Molybdate and Phosphate Cross-Linked Chitosan Films for Corrosion Protection of Hot-Dip Galvanized Steel

Environmentally friendly and sustainable methods to protect hot-dip galvanized (HDG) steel from corrosion are extensively studied. Films of the biopolymer polyelectrolyte chitosan were ionically cross-linked in this work with the well-known corrosion inhibitors phosphate and molybdate. Layers on this basis are presented as components in a protective system and could, e.g., be applied in pretreatments similar to a conversion coating. For the preparation of the chitosan-based films, a procedure involving sol-gel chemistry and wet-wet application was utilized. Homogeneous films of few micrometers thickness were obtained on HDG steel substrates after thermal curing. Properties of chitosan-molybdate and chitosan-phosphate films were compared with purely passive epoxysilane-cross-linked chitosan, and pure chitosan. Delamination behavior of a poly(vinyl butyral) (PVB) weak model top coating studied by scanning Kelvin probe (SKP) showed an almost linear time dependence over >10 h on all systems. Delamination rates were 0.28 mm h^-1 (chitosan-molybdate) and 0.19 mm h^-1 (chitosan-phosphate), ca. 5% of a non-cross-linked chitosan reference and slightly higher than of the epoxsyilane cross-linked chitosan. Immersion of the treated zinc samples over 40 h in 5% NaCl solution yielded a 5-fold increase of the resistance in the chitosan-molybdate system, as evidenced by electrochemical impedance spectroscopy (EIS). Ion exchange of electrolyte anions with molybdate and phosphate triggers corrosion inhibition, presumably by reaction with the HDG surface as well described in the literature for these inhibitors. Thus, such surface treatments have potential for application, e.g., in temporary corrosion protection.

Role of aluminium hydrides in localised corrosion of aluminium revealed by operando Raman spectroscopy

Filiform corrosion (FFC) is characteristic of metals such as aluminium and magnesium, usually takes place on coated metals, and spreads from coating defects in the form of filaments with a width on the order of 100 μm. In this work, in situ and operando Raman spectroscopy and optical microscopy were used to characterize the composition and distribution of corrosion products inside growing filaments. The filament head contains water (OH stretching modes, 3000-3600 cm^-1), and corrosion products based on aluminium oxide with both tetrahedrally (840 cm^-1) and octahedrally (600 cm^-1) coordinated Al³⁺, and with some hydroxyl group content (3075, 1420, 1164 cm^-1). Remarkable is the prominent presence of structural motifs as in γ-AlH₃ (1045, 1495 cm^-1). The tail contains predominantly aluminium oxide with octahedrally coordinated Al³⁺ and in addition carbonate (1100 cm^-1) and aluminium chloride (347 cm^-1). Video recordings of the active filigree show hydrogen evolution inside the active head and a very fast precipitation of corrosion products. Re-dissolution, transport and re-formation of the corrosion products is also observed, accompanying start-stop-cycles of the propagation of FFC; this mechanism leads to wavy surface morphologies by lifting of certain coating areas after the passage of the corrosion front as evidenced by 3D optical profilometer analysis. When exposed to the acidic head conditions for a sufficient time, the initiation of other forms of localised corrosion, such as pitting, is possible, which in turn facilitates further propagation of the filament. The in situ detection of hydride which transforms into the typical aluminium corrosion products in due course points to a prominent role of hydride as intermediate in the aqueous corrosion of aluminium.

Periodic aggregation patterns of oxide particles on corroding metals: chemical waves due to solution feedback processes

Chemical waves that produce periodic patterns are common occurrences in nature. The underlying processes involved have been studied in many disciplines of science, but rarely reported in the chemistry of corrosion. In this study of carbon steel corrosion, iron oxide crystals are observed to deposit in concentric wave patterns or in discrete bands, known as Liesegang patterns. We demonstrate that oxide growth in these patterns is preceded by the formation of a hydrogel network, which consists of a semi-stationary phase of loosely connected metal-hydroxide colloids and a mobile phase of solution saturated with metal cations. Once the hydrogel network covers the metal surface, a metal cation produced by corrosion reactions at the metal surface must diffuse through the layer into the bulk solution. While diffusing through the porous network, the metal cation undergoes adsorption-precipitation as metal-hydroxide colloids which later can dissolve back into the solution. When the kinetics of precipitation and dissolution of the metal cation can be effectively coupled with the transport flux of the dissolved metal cation, the precipitation-dissolution-diffusion cycles can be sustained over time which can lead to periodic aggregation patterns of metal-hydroxide colloids at a specific time. We also establish that for transition metal cations the precipitation-dissolution-diffusion process can couple with reversible redox reactions between the soluble and less soluble metal cations, which can affect the overall transport of banded aggregates of metal-hydroxide colloids and the growth and transformation of metal-hydroxides into crystalline oxides. If systemic feedback between different elementary processes is sustained over long durations, iron-oxide crystals of different chemical compositions and shapes aggregate in Liesegang patterns. This work demonstrates unequivocally that non-uniform deposition of metal oxides during corrosion can occur via strongly coupled solution reactions and transport processes, and not simply as a result of metallurgical non-uniformity and/or localized solution environments.

Pretreatment with a β-Cyclodextrin-Corrosion Inhibitor Complex Stops an Initiated Corrosion Process on Zinc

Metal pretreatment is typically the first step in a reliable corrosion protection system. This work explores the incorporation of complexes between the cyclic oligosaccharide β-cyclodextrin (β-CD) and the molecular organic corrosion inhibitor 2-mercaptobenzothiazole (MBT) into an oxide-based pretreatment layer on metallic zinc. The layers were produced by a precorrosion step in the presence of β-CD. The resulting films have a morphology dominated by spherical particles. X-ray photoelectron spectroscopy investigations of the surfaces show the sulfur atoms of MBT to be partially oxidized but mostly intact. Samples pretreated with such a layer were subsequently coated with a model polymer coating, and the delamination of this model coating from an artificial defect was monitored by a scanning Kelvin probe (SKP). The SKP results show a slow down of delamination after several hours of the ongoing corrosion process for surfaces pretreated with the complexes. Finally, an increase in the electrode potential in the defect was observed, with a subsequent complete stop in delamination and repassivation of the defect after ≈10 h. This repassivation is attributed to the release of MBT after the initiation of the corrosion process. Most likely, the increase of pH, combined with the availability of aqueous solution, facilitates the MBT release after the initiation of a corrosion process. Consequently, complexes formed from β-CD and corrosion inhibitors can be effectively incorporated into inorganic pretreatments, and the inhibitor component can be released upon start of the corrosion process.

Gradient in defect density of ZnO nanorods grown by cathodic delamination, a corrosion process, leads to end-specific luminescence

ZnO nanorods were grown on a zinc substrate via cathodic delamination of a polymer coating, a tailored corrosion process, at room temperature. A comparison between in situ Raman spectra and post mortem cross sectional analysis by Raman spectroscopy, photoluminescence spectroscopy and scanning electron microscopy shows that in the initial stages of the synthesis, preferentially defect rich ZnO grows. At later stages, crystalline wurtzite ZnO growth dominates. The result is nanorod arrays consisting of nanorods with a large density of point defects in the ≈500 nm range near the zinc substrate, and low defect density in the regions further away from the interface. The growth, which proceeds over several hours, can be interrupted at any time. Large salt concentrations in the corrosive medium increase the growth rate, but also the amount of point defects. The resulting rods show strongly position-dependent luminescence and Raman spectra. Different luminescence can thus be selectively excited, based on the position of excitation.

Highly Selective and Sensitive Detection of Nitroaromatic Compounds and Metal Ions by Supramolecular Assemblies of 3,3',5,5'-Azobenzenetetracarboxylic Acid and 4,4'-Bipyridine

A supramolecular compound, (H₄L)(4,4'-bpy)₂ (1) (H₄L = 1,2-bis(3,5-dicarboxyphenyl)diazene oxide, 4,4'-bpy = 4,4'-bipyridine) with 2D + 2D → 2D 3-fold parallel interpenetrated layer feature, has been prepared which was investigated as selective sensing material for detection of nitroaromatic compounds (NACs) and metal ions, and exhibits significant fluorescence quenching toward NACs and high selectivity for detection of Fe³⁺ ion. The result indicates that 1 is a promising multi-functional fluorescence probe for detecting and recognizing NACs and metal ions with high sensitivity and selectivity.

Waterborne chitosan-epoxysilane hybrid pretreatments for corrosion protection of zinc

Biopolymer-based systems are extensively studied as green alternatives for traditional polymer coatings, e.g., in corrosion protection. Chitosan-epoxysilane hybrid films are presented in this work as a chitosan-based protective system, which could, e.g., be applied in a pretreatment step. For the preparation of the chitosan-epoxysilane hybrid systems, a sol-gel procedure was applied. The function of the silane is to ensure adhesion to the substrate. On zinc substrates, homogeneous thin films with thickness of 50-70 nm were obtained after thermal curing. The hybrid-coated zinc substrates were characterized by infrared spectroscopy, ellipsometry, and x-ray photoelectron spectroscopy. As model corrosion experiments, linear polarization resistance was measured, and cathodic delamination of the weak polymer coating poly(vinylbutyral) (PVB) was studied using scanning Kelvin probe. Overall, chitosan-epoxysilane hybrid pretreated samples showed lower delamination rates than unmodified chitosan coatings and pure PVB. Electrochemical impedance spectroscopy confirmed a reduced ion permeability and water uptake by chitosan-epoxysilane films compared to that of a nonmodified chitosan coating. Even though the coatings are hydrophobic and contain water, they slow down cathodic delamination by limiting ion transport.