Improvement of high temperature oxidation and corrosion resistance of superalloy IN-738LC by pack cementation

This study systematically analyzes the experimental results of the resistance of various coatings on superalloy IN-738LC to high temperature oxidation and corrosion. Three different pack cementation processes are carried out: pack-aluminization process, two-step aluminization-chromization process and siliconization process (with or without TiB2 in the packing mixture). Rhodium or palladium is deposited on the superalloy IN-738LC specimen by electroplating prior to the pack-aluminization processes. NiAl and Ni2Al3 phases are the main constituents of the aluminized coatings. Single Cr-rich phase forms on the surface of aluminized-chromized coatings. The Ni2Si, Ni3Si and TiSi phases exist after simple siliconization while Ni2Si, Ni3Si, TiSi, TiSi2 and TiB phases form after siliconization with TiB2 in the packing mixture. Among the three different categories of pack cementation coatings, aluminized coatings have the best isothermal and cyclic oxidation resistance at high temperature. When the temperature is over 850 degreesC, aluminized-chromized coating layers lose their protection against oxidation due to the spalling or cracking of the oxide scales during the test for cyclic oxidation. Siliconized coatings are brittle and crack severely due to thermal shock. Therefore, siliconized coatings are not suitable for superalloy IN-738-LC. Hot corrosion tests were carried out at 900 degreesC, a temperature above the melting point of Na2SO4 (884 degreesC). All three categories of pack cementation coatings show degradation of the corrosion scales and internal attack. Nevertheless, the aluminized coatings were still protective under hot corrosion conditions. The ranking of high temperature oxidation and corrosion resistance of all pack cementation coatings, from best to worst, is as follows: Rh-Al, simple Al, Pd-Al, Si+TiB2, Simple Si, Pd-Al-Cr, Rh-Al-Cr and simple Al-Cr samples. (C) 2003 Elsevier B.V. All rights reserved.