Research On High Temperature Corrosion Of Candidate Alloy Of 700℃ Ultra-Supercritical Boiler Superheater

Since having good high temperature creep rupture strength, superior corrosion resistance under high temperature and good structure stability, meeting the high steam pressure and temperature needs of 700 ℃ level uitra-supercritical plants’ materials, Inconel Alloy 740H has been the mainly candidate alloy of the boiler superheater and reheater. Because of having features of pipe wall high temperature environment, corrosive atmosphere and deposition ash, corrosion and abrasion frequently happen on superheater and reheater’s pipes, thinning the pipes’ wall and resulting in failure, which brings great risk on operation safety and economic of power plant.In this article, fireside corrosion behavior and corrosion mechanisms of Inconel Alloy 740H had been researched in two simulated corrosion environment and two corrosion temperature. The dynamic curve been drew using the method of increased mass in order to analyze the corrosion rat. The X-ray diffraction been performed to analyze the corrosion products, and the scanning electron microscopy attaching the energy dispersive spectrometer been used to study surface morphology, cross sectional morphology and elements distribution of Inconel Alloy 740H.The main mechanisms of Inconel Alloy 740H are the high temperature oxidation, internal oxidation and intergranular oxidation in the superheater bottom ash. The high temperature oxidation and internal oxidation were obvious in the beginning period of corrosion, and the corrosion products are mainly Cr2O3. With extension of corrosion time, the intergranular oxidation of alloy took place, the internal oxidation becoming deepening, which damaged the alloy matrix. The intergranular oxidation product was mainly Al2O3. The corrosion products are mainly Cr2O3 and a small amount of Cr1.3Fe0.7O3 and (Cr0.88Ti0.12)2O3. Under 750 ℃, the alloy failured to form Cr2O3 layer in the beginning period of corrosion. While under 780 ℃, a low density Cr2O3 layer was observed in the beginning period, and the corrosion dynamic curve show a liner upward trend, the fitting equation being y=0.65253+2.7937*10-4x.Inconel Alloy 740H had a similar corrosion behavior in two temperature degree synthetic ash, and the corrosion mechanisms were high temperature oxidation, internal oxidation, pitting attack of sulfur and low temperature hot corrosion. The overall corrosion behavior of Inconel Alloy 740H can be divided into twe successive stages, an initial stage and a propagation stage. In an initial stage, Inconel Alloy 740H formed a continuous Cr2O3 layer, and mainly took place high temperature oxidation, internal oxidation and pitting attack of sulfur. The corrosion products were mainly Cr2O3, Fe2O3 and Al2O3, and Fe2O3 and Al2O3 were components of synthetic ash. With corrosion time prolonging, low temperature hot corrosion was observed and corrosion was gradually progressing to the propagation stage, Cr2O3 layer thinned, internal oxidation deepening. The corrosion morphology become in layers, the outer layer being molten synthetic ash, the middle layer being Cr2O3 layer, the inner layer being internal oxidation and pitting attack of sulfur. The outer of corrosion layer became loose and porous, cohering a large number of synthetic ash. The corrosion products were mainly Fe2O3, (Fe0.6Cr0.4)203, SiO2 and Al2O3, and Fe2O3 SiO2 and Al2O3 were components of synthetic ash. Inconel Alloy 740H showed a similar corrosion dynamic curve in the two corrosion environment, and took place low temperature hot corrosion leading to a propagation stage.With the increase of Cr-depletion and the relative content of Al, the Inconel Alloy 740H matrix took place seriously Al element internal oxidation or intergranular oxidation with extension of corrosion time and temperature. Because the Cr2O3 layer and the interface of matrix and Cr2O3 layer existed a large number of holes and internal oxidation products, the Cr2O3 layer cracked and spalled repeatly, which leaded to fluctuation of the corrosion dynamic curve. Then, new Cr2O3 layer formed on the surface of naked matrix. The Ti enrichment was observed within large size internal oxidation products and holes. It was obvious that Inconel Alloy 740H took place more serious corrosion in synthetic ash than the superheater bottom ash.