Storage and handling systems for concentrated (90-98%) sulphuric acid are traditionally designed from carbon steel. However there is a growing tendency for utilization of highly-alloyed stainless steels and nickel based alloys in order to overcome known limitations of carbon steel related to its susceptibility erosion corrosion a velocities above approximately 0.6 m/s (2 ft/s). The austenite stainless steels UNS S31600/03 (AISI 316/316L) and UNS N08020 (AISI Alloy 20) are usually used as the “first choice” materials due to their commonly accepted high corrosion resistance in concentrated H2SO4 environment within temperature range 20°-40°C (68-104°F at flow velocities varying from 0.6m/s to 1.5m/s (2 to 5 ft/s).High nickel-chrome-molybdenum alloys like UNS N10276 (Alloy C276) or UNS N10675 (Alloy B3) that posses superior corrosion resistance over wide range of H2SO4 concentrations and temperatures are used at flow velocities exceeding of 1.5 m/s (5 ft/s). It is commonly believed that the true scalable hydrodynamic parameter is the wall shear stress and has somewhat linear impact on corrosion rate and hence can be easily calculated and/or predicted. However experience has shown that Type 316L and Alloy 20 can occasionally exhibit unexpectedly severe corrosion due to instability of the passivation films responsible for their corrosion resistance. This phenomenon may have a serious implication on field operability of those materials due to sudden appearance of active corrosion.A very large systematic study of the corrosion behaviour of carbon steel Type 316L stainless steel Alloy 20 Alloy C-276 and Alloy B3 was completed by the authors (Sulphuric Acid Alkylation JIP) in sulphuric acid with concentrations ranging from 87% to 99.5% at temperatures between 5°C and 40°C (40° and 104°F) with wall shear stresses ranging from zero (stagnant) to 300 Pa. The following paper shows part of those results with focus on swinging active/passive behaviour of alloy 20 and stainless steel 316L versus acid concentration wall shear stress temperature and other process parameters.Key words: sulphuric acid flow regime prediction model austenitic stainless steels welding alloy 20