Atomic Force Infrared Microscopy (AFM-IR), also known as Photothermal Induced Resonance (PTIR), is a scanning probe spectroscopic technique that enables chemical characterization with submicron spatial resolution. This capability is especially valuable in corrosion science, where the chemical composition and phase distribution of metal alloy microstructures play a critical role in corrosion initiation, progression, and the formation of reaction products—key factors that determine long-term material performance. In this study, PTIR and infrared microspectroscopy were employed to investigate the influence of microstructure on corrosion product formation on ZnAlMg-coated steel exposed to SO₂-containing humid air. The analysis revealed that MgSO₄·7H₂O and MgSO₃·nH₂O predominantly form in eutectic regions. High-resolution PTIR measurements further showed preferential corrosion of microscopic MgZn₂ phases within the lamellar binary eutectic. This localized corrosion is attributed to micro-galvanic coupling, where MgZn₂ acts as the anodic site and Zn-rich phases serve as cathodic regions. These findings underscore the significant role of microstructural phase distribution in corrosion behavior and demonstrate the utility of PTIR for spatially resolved chemical analysis for complex metallic alloys.

This paper is dedicated to long-term atmospheric corrosion behaviour of magnesium alloys. Five different magnesium alloys, namely, AZ31, AM60, AZ61, AZ80, and AZ91, were exposed for 4 years under harsh conditions at the marine corrosion site of Brest (France). From the results, the corrosion performance increased in the following order: AZ31 < AM60 < AZ91 < AZ61 < AZ80. The corrosion was highly localised during the first year of exposure, but more general corrosion prevailed after long-term exposure. All materials followed a power law with rather similar kinetics of corrosion. The observed difference in the corrosion performance of the alloys was explained by the amount of secondary phases as well as that of the Al-content in the α-Mg phase

In the present study, ex situ Fourier transform infrared (FTIR) chemical imaging and in situ electrochemical impedance spectroscopy (EIS) were combined to investigate the ageing process of a polyester/melamine coil-coated steel. The samples were first subjected to a QUV accelerated weathering test for 250 h up to 2000 h, followed by immersion in a 0.5 M NaCl solution to assess water uptake and polymer matrix plasticization. FTIR analyses revealed chemical degradation, including chain scission and the formation of polar groups, between 500 h and 2000 h of QUV exposure. Degradation effects were observed throughout the whole topcoat, with more significant degradation occurring near the surface. EIS measurements indicated greater water uptake with increasing QUV exposure, highlighting two regions of water sorption: an initial rapid Fickian diffusion region and a slower non-Fickian region. The time constant (τ) analysis, which was extracted from the EIS data and related to the dielectric manifestation of the glass transition, confirmed polymer matrix plasticization due to water uptake. Despite UV-induced degradation, the polymer maintained effective protective properties, as evidenced by the high low-frequency impedance unaffected by UV exposure or immersion duration (1 week). This methodology successfully identified ageing markers, providing a framework for studying UV degradation mechanisms, water uptake, and polymer mobility in anticorrosion coatings. 

Almost all organic coatings used for corrosion protection contain various types of pigmentation, both for the sake of aesthetics and performance. By focusing on spatially resolved vibrational spectroscopy and electron microscopy, we demonstrate that the type of pigmentation has a fundamental effect on both localized and global degradation processes across the investigated coatings. Samples with white TiO<inf>2</inf> pigments display low macroscale chemical degradation across the surface but do experience a significant change in topography. SEM-EDS imaging shows that µm-scale craters are formed around the TiO<inf>2</inf> pigments. Nanoscale spatially resolved IR spectroscopy (AFM-IR) suggests that this local erosion is not triggered by reactions seen in coatings with other types of pigments. However, FTIR-ATR chemical imaging of coating cross sections confirmed that the TiO<inf>2</inf> pigments also protects material beneath the surface, resulting in very shallow chemical degradation effects as compared to the other systems. Darker coatings, containing carbon black, experience moderate to high chemical degradation across the surface, and additionally the degradation propagates deep into the coatings. Overall, this paper highlights that single criteria should not be used when comparing the degradation of different coatings.

The initial formation of corrosion products in pure humid air on magnesium alloys AZ91 and AZ31 was studied using infrared reflection absorption spectroscopy (IRRAS), infrared spectroscopic imaging, and SEM-EDS. The kinetics of corrosion product formation were monitored in situ with IRRAS during exposure to humid air (95% relative humidity) under two different CO2 concentrations: low (≤1 ppm) and ambient (400 ppm). For low CO2 concentrations, the primary corrosion product detected on both alloys was magnesium hydroxide (Mg(OH)2). In contrast, under ambient CO2 conditions (400 ppm), magnesium hydroxy carbonate was the dominant product. After 16 h of exposure, the amount of magnesium converted into corrosion products was approximately 8–10 times higher under low-CO2 conditions compared to ambient levels. The smaller formation of corrosion products but increased magnesium carbonate formation on AZ91D is attributed to its higher aluminium content compared to AZ31. Corrosion attack and product formation were largely localised to the centre of the α-phase in AZ91D, with the β-phase likely serving as sites for cathodic reactions.

Hot-dip Zn55Al-coated steel samples have been exposed for up to 6 years at 11 different weathering sites, including marine, marine-industrial, acid-rain and dry atmospheres. From the mass loss measurements, Zn55Al metallic coating showed globally long-term good corrosion resistance in all weathering conditions compared with hot-dip Zn-0.2Al-coated steel (Z). Yet, weaker performance was observed on Zn55Al in high SO2 polluted atmosphere, particularly when combined with seawater aerosols. This is explained by a more acidic surface condition linked to high SO2. Although the extent of corrosion in this phase was different at the different sites, the final corrosion products formed after 6 years were rather similar at all sites. This consists of hydrous aluminium sulphate or hydrous aluminium hydroxy sulphate and, probably also a smaller amount of sulphate-containing zinc corrosion products or Al/Zn products.

Local heterogeneities can have significant effects on the performance of anti-corrosion coatings. Even small features can act as initiation points for damage and result in corrosion of the substrate material. Analysis methods with high spatial resolution and the ability to collect information relevant to crosslinking and degradation behavior of these coatings are therefore highly relevant. In this work, we demonstrate the utility of nanomechanical AFM measurements and nano-FTIR in investigating the nanoscale mechanical and chemical properties of two polyester coil coating clearcoats before and after weathering. On the nanoscale, weathering led to a stiffer and less deformable coating with less variation in the nanomechanical properties. Chemical degradation was quantified using changes in band ratios in the IR-spectra. Macro and nano-scale measurements showed similar trends with the latter measurements showing larger heterogeneity. Our results demonstrate the usefulness of the described analysis techniques and will pave the way for future studies of local properties in other coating systems and formulations. © 2024 The Authors

The acid pickling of Al-3at.%Mg, Al-3at.%Cu, and aluminum alloy (AA) 7449-T651 in nitro-sulfuro-ferric acid was investigated using element-resolved electrochemistry (AESEC) in terms of their elemental dissolution kinetics. The influence of this acid pickling on the subsequent Zr-based conversion coating process was also demonstrated on these alloys by monitoring the dissolution rates of the alloying elements during conversion and the final elemental depth profiles from calibrated glow discharge-optical emission spectroscopy (GD-OES). The separate influence of fluoride (F−) and nitrate (NO3−) as additives on the dissolution kinetics was also investigated when added to the conversion coating bath solution. F− increased the dissolution rate of Al but no significant effect was seen on Cu, while NO3− enhanced the dissolution rates of both elements. Fourier-transform infrared reflection absorption spectroscopy (FT-IRRAS) data suggested a greater Zr-fluoride presence if the conversion coating was performed on a non-acid-pickled surface. 

An approach involving the quantification of microstructure characterized by different techniques such as SEM, EDS, and SKPFM is statistically treated to provide a deeper insight into the influence of recycling AA6063 on localized corrosion susceptibility. Particularly, the intermetallic particles and the two forms of localized corrosion – pitting and intergranular corrosion are systematically documented, measured, and analyzed. Even trace amounts of Cu and Zn introduced into the alloy from recycling had a remarkable effect on the localized corrosion susceptibility. The study found that the initiation and early evolution of the two localized corrosions are in competition, and the predominance of one over the other is closely linked to the composition of the alloy, and microstructure. Recycled variants with higher trace Cu made the alloy more susceptible to pitting attack whereas higher trace Zn is linked with greater IGC susceptibility. The trace amount of higher Zn addition has a particularly beneficial effect on pitting susceptibility as it reduces the likelihood of pitting even in alloys with a higher trace Cu content. The SKPFM results obtained in this study provided a basis for the circumferential pitting susceptibility around intermetallic particles, as a higher volta potential difference (∆V) implied a higher driving force for corrosion. ∆V differences between the different variants were further explained based on trace recycled element distribution in the microstructure. 

A mechanistic model of atmospheric bimetallic corrosion with a simplified empirical approach to the onset of localized corrosion attacks is presented. The model was built for a typical bimetallic sample containing aluminum alloy 1050 and stainless steel 316L sheets. A strategy was developed that allowed the model to be calibrated against the measured galvanic current, geometrical corrosion attack properties, and corrosion products. The pitting-onset simplification sets all pits to be formed at a position near the nobler metal and treated all pits as being of the same shape and size. The position was based on the location of the highest pitting events and the pit attributes on an average of the deepest pits. For 5 h exposure at controlled RH (85%, 91%, and 97%) and salt load (86 μg NaCl/cm2), the model was shown to be promising: both for analysis of local bimetallic corrosion chemistry, such as pH and corrosion products, and for efficient assessment of pitting damage by computing a single largest pit depth. Parametric studies indicated that the pitting-onset approximation deviated the most at the beginning of exposure and when RH was below 91%. © 2023 by the authors.