Pickling Technics Of Ultra-pure Ferrite Stainless Steel

Stainless steel has being been widely used in all aspects of our life today. Especially, austenitic stainless steel with its excellent balanced performance is the most popular one. Nickel is one of the important elements which has direct impact on the performance of austenitic stainless steel. However, nickel is scarce and expensive. Particularly in recent years, due to the increasing demand of stainless steel, the price of nickel is growing higher and higher. The variability of nickel price lead to a rise in the production costs of austenitic stainless steel, which directly affect austenitic stainless steel’s production and marketing. Therefore, a new type of stainless steel containing none or a small amount of nickel and can be an surrenal to austenitic stainless steel is of vital essential.Compared with austenitic stainless steel, ferritic stainless steel performs a high strength, contains no nickel or only a small amount of nickel. The production costs of ferritic stainless is lower and the cold hardening tendency is small. what’s more, it’s thermal conductivity is about1.3-1.5times of austenitic stainless steel, but linear expansion coefficient is only about60%-70%of austenitic stainless steel. Otherwise, it has excellent characteristic to withstand chlorine monoxide、pitting and gap. However, austenitic stainless steel is more widely used for the impurity elements included in ferritic stainless steel reduced it’s impact strength. Ultra-pure ferrite stainless steel which developed in recent years, has high toughness. The reason for that is it contains a much smaller amount of impurity elements such as carbon、nitrogen and sulphur. Even more, the content of C+N is less than0.05%. Moreover, the acid-washing technology for ferritic stainless steel is not qualified but is quite mature for austenitic stainless steel. While, for ultra-pure ferrite stainless steel, the acid-washing technology is placed in a bottleneck stage. In practice, general environment-friendly acid liquid can not react with44660ultra-pure ferritic stainless steel. According to practical experience, high concentration HF equipped with added liquid cycle system had been chosen. But even so, due to the underpickling and overpickling happened during pickling process, the quality of ultra-pure ferritic stainless steel is still unstable. When underpickling, the surface is only partially react with acid lotion which result in a discontinous passivation film.overpickling, pickling solution starts to corrode its substrate after finishing the corrosion of its surface which can reduce the corrosion resistance of ultra-pure ferritic stainless steel obviously.In this paper, imidazoline and methenamine are selected as pickling corrosion inhibitor. The acid wash formula is improved,and the best pickling process parameters is optimized by orthogonal test. Results show that:1. When select methenamine as corrosion inhibitor, the corresponding optimum process1is that the ratio of HF is40%, methenamine is1%o and pickling for5min.2. When select imidazoline as corrosion inhibitor, the corresponding optimum process2is that the ratio of HF is40%, methenamine is5%o, and pickling for5min.3. After a further verification of comparison, the best pickling process for44660ultra-pure ferritic stainless steel is choosing imidazoline as corrosion inhibitor, the ratio of HF is40%, methenamine is5‰, and pickling for5min.4. After pickling with the best acid-bath process, the surface of44660ultra-pure ferritic stainless steel has formed a layer of passive film, which is intact and compact. The appearance of the film is qualified, and ultra-pure ferritic stainless steel’s corrosion resistance is largely improved. The technology successfully resolved the problem of underpickling and overpickling during pickling process for ultra-pure ferritic stainless steel in practice and is meaningful to working out pickling process of all kinds of ultra-pure ferritic stainless steel.