The Degree of Rust Remaining on the Surface of Ball-milled Steel Parts and Its Effect on the Leakage of Hot-dip Galvanizing - 腐蚀与防护实验室

Batch hot-dip galvanizing is a mature corrosion protection method. However, the waste acid and gas generated in the pretreatment seriously pollute the environment, which urgently requires the development of an environmentally friendly hot-dip galvanizing. Ball-milling is a promising surface treatment method. The use of ball-milling instead of pickling in the traditional hot-dip galvanizing can avoid the generation of waste acid and meet the sustainable development requirements of green environmental protection The iron ingot samples (20 mm x 20 mm x 3 mm) were used as the raw material Before hot-dip galvanizing, the samples were alkaline washed in 15wt.% NaOH solution at 70-90 ℃ for 3 min to remove surface grease. Then, at room temperature, the surface oxides were removed by pickling or ball-milling. Pickling is to soak the alkali-washed sample in a 10wt.%-15wt.% hydrochloric acid solution for 3 min. Ball-milling is to put the alkali-washed samples together with 304 stainless steel grinding balls into a ball mill with a diameter of 60 mm, and remove the oxides on the surface of the samples through the mechanical action of the grinding balls. Based on theoretical calculations, the ball-milling parameters designed: the particle size of the grinding ball d = 2 mm, 5 mm, 8 mm; the rotational speed of the drum r=120 r/min, 130 r/min, 140 r/min, 150 r/min; the filling factor of the grinding ball is 0.5. The rust-removed samples were rinsed in deionized water and then fluxed. The NH4Cl-ZnCl2 solution (1.2∶1) with a concentration of 200-300 g/L using as the fluxing agent, the temperature was (75±5) ℃, and the time was 5 min. Then the samples were immediately dipped in the zinc bath after being dried in a blast drying oven at 100 °C for 10 min. The composition of the zinc bath is 99.9wt.% pure zinc, the mass is 2 kg, the temperature is (450±3) ℃, and the time is 60 s. The hot-dipped workpiece is slowly lifted out of the zinc bath, and then immediately quenched in water Optical Microscope was used to observe the effect of ball-milling parameters on the residual rust degree of the steel surface. With the increase of the particle size of the grinding ball, the residual rust degree α on the steel surface reduces to 0. Scanning Electron Microscope and High-temperature Pendant Drop Contact Angle Measuring Instrument were used to analyze the effect of residual rust degree of steel surface on hot-dip galvanizing leakage. With the decrease of the residual rust degree, the contact angle between the zinc droplet and the matrix decreases, and the wettability improves. When the residual rust degree reaches 0, the contact angle θ=84°, and the zinc liquid spreads evenly on the substrate surface, which can avoid the leakage of the coating and achieve the same effect as pickling.