Studying the onset of galvanic steel corrosion in situ using thin films: film preparation, characterization and application to pitting

Abstract

This work reports about a novel approach for investigating surface processes during the early stages of galvanic corrosion of stainless steel in situ by employing ultra-thin films and synchrotron x-radiation. Characterized by x-ray techniques and voltammetry, such films, sputter deposited from austenitic steel, were found representing useful replicas of the target material. Typical for stainless steel, the surface consists of a passivation layer of Fe- and Cr-oxides, a couple of nm thick, that is depleted of Ni. Films of ≈4 nm thickness were studied in situ in an electrochemical cell under potential control (−0.6 to +0.8 V vs Ag/AgCl) during exposure to 0.1 M KCl. Material transport was recorded with better than 1/10 monolayer sensitivity by x-ray spectroscopy. Leaching of Fe was observed in the cathodic range and the therefor necessary reduction of Fe-oxide appears to be accelerated by atomic hydrogen. Except for minor leaching, reduction of Ni, while expected from Pourbaix diagram, was not observed until at a potential of about +0.8 V Cr-oxide was removed from the steel film. After couple of minutes exposure at +0.8 V, the current in the electrochemical cell revealed a rapid pitting event that was simultaneously monitored by x-ray spectroscopy. Continuous loss of Cr and Ni was observed during the induction time leading to the pitting, suggesting a causal connection with the event. Finally, a spectroscopic image of a pit was recorded ex situ with 50 nm lateral and 1 nm depth resolution by soft x-ray scanning absorption microscopy at the Fe L2,3-edges by using a 80 nm film on a SiN membrane, which is further demonstrating the usefulness of thin films for corrosion studies.

Swelling of Steel Film by Hydrogen Absorption at Cathodic Potential in Electrolyte

“…X-ray fluorescence analysis of the steel film revealed the metal concentration ratios of Fe/Cr ¼ 4.3 and Fe/Ni ¼ 9.0. [34] This corresponded to Cr and Ni concentrations of 17% and 8%, respectively. Compared with the ranges of 18-20% and 8-12% for Cr and Ni, respectively; for SS304, [53] the film was found slightly low in nickel and chromium content.…”

“…Overall, the deduced composition of the film is comparable with the surface composition of bulk SS304. [8,12,23,33] XANES measurements, to be published in a forthcoming report, [34] revealed that about 80%, 70%, and 20% of Cr, Fe, and Ni, respectively, in the film were oxidized. To investigate the oxidation status of the metals depth-selectively, XANES measurements were carried out under XSW excitation, enhancing the X-ray intensity in three distinct regions in the steel film [35] (Figure 1a).…”

“…Details on the growth and characterization using X-ray as well as electrochemical techniques are reported in a forthcoming communication article. [34] Emission-mode XANES and XSW measurements were carried out on the P64 undulator beamline at PETRA-III, DESY, Hamburg, acquiring X-ray fluorescence with the help of an energy-dispersive solid-state detector. [54] XSW measurements were carried out above the Ni K-edge (8333 eV), scanning the energy E γ across the first-order Bragg peak of the ML.…”

“…Under control of a potentiostat, the film was exposed for 30, 120, and finally 480 s to 0.1 M KCl electrolyte at À0.6 V in a standard three-electrode electrochemical cell with a silver/silver-chloride (Ag/AgCl) reference and a Pt counter electrode. [34] The current recorded during the final 480 s exposure, resulting in a charge transfer of 0.030 A s cm À2 , is shown in Figure 3a. The total transferred cathodic charge of all three exposures was 0.038 A s cm À2 , corresponding to a total electron transfer of 2.4 Â 10 17 e 0 cm À2 .…”

Structure of the Surface Region of Stainless Steel: Bulk and Thin Films

“…[29][30][31][32] The results revealed that the metal oxidation is limited to the outer 2-3 nm surface region of the SS 304 film. [23,26] The depth distribution of Fe, Cr, and Ni, the three main elements of a 4 nm SS 304 steel in the film, was also analyzed with the help of X-ray standing wave (XSW) measurements. The XSW analysis of the 4 nm SS 304 film on a Ru/B 4 C multilayer has been published recently.…”