External coating techniques represent the most extensive group of metal corrosion protection methods. Primer is often used in the protection of automobile bodies, which are exposed to all sorts of influences that contribute to the development of rust. One of the most effective methods of such protection is a cataphoretic coating, which simultaneously combines elements of physical and chemical isolation.
Preparation of technological equipment
The technical basis for the cataphoresis treatment procedure is the limiters for body elements, including the hood, trunk and side doors. All these parts must be slightly opened so that the active mixture freely fills the cavities with the inside, leaving no gaps. From the point of view of in-line production, the solution for cataphoresis coating is thus saved on the line, since the unused composition is completely merged into a special container and used in subsequent operations. The function of the drain channels is usually performed by holes in the bottom of a car body. That they were open, you should also take care in advance.
Body wash
The next step in the preparation is a preliminary wash. Not only from external surfaces, but also from the inside of the body, all foreign particles should be removed from dirt and dust to products of welding production and factory lubrication. On professional lines, chambers with many nozzles directing water with a cleaning solution are used for such purposes. A simpler option involves the use of mini-sinks under high pressure with the connection of compressor units. In any case, the better the initial cleaning, the more likely it is to get a durable and durable protective coating on the car body with an emphasis on the formation of an anti-corrosion barrier.
Degreasing surfaces
The presence on the body surfaces of traces of technical oil or other chemicals can lead to the preservation of open areas, which often become foci of rust after damage. Therefore, it is very important to carry out a degreasing operation. Usually, before applying a cataphoretic coating, this problem is solved by spraying. The car passes through a special tunnel, where it is also supplied with water under high pressure, but with the addition of special chemicals. The method of immersion in bathtubs with degreasing solutions is also practiced, but this approach justifies itself only in conditions of a high-performance auto service with a large flow.
How to degrease metal?
So, what additives are used after complex washing before applying the cataphoresis composition? The simplest and most affordable material is alkaline solutions. These are useful active components that, together with degreasing amplifiers, give a good cleaning effect and basic protection. With a high degree of concentration of fat will have to use specialized tools. For example, how to degrease metal if the surface fat content exceeds 3 g / m2? In this case, you can use specialized chemicals from Henkel, Chemetall or ECOCHEM, made on the basis of solid and chlorinated particles of polyvinyl chloride and acrylonitrile butadiene styrene. As practice shows, the key factor in the effectiveness of a degreasing solution is not so much its concentration as the correctly selected temperature regime during processing.
Surface Activation and Phosphating
The transitional stage between the cleaning of the target surface and the creation of a base for cataphoresis coating is phosphating. In turn, the activation of the metal will be a preparatory operation, which is carried out with the aim of improving the crystal formation in the bath. For this, insoluble compounds, manganese phosphate and titanium salts are used. They provide a precipitating effect on the surface, contributing to the compaction of the phosphate layer.
Activation forms the basis of the protective coating of the car body, contributing to increased adhesion. After phosphating, the coating is monitored to evaluate properties such as total acidity, density, mass, etc. The composition of the solution can also be analyzed. A standard set of components is formed by zinc elements, fluorides and additional modifying additives, which are selected individually.
What is cataphoresis soil?
The main component of the coating is a lead-free two-component mass, which can be presented in the form of a paste or emulsion. In the latest generations, such primers are environmentally friendly and highly penetrating. With the help of modifiers, the indicators of thermal resistance and protection against corrosion processes are also improved. In addition, the operation improves rust removal without damaging the body base. For cataphoresis, emulsions are used directly, including several cationic polymers. The formed layer is an elastic film with blocked isocyanate. Also in the composition may be used pigment paste based on mineral components, binders of active substances and demineralized liquid.
Soil application
The protective structure is formed in the bathroom, in which the emulsion was previously loaded with a paste, including the necessary additives. The average volume of the bathtub for the cataphoretic method of corrosion protection is 100 m3 per one body. The simultaneous processing of several cars is also allowed. In this case, containers of 200-300 m3 are used.
Dialysis cells with a supply of metal cathode rods are connected to the bath along the perimeter. This is an important part of the technology, due to which, in principle, an electrochemical effect is provided. When voltage is applied, the cataphoresis soil is charged, followed by the formation of a protective layer up to 25-30 microns thick. As for the charge parameters, the current strength should be from 350 to 700 A with an average voltage of 400 V. The operation lasts 2-3 minutes over time.
Drying metal surfaces
Unlike other technologies of quick drying, in this case, the polymerization process also has the character of soft thermal exposure with burning. For this operation, special furnaces are used, allowing several stages of drying to be performed. At each stage, the corresponding module is connected with an independent burner (usually gas fuel), heating the air to the desired temperature. Heat flows in the direction of the body are supplied by fan units. Moreover, each thermal exposure circuit contains filters that clean the air from the smallest particles, which can harm the structure of the coating. Regarding the choice of temperature, much depends on the characteristics of a particular metal. For automotive bodywork, corrosion protection is fixed in the polymerization conditions at 180-200 ° C. Moreover, for each individual part, the temperature can vary due to special sensors. A special approach is usually applied to the roof, dashboard and bottom.
Technology benefits
Among the main advantages of this technique is the high resistance of the soil to external negative influences, which create the conditions for the formation of corrosion. Having performed rust cleaning with abrasives of the desired fraction at the first stage, in the future we can forget about the risks of such damage from high humidity and salt influences. In this case, cataphoresis is used not only as a technological protective coating, but also as a decorative one, if the primer solution is combined with powder paint. Among the other advantages of this method, there is a high processing speed, uniformity of laying of the layer, high penetration and the already mentioned environmental safety.
Alternative methods of corrosion protection
The closest anti-corrosion protection technology , which in principle can be compared with cataphoresis, is anaphoresis. It is also a type of electrolytic staining, but with some differences. In particular, the anaphoretic method of corrosion protection involves a negative charge of the primer, and the target surface is associated with a positive contour. From the point of view of the quality of the coating device, this is not of fundamental importance, but organizational processes are facilitated due to the exclusion of the mandatory grounding device for the bath and dialysis cell supply. In addition, the requirements for paint deposition and drying procedures are reduced.
Conclusion
Cataphoresis technique is complex and financially expensive. The quality of the result will be guaranteed only by the competent implementation of each processing step. The slightest violation of technology can cause the destruction of the applied soil in the first months of operation of the car. But in other cases, cataphoretic metal processing fully justifies itself as an effective way of corrosion protection for many years. The secret to the formation of positive technical and operational properties after the formation of the coating is a balanced combination of chemical components and electrical effects on the metal structure. This is far from an innovative method, since its principles have long been involved in protecting pipeline networks from the same corrosion. But with regard to the protection of automobile bodies, one can speak of an unprecedented optimization of cataphoresis processes, albeit with great organizational costs.