Coating Selection
for Structural Parts

If your part is designed for structural purposes with no sliding contact, then your main concern will be corrosion and, if there is cyclic loading, with fatigue. There may also be concern about the acceleration of mechanical or fatigue failures by corrosion. Before continuing with the analysis, you should read the section on corrosion and consequential mechanical failures. You can also consult the section on fatigue with our Problem Solver.

The base material

Those of most concern will be engineering steels and cast irons, stainless steels (most probably ferritic types), or aluminium alloys.

Selecting the surface treatment

A) No corrosion, but with concern about fatigue.

1. If there is no concern about corrosion, but a requirement for improved strength or fatigue resistance, then consider using any of the following:
    
   1. Shot peening
   2. Improving the surface quality and finish (grinding, lapping, polishing)

B) If there is concern about corrosion, then the corrosive media and the temperature are both important. Additionally, if the part being considered is in contact with another metallic component of a dissimilar material, then there is a strong likelihood of galvanically assisted corrosion.

1. For outdoor, normal atmospheric corrosion consider the following:
    
   1. Painting and Powder Coatings with appropriate preparation and priming. Consider zinc-loaded paints for extra galvanic protection
   2. Heavy Electrolytic Nickel Plating will provide a diffusion barrier provided that there are no local defects in the coating
   3. Electroless Nickel will provide good corrosion protection
   4. Galvanising or Sprayed Zinc will provide prolonged protection, even in polluted environments
   5. Electrolytic Zinc, for smaller parts
   6. Aluminium Ion-Plating, an evaporated aluminium coating
   7. Phosphating, for moderate protection
   8. Bath Aluminising, for steel strip products
   9. Anodising, for protection of aluminium alloys, preferably sealed
   10. Electro-Ceramic Coating, for protection of aluminium and magnesium alloys


2. For marine environments, including aerospace:
    
   1. Painting, use Zn or Al-loaded paints for best protection of steels
   2. Galvanising or Sprayed Zinc will provide moderate protection
   3. Aluminium Ion-Plating
   4. Electrolytic Zinc or Zn/Ni for smaller parts, chromate passivated, e.g. fasteners
   5. Cadmium Plate for maximum protection, preferably chromate passivated
   
   
3. For highly oxidising environments and high temperature situations, many traditional coatings will oxidise. The best options are:
    
   1. Chromising against oxidation at temperatures up to 800^oC
   2. Aluminising will protect against oxidation, sulphur and carburising. Can exceed 800^oC
   3. Thermochemically formed chromium oxide-based ceramics at up to 600^oC
   4. Thermally Sprayed MCrAlY and ceramic-based thermal barriers
   
   
4. For caustic environments, the best options are:
    
   1. Thermochemically formed chromium oxide-based ceramics
   2. Thermally Sprayed ceramics; chromium oxide, alumina, preferably sealed
   3. Cadmium Plate for moderate protection, preferably chromate passivated
   4. Electroless Nickel will provide good protection
   5. Heavy Electrolytic Nickel Plating will provide good protection
   
   
5. For acidic environments, the best options are:
    
   1. Thermochemically formed chromium oxide-based ceramics
   2. Thermally Sprayed ceramics; chromium oxide, alumina, preferably sealed
   
   
6. Under stress assisted corrosive conditions, the best option is:
    
   1. Shot peening followed by corrosion protection appropriate to environmental conditions

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