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Passivation of Elemental Metals and AlloysPassivation is the reduction in electrochemical or chemical activity of a metal or alloy as a result of a reactionbetween the metal (or alloy) surface and the environment. This usually involves the formation of an oxide and/ora hydrated oxide.
Passive oxides or hydroxides films can range in thickness from a few nanometers (stainless steel) to tensof microns in thickness (copper).
Importantly passive films can represent far from equilibrium structures. In fact passive films on stainless steelmay initially be amorphous and after some period of time, of order hours, they crystallize. Oxide or hydroxide formation at a certain pH and potential may not even appear on a conventional Pourbaix diagram!
Virtually all metals and alloys will form an oxide when exposed to air which can provide some often modest degree of corrosion protection. This air-formed oxide must be removed from the metal surface in order to study passivation processes in electrolytes. In acid, this can be accomplished by reducing the oxide electrochemically.
MnOm + 2mH+ + 3ne− ⇒ nM +mH2O This is accomplished at V < 0 VSHE
Passivation of Elemental Metals and Alloys
Cr potential-pH diagram showing noPassivation in acid (< ~ pH 3).
Cr passivation in acid
Passivation of Elemental Metals and Alloys
Passivation of Elemental Metals and Alloys
Nomenclature
Passivation of Elemental Metals and Alloys
pH effect on Fe passivation
Passivation of Elemental Metals and Alloys
18Cr-8Ni (Balance Fe) in 1M H2SO4 and 0.1M NaCl
Localized corrosion effects on passivation
Passivation of Elemental Metals and AlloysCharacterization techniques used to examine passive film composition and crystal structure
Passivation of Elemental Metals and AlloysCharacterization techniques used to examine passive film composition and crystal structure
Passivation of Binary AlloysHistorically Fe-Cr alloys have served as a model system for understanding alloy passivation
-0.5 0.0 0.5 1.0 1.5 2.0101
102
103
104
105
Cur
rent
Den
sity
(µA/
cm2 )
Voltage (VSHE)
5.3% 7.0% 9.4% 11.6% 14.7% 16.8% 17.4% 20.2% 22.2% 24.3%
Passivation of Fe-Cr alloys in 0.1M H2SO4.Nature Materials 2021.
Critical current density for passivation as a function of the Cr mole fraction.
-0.5 0.0 0.5 1.0 1.5 2.0101
102
103
104
105
Cur
rent
Den
sity
(µA/
cm2 )
Voltage (VSHE)
5.3% 7.0% 9.4% 11.6% 14.7% 16.8% 17.4% 20.2% 22.2% 24.3%
Passivation of Binary Alloys
Passivation of Binary Alloys
Theories of passivation in binary alloys: These fall in to two categories
• The structure and composition of the passive film• The structure and composition of the metal alloy
H. Uhlig’s electron configuration theory;
His idea was that a binary alloy would have an electron configuration as close as possible to that of an “inert noble metal gas atom”. Of course electron configurations are different in a condensed solid compared to that of an atom!
Passivation of Binary Alloys
The number of d vacancies for the metal surface atom is the number of vacancies in themetal plus 1. Fe then has 3.2 d-vacancies in the surface. So Fe has (5-3.2) 1.8 4s electrons It can donate to it’s d-band. 1 electron is donated to oxygen leaving it 0.8 electrons that it can donate to Cr. The Cr needs to complete it’s d shell and needs 4 electrons.
H. Uhlig’s electron configuration theory;
Passivation of Binary Alloys
H. Uhlig’s electron configuration theory;
“Critical” Cr mole fraction for goodpassive film behavior.
Passivation of Binary AlloysPercolation theory of passivity: https://doi.org/10.1038/s41563-021-00920-9
Cartoon illustrating undercutting of non-percolating Cr-O-Cr mer-units in binary Fe-Cr alloys. These mer-units or incipient Cr-rich oxide clusters (blue) which are less than 1 nm in size are embedded in the first metal monolayer. Selective Fe dissolution (brown arrows) results in the formation of “pits” several monolayers deep and the detachment of the mer-units from the surface.
Passivation of Binary Alloys
Percolation theory of passivity: https://doi.org/10.1038/s41563-021-00920-9
Passivation of Binary Alloys
Passivation on a 2D topological surface. a. Cartoon illustrating the 2D-3D percolation crossover that occurs on the topologically roughened surface. The top row is a plan view of the surface and the bottom row is an isometric view. The initial composition is 0.34 Cr (blue) and O.66 Fe (red). The 2D site percolation threshold for the square lattice is 0.59. The first column shows the sample intact prior to dissolving any components. The middle column shows the situation after Fe is dissolved from the top layer. The last column reveals the structure after Fe is dissolved from the top 3 layers. The planview shows percolation of Cr across the topologically roughened surface. b. Results of KMC simulations of passivation in a 17% Cr, Fe-Cr alloy in which primary passivation occurred following the dissolution of 5.4 atomic layers. This image has been topographically colored. Except for the Fe incorporated in to the primary passive film (shown as black atoms), richer shades correspond to atoms in top-most layers. Violet atoms correspond to Cr and red atoms, not part of the primary passive film, correspond to Fe.
ab
Passivation of Binary Alloys
Composite conductivity analogy and and alloy passivation. (A) Cross-section of a composite with electrically conductive elements on the surface (red), in the bulk (orange) and non-conductive elements (white). A two-point resistivity measurement placed on any of the red elements would show that they are electrically connected even though this is not apparent by examining the surface. (B) If the white elements are now considered to be Fe and the red/orange elements, Cr, selective dissolution of Fe results in a Cr structure that percolates across the surfaces. (C) Qualitative plot showing the theoretical behavior of the number of layers dissolved required for 2D percolation across the topological surface as a function of the Cr composition, p. The x-axis defines a series of h-dependent 2D percolation thresholds.
A B C
Passivation of Binary Alloysa b c
d e f
Linear sweep voltammetry and potential step integrated chronoamperometry results with numerical fits to the
theoretical equation, ! = # $# ! − $#&'()&' . a. KMC simulations of Fe-Cr passivation behavior. Sweep rate 1mV s-1 .b.
Experimental results for Fe-Cr. Sweep rate 5 mV s-1 . c. Experimental results for Ni-Cr. Sweep rate 5 mV s-1 . Integrated chronoamperometry from potential step experiments: d. KMC simulation result for Fe-Cr, potential step from 0.1 V to 0.7 V: ℎ = 1.10 ./ ℎ − 0.095 (2.343. Error bars correspond to the standard deviation in 100 realizations. e. Experimental results for Fe-Cr, potential step from -0.36 V to 0.04 V : ℎ = 1.07 ./ ℎ − 0.108 (2.343. f. Experimental results for Ni-Cr, potential step from -0.36 V to 0.02 V : ℎ = 2.29 ./ ℎ − 0.052 (2.343. Error bars in e and f correspond to the standard deviation in three data sets. The red curves in d, e and f are the fits to the theoretical equation. The dashed lines in a, b, and c indicate the location of the potential steps.
Passivation of Binary AlloysTwo length scales control primary passivation behavior of alloys. The x-axis defines the mole fraction, [M], of passivating component required to form M-O-M mer-units that percolate in 3D. The y-axis defines the [M] required for passivation set by the number of selectively dissolve layers, ℎ, necessary for M-O-M mer-unit percolation across the 2D topological surface. Both length scales are affected by any SRO that may be present in the alloy, which creates a spectrum of 3D percolation thresholds. FCC (BCC) alloys are indicated in black (blue) points.
