Wear Protection of Pumps

Surface coatings are extensively used to protect pump components from degradation due to wear, corrosion and cavitation. These coatings have traditionally been based on nickel-chrome and cobalt-chrome alloys, but ceramic metal composites (CMCs) provide improved wear resistance.

There are several coating technologies that are commonly used for applying coatings to pump components, and each has its own advantages and disadvantages. A basic understanding of coating processes and materials will assist the user in selecting the right solution for a specific pump problem.

Processes

Thermal spray coatings are commonly used for applying tungsten carbide cermet coatings onto impeller and casing wear ring surfaces, impeller inlet and outlet vane tips, anti-galling coatings onto pump shafts (e.g. to assist with assembly and disassembly on stainless steel pump components), carbide and ceramic coatings onto shaft sleeves and for the refurbishment of worn or damaged shafts. There have recently been significant material and process developments, especially the high velocity oxy-fuel (HVOF) spray process, and it is now possible to produce carbide coatings with exceptional wear (i.e. hardness of >1200 HV0.3) and corrosion ( i.e. ASTM B117 > 400 hours) resistance and high toughness (i.e. tensile adhesion strength ASTM C633 > 80 MPa).

Spray & fuse coating technology has been employed in pump applications for many years, and the Colmonoy range of spray-fuse coatings is a well-known brand, particularly in API 610 pumps.

Plasma transferred arc (PTA) and laser cladding are weld overlay processes that melt a powder consumable and weld it (i.e. form a metallurgical bond with) to the substrate. Both have low substrate heat input and substrate dilution when compared with conventional welding techniques like MMA or TIG. Laser cladding has reduced heat input and dilution compared with PTA, making it more suitable for certain problematic materials (e.g. martensitic stainless steels), but at significantly higher cost. In most pump applications there is no compelling performance advantage. The PTA process is typically used to apply relatively thick (i.e. 2-3 mm) wear and/or corrosion resistant cobalt (Stellite and Triballoy) and nickel (Inconel 625) alloys. Stellite 6 is a well know overlay in the pump and valve industry and it is also possible to apply a tungsten carbide-containing nickel based overlay. PTA and laser clad carbide coated shaft sleeves are commonly used in slurry pump applications.

Materials

Stellite 6 is a well-known cobalt-chrome-tungsten alloy that combines good corrosion and wear resistance with excellent galling and sliding wear resistance. This coating material can be applied using any of the coating processes previously described. A thermally spayed Stellite 6 will be suitable for low-stress sliding wear applications with low to moderate abrasion or erosion wear, e.g. impeller and casing wear rings in a process medium with relatively low solid particle content. Where high mechanical or thermal stresses, or high wear rates due to high solid particle levels are expected, PTA or laser cladding will be more appropriate. Both PTA and laser can achieve sufficiently low dilution in a single pass weld, reducing the required overlay thickness to approximately 3 mm. This provides a significant benefit over conventional welding techniques which require multiple passes, producing overlays of around 6 mm.

Tungsten Carbide wear resistant ceramic particles in a ductile metal matrix (typically cobalt or nickel alloys), result in a material with hardness comparable to that of a ceramic but with ductility approaching that of a metal. Thermal spray is the most flexible process in terms of binder alloy type, and it is possible to spray a wide variety of different carbide binder phase alloys, such as:

  • WC-10Co4Cr - Excellent wear and moderate corrosion resistance
  • WC-10Ni5Cr - Excellent wear and good sea-water resistance
  • WC-NiCrMo (Hastelloy C binder) - Excellent wear and sea-water corrosion resistance.
With the HVOF process it is possible to spray carbide coatings with very fine carbides and high carbide mass fraction, providing superior wear resistance in the presence of fine abrasives particles. Coating thickness is however limited to approximately 0.5 mm and these coatings are not suitable for applications where the coating will be subject to high mechanical or thermal stresses. HVOF coatings are therefore commonly used in light to medium wear applications and/or where a corrosive medium is present. All the other processes have a limitation on minimum carbide size and carbide fraction because of the high processing temperature and time. Smaller carbide particles will decompose at these higher processing temperatures, resulting in the formation of unwanted hard and brittle phases in the binder alloy and a decrease in the carbide fraction. Furthermore the binder composition is limited to nickel-boron-silicon and nickel-chrome-boron-silicon alloys, with generally relatively low chrome content, therefore limiting the use of these coatings in certain corrosive environments.

Conclusion

It is clear that the pump user will benefit from developing a relationship with a knowledgeable and reputable coating service provider to assist in ensuring that the optimum coating solution is identified and correctly applied. This can result in a significant extension to the service life of pump components and improve pump efficiency over its service life, resulting in a substantial reduction in the life cycle cost of the pump.


This is a summary of the magazine article entitled "Wear Protection of Pump Components Using Hard Coatings" which appeared in pumpindustry, the Australian pump magazine. See the full article here or download here

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