Hightemperature corrosion and erosion of heat transfer pipes in coal fired boilers,such as tubes for super heaters and water walls, are recognized as a severeproblem, resulting in tube thinning and premature failure. In the applicationof a thermal spray coating, a metal powder is heated to a high temperature andpropelled toward the boiler tube surface with a high velocity gas (HVOF) orplasma jet. Once at the surface, the particles form a coating.
Thermal spray coatings have the advantage ofbeing applied in-situ in the boiler and as a consequence, they are more easilyrepaired after exposure. Coating Requirement HighHardness (To prevent erosion because of ash and silica) Hot corrosionresistance (that will react with the environment to produce a slow growingprotection oxide scale, which should not allow the corrosion species to diffuseinto the coating) Low porosity (Less area for erodent particle to impact and abrade) Smallgrain size (small grains tend to be stronger) Absenceof cracks (Ductile structure, thermal stability, cracking lead to spalling) Uniformcoating structure (Homogenous distribution of hard constituents & ductilebehavior) Smooth Surface
Highchromium content nickel based alloy Cr3C2-NiCr & In presence of sulfur andvanadium atmosphere NiCrTi & amorphous alloy of FeCrBSi as a widely usedcorrosion & wear resistant coating material, possesses good ability againstoxidation, sulfur induced corrosion and hot corrosion.
Chromium Carbide (Cr3C2) Due to hardchromium Carbide content, it offers more erosion resistance.
Nickle Chromium (NiCr) a) Chromium is acorrosion resistance alloy that binds the continuous skeleton of erosion resistantchromium carbide constituents. b) Due to high binding energy and ductility,nickel exhibit enhance erosion resistance by their ability to dissipate theimpact force to particles by plastic deformation. Feric Chromium Boron Silicon(FeCrBSi) A FeCrBSi amorphous alloy producing a partially amorphous, hardabrasive and corrosion resistance coating, with a service environment upto920C. Typical grinding and lapping techniques can be used to obtain highchrome like finishing. It can be used in a wide variety of high wearapplications, anti skid surfacing applications and corrosion environment.