Corrosion protection since 1954 21 October 2015

For over 61 years Duroc NV has been a key player in the European market for the surface treatment of fasteners and other related articles. Thanks to its good geographical location at the gates of the port of Antwerp, Belgium, more than 800,000kg of fasteners are surface treated per week to enhance their corrosion and friction properties.

Duroc is able to offer a broad range of solutions – with an average layer thickness of 8 microns and corrosion protection up to 2,500 hours of salt spray test (SST) – and all without the use of chrome VI.

With the flexibility and order handling capability from 1kg to 100,000kg, Duroc is confident it is the ideal partner for European manufacturers and wholesalers. Duroc also holds licences from a diverse list of Triple A coating suppliers, such as Dörken MKS. In addition to coatings according to specification, the company has developed its own broad range for different industries. Applications vary from M52 x 800mm studs for windmill applications through to M8 x 40mm with TORX® insert for the automotive industry.

Surface treatments
There are four surface treatment departments within Duroc:

• Electrolytic zinc plating – up to 480 hours SST.

• Zinc flake coatings – up to 2,000 hours SST.

• (In) Organic coatings – up to 2,000 hours SST.

• PTFE coatings – up to 2,500 hours SST.

The electrolytic zinc plating is only done in barrel application, whereas the zinc flake and PTFE coatings happen both in dip spin as well as rack application. The rack application has a maximum length of 2,000mm – application is by means of a robot to guarantee constant layer thicknesses and quality. Thanks to the large diversity, Duroc is able to provide customers both flexible and above all independent advice on the methods most suited for customers’ particular applications.

Each application has its own specific properties. 300 different potential combinations mean specific coatings can be delivered per order. Yet finding the best surface treatment is very complex. That is why Duroc has extended its assortment to 15 broad applicable coatings. Below is a selection of innovative coating systems that have already proven their worth:

DUPLEX®: Developed specifically for environments where standard electrolytic zinc plating or zinc flake coatings are not applicable, such as chemical industries, stable construction, or typically the use of a carbon steel bolts in an aluminium construction to eliminate the risk of contact corrosion.

To obtain a good corrosion protection it is vital to find the correct balance between salt spray and kesternich testing results. A high salt spray test alone does not mean it is necessarily a good system. The DUPLEX system achieves a very high corrosion resistance (both salt spray and kesternich), with a constant low friction coefficient. This means that it has a broad applicability and can be used universally.

The DUPLEX coating is available in the following variants – DUPLEX 700, DUPLEX 1000, and DUPLEX 2000.

•  SEAL INOX®: Providing a low price and excellent low friction properties, SEAL INOX prevents cold welding. It can also be applied on stainless steel fasteners to prevent ‘galling’. The costs for surface treatment are only a fraction of the purchase cost of mounting paste. In addition, because only the nut has to be coated, it is very simple to keep it as a stock article, reducing time to market.

•  M5000®: Another example of a unique coating, the M5000 was developed specifically for the heaviest applications such as the mining industry and offshore. One of the most recent projects was a new pioneering mega lifting ship with a maximal lifting power of 48,000 tonnes. Bolts in such applications are pushed to their maximum limits and today’s coatings could not withstand this. With astonishing results, the M5000 proved itself as the new standard for the offshore market going forward.

Quality control
Constant quality is very important in surface treatment. Processes should be able to be monitored from a distance and corrected if needed. Maintaining the chemistry is not a case of daily or weekly control. Automated and continuous dosing, controlled via PLC, ensures Duroc is able to deliver continuous quality. Perfectly audited processes for the automotive industry are mixed with products for the construction industry. “We understand our job by drawing a single line in process control.”

All products that are surface treated are sampled, and are randomly subjected to salt spray tests and the kesternich test. Certificates of these tests can be provided, proving the products respond to the high demands surface treatments must meet.

Despite the fact that electrolytic zinc plating of fasteners with a tensile strength of more than 1,000N/mm2 is not advised, due to the risk of hydrogen embrittlement, a large volume are still demanded by the market. To minimise the risk of hydrogen embrittlement Duroc has a standard method applied after treatment – annealing.

This process drives potential penetrated hydrogen out of the material. This technique does not totally exclude the risk of hydrogen embrittlement for 100%, but thanks to automated processes and years of experience Duroc achieves a high-level of success.

As an added value to the supply chain of fasteners, Duroc can also package fasteners, so that customers receive their product without the need for extra handling.


Claire Aldridge Deputy Editor t: +44 (0) 1727 743 889

Having spent a decade in the fastener industry experiencing every facet – from steel mills, fastener manufacturers, wholesalers, distributors, as well as machinery builders and plating + coating companies, Claire has developed an in-depth knowledge of all things fasteners.

Alongside visiting numerous companies, exhibitions and conferences around the world, Claire has also interviewed high profile figures – focusing on key topics impacting the sector and making sure readers stay up to date with the latest developments within the industry.