材料期刊网

乙烯裂解炉HP-40合金弯管有时出现严重腐蚀。对用不同含硫量的煤柴油裂解时的弯管内壁锈层进行了金相、X-射线和EPMA分析。当煤柴油含硫量≥0.06wt％时,弯管内壁出现层状锈层,它是由铁及铬的氧化物和镍的硫化物所组成的多相混合物;其腐蚀的本质是高温硫化氧化侵蚀。而当煤柴油含硫量≤0.02wt％时,在弯管内壁形成主要由铬的氧化物组成的薄薄的锈层,此时只发生合金的高温氧化。文中还讨论了腐蚀产物的热力学稳定性和硫在导致保护性氧化膜的开裂和铬的贫化上的作用。

Severe corrosion failure of HP-40 superalloy tube bends in ethylene pyrolysis unit has been observed in a petrochemical plant nearby Shanghai. Preliminary survey indicates that the attack is probably associated with the sulfur content in gas oil. Scales formed on tube bends with various amount of sulfur contamination in gas oil are investigated by means of metallography, x-ray diffraction and electron microprobe analyses. Results indicate that scales formed in gas oil with various amount of sulfur contamination show distinguished difference in corrosion products. When sulfur content of gas oil reaches 0.06％ (wt), there is a large amount of pits covered by about 10mm thick stratified scale on the inside of bend. Every layer in the scale is a multiphase mixture consisting mainly of oxides of chromium or iron with sulfides of nickel. Besides, some metallic nickel as well as deposited carbon are found in the scale. Elementary sulfur in crystalling state exists in outer layer of scale. Enrichment of sulfur is found in the interface between scale and grain-boundary of alloy matrix. The alloy is attacked intergranularly and the corresponding corrosion products are sulfides and oxides of chromium. The nature of corrosion is high-temperature sulfidationoxidation. Corrosion by penetrated sulfur leads to localized breakdown of protective scale, promoting further oxidation of the substrate alloy, causing chromium depletion. The overall result is sulfidation and oxidation along grainboundaries. When sulfur content of gas oil reaches 0.02％ (wt), only a thin scale of oxide of chromium is formed on the inside of bend. For a better understanding of the corrosion processes in this study, thermodynamic stability of corrosion products and the role of sulfur in breakdown of protective oxide and chromium depletion are discussed.