Protection layer by layer

A new two-layer passivation system provides significantly improved corrosion resistance. The key feature of the process is the synergy between the two layers which results in increased protection against rust.

To ensure that small bulk parts such as bolts can function effectively over as long a period as possible, the steel they are made of must be protected against corro­sion. The parts undergo a process consisting of several stages which include pretreatment (generally cleaning and pickling), galvanising (with zinc or zinc-nickel for example), passivation and sealing or posttreatment with a topcoat.

The purpose of the passivation phase is to improve the corrosion resistance of the steel. lt involves immersing the component in an electrolyte to produce a conversion coating. Normally a thick passivation layer is formed, which is a corrosion-resistant surface treatment based on compounds containing trivalent chromate, cobalt or in some cases hexavalent chromate. This last process is known as blue, yellow, green or black chromating, depending on the colour of the coating. 

lndividually designed chromate­free systems 

Against the background of the problems caused by hexavalent chromate, the restrictions on its use and the increasing demand for more environmentally friendly solutions, several development projects were started with the aim of replacing the systems based on hexavalent chromate and cobalt. Chromate-free systems have been used for pretreatment and passivation in the steel industry for several years. Because of the high levels of mechanical stress involved in batch galvanising and the accompanying damage to the surfaces, these systems are not suitable for small bulk parts. The latest thick passivation product developed by Dörken MKS is Delta-Prozinc, an electroplating system with a high level of corrosion protection and good fastening properties. lt consists of individual media that are combined in a specially designed process, but uses no hexavalent chromate, nickel or cobalt. 

New passivation system provides excellent corrosion protection 

Dörken MKS has developed a new alternative that produces significantly better results than existing conventional systems: a passivation process with two layers. The first specially developed passivation coating is based on a trivalent chromate conversion. No other systems containing cobalt salts or chromate which are of toxicological concern are used. The product is applied as usual to small bulk parts in a galvanising barrel or a rack system. The parts are then rinsed twice and the second passivation coating is applied. This consists of a solgel-based formulation which can be applied using a dip-spin process or, as in the case of the first layer, using a rack system that immerses the parts. No further rinsing is needed and the drying process can start immediately. 

The two-layer coating that is deposited on the parts has a thickness of around 200 nm. This means that it is no thicker than a classic single-layer passivation coating. However, the second silicabased layer acts as a barrier and significantly reduces the erosion of the first layer and the attacks on it by corrosion. lt also makes the parts much less sensitive to mechanical impacts. 


Diagram showing the structure of a two-layer passivation coating containing no hexavalent chromate or cobalt.

Up to 240 hours of protection from white rust 

The corrosion resistance test used for the bolts with the two-layer passivation coating is the conventional neutral salt spray test in accordance with EN ISO 9227. The bolts are protected against white rust for up to 240 hours. By comparison, traditional galvanised and yellow-chromated bolts have 96 hours of resistance to white rust. A closer investigation of the bolts with a two-layer passivation coating indicates that each of the individual layers provides little corrosion protection. The first passivation layer shows significant amounts of white rust after 72 hours, while the second provides only 24 hours of protection against corrosion. The special feature of this system is the synergy between the two layers, which results in a considerable increase in corrosion protection. lt is not only small bulk parts that achieve very good results in the salt spray test. Rack parts are also protected against white rust for up to 360 hours. 


The SEM image shows that the two-layer passivation coating has a consistent structure.

An innovative alternative 

The two-layer passivation coating is a good alternative to single-layer systems containing hexavalent chromate or cobalt. Because of the synergy between the two layers, the process provides excellent corrosion protection. As the system contains no hexavalent chromate or cobalt, it is also much more environmentally friendly. The use of an additional topcoat can increase the protection against red rust to up to 1000 hours and also enables other requirements to be met, such as an improved coefficient of friction or chemical resistance and the option of different colours. Work is currently underway to develop completely chromate-free systems with the same level of mechanical strength and corrosion resistance. 

 


Bolts before (left) and after (right) the 240-hour neutral salt spray test. The two-layer passivation coating (below) produces considerably better results than the single-layer coating (above).