Analysis of Corrosion Data For WC-17%Co and WC-10Co-4Cr HVOF Thermal Spray Coatings

1. Analysis of Corrosion Data For WC-17%Co and WC-10Co-4Cr HVOF Thermal Spray Coatings

2. Introduction • The method of evaluation involves optical and SE microscopy of cross-sectioned corrosion samples. • Literature 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. • The data correlation is presented here.

3. Metallurgical Data • Optical Microscopy shows chemical attack of binder materials at interfacial layers. • SEM 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. • XRD evaluation of corrosion products showed the presence of CoCl2,CoO, CoOOH, Co(OH)2, WO2 (possibly W2O5) and FeCl2. • Electrochemical potential measurements revealed a higher surface corrosion current ( Icorr) for the WC-17%Co vs. the WC-10Co-4Cr which increased significantly as the pH of the solution was increased. • Also, the corrosion current increased with the same relationship as the concentration of halogen (chloride) ions increased in solution.

4. B117 Results • ASTM 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. • XRD evaluation showed the presence of CoCl2,CoO, CoOOH, Co(OH)2, WO2 (possibly W2O5) and FeCl2. • Similar Results were obtained in studies listed in reference (1) and (2) for alkaline solutions.

5. 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 CoCl2, CoO, and W2O5 or WO3. Tungsten Oxide is very robust, however the various oxidation state changes with pH.

6. B117 Cross-section

7. Atmospheric Sample Showing Heavy Oxidation

8. Atmospheric Corrosion Test • Surface condition showed a mat gray appearance with some localized pitting. • XRD of the corrosion products revealed CoCl2, CoO, in relative proportions. Analysis of the bulk surface also showed the presence of CoO, WO3, and W2O5. • 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. Surface Oxidation

9. 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.

10. Conclusions • Marine 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). • CoO is an unprotective surface film and is subject to dissolution due to Cobalt’s low affinity of oxygen and high affinity for halogen ions. • Oxides of Tungsten are very stable and cathodic to the Cobalt binder. However, Cr oxide (Cr2O3) is protective and will stabilize the the Co and CoO and enhance corrosion resistance.

11. Conclusions (con’t) • This 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. • Flight 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. • Flight testing of a P-3 Main Landing Gear Piston coated with WC-17% Co shows virtually no signs of oxidation. • Hydraulic fluid film is sufficient to protect the Cobalt binder from dissolution in marine environments.

12. References • “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 295 1999 • “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 693-704, 2000 • “Electrochemical and XPS investigations of cobalt in KOH solutions” K.M. Ismail, W.A. Badawy, Journal of Applied Electrochemistry Vol 30 n5 p 1303-1311, 2000 • “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 601-606, 2000 • “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 –2725 1998