THERMAL DIFFUSION GALVANIZING
The thermal diffusion galvanizing process (Sherardizing) consists in diffusing zinc coating into steel items. The reaction takes place in a rotating electric oven filled with zinc powder at temperatures of 360–430 °C. The process leads to the formation of an outer layer of steel impregnated with zinc on the surface of the protected steel items. This resulting surface layer is characterized by extraordinary anticorrosive protection, without the original dimensions of parts having changed. In addition, the thermal diffusion method is characterized by unbeatable abrasion resistance due to the absence of any applied layer, which will subsequently peel. Due to lower temperatures, the internal steel structure is unaffected by the process and therefore this method is also suitable, for example, for protecting springs and similarly heat-treated elements.
The zinc process is carried out in closed chambers, where the zinc diffuses into the steel structure. For this reason, the chamber size limits the maximum dimensions of the parts intended for thermal diffusion galvanizing. We can sherardize parts with a maximum length of 30 cm for our customers. Rods are an exception: they can be larger; galvanizing is possible up to a length of 1 metre (from a diameter of 10 mm).
Coating cross-section, 200x magnification using an optical microscope
Advantages of the technology
- Strong adhesive bonding of layers – without peeling or cracks.
- High corrosion resistance.
- High coverage reliability – about twice as long a period of corrosion resistance compared to hot-dip galvanizing. With thermal diffusion galvanizing, for example, the standard thickness for a layer of Class 35 intended for a corrosive environment is 30 microns, whereas the equivalent is 70 microns using hot-dip zinc.
- High durability of the resulting surface – very high abrasive resistance.
- Uniform thickness of the resulting layer.
- Smooth surface prevents the formation of zinc build-ups and greater roughness.
- Complete coverage – repeatability of processing.
- Provides the perfect surface preparation for painting or rubber coating.
- Possibility to obtain zinc layer in the range from 15 to 120 µm.
- Relatively low process temperature – special parts such as springs can be processed without affecting their mechanical or strength characteristics.
- Possibility of coloured galvanizing: grey, silver, black and green.
- Environment-friendly process. No use of chromium plating.
Comparison of different methods of zinc galvanizing
- CSN EN 13811:2003 – Sherardizing – Zinc diffusion coatings on ferrousproducts – Specification (English)
- EN 13811:2003 – Zinc diffusion coatings on steel products.
- American Standard Testing: ASTM B633, ASTM B695
- Russian Federation Standard: GOST R 51163-98
In accordance with EN 13811, thermal diffusion coating thickness is determined according to the following classes: