Corrosion behaviours of several thermal spray coatings used on boiler tubes at elevated temperatures

Purpose - Four thermal spray coatings were subjected to high temperature corrosive environments of oil-fired boiler conditions to compare their corrosion protection under simulated conditions. The coatings included FeCrAl, Tafaloy 45CT, which were arc-sprayed, 50Ni-50Cr and Cr3C2-NiCr, which were coated by high velocity oxy fuel spray (HVOF) method. Design/methodology/approach - The coating substrates used were SA213TP 347H, SA213 T11 and SA213 T22 alloys that are widely used as boiler tube materials. Specimens were covered with a synthetic ash mixture of 70 per cent V(2)O(5)(-)20 per cent Na2SO4-10 per cent NaCl and exposed to 550 degrees C and 650 degrees C degrees for 192 h (6 cycles). After high temperature corrosion tests, weight change curves were obtained; specimens were examined by metallographical techniques, scanning electron microscopy and EDX analyses. Findings - Salt deposits attacked steels and coatings during the exposure. The corrosion rates were strongly affected by the composition of the scale formed adjacent to the steels and coatings surfaces. Austenitic steel was only bare material that experienced uniform corrosion in the tests. Ferritic steels were primarily attacked by grain boundary corrosion. Thermally sprayed coatings were mainly attached through oxides and voids at splat boundaries. FeCrAl and 50Ni-50Cr were prone to spalling. Tafaloy 45CT is also a promising method for producing homogenous coatings. Cr3C2-NiCr 80/20 coating remained mostly intact. Originality/value - This paper provides useful information about corrosion behaviours of four coatings used for common boiler tubes. It shows with a practical explanation how the bare material and coatings react in corrosion simulated environments.