COATING TECHNOLOGY
SECTION TITLE
also infl uence the severity and rate of corrosion.
SELECTING COATINGS
Many factors support the addition of a properly selected coating for added corrosion resistance. Coatings act as the fi rst line of defence in contact with the corrosive media and with low porosity can eff ectively block corrosive media penetration to the substrate. Multiple types of corrosion can attack metal components simultaneously which may not be initially anticipated, making coatings a sensible added solution. Coatings are often applied when
erosion, abrasion and other wear factors are anticipated in order to extend the working life of components in demanding applications.
Advancements in materials,
technologies and equipment have allowed for a wide range of available coating options. Some conventional solutions like hard chrome plating (HCP) are now facing a global phase-out (April 2024 in the EU and 2039 in the US set by California) due to health and environmental risks. Common existing alternatives to HCP include high velocity oxygen fuel (HVOF) and physical vapour deposition (PVD) technologies. Although successful in some applications, each has its limitations.
HVOF coatings can be thick and
durable, but internal surfaces or complex shapes are not permissible. T ey are rough and porous and often require post-coat grinding. Cobalt, often used as the
binder, can wear and degrade faster which compromises the integrity, surface fi nish and life of the HVOF coating. HVOF coatings typically have at least 1-2vol% porosity, which can open a path for corrosive media to attach the substrate. T e porosity is often sealed using epoxies or other organic sealants which restrict
operating
temperatures below 200°C. PVD coatings are typically hard and thin (<5µm) but are considered brittle with reduced load-bearing capacity. Various wet electroplating and electroless coatings are more suitable for internal surfaces and complex geometries, but have lower hardness and wear resistance than HCP. T e coating technology which combines all benefi cial attributes into one solution is chemical vapour deposition (CVD).
CVD COATING TECHNOLOGY Hardide’s CVD coating uses reaction gases in a coating chamber to create a very dense tungsten/tungsten carbide layer with desired application thicknesses from 5µm-100µm. T ese coatings are binder- less, porosity-free and exhibit unique ductile properties with hardness ranging from 400Hv (41HRc) to 1,800Hv (82HRc). As applied, they
typically have a surface fi nish of ~0.5 - 0.6µm Ra, which can be polished to 0.2- 0.3µm Ra without the need for grinding. T is technology allows for the coating of complex geometries including ID’s of cylinders, odd shapes and porous materials. One
Coating Internal Diameters
example is a downhole mesh sand screen used by Chevron which boasts over a 10 times corrosion resistance improvement using Hardide CVD. T ese unique properties have helped solve many severe service challenges in
Coating of Metal Mesh
oil and gas downhole tools and pumps and in aerospace wing, door and landing gear components in several new Airbus commercial aircraft. In fl ow control valve components, Hardide CVD can withstand operating temperatures up to 400°C or cryogenic LNG valve stem applications down to -196°C to mitigate fugitive emissions. With respect to corrosion resistance,
the virtually pore-free Hardide CVD coating matrix is highly resistant to chemicals, acids, as well as H2S and CO2 related service conditions. T is off ers customers an added superior corrosion barrier, in addition to exceptional wear resistance characteristics.
Mark Hanania is the Director of Strategic Business at Hardide Coatings.
www.hardide.com
www.engineerlive.com 29
Coating of Valve Components
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