PREPARED BY
HYDROELECTIRC DESIGN CENTER,
PORTLAND DISTRICT, COE
EROSION and CORROSION - RESISTANT
THERMAL SPRAY COATINGS
The U.S. Army Corps of Engineers' Construction Engineering Research Laboratories (CERL) has identified and developed a thermal spray coating material and process that will protect hydraulic turbine and pump water passages from damage due to erosion, cavitation resulting from erosion, and dis-similar metal corrosion damage. These surface damaging phenomena may be present to some degree in all hydraulic rotating equipment, and the repair of resulting damage depletes O&M funding and burdens the ever diminishing project maintenance staff.
The R&D program was conducted for Headquarters, U.S. Army Corps of Engineers (HQUSACE) under Construction Productivity Advancement Research (CPAR) Work Unit 3121-LY4, "Development of Cavitation/Erosion-Resistant Thermal Spray Coatings." The work was performed by CERL in partnership with the Thermal Spray Laboratory at the State University of New York (SUNY) at Stony Brook. The CERL Principal Investigator was Dr. Ashock Kumar and his assistant was Dr. Jeffrey H. Boy. The independent program technical monitory were Andy Wu, CECW-EE and Craig Chapman, CECW-OM.
The resulting R&D program report gives a good overview of hydraulic machinery water passage damage which can occur as a result of erosion, cavitation, and dis-similar metal corrosion. The report further describes current weld (fusible process) and thermal spray (non-fusible process) repair processes, and repair materials used. A valuable summary of past and current comparison testing (tests performed as a result of this R&D effort) of repair processes and materials is presented.
After an extensive literature search, consultation with academia and industry, and the laboratory testing of 21 thermal spray coatings and application methods, the report concludes that the spray metal of choice is Stellite 6 and that the material should be applied using the High Velocity Oxyfuel (HVOF) process. The report further details the optimal thermal spray methodology using this material and process. Laboratory tests have shown that the application of Stellite 6 results in less material loss (5.33 mm3/h) in slurry erosion wear testing than 304 stainless steel (11.17 mm3/h loss) and ASTM A572 carbon steel (19.70 mm3/h loss). The change in a surface's roughness and geometry due to erosion, can result in the formation and collapse of cavitation vapor bubbles which result in surface damage. Minimizing erosion can minimize this resulting type of cavitation. Tests also conclude that the electrical potential differences between Stellite 6 coated specimens and both ASTM A572 and A36 carbon steels in tap water were 0.25 volts, half the potential difference between 304 stainless steel and mild carbon steel (i.e., 0.50 volts). Dissimilar metal corrosion damage usually occurs at the metals interface boundary when stainless steel weld repairs are made on carbon steel water passages. It is also important to note that the thermal spray processes avoid the inducement of thermal stresses associated with the fusion welding processes.
This R&D program has shown that the current state of the art in thermal spray processes and materials cannot provide a coating that is much better in resisting cavitation damage than a carbon steel material. The report concludes that repairs required as a result of direct cavitation damage should be performed using a fusible material by a welding process. The report has shown that the spray method of surface repair is at least half the cost of welding. With this in mind, one should keep an eye on advances in this technology, as one day a material and process may be developed that will out perform carbon steel in cavitation environments.
A field test using Stellite 6 is currently underway at the Tennessee Valley Authority's (TVA) Raccoon Mountain Pumped-Storage Plant, Chattanooga, TN. Please contact Dr. Kumar, Ph. 217.373.7235, for additional information on the testing or regarding the R&D work.
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