Atmospheric corrosion of Galfan coatings on steel in chloride-rich environments

Galfan coatings on steel in laboratory exposures with predeposited NaCl and cyclic wet/dry conditions exhibit nearly the same corrosion products as after 5 years of marine exposure. A general scenario for corrosion product evolution on Galfan in chloride-rich atmospheres is proposed. It includes the initial formation of ZnO, ZnAl₂O₄ and Al₂O₃ and subsequent formation of Zn₆Al₂(OH)₁₆CO₃⋅4H₂O, and Zn₂Al(OH)₆Cl⋅2H₂O and/or Zn₅Cl₂(OH)₈⋅H₂O. An important phase is Zn₆Al₂(OH)₁₆CO₃⋅4H₂O, which largely governs the reduced long-term zinc runoff from Galfan. A clear influence of microstructure could be observed on corrosion initiation in the slightly zinc-richer η-Zn phase adjacent to the β-Al phase.     

Cerium chemical conversion coatings for corrosion protection of stainless steels in hot seawater environments

In order to replace the hazardous chromate‐based surface treatment, a new cerium chemical conversion coating was developed on 316L stainless steel through a mixed solution of hydrated cerium nitrate, citric acid, and hydrogen peroxide. The chemical composition was characterized by energy‐dispersive spectroscopy, X‐ray photoelectron spectroscopy and atomic force microscope. The dense conversion coating is composed of CeO₂ with a small amount of Ce₂O₃ and has small grain size lower than 50 nm. Its thickness is about 47.4 nm as determined by spectroscopic ellipsometry analysis. Potentiodynamic polarization was used to study the corrosion behavior of the coatings in the concentrated artificial seawater at 72 °C. In comparison with the conventional nitric acid‐chromate passivated specimens, the cerium conversion coatings show much higher pitting potentials. It is suggested that the cerium conversion treatment is more effective than the nitric acid‐chromate passivation to improve the pitting resistance of 316L stainless steel used in the hot seawater environments.     

Polybenzoxazine/SiO2 nanocomposite coatings for corrosion protection of mild steel

A series of nanocomposite coatings (PBS) consisting of silane functional polybenzoxazine (PB-TMOS) and SiO₂ nanoparticles were developed for corrosion protection of mild steel. The influence of silica content on corrosion resistance of PBS coatings was investigated by electrochemical measurements. The surface chemistry of nanoparticles and its effect on morphology of the PBS coating was also studied utilizing Fourier Transforms Infrared Spectroscopy, ²⁹Si Nuclear Magnetic Resonance and Scanning Electron Microscopy analyses. The results indicate that the presence of the covalent bond between nanoparticles and PB-TMOS, greatly improves the interfacial interactions at the polymer/filler interfaces resulting in a better corrosion performance.     

Spectroscopic identification of protective and non-protective corrosion coatings on steel structures in marine environments

Corrosion research, and the need to fully understand the effects that environmental conditions have on the performance of structural steels, is one area in which Mössbauer spectroscopy has become a required analytical technique. This is in part due to the need to identify and quantify the nanophase iron oxides that form on and protect certain structural steels, and that are nearly transparent to most other spectroscopic techniques. In conjunction with X-ray diffraction and micro-Raman analyses, the iron oxides that form the rusts on steels corroded in different marine and other environments can be completely identified and mapped within the rust coating. The spectroscopic analyses can be used to determine the nature of the environment in which structural steels have been, and these act as a monitor of the corrosion itself. Mössbauer spectroscopy is playing an important role in a new corrosion program in the United States and Japan in which steel bridges, old and new, are being evaluated for corrosion problems that may reduce their serviceable lifetimes. Mössbauer spectroscopy has been used to characterize the corrosion products that form the protective patina on weathering steel, as well those that form in adverse environments in which the oxide coating is not adherent or protective to the steel. Mössbauer spectroscopy has also become an important analytical technique for investigating the corrosion products that have formed on archeological artifacts, and it is providing guidance to aid in the removal of the oxides necessary for their conservation.     

Low-temperature titania coatings for aluminium corrosion protection

TiO₂ coatings on AA6082 aluminium alloy were obtained at low temperature (80 and 100°C) by the sol–gel dip-coating technique starting from titanium tetra-isopropoxide solution in ethyl alcohol. The preparation was carried out in the presence of acetic acid with both functions of catalyst and chelating agent. The curing temperatures used for these coatings are between 80 and 100°C, low enough to make such coatings suitable to incorporate additives such as organic inhibitors or polymeric nanoparticles. The coated samples were characterised by scanning electron microscopy and energy-dispersive spectroscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy (EIS). Compact coatings with a thickness of 500?nm, consisting of amorphous and nearly stoichiometric titanium dioxide, were obtained. EIS results revealed an effective corrosion protection of the substrate for more than 120?h of immersion in 3.5?wt-% NaCl aqueous solution.     

Microbiologically influenced corrosion of underground pipelines under the disbonded coatings

The microbiologically influenced corrosion (MIC) of underground pipeline was identified. Corrosion was typically found under the disbonded area of girth welds coating where cathodic protection (CP) current could not penetrate. Sulfate reducing bacteria (SRB) and fermentative acid producing bacteria (APB) were confirmed as the microbes involved in the corrosion process. This corrosion occurred despite the fact that the CP level was well below the criteria of -850 mV (Cu/CuS0₄). Detailed field surveys and laboratory analysis revealed the presence of high numbers of microbes at these sites, metallurgical and mineralogical fingerprints of MIC. The results indicate that the synergistic effects of disbonding, the ineffective input of CP current under the disbonded coatings, and environments favorable to bacterial growth were the corrosion mechanism.     

Development of coatings for protection in specific high temperature environments

Metallic and intermetallic coatings are widely used in jet engines and land-based gas turbines for oxidation and corrosion protection in the hotter parts of the engines. However there is a significant number of industrial processes where the use of protective coatings at high temperatures could contribute to a significant extension of life-time or an increase in operation temperature and thus efficiency. Examples of such industries are incineration and gasification of waste, biomass and coal, chemical process industries and petrochemical plants where highly aggressive environments are encountered containing species of e.g. carbon, chlorine, sulphur, vanadium or alcalines. Since most of these process environments contain only very low oxygen partial pressures or exhibit high concentrations of extremely aggressive compounds, the conventional, uncoated materials come to their limits. In recent years in laboratory work a number of new types of coatings have been developed for high-temperature applications which include diffusion coatings, overlay coatings and nanotechnological approaches for sealing porosity. In the paper the background of this development and the thermodynamic fundamentals are discussed together with some more recent solutions based on synergistic effects of multi-element coatings. Some results of performance tests of these coatings in sulfidizing, carburizing, chloridizing and vanadate environments will be presented. At the end conclusions can be drawn on the suitability of the different types of coatings for their specific applications.     

Formulation and evaluation of nano-structured polymeric coatings for corrosion protection

The effect of the addition of multi-walled carbon nano-tubes (MWCNT) to epoxy and vinyl chloride/vinyl acetate copolymer coatings on their ability to protect the substrates was studied. Coatings were formulated from these resins with and without MWCNT reinforcement. Steel substrates were prepared and coated with each formulated coating and submerged in 5% NaCl solution to study their corrosion resistance by means of Electrochemical Impedance Spectroscopy (EIS). In addition, thin films from these polymers, with and without nano-reinforcement, were cast. Dogbone specimens were cut in order to study their mechanical properties. Some of these specimens were immersed in the NaCl solution for two weeks in order to compare their mechanical properties with samples not exposed to salt water. Optical microscopy was used to capture the progress of sample corrosion. EIS measurements showed that the addition of MWCNTs to epoxy and vinyl chloride/vinyl acetate copolymer (VYHH) coatings increased their charge transfer resistance in comparison with the neat coatings. This is an indication of the enhanced corrosion protection of the nano-coatings. In addition, mechanical strength tests, both before and after immersion in 5% NaCl solution, showed that thin films from both epoxy and VYHH resins containing MWCNTs had improved strength, an indication of an improvement in the coatings' cohesive properties.     

Electrodeposition of Zn–Ni coatings as Cd replacement for corrosion protection of high strength steel

Electrodeposition of Zn–Ni coatings performed in acidic baths are not suitable for high strength steels due to their high susceptibility to hydrogen embrittlement.In this work, Zn–Ni coatings were deposited on a high strength steel (4340) upon stirring conditions from an alkaline bath. A complete characterisation of the coatings (corrosion, morphology and composition) has been accomplished, correlating the electrodeposition conditions with these features. The best protective properties of the grown coatings were achieved for the alloys with a single phase structure of γ-Ni₅Zn₂₁ and a denser morphology. Additionally, the hydrogen content incorporated is lower than even cadmium-coated 4340 steel which has undergone a postbaking dehydrogenation treatment.     

Enhanced corrosion protection by powder coatings on hot rolled steel (HRS) for high-temperature applications

Hot rolled steel (HRS) has long been a major product to the motor industry for bodywork, as galvanised steel (zinc coatings), and it is widely used in building and as tinplate (including tin and chrome oxide coatings) for food, etc. can-making. Among more specialist uses its moderate production costs makes it suitable for the manufacturing process. In this study, newly developed corrosion protection enhanced silicone based powder coating was evaluated on hot rolled steel (HRS). The powder coating in this study was developed to withstand high temperatures up to 550°C without any degradation. In the study silicone resin was incorporated with fillers, corrosion inhibitors and fibres at various compositions to achieve maximum heat resistance and improved corrosion resistance. Protective behaviours of the differently formulated powder paints were investigated before and after heat exposure. The developed paint system was evaluated using various test methods such as heat resistance, salt spray, Electrochemical Impedance Spectroscopy (EIS), adhesion test and microstructure study using SEM. The evaluation of test results revealed that powder coating combining zinc dust particles and mineral fibres have better performance due to better corrosion and adhesion stability in the corrosive environment before and after heat exposure up to 550°C.     

Effect of plasma nitriding of electroplated chromium coatings on the corrosion protection C45 mild steel

Plasma nitriding is a promising posttreatment technique to create a nitride layer on electroplated chromium coatings for improving their corrosion resistance. In the present study, the effects of plasma nitriding on the corrosion properties of electroplated chromium/C45 mild steel were investigated using electrochemical characterization. The chromium plated samples were nitrided using a pulsed direct current glow discharge in an NH₃ atmosphere. The polarization curve measurement results showed that the plasma nitrided samples exhibited more positive corrosion potentials (E_corr), smaller corrosion currents (I_corr), and evident passivation when compared with unnitrided chromium plating/substrate system. The high value of E_corr and low value of I_corr imply an improvement of the corrosion resistance of the coating/substrate system after plasma nitriding.     

Novel cationic gemini surfactants as corrosion inhibitors for carbon steel pipelines

In the present investigation novel cationic gemini surfactants namely: bis(p-(N,N,N-octyldimethylammonium bromide)benzylidene)benzene-1,4-diamine (I), bis(p-(N,N,N-decyldimethylammonium bromide)benzylidene)benzene-1,4-diamine (II), and bis(p-(N,N,N-dodecyldimethylammonium bromide)benzylidene)benzene-1,4-diamine (III) were synthesized, characterized, and tested as corrosion inhibitors for carbon steel in 1 M HCl solution. The corrosion inhibition efficiency was measured by using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and weight loss methods. The obtained results showed that, the synthesized inhibitors are excellent inhibitors for carbon steel in 1 M HCl solution. The inhibition efficiency decreased in the temperature range 30-40 °C and then increased in the temperature range 40-60 °C. The prepared inhibitors act as mixed inhibitors. Thermodynamic and activation parameters were discussed. Adsorption of the synthesized inhibitors was found to follow the Langmuir’s adsorption isotherm. Mixed physical and chemical adsorption mechanism is proposed. The morphology of carbon steel samples was investigated by scanning electron microscopy (SEM).     

Inhibition of steel corrosion by polyaniline coatings

Polyaniline (PANI) coatings were electrosynthesised on steel samples (13% and 4.44% Cr) using sulphuric and phosphoric acids as supporting electrolytes. Protective properties of PANI coatings in the supporting electrolytes were investigated by monitoring the open-circuit potential vs. time, and by applying electrochemical impedance spectroscopy. PANI layers have been found to provide corrosion protection. Thicker PANI layers at 530 mV vs. Ag/AgCl (3 mol dm⁻³ KCl) exhibit pure capacitive behaviour at low frequencies, and in addition a small resistance at high frequencies. Thinner layers at 530 mV exhibit a much higher resistance attributed to a higher degree of PANI-free electrode surface and/or to a significant amount of PANI transformed from emeraldine to leucoemeraldine form. The layer deposited in a phosphate solution appears to have better protective properties than the layer deposited in a sulphate solution. Therefore, PANI from phosphate solution was tested also in 0.1 mol dm⁻³ HCl. However, in the chloride-containing solution, the time of protection was significantly shorter.     

Chromate replacement in coatings for corrosion protection of aerospace aluminium alloys

Mixed rare earth organophosphates have been investigated as potential corrosion inhibitors for AA2024‐T3 with the aim of replacing chromate‐based technologies. Cerium diphenyl phosphate (Ce(dpp)₃) and mischmetal diphenyl phosphate (Mm(dpp)₃) were added to epoxy coatings applied to AA2024‐T3 panels and they were effective in reducing the amount and rate of filiform corrosion in high humidity conditions. Ce(dpp)₃ was the most effective and characterisation of the coating formulations showed approximately a factor of 5 reduction in both the number of corrosion filaments initiated as well as the length of these. Mm(dpp)₃ appeared to reduce the corrosion growth rate by a factor of 2 although it was the more effective inhibitor in solution studies. Spectroscopic characterisation of the coatings indicated that the cerium based inhibitor may disrupt network formation in the epoxy thus resulting in a coating that absorbed more water and allowed greater solubilisation of the corrosion inhibiting compound.     

Effects of alternating current interference on cathodic protection potential and its effectiveness for corrosion protection of pipelines

In this work, the effect of alternating current (AC) interference on cathodic protection (CP) potential on a X65 steel in a near-neutral pH bicarbonate solution was investigated, and the CP performance under AC was evaluated by weight-loss measurements. The CP potential applied on the steel cannot be maintained in the presence of AC interference. The shift of the CP potential depends on the applied CP level and AC current density. No matter if the direct current potential of the steel is shifted negatively or positively upon application of AC, the steel suffers from increased corrosion. The AC decreases the effectiveness of CP for corrosion protection. The CP standard at ?0.850?V (copper sulphate electrode) that does not consider the AC interference is not appropriate for AC corrosion protection.     

Preparation of MgO coatings on magnesium alloys for corrosion protection

MgO coating is formed on magnesium alloy by anodic electrodeposition in 6 M KOH solution, whereas Mg(OH)₂ coating is produced by anodization in 10 M KOH solution, which could be successively converted to MgO by calcination in air at 450 °C. The evolution of morphology, structure and composition of anodic film obtained on Mg alloy is investigated using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray diffraction (XRD). Potentiodynamic polarization measurements show that the as-grown MgO protective coatings are very effective in improving the corrosion resistance of magnesium alloy compared to bare metallic magnesium.     

Nanofabricated multilayer coatings of Zn-Ni alloy for better corrosion protection

As an effort to increase the corrosion resistance of conventional monolayer Zn-Ni alloy coating, the multilayer Zn-Ni alloy coating have been done electrolytically on mild steel (MS), using gelatin and glycerol as additives. Multilayered, or more correctly composition modulated multilayer alloy (CMMA) coatings have been developed using square current pulse. Successive layers of alloys, in nanometric scale having alternately changing composition were fabricated by making the cathode current to cycle between two values, called cyclic cathode current densities (CCCD’s). The coatings having different configuration, in terms of composition and thicknesses of individual layers were developed and their corrosion performances were evaluated by electrochemical methods. The corrosion rate (CR)’s were found to decrease drastically with progressive increase in number of layers (up to 300 layers), and then increased. The coating configurations have been optimized for best protection against corrosion. The CMMA Zn-Ni coating having 300 layers was found to be about 37 times more corrosion resistant than corresponding monolayer alloy, developed from same bath for same time. High protection efficacy of the coatings were attributed to alternate layers of alloys having different surface structure and composition, supported by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) study, respectively. Optimization procedure has been explained, and results are discussed.     

Comparison between polyaniline‐phosphate and polypyrrole‐phosphate composite coatings for mild steel corrosion protection

Polyaniline, polypyrrole and their composites have been deposited on mild steel electrode from aqueous oxalic acid solution by using cyclic voltammetry (CV) technique. The resulting coatings were characterized by scanning electron microscopy and EDX analysis. The protective performance against corrosion of these coatings is evaluated by using electrochemical impedance spectroscopy (EIS) technique in corrosive solution (3.5% NaCl). Impedance data indicates a noticeable enhancement of protection against corrosion processes. Also, comparison of polyaniline‐phosphate and polypyrrole‐phosphate coatings indicates better corrosion protection with polyaniline‐phosphate coatings.     

Crevice and under deposit corrosion resistance of titanium alloys in highly aggressive environments

The outstanding resistance of titanium to corrosion in a wide range of aggressive conditions has been confirmed by successful applications in the chemical and process industry, (CPI), over many years. Crevice and underdeposit corrosion threshold temperatures frequently set the limits for wider application. Research to extend the capabilities of titanium and its alloys in extremes of temperature, stress and corrosion has pursued two objectives: 1. To reduce the cost of enhanced crevice corrosion resistance using palladium. 2. To provide higher strength alloys with improved crevice corrosion resistance. The first section of this paper details the range of enhanced corrosion resistance alloys containing nominally 0.05 % palladium, (ASTM Grades 16 and 17); and the measures taken to confirm that the homogeneity of composition is achieved in standard 4500 kilo ingots, now being melted for production of a full range of mill products. The second section covers the assessment and modification of alloys and the development of new alloys for specific performance goals under high stress in hostile environments. Applications are described in the offshore oil and gas extraction, desalination plant and equipment and flue gas desulphurisation plant. Reference is made to the challenges presented by emerging technologies for the safe disposal by wet oxidation of toxic substances and process effluents.