The chemical form (chemical speciation) of chromium (Cr) is important for human health. Hexavalent Cr (Cr^VI), present as oxyanions in water, is of great concern even at trace levels. Here, we briefly describe and discuss common and additional liquid state (often standardized) and solid state Cr speciation methods for typical samples of health concerns. This review covers common standardized, extraction-based liquid state methods and various solid state Cr speciation methods. Liquid state methods include chromatography, colorimetric methods, mass spectrometry, and electrochemical methods, with widely varying detection limit ranges to accommodate all sample needs. The most sensitive liquid state method can detect trace amounts of Cr^VI in the nanograms per litre range. Colorimetric methods can be used both for the liquid and solid state and are the simplest methods without the need for a laboratory or equipment. Other solid state methods include vibrational spectroscopy, electrochemical methods, and various laboratory-or synchrotron-based methods: X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray diffraction. No method is perfect on its own, and we therefore recommend best practices, the investigation of potential interfering agents, and validating the method with another method. However, the largest threat to accurate Cr speciation-based hazard assessments is the dynamic change of Cr speciation in a potential exposure scenario or during sample preparation for the analytical method. To avoid wrong conclusions, we recommend considering the Cr chemistry, the sample chemistry, and the method-specific interferences and detection limits.

The dream corrosion inhibitor would work for every substrate–environment combination, and the protection would be sustained indefinitely with an irreversible barrier layer when exposed to aggressive and changing environmental conditions. However our prior electrochemical experiments on AA2024-T3 have shown that despite the initial inhibition, all of the tested molecules had reversible bonds that limit their inhibition performance and applicability in dynamic environments, with the exception of 3-amino-1,2,4-triazole-5-thiol, which still showed 42% inhibition efficiency after being exposed to 0.1M NaCl only for three days. To our knowledge, this is the first mechanistic study that explains the origin of such quasi-sustained inhibition by an organic molecule under dynamic and aggressive conditions relevant to aerospace alloys. Potentiodynamic polarization, atomic force microscopy and scanning Kelvin probe force microscopy (AFM/SKPFM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) complemented by density functional theory (DFT) calculations were used to identify the molecular mechanism responsible for the quasi-stable adsorption provided by 3-amino-1,2,4-triazole-5-thiol. Our findings suggest that a sulphatization of the Al-(hydr)oxide is the key contributor to the quasi-sustained corrosion inhibition. Sustained molecule adsorption over intermetallics in trace amounts was also observed, but their presence was insufficient to inhibit corrosion.