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Volkswagen AG and General Motors have recently approved a second generation, high-performance, chrome(VI)-free microlayer corrosion-protection system (abbreviated to  MKS) according to their material specification TL 245 (VW) or GM 3359 for protecting metric threaded parts made from high-strength steel against corrosion. This non-electrolytically applied, zinc flake coating provides reliable protection where the classical methods, such as hot dip electroplating or zinc electroplating, cannot be used for technical reasons.

While zinc electroplating and hot dip electroplating provide adequate corrosion protection for non-high-strength steels, neither of these methods can be used for high-strength base materials. The hardness of the steel is predominantly achieved through precisely defined temperature control (quenching and reheating to draw the temper) when processing the steel. The reheating temperature for this process generally lies between 340 and 425 °C.  This value has a crucial effect on the other heat treatment processes. However, the temperature of hot dip electroplating is higher than that used for the heat treatment because the melting point of pure zinc is 420 °C. During the electroplating process, the hardness of the steel changes uncontrollably.

Zinc electroplating also cannot be used for coating high-strength steels because atomic hydrogen is released on the surface of the component when it is immersed in the electrolyte because the component is used as the cathode. The hydrogen diffuses into the substrate and some of it then remains in the form of dissolved hydrogen in the interstitial sites of the crystal lattice of the metal.  This produces hydrogen embrittlement - a term used for a series of failure mechanisms. The most important of these mechanisms is hydrogen-induced stress corrosion since this results in immediate failure of the component without showing any preliminary signs of damage. Although, in some cases, the hydrogen can be induced to effuse again by tempering, the success of this method is always dependent on the structure of the electroplate. It is also costly, both in time and money and the method never eliminates the problem altogether.

Zinc flake as an alternative

For this reason, zinc flake coatings are predominantly used for high-strength materials. These coatings contain flat zinc platelets about 10 µm across which are bonded to the component with a binder system. The technique used for this “painting process” varies and is mainly determined by the shape of the component:

• Dip-spin systems for medium sized mass produced small parts (such as screws)
• Spraying for large screws and stamped parts
• Dip-drain for tubes or larger panels with simple geometries

After the coating has been applied, it is cured under relatively low temperatures around 200 °C. Cathodic corrosion protection coatings process at much higher temperatures which may adversely affect the material.

When finished, the cured coating contains about 85 % metal, most of which is zinc. Because of the high metal content, the flakes overlap one another, touch one another and therefore form a conductive coating. The contact points are very small which means that high metallic conductivity, such as that of solid copper, is not achieved.  However, the conductivity is sufficient to protect the substrate by means of the same corrosion protection mechanism as that provided by other zinc coating methods. The flakes are therefore bonded sufficiently to one another and the substrate.

The zinc provides sacrificial protection

During corrosion, the individual zinc flake is converted to Zn(OH)₂. This produces the so-called white rust. The oxidation of Zn to Zn²⁺ (according to the model) releases electrons on this lamella and all other parts that are in conductive contact, including the steel substrate. Since, according to the electrochemical series, zinc is less noble (at -0.7 V) than iron (at -0.44 V), the zinc-flake layer also corrodes in preference to the steel when damaged by scratches etc. Of course, these electrons are constantly consumed by the corrosion process. The reduction of oxygen, O₂ + 4e^- +H₂O -> 4 OH^-, is the most probable reaction to take place in the presence of moisture.

The properties of the zinc-flake materials described above can be extended even further. A supplementary coating with a topcoat can be used to optimise properties selectively such as color, friction values, wear resistance and chemical resistance. There is a fundamental difference between organic topcoats that are usually epoxy based and inorganic topcoats that are usually silicate based. As a rule, the organic topcoats (such as DELTA®-Seal, with approximately 6-8 micron thickness) provide better protection against chemical attack while the inorganic topcoats (such as DELTA-PROTEKT® VH 300) can be applied in much thinner layers—approximately 2-3 microns for applications with narrow tolerances and higher heat requirements. Common among all topcoats is that the desired friction value can be obtained with the addition of an integrated lubricant.

A topcoat increases protection

Of course, a topcoat also increases the corrosion protection of the substrate by sealing the cathodically active surface of the zinc flake basecoat. Thus, the combined use of an approximately 8 µm basecoat and a 3 µm inorganic topcoat will provide a service life during a salt-spray test according to USACAR SAE 1 of more than 1000 hours and up to three cycles in the “DaimlerChrys-ler” test[1].

There are clear advantages associated with the use of second-generation MKS materials. With the same coating thickness as, for example, DELTA®-Tone  9000, which has been proven for years, the corrosion protection can be significantly improved, which can also considerably extend the service life of the finished component. Alternatively, by optimising the coating, it is also possible to reduce the coating thickness and therefore the material consumption when the stresses on the coating are not expected to be too high. The material price of the new DELTA-PROTEKT® KL 100 zinc flake systems is thoroughly comparable with that of the established system mentioned above. For the coater, therefore, there is no difference in the material costs.

References

Kayser K.: Draht 9/89, 11/89,  3/90,  5/90
DIN 50900
Gräfen H.: “Werkstofftechnik”, VDI-Verlag 1991

Last changed: Mar 14 2009 at 7:53 PM

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