Sherardizing or diffusion galvanizing is the application of a zinc layer, in a gas phase, to steel products. This zinc layer ensures that the product does not require chemical treatment. It significantly improves the durability of products that have been treated.
It is not necessary to pre-treat the products. Products that are metallic blank can be sherardized without pre-treatment. Possible contaminants or oxidation products (such as mill scale, for example) need not be a problem. These are removed beforehand by means of steel blasting.
The products are heated together with solid zinc powder in closed rotating drums. At a temperature of 300 ºC, the zinc powder starts to evaporate. The zinc powder reacts with the steel surface in this vapor phase. The process is carried out at temperatures between 350 C and 500 ºC.
The sherardised base layer (base coat) is now inextricably linked to the metal product as a zinc-iron alloy. The diffusion galvanized layer actively protects the base product. This is called cathodic protection. This cathodic protection period can be influenced by the adjustability of the layer thickness.
If you want to protect your product against corrosion, you want to preserve its value and functionality for a long time. Often because the costs for replacement or maintenance are significantly higher than the costs for good corrosion protection.
The service life of the active corrosion protection depends on the amount of zinc applied. A thicker layer protects for longer. This relationship has been demonstrated in numerous studies.
The standard EN ISO 14713 describes the service life of zinc layers in different corrosion categories. The expected life of a sherardised top layer of 35 µm on average is more than 20 years in an urban environment (C3).
Since zinc is less electrochemically than iron, it provides active, cathodic protection. This means that as long as enough zinc is in the vicinity of the base material, the latter will not be affected.
The corrosion products of the zinc bind to the outside and protect it. Scratches are filled in a self-repairing manner with these corrosion products.
In steel products, the zinc content is approximately 90 percent, so that active, cathodic corrosion protection is guaranteed. Zinc-iron layers have a significantly higher melting point than pure zinc (Tm = 419.5°C), the surface is more temperature resistant and hard.
The hardness of the layer is above 400 HV (41 HRC). Due to their iron content, sherardised coatings are more noble than pure zinc coatings, which has a positive influence on the corrosion resistance.
Almost all types of steel and geometric shapes can be provided with an even and adjustable zinc layer. Sherardization is widely applicable. There is even so much that you could say that almost nothing is impossible.
Sherardizing is a reliable and time-honored application, which was already developed in the 1900s.
The name sherardizing refers to the inventor of the technique: Sherard Cowper-Coles.
SHERART has continued to refine and improve this technique.
A historical application of sherardization is iron coins issued in 1920.
There are still copies, without rust, preserved.
Sherardising is a sustainable solution, because products last longer due to the diffusion galvanizing process. The service life is extended.
In addition, it is an environmentally friendly solution. No chemical treatment is required and there is therefore no chemical waste.
Because sherardizing is a so-called closed zinc process, no toxic zinc fumes are released.
The zinc layer that is applied is uniform and follows the contours of the product exactly. This does not change the original geometry.
Sherardized products have a neutral, matte gray appearance.
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