Effect of Pickling Treatment on the Pitting Corrosion of Hyper Duplex Stainless Steel in a Highly Concentrated Chloride Solution

Abstract

The effect of pickling treatment on the pitting corrosion of hyper duplex stainless steel was investigated in a highly concentrated chloride environment. Increasing the pickling time in 10 vol% HNO₃ + 5 vol% HF solutions at 328 K improved the resistance to pitting corrosion by increasing the pitting potential and passive region. There are two primary reasons for the considerable enhancement of the corrosion resistance of the hyper duplex stainless steel pickled for 20 h. First, the oxide scale and Cr-depleted areas which decrease the resistance to pitting corrosion were removed. Second, chromium oxide (Cr₂O₃), chromium trioxide (CrO₃), molybdenum dioxide (MoO₂), molybdenum trioxide (MoO₃), tungsten dioxide (WO₂) and tungsten trioxide (WO₃) in an oxide state, chromium hydroxide (Cr(OH)₃) and molybdenum oxy-hydroxide (MoO(OH)₂) in a hydro–oxide state, chromate (CrO₄²⁻) as corrosion inhibitor in an ion state, ammonium (NH₄⁺) elevating the pH in an ion state, and chromium nitride (CrN) acting as a precursor for the formation of Cr₂O₃ in the nitride state were increased and assisted in improving the corrosion resistance.