Corrosion properties of chemically modified metal surfaces

Study of n-Alkanethiol Self-Assembly Behavior on Iron Particles: Effect of Alkyl Chain Length and Adsorption Solvent on Resulting Iron-Based Magnetorheological Fluids

Self-assembled monolayers (SAMs) of organic molecules on metal surfaces are a type of inexpensive surface coating often used to improve metal substrate properties for sensors, electrochemistry, and nanofabrication applications. Iron, specifically, is one of the most commonly used metals, both as a pure metal and as an alloy due to its high conductivity, strong ferromagnetism, and low cost. However, magnetorheological fluids, which have shown impressive energy dampening in fields from civil infrastructure to biomedical devices utilizing iron dispersions, have suffered from low reliability and efficiency due to iron particle oxidation, corrosion, and settling. To understand the effect of self-assembled monolayers on iron and both the adsorbed particle's resistance against aggregation and performance impact, this work performs an in-depth study on alkanethiol-based self-assembled monolayers on iron particles. Adsorption of alkanethiols and the generation of SAMs on micron-sized iron particles were evaluated as a function of adsorption solvent polarity and alkanethiol chain length. Maximum alkanethiol loading, determined from appropriate isotherms, was found to strongly be a function of both parameters. Alkanethiol adsorption increased with increasing alkyl chain length and increasing solvent log P values in polar solvents. With respect to magnetorheologically relevant parameters, alkanethiol adsorption did not show any significant effect on both the magnetic properties of iron (as particles) and fluid on-state yield stress. The colloidal stability of n-alkanethiol adsorbed iron-based magnetorheological fluids (MRFs) was a function of both n-alkanethiol chain length and the iron particle adsorption solvent. MRFs composed of hexadecanethiol adsorbed iron prepared in polar solvents like methanol and ethanol showed excellent sedimentation stability compared to all other MRFs prepared in this study.

Atomic-level investigation on the oxidation efficiency and corrosion resistance of lithium enhanced by the addition of two dimensional materials

Understanding the oxidation and corrosion characteristics of Lithium (Li)-based systems is critical to their successful use as a solid fuel in spacecraft, powerplants, rechargeable batteries, submarines, and many other aquatic and corrosive environments. This study offers a systematic roadmap for engineering the oxidation efficiency and corrosion resistance of Li-based systems using ReaxFF-based Reactive Molecular Dynamics (RMD) simulations for the first time. First, we explored the oxidation mechanism of bare Li (Li/O2) at 1200 K, noticing that the oxidation process quickly ceases due to the creation of a passive oxide film on the Li surface. Afterward, we examined the effect of introducing graphene-oxide (GO) to the oxidation process of Li/O2. Interestingly, the inclusion of GO establishes a new reaction pathway between Li and O2, thus significantly improving oxidation efficiency. Additionally, we realized that when the concentration of GO increases in the system, the oxidation rate of Li/O2 increases considerably. As exposed to O2 and H2O, bare Li is observed to be highly corrosion-prone, while graphene (Gr)-coated Li exhibits excellent corrosion resistance, suggesting that Gr might be used as a promising corrosion-protective shield. Overall, this study is intended to serve as a reference for experimental investigations and assist researchers and engineers in designing more efficient Li-based functional systems.

Polar zinc oxide surface in electrolyte solutions: an atomic view of reconstruction, hydration and surface states

The stabilization mechanism of the Zn-terminated (Zn-) ZnO(0001) surface in electrolyte solutions has been investigated by using atomic-resolution liquid-environment atomic force microscopy (AFM) and an electrochemical method. The electrochemically measured pH dependence of the flat band potential of the Zn-ZnO(0001) surface indicated the adsorption of OH groups onto the (0001) surface in the wide pH range of 1-13. Atomic-scale AFM images of the Zn-ZnO(0001) surface showed a well-ordered hydroxide superstructure in an alkaline solution but a disordered structure in an acidic solution, which is probably attributed to the rapid diffusion of the adsorbed OH groups. Furthermore, the density of the O-terminated step edge on the Zn-ZnO(0001) surface in an acidic solution was higher than that in an alkaline solution. From these findings, we concluded that the excess positive charges of the Zn-ZnO(0001) surface are compensated by the adsorbed OH groups and the O-terminated step edges. In acidic solutions, a higher density of the O-terminated step edge is required for charge compensation. In addition, it was found that potential-dependent reversible surface reconstruction occurs in the local transition area with disordered step orientation by electrochemical AFM. We concluded that the reconstruction compensates the excess surface charges of the local transition area which are induced and varied by potential-dependent local surface states.

Multilayer Regulation of Atomic Boron Nitride Films to Improve Oxidation and Corrosion Resistance of Cu

The boron nitride (BN) monolayer (1L) with high impermeability and resistivity seems to hold promise as a long-term corrosion barrier for Cu under ambient condition, which is supported by recent researches. Here, we perform a complete study of the alternating temperature tests (the sample is exposed in air for 30 days and subsequently heated at 200 °C for 2 h) and electrochemical measurements on 1L and multilayer BN-coated Cu foils. Results imply that the BN-coated Cu foils are less oxidized than uncoated Cu foils after alternating temperature tests, regardless of the layers of BN. Particularly, the oxidation process proceeds slowly in multilayers because most of the underlying defects are covered with BN layers to suppress the oxygen diffusion in the vertical direction and the oxidation mainly occurs on the wrinkled region of BN films. Electrochemical analyses reveal that the BN layers provide an effective physical barrier against the corrosive medium and inhibit the electron diffusion because of their high electrical insulating behavior and the corrosion resistance of the samples increases with increasing BN layers. These findings indicate that BN films with adequate layers are good candidates for oxidation and corrosion protection at the atomic level, which is vital to many industrial and academic applications.

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.

Oxidation resistance of iron and copper foils coated with reduced graphene oxide multilayers

Protecting the surface of metals such as Fe and Cu from oxidizing is of great importance due to their widespread use. Here, oxidation resistance of Fe and Cu foils was achieved by coating them with reduced graphene oxide (rG-O) sheets. The rG-O-coated Fe and Cu foils were prepared by transferring rG-O multilayers from a SiO(2) substrate onto them. The oxidation resistance of these rG-O-coated metal foils was investigated by Raman spectroscopy, optical microscopy, and scanning electron microscopy after heat treatment at 200 °C in air for 2 h. The bare metal surfaces were severely oxidized, but the rG-O-coated metal surfaces were protected from oxidation. This simple solution process using rG-O is one advantage of the present study.

Oxidation resistance of graphene-coated Cu and Cu/Ni alloy

The ability to protect refined metals from reactive environments is vital to many industrial and academic applications. Current solutions, however, typically introduce several negative effects, including increased thickness and changes in the metal physical properties. In this paper, we demonstrate for the first time the ability of graphene films grown by chemical vapor deposition to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation. In particular, graphene prevents the formation of any oxide on the protected metal surfaces, thus allowing pure metal surfaces only one atom away from reactive environments. SEM, Raman spectroscopy, and XPS studies show that the metal surface is well protected from oxidation even after heating at 200 °C in air for up to 4 h. Our work further shows that graphene provides effective resistance against hydrogen peroxide. This protection method offers significant advantages and can be used on any metal that catalyzes graphene growth.

Electrochemical and molecular simulation studies on the corrosion inhibition of L-glutamine monolayers on an iron surface

L-Glutamine was used to form monolayers for the inhibition of the corrosion of iron in 0.50 mol dm-3 H2SO4. The protection ability of the films was examined by electrochemical impedance spectroscopy. The mechanism of adsorption is discussed using quantum chemical calculations and molecular simulations. Scanning electron microscopy was applied to confirm the formation of L-glutamine monolayers and the inhibitive effect. The results indicate that the molecules of L-glutamine are able to form films on the surface of iron and longer immersion time of the iron electrode in the solution results in a stronger inhibition ability of the films. The films are formed spontaneously by the adsorption of L-glutamine with a specific affinity of its head-group to the iron surface, hence, the densely and ordered monolayers can be considered as self-assembled.

Stabilization and acidic dissolution mechanism of single-crystalline ZnO(0001) surfaces in electrolytes studied by in-situ AFM imaging and ex-situ LEED

A combined approach of pH-dependent in-situ AFM topography and ex-situ LEED studies of the stability and dissolution of single-crystalline ZnO(0001)-Zn surfaces in aqueous media is presented. Hydroxide-stabilized and single-crystalline ZnO(0001)-Zn surfaces turned out to be stable within a wide pH range between 11 and 4 around the point of zero charge of pH PZC = 8.7 +/- 0.2. Hydroxide stabilization turned out to be a very effective stabilization mechanism for polar oxide surfaces in electrolyte solutions. The dissolution of the oxide surface started at an acidic pH level of 5.5 and occurred selectively at the pre-existing step edges, which consist of nonpolar surfaces. In comparison, the oxide dissolution along the ZnO(0001) direction proved to be effectively inhibited above a pH value of 3.8. On the basis of these microscopic observations, the mechanistic understanding of the acidic dissolution process of ZnO could be supported. Moreover, both the in-situ AFM and the ex-situ LEED studies showed that the stabilization mechanism of the ZnO(0001) surfaces changes in acidic electrolytes. At pH values below 3.8, the hydroxide-stabilized surface is destabilized by dissolution of the well-ordered radical3. radical3. R30 hydroxide ad-layer as proven by LEED. Restabilization occurs and leads to the formation of triangular nanoterraces with a specific edge termination. However, below pH 4 the surface structure of the crystal itself is ill-defined on the macroscopic scale because preferable etching along crystal defects as dislocations into the bulk oxide results in very deep hexagonal etching pits.

Alkanethiol monolayers at reduced and oxidized zinc surfaces with corrosion protection: a sum frequency generation and electrochemistry investigation

In this work, octadecanethiol (ODT) was demonstrated to form ordered monolayers at either electrochemically reduced or oxidized Zn surfaces, by means of sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG spectra of ODT-modified Zn electrodes featured three methyl group resonances in the C-H vibrational region (2800-3100 cm(-1)). A significant decrease in interfacial capacitance and an increase in charge-transfer resistance were observed in EIS measurement after ODT modification. The alkane chain tilt angle of ODT within a monolayer at the Zn surface was estimated as 0 degrees with respect to the surface normal by interfacial capacitance measurement via EIS. CV and SFG investigation revealed that ODT monolayers undergo reductive desorption from the Zn electrode in 0.5 M NaOH at -1.66 V (vs SCE) and in 0.5 M NaClO4 at -1.62 V. The integrated charge consumed to the desorption of ODT is determined as 87 mC/cm2 from the reductive peak on CV curve, resulting in a coverage of 9.0 x 10(-10) mol/cm2 (5.4 x 10(14) molecules/cm2) if assuming the reduction follows a one-electron process. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt, and acid media. However, the protection efficiency was reduced with immersion time due to the presence of defects within the monolayers.

Investigation of Solid Surfaces Modified by Langmuir−Blodgett Monolayers Using Sum-Frequency Vibrational Spectroscopy and X-ray Photoelectron Spectroscopy

Langmuir-Blodgett (LB) monomolecular layers of alkylhydroxamic acids and alkylphosphonic acids on copper and iron substrates have been studied by X-ray photoelectron spectroscopy (XPS) and sum-frequency vibrational spectroscopy. According to the XPS results, the structures of the hydroxamic acid and phosphonic acid Langmuir-Blodgett films are very similar: the thickness of the layer of the hydrocarbon tails is typically 1.9-2.1 nm, while the layer of headgroups is about 0.3-0.35 nm thick. The tilt angle of the carbon chains is estimated to be 20-30 degrees with respect to the sample surface normal, and the molecules are connected to the substrate via their headgroups. Analysis of the P 2p and N 1s lines indicates the presence of deprotonated headgroups. The substrate Cu 2p line includes a component which can be assigned to Cu(2+) ions in a thin Cu(OH)(2) layer. The deposition of LB layers led to significant decrease of the hydroxide-related signal, which indicates that binding of the headgroups to the surface is accompanied by the elimination of water molecules. The sum-frequency spectra also clearly indicate that well-ordered monolayers can be formed by the Langmuir-Blodgett technique. Since the non-resonant background from the metal substrates renders the analysis of the spectra more difficult, model system samples on glass were prepared. It was found that the alkyl chains of the adsorbed acids predominantly adopt the all-trans conformation and form an ordered structure. Upper limits for the mean tilt angle of the terminal methyl groups are approximately 10-20 degrees.

Preparation of Alkanethiol Monolayers on Mild Steel Surfaces Studied with Sum Frequency Generation and Electrochemistry

An n-alkanethiol, octadecanethiol (ODT), monolayer was successfully prepared onto an oxide-free mild steel (MS) surface under cathodic polarization in a 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT. Cyclic voltammetry (CV) and electrochemical impedance (EIS) and sum frequency generation (SFG) spectroscopy were applied to study and characterize the adsorption of ODT at a MS surface. In 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT, CV of the MS electrode shows a dramatic decrease in charging current and a positive shift in oxidation potential when compared to a solution without ODT. The interfacial capacitance was obtained as 2.52 microF/cm(2) from the impedance data. An average chain tilt angle of 48 degrees for the ODT molecules was deduced from the comparison of the interfacial capacitances of the ODT/MS and ODT/Au monolayers. X-ray photoelectron spectroscopy confirmed the formation of the ODT monolayer on mild steel. The ppp SFG spectrum of the ODT-modified MS features three strong methyl vibrational modes at 2877, 2943, and 2967 cm(-1), indicating the formation of the oriented and densely packed ODT monolayer. However, the appearance of the two weak CH(2) groups' vibrational modes at 2850 and 2914 cm(-1) implies the presence of defects in the ODT monolayer. ODT/Au films were prepared to compare with the ODT/MS films. Orientation analysis of the air/solid interface suggests that the methyl group of ODT/Au films has a tilt angle of 30 degrees , while the methyl group of ODT/MS films has a tilt angle of 23 degrees . Water was found to have an impact on the shape of the SFG spectra of ODT/MS. This suggests that the solution penetrated through the defects to reach the MS surface.