Corrosion of stainless steel coated with TiN, (TiAl)N and CrN in aqueous environments

The corrosion behaviour of TiN, (TiAl)N and CrN coated on 304 stainless steel by physical vapour deposition was examined in borate solution (pH 9.0) and in 0.5 M NaCl solution. The study was performed by using open circuit potential, potentiodynamic polarization and cyclic polarization techniques, complemented with XRD and laser microscopy. The measurements were also performed on the uncoated substrate for comparison. Nitrided coatings significantly improve the corrosion performance of the steel; however, they are prone to corrosive attack as a consequence of the presence of microstructure defects such as pinholes and pores. Galvanic corrosion between the coatings and the substrate result in significant attack of the substrate, allowed by the penetration of small pinholes into the substrate. In borate solution, (TiAl)N possesses the highest corrosion resistance. However, CrN possessed the highest corrosion resistance in NaCl solution.     

Corrosion protection of commercial steel using stainless steel coatings deposited by Cathodic Arc Plasma Deposition technique

The commercial steel AISI 1010 was coated with AISI 316L steel using Cathodic Arc Plasma Deposition (CAPD) technique. The coatings were deposited in vacuum and in the presence of nitrogen, acetylene and mixture of the two as reactive gases. The coatings were deposited as a function of time while other parameters remained constant. The coatings 0.75 to 1.3 μm thick were adherent and amorphous. The aqueous corrosion properties of the coated samples in 3.5 wt % NaCl solution were studied by Tafel, cyclic and potentiodynamic polarization measurements. The derived corrosion parameters were then compared among the various uncoated and coated samples. The study revealed that the coated samples were more corrosion resistant than the uncoated one. Similarly, the samples coated in the nitrogen + acetylene mixture atmosphere were more corrosion resistant than the samples coated in only nitrogen and acetylene atmospheres. The corrosion parameters were also compared as a function of coating time to ascertain best coating thickness.     

Corrosion Inhibition of 304 Stainless Steel by Nano-Sized Ti/Silicone Coatings in an Environment Containing NaCl and Water Vapor at 400–600°C

The corrosion behavior of 304 stainless steel (SS) and its corrosion inhibition by brushing nano-sized Ti/silicone coatings on its surface in an environment containing a solid NaCl deposit and water vapor at 400–600°C was studied. Results indicated that water vapor or NaCl, especially water vapor plus NaCl accelerated the corrosion of the steel markedly. The corrosion scales of the uncoated steel had a duplex structure at 400–500°C and internal oxidation occurred for the uncoated steel at 600°C in an environment containing NaCl and water vapor. The corrosion of the 304SS was inhibited efficiently by the coatings at 400–500°C, and the coated steel suffered corrosion to some extent and most of the coatings were destroyed at 600°C. X-ray diffraction (XRD) indicated that the corrosion products of the uncoated steel were mainly Fe₂O₃, Cr₂O₃, NiO or Na₂CrO₄, and the coatings consisted mainly of TiO₂ and SiO₂ after exposure at 400–500°C. The good corrosion resistance of the nano-sized Ti/silicon coatings was attributed to the formation of SiO₂, and TiO₂ that resulted from the decomposition of the organic components in the coating and fast oxidation of nano-Ti powder respectively during the experiments, TiO₂ mixed together with SiO₂ and formed a new coating on the steel surface that played an important role in the protection of the steel.     

The effects of a CeO2 coating on the corrosion parameters of type 304 stainless steel

Coatings of CeO₂ derived from inorganic sol-gel dispersions were applied to type 304 bright annealed stainless steel (British Standard 1449 type 304S31) coupons and subjected to heat treatments at 450 and 550 °C. The coatings, 0.5 μm thick, were transparent, adherent, and stable. The aqueous corrosion properties of the coated coupons in 1 molar NaCl were then studied by potentiodynamic polarization measurements and electrochemical impedance spectroscopy. The derived corrosion parameters were then compared with similarly heat-treated, uncoated 304SS coupons. The results indicate that the application of a CeO₂ coating, heat treated at 550 °C, improves the corrosion rate as compared to an uncoated heat-treated coupon. The polarization curve for the 450 °C heat-treated, coated coupon showed passive behavior compared to transpassive for the uncoated specimen. The impedance spectrum data for the uncoated, as-received coupon could be modeled using a resistor in series with a parallel capacitor resistor combination “Randles” equivalent circuit. A coated coupon, heat treated at 550 °C, could be modeled using a more complex equivalent circuit involving a constant phase element, due to the CeO₂ coating. Modeling of the impedance characteristics for the oxidized coupons required the addition of a second series element consisting of a parallel resistor capacitor combination to give a Chi square statistic fit of X² < 5 X 10^-4.     

AZ91镁合金脉冲电沉积Ni-SiC纳米复合涂层的磨损与腐蚀性能

为了改善AZ91镁合金的表面性能,在含0-15g／LSiC纳米颗粒的改进的瓦特槽中,采用脉冲电沉积得到不同SiC含量的Ni-SiC纳米复合涂层。采用扫描电子显微镜（SEM）研究涂层的形貌,采用能谱仪（EDs）测试涂层的SiC含量。从15g／LSiC槽中电沉积得到的样品,其涂层的显微硬度提高了600％。采用动电位极化法研究包覆AZ91镁合金的腐蚀行为。结果表明,样品的耐腐蚀性能明显提高,即腐蚀电流密度从未包覆样品的0．13mA／cm2降低到槽中含15∥LSiC电沉积包覆样品的1．74x101mA／cm2,腐蚀电位从未包覆样品的-1．6V增加到槽中电沉积包覆样品的-0．31V。使用盘销摩擦测试仪评估了包覆和未包覆样品的耐磨性能,包覆样品的磨损量比未包覆的小8倍。     

45#钢Cr-N包埋共渗涂层的组织特征及形成机理

目的研究在45~#钢表面包埋共渗沉积Cr_2N涂层提高其耐蚀性的可行性。方法采用包渗法,对在1100℃下保温不同时间,得到不同时期的氮铬共渗涂层。利用扫描电镜及能谱仪、X射线衍射仪研究氮铬共渗层的微观组织及其生长机制,利用极化曲线评估涂层耐蚀性能。结果 45~#钢氮铬包埋共渗在保温4 h时可获得最佳涂层,涂层组织为Cr_2N层(约15μm)、Cr的沉积层(约10μm)、Cr的扩散层(约15μm)。Cr_2N层呈现强烈的(002)晶面择优取向;Cr沉积层为Fe-Cr合金及铬的碳化物相(Cr_7C_3,Cr_3C_2)。在模拟燃料电池腐蚀液中,45~#钢、45涂层样品、304不锈钢自腐蚀电位和自腐蚀电流分别为-0.521 V和230.63μA·cm~(-2),-0.448 V和10.89μA·cm~(-2),-0.299 V和5.26μA·cm~(-2)。当腐蚀电位高于0.3 V时,涂层样品会二次钝化,腐蚀电流低至1.43μA·cm~(-2)。结论沉积Cr_2N的45~#钢样品相对原样其耐蚀性有很大提高,并且当腐蚀电位达到0.3 V以上时,其耐蚀性能优于304不锈钢。     

Effect of Micro Arc Oxidation Coatings on Corrosion Resistance of 6061-Al Alloy

In the present study, the corrosion behavior of micro arc oxidation (MAO) coatings deposited at two current densities on 6061-Al alloy has been investigated. Corrosion in particular, simple immersion, and potentiodynamic polarization tests have been carried out in 3.5% NaCl in order to evaluate the corrosion resistance of MAO coatings. The long duration (up to 600 h) immersion tests of coated samples illustrated negligible change in weight as compared to uncoated alloy. The anodic polarization curves were found to exhibit substantially lower corrosion current and more positive corrosion potential for MAO-coated specimens as compared to the uncoated alloy. The electrochemical response was also compared with SS-316 and the hard anodized coatings. The results indicate that the overall corrosion resistance of the MAO coatings is significantly superior as compared to SS316 and comparable to hard anodized coating deposited on 6061 Al alloy.     

The Effect of Water Vapor on the Oxidation Behavior of CVD Iron-Aluminide Coatings

The oxidation behavior of iron-aluminide coatings, Fe₃Al or (Fe,Ni)₃Al, produced by chemical-vapor deposition (CVD) was studied in the temperature range of 700–800°C in air + 10 vol.% H₂O. A typical ferritic steel, Fe–9Cr–1Mo, and an austenitic stainless steel, 304L, were coated. For both substrates, the as-deposited coating consisted of a thin (<5μm), Al-rich outer layer above a thicker (30–50 μm), lower-Al-content inner layer. In addition to coated and uncoated Fe–9Cr–1Mo and 304L, cast Fe–Al model alloys with similar Al contents (13–20 at.%) to the CVD coatings were included in the oxidation exposures for comparison. The specimens were cycled to 1000 1 hr cycles at 700°C and 500 1 hr cycles at 800°C, respectively. The CVD coating specimens showed excellent performance in the water-vapor environment at both temperatures, while the uncoated alloys were severely attacked. These results suggest that an aluminide coating can substantially improve resistance to water-vapor attack under these conditions.     

Characterization and Performance of Magnetron-Sputtered Zirconium Coatings Deposited on 9Cr-1Mo Steel

Zirconium coatings of different thicknesses have been deposited at 773 K on 9Cr-1Mo steel substrate using pulsed DC magnetron sputtering. These coatings were heat treated in vacuum at two different temperatures (1173 and 1273 K) for one hour. X-ray diffraction (XRD) analysis of Zr-coated samples revealed the formation of α-phase (HCP structure) of Zr. XRD analysis of heat-treated samples show the presence of Zr₃Fe and Zr₂Fe intermetallics. The lattice parameter of these coatings was calculated, and it matches with the bulk values when the thickness reached 2µm. In order to understand this, crystallite size and strain values of these coatings were calculated from XRD plots employing Williamson-Hall method. In order to assess the performance of the coatings, systematic corrosion tests were carried out. The corrosion current density calculated from the polarization behavior showed that the corrosion current density of the uncoated 9Cr-1Mo steel was higher than the coated sample before and after the heat treatment. Studies using electrochemical impedance spectroscopy confirmed that the coated steel has higher impedance than the uncoated steel. The corrosion resistance of 9Cr1Mo steel had improved after Zr coating. However, the corrosion resistance of the coating after heat treatment decreased when compared to the as-deposited coating. The microstructure and composition of the surface oxide film influence the corrosion resistance of the Zr-coated 9Cr1Mo steel.     

Electrosynthesis of Poly(ortho-phenetidine) Coatings on Steel and Investigation of Their Corrosion Protection Properties

Poly(ortho-phenetidine) coatings on 304 stainless steel (304 SS) surface have been synthesized by using the galvanostatic technique. The electrosynthesized coatings were characterized by Fourier Transform Infrared Spectroscopy (FT-IR), UV-visible absorption spectrometry and Scanning Electron Microscopy (SEM). The anticorrosion performances of poly(ortho-phenetidine) coatings were examined in 0.1 M HCl medium by the electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique. The corrosion rate of poly(ortho-phenetidine)-coated 304 SS was found ~10 times lower than bare 304 SS and corrosion potential increased from –0.29 V for uncoated 304 SS to –0.19 V versus Ag/AgCl (3 M KCl) for poly(ortho-phenetidine)-coated 304 SS electrode. Electrochemical measurements indicate that poly(ortho-phenetidine) coating has good inhibiting properties with an efficiency of ~93% at 1.5 mA cm^–2 applied current density in acidic corrosive media. The results of this study obviously ascertain that the poly(ortho-phenetidine) has an outstanding potential to protect 304 SS against corrosion in an acidic environment.     

Creep and corrosion testing of aluminide coatings on ferritic-martensitic substrates

Pack and chemical vapor deposited (CVD) aluminide coatings on commercial ferritic-martensitic Fe-9Cr-2W steel are being investigated by creep and corrosion testing at 650 °C. Results from different coating thicknesses show that the coated region makes no contribution to the creep strength. The creep behavior of uncoated material was studied after various heat treatments to simulate the coating process and typical secondary heat treatments. Alternating creep and corrosion exposures showed little effect on the creep strength of uncoated material but coated materials became progressively weaker. The coatings were protective in wet air at 650 °C after creep testing.     

Effect of trivalent rare earth dopants in nanocrystalline ceria coatings for high-temperature oxidation resistance

Nanocrystalline ceria (NC) and La³⁺-doped nanocrystalline ceria (LDN) particles synthesized by the micro-emulsion method were coated onto AISI 304 stainless steel (SS) in order to study their high-temperature oxidation resistance properties at 1243 K in dry air for 24 h. Results were compared with those for micro-ceria (MC) coatings. The coated samples showed 90% improvement in oxidation resistance compared to uncoated and MC coated steels as seen from SEM cross-sectional studies. X-ray diffraction (XRD) analysis showed the presence of chromia in both NC and 20 LDN (NC doped with 20 at.% La³⁺) samples which is absent in uncoated steels. From secondary ion mass spectrometry (SIMS) depth profiles, Fe, Ni depletion zones were observed in LDN-coated samples, indicating diffusion through the oxide layer. The role of oxygen vacancies in the nanoceria coatings on the formation of protective chromia layers is discussed.     

Electrochemical characteristics of steel coated with TiN and TiAlN coatings

The corrosion protection characteristics of titanium nitride (TiN) and titanium–aluminum nitride (TiAlN) coatings produced on cemented carbon steel targets were investigated in aqueous sodium chloride solution. All coatings were produced by cathodic arc plasma deposition. The results indicated that it was possible to follow the corrosion behavior of the coated systems over a period of 300–900 h of immersion. It was found that the TiN and TiAlN coatings had a lower corrosion rate (current density), about three orders of magnitude lower than the untreated steel substrates. The metal substrate was actually passive in these experimental conditions, and exhibited an electrochemical impedance response that could be described by means of the same equivalent circuit than for the coating. Nevertheless, the analysis of the impedance parameters allowed for direct information concerning the enhancement of the corrosion resistance of the coated system as compared to the passive uncoated metal substrate to be extracted. The major corrosion mechanism for the coated samples arises from electrolyte penetration in the pores of the deposits, which may eventually lead to the development of localized forms of corrosion.     

Protection of carbon steel against hot corrosion using thermal spray Si- and Cr-base coatings

A Fe75Si thermal spray coating was applied on the surface of a plain carbon steel baffle plate. Beneath this coating, a Ni20Cr coating was applied to give better adherence to the silicon coating. The baffle was installed in the high-temperature, fireside, corrosion zone of a steam generator. At the same time, an uncoated 304 stainless steel baffle was installed nearby for comparison. For 13 months the boiler burned heavy fuel oil with high contents of vanadium. The samples were studied employing scanning electron microscopy, x-ray microanalysis, and x-ray diffraction techniques. After that, it was possible to inspect the structural state of the components, and it was found that the stainless steel baffle plates were destroyed almost completely by corrosion, whereas the carbon steel coated baffle plate did not suffer a significant attack, showing that the performance of the thermal spray coating was outstanding and that the coating was not attacked by vanadium salts of the molten slag.     

In vitro electrochemical evaluation and phase purity of natural hydroxyapatite coating on medical grade 316L stainless steel

Natural hydroxyapatite (HA) has been electrophoretically deposited on the surface of medical grade 316L stainless steel (SS). Deposition was performed at various applied voltages of 30, 60, and 90 V for 3 min. After deposition the samples were dried at room temperature and sintered in a vacuum furnace (10⁻⁴–10⁻⁵ Torr) at 800 ^°C for 1 h. In vitro electrochemical studies were performed in the Hank's solution. The results showed that corrosion potential and breakdown potential of the coated samples shifted toward nobler potential compared with the uncoated 316L SS. Electrochemical impedance spectroscopy investigations revealed a higher polarization resistance, total impedance, and lower capacitance values for the coated samples compared to the bare one which indicated the improvement of corrosion resistance of the coated samples. The surface morphology of the samples was studied by a scanning electron microscope, and also phase purity and crystallinity of the coating material were investigated by X-ray diffraction. Deposition at 60 V for 3 min was found to be an optimum coating condition which led to the uniform, continuous, and crack-free coatings. The crystallinity of the coating materials was measured to be high, and quantitative analysis of the coatings after sintering showed some partial decomposition of natural HA to tricalcium phosphate.     

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.     

Hot Corrosion Studies of HVOF-Sprayed Coating on T-91 Boiler Tube Steel at Different Operating Temperatures

The aim of the present work is to investigate the usefulness of high velocity oxy fuel-sprayed 75% Cr₃C₂-25% (Ni-20Cr) coating to control hot corrosion of T-91 boiler tube steel at different operating temperatures viz 550, 700, and 850 °C. The deposited coatings on the substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 2%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of uncoated and coated samples. The corrosion products of the coating on the substrate are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for the corrosion mechanisms. It is found that the coated specimens have shown minimum weight gain at all the operating temperatures when compared with uncoated T-91 samples. Hence, coating is effective in decreasing the corrosion rate in the given molten salt environment. Oxides and spinels of nickel-chromium may be the reason for successful resistance against hot corrosion.     

Corrosion behaviour of thermally sprayed Al and Al/SiCp composite coatings on ZE41 magnesium alloy in chloride medium

Thermally sprayed Al and Al/SiCp composite coatings have been deposited on ZE41 magnesium alloy and mechanical compaction at room temperature was applied to the Al and Al/SiCp coatings to reduce their porosity. Corrosion behaviour of coated samples was evaluated and compared to that of uncoated substrate in 3.5 wt.% NaCl solution using electrochemical measurements. Al and Al/SiCp composite coatings reduced the corrosion current density of Mg-Zn alloys by three and two orders of magnitude, respectively, and reductions up to four orders of magnitude were obtained after mechanical compaction.     

Corrosion behaviour of HVOF-sprayed Co–Cr–W–C coating in NaCl solution

Stellite 6 (Co–Cr–W–C) coating was deposited on carbon steel substrate by high-velocity oxyfuel spraying. The electrochemical corrosion behaviour of the coatings was investigated in 3.5% NaCl solutions at 25°C by means of polarisation studies and electrochemical impedance spectroscopy (EIS) measurements. The immersion time before corrosion tests was 0.5 and 24?h. The results showed that the corrosion rate of the coatings was lower than that of the steel substrate by one order of magnitude. An active–passive behaviour was observed during anodic polarisation of the coatings at both immersion times. The degree of substrate attack after polarisation tests of coated samples was not considerable. EIS measurement showed that two-time constants were observed in Nyquist plots of the coatings.     

Corrosion Resistance of PAPVD TiN Coating on AISI 304 Stainless Steel

In this work the corrosion resistance of PAPVD TiN hard coatings on AISI 304 stainless steel with a titanium interlayer has been addressed. Cyclic voltammetry corrosion tests in NaCl 3.5% solutions were performed for samples prepared by depositing TiN/Ti onto steel using different deposition parameters. The surface morphology of the samples was examined by using a scanning electron microscope (SEM) and phase analysis was performed by X-ray diffraction (XRD). The cyclic voltammetry curves showed two distinct behaviours. Firstly, a reduction in corrosion resistance was observed when current density was increased for the whole potential range studied. The second observation, no less important, was the increase in corrosion resistance compared to the uncoated steel. The TiN films deposited showing (III) preferred orientation showed better corrosion resistance than films presenting other orientations.