C. L. Zeng

Electrochemical impedance models for molten salt corrosion

Taking into account the chemical stability and scaling features of metals, four electrochemical impedance models were proposed to represent their electrochemical impedance responses in molten-salt systems at the open-circuit potential. Electrochemical charge transfer for the non-active metals is the rate-limiting process. For the active metals, the transfer of ions in the scale and the diffusion of oxidants in melts become increasingly important as compared with the electrochemical process. When a non-protective scale forms on the metal surface, the impedance diagram may present the characteristics typical of a diffusion-controlled reaction. i.e.. a semi-circle at high frequency and a line at low frequency. When a protective scale forms on the metal surface, the Nyquist plot is composed of double capacitance loops, and the transfer of ions in the scale is rate limiting. An equivalent circuit of double layer capacitance in series with oxide capacitance can be used to represent this kind of impedance response. In the case of localized corrosion, the Nyquist plot also consists of double capacitance loops, which can be described by an equivalent circuit of double layer capacitance parallel to oxide capacitance. Impedance measurements of Pt. Ni3Al and FeAl intermetallics in molten-salt systems were conducted to verify the proposed impedance models

Corrosion of Ni-Ti alloys in the molten (Li, K)2CO3 eutectic mixture

The corrosion of pure Ni and of binary Ni-Ti alloys containing 5, 10, and 15 wt.% Ti respectively in molten (0.62Li,0.38K)(2)CO3 at 650 degreesC under air has been studied. The corrosion of the single-phase Ni-5Ti alloy was slower than that of pure Ni, forming an external scale composed of NiO and TiO2. The two-phase Ni-10Ti and Ni-15Ti alloys underwent much faster corrosion than pure Ni, producing an external scale containing NiO and TiO2, and a thick internal oxidation zone of titanium mainly involving the intermetallic compound TiNi3 in the original alloys. The rates of growth of the external scales for the Ni-Ti alloys were reduced with the increase of their titanium content, while the internal oxidation was significantly enhanced. The corrosion mechanism of the alloys is also discussed

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (α-Ni) and TiNi3. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO2 scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO3 and TiO2 formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.

Duplex Al-based thermal spray coatings for corrosion protection in high temperature refinery applications

The application of thermal spray coatings has been effective in preventing corrosion of steel and iron products. It has been used in a wide range of applications spreading from the petroleum to the food industry. In this work, the performance and effectiveness of a two-layered aluminum-based thermal spray coating applied to an ASTM A387 G11 steel was evaluated. The coating structure was comprised of an inner Al-Fe-Cr layer and an outer layer of aluminum. Coated samples were tested in the reactor zone of a fluid catalytic cracking unit (FCCU) of a petrochemical plant for 2.5 years. The reactor zone temperature was about 793 K (520 °C) and the environment was a mixed gas containing sulfur, oxygen and carbon. Laboratory-scale tests were also conducted on the coated samples in order to gain a better understanding of the corrosive effect of the gaseous species present in the FCCU atmosphere. Porosity present in the thermal spray coatings allowed the penetration of the atmosphere corrodents, which instigated intergranular corrosion of the steel substrate. The presence of an inner Al-Fe-Cr layer did not prevent coating spallation, which further contributed to the internal corrosion process.

Corrosion resistance of a steel under an oxidizing atmosphere in a fluid catalytic cracking regenerator

In the present work, the corrosion resistance of an ASTM A 387 G11 steel was evaluated under two conditions: an oxidizing atmosphere in a fluid catalytic cracking regenerator of a petroleum processing unit and a simulated atmosphere in the laboratory, at temperatures of 650 °C and 700 °C. The characterization of the phases present in the oxidized layer was carried out by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM) with X-ray energy dispersive analysis (EDS). Severe corrosion was observed after exposure to both the real and simulated conditions, with formation of several iron oxides (Fe2O3, Fe3O4 and FeO) in the product scale layer, as well as a slight inner oxidation and sulfidation of chromium in the substrate. Internal nitridation of the silicon and the manganese was observed only in the real condition, probably related to the long-term exposure inside the regenerator.

Evaluation of the corrosion resistance of thermal-spray coatings under oxidant atmosphere in a fluid catalytic-cracking unit

The effectiveness of several thermal-spray coatings for improving the corrosion resistance of a low-alloy steel was evaluated at 650°C under two conditions: an oxidizing atmosphere in a fluid catalytic-cracking regenerator of a petrochemical unit and a simulated laboratory atmosphere. The high porosity present in all coatings studied in the present work, inherent to the thermal-spray technique, allowed the penetration of gaseous species from the atmosphere into the substrate, leading to the formation of nonprotective oxides and sulfides, as well as internal oxidation, sulfidation, and nitridation. A protective alumina and/or chromia layer did not form, probably due to the relatively low temperature used in both the real and simulated conditions. Characterization of the phases present in the oxidized layer was carried out by X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM) with X-ray energy-dispersive analysis (EDS).

The Oxidation of Fe-2 and 5 at.% Y Alloys at 600-800°C in Air

The oxidation of Fe-Y alloys containing 2 and 5at.% Y and pure iron has been studied at 600-800°Cin air. The oxidation of pure iron follows the parabolicrate law at all temperatures. The oxidation of Fe-Y alloys at 600°C approximatelyfollows the parabolic rate law, but not at 700 and800°C, where the oxidation goes through severalstages with quite different rates. The oxide scales on Fe-2Y and Fe-5Y at 700 and 800°C arecomposed of external pure Fe oxides containingFe2O3,Fe3O4, and FeO, with FeO being themain oxide and an inner mixture of FeO andYFeO3. The scales on Fe-2Y and Fe-5Y at 600°C consist ofFe2O3,Fe3O4, andY2O3, with a minor amount of FeO.Significant internal oxidation in both Fe-Y alloysoccurred at all temperatures. The Y-containing oxidesfollow the distribution of the original intermetalliccompound phase in the alloys. The effects of Y on theoxidation of pure Fe are discussed.