Abstract

Storage and handling systems for concentrated (90-98%) sulfuric acid are traditionally designed from carbon steel. However, recent industry practices show an increased propensity for utilization of highly-alloyed stainless steels and nickel based alloys, primarily stemming from the need to overcome known limitations of carbon steel related to its susceptibility to erosion-corrosion at velocities above approximately 0.9 m/s. The austenitic stainless steels UNS(1) S31600/03 and nickel alloy UNS N08020 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 at flow velocities varying from 0.9 m/s to 1.5 m/s. It is commonly believed that the true scalable hydrodynamic parameter in sulfuric acid alkylation is the wall shear stress which has a somewhat linear relationship to corrosion rate and can be easily calculated and/or predicted. However, experience has shown that UNSS31600/03 and UNS N08020 can occasionally exhibit unexpectedly severe corrosion due to instability of the passivation films responsible for their corrosion resistance. This phenomenon may have serious implications for field operability of those materials due to sudden appearance of active corrosion. A systematic study on the corrosion behavior of different materials varying from carbon steel to nickel alloys UNS N10276 and UNS N10675 was completed by the authors (during research program “Sulfuric Acid Alkylation JIP”†) in sulfuric acid with concentrations ranging from 87% to 99.5% at temperatures between 5°C and 40°C, with wall shear stresses ranging from zero (stagnant) to 300 Pa. The results were incorporated into a predictive model for sulfuric acid corrosion. A case study of actual failure of pipeline made of UNS N08020 in concentrated sulfuric acid service is presented and compared with the sulfuric acid corrosion model predictions.

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