Electrochemical Behavior of Nickel Aluminide Coatings Produced by CAFSY Method in Aqueous NaCl Solution

Combustion-assisted flame spraying (CAFSY) is a novel method that allows in-flight synthesis of alloys during flame spraying. The in-flight synthesis of alloys by the CAFSY method during flame spraying combines two different methods: the self-propagating high-temperature synthesis (SHS) and flame spraying (FS). The present work studies the corrosion performance (by cyclic polarization and chronoamperometry in aerated 3.5 wt.% NaCl) of NiAl coatings fabricated by the CAFSY technique in relation to main process parameters (composition of the initial feedstock, spraying distance, substrate temperature, postdeposition heat treatment) and their effect on the microstructure and porosity of the coatings. Most of the coatings exhibited limited susceptibility to localized corrosion. In all cases, the steel substrate remained intact despite corrosion. Interconnected porosity was the main parameter accelerating uniform corrosion. Localized corrosion had the form of pitting and/or crevice corrosion in the coating that propagated dissolving Al and Al-rich nickel aluminides along coating defects. Substrate preheating and postdeposition heat treatment negatively affected the corrosion resistance. A short spraying distance (1.5 inches) increased the corrosion resistance of the coatings.