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Surface Protection Thread-Forming Screws: Lasting Corrosion Protection

Author / Editor: Markus Nowak / MA Alexander Stark

An appropriate surface protection including relubrication is ideally suited to prevent corroding and thus damage to thread-forming screws. What type of coating meets the requirements?

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Thread forming screws with relubrication form no red rust after 1000 h (according to salt spray test DIN EN ISO 9227).
Thread forming screws with relubrication form no red rust after 1000 h (according to salt spray test DIN EN ISO 9227).
(Source: Dörken MKS)

Since different materials come into contact with thread-forming screws, they can quickly corrode. The zinc in a zinc flake coating "sacrifices" itself on contact with water and oxygen and corrodes instead of the more precious steel. Since no hydrogen is produced in the coating process with zinc flakes, even high-strength steels can be produced without hydrogen embrittlement.

During assembly, thread-forming screws form the material in the mating component and thus create a precisely fitting nut thread. This eliminates the need for time-consuming process steps such as thread setting or chip removal in direct bolting - saving time and money. At the same time, the material solidification provides a higher load bearing capacity of the entire screw connection. Since thread-forming screws can also be flexibly screwed into all plastically deformable materials, they are becoming more and more important in numerous applications and are replacing more cost-effective standard screws - for example in the automotive industry for transmission and steering housings as well as in engines, in mechanical engineering or in constructions of all kinds.

As thread-forming screws are screwed into components made of different materials and are also exposed to changing temperatures, moisture or contact with chemical substances, corrosion can quickly occur. With the aid of corrosion protection that meets different requirements, the service life of the connecting elements and the contact components can be extended, and optimum functionality ensured.

Coating Provides Permanent Protection

Zinc flake coating consisting of a basecoat and - depending on requirements - a topcoat or relubrication can be used to protect thread-forming screws from corrosion.

Zinc-containing basecoat provides cathodic protection, i.e. the screws are coated with a barrier layer. If the surface is damaged, the less precious zinc ("sacrificial anode") "sacrifices" itself on contact with water and oxygen in favor of the more precious steel substrate. Thread-forming screws can then be coated with a special topcoat or alternatively a colorless re-lubricant. The latter extends corrosion protection and enables defined coefficients of friction as well as low creep torque.

A zinc flake coating is also particularly useful as it is applied in very thin layers (8 to 20 μm) and is resistant to chemicals. Surface protection is also suitable for high-strength steels, since no hydrogen is produced during the coating process, thus avoiding the risk of application-related, hydrogen-induced stress corrosion cracking. Last but not least, the low crosslinking temperatures of up to 240 °C prevent material damage to the steel caused by excessive heat stress.

Efficient Coating in Dip Spinning Process

Extensive tests and studies conducted by Dörken MKS have shown that thread forming screws grooved with a zinc flake system in aluminum are resistant to red rust for more than 3000 h. Grooved in steel, a corrosion protection of more than 1000 h could be verified in a salt spray test according to DIN EN ISO 9227.

The coating of thread-forming screws is carried out in accordance with DIN EN ISO 10683 and DIN EN 13858 in a non-electrolytic process such as immersion spinning. The screws are loaded into a basket or dipping drum and then dipped into the coating material. This ensures that all threads and furrows are wetted.

After the frame has been removed from the immersion bath, centrifugation is carried out to remove material excess. The last step is the drying and cross-linking process. The result is an even, adhesive and durable coating of paint on the screws.

Zinc flake coating including relubrication not only protects thread-forming screws permanently against corrosion, it also supports optimum screwing.

* Markus Nowak is Technical Engineer at Dörken MKS-Systeme GmbH & Co. KG in 58313 Herdecke

This article was first published by MM MaschinenMarkt

Original by Stéphane Itasse / Translation by Alexander Stark

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