Q420C steel plate pickling process

The metallurgical function of the Q420C steel plate pickling process is to effectively remove the oxide layer and the chromium-depleted layer, achieve surface passivation, and improve the surface quality at the same time. After the pre-pickling, there are still silicon-rich oxide layers and chromium-poor layers on the surface of the steel plate, which need to be removed by a final pickling process. In order to improve the pickling efficiency, the final pickling system consists of at least one acid and one oxidant.
 
1. Nitric acid-hydrofluoric acid pickling system design
Nitric acid-hydrofluoric acid is a widely used steel plate pickling system at present, and it has a good pickling effect on austenitic stainless steel, ferritic stainless steel and low alloy steel. Among them, nitric acid can not only provide H+, dissolve the oxide layer, but also act as an oxidizer to increase the oxidation-reduction potential of the pickling solution to improve pickling efficiency. In addition to providing a small amount of H+ by HF, ionized F- can form stable complexes with Cr3+ and Fe3+, which can depassivate and increase the dissolution rate of the chromium-depleted layer.
 
According to the research, during the mixed acid pickling process, 85% of the acid consumption is used for the dissolution of metals, and only 15% of the acid consumption is used for the direct dissolution of the oxide layer. During the pickling process, the removal of the oxide layer mainly relies on the dissolution of the chromium-poor layer and the base metal, so that the oxide layer attached to the surface peels off.
 
2. Improvement of nitric acid-hydrofluoric acid pickling system
Under the synergistic effect of HNO3, HF and metal ions, the nitric acid-hydrofluoric acid pickling system can basically completely remove the residual oxide layer and improve the surface quality of stainless steel, but the pickling process will cause more serious NOx and nitrite pollution. There are two ways to solve the environmental pollution problem caused by nitric acid: one is to add an oxidant to oxidize NO and NO2 into nitric acid again, which not only reduces pollution, but also realizes the recycling of nitric acid; one is to use other acids and oxidants Replace nitric acid.

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