Revisit the Type II Corrosion Mechanism
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Revisit the Type II Corrosion Mechanism WEI-JUN ZHANG and REZA SHARGHI-MOSHTAGHIN Since it was first proposed in the early 1980s, Type II hot corrosion has been widely cited in the literature as a process of molten sulfate attack on high-temperature alloy components. In the current study, typical Type II corrosion pits observed on field components were characterized using high-resolution TEM technique. The corrosion products in the pits were found to be mainly of nanosized sulfides and oxides, but not of the hypothesized sulfates. The results suggest a solid-phase corrosion process involving the cooperative precipitation of fine sulfides and oxides at the corrosion front. https://doi.org/10.1007/s11661-018-4755-4 The Minerals, Metals & Materials Society and ASM International 2018
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INTRODUCTION
HOT corrosion is an accelerated degradation process seen on high-temperature alloy components due to the presence of corrosive deposits or air pollutants. The degradation was first reported in the early 1950s on boilers and combustion engines.[1] During the last several decades, many studies have been carried out to understand the hot corrosion processes.[2–16] Most of the corrosion cases were related to the condensed alkali sulfates and chlorides such as Na2SO4 and NaCl. Hot corrosion is often classified as Type I and Type II corrosion based on the temperature range and attack morphology.[2–5] In general, Type I corrosion occurs at relatively higher temperatures above 800 C, while Type II takes place in the temperature range 600 C to 770 C. The Type I attack is often characterized as a broad front attack with sulfide droplets and alloy depleted zone seen at the corrosion front; Type II is normally seen as localized pitting with little or no sulfides observed at the corrosion front. As widely cited in literature, the Type II corrosion mechanism has been proposed to be a liquid-phase attack by molten mixed sulfates.[2–6] It has been proposed that molten mixed sulfates were responsible for corrosion initiation by fluxing the protective oxide layer and for rapid propagation by transporting the reactive species through the liquid in the corrosion pits. The molten sulfates (Na2SO4-NiSO4 or Na2SO4-CoSO4) were suggested to convert from NiO and CoO by reacting with SO3 in the gas. Based on this hypothesis, mixed Na2SO4-NiSO4 sulfates were often used in WEI-JUN ZHANG is with GE Aviation, One Neumann Way, Cincinnati, OH 45215. Contact e-mail: [email protected] REZA SHARGHI-MOSHTAGHIN is with the GE Global Research Center, One Research Circle, Niskayuna, NY 12309. Manuscript submitted March 12, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
laboratory tests to simulate the Type II corrosion attack.[7–11] So far, most of the studies on Type II corrosion have concentrated on understanding and characterization of corrosion seen on specimens tested in laboratory with mixed sulfates. These studies are not often compared with the actual corrosion that has occurred on engineering parts. In the current study, the microstructure of the Typ
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