Tin-induced microstructural changes and associated corrosion resistance of biodegradable Mg-Zn alloy

As a new degradable medical metal material,Mg-Zn alloy has been widely used in clinical application.However,its popularization and application are seriously res...     

Microstructure and corrosion resistance of ceramic coating on carbon steel prepared by plasma electrolytic oxidation

Ceramic coating was prepared on Q235 carbon steel by plasma electrolytic oxidation (PEO). The microstructure of the coating including phase composition, surface and cross-section morphology were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR) and scanning electron microscopy (SEM). The corrosion behavior of the coating was evaluated in 3.5% NaCl solution through electrochemical impedance spectra (EIS), potentiodynamic polarization and open-circuit potential (OCP) techniques. The bonding strength between Q235 carbon steel substrate and the ceramic coating was also tested. The results indicated that PEO coating is a composite coating composed of FeAl₂O₄ and Fe₃O_4. The coating surface is porous and the thickness is about 100 μm. The bonding strength of the coating is about 19 MPa. The corrosion tests showed that the corrosion resistance of Q235 carbon steel could be greatly improved with FeAl₂O₄-Fe₃O₄ composite coating on its surface.     

The hardness and corrosion resistance of nickel alloys with molybdenum and tungsten

Conclusions The newly-developed Ni–Mo–W corrosion resistant hard alloys (N65M20V15 and N55M20V25) have a corrosion rate of no more than 0.2 mm/year in 30% HCl at 60°C and in 70% H₂SO₄ at 90°C, with a hardness as high as HRC 52.The alloys are precipitation-hardening. To obtain a high hardness it is recommended that they be heat treated by water quenching from 1000–1050°C and aging at 800°C for 4 h.Deceased.Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 24–27, March, 1972.     

微弧氧化处理对铝合金耐霉菌腐蚀特性的影响

在2A12铝合金表面制备了微弧氧化膜层,按照国家军用标准霉菌测试方法对微弧氧化膜层的耐腐蚀性能进行了测试,通过扫描电镜(SEM)、X射线衍射分析(XRD)对铝合金基体及微弧氧化膜层霉菌腐蚀前后的微观结构、相组成进行了表征。结果表明,未经过微弧氧化处理的铝合金表面有少量的霉菌生长,表面产生了一定数量的点蚀坑,长霉等级为1级。经过微弧氧化处理试样表面未发现霉菌生长,长霉等级为0级。微弧氧化处理可以有效提高铝合金表面耐霉菌腐蚀性能,扩大其应用范围。     

Improved corrosion resistance of AA2024 alloys through hybrid organic-inorganic sol-gel coatings produced from sols with controlled polymerisation

In this work we present the development of a nanocomposite material composed by silica nanoparticles in a hybrid organic-inorganic sol-gel matrix for corrosion protection of aluminium alloys. The sol-gel matrix was produced from an inorganic precursor, tetraethoxysilane (TEOS), a hybrid precursor organically functionalized with CC groups, 3-metacryloxypropyltrimethoxysilane (MPS), and an organic bi-functional monomer, ethyleneglycol-dimethacrylate (EGDMA) used to increase the cross-linking network. Silica nanoparticles, on the other side, increase the density and provide a major mechanical performance through the reinforcement of the coating. The evolution of the sol, mainly the chemical structure, during the processes of hydrolytic condensation and organic polymerisation was studied as a function of the sol concentration through Fourier transformed infrared spectroscopy (FTIR), rheometry, laser diffraction analysis and contact angle. Mono and multilayer coatings were deposited by dipping onto AA 2024 substrates and characterised by profilometry. The corrosion behaviour was followed through potentiodynamic tests and Electrochemical Impedance Spectroscopy (EIS).     

Improved corrosion resistance of cast carbon steel in sulphur oxides by alonizing

The results of studies on the Alonizing of cast carbon steel and of testing the corrosion resistance of this cast steel in an atmosphere containing 5 to 6% SO₂ + 50% SO₃ at 853 K are described and compared with the results obtained with unalonized cast carbon steel and high-alloy 23Cr-8Ni-2Mo cast steel. The duration of the corrosion tests was 336 hours. The aluminium diffusion layer on cast carbon steel was obtained by holding the specimens in a mixture containing 99% of powdered Fe-Al and 1% of NH₄Cl at 1323 ± 20 K. The holding time was 10 and 20 hours, respectively. The aluminium layer formed on the cast carbon steel was examined by optical microscopy and an X-ray microanalysis. After Alonizing for 10 h the layer had reached a thickness of 950 μm, and contained up to 35% Al. In a mixture of sulphur oxides corrosion rate of the alonized cast carbon steel was by about 600 times lower than that of the unalonized cast carbon steel, and by about 50 times lower than that of the 23Cr-8Ni-2Mo cast steel.     

Improved surface morphology and corrosion resistance for galvannealed coatings by pre-electroplating iron

Dual-phase steel was electroplated with Fe prior to reduction annealing to improve surface quality and corrosion characteristics after galvannealing. This pre-electroplating process caused remarkable reduction in surface cracks and induced δ-phase formation over the surface of the galvannealed coating. Moreover, the degree of Fe–Zn alloying significantly increased, resulting in a thicker coating. Consequently, the corrosion potential increased and corrosion current decreased in the highly polarized anodic reaction. Further, spontaneous passivation occurred, resulting in a very low corrosion rate.     

Excimer laser surface alloying of titanium with nickel and palladium for increased corrosion resistance

In the electron beam treatment of flue gases, titanium foil is employed as an electron-transparent window. Due to its degradation in the flue gas environment and eventual failure, extension of the life of the window is being sought. Previous studies have indicated significant improvements of corrosion resistance from surface alloying with nickel or palladium, using high intensity pulsed plasma beams, but restricted size of vacuum systems prevents treatment of large surfaces. In the present work, an excimer laser was employed to surface alloy titanium foil with nickel or palladium, using fluences in the range 0.4-1.1 J cm⁻² and either nitrogen or argon as the cover gas. The resultant surfaces provided high resistance to corrosion in 0.1 M H₂SO₄ solution at 80 °C that simulates, under accelerated conditions, the degradation of titanium by the flue gas. The improved behaviour is associated with the corrosion potential being shifted to the region of passivity. Treatments at increased fluences reduced losses of nickel and increased alloying of palladium during processing of the foils. Palladium was largely retained during the subsequent immersion tests, contrasting with the depletion of nickel by corrosion that limits the durability of the treated foils. The corrosion rates of the optimum palladium-alloyed surfaces were about two orders of magnitude lower than that of untreated titanium.     

Effect of nickel alloying on crevice corrosion resistance of stainless steels

The crevice corrosion behaviour of stainless steels containing 25 mass% Cr, 3 mass% Mo and various amounts of Ni was investigated in natural seawater. The results showed that ferritic steels containing nickel were more resistant to corrosion than both ferritic steels without nickel and austenitic steels. The superiority of the Ni bearing ferritic steel over the other steels was in close agreement with the depassivation pH of those steels in acidic chloride solutions. The results showed that the addition of Ni to ferritic steel was effective in decreasing the depassivation pH and the dissolution rate in acidic chloride solutions at crevices.     

Improving corrosion resistance of pure Al through ECAP

Abstract

Significant grain refinement and corrosion resistance improvement were achieved in industrial pure Al through equal channel angular pressing (ECAP). The effect of microstructure change on its corrosion resistance was investigated by optical/electron microscopy observation, constant immersion tests, polarisation tests and electrochemical impedance spectroscopy (EIS) in aqueous NaCl solution. The ultrafine grained (UFG) bulk pure Al (with grain sizes of 300–500 nm) has a higher pitting potential E_pit, a lower corrosion current density I_corr in polarisation tests and an increased polarisation resistance R_p from EIS plots, along with reduced corrosion damage in immersion tests, compared with the as cast material. It was found that the improved corrosion resistance resulted from the uniform distribution of fine Si containing impurities and the formation of a denser oxide film. The ECAPed samples with smaller Si containing impurities have lower microgalvanic currents and reduced susceptibilities of pitting corrosion, which is consistent with the classical ‘small cathode, large anode’ mechanism. The strain induced crystalline defects, for example, high angle grain boundaries and dislocations, appear to provide more nucleation sites for the formation of a denser and thicker oxide film, thus enhancing its corrosion resistance.     

Environmental friendly plasma electrolytic oxidation of AM60 magnesium alloy and its corrosion resistance

Plasma electrolytic oxidation of Mg-based AM60 alloys was investigated using 50 Hz AC anodizing technique in an alkaline borate solution, which contained a new kind of organic. The anodic film is relatively smooth with some micro pores and cracks, while the anodic film consists of MgO, MgAl2O4 and MgSiO3. The electrochemical behavior of anodic film was studied by electrochemical impedance spectroscopy and potentiodynamic polarization. Polarization results indicate the PEO treatment can decrease corrosion current by 3-4 magnitude compared with blank AM60 alloy. The anodic film presents a good level of corrosion protection for AM60 magnesium alloy, over 272 h of the salt spray test based on ASTM B 117. The effect of micro-structure and composition on corrosion protection efficiency was also investigated.     

Unipolar plasma electrolytic oxidation: Waveform optimisation for corrosion resistance of commercially pure titanium

This study deals with the anodisation of titanium grade 2 in 0.5-M sulphuric acid using a pulsed signal in a unipolar regime. The electrical parameters investigated are voltage, frequency and duty cycle. The use of duty cycles with a high percentage of anodic polarisation (90%), combined with high frequencies (1000 Hz) and the higher voltage tested (220 V), favoured the establishment of a plasma regime involving strong dielectric discharges, allowing the growth of thicker oxides but with rough architecture. The corrosion resistance of the formed film has been characterised by potentiodynamic tests in 0.5-M NaBr for localised corrosion resistance and by immersion tests in 10% v/v sulphuric acid solution for a uniform corrosion assessment. Current–time curves, visual observations and electron microscope analysis (scanning electron microscopy, energy-dispersive X-ray spectroscopy) were the tools selected to provide a correlation between technological parameters and oxide growth mechanism. For localised and uniform corrosion, anodisation at 220 V with a high level of anodic polarisation (90%) and frequency (1000 Hz) was verified to be particularly advantageous.     

Detection of corrosion fatigue cracking through current responses induced by cyclic stressing

Corrosion fatigue cracking of iron was investigated by analyzing current responses induced by cyclic stressing in a borate buffer solution containing 5 mM NaCl. Crack initiation was detected by evolution of harmonic components in the current responses and decrease in phase shift between strain and current responses. After crack initiation, the current response due to depassivation and repassivation at crack tips merged into the current response due to charging and discharging of an electric double layer. The decay of the damage current during unloading was related to both repassivation at crack tips and decrease in surface areas by crack closure.     

Investigation of wear and corrosion resistance of nanocomposite coating formed on AZ31B Mg alloy by plasma electrolytic oxidation

Ceramic-WC coatings were prepared on AZ31 B Mg alloy by plasma electrolytic oxidation (PEO) from a phosphate based bath containing suspended tungsten carbide nanoparticles at various process times. Scanning electron microscope results indicated that increase of coating time and incorporation of tungsten carbide into the ceramic coating during the PEO process led to a decrease in the number and diameter of coating pores. Phase analysis showed that the nanocomposite coating was composed of MgO, Mg₃(PO₄)₂ and WC. Tribological properties and corrosion behaviour of uncoated AZ31 B Mg alloy and ceramic coatings were evaluated using a pin-on-disc tribometer and potentiodynamic polarisation technique in 3.5% NaCl solution, respectively. The wear and electrochemical tests showed that wear and corrosion resistance of ceramic-WC nanocomposite coatings were better than ceramic only ones. In addition, wear and corrosion behaviour of coatings improved with increasing the coating time.     

Modification of plasma electrolytic coatings on aluminum alloys with corrosion inhibitors

Protective properties of the coatings produced by plasma-electrolytic oxidation on different aluminum alloys in a phosphate-borate electrolyte are studied. Filling the coatings with corrosion inhibitors of IFKhAN family is shown to substantially improve their corrosion resistance in chloride environments. Modification of the coatings with corrosion inhibitors provides an increase in the contribution of hydrophobization to the improvement of the protective characteristics of coatings.     

Improved corrosion resistance in simulated concrete pore solution of surface nanocrystallized rebar fabricated by wire-brushing

Continuous surface nanocrystallization (SNC) of rebar was achieved through wire-brushing process. A uniform NC layer with thickness of 25 μm and average grain size of 50 nm was formed on the rebar surface. Due to the enhanced passivation performance of the NC layer, corrosion resistance of the SNC rebar was significantly improved in Cl⁻-containing saturated Ca(OH)₂ solution. High-energetic crystal defects of the nano-grains leads to the faster passivation and enhanced stability of the passive film of the SNC rebar.