Effect of Copper on Active Dissolution and Pitting Corrosion of 25% Cr Duplex Stainless Steels

• Published in: Corrosion (Houston, Tex.) Vol. 54; no. 1; pp. 40 - 47
• Main Authors: Garfias-Mesias, L.F., Sykes, J.M.
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.01.1998; NACE
• Subjects: Alloys; Applied sciences; Copper; Corrosion; Corrosion effects; Corrosion environments; Current density; Dissolution; Dissolving; Duplex stainless steels; Electrochemistry; Exact sciences and technology; Ferric chloride; Hydrochloric acid; Hydrogen chloride; Iron chlorides; Metals. Metallurgy; Pitting (corrosion); Sodium; Sodium chloride; Stainless steel; Copper addition; Austenitic ferritic steel; Composition effect; Sodium chloride; Dissolution; Iron chloride; Hydrochloric acid; Corrosion resistance; Pitting corrosion; Stainless steel; Pitting potential; Critical temperature; Dual phase steel; Current density
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3284827

ABSTRACTElectrochemical studies were made of the influence of Cu on pitting behavior and active dissolution of 25% Cr duplex stainless steels (DSS) in chloride-containing neutral and acid solutions. Alloys with increased Cu content showed higher pitting potentials (Ep) in 1 M hydrochloric acid (HCl) and 3.5% sodium chloride (NaCl) solutions. In HCl, the current density in the active state also was lower for the Cu-containing alloys. However, the critical pitting temperature (CPT) in ferric chloride (FeCl3) was not improved significantly by addition of Cu. Pitting in all environments took place preferentially in the ferrite phase. INTRODUCTIONThe effect of different alloying elements on the susceptibility of steels to pitting has been studied widely.1-18 It is accepted commonly that those elements that shift the pitting potential (Ep) to more positive values (e.g., Cr, Mo, Ni, and N) are beneficial, while those such as S and P, which shift Ep values in the negative direction, are detrimental. For duplex stainless steels (DSS), excellent corrosion resistance arises mainly from the presence of three key alloying elements: Cr, Mo, and N.The effect of Mo on localized corrosion has been studied extensively.1-4 Sugimoto and Sawada showed that addition of Mo to SS decreases the current density in the active region in acidic solutions by 2 or 3 orders of magnitude, facilitating passivation.1 The benefits from Mo were explained by Clayton and Lu, who found that Mo additions provide resistance against localized corrosion (chloride attack) by formation of a stable, amorphous passive film.2 Newman proposed that Mo enriches at kink or step sites during dissolution of Fe-Cr-Mo alloys in the active state.3 The pitting process in those alloys was inhibited because Mo reduces anodic kinetics in the pit environment by 5 to 10 times.4 N has been used extensively in austenitic and DSS to stabilize the austenite phase and to impart higher corrosion resistance. However, it has been proven that N can form deleterious precipitates in the ferrite phase of DSS and, therefore, can reduce corrosion resistance of the alloy.5 There are different theories as to why N improves localized corrosion resistance. Osozawa and Okato found ammonium in the pitting solution and proposed that N buffers the local pH in the pit through the formation of ammonium ions.6 Newman proposed that, as a result of the slow reaction of N with protons during anodic dissolution, elemental nitrogen enriches the surface, which inhibits the anodic disso-