铝改性奥氏体合金在模拟石化气氛中的高温碳化-粉化及硫效应

Gaseous environments typical of petrochemical processes contain mixtures of compounds of C, S and O under low partial pressures of O2 and S2 coupled with rather large C activities. These gases are quite aggressive to alloys used at high temperatures mostly due to formation of metal carbides or even carbon deposits, as well as the catastrophic "metal dusting". The Cr content of classical heat-resistant austenitic Fe-Ni-Cr steels for petrochemical is sufficient to form stable and slowly-growing external Cr2O3 protective scales in oxidizing environments. However, above 850 C they do not offer good protection due to evaporation and forming volatile hydroxide species. When exposed to gases with high C-activities, these alloys form Cr carbides as internal precipitates, producing a Cr depletion on alloy surface. This has a negative effect on the mechanical properties and tends to prevent the formation of Cr2O3 scales. Limited additions of Al to these steels may lead to the formation of external Al2O3-rich scales, more stable than Cr2O3 at 900-1200 C, and more resistant to C- attack. Small amounts of S can be beneficial against the C-attack, provided they are insufficient to form metal sulfides (sulfur effect). In fact, S is much more aggressive than O, because sulfides grow much more rapidly than oxides. Low O2 pressures may also reduce the C-attack by lowering the C-activity in the gas and also by favoring the growth of oxides of Cr and/or Al. The aim of this proposal is to examine the high temperature corrosion behavior of austenitic alloys based on Fe-Ni-Cr with and without Al additions in gases, typical of petrochemical industries, containing appropriate combinations of C, S and O, providing O2 and S2 pressures below the stabilities of the Fe and Ni compounds coupled with C activities ranging from values below unity, typical of carburization, to values above unity, typical of metal dusting. Mixtures I H2-CH4 are designed to study corrosion by a single oxidant (C). Mixtures II H2-H2S-CH4 and III H2-CH4-CO2, containing two oxidants C-S or C-O, are used to study the effects of S and of the formation of Cr and Al oxides on the reaction with C. Finally, mixtures IV H2-H2S-CH4-CO2 allow to establish the interactions between the three oxidants, with particular attention to the S effect also in the presence of oxygen and to the possible protection offered by formation of external oxides against the C/S attack. We intend also to define the conditions, such as temperature, alloy composition, activities of the different oxidants in the gas and especially the effect of S, which may affect the transition between carburization and metal dusting. The research will address many aspects of the gas-metal reactions examined, including corrosion kinetics for each alloy/gas combination, the nature of reaction products and their spatial distribution in sscales. The results obtained will provide a basis for the interpretation of mechanisms of these corrosion process.

石化工业的高碳、低硫和低氧活度的高温环境常导致耐热合金发生碳化、或灾难性的粉化。传统奥氏体钢表面生成Cr2O3膜，但它在850C以上易挥发、而钢易生成碳化铬致材料贫铬而失效。本项目选择可生成Al2O3膜的新奥氏体合金、研究其在组成不同的含碳、硫和氧的双或三元混合气氛中的碳腐蚀：包括在仅含碳气氛中的碳化、及其随碳活度增加和温度降低而转变为粉化的热力学与动力学特征；向气氛中添加某种氧化剂对其它两种混合氧化剂腐蚀的影响，特别是硫抑制碳化和粉化过程的临界环境条件。揭示含铝的Fe-Ni-Cr合金在高碳还原性气氛中生成Al2O3或铝-铬混合氧化膜、并抑制碳腐蚀的条件与机制。尝试界定体系各参数，如温度、合金成分、气相中各氧化剂活度对引发碳化-粉化转变的影响。澄清混合气与合金反应的各层面问题，包括反应动力学、产物的性质与空间分布和三种氧化剂间可能的交互作用。验证、拓展申请人已获多氧化剂腐蚀理论研究结果。

多数高温合金都含有足够浓度的Cr，Al，Si等保护性组元，以生成稳定性高且生长速率低的氧化膜而抑制腐蚀，尽管它们的碳化物和硫化物的稳定性远低于相应的氧化物，但在含碳或硫的还原性气氛中，这些组元却不能生成有效的保护性氧化膜。碳在合金中能扩散速渗透并与保护性组元反应生成内碳化物，令后者贫化至不足以支持形成外氧化膜而引发碳化或灾难性的粉化。. 选择Cr和Al含量不同的六种奥氏体合金Fe-19Ni-13/21Cr-x Al (x=0,2,6%) 和 三种在900℃平衡的混合气氛CH4-H2，CH4-H2-CO2和CH4-H2-H2，设计碳活度既包括aC<1 (碳化)，也包括aC>1（粉化）的情况。高碳活度气氛中: 合金的腐蚀呈多段抛物线或直线规律，发生内碳化，随碳活度的增高腐蚀加剧；添加铝6%Al时，能减缓碳侵蚀，这得益于气氛中残氧致合金局域生成Al2O3，但此Al含量尚不足以生成完整氧化铝膜；在高碳-低氧气氛中：除Fe-19Ni-13Cr(-2Al)外，其余四种合金的耐蚀性均有提升；添加6%Al促进表面局域氧化铬膜的生成，减轻碳腐蚀；在高碳-低硫气氛中：Fe-19Ni-13Cr(-Al)的腐蚀速率随硫分压增加而下降，Fe-19Ni-13Cr-6Al腐蚀速率最低。当硫分压最高时，低和无Al合金生成CrS而抑制碳化。在两种高硫气氛中Fe-19Ni-13Cr-6Al可生成外氧化铝膜防止碳化。随硫分压上升，所有含21%Cr合金可生成局域外氧化铝膜，但碳化未完全消失，Fe-19Ni-21Cr-6Al遭腐蚀程度比另两合金低许多。总之，若S分压控制得当，既有利于防止碳化又可避免硫化物生成。提高气氛的碳活度促进碳化铬生成而损害合金的抗腐蚀性。添加Al有利于抑制合金碳化，但Al含量超过6%才能生成保护膜。. 对Fe-Ni-Cr合金与含C、S和O气体反应可能形成的各种碳化物、硫化物以及氧化物相进行了热力学分析。通过合理的近似，构建三元合金与双氧化剂反应的热力学相图，同时考虑腐蚀动力学因素、在相图中绘出相应的扩散路径，诠释和预测这类复杂体系的腐蚀过程。将适于二元合金内氧化的经典理论进一步拓展以描述Fe-Cr合金在碳-氧双氧化剂、以及Fe-Ni-Cr合金在碳-硫多组元反应体系，理论上很好地诠释实验所见，并预测在碳-氧-硫气氛中生成完整保护性外氧化膜需要铬和铝的临界浓度。