Chaur-Jeng Wang

Corrosion behaviors of low carbon steel, SUS310 and Fe–Mn–Al alloy with hot-dipped aluminum coatings in NaCl-induced hot corrosion

Abstract The hot corrosion behaviors of hot dip aluminized low carbon steel, SUS310 and Fe–Mn–Al–Cr–Si–C alloy with 9 mg cm −2 of NaCl deposition were studied at 900 °C for 1–144 h in dry air. A preoxidation of as-dipped specimens at 900 °C for 2 h was also conducted. The formation of an aluminized layer and self-healing potential are responsible for the better hot corrosion resistance of as-dipped low carbon steel and SUS310. Owing to the high solubility of aluminum in the Fe–Mn–Al–Cr–Si–C alloy, the coated aluminum diluted rapidly and complex phase transformations were induced by the interdiffusion of the aluminum and alloy matrix. A nitrogen-attacked zone in the aluminized layer was also observed. The corrosion resistance of hot-dip aluminum coated Fe–Mn–Al–Cr–Si–C alloy was thus degraded. Preoxidation treatment increased the thickness of the aluminized layer, yet reduced the corrosion resistance of three alloy systems due to the formation of microcracks during the cooling and the further heating to elevated temperature.

Morphological Development of Subscale Formation in Fe–Cr–(Ni) Alloys with Chloride and Sulfates Coating

Three types of stainless steel (430, 304, and 310) with a coating of NaCl, NaCl/AlCl3, or NaCl/Al2(SO4)3 are exposed at 750 and 850°C. Results show that NaCl has a major effect on corrosion and sulfur plays an important role in intergranular corrosion. After high-temperature exposure with a 100% NaCl coating, the morphologies of alloys 304 and 310 show typical uniform subscale attack the depths of attack increasing with temperature, while alloy 430 showed a planar attack. Alloy 310 has the highest chromium content and has the least metal loss. After high-temperature exposure with a NaCl/AlCl3 coating, the corrosion morphologies and depths of attack are similar to those associated with an NaCl coating, but only voids are larger in the subscale. When coated with NaCl/Al2(SO4)3, the alloys are attacked simultaneously by sulfur and chlorine at 750°C, resulting in a typical sulfur-attack intergranular corrosion. However, as the temperature increases to 850°C, the corrosion morphology changes to a uniform subscale attack.

TEM Study of the Internal Oxidation of an Fe–Mn–Al–C Alloy After Hot Corrosion

The internal oxides formed in Fe–31.1Mn–9.07Al–0.89C at 750–850°C in air with 2 mg/cm2 NaCl deposits initially were investigated by means of transmission electron microscopy (TEM). The results showed that the volatile species generated by hot corrosion accelerated internal oxidation in the internal voids. The alloys suffered severe subscale attack at 750 and 800°C because a protective alumina scale was not formed. The morphology of attacked subscale can be divided into three layers. The interconnecting voids in the outer subscale are finer and denser than those in the inner subscale. The products inside the voids of the outer subscale are metal oxides, while the oxides inside the voids of the inner subscale are filled with fine Fe3O4 particles. However, due to the formation of an aluminum-rich oxide scale at 850°C, only a small amount of internal oxidation was generated in the subscale, which provided the best corrosion resistance in this study.

Study of wear behavior of nitride layers in Fe–Mn–Al–C alloys

Abstract The wear behavior of high toughness austenitic Fe–30Mn–(2, 5, 6 and 9 wt.%)Al–0.7C–(3Cr) alloys with/without nitriding in pure nitrogen were studied by a TE77 frictional test machine in a reciprocating sliding mode. Microstructures of alloys were also examined. The results showed that AlN was the major product of the nitriding process at 1050 °C for 25 h. Acicular AlN was found at the subsurface to be part of the morphology of all specimens. The hardness of the raw metal in the Fe–Mn–Al alloy system increased with increasing aluminum content in alloys. When oxide film was absent on the worn surface, wear resistance increased with increasing hardness of the raw metal. The wear loss of Fe–30Mn– x Al–0.7C–(3Cr) alloys ( x ⩾5) decreased one order of magnitude after nitriding due to AlN supporting an oxide film. The nitride layer is beneficial for enhancing hardness of Fe–30Mn– x Al–0.7C–(3Cr) alloys ( x ⩽7.5), to improve wear resistance of alloys due to the fact that the hardness of AlN is much higher than that of the matrix. When oxide is produced on the worn surface, the governing wear mechanism is tribooxidation. On the contrary, when oxide is absent on the worn surface, the governing wear mechanisms are abrasion, surface fatigue and shear fracture or delamination.