Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid

Abstract

The effect of copper (Cu) addition on the active corrosion behavior of hyper duplex stainless steels in sulfuric acid was investigated. The addition of Cu in the base alloy enhanced the resistance to general corrosion by decreasing the critical and corrosion current densities, and increasing the polarization resistance. There are two primary reasons for the considerable enhancement of the corrosion resistance of the experimental alloys containing Cu. First, the protective surface film was enriched with the noble metallic copper (Cu) due to the selective dissolution of the active metallic Cr, Fe, and Ni, and the electrochemical dissolution of the corrosion products such as iron-sulfide (FeS 2), iron sulfate (FeSO 4), ferrous oxide (FeO) and hydrous iron sulfate (FeSO 4 •7H 2 O). Second, chromium oxide (Cr 2 O 3), chromium trioxide (CrO 3), nickel oxide (NiO), molybdenum dioxide (MoO 2), molybdenum trioxide (MoO 3), and tungsten trioxide (WO 3) in an oxide state, molybdenum oxy-hydroxide (MoO[OH] 2) and chromium hydroxide (Cr[OH] 3) in a hydro-oxide state, molybdate (MoO 4 2¹) and tungstate (WO 4 2¹) as corrosion inhibitors in an ion state, and ammonium (NH 4 +) elevating the pH in an ion state were increased and assisted in improving the corrosion resistance.

Decrease in Pitting Corrosion Resistance of Extra-High-Purity Type 316 Stainless-Steel by Cu2+ in NaCl

“…Pitting was not caused by the increase in amount on deposited Cu or Cu compound on the surface. Chemical states of Cu on the surface were also important for the corrosion resistance for stainless-steels [14][15][16][17][18]. The chemical state of Cu was analyzed in detail by comparing the Cu 2p 3/2 XPS spectrum of the specimen polarized from 0.20 to 0.40 V with that of the specimen polarized from 0.25 to 0.86 V. Figure 7 shows the Cu 2p 3/2 XPS spectrum corresponding to the outermost surface of the specimen polarized in 0.1 M NaCl-1 mM CuCl 2 under the two aforementioned polarization conditions.…”

“…Sourisseau et al reported that the active dissolution rate inside the pits was suppressed by dissolved Cu 2+ , and that Cu-deposited insoluble copper sulfides were formed with dissolved S species [13]. The active dissolution of Cu-containing stainless-steels is known to be suppressed by Cu enrichment at the surface [12,14,15] or by the formation of a deposited Cu-containing layer [16][17][18] in acidic environments.…”

Review—Passive Film Properties and Electrochemical Response of Different Phases in a Cu-Alloyed Stainless Steel after Long Term Heat Treatment

“…-The effects of copper-alloying in corrosion resistance of stainless steels have been presented in numerous publications, showing positive and negative effects depending on the alloy, test environment and alloying level of copper [1][2][3][4][5]12,14 In reducing acids, such as sulfuric or hydrofluoric acids, copper improves corrosion resistance of stainless steels. [1][2][3][4][5] The positive effect is attributed to deposition and enrichment in metallic copper on the alloy surface as a result of dissolution of Fe, Ni and Cr. 3,5 In such environments, corrosion is reported to initiate in the ferritic phase and propagate to include the austenitic phase, 3 although the austenite may also remain intact.…”

“…[1][2][3][4][5] The positive effect is attributed to deposition and enrichment in metallic copper on the alloy surface as a result of dissolution of Fe, Ni and Cr. 3,5 In such environments, corrosion is reported to initiate in the ferritic phase and propagate to include the austenitic phase, 3 although the austenite may also remain intact.…”

“…Stainless steels alloys with additions of copper represent a small fraction of the total stainless steel production. Additions of copper can have a significant beneficial effect on the corrosion resistance of stainless steels in reducing acids; [1][2][3][4][5] but the use of copper in stainless steel is limited since it can cause hot cracking during rolling. 1,6 Long term heat-treatment-In this work a long heat-treatment at 800 • C for 6 months was performed to promote the formation of different phases in a Cu alloyed stainless steel so that these could be investigated in terms of the electrochemical response and the passive film properties.…”