Zinc flake provides reliable protection where hot dip galvanising and electroplating cannot be used say Christine Rohr and Detlef Stockert
Crypto and Banking
We recommend the following high-quality options for secure Bitcoin transactions and online banking services:
BTC and ETH QR code generator websites
CRA Login Canada Revenue Agency
MyCRA Login | CRA Tax Refund | MyCRA | CRA My Account
MyCRA Login | CRA Tax Return | MyCRA | CRA My Account
RBC Royal Bank Login
RBC Canada | RBC Online Banking Sign In| Royal Bank Canada Business Login
CIBC Online Banking Login/span>
TD EasyWeb Canada Trust Login
TD EasyWeb Login | TD EasyWeb Banking Login | TD Canada EasyWeb Login| Login TD EasyWeb | TD Online Banking Login
While electroplating and hot dip galvanising 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 well-defined temperature control (quenching and tempering) when processing the steel. As a rule, the reheating temperature for this process lies between 340 and 425 degree Celsius. This value has a crucial effect on other heat treatment processes. However, the temperature of hot dip galvanising is higher than that used for the heat treatment because the melting point of pure zinc is 420 degree Celsius. During the galvanising process, the hardness of the steel changes uncontrollably. Electroplating cannot be used for coating high strength steels.
Since the component is used as the cathode, atomic hydrogen is generated on the surface when it is immersed in the electrolyte. The hydrogen diffuses into the substrate and some of it then remains in the form of dissolved hydrogen in the interstitial sites of the metal’s crystal lattice. This leads to hydrogen embrittlement-a term used for a series of failure mechanism. 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. However, depending on the structure of the electroplated layer, the effusion of the hydrogen is possible by means of tempering. This process is costly, both in terms of time and money, and the method never eliminates the problem altogether.
Zinc flake coating makes an alternative. Zinc flake coatings are used for high strength materials. These coatings contain flat zinc plates about 10 Ã?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, i.e, dip-spin systems for medium sized mass produced small parts such as bolts and screws, spraying for large bolts and stamped parts and dip-drain for tubes or larger panels with simple geometries.
After the coating has been applied, it is cured at relatively low temperatures (Delta-Tone at 180 to 200 degree Celsius, Delta-Protekt KL100 at 200 to 230 degree Celsius and competitor’s product at 300 to 360 degree Celsius). When finished, the cured coating contains about 85 per cent 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.
Zinc also provides sacrificial protection. During corrosion, the individual zinc flake is converted to Zn(OH)2. This produces what we call white rust. The oxidation process (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 mechanically damaged (scratches). These electrons are of course constantly consumed by the corrosion process. The reduction of oxygen is the most probable reaction to take place in the presence of moisture.
The properties of 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 colour, 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) provide better protection against chemicals (Figure 1), while the inorganic topcoats (such as Delta-Protekt VH 300) can be applied in much thinner layers – and therefore with tighter tolerances (Figure 2). Common to all topcoats, however, is that the desired friction value can be obtained with the addition of an integrated lubricant.
Topcoats increase 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 Ã?µm basecoat and a 3 Ã?µm inorganic topcoat will provide a service life during a salt Ã?Âspray test according to DIN 50021 SS of more than 1000 hours and up to three cycles in the “Daimler Chrysler” test, (condensate test), where one cycle corresponds to 14 days.
There are clear advantages associated with the use of third-generation MKS materials. Using 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.
The new siliceous topcoats are also associated with a great range of advantages. In contrast to conventional, epoxy-resin based organic topcoats, for example, the lubrication of these topcoats is not diminished even following long exposure to thermal stress (up to 100 hours at 180 DC). Since these coats are applied in significantly thinner layers of around 2 to 3 �µm, they offer significant corrosion protection even where tolerances are low. This not only creates the added benefit of very high yields associated with these inorganic topcoats, but the layers of organic topcoats required are generally thicker by as much as 8 �µm.
Volkswagen and General Motors recently approved a third generation, high performance chromium (VI)-free microlayer corrosion-protection system (MKS) that complies with their material specifications TL 245 (VW) and GM 3359 respectively for protecting metric threaded parts made of high strength steel against corrosion.
Article courtesy: D�¶rken MKS-Systeme GmbH & Co. KG,WetterstraBe 58, 58313 Herdecke, Germany. For further information contact D�¶rken MKS-Systeme India representative: Tel: 022-6590 7137, Cell: 91- 98672 89420, E-mail: mks@in. doerken.com
Leave a Reply