ELECTROCHEMICAL CORROSION BEHAVIOR OF NICKEL ALLOYS IN SULFATE-CONTAINING MOLTEN SALTS AT 700 °C

Restricted (Penn State Only)
Author:
Kumar, Kuldeep
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 26, 2018
Committee Members:
  • Hojong Kim, Dissertation Advisor
  • Hojong Kim, Committee Chair
  • Kwadwo Osseo-Asare, Committee Member
  • Zi-Kui Liu, Committee Member
  • Barbara Shaw, Outside Member
  • Ismaila Dabo, Committee Member
Keywords:
  • Nickel alloys
  • Hot corrosion
  • Molten salts
  • Electrochemical corrosion measurements
  • Passivity
  • High temperature corrosion
  • Nickel-aluminum
  • Chromium addition in nickel alloys
  • Sulfate reduction
  • Potentiodynamic polarizations
  • LiCl-KCl-Na2SO4
Abstract:
Hot corrosion on nickel based alloys has been the focus of extensive research due to their applications in high temperature operations such as gas turbines, petrochemical, and fossil fuel power engines. The demand for good mechanical and corrosion-resistant properties in construction materials for high temperature components has led to the development of several nickel-based superalloys. While these novel alloys are well known to exhibit sufficient mechanical properties, they lack corrosion resistance. As a result, components in high temperature processes do not exhibit the necessary degree of durability. During operation, the combustion zone in high temperature engines contains contaminants including sulfur (S) and sodium chloride (NaCl), which enter due to contamination in oil and polluted air. As a result, molten sulfate salts (e.g. sodium sulfate, potassium sulfate) are formed within the combustion zone, which accelerate degradation reactions on metallic surfaces. To provide higher stability against the corrosive effect of alkali sulfates, a protective metallic coating is applied over the components in these high temperature environments. Corrosion behavior of metallic coatings is reported in the literature using burner-rig tests, exposure tests, and thermodynamic calculations; however, a fundamental study on the electrochemical corrosion behavior of nickel-based alloys has yet to be performed. Therefore, this dissertation aims to provide an outlook on the corrosion properties of nickel-based metallic coatings in corrosive environments. Specifically, an electrolyte composition ‒ LiCl-KCl-Na2SO4 (53.3-36.7-10 mol%) was used to elucidate the corrosion reactions at 700 ºC. During molten sulfate induced corrosion (hot corrosion), metal elements undergo oxidation reactions coupled with the reduction of sulfate ions. To further our understanding of the corrosion properties of metallic coatings, it is important to investigate the reduction mechanism of sulfate ions in hot corrosive environment. In a three electrode electrochemical cell configuration, sulfate ions were observed to undergo reduction reactions at sufficiently large negative overpotentials (< ‒1.1 V vs. Ag/Ag+) under inert argon atmosphere. It was found that sulfate ion reduction reactions produce elemental sulfur (S) and sulfide (S2‒) species, with charge-controlled, sluggish kinetics. Cyclic voltammograms remained invariant on switching the argon atmosphere to O2-0.1%(SO2+SO3), suggesting that electrochemical reduction potential of sulfate ions is not dependent on the gaseous environment . However, the open circuit potential became more positive by ~0.60 V upon switching the argon atmosphere to oxidizing O2-0.1%(SO2+SO3) gas, implying the presence of stronger oxidants [O2, SO2, SO3] which can be coupled with the metal corrosion reactions during the hot corrosion processes. To evaluate the corrosion behavior of nickel alloys in sulfate ion containing solutions, electrochemical measurements with pure nickel in LiCl-KCl-Na2SO4 solution were performed under gaseous atmospheres of inert argon, pure O2, and O2-0.1%(SO2+SO3). In inert argon, nickel corrosion developed an adherent NiO passive layer, resulting in a decreasing trend in open circuit potentials, and a low corrosion rate (~0.26 mA/cm2). In oxidizing atmospheres, Ni rapidly degraded due to the instability of NiO, which formed a dispersed NiO-NiSx phase, and showed significantly higher corrosion rates (7‒12 mA/cm2). With the baseline behavior of pure nickel established, oxygen active elements (e.g. Al, Cr) were alloyed with nickel to substitute the NiO passive layer with alumina (Al2O3) and chromia (Cr2O3). Hot corrosion studies with β-NiAl (BCC phase) demonstrated a strong corrosion resistance under inert gaseous environment due to the formation of an adherent and protective alumina scale. However, oxidizing gaseous atmospheres led to localized damages in the alumina layer. Particularly, in the SO2-SO3 containing environment, the Al2O3 layer showed several cracks, which led to a corrosion rate ~35 times higher in comparison to that under inert argon. Upon the addition of 5 at% Cr in β-NiAl, the passive layer consisted of a bi-layer structure: pure Al2O3 on the alloy surface and Al2O3‒Cr2O3 solid solution on the outer part. The lower corrosion current densities of the Ni-Al-Cr alloys compared to β-NiAl under pure O2 as well as O2-0.1%(SO2+SO3) environments suggested the beneficial effect of Cr addition in β-NiAl. Considering the fact that Cr addition in β-NiAl improves the corrosion resistance, Cr content was varied in Ni-Al-Cr alloys. It was observed that a small fraction of Cr is sufficient to improve the integrity of passive layer [Al2O3-Cr2O3] under a pure O2 environment. Moreover, corrosion current densities decreased with higher Cr content in Ni-Al-Cr alloys [β-NiAl: 0.78 mA/cm2, NiAl-10Cr: 0.30 mA/cm2], In general, the corrosion properties of Ni-Al-Cr alloys improved with increasing content of Cr in hot corrosive environments containing pure O2. To elucidate the effect of sulfur oxides in corrosion behavior of Ni-Al-Cr alloys, similar measurements were performed under a O2-0.1%(SO2+SO3) environment. It was observed that Cr addition helps in repassivating the damaged sites on alloy surfaces. However, the repassivation time was dependent on Cr content. With increasing Cr, the recovery time decreased. Furthermore, corrosion potential measurements in SO2-SO3 containing environments demonstrated an increasing trend of stability up to Cr = 5 at%; with any more addition of Cr reducing the magnitude. In agreement with the passivation stability predicted by corrosion potentials, Ni-Al-Cr alloys with Cr = 5 at% showed the least corrosion current density (~0.09 mA/cm2), which was approximately two orders of magnitude lower in comparison to β-NiAl. As a result, NiAl-5Cr is considered a prime composition for further enhancement of Ni-Al-Cr alloys in SO2-SO3 containing hot corrosive environments.