Abstract: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 Cr₂O_{3 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.}