Pekka Pohjanne

The transpassive dissolution mechanism of highly alloyed stainless steels. I. Experimental results and modelling procedure

The transpassive dissolution of austenitic stainless steels (AISI 316L, AISI 904L, 254SMO and 654SMO) in a 0.5 M sulphate solution with pH 2 was studied by conventional and rotating ring–disc voltammetry, as well as electrochemical impedance spectroscopy. The main process in the transpassive potential region was found to be the release of soluble Cr(VI), while small amounts of lower-valency Cr or Mo species are released as well. Secondary passivation readily occurs for AISI 316L, whereas the remaining highly alloyed steels dissolve at high current densities in the whole potential range studied. The dissolution rate was found to increase in the order AISI 904L < 254SMO < 654SMO. Thus it can be correlated to the increase in the Cr and especially Mo content of the steel substrate. The impedance spectra contain contributions from the transpassive dissolution of Cr and secondary passivation, probably due to enrichment of Fe in the outermost layer of the surface film. A kinetic model of the process is proposed, including a two-step transpassive dissolution of Cr via a Cr(VI) intermediate and the dissolution of Fe(III) through the anodic film. The model was found to be in quantitative agreement with steady state current vs. potential curves and electrochemical impedance spectra. The kinetic parameters of transpassive dissolution were determined and the relevance of their values is discussed. � 2002 Elsevier Science Ltd. All rights reserved.

The transpassive dissolution mechanism of highly alloyed stainless steels: II. Effect of pH and solution anion on the kinetics

The effect of pH and solution anion on the kinetics of transpassive dissolution of highly alloyed austenitic stainless steels (AISI 904L, 254SMO and 654SMO) was studied by a combination of electrochemical techniques. The experiments were performed in 0.5 M sulphate and 0.5 M chloride solutions, and in an equimolar mixture of the two. The transpassive dissolution was found to start at higher potentials in solutions with higher pH. The rate of transpassive dissolution was shown to decrease with increasing pH and to be the lowest in chloride solutions and the highest in sulphate electrolytes. The steady-state current vs. potential curves and the impedance spectra of the studied materials in the transpassive potential region were found to be consistent with a proposed kinetic model. The model describes the process as dissolution of Cr as Cr(VI) and Fe as Fe(III) through the anodic oxide film via parallel reaction paths. The kinetic parameters of the model in solutions with different pH values and different anions were determined. The role of pH and solution anion in the transpassive dissolution process is discussed in relation to changes induced by these parameters in the composition of the anodic passive film. The factors determining the efficiency of Fe as a secondary passivating agent are also considered.

Mechanism of transpassive dissolution of nickel-based alloys studied by impedance spectroscopy and rotating ring-disc voltammetry

A generalised kinetic model of transpassive dissolution of industrial nickel-based alloys is presented in this paper. It is based on electrochemical impedance spectroscopic (EIS) and rotating ring-disk (RRDE) measurements on several nickel-based alloys in 0.5 M sulphate and chloride solutions with pH ranging from 2 to 7. The model assumes that the dissolution of Cr from the transpassive film occurs via a Cr(VI) surface intermediate species. The dissolution of Ni as Ni(II) is proposed to occur via two parallel paths featuring two reaction intermediates. The role of Fe as a secondary passivating agent and Mo as an accelerator of transpassive dissolution is also taken into account. The kinetic model has been found to reproduce quantitatively the current vs. potential curves and the impedance spectra for the studied nickel-based alloys in the transpassive region. The proposed mechanism is sensitive to the impact of alloy and electrolyte composition on the dissolution process. Thus it can serve as a starting point to describe the transpassive behaviour of engineering alloys.

Transpassive Dissolution Mechanism of Ni-Based Alloys in a Simulated Bleaching Solution Effect of Alloying Elements

The transpassive dissolution of several nickel-based alloys has been studied in a simulated bleaching solution (0.014 M SO 2- 4 , 0.017 M Cl - , and 0.0025 M diethylenetriaminopentaacetic acid, DTPA, pH 2.8) by electrochemical techniques. The alloys have been prepared by welding superaustenitic stainless steels (UNS S31254 and S32654) with nickel-based filler metals (UNS N06625 and N06059). According to the rotating ring-disk voltammetric studies, the release of soluble high-valency products from the alloys plays a significant role in the transpassive potential region. Electrochemical impedance spectroscopic data point to a competition between transpassive dissolution and secondary passivation reactions. The experimental data have been compared to a previously proposed model for the transpassive dissolution of nickel-based alloys. The model treats the transpassive dissolution of Cr as a two-step reaction featuring a Cr(VI) intermediate. It assumes that the dissolution of Ni follows two parallel reaction paths, one of which involves a self-catalytic step. The kinetic parameters of the process for the studied alloys have been determined, and a discussion of the role of alloying elements such as Fe and Mo on the transpassive dissolution mechanism is given.

Effect of Electrochemical Potential on Stress Corrosion Cracking Susceptibility of EN 1.4301 (AISI 304) Austenitic Stainless Steels in Simulated Hot Black Liquor

Stress corrosion cracking (SCC) susceptibility of austenitic EN 1.4301 (AISI 304, UNS S30400) stainless steel was studied as a function of electrode potential at T = 190°C in 15 g/kg NaOH + 150 g/kg Na2S containing caustic environment simulating heavy black liquors (HBL) of the pulp industry. Severe cracking was detected at the corrosion potential and at the cathodic potential of −0.11 VMo/MoS2 reference electrode. On the other hand, at anodic potentials of 0.03 VMo/MoS2 to 0.3 V Mo/MoS2 no cracking was observed. Thus, SCC of EN 1.4301 steel can potentially be mitigated in HBL environment by applying anodic protection. At the corrosion potential, selective dissolution of Fe and slight localized enrichment of Ni and Cr, as well as Na, S, and O, was observed. At anodic potentials, Fe was selectively dissolved and marked enrichment of both Ni and Cr, as well as Na, S, and O, took place in the corrosion product. The simultaneous enrichment of Ni and Cr in the corrosion product film was concluded to be the pre...

Interplay Between Surface Characteristics, Initial Oxidation Behavior, and Corrosion Performance of Ferritic Stainless Steels Under Simulated Automotive Exhaust Gas Condensate Conditions

This paper reports on the interplay between surface characteristics, initial oxidation behavior, and corrosion performance of three ferritic stainless steels, grades EN 1.4512, EN 1.4510, and EN 1....

Mechanical and corrosion properties of welded joints in new generation ferritic and duplex stainless steels

This study covers the very preliminary results of welding and characterisation of one novel duplex and two ferritic stainless steel grades. The so-called ”lean duplex” grade EN 1.4162 (UNS S32101) has 21.5% Cr-5% Mn-1.5% Ni-N, the ferritic EN 1.4509 (UNS S43940, AISI 441) 17% Cr, and the other, improved 21% Cr ferritic grade that fulfills also UNS S44330 standard requirements, but has no standard EN designation yet. Both ferritic stainless steels have low (< 0.02%) carbon content and double (Nb+Ti) stabilisation. The materials were used as 1.5 and 2 mm sheets, hence laser and resistance spot welding were selected for welding experiments. The joints were subjected to mechanical testing and critical pitting temperature (CPT) corrosion tests, which were performed on both the base materials and welds. The mechanical tests of the welds did not reveal any significant softening effect due to welding operations. Comprehensive CPT data were achieved for base materials and their welds using two different polarisation potentials, and the new 21% Cr ferritic grade shows a great promise in both as-received and welded conditions. This paper was written as part of the Finnish Metals and Engineering Competence Cluster (FIMECC)’s Demanding Applications (DEMAPP) program.