Aqueous Chloride Environments Research Articles

Insight into the corrosion inhibition of Ce-MOF on copper in aqueous chloride environment: From experimental validation to molecular-level prediction

Currently, the demand for environmentally friendly corrosion inhibitors for marine environment is on the increase due to government regulations to phase out toxic ones. A new insight into room temperature incorporation of organic and inorganic materials to form an environmentally benign Ce-based metal organic framework (Ce-MOF) for corrosion inhibition of copper alloys in chloride rich environment becomes imperative. Ce-MOF contains O and N heteroatoms which can easily form compact-uniform layer on metallic surface to prevent corrosion attack. The inhibition potential of Ce-MOF was thus examined via combined electrochemical method, density functional theory (DFT) and molecular dynamics (MD) simulation. The overall results revealed a strong protection of copper in the presence of the formulated Ce-MOF compared to its absence.

Molecular-level insights into the corrosion protection mechanism of Mg/Al CO32−-LDH films on steel in aqueous chloride environments

Despite their thickness of only a few microns, layered double hydroxide (LDH) films offer excellent corrosion protection for steel. However, the lack of molecular-level comprehension of this protective mechanism has hindered the optimal design of LDH films. Herein, we developed a molecular model of Mg/Al-CO32−-LDH and investigated the nanoscale characteristics of water and ions in proximity to the LDH surface. Results underscore a robust attachment of water and ions to the LDH surface, accompanied by the attenuation of diffusion as they approach. The diffusion of interfacial water is predominantly influenced by H-bond interactions stemming from hydroxyl groups on the LDH surface. Additionally, the impediment of Cl− diffusion can be attributed to both anionic competition adsorption and the chemical bonding interactions of LDH.

Insights into the characteristics of corrosion products formed on the contact and exposed regions of C1045 steel bolt and nut fasteners exposed to aqueous chloride environments

• Corrosion products on different regions of C1045 bolt are characterized for the first time. • Differential aeration/concentration effects induced heterogeneity in the corrosion products. • Reactive phases, γ-FeOOH and β-FeOOH dominated at the exposed and contact region, respectively. • Magnetite, Fe 3 O 4 was detected in both regions under low oxygen conditions. • Due to the low amount of α-FeOOH, the rusts in both regions lack stability and protectiveness. This study investigated the characteristics of corrosion products formed on the contact and exposed regions of C1045 steel bolt and nut fasteners used in structures exposed to aqueous chloride environments. The corroded surface morphology, rust compositions, and corrosion kinetics of the bolt specimen were studied by visual observation, optical microscopy (OM), scanning electron microscopy (SEM), X-Ray diffractometry (XRD), micro-Raman, electron probe micro-analyzer (EPMA), and potentiodynamic polarization techniques. Results obtained showed a variation in corrosion kinetics, morphology, and composition of the rust layer which were driven by differential aeration and concentration effects. Due to the availability of sufficient dissolved oxygen, the oxyhydroxide compound, lepidocrocite (γ-FeOOH) was detected in the outer rust layer in the exposed region, whereas the inner rust layer was composed of magnetite (Fe 3 O 4 ). However, the oxygen-deficient contact surface revealed the presence of akaganeite (β-FeOOH) and magnetite (Fe 3 O 4 ) as dominant oxide phases. The most stable phase, goethite (α-FeOOH) was also detected in the rust formed in both regions, though in significantly low amounts. Furthermore, owing to variation in environmental conditions, the amount and density of the rust layer varied in the different regions. The estimated corrosion stability values for the different regions revealed that the corrosion products formed on the steel surfaces were non-protective, suggesting the need for specific surface treatment as a protective measure.

Localized Corrosion of Chromium-Containing Conventional and Multiprincipal Element Alloys in Methanol and Aqueous Chloride Environments

The localized corrosion behavior of Type 304L stainless steel (UNS S30400), Alloy 625 (UNS N06625), and three experimental NiCoCrMo multiprincipal element alloys (MPEA), exposed to methanol/lithium chloride solutions with different water contents, is examined in this paper. Cyclic potentiodynamic polarization tests indicated that water increased the pitting and repassivation potentials and the critical temperature at which localized corrosion occurred in chloride solutions. The effect of water on repassivation potential in methanol-chloride solution depended on the chloride concentration and alloy content. At lower chloride concentration and higher alloy content, water had a greater inhibiting effect on repassivation potential, suggesting a competitive reaction between chloride and water on the metal surface in the pit. The localized corrosion resistance of the MPEAs followed the same trend as that of commercial stainless steels and Ni-base alloys in terms of the effect of alloying elements. The implications of these results on the mechanisms of localized corrosion are discussed.

Effect of austempering temperature on corrosion evolution during immersion of high carbon nano-structured bainitic steel in aqueous chloride environment

Nano-structured bainitic (NSB) steels have a dual-phase structure comprising carbon-rich retained austenite (RA) and bainitic ferrite (BF). These steels exhibit high strength as well as sufficient ductility and damage tolerance and hence can find potential application in marine areas. However, a systematic study of the corrosion behavior is required to assess the reliability and longevity of these steels for the above applications. The role of austempering temperature (250 and 350 °C: specimens NB250 and NB350, respectively) and in turn the microstructural constituents on the corrosion response in aqueous chloride environment has been studied using immersion tests at different intervals of time (10, 20 and 30 days). Characterization of corrosion products has been done using microscopy and multiple spectroscopy techniques to reveal the attack mechanism and corroded layer composition. The corrosion rate, measured using weight loss experiments, decreases with a reduction in austempering temperature of the specimens and with an increase in immersion duration. Cross-sectional imaging of the corroded regions shows highly non-uniform but deeper penetration of corrosion with an increase in austempering temperature. In contrast, specimen transformed at the lowest temperature showed uniform and shallow penetration of corrosion. The corroded layer formed over NB350 after 10 days mostly comprised of oxyhydroxides but oxides took predominance after 30 days. However, NB250 showed mostly oxyhydroxides on the surface after 10 as well as 30 days. Hence, decreasing the austempering temperature while transforming austenite to nano-bainite not only provides higher strength but also greater corrosion resistance in saline environments.

Reflections on Early Stages of Environmentally Assisted Cracking from Corrosion Pits

A perspective is presented on the evolution of damage due to environmentally assisted cracking (EAC), from crack precursor development through to long crack growth. The variable nature of crack precursors is highlighted with an observation that uncontrolled chemistry excursions or fabrication defects could eliminate any significant delay associated with that step in the damage evolution process. Specimen preparation by machining and grinding can be critical in determining the apparent susceptibility of the metal to EAC and corrosion, and an example for 316L stainless steel is given to show how physical defects generated by the grinding wheel can become the dominant site for pitting attack relative to MnS inclusions. Corrosion pits are the most commonly observed precursor to cracks in aqueous chloride environments. The loci of sites of crack initiation around a pit are discussed and the inherent challenges in quantifying the growth of cracks smaller than the pit depth described with implications for modelling of the pit-to-crack transition. The remarkably enhanced stress corrosion crack growth rate data for short and small cracks in a 12Cr steam turbine blade in a simulated condensate environment are discussed in the context of crack electrochemistry modelling and the implications for engineering integrity.

Comparative electrochemical impedance spectroscopy study in chloride environment of nano-structured bainitic steels prepared by austempering at varied temperatures

Carbide free nano-bainitic (NSB) steels have a dual phase structure comprising of retained austenite (RA) and bainitic ferrite (BF). The content and morphology RA and BF in NSB steels can be easily modulated by changing the austempering temperature. This results in significant changes in the strength and is expected to influence the corrosion resistance as well. The corrosion behavior of three NSB steel blocks austempered at 250, 300 and 350 °C has been studied in aqueous chloride (3.5 wt% NaCl) environment. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization have been used to investigate the role of austempering temperature on the corrosion response. It is found that the corrosion current density and hence the corrosion rate decreases with a reduction in austempering temperature. Moreover, a reduction in the austempering temperature shifts the corrosion potential towards the nobler side. The EIS studies have shown that the charge transfer resistance gets enhanced for specimens austempered at lower temperatures. This comes with a concomitant lowering in the capacitance of the electrical double layer formed between the steel specimen and electrolyte. Thus, it is possible to achieve a combination of high hardness and corrosion resistance by fabricating the NSB steels at low austempering temperatures.

Development of Superhydrophobic Coating on Copper for Enhanced Corrosion Resistance in Chloride Medium

In this paper, a simple and economic method of superhydrophobic (SHP) surface modification of copper surfaces with water contact angle (WCA) of 147° ± 0.1° and roll-off angle of 5° is reported. SHP copper surfaces were synthesized by annealing copper foil in air and then coated with silica nanoparticles dispersed in silane solution. Scanning electron microscopy analysis of the SHP Cu surfaces showed the uniform distribution of spherical micron-sized CuO particles throughout the surface. Atomic force microscopy images of silane-coated surfaces showed increased valleys and peaks with higher root mean square and average roughness contributed by the silica nanoparticles. X-ray photoelectron spectroscopy results further confirmed the micron-sized CuO particles in the outer layer with silane containing silica nanoparticles, which contributed to the micro-/nano-roughness causing a phenomenal improvement in the WCA values. Electrochemical studies carried out in aqueous chloride environments demonstrated the corrosion resistance of superhydrophobic copper surface as evident from the shift in open-circuit potential values towards the nobler direction, increase in the charge transfer resistance and lower anodic current as compared with the fresh copper foil.

The effect of pit size and density on the fatigue behaviour of a pre‐corroded martensitic stainless steel

AbstractUNS S17400 steel is used in turbines for the aerospace and utility industries. While it is generally corrosion resistant, it is susceptible to pitting when exposed to aqueous chloride environments. Effects of pitting characteristics, such as depth, width, and local density on fatigue life, have been studied in this work to better inform criteria for component replacement or repair. While pit depth correlates well with cracking, the deepest pit never initiated the crack that ultimately led to failure. The clustering of pits, or local pitting density, also correlated well with crack initiation location; however, the densest region of pitting was not always the location where cracking occurred. There is likely no single metric that directly correlates pitting with fatigue cracking, rather there is a combination of pitting characteristics that ultimately lead to cracking. The results from this work suggest that pit depth and local pitting density are among the more important metrics.

Experimental and theoretical studies on the corrosion inhibition of vitamins – Thiamine hydrochloride or biotin in corrosion of mild steel in aqueous chloride environment

The corrosion inhibition of thiamine hydrochloride (vitamin B1) or biotin (vitamin B7) on corrosion of mild steel in 240ppm chloride ions solution was studied using weight loss method, potentiodynamic polarization curves and electrochemical impedance spectroscopy. UV–Vis, FTIR and surface analysis using SEM and EDS characterization were carried out to understand the corrosion inhibition property of these inhibitors in 240ppm aqueous chloride medium. Experimental results show that both thiamine hydrochloride and biotin act as a good corrosion inhibitors. Quantum chemical parameters obtained from density functional theory (DFT) calculations were discussed with experimental results.

Phosphorylated chitin as a chemically modified polymer for ecofriendly corrosion inhibition of copper in aqueous chloride environment

Heterocyclic organic molecules containing oxygen, nitrogen or sulfur are generally used as inhibitors for corrosion protection of copper. These compounds form protective films by adsorption onto the metal surface. Nevertheless, most such inhibitors are biologically toxic, hence there is a need for development of ecofriendly inhibitors. Chitin is a nontoxic biological polymer that is known to be enzymatically biodegradable. In the present study chitin was chemically modified to phosphorylated chitin (PCT) to make it soluble in aqueous medium and increase its complexing ability with metal ions. The ability of this modified polymer to inhibit corrosion of copper in aqueous NaCl and the nature of the inhibition process were investigated by gravimetric analysis, electrochemical impedance spectroscopy, and polarization measurements. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) analysis were used to examine the surface morphology and analyze the chemical composition of the film formed on the metal surface. Maximum efficiency of 92 % was achieved by addition of 200 ppm PCT as inhibitor to an aqueous corrosive environment containing 200 ppm chloride at 30 ± 0.1 °C. SEM studies revealed that the smoothness of sample surface was retained on PCT addition to the aggressive medium. EDX and XPS studies on the surface film revealed the presence of PCT. A plausible mechanism for the inhibition by PCT is suggested.

Corrosion behaviour of triode plasma diffusion treated and PVD TiN-coated Ti–6Al–4V in acidified aqueous chloride environments

The corrosion resistance of triode plasma-nitrided and oxynitrided Ti–6Al–4V was investigated in a hydrochloric acid solution using potentiodynamic polarisation, cyclic voltammetry, and immersion testing at OCP. The untreated and the PVD TiN-coated alloy were used as benchmarks. When compared to the untreated alloy and a PVD-TiN coating, the diffusion treated alloys proved to be more durable due to the presence of a Ti2N compound layer. The corrosion barrier properties of the nitrided alloy depend on both thickness and integrity of the nitride compound layer; the thicker the layer, the better the performance in these tests.

The influence of organic structure and rare earth metal cation on the corrosion efficiency observed on AS1020 steel compared with La(4OHCin)3

Whilst the corrosion protection of steel in aqueous chloride environments by the rare earth inhibitor lanthanum 4-hydroxycinnamate is well known, the influence of the structural variation of the organic component as well as the nature of the metal centre has not previously been addressed. Herein we show that praseodymium 4-hydroxy cinnamate is comparable to its lanthanum counterpart in aqueous solution. On the other hand, cerium 4-hydroxycinnamate and lanthanum 2-hydroxycinnamate show poor corrosion protection performance while lanthanum 3-hydroxycinnamate provides a level of inhibition between these. These differences are shown to be related to the speciation in solution and are postulated to be linked to steric influences which are likely to affect the bonding environment within the rare earth compound itself, as well as its bonding with the steel substrate.

Nanoscopic view on the initial stages of corrosion of hot dip galvanized Zn–Mg–Al coatings

The onset and initial progress of atmospheric corrosion of ternary alloyed Zn–Mg–Al coatings induced by an aqueous sodium chloride environment was studied in detail by means of surface sensitive techniques, X-ray photoelectron spectroscopy and Auger electron spectroscopy. Two fundamental processes were found to be involved in the initial corrosion attack: (i) dissolution of the surface layer consisting of MgO and (ii) preferential anodic dissolution of both, binary and ternary eutectic phases constituting the coating, which led to the formation of characteristic cracks/holes and was followed by a predominant deposition of zinc corrosion products around anodic regions.

Effect of N and C on stress corrosion cracking susceptibility of austenitic Fe18Cr10Mn-based stainless steels

Stress corrosion cracking (SCC) susceptibility of austenitic Fe18Cr10Mn alloys with 0.3N, 0.6N and 0.3N0.3C was investigated in aqueous chloride environment using a slow strain rate test method. The SCC susceptibility of Fe18Cr10Mn alloys in 2M NaCl solution at 50°C under constant anodic potential condition decreased with increase in N content from 0.3 to 0.6wt%, and with addition of 0.3wt% C to the Fe18Cr10Mn0.3N alloys. The present study strongly suggested that the beneficial effects of N and C on the SCC behavior of Fe18Cr10Mn alloys would be associated with the resistance to pitting corrosion initiation and the repassivation kinetics.

Behaviour of Stress Corrosion Cracking of Austenitic Stainless Steels in Sodium Chloride Solutions

Click Stress Corrosion Cracking (SCC) is an environmentally well-known as a failure caused by exposure to a corroding while under a sustained tensile stress. SCC is most often rapid, unpredictable. Failure can occur in a short time as a few hours or take years and decades to happen. Most alloys are liable to SCC in one or more environments requiring careful consideration of alloy type in component design. In aqueous chloride environments austenitic stainless steels and many nickel based alloys are common to perform poorly. Stress corrosion cracking (SCC) of austenitic stainless steels of types 316 was investigated as a function of applied stress at room temperature in sodium chloride solutions using a constant load method. The experiment uses a spring loaded fixture type and is based on ASTM G49 for experiment method, and E292 for geometry of notched specimen. The stress dependence of fracture appearance and parameters of time to cracking, and cracking growth. The results are explained in terms of comparison between the two concentrations of sodium chloride solutions.

Corrosion behaviour of carbon fibres/magnesium metal matrix composite and electrochemical response of its constituents

The corrosion behaviour of carbon fibres reinforced magnesium metal matrix composite (MMC) was investigated with emphasis on the galvanic corrosion arisen between the magnesium matrix alloy and the carbon fibres. The susceptibility of carbon/magnesium interface to degradation was also ascertained. The corrosion behaviour was studied in both neutral and alkaline aqueous solutions containing different concentrations of NaCl, using electrochemical techniques, hydrogen evolution test, optical microscopy and scanning electron microscopy (SEM) coupled with EDX and WDX capabilities. Two matrix alloys were used, AS41 and AS41(0.5% Ca). The latter monolithic alloy had a significantly higher corrosion resistance marked by Ca-addition, but the presence of carbon fibres in MMCs invalidated the beneficial effect of Ca. Carbon, as an electrically conductive element, is electrochemically unstable despite its almost chemical stability in aqueous chloride environments. Therefore, the polarisation diagrams of MMCs were demonstrated in terms of the electrochemical activity of carbon fibres. This necessitated performing the electrochemical polarisation tests, individually, on both Mg MMC, monolithic magnesium matrix alloy and carbon fibres electrode.

Inhibition of Corrosion on AA2024-T3 by New Environmentally Friendly Rare Earth Organophosphate Compounds

Abstract The combination of rare earth metals (REM), such as Ce, La, Pr, or the less-refined mischmetal (consisting of a combination of REM) with dibutyl phosphate or diphenyl phosphate, provides a complex that shows excellent corrosion inhibition for AA2024-T3 (UNS A92024) in aqueous chloride environments. In some instances, it is equal to the inhibition provided by the chromate compounds, which are still used by the aircraft industry. The addition of these compounds as an inhibiting pigment in an epoxy coating also shows efficient suppression of filiform corrosion from scribes in the coating, under both alternate immersion and high-humidity conditions. Surface characterization of AA2024-T3 specimens immersed in solutions of these complex salts suggests that a film containing both the organic and rare earth components deposits on the alloy surface. The electrochemical data indicate that this film is acting as a mixed inhibitor, suppressing both the anodic and cathodic processes on the surface.

Corrosion properties of HVOF cermet coatings with bond coats in an aqueous chloride environment

WC–17Co coatings with Ni–5Al bonding layers were deposited on Al–7075 by HVOF spraying. The top-coat consisted of layers comprising tungsten carbide particles embedded in a Co(W,C) matrix of varied composition.The coated specimens were subjected to potentiodynamic polarization in 3.5% aqueous NaCl at 25, 35 and 45 °C. The coatings exhibited pseudopassivity caused by the oxidation of tungsten, carbon and possibly cobalt. Chronoamperometric measurements indicated that the inhomogeneous binder composition induced active corrosion processes taking place simultaneously with pseudopassivity. Cyclic polarization suggested that the coatings were not susceptible to pit corrosion in the temperature range of 25–45 °C. The likely “critical pitting” temperature of the coatings was 60 °C. Higher testing temperatures led to lower corrosion potentials and faster corrosion kinetics.

A Comparison Study of Stress Corrosion Crack Growth Rates for AA7XXX Alloys as a Function of Bulk Aqueous Chloride Concentrations

Abstract Alternative aluminum alloys are under consideration for drop-in replacement and refurbishment of corrosion and stress corrosion cracking prone AA7075-T6 (UNS A97075) components on aging military aircraft. An assessment of new materials for refurbishment and replacement of older generation, aging components is necessary for a viable life extension program. A quantitative database of stress corrosion cracking (SCC) properties of old generation materials compared to currently available materials is needed as a guideline/justification for part replacement. This effort evaluates AA7150-T7751 (UNS A97150) and AA7040-T7651 (UNS A97040), both which meet mechanical property requirements and are thought to be superior to AA7075-T6 in terms of corrosion performance. Specifically, stress corrosion crack growth rates for these plate alloys were documented and found to be similar in various bulk aqueous chloride environments and to represent a significant improvement over measured rates for AA7075-T651.