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PICKLING STAINLESS STEEL IN REDUCING SALT BATH
The finishing activities during the production of the alloyed and stainless steel like annealing or rolling are creating the layer of the metal oxides on the surface of the steels. This layer of the oxides called scale must be removed before the further operation. The chemical way of the removal of the scale by the dissolving in concentrated acids is called pickling. The problem is that a lot of metal oxides in the scale , which are result of the heat treatment of alloyed, special and stainless steel, are very slowly or even unsoluble in acids. Therefore the scale must be destroyed or changed before acid pickling. The scale can be changed in molten sodium hydroxide (salt bath).
Article chapters
- The technology
- References and history
There are two technologies of the salt bath:
- oxidizing
- reducing
Our unique technology is reducing salt bath technology. The salt bath contains sodium hydride (NaH) and sodium hydroxide (NaOH) at the temperature of 400°C. Sodium hydride( dosed in FEROPUR ) can make the reduction of the metal oxides by the removal of molecules of oxygen from the scale. This reduction of the scale prepares the scale for simple, fast and easy acid pickling. This tretreatment gives, as the universal technology, the guarantee of the excellent pickling of all steel grades, alloys or special metals. The techology is also very friendly to the environnment and it can cut the total costs of the operation.
The pickling in reducing salt bath has got many advantages in comparison with oxidizing salt bath:
- The usage of reducing salt bath can easily pickle high Cr alloys and steels, like ferritic and martensitic steels grades.
- The acid pickling period after reducing salt bath is substantially shorter, because metals in reduced scale are dissolving very quickly.
- There is no danger of overpickling of the steel or higher weight losses of basic steel after the pickling.
- There is low production of harmfull NOx gases during the pickling in HNO3+HF acid.
- The consumption of pickling acids is substantially lower.
- Reducing salt bath is not a source of dangerous and cancerogenous hexavalent Cr, which is the product of oxidizing salt bath.
The whole technology of reducing salt bath deliveres Bochemie together with daughter´s company EKOMOR s.r.o., Frýdek Místek. We are the only company in the world , which is offering this special technology for the stainless steel producers as a whole. This technology is the fastest spreading metal surface treatment technology in the world. In 1996 , there was market share of this technology 6%, in 2009 it is 85%.
This technology is used by the biggest stainless steel producers in the world (Outokumpu, POSCO, Baosteel, Dongbei Dalian, Acerinox, Viraj Profiles etc.). The instalation in other big companies like Ugitech, Allvac ATI, TISCO, Nippon Steel or Dido Steel are in the process of negotiation. This technology is used in the Czech Republic, Germany, the UK, Sweden, Spain, Italy, Russia, the USA, South Korea, India and China.
