DOI: 10.17033/data.00000300
title: SOCORRO:Seeking out corrosion

description: This project aimed to establish environmental parameters that could be used to monitor for corrosion risk in a range of environments, including marine and intertidal structures. As part of the project X-ray photoelectron spectroscopy (XPS) was used to determine the mineralogy and chemistry of corrosion products, and then to infer the corrosion mechanism. This is important as steel in the intertidal and marine zone is subject to both abiotic and microbially influenced corrosion. This archive present raw and interpreted XPS data obtained using a Thermo Scientific ESCALAB 250xi from this project. The raw data files are in ThermoAvantage format.
description: Samples were collected from the steel surface at each site using cleaned metal scrappers, and immediately placed in sealed plastic boxes with sodium metabisulphite (AnaeroGenTM) sachets to prevent oxidation of the interior of the corrosion features. Samples were classified by colour to select materials for analysis and contrasting analysis sites. Prior to analysis samples were stored frozen at -80°C (as splits were also used for metagenomic analysis), and defrosted in sealed containers, again in the presence of sodium metabisulphite. XPS depth profile measurements were conducted using an ESCALAB 250xi X-ray photoelectron spectrometer (Thermo Scientific, UK) with aMAGCIS™ Dual Beam Ion Source. XPS analysis was carried out using selected area analysis mode with a nominal width of analysis of 400 μm and monochromated Al Kα X-rays at 1486.6 eV. The MAGCIS gun was selected in monoatomic mode with 3 keV ion energy and raster size 1.2mm. Sputtering cycle was kept at 120 s each time with a total of 100 levels for the depth profile analysis. The charge neutraliser and X-ray source were only used during the acquisition of spectra, both being turned off during the sputtering cycle. Survey (wide) scans (step size 1 eV, pass energy 150 eV,dwell time 50ms, number of scans 2) and narrow scans (step size 0.1 eV, pass energy 20 eV, dwell time 100 ms, number of scans 3) of the Fe 2p (binding energy, BE ∼ 708 eV), C 1s (BE ∼ 285 eV), O 1s (BE ∼ 531 eV), N 1s (BE ∼ 399 eV), Mg 1s (BE ∼ 1304 eV), S 2p (BE ∼ 164 eV) and Si 2p (BE ∼100 eV)) regions were acquired. Data analyses were carried out using Thermo Avantage software version 5.952. Chemical peak shift due to the sample charging under X-rays and corresponding charge neutralisation, was calculated using the advantageous C1s hydrocarbon peak at 285.0 eV following standard practice. However, during sputtering, where carbon was not present, the metallic Fe 2p binding energy of 706.6 eV was used as a reference point (Biesinger et al., 2011). Where multiple compounds have similar chemical shift reported in the literature, peaks were assigned to the corresponding Fe–S phase identified form the S 2p spectrum. Full width half maximum (FWHM) peak values of Fe 2p spectra were allowed to vary between 1.0 and 1.5 eV (Beisinger et al., 2011); and for S 2p3/2, they were constrained to vary between 1.1 and 1.6 eV (Pratt et al., 1994). Previous work has shown that Ar+ sputtering does not induce redox reactions in iron sulphate compounds (Smith et al., 2019). For this study we also tested Ar+ sputtering of well crystalline pyrite (FeS2 – e-Appendix 1). For Fe2p electrons no redox interaction with the ion beam is identifiable, although peaks are asymmetric because of the effects of electron spin coupling between the Fe 2p and 3d electrons resulting in the presence of high spin and low spin states (Beisinger et al., 2011) and the presence of the FeII-S multiplet at 708.4eV (Pratt et al., 1994). The presence of oxidation products on the mineral surface and along cleavage planes to depth is also detectable, but does not become more pronounced with depth. The S2p spectra do show evidence of interaction between the sample and the ion beam, with growth of a peak at 161.2eV with increasing ablation into the sample. This peak can be related to the formation of FeS as a result of Ar+ ablation and removal of S2- (Nesbitt and Muir, 1991; Chaturvedi et al., 1996) or the breakage of S-S bonds (Leiro et al., 2003) on cleaved pyrite surfaces.
description: All raw data files are in VGD or VGX format suitable for use with Thermo Avantage software, which was used for processing. Processed data is presented as excel files.