The right pre-machining quality (part 2)

To avoid this here are guidelines to check the quality of the base material of any rod before shipping it for coating:

1. Porosity in the material can cause holes up to the coating surface. Also the bonding strength on and around porosity can be substandard. Porosity may occur after welding. With a dye check porosity can be identified.
   Cracks cause the same issues as porosity. Cracks may form at rod ends on forged rods.
2. Corrosion should be avoided at all times. Corrosion is often a sign of low quality material. Flash rust can be removed with sand/grit blasting. Deeper corrosion, however, remains in the material under the coating and can eventually push the coating up causing the coating to fail. Under most circumstances corroded rods are not coated.
3. Impurity has the tendency to break out. Impurities most often occur from a less noble material that may lead to or accelerate a corrosion process. Before coating a rod all impurities have to be removed.
4. Roughness is a problem when it is too high on a pre-machined rod. A high roughness decreases coating bonding because of two reasons:

a. The blasting medium will not hit the entire rod as it cannot reach the bottom of the machining groove. This may lead to alien material remaining on the rod surface under the coating.
b. Coating particles will not have their maximum bonding strength due to the sharp micro-edges and micro-corners in the machining roughness.

1. Porosity and/or cracks       2. Corrosion / mill scale         3. Impurity          4. Too rough

Many of the material non-conformities can be repaired, however, a final check and correction of material non-conformities are important not only to the coating quality but also to the processing time and coating costs.