Effect of surface treatment and synergistic effect of O and S elements on hot corrosion behavior of GH738 superalloy

The hot corrosion behavior of GH738 superalloy after solution heat treated (SHT), room temperature machining (RTM) and low temperature burnishing (LTB) coated mixed salt (25% NaCl + 75% Na₂SO₄) on its surface was investigated at 700 °C. The results showed that the surface grain refinement of the LTB specimens and produced numerous multiple nano-twins, which was conducive to the diffusion of Cr and Al elements from the inside to the outside of the matrix. Dense and continuous Cr₂O₃ and Al₂O₃ layers were formed on the surface of the corrosion product layer and between the external corrosion layer and the surface of the matrix, respectively, which improved the hot corrosion resistance. The mechanism of the synergistic effect between O and S elements during corrosion process was found. The sulfide (SO₃) reacted with the alloy elements in the matrix to form metal oxides and S, and this process promoted the diffusion of O element into the matrix. The O₂ entering the internal corrosion layer and the O₂ generated by the decomposition of SO₃ together cause the partial pressure of oxygen in the matrix to rise, promoting the reoxidation of the metal sulfide to release S element. The oxygen in the matrix is consumed, resulting in the decrease of oxygen partial pressure. This leads to the reaction between S element and metallic elements with strong affinity to generate metal sulfide (such as TiS). In this process, O element promotes the diffusion of S element to the matrix.