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.

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.

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

In light of the need to redesign industrially produced materials by utilizing renewable resources, nanoscopic forms of cellulose are regarded as a valuable asset. In this work, the potential of cellulose nanofibrils (CNFs) as an unconventional nanoadditive for a polyester–melamine coil coating formulation is explored. CNFs, hydrophilic nanomaterials with high aspect ratios, were successfully incorporated in the solvent-borne formulation. Coatings were prepared with extremely low loadings of the nanoadditive (0.5 and 0.7 wt%). At these concentrations, the volume fraction of the nanofibrils in the coatings was calculated to be close to their percolation threshold. Two different pathways of CNF surface modification were applied to allow for a good compatibilization in the resin matrix, and the formulations were cured in two different settings. The effective compatibilization of the nanoadditive led to a significant variation of the viscoelastic properties in the coatings containing CNFs. The results from dynamic mechanical analysis (DMA) highlighted the effect of CNFs on the crosslinked network at the nanoscale, resulting in an increase in Tg. Additionally, an increase in the stress at break and Young's modulus was determined by tensile testing, while satisfactory elongation at break was preserved. Other relevant effects induced by the presence of CNFs on the properties of the coatings were highlighted, such as a significant matt effect, increased surface roughness and lower scratch resistance. A preliminary evaluation of the water barrier properties by electrochemical impedance spectroscopy (EIS) is also presented, suggesting that the incorporation of a hydrophilic nanoadditive did not lead to a deterioration of the coatings' performance. © 2023 The Authors

Improved methods to assess the degradation of coil coatings to approximate lifetime have been an area of academic and industrial interest for decades. This work aims to elucidate the differences in the degradation behavior of two coil coating systems: one standard commercial formulation and one formulation with a significant addition of biorenewable reactive diluents. Depth-resolved degradation behavior of samples exposed to accelerated and natural field weathering is assessed. Focal plane array attenuated total reflection-Fourier transform infrared spectroscopy was used to acquire high-resolution chemical data from a sloping cross section. The results agreed with conventional photoacoustic spectroscopy. Degradation profiles for the two coatings were significantly different, with the biobased samples showing a more durable behavior. This study provides a method for detailed assessment of coating degradation, giving a good estimation of its durability. This is both a way to compare the performance of coating systems and to improve the understanding of the impact of exposure conditions, paving the way for the development of more sustainable coil coatings.