Structure stability and corrosion resistance of nano-TiO2 coatings on aluminum in seawater by a vacuum dip-coating method

A stable nano-TiO₂ coating was prepared by vacuum dip-coating TiO₂ sol-gel onto the anodized aluminum surface. The effect of vacuum dip-coating method and anodization pretreatment on compactness and stability of the coatings was proved. The structure and composition of the coatings were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicate that the surface of coatings is uniform and compact, with high load capacity. The particles of TiO₂ anatase is filled into the Al₂O₃ nano-pores formed by anodization. The electrochemical measurements show that the nano-TiO₂ coatings significantly decrease the corrosion currents densities (i_corr), as simultaneously increased the values of polarization resistance (R_t) of aluminum. It indicates the nano-TiO₂ coatings exhibit excellent anticorrosion properties in sterile seawater at the room temperature.     

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。     

The effects of immersion time on morphology and electrochemical properties of the Cr(III)-based conversion coatings on zinc coated steel surface

The main purpose of this paper is to develop a dynamic and non-destructive method to quantify and correlate the microstructure changes of the Cr(III) layer by electrochemical techniques. The open circuit potential (OCP) analysis reveals the nucleation growth mechanisms of the Cr(III) layer and the dissolution phenomena of Zn. In addition, the effects of immersion time to the corrosion behavior of Cr(III)-based conversion coatings (TCCCs) on electrogalvanized steel were studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution. Furthermore, surface morphology of the Cr(III) coatings under different immersion times was examined using both a scanning electron microscope and an atomic force microscope.From the potentiodynamic polarization experiment, the corrosion current density (I_corr) of the specimen with immersion time of 60 s was found appreciably small, representing the inheritance of the best anticorrosion performance. Additionally, the corrosion resistance of the Cr(III)-coating for the specimens obtained between 30 s and 60 s is two order higher than those of the untreated specimen from the EIS experiments. Results show that the quality of Cr(III)-based conversion coatings was strongly influenced by the immersion time of Cr(III) solution. And the optimal immersion time is recommended in the range of 30–60 s.     

Structure, morphology and chemical composition of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

WS₂ and Cr-WS₂ nanocomposite coatings were deposited at different Cr contents (approximately 15-50 at.%) on silicon and mild steel substrates using an unbalanced magnetron sputtering system. X-ray diffraction (XRD) was used to study the structure of Cr-WS₂ coatings and the bonding structure of the coatings was studied using X-ray photoelectron spectroscopy (XPS). The characterization of different phases present in Cr-WS₂ coatings was carried out using micro-Raman spectroscopy. The XPS and Raman data indicated the formation of a thin layer of WO₃ on the surface of Cr-WS₂ coatings and the intensity of the oxide phase decreased with an increase in the Cr content, which was also confirmed using energy-dispersive X-ray analysis results. The surface morphologies of WS₂ and Cr-WS₂ coatings were examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy. It has been demonstrated that incorporation of Cr in WS₂ strongly influences the structure and morphology of Cr-WS₂ coatings. The XRD and FESEM results suggested that increase in the Cr content of Cr-WS₂ coatings resulted in a structural transition from a mixture of nanocrystalline and amorphous phases to a complete amorphous phase. The cross-sectional FESEM data of WS₂ coating showed a porous and columnar microstructure. For the Cr-WS₂ coatings, a mixture of columnar and featureless microstructure was observed at low Cr contents (≤ 23 at.%), whereas, a dense and featureless microstructure was observed at high Cr contents. Detailed cross-sectional transmission electron microscopy (TEM) studies of Cr-WS₂ coatings prepared at Cr content ≤ 23 at.% indicated the presence of both nanocrystalline (near the interface) and amorphous phases (near the surface). Furthermore, high-resolution TEM data obtained from the nanocrystalline region showed inclusion of traces of amorphous phase in the nanocrystalline WS₂ phase. Potentiodynamic polarization measurements indicated that the corrosion resistance of Cr-WS₂ coatings was superior to that of the uncoated mild steel substrate and the corrosion rate decreased with an increase in the Cr content.     

An efficient protection of stainless steel against corrosion: Combination of a conversion layer and titanium dioxide deposit

In the present work, a novel process has been developed to improve the corrosion properties of ferritic stainless steels. Titanium oxide coatings have been deposited onto stainless steel by sol-gel process after a pre-functionalization of the substrate in a conversion bath. Gel titania was prepared by hydrolysis of a titanium butoxide through a sol-gel process. Duplex systems “conversion layer/uniform TiO₂ coating” have been prepared on stainless steels using a dipping technique and thermal post-treatments at 450 °C. The preparation of sol-gel coatings with specific chemical functions offers tailoring of their structure, texture and thickness and allows the fabrication of large coatings. The morphology and structure of the coatings were analysed using scanning electron microscopy with field effect gun (SEM-FEG), Mass spectroscopy of secondary ions (SIMS) and X-ray diffraction (XRD). The anticorrosion performances and the ageing effects of the coatings have been evaluated in neutral and aggressive media by using several normalized tests.The results show that the conversion layer was not sufficient to protect steel but sol-gel TiO₂ coatings, anchored on the metal substrate via the conversion layer, show good adhesion with the substrate and act as a very efficient protective barrier against corrosion. So, duplex layers with TiO₂ nanoparticle coatings on steels exhibit an excellent corrosion resistance due to a ceramic protective barrier on metal surface. Analysis of the data indicates that the films act as geometric blocking layers against exposure to the corrosive media and increase drastically the lifetime of the substrate.     

Corrosion resistance of Fe2O3-Cr2O3 artificial passive films formed with LP-MOCVD

Fe₂O₃-Cr₂O₃ artificial passive films were formed with a low pressure MOCVD technique using iron (III) acetylacetonate and chromium (III) acetylacetonate. The relationships between the crystal structure, the chemical state of the constituent elements, and the corrosion resistance of the films were examined in acid solutions. The films deposited above 300°C hardly dissolved in 1.0 M HCl and those deposited below 250°C, however, easily dissolved in the same solution. The dissolution rate of the films in solution increased with decreasing substrate temperature. When polarized cathodically in 1.0 M H₂SO₄, the films deposited below 250°C dissolved due to the reduction of the Fe₂O₃ component in the films. The reduction of the Fe₂O₃ component was, however, suppressed on the films deposited above 300°C. Therefore, with increasing crystallinity and the amount of M-O type chemical bonds, the corrosion resistance of the films increases in HCl and H₂SO₄ solutions.     

Corrosion behavior of Cr electrodeposited from Cr(VI) and Cr(III)-baths using direct (DCD) and pulse electrodeposition (PED) techniques

A comparison was made between the electrochemical corrosion behaviors of chromium deposited from hexavalent [Cr(VI)] and trivalent [Cr(III)] chromium baths using direct current (DCD) and pulse electro deposited (PED) techniques. Chromium coatings were deposited on mild-steel (MS) substrate. The corrosion behavior of both DCD and PED chromium from Cr(VI) and Cr(III)-baths in 3.5%NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results indicated that PED chromium from Cr(VI) and Cr(III)-baths have higher charge-transfer resistance R_ct and very low I_corr than that of DCD chromium on mild-steel substrate.     

Preparation of Fe/Cr doped SiO2 thin film on 304 stainless steel substrate originated from corrosion

Abstract

A Fe/Cr doped SiO₂ thin film which can improve the anticorrosion characteristic of materials was prepared on SUS304 stainless steel substrates by a modified hydrothermal method that constitutes a two part preparation process. The corrosion of stainless steel and the dissolution of silica from Na–Ca glass are simultaneously stimulated by an aqueous lithium bromide solution, and then a Fe/Cr doped SiO₂ thin film is coated onto the substrate. The Fe/Cr doped SiO₂ film was characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electron probe X-ray microanalyser. The results showed that the corrosion products and the silica are combined to form a Fe/Cr doped SiO₂ thin film. Furthermore, a chemical bond is formed between the silicon coating and the metal that improves the compactness and the adhesive property of the film. The anticorrosion characteristics of the film were measured by the methods of weight loss and electrochemical potentiodynamic polarisation, indicating that the Fe/Cr doped SiO₂ thin film can improve the anticorrosion performance of stainless steels effectively.     

Characterisation of calcareous deposits on freely corroding low carbon steel in artificial sea water

Abstract

Calcareous deposits were formed on steel under cathodic protection conditions in artificial sea water at various potentials from ?0·900 to ?1·400 V(SCE). The deposition calcareous layers were characterised by electrochemical impedance spectroscopies, scanning electron microscopy observations and X-ray diffraction analyses. At 20°C, the deposits were composed of calcite CaCO₃ when formed at various potentials in solution 1, of brucite Mg(OH)₂ and aragonite CaCO₃ when formed at potentials from ?0·900 to ?1·200 V(SCE) and only of brucite when formed at potentials E??1·300 V(SCE) in solution 2. Magnesium seems to influence the corrosion behaviour of freely corroding steel by causing calcium carbonate to precipitate as aragonite. Aragonite is more effective in covering the surface than calcite and is therefore more functional in preventing oxygen from reaching the steel surface, thereby lowering the corrosion rate.     

不同(Mo+B)/(Ni+Cr)质量比的原位合成MoB/NiCr涂层的组织结构与性能

采用超音速火焰喷涂技术沉积含3种不同(Mo+B)/(Ni+Cr)质量比（1:1,2:1和3:1）的Mo-B-Ni-Cr球磨复合粉末以原位反应制备获得MoB/NiCr涂层。采用扫描电子显微镜（SEM）和X射线衍射仪（XRD）分析了MoB/NiCr涂层的组织结构和物相。同时讨论了不同(Mo+B)/(Ni+Cr)质量比对涂层的组织结构、硬度、结合强度和耐腐蚀性能的影响。研究结果表明,(Mo+B)/(Ni+Cr)质量比为1：1的MoB/NiCr涂层孔隙率最低及涂层厚度最大。在3种涂层中均原位反应生成了Mo₂NiB₂三元硼化物,且随着(Mo+B)/(Ni+Cr)质量比的增加,涂层中三元硼化物含量随之增加,涂层的硬度值增加,结合强度反而随之降低;由于涂层中三元硼化物的原位生成,MoB/NiCr涂层的硬度值均高于316L不锈钢基体。通过能谱和XRD分析发现,经过360 h熔融锌腐蚀试验后,涂层表层中没有发现锌元素及其金属间化合物,然而随着(Mo+B)/(Ni+Cr)质量比的增加,涂层的孔隙率增加及厚度降低。最后,综合分析可得,相比其他涂层,(Mo+B)/(Ni+Cr)质量比为1:1的MoB/NiCr涂层具有更好的耐熔融锌腐蚀能力。     

Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering

MoS_x-based films were deposited using magnetron sputtering from a pure MoS₂ target. Alloying was accomplished by “co-deposition” from separate targets onto substrates having a two-fold rotation. An additional experiment had also a Cr⁺-ion etch for surface preparation, followed by a Cr adhesion layer, made using a Cr target mounted on a cathodic arc evaporation source. MoS_x and Al- and Ti-alloyed MoS_x coatings have been deposited onto high speed steel (HSS) and glass substrates for corrosion investigations.The coatings were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, hardness and adhesion measurements. The corrosion behaviour of the samples was electrochemically measured by open-circuit-potential (OCP) measurements and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7). Additionally the MoS_x-based coatings on HSS have been exposed to salt spray tests. The corrosion investigations revealed that the addition of Al and Ti to MoS_x shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoS_x coating a little bit less noble. In agreement with the OCP measurements, the corrosion potential E_corr in potentiodynamic corrosion tests was the highest for non-alloyed MoS_x coatings on HSS substrates. After the potentiodynamic corrosion tests, a strong corrosive attack could be observed for all coated samples. In salt spray tests the lowest number of corrosion pits was found for the MoS_x-Al (Cr⁺) coating on HSS.     

Influence of pulse parameters on electrocodeposition of Cr–Al2O3 nanocomposite coatings from trivalent chromium bath

Abstract

Cr and Cr–Al₂O₃ coatings were electrodeposited from Cr(III) bath with both pulsating and direct current onto copper substrates. Pulsating current resulted in homogeneous films of higher Al₂O₃ content and lower particle agglomeration than the direct current. Differences were more tangible at shorter duty cycles and pulse frequencies. Pulsating current improved both microhardness and corrosion resistance. The presence of alumina nanoparticles resulted in greater current efficiency, higher film microhardness and better corrosion resistance. Maximum current efficiency, highest microhardness and densest electrodeposited coatings were achieved at current density of 20 A dm^?2, duty cycle of 40% and pulse frequency of 10 Hz.     

Dry sliding tribological behaviour of bilayer Cr/Cr coatings obtained in sulphate Cr(III) baths

This study was intended to investigate the properties of Cr/Cr bilayer coatings. These coatings were deposited on a copper substrate by the DC electrodeposition method from Cr(III) sulphate baths with additions of formate-urea or glycine as complexing agents. Examination of the surface morphology of Cr coatings with SEM has shown that both single and bilayer Cr coatings obtained in the Cr(III) bath with formate-urea, and those obtained in the Cr(III) bath with glycine are cracked. It has been determined that the surface microhardness (HV) of bilayer Cr coatings obtained in the Cr(III) bath with glycine is higher compared with that of single-layer Cr coatings. Wear testing of the coatings was undertaken against an Al₂O₃ ball counterface (6?mm diameter) at 1N load. The results indicate that the friction coefficients (COF) of bilayer Cr/Cr coatings obtained in the Cr(III) bath with formate-urea increased from 0.2 to 0.5 compared with that of single-layer Cr coatings, while their wear resistance deteriorated. However, bilayer Cr/Cr coatings obtained in the Cr(III) bath with glycine exhibit wear resistance close to that of single coatings with COF equal to 0.05.     

Corrosion performance of a hot-dip galvanized steel treated by different kinds of conversion coatings

The anticorrosive performance of various kinds of conversion coatings including Cr(VI) and Cr(III) were studied on hot galvanized steel (HDG). The effect of Co(II) and Ni(II) ions was also examined on the Cr(III) coating performance. DC and electrochemical impedance spectroscopy (EIS) were conducted when the specimens were immersed in 3.5 wt.% sodium chloride solution. The coating morphology was observed using SEM. Micro-cracks were observed on both Cr(III) and Cr(VI) treated samples. The results indicated a decrease in the corrosion rate of HDG when the Cr(III) or Cr(VI) conversion coatings were found on the surface. The greater anticorrosion resistance of a Cr(VI) treated sample (compared with a Cr(III) one) was obtained at low immersion times. However, a significant decrease in self healing behavior and therefore corrosion resistance of the Cr(VI) treated sample was observed at longer immersion times. The anticorrosion resistance of Cr(III) treated samples was significantly improved using Co(II) or Ni(II) conversion coatings on Cr(III) treated samples. On the other hand, a lower decrease in corrosion performance of samples treated by Cr(III) and Co(II) or Ni(II) was obtained after a long immersion time in corrosive environment. It was found that, the passive and barrier layer which the Co(II) or Ni(II) conversion coating can produce on the Cr(III) coating can superiorly improve its anticorrosion resistance.     

The preparation and characteristics of a rare earth/nano-TiO2 composite coating on aluminum alloy by brush plating

Nano-TiO₂ modified rare earth composite coatings are prepared on 2024 aluminum alloy by brush plating. The composite coating is composed of mainly Ce(OH)₃, Ce(OH)₄, CeO₂ and TiO₂, with less cracks and lower porosity. The addition of nano-TiO₂ enhances the adhesive strength of the rare earth coating to Al substrate, results in refined coating grains and increases the micro-hardness of the coating. The nano-TiO₂ modification obviously improves the corrosion resistance of the rare earth coating. For the composite coating containing 2% TiO₂, both the corrosion current density and the impedance are reduced by more than one order of magnitude in contrast to the values for the pure rare earth coating. The higher barrier ability and increased Ce³⁺ content in the Ce oxides may explain the increase of the corrosion resistance.     

Effects of nitrogen content on structure and mechanical properties of multi-element (AlCrNbSiTiV)N coating

AlCrNbSiTiV metallic and nitride films were deposited by reactive radio-frequency unbalanced magnetron sputtering. The composition, microstructure and mechanical properties of the coatings deposited at different nitrogen flow rates were evaluated. The deposited AlCrNbSiTiV metallic film has an amorphous structure. The nitride films, regardless of the nitrogen flow ratio, were found to have only an FCC structure register on the XRD profiles. A Stoichiometric nitride ratio, i.e. (Al,Cr,Nb,Si,Ti,V)₅₀ N₅₀ is attained for a nitrogen flow ratio (R_N) of 10% and higher. At the lowest nitrogen flow ratio there is a preferred (200) orientation; however the films become less textured at higher nitrogen flow ratios. Nano-grained structures are obtained for all flow ratios, with grain sizes ranging from 8.7 to 12.3 nm. At the highest nitrogen flow rates the coatings have a compressive stress of around 4.5 GPa. The (Al,Cr,Nb,Si,Ti,V)₅₀ N₅₀ nitride coatings have both a high hardness and elastic modulus of 41 and 360 GPa, respectively. The maximum H/E ratio occurs at a nitrogen flow ratio of 20%.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

Ce 盐封孔对 SiCp/ Al 复合材料表面 Ni-P 镀层的影响

为提高SiCp/Al复合材料的耐腐蚀性能,先化学镀镍,再沉积稀土封孔,讨论了稀土溶液主盐Ce(NO3)3浓度和沉积时间对镍-稀土多层膜耐蚀性能的影响。结果表明:化学镀镍的SiCp/Al复合材料在室温下沉积稀土时,采用Ce(NO3)3含量1 g/L、成膜时间2 h的条件获得的多层膜耐蚀性最好,其腐蚀电位为-0.48mV,腐蚀电流密度为3.54×10-8A/cm2;稀土在膜层中以Ce的氧化物颗粒堆积状态存在,起到了封孔的作用;膜层中的镍磷合金呈多晶态,而稀土含量少,未能测出;稀土溶液浓度越高,沉积速度越快,而在相同浓度下,膜层厚度随着时间的延长而增加,越厚则膜层结合力越差。     

Corrosion and passivation mechanism of chromium diboride coatings on stainless steel

The corrosion resistance of fully crystalline CrB₂ coatings magnetron sputtered onto AISI 316L stainless steel was tested in acidic solutions. CrB₂ coatings showed excellent corrosion protection, but suffered a breakdown when an anodic potential of greater than about +1 V (SHE) was applied to the surface in a 1 M HCl electrolyte. The coating failure at high potentials is attributed to transpassive dissolution of the coating at volume defects, enabling the electrolyte to reach the underlying 316L substrate, resulting in its rapid corrosion and subsequent fracturing of the coating. Electrochemical data and potential-pH (Pourbaix) diagrams, constructed from thermodynamic data, indicate that the corrosion resistance of CrB₂ is due to the formation of a Cr(III) oxide passive film in the absence of activation corrosion.     

Influence of cold gas spray parameters on the corrosion resistance of Al-Al2O3 coatings sprayed on carbon steel

ABSTRACT

This work describes the influence of standoff distance (SoD), and gas temperature on the morphology and corrosion resistance of Al-10%Al₂O₃ coatings deposited by cold gas spray (CGS) on carbon steel. The results showed that the standoff distance had little effect on the thickness and microstructure of the coating. However, a 100 °C decrease of the spraying temperature reduced the coating thickness by 300?µm. The use of electrochemical analyses and SEM images showed that all the coatings studied were able to protect the substrate during at least 1300?h of immersion, due to the dense microstructure obtained by CGS.