A comparison between corrosion resistances of some HVOF-sprayed metal alloy coatings

This study compares the corrosion resistance of one Co–based alloy coating, namely Co–28Mo–17Cr–3Si (similar to Tribaloy-800), four Ni-based alloy coatings, namely Ni–17Cr–4Fe–4Si–3.5B–1C (Diamalloy-2001), Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006), Ni–22Cr–9Mo–4Nb–5Fe (similar to Inconel-625), Ni–32Mo–16Cr–3Si–2Co (similar to Tribaloy-700), and a (WC-12Co)–33Ni–9Cr–3.5Fe–2Si–2B–0.5C cermet–Ni alloy blend coating. They were produced by liquid-fuelled HVOF spraying onto AISI1040 steel plates. Electrolytic hard chrome (EHC) plating was characterised as a reference material, to verify whether some HVOF coatings are suitable as an EHC replacement. The microstructure of the coatings was examined by SEM and XRD. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution; the corrodkote test (ASTM B380-97R02) was also performed, to rank coatings qualitatively.The lowest corrosion current densities (Icorr) were recorded for EHC and Tribaloy-700. The latter coating contained few secondary phases and little porosity; the damage was mainly due to corrosion activation along lamellae boundaries. Diamalloy-2001 exhibited the highest Icorr and was significantly damaged after the polarization test, as its multi-phase microstructure had triggered severe galvanic corrosion. During free corrosion in 0.1 M HCl, Tribaloy-700 and Diamalloy-4006 retained rather stable polarization resistance (Rp), whereas the Rp of EHC decreased significantly. Tribaloy-700 survived 40 h of corrodkote test with no apparent damage and EHC underwent limited pitting corrosion. All other coatings had visible corrosion. The Inconel-625 coating failed to protect the substrate after 20 h of testing, due to inadequate processing conditions.