In machining processes such as milling, cutting or drilling, the cathodic dip coating does not usually disturb. However, subsequent mechanical processing should be avoided, as corrosion protection is no longer provided where the cathodic layer of the dip coating is damaged, or the base material is exposed. In addition, there is also the risk of the surface being scratched and damaged by the necessary clamping.