304 stainless steel pipe and fittings has a more complex composition of the oxide scale inner layer, with increased chromium content and nickel oxide, but the composition of the oxide scale inner layer of S31803 stainless steel pipe and fittings is more complicated.
The most obvious difference is the appearance of molybdenum oxide in S31803. Mo is an important element that significantly improves the corrosion resistance of stainless steel. Mo is enriched in the oxide film close to the substrate, which improves the stability of the oxide film.
When the nitrogen content is increased to a certain amount, the enrichment of nitrogen is very obvious, and the maximum enrichment amount can reach 14 times the nitrogen content of the matrix. The enrichment of nitrogen can make the oxide film difficult to damage.
The combination of molybdenum and nitrogen can significantly improve the stability of the oxide film. In the oxide layer, the more alloying elements it contains, the more complex the structure of the oxide. This difference in oxide scale is the root cause of the difficulty of pickling. The effect of hot working process and slab surface finish on the quality of pickling. S31803 stainless steel pipe is heated at different temperatures for the same holding time, and the resulting surface oxide thickness is different. The general trend is that with the increase of temperature, the oxide scale becomes thicker. The pickling increases the difficulty.
After measurement, the surface roughness of the slab after grinding with 16# grinding wheel is Ra=160μm, and the surface roughness of slab after grinding with 20# grinding wheel is Ra=60μm. For the same pickling process, the latter corresponds to The pickling quality of the board surface is significantly better than the former.
Adding Cl- to the acid solution improves the corrosion ability of the acid solution, and introduces trace oxygen to increase the corrosion rate of the acid solution system to duplex stainless steel. Orthogonal experiment was used to optimize the design of sulfuric acid, sodium chloride, pickling temperature and additives, and mixed acid (nitric acid + hydrofluoric acid) was used for passivation. The specific test conditions and results are shown in the table. The pickling process is determined by the pickling effect as follows: sulfuric acid 220g/L, sodium oxide 30g/L, temperature 60℃, additive 10g/L, pickling time 50 minutes, passivation time 4 minutes.
Organize mass production according to laboratory results, and configure pickling tanks for S31803 stainless steel pipe production according to laboratory ingredients. Acid composition: 220g/L sulfuric acid, about 30g/L sodium oxide, and about 10 g/L additives. Soak in the bleaching tank for 5 minutes. Adopting a new thermal processing technology, controlling the surface finish of the cast slab after grinding and a new acid solution ratio, a total of 34 batches of tests were organized, the average pickling time was 57.2 min, the surface color was uniform, and the expected goal was achieved.
Through the specification of the pickling quality-related processes, the goal of a 100% pass rate of pickling has been achieved. At the same time, it has passed the certification of S31803 stainless steel pipes by China Classification Society, Delaunay, Inlaw, France and other classification societies. Through the test of the pickling process of S31803 stainless steel pipe and fittings, the acid solution composition ratio, specific acid temperature and effective additives suitable for high Cr, high Mo, and high N duplex stainless steel are proposed. In the test results, the board surface has a uniform color, which reduces acid consumption and improves production efficiency.