Fabrication And Research On Materials With Corrosion Resistance To Molten Zinc

The hardwares?eg., sink roll, stablizing roll, bushes and sleeves? have to experience severe environment condition in the zinc pot of the continuous galvanizing line. They would be corroded with the molten zinc through complicated interactions, like diffusion, dissolution and reaction. The result is that it would lead to a great dimensional change. Meanwhile, wear does also exist in this situation between the hardwares as well as the strip steel. It would further accelerate the damage and failure of the hardwares. Therefore, it is essential to develop new materials with superior corrosion resistance in liquid zinc to solve the above mentioned issue. It will definitely enhance their service life and reduce the production cost.In our study, we provide two different methods to improve the corrosion resistance in liquid zinc, including developing an oxide layer by heat treatment and preparing TiC composite coatings by air plasma spraying. The corrosion behavior and mechanism are studied emphatically.The oxidizing treatment?OT? and nitriding-oxidizing treatment?NOT? both can hinder the attack of molten zinc and improve the corrosion resistance of the Fe-20Cr-10 Mn alloy effectively. Unlike the non-treated alloy showing a uniform corrosion, the OT alloy and NOT alloy both show an incubation time in liquid zinc owing to the existence of an unwettable oxide layer. As the test continues, the corrosion rate would increase dramatically once the corrosion time exceeds the incubation time. By comparison, NOT alloy shows a longer incubation time than OT alloy due to a more compact and homogeneous oxide. Moreover, a better combination of the NOT oxide would contribute to improving the adhesion strength and reducing the probability of peeling off. It is also very helpful to protect the alloy and to improve the corrosion resistance. Overall, NOT alloy shows the best corrosion resistance in liquid zinc. As corrosion continues, the liquid zinc would diffuse into the oxide and the chromium would dissolve into the zinc along the reversed path. The defects in the oxide will become diffusion channels to provide lots of paths for the permeation of liquid zinc. The diffusion-substitution stress, thermal stress and the seepage pressure from the liquid zinc will cause the crack initiation and propagation, and finally lead to the damage of the protective oxide.Two Fe-20Cr-10Mn/TiC composite coatings?FTB30, FTB50? with varied TiC content?x=30%, 50%? are prepared via air plasma spraying. An identical heat treatment is carried out for these composite coatings?FTA30, FTA50?. The porosity, phase constituent, adhesion strength, micro-hardness, thermal shock resistance and dry sliding wear resistance of the coatings before and after heat treatment are investigated, respectively. The results reveal that after heat treatment, the porosity of FTB30 and FTB50 falls by 39.64% and 50.65%, the adhesion strength falls by 19.82% and 44.12%, the thermal shock cycles fall by 73 and 192 times and the micro-hardness falls slightly. The TiC phase in the composite coatings can be oxided to be rutile-TiO₂ during heat treatment accompanied with a volume expansion?53%?. The pores and cracks can be filled and sealed by such an oxidation product?rutile-TiO₂?. The TiC phase in the composite coatings is regarded to be a self-healing agent to decrease porosity and to compact the structure concurrently. The wear resistance of the coatings is also improved after heat treatment. Considering the decreased TiC relative content?TiCR%?, the enhancement of wear resistance are ascribed to the more compact morphology after heat treatment.The non-treated TiC composite coatings do also exhibit an incubation period in molten zinc. The incubation time is very short?10 days? for FTB50 because of its high porosity and loose structure. After this incubation period, the samples will suffer a localized corrosion accompanied with an obviously increased corrosion rate.The corrosion of the non-treated coatings is controlled by the internal defects?eg., pores, cracks? and the coatings will be stripped under the permeation of the liquid zinc. However, TiC composite coatings have been proved to be more compact and uniform after heat treatment. Based on this, localized corrosion is very unlikely to happen in this situation and the coatings show a better corrosion resistance by contrast. All in all, FTA50 reveals the best corrosion resistance due to the larger amount of TiC/TiO₂ layered structure, which is extremely inert in molten zinc.The corrosion process of the heated TiC composite coatings contains the preferential corrosion of the binder phase, the formation of micro-cracks and the peeling-off of brittle TiC/TiO₂ phase under the phase transformation stress and seepage pressure. In summary, the heated TiC composite coating?FTA50? manufactured by plasma spraying is much thicker and shows better corrosion resistance in liquid zinc than the oxide layer prepared by heat treatment. It will be more difficult for them to be utterly destroyed under coexist condition of both corrosion and wear. The heated TiC composite coatings are more likely to be used to protect the hardwares from corrosion wear in liquid zinc.