XPS evaluation of oxidation degrees

Characterization of chromium oxidation states using XPS


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.

Chromium VI, mainly derived from industrial waste, is one of the most widely used cations in surface treatment, metrology and leather tanning.

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.

Because of its very high reactivity, chromium VI has carcinogenic, mutagenic and repro-toxic (CMR) properties. These characteristics call for the adoption of strict precautionary measures to protect people and the environment.

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).

X-ray photoelectron spectroscopy (XPS) gives access to the elemental and chemical composition (except for H and He) of the extreme surface over 3 to 10 nm.

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).

TESCAN ANALYTICS has over 30 years' expertise in the use of XPS for surface analysis of all types of materials, whether insulating or conductive... With state-of-the-art instruments, our team of experts works with all industrial sectors.


Objective of the analysis

XPS evaluation of the degree of oxidation of chromium.


Sample preparation

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.



Figure 1: XPS spectrum of corrosion-resistant steel plate

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.


XPS provides information on the degree of oxidation of the elements present in the first atomic layers of a sample through chemical shifts. This quantitative technique also provides information on the relative proportions of these elements.

XPS is an ideal method for determining the oxidation state of elements present on the surface of a sample.

In profile mode, it is also possible to access layer information down to µm thickness by alternating analysis and abrasion sequences.

Other studies have demonstrated the power of XPS for assessing the homogeneity of a treatment on different substrates.

For more information on XPS analysis or our other techniques, please consult our thematic page.

Other complementary techniques can be used to study the molecular chemical nature of a treatment (TOF-SIMS) as well as the morphological modifications induced at the extreme surface (AFM) or the thickness of nanometric deposits (TEM).