我国常用的锅炉用钢为20G钢[7]，但这种低成本钢通常不能抵抗实际腐蚀环境，因此,在其上涂覆耐蚀涂层是常用做法。目前主要采用表面堆焊[8]、喷涂[9]其他耐蚀合金方法,但这种金属类涂层在长期服役过程中也会与腐蚀介质发生反应从而出现涂层退化问题，同时这也会使得原材料成本大幅上升。近年来，有机与无机非金属涂层因原料便宜，耐蚀性高等优点而受到研究者越来越多的关注，传统有机涂层可解决常温下盐雾腐蚀问题，但高温下清漆易发生严重的热分解导致涂层失效[10,11]。搪瓷涂层作为无机非金属氧化物的复合体，服役时不存在类似金属类涂层的退化问题，具有高热化学稳定性[12]。目前对于搪瓷涂层的研究主要集中在高温领域的发动机叶片[13,14]、燃气轮机[15,16]，以及常温领域的石油管道[17]、锅炉热交换器 (GGH)[18]、生物活性玻璃等[19,20]，而在中温领域的研究为数不多。针对于此，本文选取与20G成分相似的20钢为基体，探索在其上施加搪瓷涂层的可行性，并试验了涂层在400 ℃下恒温氧化以及氯盐加速腐蚀环境中的防护能力表现。
1 实验方法
20钢的化学成分 (质量分数，%) 为：C 0.17~0.23, Si 017~0.37, Mn 0.35~0.65, P≤0.035, S≤0.035, Ni≤0.3, Cr≤0.25, Fe为余量。其碳含量为0.2%左右，而搪瓷用钢碳含量一般不超过0.1%[21]。碳含量过高会使得搪烧过程中产生大量气泡，造成搪瓷表面发泡、开裂等缺陷。针对这个问题可以适当提高Na2O、K2O等助溶剂含量来解决，同时加入Al2O3等原料兼顾耐蚀性[22]。详细瓷釉配方 (质量分数，%) 为：SiO2 52，B2O3 12, Al2O3 6，K2O 5.5，Na2O 11，CaF2 6，Li2O 6，CoO 1.5。经前期探索，该涂层最佳烧结工艺为：780 ℃下烧结2 min。
将上述原料装入氧化铝坩埚并加热至1300 ℃形成熔融玻璃，水淬和研磨后得到搪瓷粉末。将基体切割成15 mm×10 mm×2.5 mm大小的试样，并使用SiC砂纸研磨至1000粒度。首先对基体进行喷砂处理，然后在丙酮中进行超声波清洗，将混合有乙醇的搪瓷粉末利用空气压缩机喷涂到试样的粗糙表面上，然后在100 °C下干燥5 min，在780 ℃马弗炉里烧结2 min以形成搪瓷涂层。
氧化在马弗炉里进行，经称重和测量表面积后，将样品放在预先烧至恒重的刚玉坩埚中，在实验温度下的静止空气中氧化，间隔一定时间取出，空冷至室温，在精度为10-5 g的电子天平上称重得到动力学曲线，然后放入炉中继续氧化。实验温度为400 ℃，氧化时间为1000 h。为了得到氧化规律，前期取样频次适当增多，后500 h每隔100 h取一次。
氯盐腐蚀实验在马弗炉中进行，选用饱和NaCl溶液。首先将饱和盐溶液均匀地涂在预热的试样表面，涂盐量为1.5~2.5 mg/cm2，然后将试样置于400 ℃的马弗炉内保温一定时间后取出，空冷至室温后置于沸腾的去离子水中洗去残留盐，最后烘干、称重，以此作为一个循环。为减小误差，每组样品准备了3个平行样，用精度为10-5 g的电子天平记录样品的重量变化。
使用Inspect F50扫描电子显微镜 (SEM) 并结合X-Max能谱仪 (EDS) 分析涂层形貌及成分，利用X'Pert PRO X射线衍射仪 (XRD) 分析相组成 (Cu Kα，40 kV)。
2 结果与讨论
2.1 搪瓷涂层制备态微观形貌
图1
图1
制备态搪瓷涂层的表面和截面形貌以及XRD谱
Fig.1
Surface (a) and cross-sectional (b) morphologies and XRD spectra (c) of as-prepared enamel coating on 20 steel
2.2 400 ℃恒温氧化
图2为带涂层和不带涂层的20钢在400 ℃下氧化1000 h后的动力学曲线。从中可看出，施加搪瓷涂层后明显降低了20钢的氧化速率。对于20钢基体，氧化1000 h后，增重达到了0.64 mg/cm2，而带涂层的样品增重仅0.02 mg/cm2，几乎不增重。裸钢的氧化动力学曲线前期遵循抛物线规律，后期氧化速度则有所放缓，而带有涂层的样品则近似处于一条水平直线上。
图2
图2
不带及带有搪瓷涂层的20钢在400 ℃下的氧化动力学曲线
Fig.2
Oxidation kinetics of 20 steel with and without enamel coating at 400 ℃
图3
图3
20钢基体氧化不同时间后背散射扫描像
Fig.3
Cross-sectional BSE-mode images of the bare 20 steel after oxidation at 400 ℃ for 10 h (a), 100 h (b), 500 h (c) and 1000 h (d). The inset in Fig.3a shows the surface morphology
图4
图4
400 ℃下氧化1000 h后搪瓷涂层微观表征
Fig.4
Surface (a), cross section (b) and corresponding EDS line scannings (c) for the enamel coating oxidized at 400 °C for 1000 h
2.3 400 ℃氯盐腐蚀
动力学曲线是腐蚀产物生成与剥落的综合结果。从图5看出，基体的增重曲线在初期波动很大，结合扫描照片可知此时便已发生较为严重的腐蚀，表面已形成较厚腐蚀膜，从图6a可知基体已经开始出现剥落趋势。腐蚀10 h便开始出现失重现象，表明腐蚀产物剥落量大于产生量，从图6b和d腐蚀200 h后的扫描照片来看，基体表面已经遭受了严重的破坏，形成分散破裂的腐蚀膜，从截面也可看出腐蚀膜呈多层相互分离状，XRD结果表明腐蚀膜主要成分为Fe3O4，表面晶须状为Fe2O3。表面分散的多层腐蚀膜也表明基体受到了反复的氯盐腐蚀，这也解释了动力学曲线上先失重后增重现象。而带有搪瓷涂层的样品动力学曲线一直很平稳，腐蚀200 h后的扫描结果如图7所示，可看出涂层腐蚀后仍旧致密平整，未遭到破坏。从图7c中XRD结果可知搪瓷涂层在整个腐蚀期间并未生成其他相，表明涂层能稳定存在于氯盐环境中，表现出优异的耐蚀性能。
图5
图5
不带及带有搪瓷涂层的20钢在400 oC下表面沉积NaCl时增重曲线
Fig.5
Mass changes of 20 steel without and with enamel coating at 400 ℃ under the condition of NaCl deposition
图6
图6
20钢表面沉积NaCl时400 °C下腐蚀后的形貌
Fig.6
Surface morphologies (a, b) and cross sections (c, d) of 20 steel after NaCl pre-deposited corrosion at 400 °C for 5 h (a, c) and 200 h (b, d)
图7
图7
带搪瓷涂层的20钢在表面沉积NaCl时腐蚀200 h后微观表征
Fig.7
Surface (a), cross section (b) and XRD pattern (c) of enamel coating after NaCl pre-deposited corrosion at 400 ℃ for 200 h
氯盐腐蚀属于工业上常见的腐蚀类型，在实验温度下，NaCl可直接与金属发生反应：
释放出Cl2。通常氯在氧化膜中有很强的渗透性,能够穿过氧化膜到达氧化膜/金属界面与金属反应：
生成挥发性氯化物FeCl2，FeCl2在向外扩散过程中又会发生如下氧化反应：
生成的Fe2O3与无氯环境下氧化膜相比黏附性差，疏松多孔，极大破坏了原有氧化膜的致密性，最终导致氧化膜更严重的开裂与脱落，部分生成的Cl2又会回到氧化膜/金属界面重复上述腐蚀过程，直至氯被耗尽为止，这个过程称为氯的“活化氧化”[28]。搪瓷涂层本质为惰性氧化物的结合体，在氧化和热腐蚀过程中本身不会继续被氧化，其连续致密的微观网络结构能在保持较高热稳定性的同时可以显著地抑制外界环境中Cl-等腐蚀介质向基体中的扩散。搪瓷涂层的防护机制主要为惰性氧化物的物理阻隔作用。
3 结论
20钢基体在400 ℃长期恒温氧化以及氯盐腐蚀环境中均表现出较差的耐蚀性，氯盐腐蚀尤为严重，初期便已失去抵御能力。
搪瓷涂层在两种环境下均表现出极高的热稳定性与极低的腐蚀速率，涂层在实验过程中保持完整致密，有效地阻挡了O、Cl等腐蚀介质向涂层/基体界面扩散，在盐膜与合金基体之间保持长期稳定的阻隔作用，极大地提高了20钢在氯盐环境中的耐蚀能力。搪瓷涂层的耐腐蚀机制主要为物理阻隔作用。
参考文献
Municipal solid waste (MSW) as a renewable source of energy: current and future practices in China
[J].
A review on municipal solid waste as a renewable source for waste-to-energy project in India: current practices, challenges, and future opportunities
[J].
A review of China's municipal solid waste (MSW) and comparison with international regions: management and technologies in treatment and resource utilization
[J].
Study on high temperature chlorination corrosion of metallic materials on the waste incineration boilers
[J].
High temperature chlorine corrosion of nickel based alloy coating for piping of waste incineration power plant
[J].
垃圾焚烧电站水冷壁镍基合金涂层高温氯腐蚀性能研究
[J].
垃圾焚烧电站水冷壁高温腐蚀是影响垃圾焚烧炉稳定运行的重要因素,为了提高垃圾焚烧炉运行的安全性,研究者大多从基材的选择、施加表面防护涂层、温度控制等方面进行了研究。本文则主要针对施加表面涂层方面,研究了锅炉水冷壁镍基涂层高温氯腐蚀性能,利用SEM、EDS、XRD和光学显微镜等对合金涂层进行表面形貌和组织结构分析,并沿涂层深度方向进行维氏硬度测试和组织成分检测,探究其抗氯腐蚀机理。结果表明,镍基涂层在高温下耐腐蚀能力强,组织致密,没有孔隙、裂纹等较大缺陷,能够较好地耐氯盐腐蚀,保护垃圾焚烧炉水冷壁。
Comparative investigation on thermal corrosion of alloy coatings in simulated waste incinerator environment
[J].
A review of technologies and performances of thermal treatment systems for energy recovery from waste
[J].The aim of this work is to identify the current level of energy recovery through waste thermal treatment. The state of the art in energy recovery from waste was investigated, highlighting the differences for different types of thermal treatment, considering combustion/incineration, gasification and pyrolysis. Also different types of wastes - Municipal Solid Waste (MSW), Refuse Derived Fuel (RDF) or Solid Refuse Fuels (SRF) and some typologies of Industrial Waste (IW) (sludge, plastic scraps, etc.) - were included in the analysis. The investigation was carried out mainly reviewing papers, published in scientific journals and conferences, but also considering technical reports, to gather more information. In particular the goal of this review work was to synthesize studies in order to compare the values of energy conversion efficiencies measured or calculated for different types of thermal processes and different types of waste. It emerged that the dominant type of thermal treatment is incineration associated to energy recovery in a steam cycle. When waste gasification is applied, the produced syngas is generally combusted in a boiler to generate steam for energy recovery in a steam cycle. For both the possibilities--incineration or gasification--co-generation is the mean to improve energy recovery, especially for small scale plants. In the case of only electricity production, the achievable values are strongly dependent on the plant size: for large plant size, where advanced technical solutions can be applied and sustained from an economic point of view, net electric efficiency may reach values up to 30-31%. In small-medium plants, net electric efficiency is constrained by scale effect and remains at values around 20-24%. Other types of technical solutions--gasification with syngas use in internally fired devices, pyrolysis and plasma gasification--are less common or studied at pilot or demonstrative scale and, in any case, offer at present similar or lower levels of energy efficiency.Copyright © 2014 Elsevier Ltd. All rights reserved.
Alleviation of thermal corrosion caused by molten ash on heat-exchange tubes in MSW incinerators: effects of Ni-, Co-, Fe-based HVOF coatings
[J].
Development of thermal spraying materials through several corrosion tests for heat exchanger tube of incinerators
[J].
Comparative study of micro- and nano-structured coatings for high-temperature oxidation in steam atmospheres
[J].
Enhanced oxidation and corrosion inhibition of 1Cr11Ni2W2MoV stainless steel by nano-modified silicone-based composite coatings at 600 ℃
[J].
Long-term high temperature oxidation and hot corrosion behavior of an enamel coating on γ-TiAl intermetallics at 700 ℃
[J].
搪瓷涂层700 ℃长期抗高温氧化和热腐蚀行为研究
[J].
Effect of Al2O3 content on microstructure and oxidation behavior of silicate enamel coatings on a Ni-based superalloy at 1000 ℃
[J].
Glass-ceramic coatings on titanium alloys for high temperature oxidation protection: oxidation kinetics and microstructure
[J].
Oxidation behavior of glass-based composite thermal barrier coating on K417G superalloy with a NiCoCrAlY bond coat at 1000 ℃
[J].
Oxidation behavior of bilayered ceramic particle-reinforced enamel composite coating on nickel-based superalloy K38G at 1000 ℃
[J].
镍基高温合金上双层搪瓷-陶瓷复合涂层1000 ℃氧化机理
[J].
Simulating sulfuric acid dew point corrosion of enamels with different contents of silica
[J].
Permeability of coal tar enamel coating to cathodic protection current on pipelines
[J].
Studies on novel bioactive glasses and bioactive glass-nano-HAp composites suitable for coating on metallic implants
[J].
Bioactive glass coatings on metallic implants for biomedical applications
[J].
Protection mechanism of enamel-alumina composite coatings on a Cr-rich nickel-based superalloy against high-temperature oxidation
[J].
Bubble structure─the unique structure of enamel coating
[J].
气泡结构—搪瓷层的独特结构
[J].
Glass coatings on stainless steels for high-temperature oxidation protection: mechanisms
[J].
Kinetics and morphological development of the oxide scale on iron at high temperatures in oxygen at low pressure
[J].
Oxygen and silicon diffusion-controlled processes in vitreous silica
[J].
Diffusion-controlled processes in glass forming melts
[J].
