金属学报, 2024, 60(6): 731-742 DOI: 10.11900/0412.1961.2023.00498

综述

Ni-Cr合金在富CO2 气氛中的高温腐蚀研究进展

谢云,1, Zhang Jianqiang2, 彭晓1

1 南昌航空大学 材料科学与工程学院 南昌 330063

2 School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia

Research Advances in High Temperature Corrosion of Ni-Cr Alloys in CO2-Rich Environments

XIE Yun,1, Zhang Jianqiang2, PENG Xiao1

1 School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China

2 School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia

通讯作者: 谢 云,yun.xie@nchu.edu.cn,主要从事金属材料高温腐蚀的研究;

责任编辑: 毕淑娟

收稿日期: 2023-12-27   修回日期: 2024-01-29  

基金资助: 国家自然科学基金项目(52301089)
江西省重点研发计划项目(20232BBE50007)
江西省自然科学基金项目(20224BAB214018)

Corresponding authors: XIE Yun, associate professor, Tel: 15827996962, E-mail:yun.xie@nchu.edu.cn

Received: 2023-12-27   Revised: 2024-01-29  

Fund supported: National Natural Science Foundation of China(52301089)
Jiangxi Provincial Key Research and Development Program(20232BBE50007)
Jiangxi Provincial Natural Science Foundation(20224BAB214018)

作者简介 About authors

谢 云,男,1988年生,副教授,博士

摘要

发展富氧燃烧技术和提高锅炉的蒸汽参数可以有效减少燃煤电厂的CO2排放,有助于火力发电行业实现“碳达峰-碳中和”的目标,但采用上述技术对锅炉材料的抗CO2高温腐蚀性能和高温蠕变强度提出了更高要求,镍基合金有望成为优选材料。针对先进超超临界发电机组富氧燃烧烟气高含CO2的特点,本文综述了目前关于Ni-Cr合金在富CO2气氛中的高温腐蚀研究成果,重点介绍了CO2气氛对Ni-Cr合金热生长Cr2O3保护膜的影响,阐明了Ni-Cr合金在CO2气氛中发生碳化的机理,总结了富CO2气氛中的H2O(g)、SO2、环境温度和合金元素对Ni-Cr合金抗高温腐蚀性能的影响,进而提出未来关于Ni-Cr合金在富CO2气氛中的高温腐蚀需深入研究的问题,包括:解析Ni-Cr合金在不同气氛中热生长Cr2O3膜的微观精细结构,探明CO2、H2O(g)和SO2与稀土元素在氧化膜晶界处的交互作用,关注富CO2气氛中的HCl(g)组分对Ni-Cr合金形成Cr2O3保护膜的影响。

关键词: Ni-Cr合金; 富CO2气氛; 高温腐蚀

Abstract

The thermal power generation industry in China is facing heavy pressure from environmental protection sectors as the “emission peak-carbon neutrality” goal has been proposed. Oxyfuel combustion and operating with high steam parameters are considered promising technologies that can effectively reduce CO2 emissions from coal-fired power plants. However, using these two technologies, the currently used ferritic/martensitic heat-resistant steels and austenitic stainless steels in traditional boilers cannot meet the requirements of good creep strength and corrosion resistance to hot CO2-rich gasses. Thus, nickel-based alloys must be considered. Given that flue gasses related to oxyfuel combustion are characterized by high CO2 concentrations, this work reviews recent research progress on the high-temperature corrosion of Ni-Cr alloys in CO2-rich gasses. Herein, the effect of CO2 on protective Cr2O3 scale formation is introduced, and the related carburization mechanism caused by CO2 ingress is clarified. Moreover, the impacts of H2O(g), SO2, temperature, and alloying elements on the high temperature resistance of Ni-Cr alloys in CO2-rich gasses are summarized. Based on the current findings, future research on high-temperature corrosion of Ni-Cr alloys in CO2-rich gases should focus on the following key points, such as analyzing the microstructure of Cr2O3 scales formed in different gasses; elucidating the interaction of CO2, H2O(g), and SO2 molecules with rare earth elements at grain boundaries of oxide scales; and investigating the effect of HCl(g) impurities in CO2-rich gasses on Cr2O3 scaling of Ni-Cr alloys.

Keywords: Ni-Cr alloy; CO2-rich environment; high temperature corrosion

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本文引用格式

谢云, Zhang Jianqiang, 彭晓. Ni-Cr合金在富CO2 气氛中的高温腐蚀研究进展[J]. 金属学报, 2024, 60(6): 731-742 DOI:10.11900/0412.1961.2023.00498

XIE Yun, Zhang Jianqiang, PENG Xiao. Research Advances in High Temperature Corrosion of Ni-Cr Alloys in CO2-Rich Environments[J]. Acta Metallurgica Sinica, 2024, 60(6): 731-742 DOI:10.11900/0412.1961.2023.00498

能源电力行业是关乎国民经济发展的重要支柱行业,虽然近年来我国大力发展非化石能源,加速能源清洁低碳转型[1],但我国的资源和能源结构特点决定了燃煤火力发电在未来很长一段时间内仍将在我国电力结构中占据较大的比重(约超过70%)[2],而燃煤发电会产生大规模的CO2排放。因此,火电行业要实现“碳达峰-碳中和”的目标面临巨大挑战。

富氧燃烧技术利用高纯度O2代替空气在锅炉内与煤粉进行燃烧反应,并且将大部分烟气再循环进入锅炉以调节炉膛燃烧[3~5]。与传统的空气燃烧相比,富氧燃烧最终产生的烟气中CO2含量显著提高至60% (体积分数)甚至更高[6,7],而且通过适当的分离处理技术,可以较方便地对CO2进行收集和储存,大幅度减少燃煤电厂的CO2排放量。因此,富氧燃烧技术被视作最具潜力的燃煤电厂大规模碳减排技术之一。然而,研究[8~10]表明,高含CO2的富氧燃烧烟气具有极强的腐蚀性,会对锅炉换热器和再热器等热量交换部件使用的T/P91、T/P92等铁素体/马氏体耐热钢甚至TP304、TP347等奥氏体不锈钢造成严重的高温腐蚀[11~16],锅炉的安全稳定运行受到严重威胁。此外,提高燃煤发电机组的蒸汽参数,如发展700℃先进超超临界发电技术,也是提高机组发电效率、降低CO2排放的有效途径之一[17~19],但高蒸汽参数火力发电机组在带来较高系统效率的同时,也对锅炉材料的高温持久强度提出了更高的要求(> 100 MPa、105 h,700~750℃)[20,21]。因此,对于采用富氧燃烧技术的700℃先进超超临界发电机组,从蠕变强度和高温耐蚀性角度考虑,目前在600℃蒸汽参数锅炉中使用的铁素体/马氏体耐热钢和奥氏体不锈钢已经不能满足要求,必须考虑使用镍基合金[22,23]

抗氧化合金是通过在高温下形成一层致密、连续、生长缓慢的氧化膜(如Cr2O3、SiO2或Al2O3)来保护合金基体免受腐蚀性介质的侵蚀[24]。大量研究[13,25~28]表明,在空气或O2中能够热生长Cr2O3保护膜的(9%~12%)Cr系耐热钢在富CO2气氛中发生快速的“失稳”氧化,氧化产物主要由非保护性的铁氧化物组成,氧化速率显著增大,且在氧化膜下方的基体中还形成了大量的碳化物,大量消耗基体中的Cr,使基体发生Cr贫化而无法维持Cr2O3膜的稳定生长。此外,有研究[29~31]发现,当CO2气氛中存在H2O(g)时,它会改变Cr2O3膜的形态,在合金表面生长薄片状Cr2O3,破坏Cr2O3膜的稳定生长。总之,合金需要更高的Cr含量才能在富CO2气氛中形成稳定而致密的Cr2O3膜。但是,以上这些发现主要是基于各种Fe-Cr抗氧化合金在富CO2气氛中的高温腐蚀研究获得的,考虑到O和C在Fe和Ni中具有不同的溶解度和扩散速率,这些结论是否适用于Ni-Cr合金尚不明确,急需有针对性地开展相关研究。目前,有限的研究主要关注商用镍基合金在富氧燃烧烟气环境中的高温腐蚀性能评价[8,9,12,32~35],实验环境同时含有CO2、H2O(g)、O2、SO2甚至盐或煤灰等,商用合金中添加有Al、Co、Mn、Si等多种合金元素,这些都导致研究结论难以为阐明Ni-Cr合金抗CO2高温腐蚀的机理提供基础性的参考。

基于此,本文综述了目前关于Ni-Cr合金在富CO2气氛中高温腐蚀方面的研究成果,首先阐述Ni-Cr合金在CO2中的高温腐蚀行为,其次介绍CO2引起的Ni-Cr合金碳化机理,再次介绍H2O(g)和SO2的影响,然后介绍温度和合金元素的影响,最后对Ni-Cr合金在富CO2气氛中的高温腐蚀方面的研究进行了简要总结,提出了该领域目前亟待解决的问题,并对未来的研究方向进行了展望。以期为采用富氧燃烧技术的700℃先进超超临界火电机组锅炉系统内部件的选材和新材料的开发提供指导。

1 Ni-Cr合金CO2 高温腐蚀的特点

目前对Ni-Cr合金CO2高温腐蚀的研究主要集中于CO2对合金热生长Cr2O3保护膜行为的影响。为了揭示CO2不同于O2或空气的特殊性,图1[36,37]直观地对比了Ni-30Cr (质量分数,%,下同)合金在650℃下Ar + 20O2和Ar + 20CO2 (体积分数,%,下同)气氛中腐蚀310 h后的截面形貌。从图中可以很明显看出,该合金在O2中形成了不连续的Cr2O3外氧化膜,氧化膜较薄;而在CO2中则形成了保护性较差的多层氧化膜,氧化膜厚度明显增大。Nguyen等[38]在研究700℃下Ni-30Cr合金在上述2种气氛中的氧化膜结构时,也发现了类似的结果。

图1

图1   Ni-30Cr合金在650℃下Ar + 20O2和Ar + 20CO2 (体积分数,%)气氛中腐蚀310 h后的截面SEM像[36,37]

Fig.1   Cross-sectional SEM images of Ni-30Cr exposed at 650oC for 310 h in Ar + 20O2 (a)[36] and Ar + 20CO2 (b)[37] atmospheres (volume fraction, %)


根据金属高温氧化的Wagner[39]理论,Ni-Cr二元合金发生Cr的选择性氧化,即Cr由内氧化转变为外氧化,形成连续的Cr2O3外氧化膜所需的临界Cr含量(NCr(1),摩尔分数)可由下式计算:

NCr(1)=πgVmNO(S)DO2vVCrO1.5D˜Cr1/2

式中,g为内氧化向外氧化转变时的氧化物临界体积分数,通常为0.3[40]vCrO1.5的化学计量系数(1.5);NO(S)为氧化膜-合金界面处氧分压对应的合金中的氧摩尔分数;VmVCrO1.5分别为合金和Cr2O3的摩尔体积,DO为氧在合金中的扩散系数,D˜Cr近似为Cr在合金中的扩散系数。

同时,Cr2O3外氧化膜一旦在合金表面形成后,为了维持其长期稳定生长,从合金基体内部扩散到达氧化膜-合金界面的Cr通量必须大于氧化膜生长所需的Cr通量。为满足这个条件,合金中的Cr含量必须达到第二个临界值[41]

NCr(2)=VmVCrO1.5πkp2D˜Cr1/2

式中,NCr(2)为合金维持单一Cr2O3膜生长所需Cr元素的摩尔分数;kp为Cr2O3膜的生长速率。

一般来说,NO(S)DOD˜Cr在固定温度下都为常数,即由 式(1)计算所得NCr(1)的数值不随腐蚀气氛的变化而变化[37,42]。所以,Ni-30Cr合金在O2和CO2中呈现不同的Cr2O3膜生长行为应该与NCr(2)的变化有关,而实验结果也确实证明CO2气氛增大了Cr2O3膜的kp[38],导致NCr(2)增大,即在CO2气氛中氧化时Ni-Cr合金需要更高的Cr含量才能维持单一Cr2O3膜的稳定生长。

经典高温氧化理论[43]指出,金属表面氧化膜的生长动力学与O原子和金属原子穿过氧化膜的扩散过程密切相关,所以Cr2O3膜生长行为的变化必然与CO2对其生长机理的影响密不可分。为阐明该机理,Nguyen等[44]和Young等[45]利用透射电子显微镜(TEM)分析了在空气和CO2中热生长的Cr2O3膜的微观结构,发现在后者中形成的Cr2O3膜具有更细小的晶粒,同时三维原子探针(APT)发现了C在Cr2O3膜晶界处的偏聚,并基于此提出C的这种偏聚抑制了晶界移动,从而导致氧化膜晶粒更细小、生长速率更快。近期,Zhu等[46]利用原位透射电镜技术和密度泛函理论(DFT)研究了Ni-Cr合金在500℃下O2和CO2中的氧化行为,发现CO2分解产生的C进入Cr2O3膜晶体点阵的间隙位置,显著促进了Cr2O3膜中Cr和O原子空位的形成、聚集和移动,提高了它们的晶格扩散速率,最终导致Cr2O3膜的生长速率增大。该研究为从原子尺度解释CO2气氛增大Cr2O3膜生长速率的深层次机理提供了最直接的证据,但考虑到该原位研究是在500℃下完成的,其对于600℃甚至更高温度下实验结果的适用性仍需进一步检验。

2 Ni-Cr合金在CO2 中的碳化机理

不同于金属发生粉化时气氛中较高的C活度(aC > 1),金属的碳化主要指aC < 1时,气氛中的C溶解在金属表面后向内部扩散,然后与金属元素反应形成碳化物的现象[47]。并且,由于C的原子半径小,其扩散系数与O和常见的合金元素相比要高出很多,所以金属材料碳化的主要形式为内碳化[48]。虽然CO2气氛的平衡C活度很低,理应不能导致合金碳化,如热力学计算[36]表明,Ar + 20CO2气氛在800℃处于平衡状态时的aC仅为6.8 × 10-14,但是研究[36,38,49]发现,Cr含量大于20% (质量分数)的Ni-Cr合金在上述气氛中暴露后仍发生了轻微的内碳化,沿晶界析出了少量不连续分布的铬碳化物(图2[37])。

图2

图2   Ni-25Cr和Ni-30Cr合金在700℃下Ar + 20CO2气氛中腐蚀500 h后晶界碳化物的SEM像[37]

Fig.2   SEM images of carbides precipitated along the grain boundaries in Ni-25Cr (a) and Ni-30Cr (b) exposed at 700oC for 500 h in Ar + 20CO2 atmosphere[37]


对于Ni-Cr合金的碳化机理,需要解释以下3个问题:(1) 如此低的碳活度如何能够导致合金的碳化;(2) 气氛中的C怎样穿过合金表面的氧化膜;(3) 为何Ni-Cr合金内部没有像Fe-Cr合金[13,27,50,51]那样发生大面积的碳化物析出。首先,根据目前被广泛接受的Gheno等[52]提出的碳化热力学模型,CO2分子可以穿过合金表面的氧化膜,并且通过CO2/CO氧化还原反应在氧化膜-合金界面建立起热力学平衡,该处的aC相比气氛的aC明显增大,足以使合金发生碳化,该模型还成功解释了Ni-Cr (Cr ≥ 14%)合金[53]和Alloy 600、690合金[54]经超临界CO2高温腐蚀后,在Cr2O3膜-合金界面形成的一层非晶态碳膜;其次,由于C在氧化物中的溶解度几乎为零[55,56],晶格扩散可以忽略不计,所以研究[26,27,44,57~59]普遍认为C是以CO或CO2气体分子的形式通过氧化膜中的孔洞、裂纹或晶界等微观通道穿过氧化膜,Young课题组[30,44,45,60]利用APT技术对CO2气氛中生长的Cr2O3膜进行了系统研究,证明了C能够以晶界扩散的形式穿过合金表面形成连续致密的Cr2O3膜(图3[60]);Lee等[54]及Becker和Young[61]指出,Cr在镍基合金中更低的活度,Ni相比Fe在Cr23C6中更小的固溶度,以及C在合金中的溶解度和扩散系数随Ni含量增大而减小,这些因素都使铬碳化物在Ni-Cr合金内部的析出变得困难,所以Ni-Cr合金没有像Fe-Cr合金那样发生明显的内碳化。

图3

图3   Fe-20Cr合金先后在650℃下Ar + 20O2和Ar + 20CO2气氛中腐蚀所形成Cr2O3膜的三维原子探针像[60]

Fig.3   Atom probe tomography (APT) images of Cr2O3 scale grown on Fe-20Cr exposed at 650oC in Ar + 20O2 and subsequently in Ar + 20CO2[60]


3 气氛中H2O(g)SO2 的影响

3.1 H2O(g)的影响

富氧燃烧除了使烟气中富集CO2外,还显著提高了H2O(g)的浓度(约30%,体积分数)[6]。目前,基于Fe-Cr合金在含H2O(g)气氛的高温腐蚀研究普遍认为H2O(g)的影响主要体现在以下2个方面,最终都导致Fe-Cr合金的高温腐蚀速率增大:(1) 在低氧分压环境(Ar + H2O)中,H2O(g) 通过分解产生的H扩散进入合金中,增大了O在合金中的扩散系数,导致Cr更易于发生内氧化而不是形成Cr2O3[13,28,62~65];(2) 在高氧分压环境(Ar + O2 + H2O)中,H2O(g)可与Cr2O3和O2反应形成挥发性的CrO2(OH)2,致使合金因Cr被快速消耗而生长非保护性的铁氧化物[66~69]。但是,H2O(g)的上述影响对Ni-Cr合金却并不明显,关于这种差异的原因,Essuman等[70]认为这与NiO在H2O(g)中的低生长速率有关,而Mu等[71,72]认为这与Ni比Fe在Cr2O3中的固溶度更小有关。因此,有关H2O(g)对Ni-Cr合金高温腐蚀的影响比较复杂,仍需深入研究。

研究[42,73~75]表明,H2O(g)加入CO2中对气氛的氧分压(pO2)并无太大影响(650℃, Ar + 20CO2和Ar + 20CO2 + 20H2O的pO2分别为5.2 × 10-4和1.1 × 10-3 Pa),但却降低Ni-Cr (Cr ≤ 20%)合金的高温氧化速率,并改变氧化产物的形貌。如图4[42]所示,Ni-15Cr合金在650℃下Ar + 20CO2中形成了连续的NiO外层,而在Ar + 20CO2 + 20H2O中NiO呈零散堆积的片状或羽毛状形态,其中还有未被完全氧化的金属Ni。并且,通过对比纯Ni在这2种气氛中的氧化行为发现[75],H2O(g)的加入使氧化速率降低近1个数量级;在Ar + 20CO2中形成的NiO层较厚且疏松多孔,在Ar + 20CO2 + 20H2O中形成的NiO层薄而且较致密,类似的结果在对比纯Ni在O2和H2O(g)[70]中,以及干和湿空气中[76]的氧化时也有发现。还有研究[36,77~79]显示,Ni-Cr合金在H2O(g)与CO2 + H2O(g)中形成氧化膜的成分和结构基本相同,而这些都被认为与H2O(g)比CO2和O2的吸附能力更强有关[13,80]。所以,在上述混合气氛中的H2O(g)通过优先吸附,抑制了O2和CO2与Ni、Ni-Cr合金的反应,导致氧化动力学和氧化膜结构呈现与在H2O(g)中反应时类似的特征。同时,Galerie等[81]指出NiO在H2O(g)中生长速率很慢,这与其非酸性p型半导体氧化物的本质有关,这与上述Essuman等[70]的观点一致。

图4

图4   Ni-15Cr合金和纯Ni在650℃下Ar + 20CO2和Ar + 20CO2 + 20H2O气氛中腐蚀后的截面SEM像[42]

Fig.4   Cross-sectional SEM images of Ni-15Cr alloy exposed at 650oC for 310 h in Ar + 20CO2 (a) and Ar + 20CO2 + 20H2O(b), and pure Ni exposed at 650oC for 150 h in Ar + 20CO2 (c) and Ar + 20CO2 + 20H2O (d)[42]


此外,因H2O(g)加入CO2中,Ni-Cr (Cr ≥ 25%)合金生长Cr2O3膜的行为也受到了较大影响。如图5[75]所示,Ni-30Cr合金在CO2中形成单一的外Cr2O3膜,而在CO2 + H2O(g)中形成的外Cr2O3膜表面还含有少量富Ni氧化物,且TEM分析结果表明H2O(g)的加入使Cr2O3晶粒由粗大的柱状晶转变为细小的等轴晶。H2O(g)这种细化Cr2O3膜晶粒的影响已经被广泛报道,目前普遍认为[67,82~86]这主要是因为H2O(g)或其分解产物(OH-)沿Cr2O3膜晶界的吸附和扩散抑制了Cr2O3晶粒的长大,所以气氛中的氧化性物质可以沿着Cr2O3膜内的高密度晶界向内快速扩散而使kp增大,根据 式(2)可知Ni-Cr合金在CO2 + H2O(g)中需要更高Cr含量才能维持外Cr2O3膜的生长[75]

图5

图5   Ni-30Cr合金在800℃下Ar + 20CO2 + 20H2O和Ar + 20CO2气氛中腐蚀500 h后的截面SEM像和TEM像[75]

Fig.5   Cross-sectional SEM images of Ni-30Cr alloy exposed at 800oC for 500 h in Ar + 20CO2 + 20H2O (a) and Ar + 20CO2 (b), and cross-sectional BF-TEM images of corresponding scales formed in Ar + 20CO2 + 20H2O (c, d) and Ar + 20CO2 (e) (Fig.5c is the magnified image at the scale-alloy interface formed in Ar + 20CO2 + 20H2O)[75]


3.2 SO2 的影响

富氧燃烧产生的烟气中含有少量的SO2 (约0.5%,体积分数)[6],虽然其浓度比CO2和H2O(g)低的多,但比空气燃烧产生烟气中的SO2浓度增大3~4倍[58]。SO2对锅炉受热面合金高温腐蚀的影响主要分为2类。

(1) SO2与金属反应,导致后者发生硫化-氧化,加速金属的高温腐蚀,反应如式(3)~(5)所示[1]

2xM+2SO2=2MxO2+S2
2xM+S2=2MxS
2MxS+2O2=S2+2MxO2

由于Ni、Cr、Fe等大多数金属硫化物的缺陷浓度明显高于对应金属氧化物[87],所以同等条件下它们的硫化速率比氧化速率更快(如Cr的硫化速率比相应的氧化速率高4~5个数量级),大多数含Cr耐热合金在含SO2气氛中难以形成保护性氧化膜。有报道[88~90]指出,Inconel 617合金在550和650℃不含SO2的烟气中形成了一层连续的Cr2O3保护膜,而在含有SO2的烟气中形成了许多由硫化物、氧化物及硫酸盐组成的瘤状物,破坏了Cr2O3膜的连续性,导致合金的高温腐蚀速率明显增大。

(2) SO2与煤中的碱金属反应形成Na2SO4、K2SO4等,沉积在受热面合金表面,当温度超过它们的熔点后就会导致合金发生I型热腐蚀(温度> 900℃)[91];同时,如 式(6)⁓(10)所示[34,92],SO2还可与煤灰、氧化物反应形成Na3Fe(SO4)3和K3Fe(SO4)3三元硫酸盐[9,34]以及(Na, K)3Fe(SO4)3、Na2SO4-NiSO4和Na2SO4-CoSO4等共晶物[93,94],它们的熔点均较低(< 700℃),可导致部分温度较低的受热面合金发生II型热腐蚀。依据熔融Na2SO4的相对酸度/碱度[95],Ni-Cr合金表面形成的保护性Cr2O3膜与上述熔盐接触会发生碱性(式(11))或酸性(式(12))电化学腐蚀溶解而失效,Meier[96]已对此进行较为系统的总结展望。研究结果[10,34,97,98]表明,在含SO2烟气+煤灰环境中,Inconel 617、625、740等镍基合金呈现出比T92耐热钢和TP347不锈钢更优异的抗高温腐蚀性能,这主要与前3者较高的Cr含量有关。与此类似,Zeng等[9]的研究也指出,耐热合金中的Cr必须至少达到25%才能在750℃下有效抵御热腐蚀。

Fe2O3+3Na2O+6SO2+3O2=2Na3Fe(SO4)3
Fe2O3+3K2O+6SO2+3O2=2K3Fe(SO4)3
2SO2+O2=2SO3
SO3+NiO=NiSO4
SO3+CoO=CoSO4
Cr2O3+Na2O+2O2=2NaCrO4
Cr2O3+3Na2SO4=Cr2(SO4)3+3Na2O

此外,对于Inconel 617、GH984G等镍基合金在含SO2环境中发生硫化的机理[35,99,100],可以利用类似前述碳化机理[52]进行解释:在氧化膜-合金界面由SO2分解产生的硫分压(pS2)pO2成反比,由于Cr2O3分解产生的pO2很低,所以界面处的pS2显著增大,S渗透进入合金内部并形成相对较为稳定的Cr、Ti、Nb的硫化物,使氧化膜内侧发生Cr元素贫化[20,35,101]。并且,Sha等[102]最新的研究结果已经证实S可以通过晶界扩散而穿过Cr2O3膜。值得注意的是,SO2通过强竞争吸附可以抑制O2、CO2、H2O(g)在氧化膜表面的吸附,这对降低含Cr耐热合金在H2O + CO2 + O2 + SO2多组元混合气氛中的碳化和Cr2O3的挥发产生一定的有益影响[31,88,103],未来对Ni-Cr合金在上述多组元混合气氛中腐蚀时各组分气体相互作用的研究应给与更多关注。

4 温度和合金元素的影响

4.1 温度的影响

根据Wagner[39,41]提出的高温氧化理论模型,由 式(1)和(2)可知,Ni-Cr合金表面热生长Cr2O3保护膜的临界Cr含量将随NO(S)DOkpD˜Cr的变化而变化,而它们都是温度的函数,所以温度对Ni-Cr合金的高温腐蚀,尤其是Cr2O3保护膜的热生长具有显著的影响。表1[36,37,75]列出了基于Wagner理论模型计算出的Ni-Cr合金CO2高温腐蚀时NCr(1)NCr(2)在不同温度下的数值[37],可见在650~800℃范围内,升高温度有助于降低Ni-Cr合金在CO2气氛中热生长Cr2O3保护膜所需的临界Cr含量,这主要与升高温度增大了D˜Cr有关。实验结果表明(图6[37]),Ni-25Cr合金CO2高温氧化时,随着温度从650℃逐渐升高至700和800℃,Cr的氧化产物逐渐从内氧化析出的Cr2O3颗粒,转变为氧化前沿的一层连续的Cr2O3薄层,直至单一的Cr2O3保护膜,验证了该模型的准确性。目前,该模型已被广泛用于解释Fe-Cr合金[28,71,72,104,105]和某γ/γ'型CoNi基高温合金[106]热生长氧化膜的成分随温度的变化规律。考虑到先进超超临界锅炉采用富氧燃烧技术时受热面合金表面温度的大致范围(600~800℃)[107~109],该温度影响规律对指导抗CO2高温腐蚀Ni-Cr合金的成分设计和性能评价具有重要的借鉴意义。

  

Table 1  Values of D˜Cr, NCr(1), and NCr(2) of Ni-Cr alloys corroded in Ar + 20CO2 at different temperatures[36,37,75]

T / oCD˜Cr / (cm2·s-1)NCr(1)NCr(2)
6502.6 × 10-150.31> 0.68
7001.4 × 10-140.280.39
8002.3 × 10-130.240.16

Note:T—temperature, D˜Cr—diffusion coefficient of Cr, NCr(1)—the critical Cr content corresponding to the transition from internal Cr2O3 precipitation to external Cr2O3 scaling, NCr(2)—the critical Cr content required for maintaining exclusive Cr2O3 scale growth

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图6

图6   Ni-25Cr合金在不同温度下Ar + 20CO2气氛中腐蚀500 h后的截面SEM像[37]

Fig.6   Cross-sectional SEM images of Ni-25Cr alloy exposed at 650oC (a), 700oC (b), and 800oC (c) for 500 h in Ar + 20CO2[37]


需注意的是,水蒸气对Ni-Cr合金CO2高温腐蚀的影响也随温度而变化,并且这种影响随温度升高而逐渐减弱[75,79],这主要是由于H2O(g)比O2等其他大多数含氧气体分子的吸附和分解能力更强,并且这种差异在低温下表现得更明显[110~112]。但是,目前还比较缺乏关于温度对H2O(g)这种影响的机理性解释,需要更进一步深入研究。此外,若气氛的SO2含量固定,温度升高将导致SO3的分压降低,同时形成硫酸盐所需的SO3分压也将升高[91],所以温度的变化也会对Ni-Cr合金在含SO2气氛中的高温腐蚀产生影响。Huczkowski等[88,89]已经报道Inconel 617合金在550和650℃的含SO2烟气中形成了许多含有硫酸盐/硫化物的瘤状物,而在700℃却形成了Cr2O3保护膜,这是由于700℃下Cr2O3比Cr的硫酸盐和硫化物的稳定性更高所致。

1 Ni-Cr合金在不同温度下Ar + 20CO2高温腐蚀时Cr在合金中的扩散系数(D˜Cr)、形成连续的Cr2O3外氧化膜所需的临界Cr含量(NCr(1))和合金维持单一Cr2O3膜生长所需Cr含量(NCr(2))的数值[36,37,75]

4.2 合金元素的影响

为了进一步提高Ni-Cr合金的抗CO2高温腐蚀性能,Nguyen等[49,73]初步探明了添加Si、Al和Ti等合金元素的影响,发现Ni-Cr合金中添加一定量的Si (1%)、Al (1.5%)有助于在Cr2O3膜底部形成一层SiO2或Al2O3薄层,它们能够抑制Cr向合金表面的扩散,减慢合金中Cr的贫化速率;添加2%的Ti能促进Cr2O3膜在氧化初期的快速形成,所以它们都能起到提高Ni-Cr合金抗CO2高温腐蚀的作用。但同时上述合金化也存在以下难题:(1) 由于Si、Al添加量有限,它们在Cr2O3膜底部形成的SiO2或Al2O3膜并不连续,导致其不能有效阻止C向合金中的扩散,所以合金仍然发生了沿晶界的碳化;(2) Ni-30Cr-2Ti合金在CO2 + H2O(g)气氛中的碳化程度显著增大,导致晶界和晶内形成了大量的富Cr/Ti的碳化物(图7[73]),这被认为与H2O(g)改变了Cr2O3膜晶界的性能有关。

图7

图7   Ni-30Cr-2Ti合金在700℃下Ar + 20CO2 + 20H2O气氛中腐蚀不同时间后的碳化物SEM像[73]

Fig.7   SEM images of carbide precipitates in Ni-30Cr-2Ti alloy exposed at 700oC for 20 h (a) and 500 h (b) in Ar + 20CO2 + 20H2O[73]


Fe作为一种重要的金属元素,加入Ni-Cr合金中不仅有助于降低成本,还可以提高合金的工艺性能,所以Ni-Cr-Fe基高温合金作为一种潜在的适用于700℃超超临界火电机组的材料,近来也获得了较多的关注[22,23,113,114]。本文作者前期研究了Ni-(20, 30)Cr-(0, 1, 5, 15)Fe系列合金在CO2和CO2 + H2O(g)气氛中的高温腐蚀行为[115~117],发现Fe对Ni-Cr合金抗高温腐蚀性能的影响随温度和Cr含量而变化:当温度相对较低时(650和700℃),Ni-20Cr-xFe系列合金因Cr含量远低于NCr(1)NCr(2) (表1[36,37,75]),其高温腐蚀随Fe含量增大变得更严重;而Ni-30Cr-xFe合金因Cr含量接近NCr(1)NCr(2),再加上Fe对Cr在Fe-Ni-Cr合金中扩散的加速作用[118,119],合金随Fe含量的增大而更容易形成单一的外Cr2O3膜,类似的作用在800℃下Ni-20Cr-xFe系列合金的氧化膜结构随Fe含量的变化趋势中也有体现。Xu等[114,120]也认为当镍基合金中Cr含量较低时,添加较多的Fe将降低合金的抗高温腐蚀性能。此外,研究[115]还发现,Ni-(20, 30)Cr-(0, 1, 5, 15)Fe系列合金内碳化的程度随Fe含量的升高而有所增大,热力学计算也表明Fe含量增大有助于降低上述Ni-Cr-Fe合金内部形成碳化物所需的碳活度,而这与Fe在Cr23C6中的大量固溶有关[59,121]

综上所述,通过添加一定量的Fe、Si、Al和Ti对Ni-Cr合金进行合金化能在一定程度上提高其抗CO2高温腐蚀性能,但合金元素的最佳含量仍有待进一步研究。

5 总结与展望

本文主要介绍了Ni-Cr合金在富CO2气氛中的高温腐蚀特点和碳化机理,归纳总结了H2O(g)、SO2、温度和合金元素对其高温腐蚀影响的研究现状,气氛中的CO2、H2O(g)和SO2都能够阻碍Ni-Cr合金在高温腐蚀时形成Cr2O3保护膜,升高温度可通过加速合金中Cr的扩散而有助于合金生长Cr2O3保护膜,合金元素对Ni-Cr合金抗高温腐蚀性能的影响则与合金的具体成分有关。未来针对Ni-Cr合金在富CO2气氛中的高温腐蚀研究可以重点围绕以下几个方面展开。

(1) 目前对Ni-Cr合金在不同气氛高温腐蚀后形成的Cr2O3膜的具体结构研究仍不系统,这给准确厘清高温腐蚀气氛中各组分对Cr2O3膜保护性能的影响机理造成了很大困难,后期应注重开展Ni-Cr合金在相同温度、不同腐蚀气氛(O2、CO2、H2O、CO2 + H2O、H2O + O2等)下热生长Cr2O3膜微观结构的深入研究,并利用高分辨透射电镜和三维原子探针等先进技术精确表征Cr2O3膜的微观精细结构,以期为Ni-Cr合金在多组元高温腐蚀气氛中Cr2O3膜的生长行为提供更加全面和深入的认识,从而为针对不同腐蚀气氛设计和优选具有更优异抗高温腐蚀性能的镍基合金提供理论基础。

(2) 镍基合金中通常添加有微量的稀土元素以提高其抗高温腐蚀性能,这主要是因为稀土元素可以在氧化膜晶界处偏聚,从而阻碍物质沿氧化膜晶界的扩散。目前的研究已经表明,CO2、H2O(g)和SO2分子或它们的分解产物主要也是通过晶界扩散而穿过氧化膜导致合金发生碳化、氧化和硫化,未来的研究需要探明它们与稀土元素在氧化膜晶界处的交互作用以及这些作用对Ni-Cr合金在富CO2气氛中的抗高温腐蚀性能的影响规律。

(3) 考虑到电力行业目前正在蓬勃发展的生物质发电、生物质-燃煤混烧发电和垃圾焚烧发电技术,这些电站锅炉中燃烧产生的富CO2烟气中还会混合有酸性HCl(g)组分,虽然对耐热钢的高温氯腐蚀已研究较多,但围绕含氯的富CO2气氛对镍基合金抗高温腐蚀性能的影响仍未获得足够的关注,后期可考虑在CO2 + H2O(g)中添加少量HCl(g),研究该含氯气氛对Ni-Cr合金形成Cr2O3保护膜的影响机制。

参考文献

Zhang S H, Hu K, Liu X, et al.

Corrosion-erosion mechanism and research prospect of bare materials and protective coatings for power generation boiler

[J]. Acta Metall. Sin., 2022, 58: 272

DOI      [本文引用: 2]

Since the “emission peak-carbon neutrality” goal was proposed, coal-fired boilers, which are major power supply equipment and CO2 emission sites, have been gradually developed to zero-carbon emission biomass and low-carbon coal/biomass cofired boiler. The corrosion and erosion behavior of the sulfur and chlorine components of coal and biomass poses a serious threat to the safety and long-term operation of boilers, and protective coatings have become a convenient and efficient way to improve the corrosion and erosion resistance of boilers. This paper reviews recent research progress on high-temperature corrosion and erosion of bare materials and protective coatings used in coal-fired, biomass, and coal/biomass cofired boilers. The mechanism of sulfur corrosion and alkali chlorine corrosion in coal-burning and biomass combustion environments is summarized. The ash deposition-impaction mechanism in coal/biomass cofired environments is described. The current application status of boiler bare materials is introduced, and the design principles, preparation processes, and application status of alloy, ceramic, and metal-ceramic coatings in corrosive and erosive environments are summarized. Based on the current findings, future research on corrosion and erosion of boilers should focus on imperfect hot corrosion mechanisms, accurate corrosion-wear prediction models and types of protective coatings. Finally, material genome engineering and machine learning are proposed to accelerate material research/development and study the corrosion-erosion mechanisms as well as multifactor coupling models. There is a need to integrate powder synthesis methods, coating structure designs, and in-service performance into the development of new protective coatings.

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[J]. 金属学报, 2022, 58: 272

DOI      [本文引用: 2]

作为目前主要的电力供应设备和CO<sub>2</sub>排放场所,随着“碳达峰-碳中和”目标的提出,燃煤发电锅炉逐渐向零碳/低碳排放的生物质发电锅炉和生物质-燃煤混烧发电锅炉方向发展。燃煤及生物质中含硫、含氯组分的腐蚀及磨损行为对锅炉安全长效运行构成严重威胁,因此,沉积防护涂层成为提高锅炉耐蚀耐磨性能的便捷高效手段。本文综述了目前燃煤锅炉、生物质锅炉及生物质-燃煤混烧锅炉基体材料和防护涂层在高温腐蚀及冲蚀磨损方面的研究成果,针对燃煤及生物质燃烧环境,总结了高温硫酸盐腐蚀及碱金属氯化物腐蚀机理,阐述了生物质-燃煤混烧环境中灰沉积-冲击机制,介绍了目前锅炉基体材料的应用现状,归纳了在腐蚀和磨损不同环境下合金涂层、陶瓷涂层、金属陶瓷涂层的设计原则、制备方法及应用现状。指出了当前锅炉防护研究亟待解决的问题,包括:热腐蚀机理研究不深入,缺乏准确的腐蚀-磨损预测模型,防护涂层种类较少。最后,提出采用材料基因组工程及机器学习方法来加速材料研发,开展腐蚀-磨损机理及多因素耦合模型的研究,同时强调粉末合成设计-涂层结构设计-服役性能评价一体化研究对新型防护涂层研发的重要性。

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[J]. Acta Metall. Sin., 2022, 58: 311

DOI      [本文引用: 1]

Improving the steam temperature and the pressure of the boiler applied in the thermal power could enhance the coal-fired efficiency and reduce the emission of harmful gases. Due to the dual impact of dwindling fossil resources and an exacerbated global greenhouse effect, it is critical to develop new heat-resistant boiler materials for ultra super-critical (USC) units at temperatures of 650oC and higher. With great thermal conductivity, good fatigue resistance, and low cost, martensitic heat-resistant steel G115, based on P92 steel applied in 600oC USC units, is a promising steel to be applied to this among all candidate materials. This paper introduces the main chemical composition and the microstructure feature of G115 steel, and the research progress in the areas of microstructure stability, creep performance, fatigue resistance, steam oxidation resistance, and industrial pipe production are summarized, with a focus on the role of Cu-rich phase in G115 steel. Finally, some key points on G115 steel are proposed to provide ideas for future research.

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提高火电机组中耐热锅炉的蒸汽温度和压力参数可以有效提升燃煤效率,减少有害气体排放。受煤炭资源紧缺和温室效应的双重影响,发展650℃及更高温度超超临界(ultra super-critical,USC)机组中的耐热锅炉材料已迫在眉睫。我国在600℃ USC机组用耐热材料P92钢基础上研发的马氏体耐热钢G115有望成为优选材料之一。本文介绍了G115钢的成分特点、形貌特征,综述了其在组织稳定性、蠕变性能、抗疲劳性能、抗蒸汽氧化性能以及工业管材制备等方面的研究进展,重点归纳了G115钢中富Cu相的作用,展望了未来研究重点,以期为更深入研究G115钢提供可行思路。

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DOI     

迄今,600 ℃超超临界是世界最先进商用燃煤电站技术。630~700 ℃超超临界燃煤电站研发将奠定我国火电技术的国际领先地位,对实现国家节能减排目标具有重要战略意义。耐热材料是制约火电机组蒸汽温度进一步提升的技术瓶颈,本文简述了国内外630~700 ℃超超临界电站耐热材料研制现状,指出了我国急需研发的关键耐热材料。阐述了作者团队在多年实践中总结的电站耐热材料“全流程选择性冶金过程设计和选择性强韧化设计”观点,重点介绍了在该设计观点指导下,我国成功研发了用于630~650 ℃的马氏体耐热钢G115<sup>?</sup>,用于650~700 ℃的固溶强化型镍基耐热合金C-HRA-2<sup>?</sup>、C-HRA-3<sup>?</sup>,以及用于700~750 ℃的析出强化型镍基耐热合金C-HRA-1<sup>?</sup>,系统构建了我国630~700 ℃超超临界燃煤锅炉耐热材料体系,并已成功制造了上述新型耐热材料锅炉管。

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尽可能提高电厂效率,大力发展超超临界燃煤火电技术,是当前降低火电CO2排放最现实、最可行、最经济、最有效的途径。实现700 ℃ 超超临界技术的关键是开发能够在蒸汽温度700 ℃条件下长期安全运行的高温金属材料,这些材料必须要具有:(1)耐高温的持久强度;(2)耐烟气侧的高温腐蚀;(3)抗蒸汽侧氧化及氧化层剥落;(4)良好的抗热疲劳性能。为此,介绍了欧盟AD700和COMTES700等计划,以及美国和日本的700 ℃ 超超临界计划对高温材料研发的最新进展,其中包括材料试验、高温材料部件挂炉试验结果和示范厂计划。与此同时,还就高温金属材料用于700 ℃超超临界机组的制约因素的解决方案进行了探讨。

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