SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Analysis of Corrosion Data For WC- 17%Co and WC-10Co-4Cr HVOF Thermal Spray Coatings

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Introduction lThe method of evaluation involves optical and SE microscopy of cross-sectioned corrosion samples. lLiterature searches have provided extensive historical metallurgical data on chemical reactivity and electrochemical stability in various environments for the Cobalt bound Tungsten Carbides both sintered and sprayed. lThe data correlation is presented here.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Metallurgical Data lOptical Microscopy shows chemical attack of binder materials at interfacial layers. lSEM revealed grain removal (Both Co grains and WC particles) is accomplished by Cobalt Oxide dissolution at interfacial layers in the matrix and along phase and grain boundaries. lXRD evaluation of corrosion products showed the presence of CoCl 2, CoO, CoOOH, Co(OH) 2, WO 2 (possibly W 2 O 5 ) and FeCl 2. lElectrochemical potential measurements revealed a higher surface corrosion current ( I corr ) for the WC-17%Co vs. the WC-10Co-4Cr which increased significantly as the pH of the solution was increased. lAlso, the corrosion current increased with the same relationship as the concentration of halogen (chloride) ions increased in solution.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center B117 Results lASTM B117 proved to be the most aggressive test regardless of surface condition. The cobalt binder was leached and penetration to the base metal occurred. As shown in the metallographic cross-sections. lXRD evaluation showed the presence of CoCl 2, CoO, CoOOH, Co(OH) 2, WO 2 (possibly W 2 O 5 ) and FeCl 2. lSimilar Results were obtained in studies listed in reference (1) and (2) for alkaline solutions.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center GM Cyclic Tests Results Surface condition showed a mat gray appearance with some localized pitting. The microstructure was unremarkable except for some small pits evident on the surface. XRD analysis revealed CoCl 2, CoO, and W 2 O 5 or WO 3. Tungsten Oxide is very robust, however the various oxidation state changes with pH.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center B117 Cross-section

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Atmospheric Sample Showing Heavy Oxidation

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Atmospheric Corrosion Test Surface Oxidation Surface condition showed a mat gray appearance with some localized pitting. XRD of the corrosion products revealed CoCl 2, CoO, in relative proportions. Analysis of the bulk surface also showed the presence of CoO, WO 3, and W 2 O 5. The microstructure showed heavy oxidation at the surface and along lamellar planes with increased porosity and roughness at the surface. However, there was no intergranular separation or leaching evident.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Summary The finish ground coatings showed corrosion products that contain chlorides, several oxide and hydroxides of Co. Whereas the as sprayed samples only showed the initial oxide and chloride of cobalt over the same exposure cycle. Anodic oxidation and dissolution of cobalt binder takes place in two or three stages: Formation of protective CoO. Formation of CoCl and CoOOH. Formation of Co(OH) 2 or Co(OH) 3 (in solution). The rate of this dissolution and leaching from the matrix is a function of the pH and halogen content, the higher the pH or halogen concentration, the faster dissolution occurs. The more neutral, or even slightly acidic the slower it occurs.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Conclusions lMarine Corrosion of WC – Cobalt coatings is not a function of surface finish but is a function of chloride concentration and solution pH and therefore coatings must be protected in service by organic barrier films (hydraulic fluid). lCoO is an unprotective surface film and is subject to dissolution due to Cobalt’s low affinity of oxygen and high affinity for halogen ions. lOxides of Tungsten are very stable and cathodic to the Cobalt binder. However, Cr oxide (Cr 2 O 3 ) is protective and will stabilize the the Co and CoO and enhance corrosion resistance.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center Conclusions (con’t) lThis data only provides for a thorough understanding of the corrosion mechanism for these coating and in no way reduces their ability to perform in service. lFlight testing of a Coast Guard H-60 Tail Landing Gear Piston coated with WC-17% Co showed only oxidation of the surface with minimal changes in surface roughness (still less than 8 Ra) after more than 2 years of service in a hostile marine environment. lFlight testing of a P-3 Main Landing Gear Piston coated with WC-17% Co shows virtually no signs of oxidation. lHydraulic fluid film is sufficient to protect the Cobalt binder from dissolution in marine environments.

SPACEPORT ENGINEERING & TECHNOLOGY National Aeronautics and Space Administration John F. Kennedy Space Center References l“Performance of HVOF-sprayed Coatings in Aqueous Environments” S. Simard, B. Arsenault, CNRC, Montreal, Canada; K. Laul, M. Dorfman, SulzerMetco Ltd. Westbury New York. Published in the conference proceedings of the NACE International no l“Electrochemical behaviour of cobalt in aqueous solutions of different pH” W.A. Badawy, F.M. Al-Kharafi, J.R. Al-Ajami Journal of Applied Electrochemistry, Vol 30 n6 p , 2000 l“Electrochemical and XPS investigations of cobalt in KOH solutions” K.M. Ismail, W.A. Badawy, Journal of Applied Electrochemistry Vol 30 n5 p , 2000 l“Electrodeposition and dissolution of Co-W alloy films” C.L. Aravinda, V.S. Muralidharan, S.M. Mayanna, Journal of Applied Electrochemistry, Vol 30 n6 p , 2000 l“The Nature of Oxide Films on Tungsten in Acidic and Alkaline Solutions” R.S. Lillard, G.S. Kanner, and D. P. Butt, Journal of Electrochemical Society Vol 145, n8 p 2718 –