Perforated parts - a special case for the treatment of surfaces

Kirchheim/Teck, Germany, July 2018. To ensure the quality of coated, perforated parts, SOLVARO GmbH carries out more extensive corrosion testing. With the clear results from these tests, a better choice can be made from the wide-ranging standards available for the entire coating process, so that the requirements can be more precisely met. 

A safe path through the standards jungle

The existing standards and tests for coatings only have a limited relevance for the manufacturers and end users of perforated parts such as engine bonnets or ventilation grilles for agricultural and construction vehicles. There are particular challenges with perforated metal. SOLVARO GmbH has taken a proactive approach to the subject of surface treatment, as developers of perforated metal parts are now confronted with over 2800 separate standards. Even the most comprehensive customer standards often still lag behind today’s dynamic developments. The team around Martin Schneider, the head of application engineering at SOLVARO, asked the questions: “Who needs which standard?”, “What meets whose requirements?” and “Which test results are relevant for whom?”, however there are specific challenges in the coating itself before a suitable, useful quality test can be chosen for perforated parts. The coating system must be ideally adapted to the customer’s requirements. 

Coating holes and edges is a tricky matter 

A hexagonally perforated SOLVARO part can have up to 184,000 holes per square metre. The areas between the holes are then only 0.5 mm wide. If cathodic dip coating and powder coating are used as corrosion protection, then the surface of these parts is not comparable with that of solid metal parts. One of the challenges when coating is to ensure that the total thickness of the coating layer is not too high. Deposits in holes can partially or even fully block them. Otherwise, one of the part’s vital functions, of letting air through, may be impaired. Another problem arises from the edge effect, where insufficient coating adheres to the edges of holes. The thickness of the cathodic dip coating and powder coating is thus reduced towards the edges and, in extreme cases, both layers can be too thin at the edge. This means there is no longer sufficient rust protection.

If these coating hurdles have been overcome, then the quality assurance process must be adapted for the special features of the parts.   

Quality assurance – Surface rust as a measurement criterion 

For a fully perforated part without an edge or a frame, the results from a salt-spray test to DIN EN ISO 9227 cannot be evaluated with all of the common standards in the DIN EN ISO 4628 series. Some of the standards are not meaningful for perforated sheet metal. This is especially true if subsurface corrosion is measured at a scribed line, as this cannot be done with perforated parts. During the tests which SOLVARO did at the Institute of Surface Engineering (IFO) in Schwäbisch Gmünd, Germany, the perforated parts were thoroughly tested. The corrosion of perforated parts was measured to DIN EN ISO 4628.1-10 over a duration of 500, 650, 800 and 1000 hours. The most useful standard for perforated metal is DIN EN ISO 4628.3, which refers to the degree of corrosion over the surface. Depending upon the requirement, the results are available for the entire series of tests. They have been documented in an interactive table and can be called up in an instant when advising customers. “We can now, quickly and reliably establish whether the category of corrosion protection required by the designer for our parts is actually possible, even in the perforated area of the component,”explained Martin Schneider. 

Picture 1
The correct coating also provides outstanding protection from rust for perforated parts, such as this ventilation grille for a tractor.

Picture 2
Blocked holes – a problem when coating perforated metal.

Pictures 3 and 4
The difference between adequate and inadequate edge coverage is clear to see under a microscope.

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