ALLOY 700-SI, A NEW CORROSION-RESISTANT MATERIAL FOR HANDLING OF HOT,HIGHLY CONCENTRATED MINERAL ACIDS

By means of a suitable adjustment of chromium and nickel it has been a chieved to create a malleable austenitic nickel-chromium steel with ab out 7 wt.% silicon. This new steel si corrosion resistant in highly co ncentrated sulfuric acid up to 150 degrees C (96% H2SO4) or up to abou t 200 degrees C (98.5% H2SO4). In contrast to other steels being used for handling of hot highly concentrated sulfuric acid (e.g. AISI 310 L ), the new steel is able to withstand short-time excursions as to high er temperatures as to lower acid concentrations without the consequenc e of catastrophic corrosion. The new steel suffers nearly no attack at all in boiling 100% HNO3. Due to its excellent ductility in the solut ion annealed condition the new steel lends itself to the manufacture o f plate-type heat exchangers. According to experience gained hitherto the new 7% Si steel is weldable without degradation of its corrosion r esistance. However, in order to obtain a sufficient ductility of the w eldments special measures might be required as e.g. welding with very low heat input, solution annealing of the welded components, or use of roll-clad material where the new 7% silicon steel is present as a cor rosion resistant cladding on a load bearing structure of carbon steel, the latter with the advantage of substantial cost savings. Welding wi th very low heat input (e.g. laser beam welding) is most effectively m ade on thin sections, e.g. when manufacturing welded tubes with a wall thickness of some mm whereas the use of roll-clad material has to be considered for larger installations. The new steel is also available a s cast material.