Surface Finishes

After your investment castings have been through our process, the time will come to select the appropriate surface finishes.

The most common finishing process is pickling and passivation, though there are more options available to you.

Pickling and Passivation


  • Hot steel acquires chromium oxide on the surface during pouring and also grinding and finishing stages. This leaves a chromium depleted layer at the surface which must be removed in order to improve corrosion resistance.
    Removing this mechanically leads to additional vulnerability caused by inclusions or scratches.
  • Instead, the low Cr layer can be removed chemically using a mixture of Nitric and Hydroflouric acids. These also remove reactive ferrous ions on the surface.
  • This process dulls the surface finish on the parts. An alternative process that does not affect the finish is electropolishing.


  • After pickling is completed, contaminants are left on the surface that need to be removed to leave a passive outer layer.
  • Different solutions of nitric acid are used to remove iron free radicals and other contaminants but these must be selected specifically in order to remove the appropriate contaminants.
  • This does not have any effect on the parts’ appearance.

High Velocity Oxygen Fuel Coating

  • This is a coating process where powder is propelled onto material by a high velocity flame.
  • A mixture of gaseous or liquid fuel is fed into a combustion chamber with oxygen and ignited.
  • The resulting flame has pressure up to 1MPa and a velocity greater than 1 km/s.
  • Powder is then poured into the stream and sprayed onto the material to leave a non-porous and tightly bonded coating that are resistant to wear and corrosion.


  • This is a coating process where a metallic plating is applied to a part.
    Parts are connected to the positive electrode (cathode) in an electrical circuit and immersed in an electrolytic solution.
  • At the opposite end of the circuit, the anode is made from the material to be coated on the part.
  • DC current is applied, and ions begin to oxidise from the anode. These dissolve into the electrolyte and travel to the cathode (ie. The parts).
    On the part surface, these anions reduce and become stable, leaving a layer of metal over the original part.
  • This process can improve hardness and corrosion resistance using nickel as the anode.


  • This is the direct opposite of electroplating. In this process, parts are connected to the anode and oxidise to remove material that travels to the anode.
  • This is useful for removing ferrous iron ions much like pickling, but it does not affect the surface finish.

Powder Coating

  • In this process, a powder based plastic coating is applied electrostatically (the part is charged opposite to the powder particles) and cured under heat to flow and form an impermeable skin over the part.
  • This is a rust preventative measure and can be applied in much thicker layers than other coatings.

Ultrasonic Cleaning

  • A solution is agitated using high frequency pressure waves (sound waves) to create cavitation bubbles.
  • These come into contact with a submerged part, penetrating cracks and blind holes and generating high localised force.
  • This force is generated by the bubbles imploding against the surface of the part, forcing localised jets of water to remove surface contaminants.