Based on the existing Cr Mo steel, Fe Cr Ni, Fe Cr Al and other steel grades, most of the foreign high-temperature vulcanization and oxidation-corrosion-resistant materials further improve their high-temperature corrosion resistance by means of microalloying, surface protection and other measures. By adding CR, Al, Si and other alloy elements, y, CE and other rare earth elements, the high-temperature oxidation and vulcanization resistance of the steel is improved. The effects of various alloy elements on high temperature oxidation and vulcanization are as follows:
1 Cr
In the process of high-temperature sulfurization corrosion, Cr is preferentially sulfurized than Fe, Ni and other elements. A layer of dense chromium sulfide is formed on the alloy surface first. Compared with the sulfide formed by Fe and Ni, the crystal defects of chromium sulfide are less, complete and uniform, which can prevent the diffusion of metal ions and the penetration of sulfur into the alloy matrix, thus reducing the corrosion rate. The higher the chromium content, the thicker and more complete the dense CRS. Therefore, the Cr content must be increased to ensure the corrosion resistance of the material. The research shows that when the Cr content of the alloy is 30%, the alloy has a good corrosion resistance to high temperature vulcanization.
2 Al
As an anti-oxidation and sulfurization element, the mechanism of aluminum is similar to that of Cr. however, when aluminum is added to steel as a corrosion-resistant alloy element, it is restricted by the problems such as the decrease of plasticity, toughness and the increase of superheat sensitivity. The fe-25cr-6ai alloy containing 6% Al, invented in the United States, is resistant to oxidation and sulfurization by forming Al2O3 at the interface between the oxide film and the matrix, and the barrier layer inhibits the outward diffusion of the matrix metal cations to increase the corrosion resistance, especially for hydrogen sulfide. Compared with Cr Mo steel, Al containing steel has some advantages. The solid solution Cr in Cr Mo steel matrix will be enriched into carbides during use, resulting in poor Cr in the matrix, uneven distribution of Cr, thus reducing the corrosion resistance. The distribution of Al in the steel is always uniform. The representative Al containing steels are 12Cr2AlMoV, cr6almo, 15al3mowti, etc.
3 Si
The effect of Si on oxidation and vulcanization is mainly due to the formation of SiO2 barrier layer at the interface between the oxide film and the substrate, thus inhibiting the diffusion of ions on the metal surface, and achieving the purpose of preventing high temperature oxidation and vulcanization. However, considering the influence of Si on the welding performance, the content should not be too high.
4 Ni
Ni is very sensitive to high temperature vulcanization, and it is easy to form sulfide with fast growth rate and loose. The higher the content of Ni is, the greater the thickness is. Ni sulfide is mainly distributed in the outer layer. Due to its large specific volume and high defect concentration, it has little effect on the diffusion of metal ions and has no protective effect. Therefore, the increase of Ni content is disadvantageous to resist sulfur corrosion. On the premise of ensuring high temperature strength and structural stability of the alloy, the Ni content should be as low as possible. In terms of oxidation resistance and thermal strength, nickel has good corrosion resistance to oxidizing media due to the formation of passive film. In general, sulfidation and oxidation coexist in the process of high-temperature corrosion, mainly high-temperature oxidation, but sulfidation occurs before oxidation. Therefore, in consideration of the comprehensive performance of anti oxidative sulfur of materials, Ni content should be controlled at an appropriate level.
5 Y
Rare earth elements can form oxide pinning, which makes the oxide and the substrate bond firmly and not easy to peel off. The results show that the corrosion rate increases when y content is less than 0.1%, and decreases when y content is more than 0.1%. In the process of alloy oxidation, y element can promote the diffusion of Cr in the alloy, thus forming Cr2O3 layer. In the process of vulcanization, y also plays a similar role. When y content is less than 0.1%, the accelerated diffusion rate of Cr increases the corrosion rate of vulcanization. At the same time, the promoted effect of Y on the diffusion rate of Cr is not enough to enrich enough Cr on the surface of the matrix to form the vulcanization layer of Cr. at this time, the vulcanization rate increases with the increase of Cr content. When the Y content is more than 0.1%, the diffusion of Cr is further accelerated, which can enrich on the surface of the matrix, thus forming a protective inner layer of chromium sulfide. At the same time, an inner sub layer composed of fecr2s4 and nicr2s4 will be formed due to the external diffusion of Fe and Ni being blocked, so that the alloy with the Y content greater than 0.1% can significantly improve its resistance to sulfide and oxidation corrosion.
6 Ce
The results show that CE can reduce the oxidation rate of fe-27cr-7al alloy at high temperature. In the early stage of oxidation, CE promotes the formation of β - Al2O3, and makes it rapidly transform into α - Al2O3, and makes the oxide film more flat.
Among Fe, Ni and Co based alloys, Co based alloy has the best resistance to high temperature vulcanization. However, due to the high cost, the whole structural parts are rarely used, but only used on the surface. For example, 316 stainless steel used in the environment with a large amount of sulphate coal ash at 800 ℃ or above, after surfacing a layer of Stellite, the high temperature vulcanization corrosion is significantly reduced.
This paper is excerpted from the book "application and development of petroleum, petrochemical and metal materials"