The synergy between deformation and anodic dissolution of an austenitic stainless steel in acidic chloride solution

In corrosion medium, metals can deform under tensile stress and form a new active surface with the anodic dissolution of the metals being accelerated. At the same time, the anodic dissolution may accelerate the deformation of the metals. The synergy can lead to crack nucleation and development and shorten the service life of the component. Austenitic stainless steel in acidic chloride solution was in active dissolution condition when stress corrosion cracking (SCC) occurred. It is reasonable to assume that the anodic dissolution play an important role, so it's necessary to study the synergy between anodic dissolution and deformation of austenitic stainless steels. The synergy between deformation and anodic dissolution of AISI 321 austenitic stainless steel in an acidic chloride solution was studied in this paper. The corrosion rate of the steel increased remarkably due to the deformation‐accelerated anodic and cathodic processes. The creep rate was increased while the yield strength was reduced by anodic dissolution. The analysis by thermal activation theory of deformation showed a linear relationship between the logarithm of creep rate and the logarithm of anodic current. Besides, the reciprocal of yield strength was also linearly dependent on the logarithm of anodic current. The theoretical deductions were in good agreement with experimental results.     

Layer-by-layer analysis of hot-dip zinc coating by anodic dissolution in acetate bath

To analyze the distribution of the alloying elements in hot-dip zinc coatings, a layer-by-layer electrochemical dissolution method was studied by using an acetate bath. The research was performed by means of galvanostatic anodic polarization, chemical analysis and metallographic analysis. A suitable cell was constructed to perform this study. Results show that the electrochemical dissolution of η-phase is uniform, while δ-phase dissolves partially together ζ-phase, due to the formation of cracks in the coating during polarization. Faraday's law was used to demonstrate that galvanostatic dissolution of hot-dip zinc coating at 12.7 mA cm^− 2 proceeds by electrochemical mechanism without chemical dissolution; on the contrary, the occurrence of this second process was found at lower stripping current density. During polarization, the formation of a black film which interferes with the subsequent dissolution process was found. Mechanical removal of black film from electrode surface leads to the decrease of cracks formation and to a more uniform layer-by-layer dissolution. A method to obtain layer-by-layer analysis of the alloying elements of a hot-dip zinc coating, as a function of its thickness, by controlled electrochemical dissolution is suggested.     

Effect of molybdate post-sealing on the corrosion resistance of zinc phosphate coatings on hot-dip galvanized steel

The technique of post-sealing the phosphated hot-dip galvanized (HDG) steel with molybdate solution was addressed. The composition and corrosion resistance of the improved phosphate coatings were investigated by SEM, EDS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements, and neutral salt spray (NSS) test. The results showed that molybdate films were formed in the pores of phosphate coatings, and the compact and complete composite coatings composed of phosphate coatings and molybdate films were formed on the zinc surface, resulting in that both the anodic and cathodic processes of zinc corrosion were inhibited remarkably; the corrosion protection efficiency values were increased; and the electrochemical impedance values were enhanced at least one order of magnitude. The low frequency impedance values for the composite coatings were increased at the initial stages of immersion in 5% sodium chloride solution, indicating the self-repairing activity of the composite coatings.     

The effects of niobium and nickel on the corrosion resistance of the zinc phosphate layers

In this investigation the viability of nickel substitution by niobium in zinc phosphate (PZn) baths has been studied. Samples of carbon steel (SAE 1010) were phosphated in two baths, one containing nickel (PZn + Ni) and the other with niobium substituting nickel (PZn + Nb). Potentiodynamic polarization curves (anodic and cathodic, separately) and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion resistance of the phosphated carbon steels in a 0.5 mol L^− 1 NaCl electrolyte. The phosphate layers obtained were analysed by X-ray diffraction and it was found that they are composed of Zn₃(PO₄)₂.4H₂O (hopeite) and Zn₂Fe(PO₄)₂.4H₂O (phosphophylite). Surface observation by scanning electron microscopy (SEM) showed that the PZn + Ni layer is deposited as needle-like crystals, whereas the PZn + Nb layer shows a granular morphology. The electrochemical results showed that the PZn + Nb coating was more effective in the corrosion protection of the carbon steel substrate than the PZn + Ni layer. The results also suggested that nickel can be replaced by niobium in zinc phosphate baths with advantageous corrosion properties of the layer formed.     

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.     

Morphology and characterization of the anodic coating on galvanized steels prepared by alternating currents

A novel technique for producing anodic coatings on galvanized steels was developed in this study. Several coloured coatings, with thickness varying from 20 to 50 μm, were produced by adding a specific transition metal salt to the basic electrolyte of silicate. The anodic coating was composed mainly of silicon, zinc, sodium and oxygen, doped with transition metal ions. Results from electron probe microanalysis and X-ray diffraction studies indicated that the island-like structure is silicon-enriched and that a fused glass structure is a major part of the anodic coating. The colour imparted to the anodic coating depended on the specific metal incorporated during its formation. The coating significantly increased the corrosion resistance of galvanized steels.     

Microstructure and mechanical properties of HVOF sprayed nanocrystalline Cr3C2-25(Ni20Cr) coating

The objectives of this work are to deposit nanocrystalline Cr₃C₂-25(Ni20Cr) powder by thermal spraying and to compare the performance of this coating with that obtained using conventional powder. Towards that purpose, Cr₃C₂-25(Ni20Cr) powders with nanocrystalline grain size and with conventional grain size were deposited using OSU-SJS high-velocity oxyfuel (HVOF) system. The microstructural features, such as morphology of the coated surface, thickness of the coating, the interface of the coating with the substrate, distribution of various phases, and grain sizes etc, were characterized with the help of optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM). The amount of oxide phases and pores were determined by means of image analyzer. The presence of various phases was identified by x-ray diffraction (XRD) technique. Hardness, elastic modulus, and indentation toughness were evaluated employing micro indentation technique. The results indicate the presence of three different zones containing only orthorhombic Cr₃C₂ phase, FCC NiCr phase, and mixture of Cr₃C₂ and NiCr phases in, both coatings. The grain sizes in the nanocrystalline coating were in the range of 80 to 100 nm. Nanocrystalline coating exhibits 20% increase in hardness, 40% decrease in surface roughness, and comparable fracture toughness and elastic modulus with respect to conventional coating.     

Study of corrosion behaviour of A106 carbon steel absorber for CO2 removal in amine promoted hot potassium carbonate solution (Benfield solution)

The CO₂ absorber is one of the largest pressure vessels in ammonia plants, which are suffering from severe corrosion problems worldwide. The aim of the present study is to examine the corrosion behaviour of A106 carbon steel absorber for CO₂ removal in amine promoted hot potassium carbonate solution (Benfield solution). This study simulates CO₂ removal unit in ammonia production process at Abu Qir Fertilizers and Chemical Industries Company (Alexandria, Egypt) and many other plants all over the world. A typical Benfield solution contains hot potassium carbonate K₂CO₃, potassium bicarbonate KHCO₃, diethanol amine (DEA) as a promoter and potassium metavanadate KVO₃ as corrosion inhibitor. The rate of galvanic corrosion of carbon steel absorber/stainless steel pall packings couple in Benfield solution was measured without adding the corrosion inhibitor KVO₃ in order to measure the influence of corrosive solution. The corrosion rate was measured by weight loss technique in relation to different operating parameters such as solution velocity, solution temperature, %K₂CO₃, CO₂ loading, %DEA and the effect of the presence of solution contaminants. In general, increasing solution velocity, solution temperature, %K₂CO₃, CO₂ loading and the presence of solution contaminations increase the corrosion rate. However, the increase in %DEA in solution decreases the corrosion rate. The strong dependence of corrosion rate on both solution and gas velocities indicates the diffusion controlled nature of the corrosion process. In addition, estimation of activation energy revealed a value of 4·8 kcal mol^?1. Surface morphology study depicted the presence of a porous solid film of corrosion products on carbon steel surface. It has been found that the liquid phase diffusion of bicarbonate to the steel solution interface is the rate determining step.     

Synthesis and corrosion protection study of poly(o-ethylaniline) coatings on copper

Poly(o-ethylaniline) coatings were synthesized on copper (Cu) by electrochemical polymerization of o-ethylaniline in an aqueous salicylate solution by using cyclic voltammetry. The characterization of these coatings was carried out by cyclic voltammetry, UV-visible absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results of these characterizations indicate that the aqueous salicylate solution is a suitable medium for the electrochemical polymerization of o-ethylaniline to generate strongly adherent and smooth poly(o-ethylaniline) coatings on Cu substrates. The performance of poly(o-ethylaniline) as protective coating against corrosion of Cu in aqueous 3% NaCl was assessed by the potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). The results of these studies demonstrate that the poly(o-ethylaniline) coating has ability to protect the Cu against corrosion. The corrosion potential was about 0.078 V versus SCE more positive in aqueous 3% NaCl for the poly(o-ethylaniline) coated Cu (∼ 15 μm thick) than that of uncoated Cu and reduces the corrosion rate of Cu almost by a factor of 70.     

The anodic dissolution of Mg in NaCl and Na2SO4 electrolytes by atomic emission spectroelectrochemistry

The dissolution of Mg has been investigated with atomic emission spectroelectrochemistry at open circuit and during potentiostatic control in chloride (NaCl) and sulfate (Na₂SO₄) electrolytes. A dissolution stoichiometry of approximately n = 2 is observed under all the conditions of these experiments with no evidence for the commonly invoked Mg⁺ intermediate. Nevertheless, in chloride electrolyte it is shown that anodic polarization results in a cathodic activation of the surface with increased hydrogen production during the polarization and an increased corrosion rate following the pulse. The formation of insoluble Mg oxides/hydroxides was also investigated in the sulfate electrolyte.     

Thermodynamic characterization of metal dissolution and inhibitor adsorption processes in mild steel/2,5-bis(n-thienyl)-1,3,4-thiadiazoles/hydrochloric acid system

The corrosion inhibition of mild steel in 1 M hydrochloric acid solution by some 2,5-bis(n-thienyl)-1,3,4-thiadiazoles (n-TTH) has been studied in relation to the concentration of the inhibitors as well as the temperature using chemical (weight loss) and electrochemical (ac impedance and dc polarisation) techniques. All the methods employed are in reasonable agreement. The protection efficiency increases with increasing inhibitors concentration and with increasing temperature. The thermodynamic functions of dissolution and adsorption processes were calculated from experimental polarisation data and the interpretation of the results are given. Adsorption of n-TTH was found to follow the Langmuir’s adsorption isotherm and the ability of the molecule to chemisorb on the steel surface was dependent on the position of the sulphur atom on the thienyl substituent.     

XRD and SEM studies of the layer of corrosion products for carbon steel in various different environments in the province of Las Palmas (The Canary Islands, Spain)

A study was carried out in order to identify the various elements which make up the layer of rust which is produced on carbon steel exposed to the atmosphere in the province of Las Palmas (The Canary Islands, Spain). XRD and SEM analyses were used in order to identify and describe the various corrosion products produced over different areas on the islands: rural, urban, marine and marine-industrial. The main product found was akaganeite, which is typical of marine areas, followed closely by lepidocrocite, goethite and magnetite. Likewise, another formerly unidentified corrosion product for carbon steel, such as is green rust II, was detected. It has been found that the protective capacity of corrosion products increases over the first year of exposition and then levels off.Likewise, a hyperbolic law was used to model the protective effect of the layer of corrosion products which gave extremely good regressive values both in the case of each station individually and when grouped together by corrosion category (r²>0.98).     

液体中脉冲放电制备(Ti,Al)C陶瓷涂层的研究

用粉末模压成型工艺和无压烧结方法制备了Ti-Al混合烧结工具电极,采用所制备的电极在煤油中对45钢表面进行脉冲放电沉积(Ti,Al)C陶瓷涂层。探讨了粉体配比对工具电极致密度和相组成以及对液相放电所制备的涂层组织与表面形貌的影响。结果表明,随着Al含量的提高,工具电极密度和相对密度先下降后升高,当Al含量为60%时,电极密度达到最小;XRD结果显示制备出涂层的物相主要为(Ti,Al)C,而制备出的涂层表面形貌明显不同,随着电极中Al元素的升高,熔滴状的熔池尺寸越来越大,宏观上越来越粗糙,截面涂层厚度约为20μm。     

(Ti,Al)N涂层的微观组织和性能

采用EPMA、XRD、SEM、TEM、HR-TEM、EDX、纳米压痕、氧化实验和切削实验研究了磁控溅射在硬质合金基体上沉积TiN涂层和(Ti,Al)N涂层的微观组织结构和性能。结果表明:TiN涂层晶粒为喇叭口柱状晶,(Ti,Al)N涂层为面心立方平直柱状晶,由于固溶了Al元素,(Ti,A l)N涂层呈(200)面择优生长;(Ti,Al)N涂层在硬质合金基体上无外延生长;(Ti,Al)N涂层在800℃氧化后形成Al2O3/TiO2/(Ti,Al)N的分层结构;(Ti,Al)N涂层具有更高的硬度和更好的切削性能。     

空心阴极辅助多弧镀(Ti,Al)N涂层物相组成及性能

采用空心阴极离子束辅助多弧离子镀技术,通过变换靶材中Ti-Al比例(Al含量分别为30 at%、50 at%和70 at%)沉积了(Ti,Al)N涂层。利用XRD、激光共聚焦显微镜、材料表面微纳米力学测试系统等检测了涂层的质量和性能。研究了涂层的相组成、表面粗糙度、摩擦系数和硬度、以及涂层的耐高温性能等随靶材成分比例改变而变化的规律。结果表明,当靶材中Al含量为50 at%时,沉积层相组成以Ti_(1-x)Al_xN为主,涂层粗糙度和摩擦系数最小,分别为0.3188和0.6074,而硬度值最高,为34.19 GPa;加热温度低于800℃时涂层物相组成没有发生明显变化,900℃时涂层产生开裂基体被氧化;空心阴极离子束辅助沉积时能够形成"混合界面",并减轻涂层表面"大颗粒"现象。     

Electrochemical and analytical study of corrosion inhibition on carbon steel in HCl medium by 1,12-bis(1,2,4-triazolyl)dodecane

1,12-bis(1,2,4-triazolyl)dodecane (dTC12) is an excellent corrosion inhibitor for carbon steel in deaerated 1 M HCl solution. In this work electrochemical and analytical techniques were used to study the inhibition of corrosion on carbon steel in acidic medium. The carbon steel corrosion inhibition of dTC12 was attributed to the synergistic effect between chloride anion and quaternary ammonium ion. The protective efficiency of the film was higher than 90%, indicating that corrosion of carbon steel in 1 M HCl is reduced by dTC12.The effect of dissolved oxygen on the inhibition efficiency was also investigated. The results show that the inhibition efficiency increases in early stage and decreases for a long immersion time.     

Thermodynamics and kinetics of fluoride removal from simulated zinc sulfate solution by La(III)-modified zeolite

To understand the mechanism of fluoride removal from the simulated zinc sulfate solution by the La(III)-modified zeolite, the adsorbent was characterized by XRD, SEM and EDS. The effects of absorbent dose and contact time, the adsorption isotherms and the sorption kinetics were investigated. The experimental results were compatible with the Langmuir isotherm model. The theoretical maximum adsorption capacities are 20.83 and 23.04 mg/g at 303 and 313 K, respectively. And the physisorption is revealed using the Temkin isotherm model and the D–R isotherm model. The sorption process is more suitable by the pseudo-second-order kinetic models. Thermodynamic parameters such as standard free energy change (ΔG^Θ<0 kJ/mol), standard enthalpy change (ΔH^Θ=8.28 kJ/mol) and standard entropy change (ΔS^Θ=0.030 kJ/(mol·K)) indicate the spontaneity of adsorption and endothermic physical sorption. Furthermore, the fluoride concentration in the industrial zinc sulfate solution decreases from 98.05 to 44.09 mg/L with the adsorbent dosage of 15 g/L.     

The inhibition of mild steel corrosion in acidic solutions by 2,5-bis(4-pyridyl)-1,3,4-thiadiazole: Structure-activity correlation

The effect of 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (4-PTH) on the corrosion of mild steel in acidic media (1 M HCl, 0.5 M H₂SO₄, 1 M HClO₄) has been investigated using weight loss measurements, electrochemical impedance spectroscopy and potentiodynamic polarisation. These studies have shown that 2,5-bis(4-pyridyl)-1,3,4-thiadiazole is good inhibitor for mild steel in 1 M HCl, 0.5 M H₂SO₄ and 1 M HClO₄ solutions, the better performances are seen in the case of 1 M HCl solutions. But in 1 M HClO₄, the 4-PTH stimulates corrosion at low concentrations. Polarisation curves indicate that the 4-PTH is a mixed-type inhibitor in all acidic media and E (%) is temperature-dependent. Adsorption on the mild steel surface follows the Langmuir isotherm model in all acidic media. The electronic properties obtained using the Hartree-Fock AB initio 3-21G quantum chemical approach, were correlated with the experimental efficiencies.     

Cobalt(III) complexes of macrocyclic-bidentate type as a new group of corrosion inhibitors for iron in perchloric acid

A new class of corrosion inhibitors for iron demonstrated by group of cobalt complex compounds, of the general formula [Co^III(Rdtc)cyclam](ClO₄)₂ (1)-(4) where cyclam is 1,4,8,11-tetraazacyclotetradecane and Rdtc⁻ refer to piperidine-, 2-, 3- or 4-methylpiperidine-dithiocarbamates, respectively, has been studied. Investigation of inhibiting properties of the ligands themselves as well as their corresponding complexes with Co(III) ion, was performed in 0.1 M HClO₄ by potentiodynamic polarization and EIS measurements. Cobalt complexes inhibit the corrosion of iron better than the uncomplexed ligands. Polarization curves suggest that complexes and amino-ligands are mixed-type and cathodic-type inhibitors, respectively. The impedance of the inhibited substrate increased with increasing concentration of inhibitor. An equivalent circuit is suggested, based on the analysis of EIS spectra. Using molecular modeling calculations, the complex compounds were characterized in terms of their structural properties and inhibition activity.