The effect of heat treatment temperature on the structure and barrier performance of a zirconia coating electrodeposited by pulse current

Zirconia coatings were prepared by heat-treating the electrodeposited zirconium hydroxide produced by pulse current on 316L stainless steel. The results showed that a coating with amorphous structure obtained after heat treating at 200 °C, reveals no corrosion barrier performance. Heat treating at 400 °C resulted in zirconia coating with nanocrystalline/amorphous structure, which reveals a strong corrosion barrier performance. This coating shows a high value of pore resistance and Warburg behaviour in electrochemical impedance spectroscopy. As the temperature was raised to 600 °C, an entirely nanocrystalline structure was found. However, this coating revealed muddy shape cracks, with inferior corrosion barrier performance.     

Corrosion characteristics of polyaniline-coated 316L stainless steel in sulphuric acid containing fluoride

The corrosion resistance of conducting polyaniline (PANi) coatings deposited on 316L stainless steel (316L SS) at various cycle numbers of cyclic voltammetry (2-, 3- and 4-cycles) by electro-polymerization in sulphuric acid solution containing fluoride was investigated by electrochemical techniques. The corrosion resistance of the 316L SS substrate was considerably improved by the PANi coating. The increase of the cycle number of cyclic voltammetry increased the thickness and enhanced the performance of the PANi coating due to low porosity.     

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.     

Effects of Ti/TiN multilayer on corrosion resistance of nickel-titanium orthodontic brackets in artificial saliva

The effects of multilayered Ti/TiN or single-layered TiN films deposited by pulse-biased arc ion plating (PBAIP) on the corrosion behavior of NiTi orthodontic brackets in artificial saliva are investigated. The multilayered Ti/TiN coating is found to exhibit a greater free corrosion potential, much lower passive current density, and no breakdown up to 1.5 V. Moreover, electrochemical impedance spectroscopy (EIS) results indicate that the multilayered Ti/TiN coating has a larger impedance and lower porosity which is believed to be responsible for the exceedingly low metal ion release rate during 720 h exposure in the test solution. Visual inspection of the surfaces reveals different corrosion processes for the TiN and multilayered Ti/TiN coatings.     

A study on mechanism of corrosion protection of polyaniline coating and its failure

Polyaniline (PANI) coatings were electrochemically deposited on substrates of stainless steel and platinum in solutions of 0.2 M H₂SO₄ and 0.1 M aniline by cyclic voltammetry. The corrosion protection of the PANI coatings and their failure were investigated in 0.2 M H₂SO₄ solution. It was observed that the corrosion protection ability of the coating to steel substrate was increased with the increase of the coating thickness. The corrosion protection ability was mainly attributed to the passivating effect of PANI due to its oxidizing ability in its emeraldine state. During its operation, the PANI coating in emeraldine state tended to gradually lose its corrosion protection ability. This gradual failure of the PANI coating, but faster than expected, was confirmed to be related to a gradual reduction of the emeraldine PANI and a gradually increased resistance between the PANI coating and the stainless steel substrate. These findings lead to a new mechanism for the corrosion protection of PANI coating and its failure.     

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.     

Effects of plastic strain of diamond-like carbon coated stainless steel on the corrosion behavior in simulated body fluid environment

We have investigated the effect of plastic deformation of diamond-like carbon (DLC) coated and uncoated stainless steel on the corrosion resistance in a simulated body fluid environment to measure its protective efficiency as a biomedical coating material. We deposited the DLC film on 304 stainless steel specimens by radio frequency plasma assisted chemical vapor deposition(R.F.-PACVD) method, followed by a tensile test to apply plastic strain on the coated specimen. Corrosion behavior in the simulated body fluid environment was studied by a potentiodynamic polarization test. As the tensile deformation progressed, the cracks of the film were observed to be perpendicular to the tensile axis. Further deformation increased both cracking and the spallation. Estimated porosity and corrosion current density increased, and thus the protective efficiency decreased at a strain of 2%. In spite of the degradation, the anti-corrosion properties were significantly improved relative to the uncoated stainless steel. However, a significant increase in porosity and corrosion current density was observed at a strain of 4%. This study showed that increasing the thickness of the Si interlayer of film improved the corrosion resistance with reduction of spallations and cracks.     

Corrosion performance of HVOF and APS thermally sprayed NiTi intermetallic coatings in 3.5% NaCl solution

Amorphous/nanocrystalline Ni-Ti powders produced by low energy mechanical alloying were used as feedstock to deposit NiTi intermetallic coatings on 316L stainless steel substrate using high velocity oxy-fuel (HVOF) and air plasma spraying (APS) processes. Electrochemical impedance spectroscopy (EIS) and polarization tests indicated that the corrosion performance and passive behaviour of HVOF coating were far better than those of APS coating. The study also showed that the solution had penetrated through the coating microcracks and caused interior corrosion of APS coating, while the HVOF coating was immune from interior corrosion attack and consequently exhibited a good passive behaviour during long-term immersion.     

Determination of the critical flow velocities for erosion–corrosion of passive materials under impingement by NaCl solution containing sand

Transition from passivation to erosion–corrosion at the critical flow velocity (CFV) is of interest for corrosion engineers. This transition phenomenon on 304 stainless steel and a Fe-based amorphous coating was investigated under impingement by sand-containing NaCl solution. Mass loss measurement, open-circuit potential monitoring and potentiostatic test were employed to determine the CFV. The CFV values defined via the three approaches are consistent, about 10 m/s for 304 stainless steel and 15 m/s for the coating. The CFV decreases with increasing the sand concentration. It may serve as an effective tool for evaluating the erosion–corrosion resistance of passive materials.     

Depassivation-repassivation behavior of type-312L stainless steel in NaCl solution investigated by the micro-indentation

Repassivation behavior of type-312L stainless steel containing 6% of molybdenum was examined in NaCl solution using in situ micro-indentation technique, together with type-304 and 316L stainless steels. High stability of the passive film formed on the type-312L stainless steel was also examined by depth profiling analysis of passive films using glow discharge optical emission spectroscopy (GDOES). In 0.9 mol dm⁻³ NaCl solution at 296 K the type-304 and 316L stainless steels are passive only up to 0.3 V (SHE), above which pitting corrosion occurs. In contrast, no pitting corrosion occurs on type-312L stainless steel. Despite the significant difference of the pitting corrosion resistance, the repassivation kinetics of the three stainless steels, examined by micro-indentation at 0.3 V (SHE), is similar. The presence of molybdenum in the stainless steel does not influence the repassivation kinetics. The charge required to repassivate the ruptured type-312L stainless steel surface increases approximately linearly with the potential, even though the passivity-maintaining current increased markedly at potentials close to the transpassive region. Repassivation occurs without accompanying significant dissolution of steel, regardless of the stability of passive state. Depth profiling analyses of the passive films on the type-312L stainless steels formed at several potentials revealed that molybdenum species enrich in the outer layer of the passive film, below which chromium-enriched layer is present. The permeation of chloride ions may be impeded by the outer layer containing molybdate, enhancing the resistance against the localized corrosion of the type-312L stainless steel.     

Corrosion behaviour of AISI type 304L stainless steel in nitric acid media containing oxidizing species

The corrosion behaviour of AISI type 304L stainless steel (SS) in different concentration of 0.01 M, 1 M and 5 M HNO₃ in presence of oxidizing ions at different temperatures has been evaluated. The main objective of this study is to assess the corrosion resistance of type 304L SS in non-radioactive conditions encountered during storage of liquid nuclear waste. Electrochemical impedance spectroscopy (EIS) and laser Raman spectroscopy (LRS) has clearly brought out the deleterious effect of oxidizing species on the passive film leading to increased corrosion along with increase in HNO₃ concentration and higher temperature.     

Behaviour in hot ammonia atmosphere of SiO2-coated stainless steels produced by a sol-gel procedure

AISI 304 stainless steel coated with silica films obtained by a sol-gel procedure was tested together with uncoated 304 and 316 samples in anhydrous ammonia at high temperature. The highest rate of attack was experienced by all uncoated samples; the ammonia attack develops a nitride scale that embrittles the metal. After 115 h of testing at 500°C uncoated samples were completely degraded, while the coated 304 samples were lightly attacked. The SiO₂ coating is a good barrier against ammonia corrosion. The multilayer coatings provide lower corrosion protection than single-layer coatings.     

XPS analysis of the passive film formed on austenitic stainless steel coated with conductive polymer

Improvement of the passivation behavior of Type 304 austenitic stainless steel (SS) by coating with conductive polymers (CPs), like polyaniline (PANI) and poly(o-phenylenediamine) (PoPD), followed by exposure in an acid solution has been demonstrated. The passive films formed on SSs (after peeling off the polymer layer) are compared with those formed during anodic polarization under the same exposure condition. The passive films beneath the CPs are thicker and less hydrated than those formed on uncoated stainless steel. The polymer layer enhances the enrichment of chromium and nickel in the entire passive oxide, forming a more protective film than that formed during anodic polarization. The elemental distribution within the passive film is different in the two modes of passivation. The type of the polymer influences on the composition of the passive film. The best passivation is obtained by PoPD, with the passive film resulting in significant resistance of the SS to pitting corrosion in the 3% NaCl solution. The oxide film of this steel is characterized, in its inner and outer layers, by the highest ratio of Cr(OH)₃/Cr₂O₃ and the lowest content of iron species.     

Electrochemical noise from oxygen reduction on stainless steel surfaces

Oxygen reduction occurring at the passive layer is probably the most important cathodic reaction involved in corrosion processes on stainless steel. Furthermore, the influence of the surface state on the oxygen reduction reaction is a key point for the understanding of the mechanism of localized corrosion on stainless steel. In this study, electrochemical noise measurements under cathodic polarization were carried out to obtain new information about this influence. It has been confirmed that the surface state of stainless steel plays a very important role in the kinetic of this cathodic reaction. Oxygen reduction kinetics was significantly reduced on passivated surfaces and improved on pre-reduced and ground surfaces. In addition, electrochemical current noise measurements allowed to differentiate between the electrochemical activity produced by the oxygen reduction reaction and that due to the reduction of the passive layer, in direct dependence on the characteristics of the different surface states investigated.     

Corrosion and metal release rates of Ni substrates under a porous gold coating

Noble metal coatings intended to protect devices from corrosion may have small porosity that allows corrosive fluid to attack the substrate alloy. A technique combining electrochemical tests and chemical analysis is described that measures low levels of porosity and low corrosion product release rates from the substrate. The technique has been applied to an electrodeposited gold coating on a nickel substrate. The results yielded a porosity of 1.8 × 10⁻³, defined as the area of substrate exposed to corrosion/area of the coating. Galvanic coupling between gold and nickel was found to increase the rate of nickel corrosion by about a factor of 10. A simplified model showed that if the substrate alloy does not passivate, the corrosion product release rate is expected to increase with exposure time.     

Corrosion behaviour of magnetron sputtered α- and β-Ta coatings on AISI 4340 steel as a function of coating thickness

The corrosion behaviour of magnetron sputtered α- and β-Ta coated AISI 4340 steels was studied with potentiodynamic polarization and electrochemical impedance spectroscopy. The coating porosity was observed to decrease with increasing coating thickness. For coatings less than 10 μm thick (α- or β-Ta), porosity was significant and open pores resulted in severe localized corrosion of the steel substrate, coating delamination, and overall coating failure. Additionally, the β-Ta coatings were more susceptible than the α-phase to delamination. As for the 50 and 100 μm thick α-Ta coatings, the electrochemical impedance behaviour was comparable to that of Ta foil, demonstrating the coating viability and corrosion resistance.     

Corrosion stability of epoxy coatings on aluminum pretreated by vinyltriethoxysilane

The electrochemical and transport properties and adhesion of epoxy coatings electrodeposited on aluminum pretreated by vinyltriethoxysilane (VTES) were investigated during exposure to 3% NaCl. The VTES films were deposited on aluminum surface from 2% and 5% vinyltriethoxysilane solution. The electrochemical results showed that the pretreatment based on VTES film deposited from 5% solution provides enhanced barrier properties and excellent corrosion protection. The values of diffusion coefficient of water through epoxy coating on this substrate and water content inside the epoxy coating were the smallest, indicating the low porosity of the coating. In addition, the good adhesion was maintained throughout the whole investigated time period.     

Erosion-corrosion of stainless steel under impingement by a fluid jet

The localised corrosion of 304L stainless steel is described by electrochemical measurements made under impact of a fluid jet of sodium chloride solution, in the absence of the entrained solid particles. Impingement by the fluid jet causes the open-circuit potential of 304L stainless steel in 0.6 M NaCl solution to increase with time until it attains the pitting potential. This is due primarily to acceleration of the cathodic reaction because the mass transfer rate of oxygen is increased under impingement by the fluid jet. Once the pitting potential has been achieved, the open-circuit potential decreases because of growth of stable pits. Fluid flow can itself thereby initiate stable pitting corrosion in open circuit.     

Poly(N-methylaniline) coatings on stainless steel by electropolymerization

Poly(N-methylaniline) (PNMA) coatings have been electropolymerized on 304 stainless steel alloy by potentiodynamic, galvanostatic and potentiostatic synthesis techniques from aqueous solutions of 0.1 M N-methylaniline (NMA) and 0.3 M oxalic acid. Characterization of PNMA coatings was carried out by cyclic voltammetry, UV-Vis and FTIR spectroscopy techniques. Corrosion behavior of PNMA coated stainless steel electrodes was investigated using linear anodic potentiodynamic polarization, Tafel test, chronoamperometry and electrochemical impedance spectroscopy (EIS) techniques in 0.5 M aqueous HCl solutions. Corrosion test results showed that PNMA coatings possessed protection to uncoated stainless steel against corrosion.     

Corrosion performance of epoxy coatings containing silane treated ZrO2 nanoparticles on mild steel in 3.5% NaCl solution

Clear epoxy coatings were modified by adding various levels of ZrO₂ nanoparticles. In order to achieve proper dispersion of nanoparticles in the epoxy-based coating and making possible chemical interactions between nanoparticles and polymeric coating, the surface of the nanoparticles was treated with amino propyl trimethoxy silane (APS). Corrosion performance of mild steel coated specimens was investigated employing EIS, electrochemical noise (ECN) techniques and salt spray test. Coatings with 2–3 wt% ZrO₂ nanoparticles possessed the best corrosion performance among the coating specimens. Possible chemical interactions between polymeric matrix and treated nanoparticles in nanocomposites cause high barrier properties and ionic resistances.