Adsorption of sodium dodecyl sulfate and sodium dodecyl phosphate at the surface of aluminium oxide studied with AFM

Within the framework of water-based aluminium pigments the difference in inhibition offered by the adsorption of sodium dodecyl sulfate (SDS) and sodium dodecyl phosphate (SDP) was studied by atomic force microscopy (AFM). While SDS did not show strong adsorption on aluminium oxide, SDP adsorbed strongly forming bilayers which patches did not change noteworthy over several hours of SDP exposure, indicating irreversible binding to the surface. Quantization of the surface coverage of the adsorbed SDP revealed that only 63% coverage was achieved, which shows surprisingly that a complete coverage is not necessary to inhibit oxidation of aluminium pigment particles.     

Corrosion resistance of lamellar aluminium pigments coated by SiO2 by sol–gel method

The aluminium pigments were coated with SiO₂ by sol–gel method to improve their stability. The effects of formulation factors, such as medium of reaction, adding sequence of catalyst and number of coating, were investigated. The stability of the coated aluminium pigments in acid was examined by measuring the hydrogen generation amount. It was found that the coating layer formation is due to the condensation of tetraethyl orthosilicate (TEOS) to form a dense 3D cross-linked layer on the surface of aluminium. The optimized sequence of adding catalysts would be hydrochloride first, then ammonia. Stability tests confirmed that the aluminium pigments have better corrosion resistance after coating with SiO₂.     

Chlorinated rubber paints for corrosion prevention of mild steel: A comparison between zinc phosphate and polyaniline pigments

The anticorrosion performance of plasticized chlorinated rubber coated mild steel sheets incorporating polyaniline emeraldine salt or zinc phosphate as active pigments were compared using salt spray and immersion in 3.5% NaCl solution. The results obtained by different electrochemical methods indicate the superiority of polyaniline in comparison with zinc phosphate in terms of corrosion protection. The time of the emergence of the first rust spot in 3.5% NaCl solution for the film containing 1.5 wt% polyaniline reach 960 h, which is six times higher than that of the film without polyaniline. The protective mechanism by zinc phosphate and polyaniline pigments were discussed.     

Preparation and characterisation of aluminium pigments coated with silica for corrosion protection

Aluminium pigments with a layer of silica were prepared by a sol–gel method using tetraethoxysilane as precursor and ethylenediamine as catalyst. Under the optimum conditions, the corrosion protection factor can reach 99.3% and average grain almost remains the same size after coating, indicating that the coated aluminium pigments have excellent chemical stability and good dispersibility. FTIR and EDS analyses demonstrate that a layer of silica coating has been formed on the flaky aluminium particle. SEM, AFM and BET analyses show that a smooth and dense silica coating layer has been formed.     

Die Eigenschaften chromathaltiger und phosphathaltiger Korrosionsschutzpigmente

The properties of anti-corrosion pigments containing chromate or phosphate Chromate pigments with chemically and electro-chemically active groups such as potassium zinc chromate, strontium chromate, zinc tetroxychromate have a great practical importance as anti-corrosion pigments. Their effect is due to their chemical, electrochemical and physical behaviour, i.e. to their capacity of forming passivating cover films on metal. This happens with their application to anti-corrosion primers. When the coating is exposed to moisture, chromium ions are generated in such a quantity that an optimum corrosion protection effect can be attained without incurring the risk that other useful properties are detrimentally affected by the solubility of the chromate in water. The chemical composition and physical condition of these pigments have reached a standard to progress which meets all the requirements for practical applications. Practical importance as an anti-corrosion pigment also attaches to phosphate pigments, especially zinc phosphate, if these are combined with suitable binders and if the fillers are carefully selected. The chemical behaviour of phosphate pigments is characterized by hydratation, when secondary products are formed which give rise to passivating metal phosphate films.     

Corrosion protection of aluminum pigments by sol-gel coatings

New sol-gel coatings for the corrosion protection of aluminum pigments were developed. To this end, the pigment particles are first coated with a silica layer by phosphoric acid-catalyzed sol-gel processing of Si(OEt)₄, to which either hexadecyltrimethoxysilane or dimethyldimethoxysilane is condensed. The coated pigments have excellent anticorrosive properties in alkaline solutions as well as under boiling water conditions.     

Electrochemical behaviour of carbon steel in acid chloride solution in the presence of dodecyl cysteine hydrochloride self-assembled on gold nanoparticles

In this work, the dodecyl cysteine hydrochloride surfactant was synthesized. The surface properties of this surfactant were studied using surface tension technique. The nanostructure of this surfactant with the prepared gold nanoparticles was investigated using TEM technique. The synthesized surfactant and its nanostructure with the prepared gold nanoparticles were examined as non-toxic corrosion inhibitors for carbon steel in 2 M HCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The results show that the percentage inhibition efficiency (η%) for each inhibitor increases with increasing concentration until critical micelle concentration (CMC) is reached. The maximum inhibition efficiency approached 76.6% in the presence of 175 ppm of dodecyl cysteine and 90.8% in the presence of the same concentration of dodecyl cysteine hydrochloride self-assembled on gold nanoparticles. Polarization data indicate that the selected additives act as mixed type inhibitors. The slopes of the cathodic and anodic Tafel lines (β_c and β_a) are approximately constant and independent of the inhibitor concentration. Analysis of the impedance spectra indicates that the charge transfer process mainly controls the corrosion process of carbon steel in 2 M HCl solution both in the absence and presence of the inhibitors. Adsorption of these inhibitors on carbon steel surface is found to obey the Langmuir adsorption isotherm. From the adsorption isotherms the values of adsorption equilibrium constants (K_ads) were calculated. The relatively high value of (K_ads) in case of dodecyl cysteine hydrochloride self-assembled on gold nanoparticles reveals a strong interaction between the inhibitor molecules and the metal surface.     

A two step anodizing process of aluminium as a means for improving the chemical and physical properties of oxide films

An anodizing process was developed to form corrosion resistant and hard oxide films on aluminium. The process consists of two steps: first the formation of chromate/phosphate treated layer on the surface of aluminium and secondly anodizing in a sulphuric acid solution. The anodic oxide films formed by the present process contain Cr(III) and phosphate species mostly in the outer part of the porous layer. The films formed by the present process provided a better corrosion resistance to the substrate aluminium from pitting in a chloride medium than the films formed by conventional anodizing and sealed in a boiling chromate solution. Further, Vickers hardness on the cross section of film increased compared with the films formed by conventional anodizing. This two step process can be developed to form other composite oxide films by using different treatments for the first step.     

Corrosion and electrochemical behavior of aluminium treated with high-temperature pulsed plasma in CsCl–NaCl–NaNO3 melt

Dense protective layers of aluminium corrosion products, whose composition depends on the oxidation temperature, are formed on the surface of aluminium treated with high-temperature pulsed plasma (HTPP) without visible remelting and then held in a chloride–nitrate melt in conditions of anodic polarization. Modification of aluminium treated with HTPP changes the properties of 20 μm layer under its surface and the oxide layer formed by such treatment has different morphology: it consists of smaller crystals and so has the other protective properties as compared with aluminium untreated by plasma.     

SVET and SECM imaging of cathodic protection of aluminium by a Mg-rich coating

The mechanism of cathodic protection of an aluminium substrate by a Mg-rich coating was investigated using localized techniques. Both scanning vibrating electrode techniques (SVET) and scanning electrochemical microscopy (SECM) were used to investigate the processes occurring at the surface of exposed metal when electrically connected or disconnected in a galvanic couple with the Mg-rich coating. The SVET has shown the evolution of the pit activity with time under conditions of sacrificial protection, whereas the SECM allowed indirect sensing of the cathodic activity above the electrodes. It was shown that the cathodic protection provided by magnesium to aluminium substrates acts by both preventing pit nucleation and inhibiting the growth of the pre-existing pits.     

Recent developments in corrosion inhibitors based on rare earth metal compounds

Abstract

Rare earth organic compounds can provide an environmentally safe and non-toxic alternative to chromates as corrosion inhibitors for some steel and aluminium applications. For steel lanthanum 4-hydroxy cinnamate offers corrosion protection and reduces the susceptibility to hydrogen embrittlement. Recent work has also indicated that it inhibits the corrosion of steel in environments containing high levels of carbon dioxide. For aluminium alloys, cerium diphenyl phosphate provides excellent corrosion inhibition in chloride environments, and reduces susceptibly to stress corrosion cracking. Furthermore, for both steel and aluminium alloys filiform corrosion can be suppressed when rare earth inhibitor compounds are added as pigments to polymer coatings. The levels of inhibition observed are thought to be due to synergistic effects between the rare earth and organic parts of these novel compounds, and are related to the various species that may be present in the complex chemical conditions that develop in solution close to a metal surface. This paper reviews some of the published research conducted by the group at Deakin University over recent years.     

Chlorhexidine digluconate as corrosion inhibitor for carbon steel dissolution in emulsified diesel fuel

Nitrogen oxide (NO_x) and particulate matter (PM) emissions from diesel engines are reduced by mixing water in the diesel fuel in the form of water-in-diesel emulsion. The results of experiment showed that blend of span 80 and tween 80 at HLB 6 was found to be the most suitable emulsifier for water/diesel emulsion. The effect of chlorhexidine digluconate on the corrosion of carbon steel electrode in aerated stagnant water/diesel emulsion solution has been studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The inhibition efficiency was found to increase with increasing inhibitor concentration. The inhibition is attributed to adsorption of the inhibitor on the carbon steel surface. The adsorption behavior of chlorhexidine digluconate on the carbon steel surface follows Temkin-type isotherm. The standard free energies of adsorption are lower than 40 kJ mol⁻¹ confirming the physical adsorption of chlorhexidine digluconate on the electrode surface. Chlorhexidine digluconate has a significant inhibiting effect on the growth of micro-organisms in the liquid fuel and protection efficiencies up to 99.9% were measured.     

Multilayered coatings: Tuneable protection for metals

The role of oxide bi-layers in controlling the onset of corrosion has been explored. A high-throughput electrochemical approach was employed to determine the breakdown potential of aluminium metal over-coated with combinations of silicon, titanium, aluminium and magnesium oxides. Bi-layered coatings consisting of two 100 nm thick metal oxide layers provided increased protection against breakdown, and combinations with vastly different iso-electric point of solid (IEPS) were found to exhibit improved barrier properties in comparison to single-component oxides. Furthermore, the most protective oxide bi-layers were produced when a high IEPS oxide was deposited directly onto the metal surface and subsequently over-coated with a low IEPS oxide. The barrier properties of bi-layer coatings appear to be tuneable, with notable dependencies on surface charge and thickness.     

Copolymere als Korrosionsinhibitoren für verschiedene Metallpigmente

Copolymers as corrosion inhibitors for different metal pigments The base aluminium and zinc pigments corrode in aqueous alkaline media with the evolution of hydrogen whereas the more noble copper and brass pigments react with the absorption of oxygen. Both types of corrosion reactions can be examined gasvolumetrically. The corrosion reactions of all examined metal pigments can be inhibited more or less by addition of styrene‐maleic acid‐acrylate copolymers with different acrylate monomers (ethyl‐ n‐butyl‐ and n‐hexylacrylate as well as n‐dodecyl‐ and n‐octadecylmethacrylate). A surprising result is that the oxygen corrosion of the brass pigment is inhibited more effectively by the copolymers when compared to the copper pigment. With respect to the interaction of the metals with the copolymers there are two groups of metal pigments. The first group is aluminium pigment only; the corrosion inhibiting effect of the copolymers increases with increasing side chain length of the acrylate monomer. The second group consists of the base zinc pigment and the more noble copper pigment; the corrosion inhibiting effect of the copolymers decreases with increasing side chain length of the acrylate monomer. These connections between the chemical composition of the copolymers and their corrosion inhibiting effect can be correlated mathematically with the help of potential functions.     

Inhibition by tetradecylpyridinium bromide of the corrosion of aluminium in hydrochloric acid solution

The inhibition effect of tetradecylpyridinium bromide (TDPB) on the corrosion of aluminium in 1.0 M HCl solution was studied by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) methods. The results show that TDPB is a very good inhibitor at little concentrations, and the adsorption of TDPB on aluminium surface obeys Langmuir adsorption isotherm. Polarization curves reveal that TDPB behaves as a cathodic inhibitor. EIS spectra consist of large capacitive loop at high frequencies followed by a large inductive one at low frequency values, and confirm the inhibitive ability.     

The role of environmental exposure in the fatigue behaviour of an aluminium alloy

This paper reports measurements of fatigue crack velocities for an aluminium alloy 7017-T651 in high purity gaseous environments with different values of water vapour pressure and frequency. Also a detailed fractographical analysis by SEM is presented and the fracture surface morphology is correlated with the testing parameters and with the crack stress intensity level. Within a range of environmental exposure (defined as the product between the pressure and the time available for surface reaction during one fatigue cycle) crack propagation rates depend linearly on the exposure. The experimental fatigue crack propagation data are satisfactorily explained in terms of a corrosion-fatigue model for gaseous environments.     

Influence of 1,2-diaminoethane on the mechanism of aluminium corrosion in sulphuric acid solutions

The influence of 1,2-diaminoethane (DAE) on aluminium corrosion in H₂SO₄ solutions (pH 3) was investigated. In pure H₂SO₄, rapid uniform corrosion is followed by inhibition due to the formation of stable Al-sulphate binuclear bidentate metal bound surface complexes via a ligand exchange mechanism with two neighbouring sites. Metastable pitting is also observed. DAE acts as a strong corrosion inhibitor for both uniform and localised corrosion, due to the formation of Al-DAE monodentate hydrogen-bond surface complexes either by direct adsorption of the protonated molecule on Al-OH sites or via a ligand exchange mechanism with the proton of an site.     

Diphenolic Schiff bases as corrosion inhibitors for aluminium in 0.1 M HCl: Potentiodynamic polarisation and EQCM investigations

The effects of novel synthesised two Schiff bases on the corrosion of aluminium in 0.1 M HCl were investigated using potentiodynamic polarisation and electrochemical quartz crystal microbalance measurements. Results show that inhibition efficiencies increase with increase in inhibitor concentration. This reveals that the inhibition occurs through adsorption of the inhibitor molecules on the metal surface. Adsorption of these inhibitors follows Temkin adsorption isotherm. The correlation between the inhibitor performances and their molecular structures has been investigated using quantum chemical parameters obtained by MNDO (modified neglect of diatomic overlap) semi-empirical method. Calculated quantum chemical parameters indicate that Schiff bases adsorbed on aluminium surface by chemical mechanism.     

Korrosionsinhibierung von Aluminiumpigmenten durch Ester der Gallussäure

Corrosion inhibition of aluminium pigments by esters of gallic acid Aluminium pigments corrode in mixtures of water and butyl glycol with the evolution of hydrogen, which can easily be measured gasvolumetrically. This corrosion reaction can be inhibited with addition of esters of gallic acid (gallates). The corrosion inhibiting effect of the gallates increases with increasing chain length of the ester alcohol; this connexion between the chemical structure of the inhibitors and the evolved volumes of hydrogen can be correlated mathematically with the help of an exponential function. The best inhibitor dodecyl gallate, which is an amphiphilic molecule with a chelating head group, showed protective values of about 99 %. To corroborate the assumption, that amphiphilic molecules with chelating head groups are in general effective inhibitors for the examined corrosion reaction, a second example, the amphiphilic agaric acid (α-hexadecyl citric acid) was tested in comparison to the head group (citric acid); again the amphiphilic molecule inhibited the corrosion reaction of aluminium pigment more effectively as the head group alone.     

Synergistic effect between cationic gemini surfactant and chloride ion for the corrosion inhibition of steel in sulphuric acid

Corrosion inhibition of cold rolled steel in 0.5 mol L⁻¹ sulphuric acid by a quaternary ammonium gemini surfactant, l,3-propane-bis(dimethyl dodecylammonium bromide) (designated as 12-3-12), in the absence and presence of chloride ions was investigated at different temperatures. The results revealed significant synergistic effect between gemini 12-3-12 and chloride ions for the corrosion protection of cold rolled steel in sulphuric acid, and that the novel composite inhibitor system containing cationic gemini surfactant and chloride ions was efficient and low-cost for steel corrosion inhibition in sulphuric acid medium, even when concentration of 12-3-12 was as low as 1 × 10⁻⁶ mol L⁻¹. By fitting the obtained experimental data with Langmuir adsorption model and Arrhenius equation, some thermodynamic and kinetic parameters such as adsorption free energy, the apparent activation energy, and the pre-exponential factor were estimated. The adsorption mechanism of the gemini surfactant onto steel surface in acid medium in the absence and presence of chloride ions was also discussed, respectively.