曹沁,袁洁燕,王进双,曹学强.热障涂层表面腐蚀斑点的形成机理研究[J].热喷涂技术,2019,11(1):9~22.
热障涂层表面腐蚀斑点的形成机理研究
Formation Mechanism of Corrosion Spots on Thermal Barrier Coatings
  
DOI:10.3969/j.issn.1674-7127.2019.01.002
中文关键词:  热障涂层  氧化钇稳定化的氧化锆  腐蚀斑点
英文关键词:Thermal barrier coatings  Yttria stabilized zirconia  Corrosion Spots
基金项目:41422040104
           
作者单位
曹沁 武汉理工大学
袁洁燕 武汉理工大学
王进双 武汉理工大学
曹学强 武汉理工大学
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中文摘要:
      热障涂层(Thermal Barrier Coatings,TBCs)已广泛应用于航空、航天和地面燃机等发动机热端部件和 武器装备的防护。TBCs 应用的基底材料种类繁多,常见的是镍基高温合金,还有碳钢、铜合金、铝合金、镁合 金、复合材料等。氧化钇稳定化的氧化锆(Yttria Stabilized Zirconia,YSZ)涂层的常见颜色是白色和灰色,颜 色决定于氧空位含量。TBCs 产品的表面可能出现腐蚀斑点现象,特别是在高温潮湿环境下,腐蚀斑点出现机会 多、现象更明显。腐蚀斑点的颜色取决于基底或粘结层材料,碳钢和铜合金产生棕色腐蚀斑点,镍基高温合金产 生绿色腐蚀斑点,钴基高温合金产生浅红色腐蚀斑点,铝合金、镁合金、树脂则不会出现彩色腐蚀斑点。在镍基 高温合金基底表面,依次用大气等离子喷涂(Atmospheric Plasma Spraying,APS)方法制备钴基高温合金粘结层 (Bond Coat,BC)和纳米 YSZ(n-YSZ)涂层,在自然环境下贮存一段时间后,部分产品的表面出现了大量腐 蚀斑点。采用体式显微镜、扫描电镜(Scanning Electron Microscopy,SEM)和激光烧蚀 - 耦合等离子 - 质谱(Laser Ablation-Inductively Coupled Plasma-Mass Spectroscopy,LA-ICP-MS)方法证明,腐蚀斑点来源于 BC 的腐蚀产物。 BC 的主要成分是 Ni、Co 和 Cr,在涂层制备过程中分别被氧化生成氧化物 NiO、CoO 和 Cr2O3。YSZ 粉末中含有 大量残余的原料 ZrOCl2 和 YCl3,在潮湿空气中水解产生 HCl,将 BC 的氧化物溶解,便出现了 Ni2+(翠绿色)、 Co2+(浅红色)和 Cr3+(墨绿色)特有的颜色。
英文摘要:
      Thermal barrier coatings (TBCs) have been widely used for the protection of high-temperature metallic components of aero space land-based gas turbine engines, and other weapons, etc. There are a lot of substrate materials for TBCs, including Ni-based super alloys, carbon steel, copper alloy, aluminum alloy, magnesium alloy and composite materials. Yttria stabilized zirconia (YSZ) coatings usually look white or gray, depending on the content of oxygen vacancy. Corrosion spots are frequently observed on TBCs products, especially in hot and humid environment, corrosion would be more serious. The color of corrosion spots is dependent on the substrate or metallic coating materials, corrosion spots on TBCs with carbon steel or copper alloy substrates look brown, green with Ni-based super alloy, light red with Co-based super alloy, and those with Al-alloys, Mg-alloys or polymer substrates do not show corrosion spots. In order to protect TBCs against the humid effect, polymer sealing reagent is usually applied. Co-based super alloy bond coat (BC) and nano YSZ (n-YSZ, Cl content 0.053 wt%) coatings were deposited by atmospheric plasma-spraying (APS) on Ni-based super alloy substratesof a space engine, and some of the TBCs products were coated with polymer sealing reagent. However, after being stored for a few months in natural environment, numerous corrosion spots on some TBCs products were observed. Since 2011, we have collected more than 100 samples of YSZ in the form of powder, coating and bulk material, some of them were from world- famous companies including Inframat, Praxair, Sigma-Aldrich, Wieland Dental, Saint-Gobain, Sulzer-Metco and Tosoh. Corrosion spots on TBCs usually occur in spring and summer when it is hot and humid in Shenyang, Beijing, Guiyang and Xi’an. Sometimes, the coating surface looks normal, but numerous corrosion spots would appear when heated at 400~600°C for a short time. For some serious corroded TBCs products, even holes (corrosion holes) are formed due to the coating spallation. We have studied the formation mechanism of corrosion spots deeply, and have proved that the corrosion spots are induced by the residual Cl-, high contend Cl- in n-YSZ powder leads to the corrosion of BC or the metallic substrate. In this paper, we try to explore the formation mechanism of corrosion spots and find out the controlling method which is very helpful to the improvement of TBCs quality. With stereo microscope, we observed that the corrosion spots on TBCs were originated from BC, the corrosion products diffused outwards from BC to the top coating. The most important characterization method for corrosion spots is laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS), whose laser beam is concentrated into a diameter of only 20 μm. When a corrosion spot is abladed by the laser beam, only the compositions in the spot are evaporized and analyzed by MS. It is proved that concentrations of Ni, Co, Cr and Cl in corrosion spots are at least one order of magnitude higher than those in the blank area, and other impurities including Mg, Ca, Si and Al are equally distributed in the coating. On the other hand, it is also proved that concentrations of Ni, Co, Cr and Cl in corrosion spots become higher from the top surface down to BC. Finally, we conclude that the formation mechanism of corrosion spots is: main compositions of BC are Co, Ni and Cr, and oxides of CoO, NiO and Cr2O3 are formed during APS. The residual ZrOCl2 and YCl3 in n-YSZ powder are very high, both ZrOCl2 and YCl3 are hydrolyzed in humid environment, leading to the formation of HCl. Oxides of CoO, NiO and Cr2O3 would inevitably react with HCl, and therefore, corrosion spots look jade green (Ni2+), light red (Co2+) or dark green (Cr3+). Other mechanisms, such as metal powder as impurity in n-YSZ powder during APS, tiny sands absorbed into the coating from the air, mould formed during storation in hot and humid air have been proved to be totally impossible.
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