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Special steels resist 25 years of corrosion
Keywords Sandvik Steel, Stainless steel, Tubing, Corrosion prevention
Stainless steel tubes from Sandvik Steel, specially developed by Sandvik's R&D Department for use in critical process areas in the manufacture of urea, are reported to be still in operation after 25 years at the Indian Farmers Fertiliser Co-operative Limited (IFFCO) fertiliser manufacturing plant at Kalol in the state of Gujarat.
The tubes, manufactured from special steel grade Sandvik 2RE69, were installed in the stripper, which is the most aggressive part of the urea process, and typically therefore have a much shorter life.
In modern urea plants, urea is manufactured by a reaction of carbon dioxide (CO2) and ammonia (NH3) at high pressure and temperature. CO2 and NIL are mixed in a reactor at 160-220atm and 170°C-190°C, at which point ammonium carbamate is formed. The ammonium carbamate is converted to urea and water in an endothermic reaction. Although urea in itself is not a very corrosive material, the intermediate ammonium carbamate is extremely corrosive. For this reason, special stainless steels have to be used in high pressure parts of the plant. To maintain stainless steels in a passive state, it is necessary to add oxygen to the process - usually 0.4 - 0.6 volume percentage - in the CO2 feed.
Two main types of stainless steel have been developed by Sandvik for urea plants. The first, a standard austenitic grade of type 316L, modified with a high nickel content, is widely used in plate form for lining the reactor. Called Sandvik 3R60 Urea Grade, it is also used for high pressure piping to connect different stages of the production process such as the reactor, condenser and stripper.
The second urea grade is Sandvik 2RE69, a special high chromium grade with a very low or zero ferritic content which reportedly passivates easily and sustains passivity. This alloy is said to be more corrosion resistant than the modified 316L grade and, according to Sandvik, a general corrosion rate of 0.12mm per year maximum can be guaranteed in the Huey test - a test widely used to reject material with unsatisfactory corrosion resistance.
There are two key players in the design of urea plants - Stamicarbon in The Netherlands and Snamprogetti in Italy.
The IFFCO plant has a Stamicarbon designed urea stripper using Sandvik's 2RE69 for the tubes with the tube sheets overlay welded with the corresponding overlay weld metal of type 25/22/2. Under normal service conditions the stripper corrosion rate is claimed to be in the range 0.05mm-0.09mm per year at the upper part of the tubes which are exposed to the most severe conditions. A combination of high quality material and good process control by the operator are essential to achieve low corrosion rates and long service life. A 25-year life is therefore thought to be a remarkable performance. Subsequent installations in plants in India, China and Germany have reportedly enjoyed a comparably long life.
According to Sandvik, further development of its materials for use in the stripper have seen the use of bimetallic tubes, replacing titanium tubes, in a new and patented method introduced by Snamprogetti. The bimetallic tubes consist of an outer tube in 2RE69 with an inner lining of Zirconium 702. Bimetallic tubes are fabricated with a mechanical bond between the two components ensuring good heat.
Details available from: Sandvik Steel. Tel: +44 (0) 1922 728800; Fax: +44 (0) 1922 728827.