Zn-5Al-2Mg coated steel plate has excellent corrosion resistance and is widely used in construction, photovoltaics, and home appliances. The industrial production process and technical difficulty of Zn-5Al-2Mg coated steel plates are much higher than those of GI coated steel plates. The surface of the coating is prone to defects such as tooth marks, cloud patterns, and black spots, among which black spot defects are a very difficult surface quality problem in this series of coatings. This article conducts an in-depth analysis on the characteristics of black spot defects on the surface of hot-dip Zn-5Al-2Mg coatings generated in industrial production through electron microscopy analysis and FIB sample preparation technology, elucidating the causes and underlying mechanisms of the black spot defects.
An industrially produced Zn-5Al-2Mg medium aluminum zinc aluminum magnesium coated steel plate with a coating composition of 5% Al, 2% Mg, and with Zn (mass fraction) for balance, is selected for the experiment. The substrate thickness is 2.0 mm, and the weight of the single-sided coating is 138 g/m2, and the sample size is 20 mm×20 mm. For the surface analysis sample of the coating, a layer by layer polishing method is adopted to remove the light shaped surface of the coating. During the polishing process, the grinding condition of the skin pass rolling morphology on the coating surface is observed under an optical microscope every 5 seconds until the coating surface is sufficiently flat and no skin pass rolling morphology can be observed under the optical microscope; After marking the defect positions and hot embedding, the cross-sectional analysis samples of the coating are ground with emery paper and polished. To prevent oxidation of the coating, alcohol and 0.2 μm diamond polishing solution are used as auxiliary media during the polishing process of the surface and cross-sectional samples. After the sample preparation is completed, the microstructure and phase composition of the coating surface and cross-section are analyzed with a ZEISS Sigma500 field emission scanning electron microscope and its accompanying energy spectrometer; A comparative sample of FIB is prepared with a ZEISS AURIGA dual beam electron microscopy for the nanoscale ternary eutectic structure in the coating, and the differences in the ternary eutectic structure of the coating are analyzed in depth; The phase composition and solidification process of Zn-5Al-2Mg coating is calculated by thermodynamic calculation software.
When the local cooling rate on the surface of the coating is too high, a ternary eutectic transformation (abnormal eutectic) of L→Zn+Al+Mg2Zn11 occurs, while at positions with uniform cooling rate, a ternary eutectic transformation (normal eutectic) of L→Zn+Al+MgZn2 occurs. Studies show that this selective transformation depends on a higher critical cooling rate. When the cooling rate after plating is lower than the critical cooling rate, the transformation of the supercooled liquid phase tends to form a Zn/Al/MgZn2 ternary eutectic structure depending on the structural similarity with the MgZn2 phase; When the cooling rate after plating is higher than the critical cooling rate, the transformation of the supercooled liquid phase relies on extremely high driving forces and tends to form a Zn/Al/Mg2Zn11 ternary eutectic structure. Therefore, it can be concluded that when the Zn-5Al-2Mg composition coating undergoes a change from extremely low cooling rate to rapid cooling and then to extremely high cooling, the coating will undergo a process of transition from equilibrium eutectic to non-equilibrium eutectic containing metastable phase, and then to non-equilibrium eutectic containing steady-state phase. The white Zn rich phase and gray MgZn2 phase in the Zn/Al/MgZn2 ternary eutectic structure are distributed in a layered pattern; In the Zn/Al/Mg2Zn11 ternary eutectic structure, the white Zn rich phase becomes a discontinuous granular morphology with a significantly reduced proportion, while the gray Mg2Zn11 phase becomes a coarse discontinuous rod-shaped or blocky morphology with a significantly increased proportion. Due to the differences in phase composition and content in the abnormal ternary eutectic structure, it is manifested as black spot defects at the macroscopic level due to slow light emission.
In response to the black spot defects generated during the cooling process of Zn-5Al-2Mg coatings produced in industrial production, the mobile fan after plating can be upgraded and modified to allocate output power reasonably to avoid local uneven cooling rate during overload operation, thereby improving the uniformity of cooling rate of thick Zn-5Al-2Mg coating products in the mobile fan and suppressing the occurrence of such defects.
Key words
zinc aluminum magnesium coating /
black spot defect /
ternary eutectic structure /
cooling rate /
Mg2Zn11 phase /
MgZn2 phase
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