High temperature corrosion analysis of TP310S stainless steel pipe
The corrosion of TP310S stainless steel pipe in high temperature environment was analyzed by energy spectrum and X-ray diffraction, and the vulcanization mechanism of TP310S stainless steel in high temperature environment was analyzed.
TP310S (national standard 0Cr25Ni20) has excellent high temperature mechanical properties and high temperature oxidation resistance. As a structural material, TP310S is often used in some high temperature environments. The 310S seamless pipe used in a chemical plant had steam leakage after one month of use, and then the inlet and outlet valves of the steam superheater were closed and stopped using; the boiler shutdown detection found that the steel pipe had fallen off due to serious oxidation and corrosion, and the corrosion location of the fallen steel pipe was located at the end, as shown in Fig.1. In this paper, the causes of corrosion are analyzed.
Fig.1 steel pipe falling off due to serious oxidation and corrosion
Experimental analysis of pipe material
The chemical composition of the same batch of unused steel pipes was analyzed. The analysis results are shown in Table 1.
Table.1 chemical composition of 310S pipes /%
Compared with the national standard of seamless pipe, it is found that the chemical composition of this batch of steel pipe fully meets the requirements of GB/T-2012 and GB 13296-2013.
Fig.2 energy spectrum of sample outer wall
Fig.3 energy spectrum of inner wall of sample
The results are shown in Fig.2 and Fig.3. Energy spectrum analysis shows that there are a lot of 0 and S elements in the steel pipe sample.
X-ray diffraction analysis
The results of X-ray diffraction analysis show that the steel tube samples are mainly composed of Fe1 XS, Fe slave, NiO and (Fe, Ni) centers. It can be seen that serious oxidation and vulcanization reaction occurred in the process of using steel pipe.
Fig.4 X-ray diffraction pattern of sample outer wall
Corrosion mechanism analysis
Due to its excellent high temperature mechanical properties and high temperature oxidation resistance, 310S is commonly used in some high temperature environments as a structural material. In practical production, most industrial high-temperature environment contains many harmful components, among which oxygen sulfur is the most common two harmful elements.
At high temperature, under the joint action of oxygen and sulfur, many nodular protuberances will grow on the surface of the alloy. With the extension of time, the nodular protuberances will inCrease and overlap with each other. With the inCrease of temperature, the corrosion will intensify and the corrosion product layer will become easy to peel off. With the corrosion going on, the alloy will become loose and easy to peel off, such as the peeling off of protruding corrosion products, leaving corrosion inner layer, and corrosion Cracks and corrosion holes can be observed obviously.
At high temperature, the thin oxide film only occurs at the initial stage of oxidation. When the oxide and sulfide nuclei grow laterally on the surface and form continuous oxide films covering the substrate, the subsequent corrosion mechanism of 310S is different from that of the initial stage. The thin oxide film separates the metal matrix from the corrosive gas, and the subsequent formation of corrosion product layer is mainly realized by the diffusion of reactants through the corrosion layer. In high temperature corrosion, the growth of polyCrystalline oxide film is the most important aspect. One of the important aspects of the oxidation behavior of polyCrystalline oxide film is the existence of grain boundaries between the oxide grains, which provide diffusion channels for large particles.
At the beginning of corrosion, Cr is first oxidized to Cr2O3, but Cr2O3 is discontinuous and unprotected. At the same time, a small amount of Cr is sulfurized to form part of Cr2S3, but it will soon peel off or be replaced by FEL XS, which will become the main component of the outermost corrosion layer. The outer layer of FEL XS is loose and porous, brittle and easy to peel off. With the inCrease of temperature, the migration rate of Fe2+ and Fe3+ and the vulcanization rate inCrease. With the extension of corrosion reaction time, the rapid corrosion caused by sulfurization causes the alloy interface to shrink rapidly (especially in the early stage of corrosion), which makes the concentration of Ni and Cr alloy elements in the alloy higher than that in the alloy, leading to the diffusion of Ni and Cr and the formation of sulfide with S through the product layer. However, the diffusion rate of Ni is faster than that of Cr. compared with Ni, the formation energy of sulfide in Cr is greater than that of Ni. The sulfide of Ni dissolves into Fe3+ or Fe2+ to form (Fe, Ni) xSy phase, which is dispersed in the corrosion layer and is gradually contained by the later formed Fe sulfide to form an outer corrosion layer. After continuous supplement of Fe2+ and Fe3+, the outer corrosion layer grows rapidly, as shown in Fig.5.
Fig.5 Schematic diagram of material migration during vulcanization
With the extension of corrosion reaction time, the alloy interface shrinks, and the concentration of Cr alloy elements in the alloy is higher than that in the alloy. The enrichment of Cr leads to the diffusion of Cr into the alloy. However, the diffusion rate of Cr into the alloy is very slow. At the same time, the rapid corrosion caused by vulcanization makes the alloy interface shrink rapidly, especially in the early stage of corrosion. The result is that the accumulation of Cr exceeds the diffusion into the alloy. The main corrosion products of the intermediate layer are the Cr sulfide formed by the reaction of Cr and S at the alloy interface. At the same time, some Cr sulfides react with Fe sulfides to form FeCr2S4 with spinel structure
- FeS+2CrS+(I/2)S2=FeCr2S4 Or 2FeS+ 2CrS = FeCr2S4+Fe
The corrosion intermediate layer composed of spinel FeCr2S4 etc. hinders the diffusion of Fe and Ni and controls the whole corrosion rate. With the reaction going on, the outer layer is easy to peel off and decompose. From the morphology, it is the shift from the outer layer to the middle layer. The oxygen and sulfur in the corrosion atmosphere and the oxygen and sulfur produced by the decomposition of the intermediate layer diffuse to the corrosion product / alloy interface through the Cracks and cavities of the corrosion products, and react with the alloy element Cr to form the internal corrosion layer. The intermediate layer decomposes to produce some Cr sulfide, which together constitute the main component of the internal corrosion layer.
In the case of low S content, some Ni and Cr can form sulfide niCrs4, while Fe does not form stable sulfide.
According to the chemical composition analysis of the same batch of unused steel pipes, the chemical components fully meet the requirements of GB/T-2012 and GB 13296-2013 standards; through energy spectrum analysis and corrosion analysis, TP301S seamless steel pipe contains a lot of sulfur and oxygen in the working environment, and the steel pipe has been seriously oxidized and vulcanized in the working environment.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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