Durable rust protection: Special challenges with perforated metal
Perforated metal components such as cooler grills or engine bonets are exposed to particular hardships. The coating largely determines the service life of the parts, it is however subject to special challenges.
Perforated sheets can fulfil different tasks. They are also indispensable in the automotive sector. Because here their main task is to cool the engine compartment by dissipating process heat. The free cross-section of perforated sheets ensures a maximum passage through which air can be reliably dissipated and the engine cooled. Without the use of energy or additive liquid cooling media such as water or coolant. Especially in commercial vehicles, the energy efficiency and performance of engines depends significantly on the cooling concept.
However, perforated sheets also have to cover an extremely demanding range of technical, functional and optical requirements. They should be as permeable to air as possible, but at the same time reliably filter unwanted particles from the air flow. After all, dust, stones or litter pose a danger to the sensitive engine compartments of tractors or harvesters during harvesting, excavator engines must be protected from dirt and soil, and buses and trucks must be protected from stone chips or rubbish from the road. Accordingly, perforated sheets require a high degree of stability – despite filigree webs that are sometimes only 0.5 mm thin.
The service life of such heavily stressed perforated sheets depends to a large extent on the quality of the surface coating. “The demands on a coating are constantly increasing,” Marc Holz from the IFO Institute for Surface Technology also knows to report. “On the one hand, corrosion must be reliably prevented, on the other hand, the optical requirements are increasing. Customers are accepting fewer and fewer irregularities in the coating due to inclusions or unevenness in the base material.”
On the one hand, corrosion must be reliably prevented, on the other hand, the optical requirements are increasing. Customers are accepting fewer and fewer irregularities in the coating due to inclusions or unevenness in the base material.
Excellent edge coverage for maximum protection
After all, in practical use, the components must permanently withstand the most adverse weather conditions, stone chipping and aggressive dirt. An almost immense variety of DIN standards, European standards and, last but not least, customer standards regulate the use and production of surface technology. At a company like Solvaro alone, a supplier of perforated metal components for agricultural and construction machinery and buses based in Kirchheim unter Teck, there are around 2,800 individual standards on ingredients, processes and the testing and evaluation of common coating methods.
The resistance of the corrosion protection is of particular importance. However, standards that specify the base material, the manufacturing process, the surface treatment method, the paint or powder manufacturer and the testing method for solid material cannot usually be applied. For example, sufficient edge coverage must be ensured for reliable rust protection – but in contrast to solid sheet, perforated sheet consists practically only of edges. Even common means for evaluating coatings such as the salt spray test according to DIN EN ISO 9227 do not say anything about components that are perforated over their entire surface. Therefore, individual approvals outside the standard inevitably become the reference for components.
Examples for edge coverage
Adequate edge coverage
Inadequate edge coverage
The durable coating of a hexagonal perforated metal component with up to 184,000 openings per square metre is indeed a science in itself. To achieve maximum reliability and safety in the process, manufacturers have to go their own way. Solvaro, for example, gathers independently proven, comprehensive knowledge by developing its own test procedures at the Institute for Surface Technology (IFO) in Schwäbisch Gmünd.
Marc Holz, Managing Partner of the IFO, states: “Our institute has an extensive materials science test laboratory where corrosion protection systems of various types are tested and evaluated. The use of accelerated corrosion test methods allows an assessment of the qualities. This allows the suitability for the application to be assessed.” The results of such test procedures have been documented by Solvaro for years. This results in a comprehensive overall picture of the application possibilities and the limits of using them for perforated plates.
The individual steps of anti-corrosion coating at Solvaro
In order to become a perforated sheet in the first place, sheets or plates of solid steel, aluminium or stainless steel material must be perforated. Whether round holes, square holes, hexagonal holes, slotted holes or a completely individual special perforation is used depends on the later application of the sheet. No matter which hole pattern is created – the metal breaks out when the holes are punched. This creates a ridge whose surface tension pushes an applied coating back towards the flat surfaces. This effect is called edge thinning and decisively impairs the quality of the surface coating. Therefore, the higher surface tension of sharp edges is reduced as far as possible by clean deburring of the metal parts. Here, a precise, possibly multiple tool grinding is required, with which all holes are rounded equally in a controlled process.
The first step for the actual corrosion coating of the material is the pre-treatment by so-called phosphating, a chemical or electrochemical process in which thin, finely crystalline and water-insoluble phosphates are produced from phosphoric acid solutions. They form a conversion layer of metal phosphate on the surface of the perforated sheet, which is firmly anchored in the surface of the component. Since the conversion layer contains numerous cavities and capillaries, it offers optimal absorption capacity for oils, waxes, colour pigments and lacquers and has thus also proven itself as an adhesive base for corrosion protection.
Marc Holz of the Institute for Surface Technology adds: “The DIN standards of the OEMs permit both iron and zinc phosphating, with the latter being said to have better rust protection properties.” Solvaro therefore also uses zinc phosphate.
In the next processing step, the component is coated with rust protection lacquer. Cathodic dip painting (CDP) has established itself as an electrochemical process for the full-coverage coating of complex shapes and structures. While the thickness of the paint layer on solid sheets is usually between 15 and 20 µm, the situation is quite different with perforated sheets: Due to the effect of edge thinning at the cut edges, often only a fraction of the cathodic dip-paint coating applied arrives there. In order to be able to counteract this, Solvaro tested various possibilities. Finally, a coating standard was derived from the results obtained, which specifies coating thicknesses of more than 25 µm for perforated metal components.
The final step towards permanent rust protection is the powder application. Since the powder coating contributes significantly to the visual appearance of the component, the desired powder manufacturer, the colour and the gloss level of the powder are usually specified by the customer. The challenge lies in the correct dosage. This requires a sure instinct and experience. Too much paint clogs the holes and restricts the air circulation, too little paint allows the KTL lacquer to shine through due to the edge thinning. For this reason, manufacturers are constantly testing new powder compositions, for example, to increase the surface tension of the coating material through higher viscosity and to minimise edge thinning. The result: ideal powder coating thicknesses and long-lasting rust protection.
The durable coating for rust protection of perforated sheets demands a high level of know-how and experience. Manufacturers such as the metal component specialist Solvaro, together with companies from the surface technology sector, are constantly developing new, advanced and holistic processes in order to perfectly fathom the seemingly contradictory demands for sufficient coating thickness for high corrosion resistance on the one hand and the best possible air permeability on the other.
In practice, it has been shown that an optimal rust protection finish can be planned in advance of production through detailed consultation and close cooperation with the customer. Because then the three essential parameters of rust protection, functionality and design can be harmonised in the best possible way.
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IFO Institut für Oberflächentechnik GmbH, Managing Director