August 17, 2020 - Gelsenkirchen. At its final meeting, the Standards Committee for Materials Testing (NMP/Working Committee for Hot-Dip Coatings) approved the DIN 50997 standard, paving the way for its publication, which has now taken place. In analogy to DIN EN ISO 1461, zinc-aluminium hot-dip galvanized coatings are now covered by a generally applicable process standard: The new hot-dip galvanizing standard describes the requirements for the properties and testing of zinc-aluminium coatings applied to manufactured iron or steel parts by means of thin-film galvanizing. The scope of DIN 50997, which is now valid, also includes the microZINQ hot-dip galvanised surface from the Gelsenkirchen-based surface specialist Voigt & Schweitzer (ZINQ), which has already received several awards for innovation and sustainability.

Strong interest in standardisation work

The German committee responsible for developing the standard was the NA 062-01-75 AA “Hot-dip coatings” working committee of the DIN NMP standards committee, which was composed mainly of industry representatives from the steel processing and galvanizing sectors.

The very high level of participation in the standardisation work, both on the user side and in the public sector, science, and research, as well as the rapid implementation from draft to finished standard within just under a year and a half, demonstrates the strong interest and necessity of standardizing innovative processes in the field of surface technology.

What the standard regulates: Less is more

The DIN 50997 standard is a national standard that describes the requirements for the properties and testing of zinc-aluminium coatings applied to manufactured iron or steel parts by means of thin-film galvanizing. The aluminium content of the zinc-aluminium melt must be a minimum of 4.0% by mass and a maximum of 6.0% by mass.

Since influencing factors such as the silicon or phosphorus content of the steel have no effect on the optical appearance of zinc-aluminium coatings, in contrast to hot-dip galvanised surfaces according to DIN EN 1461, the entire surface of the component will appear uniformly metallic-silvery-shiny, regardless of the steel composition.

The local coating thickness for zinc-aluminium coatings must be at least 5 μm (micrometers) and the average coating thickness for zinc-aluminium coatings must be at least 6 μm. For hot-dip galvanised coatings in accordance with DIN EN 1461, much higher minimum coating thicknesses apply, which, due to the reactivity of the molten zinc, can rarely be fallen short of for process-related reasons. The thickness of the zinc coating is less decisive for corrosion protection performance than the ability of the zinc coating to form stable top layers. Cover layers form differently in zinc and zinc-aluminium coatings: Studies by the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) have shown that zinc-aluminium coatings are capable of forming extremely stable cover layers under atmospheric stress. The results showed that corrosion-promoting elements such as chlorides from the atmosphere are used to build up crystalline top layers, so that corrosion becomes protection. The corrosion-inhibiting property of the top layer formation applies to all zinc coatings, but especially to zinc-aluminium coatings such as microZINQ, because here, closed top layers form under corrosive stress, which can inhibit the corrosion process on the surface to a standstill in the so-called transpassive range.

DIN 50997: market- and customer-driven

“The impressive properties of zinc-aluminium coatings have led to strong interest in innovative technologies in the field of surface engineering,” says Dr Birgitt Bendiek, Managing Director of Technology and Development at ZINQ. „The reason for developing the standard is the increasing demand for modern, innovative zinc coatings that open up new fields of application thanks to the optimized product properties of piece galvanizing and thus the combination of steel and zinc materials. The standard is customer-driven. The application for the new standard was submitted by Welser, the market-leading profile manufacturer in Europe, which will use the new standard as a basis for developing further products that are protected against corrosion with zinc-aluminium coatings.“

In fact, the new DIN 50997 standard takes into account the range of innovative, highly effective corrosion protection systems already on the market, including microZINQ piece galvanizing. microZINQ received general building approval (AbZ) from the German Institute for Building Technology (DIBt) back in 2015. The AbZ was the first step for microZINQ to be used as a surface in load-bearing applications for building products.

“With the new DIN 50997 standard, we now have a nationally valid framework for the application of zinc-aluminium coatings that incorporates the proven results of science, technology, and practical experience-this opens up new potential for microZINQ,” summarises Lars Baumgürtel, managing partner of ZINQ.

“And we will continue to work on developing innovative and sustainable corrosion protection solutions for our customers. Our NextZINQ innovation programme focuses not only on new, efficient zinc coatings with a long service life, but also on the recyclability of zinc-coated products in terms of circular quality.”