Chlorine-induced high temperature corrosion of HVAF-sprayed Ni-based alumina and chromia forming coatings

Abstract

Chlorine-induced corrosion of HVAF-sprayed Ni21Cr and Ni5Al coatings was investigated in 5 vol.% O2 + 500vppm HCl + N2 with and without KCl at 600°C up to 168 h. Both coatings were protective in the absence of KCl. With KCl, Ni21Cr degraded through a two-stage mechanism: 1) formation of K2CrO4 followed by diffusion of Clthrough the oxide grain boundaries to yield chlorine and a nonprotective oxide, and 2) inward diffusion of chlorine though defects in the non-protective oxide, leading to breakaway oxidation. Cl-/Cl2 could not diffuse through the protective alumina scale formed on Ni5Al, hence the corrosion resistance increased.

Effect of SiO2 Dispersion on Chlorine-Induced High-Temperature Corrosion of High-Velocity Air-Fuel Sprayed NiCrMo Coating

“…3) also confirmed the relatively low level of oxygen pick up during spraying. Such low levels of porosity, low oxygen pick up during coating and high adherence of the coating to substrate can exclusively be attributed to the high velocity and low temperature of the in-flight particles in HVAF [30]. A higher level of oxygen detected in NiCrMo-SiO2 was due to the presence of the oxygen-containing dispersoid (SiO2) in the coating.…”

“…Thirdly, a significant drop in the weight change of NiCrMo from 96 h to 168 h exposure can be seen, whereas no such drop was observed in the weight change of NiCrMo-SiO2. resulting in a drop in weight, whereas in the early stages oxide formation led to weight gain [30]. It can be assumed that the oxide formed on the SiO2-free NiCrMo coating exposed to O2+HCl with KCl was not dense and protective enough which needs to be confirmed by SEM/EDS analysis.…”

“…7a along with wrinkled Cr2O3 according to their topographical features reported in the literature [30], [48].…”

“…3.2 Weight change in coatings after high temperature corrosion exposure at the coating/substrate interface [30], and f) Sintering of KCl on the surface during the exposure [43], while the problem with the spallation was coped using the crucibles to collect corrosion products in case of possible spallation, the total weight change reported for each time step in Fig. 5 was a combination of the above sources.…”

“…A mass flow controller was used to introduce 35 cm 3 /min of gas through the chamber during the hightemperature corrosion test. More details about the furnace can be found in [30]. The samples were under the flow of N2 throughout the course of exposure to prevent corrosion during cooling.…”