Hot corrosion of materials: Fundamental studies

Abstract

Hot corrosion is the accelerated oxidation of materials at elevated temperatures induced by a thin film of fused salt deposit. Because of its high thermodynamic stability in the mutual presence of sodium and sulfur impurities in an oxidizing gas, Na 2 50 4 is often found to be the dominant salt in the deposit. The corrosive oxyanion-fused salts are usually ionica/ly conducting electrolytes that exhibit an acid/base chemistry, so that hot corrosion must occur by an electrochemical mechanism that may involve fluxing of the protective oxides. With the aid of hightemperature reference electrodes to quantify an acid/base scale, the solubilities for various metal oxides in fused Na 2 50 4 have been measured, and these show remarkable agreement with the theoretical expectations from the thermodynamic phase stability diagrams for the relevant Na-Metal-5-O systems. The solubilities of several oxides in fused Na 2 50 4 -NaV0 3 salt solutions have also been measured and modeled. Such information is important both in evaluating the corrosion resistance of materials and in interpreting any oxide flu xing/reprecipitation mechanisms. Various electrochemical measurements have identified the 5P/-anion (dissolved 503) as the oxidant that is reduced in the hot corrosion process . Electrochemical polarization studies have elucidated the corrosion reactions and clarified the corrosion kinetics of alloys. Mechanistic models for Type I and Type II hot corrosion are discussed briefly .

Solubility Measurements of Fe₂O₃ and Cr₂O₃ in Fused Na₂B₄O₇-B₂O₃ in Air at 1173 K

“…4, Fe 2 O 3 and Cr 2 O 3 are expected to dissolve via a basic dissolution reaction in the basicity range of 1214.5 for Fe 2 O 3 and 11.5 13 for Cr 2 O 3 . Basic dissolution reactions of both oxides are considered as follows 5) .…”

“…In this study, the electrochemical electro motive force (emf ) measuring cell used to measure the basicity of sodium sulfate melt 5) was tested whether the basicity of the Na 2 B 4 O 7 -B 2 O 3 melt can be measured at 1173 K. And the results were compared with the previous study. 6) Then, correlation between the basicity and the solubility of the relevant oxides in the Na 2 B 4 O 7 -B 2 O 3 melt was investigated.…”

“…It comprised two types of sensors which were taken from the one used successfully for the Na 2 SO 4 melt. 4,5,9) Details of these sensors are explained as follows. Mullite, 4,5,9) ¢-alumina, 6) and quartz 10,11) are well known as the sodium ion electrolyte, and they were used to measure the sodium potentials of the melts at high temperatures.…”

“…To clarify the dissolution behavior of protective oxide scales in molten sulfate film under the corrosion environment of gas turbine blades and vanes, the dissolution behavior of metal oxides such as Fe 2 O 3 and Cr 2 O 3 in molten Na 2 SO 4 was investigated. 4,5) For the sodium borate salt, Itoh et al 6) measured its basicity by the following cell arrangement using ¢-alumina as the solid state sodium cation conductor at around 1123 K. …”

Investigation of Hot Corrosion Behaviour of Uncoated and WC-17CO Coated Samples in Aggressive Molten Salt Environment

“…The surface coating method has been given more importance to meet this challenge in the area of technology; where materials have to operate in harsh conditions of corrosive surroundings, pressure and temperature etc [4]. The thermal spray is a recent technology to deposit coatings of metallic, ceramic and some polymeric materials either in the form of powder or wire [5].…”