Sem And Hot Corrosion Evaluation Of 80Ni-20Cr Coating Applying Electrochemical Techniques.

The corrosion resistance of 80 wt. % Ni-20Cr coating applied by the powder flame thermal spray technique on AISI 304 stainless steel has been evaluated in 80 mol % Na₂SO₄-20V₂O₅ molten salt using electrochemical techniques. The testing temperatures were 700, 800 and 900°C, and the electrochemical techniques included potentiodynamic polarization curves (PPC) and electrochemical noise (EN). The objective of this research was to obtain the corrosion mechanism from the EN technique and polarization curves. The current and potential time series data were analyzed in the time domain. A SEM analysis of the corroded samples demonstrated that at 900°C a mechanism of sulfur penetration into the substrate was the most important corrosive attack. According with the electrochemical results, the 80 wt. % Ni-20Cr coating suffered localized corrosion and mixed corrosion, determining that the major localized corrosion was observed at the higher temperature.

The corrosion of metallic components resulting from molten salts is a serious problem in systems that burn fossil fuels as a source of energy such as boilers, gas turbines furnaces, etc. Fossil fuels have some chemical elements such as sulfur, vanadium and sodium, which during combustion form low melting point compounds such as sodium sulfate (Na_2/SO₄), sodium metavanadate (NaVO₃) and vanadium pentoxide (V₂O₅) [1]. These compounds are deposited on the hot surfaces of the metallic elements forming a stable electrolyte; at high temperatures, this may lead to severe corrosion [2,3]. Since the molten salt induces corrosion by means of one oxidation and one reduction reaction, hot corrosion should be considered an electrochemical phenomenon. Thermal-spray coatings may be used to protect alloy substrates against high temperature corrosion. Coatings deposited by conventional thermal-spraying systems possess pores, interconnected paths, and other defects that reduce their protective effect [4,5]; that is why it is important to make a corrosion evaluation of thermal spray coatings in presence of molten salts at high temperatures. In the present work, 80 wt. % Ni-20Cr thermal spray coatings applied to an AISI 304 stainless steel were studied in the presence of 80 mol % Na₂SO₄-20V₂O₅ molten salt using electrochemical techniques at temperatures of 700, 800 and 900°C. The electrochemical techniques were electrochemical current and potential noise and polarization curves. The purpose of this work was to determine the corrosion resistance of the coating and its performance as a substrate protector. The type of corrosion suffered by the 80 wt. % Ni-20Cr coating-substrate is also an objective of this work. The morphology and possible internal corrosion of the corroded samples were evaluated by SEM analysis.

The electrodes were made of a cylindrical piece sized 5 mm diameter and 10 mm long of AISI-304 stainless steel coated with 80 wt. % Ni-20Cr. The coating was applied by the powder flame thermal spray technique. For electrical connection, the specimens were spot welded to an 80 wt. % Cr-20Ni wire, 150 mm long and 1.0 mm in diameter. Ceramic tubes were used for isolating the electrical wire from the molten salt, the gap between the ceramic tube and electrical connection wire was filled with refractory cement.