Comparative Coating Porosity Analysis

Through pores and pinhole defects in coatings create paths for corrosive fluids or aggressive chemicals to attack the substrate material.

This local damage can quickly lead to critical component failure and inflict substantial repair and downtime costs. Low coating porosity is essential for gas and fluid control components as a diffusion of gases through a porous coating on valve components can result in an explosive gaseous mixture in the pipeline. Porosity can also affect the performance of the coated part in cyclic fatigue conditions where such defects can become stress concentrators which initiate mechanical failures.

Two methods were used to analyse the porosity of Hardide:

1. Microscopic optical measurements of porosity were recorded on cross-sections of samples of HVOF, D-Gun, welded overlay and Hardide. The image analysis measured the porosity of HVOF, D-Gun and welded overlay at between 1.5% and 2.5%, which is close to typical values. Measurements on several Hardide coated samples produced values of 0.0% and one of 0.05%, the latter probably an artefact due to dust on the sample or in the imaging system.
   
   
2. Chemical porosity detection involving the application of paper impregnated with chemicals that react with iron and visual checks of the coating surface for blue stains. This method is often used to detect pores in Hard Chrome plating. This test and analysis did not detect any through porosity on any of the tested Hardide samples.

It is concluded that the porosity of the Hardide coating is typically less than 0.05%. This exceptionally low porosity is a result of the CVD (chemical vapour deposition) process where the coating is crystallised atom-by-atom from the gas phase. The atoms have high surface mobility and are attracted to the areas of lowest energy such as holes or pores. Any defects are filled as the coating grows. This produces a very dense and impervious coating layer which protects the substrate from corrosion and aggressive chemicals.

Above: Hardide coating on a spray welded surface (Colmonoy 6, Ni/CrB)

Above: Analysis of an HVOF coating sample showing 2.55% porosity

Above: Analysis of a Hardide coating sample S16 showing 0.0% porosity