SheraTop®-System

Sherardizing

Special features of Sherardizing

  •   Sherardizing produces a layer from the vapor phase
  •   The process is diffusion controlled
  •   The coating is build up by a surface reaction
  •   The layer grows from the surface
  •   The result is a zinc-iron alloy layer

In contrast to other galvanizing processes all kind of steel like spring-, stainless- or high-strength steel, cast iron, forged steel or sintered metal can be sherardised, as well as other metals such as copper or nickel.
Zinc diffusion layers are...

  •   adjustable in thickness
  •   contour-fit, variable and evenly
  •   between 10 microns and 100 microns thick
  •   alloyed by a diffusion bond
  •   formable - no flaking
  •   hard, impact - and wear resistant
  •   temperature resistant
  •   100% inorganic - free of toxic substances
  •   UV-resistant
  •   weldable
  •   hydrogen-free
  •   perfect primer
  •   no crack infiltration
  •   adjustable friction by Topcoat

 History

Around 1900 the Englishman Sherard Cowper-Coles developed a special galvanizing method in analogous to the powder pack carburizing method.  The process was later named after his first name Sherard-izing, to honour his invention.
Sherardizing produces uniform protective zinc-iron-layers with a good protection against corrosion and wear.The reliable corrosion protection was used for example on iron-coins around 1920, until today some of them do not show any rust.

Method

The treatment is carried out by heating zinc powder and goods in a closed rotating drum.

From about 300° C zinc vaporizes from the solid powder and reacts via the gas phase with the steel substrate. The process temperatures are usually between 350°C and 500°C.

If the pre-treatment is carried out by shot blasting, the process is completely dry. Therefore Hydrogen embrittlement can be fully excluded.

Layer structure

For steel products the zinc iron protective coating contains about 90 wt.-% zinc. This ensures an active cathodic corrosion protection. Zinc-iron alloyes have a much higher melting point than pure zinc (Tm = 419.5 ° C), the surface is heat resistant and hard. The coating hardness is about 400 HV (41 HRC).
Due to the iron content are Sherardised layers nobler as pure zinc layers, this has a positive influence on the corrosion rate, and corrosion products.

Apperiance

The matte gray metallic appearance differs significantly from other zinc layers. This difference is one of the key benefits of Sherardisierens:

The microcrystalline surface structure is a perfekt primer for post treatmens like passivations, topcoats or other coatings such as rubber linings. Sherardised duplex systems are therefore very resistant.

Standards

Sherardizing is internationally specified in DIN EN ISO 17668 and EN 13811 Sherardizing / Zinc diffusion coatings.