Kyoo Young Kim

A study of aluminide coatings on TiAl alloys by the pack cementation method

Abstract The lower case halide-activated pack cementation method was utilized to deposit aluminide coatings on TiAl alloys. Emphasis was placed on the effect of alloying elements on the aluminizing behavior of TiAl alloy. The addition of a small amount of Nb or Cr in the TiAl improved significantly the aluminizing kinetics of TiAl alloys by increasing the solid-state diffusion of Al through the formation of stable TiAl 3 layer. The TiAl 3 layer formed on the TiAl alloyed with Nb or Cr had better toughness than the TiAl 3 formed on the non-alloyed TiAl. The reason for better toughness of the coating formed on TiAl was that partial TiAl 3 with tetragonal structure was changed to high symmetry cubic L1 2 structure since Nb or Cr was dissolved into TiAl 3 . The TiAl 3 layer formed on the TiAl alloyed with Nb or Cr had much better oxidation resistance than the TiAl 3 layer formed on the non-alloyed TiAl. It was attributed to change in the crystal structure of TiAl 3 from the brittle tetragonal DO 22 to the ductile cubic L1 2 by addition of small amount of Nb or Cr.

Effect of Cr addition on the properties of aluminide coating layers formed on TiAl alloys

Abstract TiAl alloys were coated by two different processes with a simple aluminide coating and a Cr+Al-type composite coating for the improvement of oxidation resistance of the alloy. Simple aluminizing was applied to TiAl– X Cr ( X =1, 4, 6 at.%) alloys. A Cr+Al-type composite coating was performed by Cr evaporation on the TiAl substrate followed by pack aluminizing. A simple aluminide coating applied to the Cr-added TiAl alloys showed improvement of ductility and oxidation resistance. This was mainly due to grain refinement of the Cr-added coating layer. The coating layer formed by the composite coating process consisted of the Al 4 Cr outer layer and TiAl 3 inner layer. However, these coating layers were transformed to a Ti(Al,Cr) 3 layer with a L1 2 structure during oxidation. This layer showed much better ductility compared to other coating layers and superior oxidation resistance in cyclic oxidation tests.

A study on aluminide and Cr-modified aluminide coatings on TiAl alloys by pack cementation method

Abstract The halide-activated pack cementation method was utilized to deposit a suitable coating on TiAl to ensure its oxidation resistance. TiAl alloys were coated by two different pack cementation processes with simple aluminide coating and Cr-modified aluminide coating. The coatings formed on the TiAl alloyed with Nb and Cr were thicker and tougher than those formed on the non-alloyed TiAl. The TiAl 3 layer formed on the TiAl alloyed with Nb and Cr had much better cyclic oxidation resistance than that formed on the non-alloyed TiAl. This is attributed to a change in the crystal structure of the TiAl 3 from the brittle tetragonal DO 22 to the ductile cubic LI 2 by addition of a small amount of Cr and Nb. The Cr-modified aluminide coating on the TiAl alloyed with Nb and Cr exhibited excellent cyclic oxidation resistance due to formation of the Al 67 Ti 25 Cr 8 phase as the coating layer.

High-temperature corrosion of recuperators used in steel mills

Abstract High-temperature corrosion is a serious problem on the heat exchanger pipes of recuperators used in steel mills, because they encounter severe corrosive environments at high temperatures. These pipes are found to be corroded via oxidation, sulfidation, and molten salt corrosion. Molten salt corrosion occurs by the formation of alkali-iron sulfate as reaction products of sulfur oxide in fuel gas and sodium chloride from seawater. Many integrated steel mills are located near the sea for convenience of transportation. Salt from the seawater can affect the corrosion of the heat exchanger pipes by inducing hot corrosion. Through inspection of the used pipes of recuperators, a corrosion mechanism of the heat exchanger pipes is proposed. To evaluate the corrosion resistance of possible protective coatings for recuperators, nickel- and cobalt-based self-fluxing alloys, iron-based alloys, and chromium carbide cermet coatings were tested. The coatings with high chromium content, either as a form of carbide or as an alloying element, had excellent corrosion resistance. A Cr 3 C 2 –NiCr coating in carbide form, and Ni–45Cr–4Ti as an alloy coating can be recommended as promising coatings for the heat exchanger pipes, even in cases where they may encounter molten salt corrosion attack. The microstructure of a coating is as important as the chemical composition of the coating material. The inter-lamella pores may act as passages for molten salts. Therefore, the coatings should be carefully sprayed so that the molten salts do not penetrate into the coating layer.

Effect of Line Pipe Steel Microstructure on Susceptibility to Sulfide Stress Cracking

Abstract The purpose of this experiment was to evaluate the effect of microstructure on sulfide stress cracking (SSC) properties of line pipe steel. Different kinds of microstructures, with chemical compositions identical to one steel heat, were produced by various thermomechanically controlled processes (TMCP). Coarse ferrite-pearlite, fine ferrite-pearlite, ferrite-acicular ferrite, and ferrite-bainite microstructures were investigated with respect to corrosion properties, hydrogen diffusion, and SSC behavior. SSC was evaluated using a constant elongation rate test (CERT) in a NACE TM0177 solution (5% sodium chloride [NaCl] + 0.5% acetic acid [CH3COOH], saturated with hydrogen sulfide [H2S]). The corrosion properties of steels were evaluated by potentiodynamic and linear polarization methods. Hydrogen diffusion through steel matrix was measured by an electrochemical method using a Devanathan-Stachurski cell. The effect of microstructure on cracking behavior also was investigated with respect to crack nu...

Effect of Ternary Elements on the Oxidation Behavior of Aluminized TiAl Alloys

The effects of ternary elements added to TiAl on the coating layer formed by the pack-aluminizing process was studied with respect to oxidation resistance and mechanical properties. All the TiAl specimens, with various amounts of Nb, Cr, Fe, and V, were pack aluminized under identical conditions using a high-activity process. Among the ternary alloying elements tested, Nb showed the best property of the TiAl3 coating layer formed on the surface and, consequently, the best oxidation resistance. The TiAl3 coating layer becomes thicker and has a finer grain size as the content of Nb or Cr is increased. Microhardness tests revealed that the addition of Nb or Cr improved the toughness of the coating layer and thus improved the cracking resistance. Cyclic oxidation tests showed that the TiAl3 coating layer formed on the TiAl alloy has better oxidation resistance with increasing Nb content. The ductility and oxidation resistance of the TiAl3 coating layers improved with Nb addition, which contributes to the grain refinement of TiAl3. The Nb present in the TiAl3 coating layer inhibits grain growth by the solute-drag effect and retards inward diffusion of Al to the TiAl matrix by forming (Nb, Ti)Al3 precipitates during high-temperature oxidation.

Effect of yttrium coating on the oxidation behavior of Ni3Al

Yttrium-coated Ni3Al with post heat treatment has shown good high-temperature oxidation resistance. To understand the effect of the Y-coating and post heat treatment on the oxidation resistance of Ni3Al, the specimens were coated with Y by an ion-plating method, and heat treatment was performed at low oxygen level before or after the Y-coating was applied. Performance of the Y-coated Ni3Al was evaluated by isothermal and cyclic oxidation tests. A simple deposition of Y on Ni3Al did not change the oxidation kinetics, but the post heat treatment after Y-ion plating significantly decreased the oxidation rate of Ni3Al. The scale formed on Y-coated Ni3Al with post heat treatment after Y-ion plating showed a fine and dense structure which was grain refined by the presence of a (Y, Al) O-type oxide in the scale. The coated Y layer becomes a Y-Al compound during heat treatment. The presence of the (Y, Al)O-type oxide in grain boundaries or the lattice of Al2O3 modify the diffusion rate of Al and oxygen, and the oxide microstructure during oxidation. Improvement of cyclic-oxidation resistance of Ni3Al by the Y-coating occurs because the presence of (Y, Al)O-type oxide develops fine-grain oxides which can easily relieve the growth stress.

Effect of yttrium on the stability of aluminide-yttrium composite coatings in a cyclic high-temperature hot-corrosion environment

A composite coating of aluminide-yttrium has shown excellent corrosion resistance in a cyclic high-temperature hot-corrosion environment. To understand the effect of yttrium on the stability of the composite coating, the specimens were prepared with various coating parameters of Y thickness, sequence of post heat treatment and surface condition before Y-ion plating. Performance of the composite coating was evaluated by isothermal oxidation and cyclic high-temperature hot corrosion. Isothermal-oxidation-test results show that the Y in the composite coating helps to form a thick and dense Al2O3 scale which is ductile and resistant to thermal stress. The Y in Al2O3 may act as a donor which leads to an increase in concentration of interstitial oxygen and, thus, increases in oxidation rate. The presence of Y2O3 and (Y, Al) O-type compounds in grain boundaries of Al2O3 and boundaries between the Al2O3 and NiAl effectively prohibits the fast diffusion of oxidants (such as O and S) and Al along grain boundaries. Consequently, it may induce slow diffusion through the matrix, and thus the corrosion resistance of the composite coating under cyclic hot corrosion increases substantially.

A novel photoelectrochemical method of metal corrosion prevention using a TiO2 solar panel

By using a simple TiO2 solar panel connected to a steel electrode, the concept of photoelectrochemical metal corrosion prevention has been demonstrated to be feasible in utilizing solar light for corrosion inhibition.

Effect of Microstructure on Hydrogen-Induced Cracking of Linepipe Steels

Abstract Hydrogen-induced cracking (HIC) has been studied phenomenologically. The effect of microstructure on HIC is discussed for steels having two different levels of nonmetallic inclusions. Steels with different microstructures were produced by a thermomechanically controlled process (TMCP) from two different heats, which had different levels of nonmetallic inclusions. Ferrite/pearlite (F/P), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) were three representative microstructures of the tested steels. For the steels with higher inclusion levels, the minimum permissible inclusion level for HIC not to develop varied according to the steel microstructure. On the other hand, HIC occurred in the martensite/austenite (M/A) constituents regardless of steel microstructure above a certain concentration. It was shown that M/A constituents were easily embrittled by hydrogen atoms. Steels having a F/AF were resistant to HIC in the tested service condition; they exhibited a wide range of diffusible hydr...