Chromium, a natural metallic element in the earth's crust, can be found in many different forms. Only trivalent (chromium III) and hexavalent (chromium VI) compounds are found to any significant extent in the environment. Chromium VI does not exist in nature, but is produced by the anodic oxidation of chromium III. This process gives it very high potential energy, which it releases by oxidizing the elements it comes into contact with.
The steel industry accounts for 90% of chromium use in the production of stainless steels, special steels and alloys. It improves metal hardness and corrosion resistance. While chromium III is an essential trace element for our bodies, chromium VI is considered a carcinogen. It has been listed in Annex XIV of the REACH regulation since 2013, providing for a ban and gradual substitution of its use.
Although alternative processes have proved their worth (vacuum deposition, etc.), they still have many limitations and do not fully meet expected performance levels. Some applications, such as electrolytic chromium plating of parts with complex geometries, or surface preparation, particularly for magnesium, remain without replacement...
These regulations require a number of analyses to identify and/or quantify the degree of oxidation. XPS is the method of choice for this application. In XPS, the sample surface is irradiated with X-ray photons from a monochromatic source. Atoms in the first few nanometers emit photoelectrons with energies characteristic of each element and its chemical environment. The spectra obtained most commonly show binding energy (the difference between X-ray energy and photoelectron kinetic energy) as a function of intensity (the number of photoelectrons emitted).
Chemical information on samples, such as their oxidation state, is obtained by plotting variations in binding energy. The XPS offers several analysis modes, one of which, the spectroscopy mode, enables elemental and chemical analysis of the extreme surface. This mode is available in different configurations: point analysis (a few tens of µm2 to 300 x 700 µm2), linescan (a few mm per cm in x or y direction) and grid (for homogeneity studies on surfaces down to cm2). Analysis depth varies between 3 and 10 nm (depending on the angle between the detector and the surface normal).
An anti-corrosion treated steel plate is cut to 1 cm2 , then fixed to the XPS stage. XPS analysis was carried out in normal detection mode (analysis depth close to 10 nm) and in high-resolution mode.
XPS data are collected with a monochromatic AlKα source. The high-resolution Cr2p3/2 spectrum of chromium is shown in Figure 1. Chemical analysis of the chromium Cr2p3/2 peak reveals 5 contributions, 4 of which are related to each other in terms of binding energy, relative intensity and half-value width. According to the literature, these correspond to a chromium III fingerprint. The fifth contribution is detected at around 579.5 eV, a binding energy often attributed to chromium VI. Chemical analysis by XPS in high-resolution mode gives access to the quantification of chromium VI in relation to chromium III, within a detection limit close to a few tenths of an atomic %. This makes it possible to evaluate surface treatment processes in terms of their ability to generate chromium VI.
Other studies have demonstrated the power of XPS for assessing the homogeneity of a treatment on different substrates.