Mechanistic changes in cut-edge corrosion induced by variation of organic coating porosity

The scanning vibrating electrode technique (SVET) has been used to investigate the mechanism of corrosion occurring at exposed metallic cut edges of galvanised steel materials coated with identical thickness organic polymeric layers. Organically coated galvanised steel samples have been prepared by application of a 30 μm polyester coating to both sides of a pre-treated hot dip zinc (99.85%) aluminium (0.15%) galvanised steel substrate. In separate samples the coatings were applied to both sides simultaneously and then cured or applied sequentially to each side curing after each application. Electrical impedance spectroscopy (EIS) was used to probe the porosity of the resulting organic coatings. This revealed that simultaneous coating application and curing resulted in both polymer layers showing EIS pore resistance of 0.6 MΩ whereas sequential coating resulted in one side showing a pore resistance of 0.6 MΩ and the other showing a capacitive EIS response with no measurable porosity. When the metallic cut edges of organically coated steel materials, coated with symmetric thicknesses of organic layers of identical pore resistance, are exposed to 5% NaCl the SVET iso-current contour maps show that both zinc layers behaving as local anodes. In addition, there is significant sample passivation in the 24 h exposure period. In the samples coated with materials of identical thicknesses but different porosity, the SVET iso-current contour maps show anodic activity focussed beneath the organic coating with the capacitive EIS signature. The greater porosity of the coating on the opposite side of the exposed cut edge enables oxygen transport to the delaminating surface and this forces anodic activity onto the other zinc layer.     

Poly(ortho-ethoxyaniline) in corrosion protection of stainless steel

Corrosion protection of stainless steel (13% Cr) coated with poly(ortho-ethoxyaniline) (POEA) has been investigated. The layers of POEA were synthesised from sulphuric and phosphoric acid solutions by means of cyclic voltammetry. The protecting properties of the layers in supporting electrolytes were investigated by monitoring the open circuit potential (E_oc) vs. time, and by electrochemical impedance spectroscopy (EIS). It was found that polymer layers provide corrosion protection, i.e. they help to stabilise the potential of the metal in the passive potential region. The protective properties of POEA layers have shown to be superior to polyaniline (PANI) layers, which is explained by a denser morphology of POEA. The behaviour of POEA obtained on stainless steel, examined by EIS, is different from the one obtained for POEA on Pt electrode. The registered resistance in the case of POEA on stainless steel represents the charge transfer resistance at polymer/solution interface, i.e. the polymer resistance. These measurements show that a part of the layer is reduced, i.e. that there is an interaction between polymer layer and stainless steel. EIS measurements prove the influence of monomer on stainless steel oxide formation and suggest that polymer is partly incorporated into the oxide film.     

Role of thickness on electrochemical behaviour of Ni2Si coatings in NaCl solution

Abstract

Ni₂Si intermetallic coatings were deposited on a substrate of 420 stainless steel using a high velocity oxy-fuel (HVOF) process. The electrochemical corrosion behaviour of the coatings in 3.5% NaCl solution was investigated by means of Tafel polarisation tests and electrochemical impedance spectroscopy (EIS) measurements. Two coatings with different thicknesses, 55 and 115?μm, were investigated. The results showed that the corrosion rate of the thicker coating was lower than that of stainless steel by one order of magnitude. Local attack of the substrate was observed after the polarisation test of this coating, while the coating was still intact. The thinner coating and the stainless steel substrate showed similar corrosion rates. In this case, the substrate was severely attacked after the polarisation test. Two time constants were observed in the EIS spectra of both coatings which were related to charge transfer processes and pore resistance, respectively.     

Electrochemically deposited polypyrrole films and their characterization

Ionically conductive polypyrrole films have been deposited at 295 K from anhydrous acetonitrile, acetonitrile/H₂O and NaBF₄ aqueous solutions onto platinum, mild steel and stainless steel discs, using cyclic voltammetry, potentiostatic and galvanostatic techniques. Cyclic voltammetry of the polymer films has been studied as a function of water content of the acetonitrile solvent, polypyrrole concentration and potential sweep rate. Potentiostatic growth of thicker (< 30 micron) films on stainless steel allowed free-standing polypyrrole membranes to be produced. Well adherent and conductive films were deposited at constant potential in stirred solutions from acetonitrile electrolytes containing 1% (v/v) of water. The membrane resistivity of the reduced films in 0.5 mol dm^− 3 KCl_(aq) at 295 K was ≈ 1 × 10⁶ Ω cm, while the resistivity of the oxidised membrane was 2700 Ω cm.     

Pitting of AISI 304L stainless steel coated with nano structured sol–gel alumina coatings in chloride containing acidic environments

Sol–gel coatings were applied on AISI 304L stainless steel and their pitting resistance was evaluated in different concentrations of chloride ions in 0.01 M H₂SO₄ solution. AFM and XRD analyses of the coated surface confirmed the presence of nano structured α-alumina phase in the coating. Anodic polarisation indicated a considerable shift of pitting potential of the substrate in the noble direction after applying coatings. The SEM micrographs of the polarised specimens revealed the presence of relatively less number of pits on the coated surface. EIS analysis demonstrated the occurrence of diffusion process at the coating/metal interfaces in chloride ions presence.     

Combination of zinc alloy coating with thin plasma polymer films for novel corrosion protective systems on coated steel

Steel sheet used in automotive applications has to be corrosion protected effectively, which is usually realized by zinc or zinc alloy coatings with a thickness range of 5–10 μm. Steel sheet for areas of a car body which are exceptionally stressed by corrosion, e.g. cavity flanges or joints, may be protected additionally by a thin weldable organic coating with a thickness of 2–4 μm. A very promising approach to a significantly reduced use of resources is the combination of zinc alloy coatings with thin plasma polymer films deposited by means of plasma-enhanced chemical vapour deposition (PECVD). Such plasma polymer films of just a few 100 nm thickness show excellent barrier and adhesion properties as well as a high mechanical stability.Within this work thin plasma polymer films were deposited on zinc alloy coated steel substrates using the strip hollow cathode (SHC) method, which was modified for application on grounded substrates. A pulsed DC glow discharge in a mixture of argon and an organosilane precursor was used for the deposition of films with a thickness of 100–500 nm.The chemical compositions of the coatings were determined by means of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The morphologies of the coating systems were studied by means of scanning electron microscopy. The performance of the coating systems has been studied in different specific tests of corrosion and processing behaviour. The investigated coating systems show a corrosion resistance comparable to reference samples of electro-galvanised steel sheet with additional organic coating even with a coating thickness less of half of the reference samples.     

Behavior of Calcia-Stabilized Zirconia Coating at High Temperature, Deposited by Air Plasma Spraying System

Thermal barrier coatings (TBCs) are employed to protect metallic components from heat, oxidation, and corrosion in hostile environments. In this paper Ni-20Cr bond coat followed by CaZrO₃ top coat was deposited on 316 stainless steel substrates by air plasma spray coating technique. Isothermal treatment of coated samples was carried out to investigate the effect of heat exposure on the microstructure and metallurgical phase changes of TBCs system. The fractured surface of as-sprayed and delaminated CaZrO₃ coatings was also studied to observe the splats morphology, structural defects, and lamellas internal microstructure. CaZrO₃ coating was found to be stable for 100 h at 700 °C but accelerated degradation was observed at 900 °C even at 20 h and lead to delamination after 60 h of exposure time. Chromium rich oxide formation was found to be responsible for the complete delamination of the top coat. Further, the formation of meta-stable monoclinic phase was also observed on the top surface of the top coat.     

Poly(3‐octylthiophene) thermally treated as coating for corrosion protection of SS304 in sulfuric acid media

Poly(3‐octylthiophene) (P3OT) was synthesized by direct oxidation of the 3‐octylthiophene monomer using ferric chloride (FeCl₃) as an oxidant. Using the drop‐casting technique, P3OT coatings were deposited onto 304 type stainless steel electrodes. For the purpose of determining the effect of thermal annealing on the corrosion protection of stainless steel with P3OT coatings, the coated electrodes were thermally annealed for 30 h at two different temperatures, 55 and 100 °C. The corrosion behavior of P3OT coated stainless steel was investigated in 0.5 M sulfuric acid (H₂SO₄) at room temperature using potentiodynamic polarization curves (PPC), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). The results indicated that the thermally treated P3OT coatings improved the corrosion resistance of the stainless steel in 0.5 M H₂SO₄. The best corrosion protection was obtained by the P3OT coating annealed at 100 °C. In order to study the temperature effect on the morphology of the coatings before and after the corrosive environment and compare it with corrosion protection, atomic force microscopy (AFM) and scanning electronic microscopy (SEM) were used.     

Cyclic oxidation resistance of Ni–Al alloy coatings deposited on steel by a cathodic arc plasma process

The cyclic oxidation resistance of nickel-aluminide coatings deposited on steel using a cathodic arc plasma (CAP) process has been investigated. Our results show that nickel-aluminide films can be successfully deposited on carbon steel and stainless steel substrates by this process; NiAl₃ is the major phase in the deposited films. The thermal cycling behaviour suggests that such coatings can resist oxidation through physical blocking of oxygen, either by the coating itself or by the aluminium oxide scale subsequently formed in-service. Aluminium diffusion inwards to the substrate may also be beneficial to the thermal oxidation resistance. The coating protects stainless steel substrate materials at 500°C by transforming the NiAl₃ phase into NiAl, producing aluminium oxide on the open substrate surface. At 800°C, oxide flaking is suppressed by the trace amounts of nickel or aluminium which have partially diffused into the substrate.     

Electrochemical characteristics and interfacial contact resistance of multi-layered Ti/TiN coating for metallic bipolar-plate of polymer electrolyte membrane fuel cells

Metallic bipolar-plates have advantages over non-porous graphite ones due to their higher mechanical strength and better electrical conductivity. However, corrosion resistance and interfacial contact resistance are major concerns that remain to be solved, since metals such as stainless steels may develop oxide layers that decrease electrical conductivity, thus lowering fuel cell efficiency. In this study, multi-layered nitride coatings consisting of Ti and TiN were deposited on 316L stainless steel (SS316L) by a D.C magnetron sputtering method to enhance the corrosion resistance and to lower the interfacial contact resistance (ICR) of metallic bipolarplates for a polymer electrolyte membrane fuel cell (PEMFC). Electrochemical methods were conducted and ICRs of the coated specimens were measured to investigate the potential of the coated metallic bipolar-plate for use in PEMFCs. The multi-layered Ti/TiN coating deposited on SS316 showed lower ICR values than the single-layered TiN coating, and improved corrosion resistance when the PEMFC was not in operation while the degradation of the coating layer was observed in both cathodic and anodic working environments.     

The corrosion protection of mild steel by polypyrrole/polyphenol multilayer coating

Electrochemical synthesis of very thin polyphenol (PPhe) film was achieved on polypyrrole coated mild steel electrode (MS/PPy) and a multilayer coating was obtained, cyclic voltammetry technique was used for the synthesis. The corrosion performance of this multilayer coating and single PPy coating were investigated in 0.05 M H₂SO₄ solution by using electrochemical impedance spectroscopy (EIS), anodic polarization curves and open circuit potential (E_ocp)-time curves were used. It was found that the multilayer coating could provide much better protection than the single PPy coating for corrosion of MS for much longer periods and an efficiency of 98.3% was determined for this coating after 340 h exposure time in corrosive medium. It is proposed that the very thin PPhe film coated on top of PPy coating lowered the porosity and improved the barrier effect of the coating significantly.     

Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti-Al-Cr-Y-N and Ti-Al-Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti-45Al-8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD).Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti-Al-Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti-Al-Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti-Al-Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.     

Effect of electrolyte and monomer concentration on anticorrosive properties of poly(N-methylaniline) and poly(N-ethylaniline) coated mild steel

The electrosynthesis of poly(N-methylaniline) (PNMA) and poly(N-ethylaniline) (PNEA) coatings on mild steel in aqueous oxalic acid solutions was carried out by potentiodynamic synthesis technique. The effects of monomer and electrolyte concentrations on electrochemical growth of PNMA and PNEA coatings on mild steel substrates were investigated. Repassivation peak did not appear during electrosynthesis of PNMA and PNEA coatings from solutions containing 0.1 M monomer and 0.1 M electrolyte. The tests for corrosion protection of the polymer coated and uncoated mild steel substrates were done in 3% NaCl solutions by dc polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion tests revealed that PNMA and PNEA coatings exhibited effective anti-corrosive properties. The acidity of the polymerization solution was found to influence the anticorrosive behavior of the polymer coating.     

Poly(N-methyl pyrrole) and its copolymer with pyrrole for mild steel protection

In this study, it was aimed to overcome disadvantages of polypyrrole films like water up taking and resulting low stability, via copolymerization of pyrrole and N-methyl pyrrole and preparing their bilayer films. Poly(N-methyl pyrrole) coating was synthesized electrochemically on mild steel and its corrosion performance has been investigated in 3.5% NaCl aqueous solution, by using electrochemical impedance spectroscopy (EIS), anodic polarization curves and open circuit potential-time relation. This coating was found to have lower protection efficiency than single poly(pyrrole) coating. This case was related to weak adsorption behavior of poly(N-methyl pyrrole) film on mild steel. On the other hand, when this film was applied as a top coat on polypyrrole coated sample; the obtained coating system hindered the attack of corrosive environment and protected mild steel efficiently. The underlying poly(pyrrole) film was strongly adherent and the top poly(N-methyl pyrrole) coat improved barrier property by decreasing permeability and water mobility. Poly(pyrrole-co-N-methyl pyrrole) has also been synthesized from aqueous oxalic acid solutions containing various ratios of these two monomers. The copolymerization rate and protective behavior of copolymer were strongly affected by the monomer concentration ratio. The ratio of 1:3 (N-methyl pyrrole/pyrrole) gave the most protective copolymer coating and it exhibited better barrier property than single PPy. This was related to introduction of -CH₃ group which creates hydrophobic effect, when compared to -NH group of pyrrole unit. However, the solution with the ratio of 1:1 yielded a coating which had lower protection efficiency than single poly(pyrrole) film. This behavior was attributed to significant decrease in adsorption strength as the ratio of n-methyl pyrrole increased.     

Anticorrosive properties with catalytic behaviour of primer PANI film and top PPy coating synthesised in presence of novel norephedrine based amino alcohol compound

Polyaniline film as primer coating was deposited on stainless steel (SS) in aniline containing aqueous oxalic acid solution, and subsequent synthesis of top PPy film with 2-((5-ethylthiophen-2-yl) methylamino)-1-phenylpropan-1-ol (AAN) compound of different concentrations was achieved in acetonitrile-LiClO₄ successfully. The corrosion performances of coated and uncoated electrodes in 3·5%NaCl solution were evaluated with the help of AC impedance spectroscopy, anodic polarisation plots and open circuit potential time curves. The protective effect of bilayer coatings with AAN compound on SS electrode grew in parallel with extended exposure time. The regular increase in the charge transfer resistance of SS/PANI/PPy-AAN17 electrode was attributed to allowing the limited ion diffusion of top PPy coating with AAN17 compound. The high 21764 Ω s^?1/2 value of the Warburg coefficient showed that PPy-AAN17 film on the SS/PANI coating led to the formation of protective oxide layers due to the catalytic behaviour of PANI film.     

EIS study on degradation of polymer-coated steel under ultraviolet radiation

Degradation of heavy-duty steel coatings when exposed to ultraviolet (UV) radiation was investigated by electrochemical impedance spectroscopy (EIS) and additional techniques in order to clarify the feasibility of evaluation of the UV degradation by EIS. Two coatings were considered: polyester-urethane topcoat plus epoxy primer (PU/E) and epoxy topcoat plus epoxy primer (E/E). Each was applied to a steel substrate and exposed to cyclic wetting-drying under UV radiation. The PU/E coating developed topcoat cracks but did not delaminate from the substrate; capacitive behaviour was evident, and corrosion of the underlying steel was not observed. The E/E coating showed topcoat chalking and partial disappearance, exposing the primer, but corrosion of the underlying steel was not observed. The morphology and chemical changes were compared with the results of EIS.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

Zinc modified polypyrrole coating on mild steel

Polypyrrole (PPy) films (∼ 1.7 μm thick) have been electrodeposited on mild steel (MS) substrates from 0.1 M pyrrole containing aqueous oxalic acid solution, by using cyclic voltammetry technique. Then, the polymer coatings were modified with deposition of zinc particles (∼ 1 mg/cm²), at a constant potential value of − 1.20 V in 0.2 M ZnSO₄ solution. The corrosion performance of zinc modified PPy coating has been investigated in 3.5% NaCl solution, by using electrochemical impedance spectroscopy and anodic polarisation curves. Also, the corrosion behaviours of zinc modified PPy coated platinum and single PPy coated MS samples have been investigated, for comparison. It was shown that zinc modified coating exhibited very low permeability and provided important cathodic protection to MS for considerably long immersion period. The voluminous zinc corrosion products are formed during exposure time in aggressive solution, giving rise to a blocking effect on the porous structure and led to effective barrier behaviour of zinc modified PPy coating, even after 96 h of exposure time to corrosive solution.     

Corrosion and surface study of sputtered Al–W coatings with a range of tungsten contents

The corrosion behaviour of sputter-deposited, Al–W coatings with three different concentrations of tungsten (0.6, 3.5 and 11 at.% W) on AA7075 aluminium alloy substrates was investigated in chloride media using EIS, salt-fog tests and surface analyses. The surface analyses were performed with XPS/AES, SEM/EDS and TEM microscopy. The presence of W reduces the thickness of the oxide layer formed after the EIS test, as proved by the compositional depth profile. From the EIS data of the investigated Al–W_x coatings, the polarisation resistance (R_p) and the capacitive behaviour as a function of the immersion time were obtained. A small increase in R_p suggested improved corrosion properties over time. However, the extent of the improvement depends on the content of W and the coating-surface morphology (the presence of growth defects on the coating surface). The capacitive behaviour observed at high frequencies was related to the dielectric properties and the thickness of the barrier oxide film formed. The salt-fog tests to some extent supported the results from the EIS measurements. However, there was a difference between the corrosion EIS and salt-fog test results in the case of the Al₈₉W₁₁ coatings. The thickest oxide layer and the “layering” into the three regions (porous, semi-porous and intact) were observed when the salt-fog corrosion test was performed on the Al₈₉W₁₁ coating. Galvanic corrosion, probably due to the local compositional and structural inhomogeneities, takes place. However, this was not unambiguously observed during the EIS corrosion test.     

Pack Cementation Coatings for High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450???m each, the titanized coating consisted of two layers with a thickness of 100???m each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200???m each. The XRD investigation of the coatings showed that the aluminized coating consisted of Al₂O₃, AlCr₂, FeAl, and Fe₃Al phases; the titanized layers contained TiO₂, Ni₃Ti, FeNi, and Fe₂TiO₅ phases; and the two-step coating contained AlNi, Ti₃Al, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050?°C for 100?h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones.