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Adherent coating of stainless steels

Stainless steels are extremely corrosion-resistant materials. They are hardy and ductile while having good strength parameters.

Stainless steels consist mainly of iron, chromium and nickel.

They can be welded easily but soldering poses as a problem. They need special preprocessing before galvanic coating is possible.

The preprocessing can be done in numerous ways.

All methods have in common that an adherent layer (“bonding layer”) needs to be applied to the steel. The purpose of the bonding layer is the reduction of chromium oxide into chrome.

Iron oxide and nickel oxide can be activated easily under acidic conditions which means that we work under these conditions primarily.

There are three procedures:

  • nickel strike – a very hydrochloric nickel chloride electrolyte
  • gold flash – a sulfuric gold electrolyte
  • copper flash – a sulfuric copper electrolyte

Evaluation of the three procedures

In the following tables we want to discuss the pros and cons of the aforementioned procedures.

Nickel strike


  • Relatively economic procedure
  • Good dispersal capabilities
  • Tried-and-tested procedure

  • Very corrosive
  • Works only with nickel anodes
  • Emits chlorine gas if insoluble anodes are used (e.g. PT, Pt-Ti, MOXTi)
  • Deposits are ferromagnetic (questionable if stainless steels with low permeability are used)
  • Permeability increases with rising temperatures (350°C, vacuum)
  • The deposits are vulnerable to corrosion (the material has to be coated in a timely manner before that happens)
  • Optical inspection of the layers is difficult because stainless steel looks very much alike without coating

Gold Flash


  • Good dispersal capabilities
  • Not ferromagnetic
  • Very good indication in terms of color (yellow in combination with stainless steels)
  • Coating not corrosive (can be dried, inspected and stored)
  • Multiple, different anodes possible (Pt, PtTi, MOXTi, graphite, gold)
  • Tampon processing is easily possible

  • Expensive (typical electrolyte consists of 3g gold per liter)
  • Poisonous electrolyte

Copper Flash


  • Good dispersal capabilities (can be varied via copper concentration)
  • Not ferromagnetic
  • Quick procedure (1 – 2 minutes)
  • Very good indication in terms of color (red in combination with stainless steels)
  • Multiple, different anodes possible (Cu,Pt,PtTi,MOX-Ti,Gold)
  • Tampon processing is easily possible
  • Affordable

  • Not yet field-tested (although internal testing was reproducibly successful)

Internal testing procedure for strikes and flashes

Basis material: GT-typical strips of 1.4301 (ideally materials supplied by clients)

Squench Test:

  • Degrease the strips
  • Pickle / anodize the strips
  • Flash treatment according to internal regulations
  • Wet on wet coating with 25µm “bright” copper
  • Repeatedly (3 cycles):
    • bring the coating to a red heat
    • chill in cold water
    • inspect for blisters or disbondment
  • bending test of the basis material and the coating until brittle fracture occurs
  • inspection of the site of fracture under a microscope
  • there must not be any disbondments visible