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Titanium and its alloys show outstanding application prospects in many industrial fields due to their high specific strength, good corrosion resistance, high temperature resistance, etc. However, at present the corrosion failure of Ti alloys is attracted great attention with gradual increasing applications. The different localized corrosion forms are introduced in this review, including galvanic corrosion, crevice corrosion, pitting corrosion, hydrogen-induced cracking, stress corrosion crack, microbiological corrosion, corrosion fatigue, and corrosion wear. Among them, the corrosion failure caused by stress corrosion crack and hydrogen-induced cracking is the most serious, accounting for about 40–60% of the actual failure accidents. The effects of temperature, pressure, corrosive media and microstructure on the failure of titanium alloys are discussed. Particularly noteworthy, F− is extremely aggressive to the passive film of titanium alloys. The corrosion behavior of titanium alloys applied in aerospace, marine, petrochemical industries and biomedical environments are introduced. In the above environments, the passive film is most easily damaged and difficult to passive in high temperature and oxygen-deficient environments. The current controversies in the corrosion mechanism have also been proposed, especially the stress corrosion that has the most serious impact on titanium alloys, and its mechanism will be the focus of future research.

The destruction of Ti passive film cannot occur if fluorine ions (F−) are below a critical concentration in an open electrolyte with oxygen acting as a passivating agent. However, in an occluded crevice, few F− combined with oxygen deficiency environment may synergistically affect the crevice corrosion of Ti alloys. Herein, the role of few F−, which is far below the critical concentration in open solution, in the crevice corrosion of Ti-6Al-4V alloy was studied using a galvanic coupling technique combined with Kelvin probe force microscopy, atomic force microscope, and X-ray photoelectron spectroscopy. The results indicate that the F− dominate the crevice corrosion process of Ti alloys by occupying oxygen vacancies to form fluoride-titanium-oxygen compounds. A multicolored product film is generated in the crevice area, and its colors vary in the order of black-blue-purple-light yellow from the crevice edge to the interior, corresponding to the corrosion degree from severe to weak. Multicolor is formed by the interference phenomena due to the different surface roughness, thickness and chemical composition of the product film.

Purpose The formation and corrosion processes of a conversion film on the AZ80 Mg alloy with different second phases were compared to clarify the effect of microstructure on the quality of protective coatings. Design/methodology/approach The size and distribution of second phases in the edge and central regions of the AZ80 cast ingot exhibit a great difference. The film growth processes and their corrosion resistance on the edge and central regions of the AZ80 cast ingot were investigated by scanning electron microscope observations, immersion tests and electrochemical measurements. Findings The results indicate that second phases act as micro-cathodes and hydrogen evolution reaction occurs on their surface, which is not beneficial for the deposition of the conversion film. Originality/value The conversion film formed on the central regions of AZ80 cast ingot with a low volume fraction of second phases exhibits a more uniform surface and higher corrosion resistance than that formed on the edge regions of the sample with a higher volume fraction of second phases.

The galvanic corrosion of 2024/Q235/304 tri-metallic couple with different area ratios was systematically investigated in deoxygenated, aerated, and oxygenated 3.5 wt.% NaCl. It is found that the galvanic corrosion deteriorates with increasing of oxygen concentrations. Moreover, the variation of oxygen concentrations and area ratios can result in the cathode/anode role change of Q235. Galvanic corrosion may lead to the change of the corrosion forms of anodic metals, which depends on their own microstructure: Both uncoupled and coupled Q235 show uniform corrosion. However, uncoupled 2024 shows pitting corrosion and coupled 2024 shows intergranular corrosion.

The deterioration process of a self-sealing pore micro-arc oxidation (MAO) film was investigated. The surface and cross-section corrosion morphologies were observed by scanning electron microscopy (SEM). Chemical composition was detected by EDS elemental mapping and XRD. The corrosion process was analyzed by electrochemical impedance spectroscopy (EIS). The surface of the film in dark and light regions exhibits different corrosion behavior. In the dark regions, the corrosion process mainly concentrates on the migration of corrosion media through the pores inward. In the light regions, the transverse expansion of cracks plays a key role, accompanying the exfoliation of film constituents.

The influence of potassium fluoride (KF) on the in-situ sealing pores of plasma electrolytic oxidation (PEO) film on AM50 Mg alloy has been investigated using a stereo–zoom optical microscope, scanning electron microscope (SEM) and energy dispersive X–ray spectroscope (EDS). The films formed in the solutions with and without KF were compared. The sketch map for the formation process of in-situ sealing pores was established. The in-situ sealing pore PEO film can be obtained in the solution with KF, while the PEO film with open pores is obtained in the solution without KF. It indicates that the addition of KF plays a key role in the formation of in-situ sealing pores. Moreover, KF can change the initial film and promote the film growth rate.

A novel manasseite/carbonates composite conversion film on Mg2Zn alloy has been developed by a two-step in situ growth method. A precursor film with cracks is first formed and then it is transformed into a compact, uniform and well adherent manasseite/carbonates composite film. The corrosion tests show that the film can provide an excellent protection to the Mg2Zn alloy. From the comparison between the manasseite and its isomeride hydrotalcite, it can be found that the platelets of the manasseite film stack a little more densely, and the manasseite/carbonates composite film delays the initial corrosion of the Mg substrate more effectively than the hydrotalcite film.

The influence of alloying elements and microstructure of Mg substrates on the formation of hydrotalcite film has been investigated. It is found that the two-step process is also available for the pure Mg and other alloys after modification. A small amount of Zn does not impact the composition of the hydrotalcite film much; whereas the highly active rare earth (RE) affects the constituents of the precursor film as well as the final film on WE54 alloy significantly. The microstructure impacts the initial nucleation and the film morphology depending on the size and chemical activity of the intermetallic particles.

The influence of assembled time on the corrosion behaviors of SAMs film on the AM60B alloy and its assembled mechanism have been investigated by electrochemical measurements, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. The self-assembled experiment on the AM60B magnesium alloy indicates that the corrosion susceptibility decreases with increasing assembled time until 24 h on cast AM60B alloy and then increases with increase of the assembled time proved by EIS measurement and potentiodynamic curves. The self-assembled experiments on pure magnesium and various heat treated cast AM60B magnesium alloy illuminate that the dissolved aluminum in magnesium solid solution is the key factor for assembled efficiency and is hard to self-assemble on the pure magnesium without aluminum. The corrosion resistance of self-assembled film on AM60B magnesium alloy is monotonically increasing with the dissolved aluminum. The results of XPS analysis reveal the assembled mechanism on AM60B and corroborate the function of Al element.

The corrosion product films formed on the surface of Mg–2Zn and Mg–5Zn alloys in NaCl solution were investigated by electrochemical measurement, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. It is found that a compact corrosion product film is formed in the initial stage of immersion, and then the film gradually degrades due to dissolution reaction. The product film formed on Mg–2Zn alloy presents better protection property than that on Mg–5Zn alloy, which can be attributed to the different chemical composition and microstructure of the both alloys.

The austenite reversion process and the distribution of carbon and other alloying elements during tempering in 0Cr13Ni4Mo martensitic stainless steel have been investigated by in-situ high temperature X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The microstructure of the reversed austenite was characterized using transmission electron microscopy (TEM). The results revealed that the amount of the reversed austenite formed at high temperature increased with the holding time. Direct experimental evidence supported carbon partitioning to carbides and Ni to the reversed austenite. The reversed austenite almost always nucleated in contact with lath boundary M23C6 carbides during tempering and the diffusion of Ni promoted its growth. The Ni enrichment and the ultrafine size of the reversed austenite were considered to be the main factors that accounted for the stability of the reversed austenite.

An environmentally friendly corrosion inhibitor phytic acid (PA) has been successfully added to modify the hydrotalcite (HT) film by adsorption and ion exchange approaches. In alkali PA solution, PO 4 3 - cannot substitute CO 3 2 - in the HT, but adsorb to the external surface; H 2 PO 4 - / HPO 4 2 - can insert into the HT after decarbonation under acidic condition. Partial decomposition occurs under both conditions. The PA/pH 11-HT film shows superior stability and lasting protective efficiency, and also can provide self-healing effect associated with the P adsorption in the filament. The worse resistance of the PA/pH 6.5-HT film may be related to the micro-cracks.

The naturally formed oxide films on Mg–2Zn and Mg–5Zn alloys were investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The oxide films on the both alloys present a similar chemical composition, consisting of surface layer of basic magnesium carbonate and MgO following with MgO and ZnO, but the oxide film on Mg–5Zn is thicker and contains more defects. The protection performance of the oxide film on Mg–5Zn is worse under open circuit potential but better in a suitable anodic potential scope compared with that on Mg–2Zn alloy.

The effect of Zn concentration on the corrosion behavior of Mg-xZn alloys is investigated. The microstructure is observed by scanning electron microscopy (SEM). The corrosion resistance is evaluated by electrochemical and immersion tests. The initiation and propagation of corrosion are analyzed based on the electrochemical measurements and corrosion morphologies observation. It is found that the MgxZny second phases are precipitated on the boundaries and their volume fractions rise with increasing Zn concentration. As a result, the corrosion resistance of Mg-xZn alloys reduces with increasing Zn concentration, which is greatly associated with the micro-cathodic effect of the MgxZny second phases. (C) 2012 Elsevier Ltd. All rights reserved.

The corrosion mechanism of Mg-Al-CO32- hydrotalcite film is discussed based on its corrosion behavior and the anion-exchange or adsorption effect of Cl-. The results demonstrate that the film is very compact and acts as a barrier against Cl- attack in the early stage. Then the surface of the film is dissolved gradually due to its instability in aqueous solutions. Only after eliminating CO32-, Cl- can insert into the hydrotalcite interlayer to balance charges. Cl- cannot substitute CO32- just in NaCl solution, but can adsorb on active spots of the film to accelerate the decomposition. (C) 2012 Elsevier Ltd. All rights reserved.

By means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), open circuit potential (OCP) measurement and scanning electronic microscope (SEM), the in situ growth mechanism of the Mg-Al hydrotalcite film has been proposed. The composition and morphology of the films undergo a series of variations during the growth processes. The added Al compounds are a vital contribution to the formation of hydrotalcite. The film formation involves the dissolution of the AZ31 substrate, adsorption of the ions from solution, nucleation of the precursor, followed by the dissolution of Al3+, exchanging of OH- by CO32- and growth of the hydrotalcite film. (C) 2012 Elsevier Ltd. All rights reserved.

The MgxZny second phases in Mg-5Zn alloy were adjusted by T4 and T6 treatment, and then the role of second phases in the corrosion behavior of Mg-5Zn alloy was investigated. The dimension of the MgxZny second phases is only some hundred nanometers. They present discrete distribution on the boundaries of the as-received sample and continuous distribution in the T6 sample. The T4 sample in absence of second phases exhibits the best corrosion resistance while the T6 sample with the largest volume fraction of second phases exhibits the worst one, which can be attributed to the micro-cathodic effect of second phases. (c) 2012 Elsevier Ltd. All rights reserved.

The deterioration process of a plasma electrolytic oxidation (PEO) coating containing zirconium oxides on AM30 magnesium alloy in 3.5 wt.% NaCl solution have been investigated. The coating consists of an outer porous layer and an inner dense layer. The content of MgF(2) is high in the pores and an MgO-rich layer is evident in the inner layer. The corrosion resistance of the outer layer gradually decreases in the initial immersion stage (96 h) due to the decomposition of MgO. and the deterioration of the inner layer is delayed by the blocking effect of the outer layer. (C) 2011 Elsevier Ltd. All rights reserved.

In situ growth of Mg–Al hydrotalcite conversion film on AZ31 alloy has been developed by a two-step method. The characteristics of the films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electronic microscope (SEM) observation, electrochemical and immersion tests. The film formation process was proposed based on the open circuit potential (OCP) measurements and surface analysis. A precursor film with network cracks is first formed and then this film is transformed into a compact and uniform hydrotalcite (Mg6Al2(OH)16CO3·4H2O) film after the post treatment. This dense Mg–Al hydrotalcite film can provide effective protection to the AZ31 alloy.

Product/metal ratio (PMR) was introduced as a novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. The criterion initially sprang from Pilling-Bedworth ratio (PBR), focused on the roles of electrolytes for the compactness of the fabricated coatings, and took attention on properties of reactants/products during MAO. Meanwhile, based on our experiments as well as the results from literatures, the effects of electrolyte additives on morphologies and compositions of the fabricated MAO coatings of Mg alloys were exploited for verification and supplement of the initial criterion. In combination of the initial PMR criterion and experimental verification, PMR could be represented by special mode (PMR s=V oxide products/V alloy substrates) and general mode (PMR g= PMR s+ PMR d). The ideal PMR s should be between 1 and 2, while PMR d is related to the coating deposition during MAO. PMR d is a supplement to PMR s when the effect of the overlaying property (O) of the coatings and the effective deposition (D) of electrolyte composites are considered (PMR d=f(O, D). O is related to the melting point (MP) and boiling point (BP) of the MAO products. D is related to the effective reactions between alloy substrates and electrolytes during MAO.

The corrosion mechanism of Mg-8Li alloy in NaCl solution was investigated by electrochemical testing and SEM observation. The electrochemical results indicated that the corrosion resistance of Mg-8Li alloy in 0.1 M NaCl solution gradually deteriorated with increasing of immersion time expect for 2 h immersion, which was consistent with the SEM observation of corrosion morphology. Mg-8Li alloy exhibited filiform type of attack under significant anodic control of magnesium solution reaction. The cathodic reaction was driven by hydrogen evolution reaction. The presence of filiform corrosion also proved a resistant oxide film naturally formed on the surface of Mg-8Li alloy. (C) 2009 Elsevier Ltd. All rights reserved.

A novel phosphate conversion film was obtained on Mg–8.8Li alloy. The film formation mechanism was studied by electrochemical measurements, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. The deposition of conversion film contained three stages. Stage I: loose surface film consisting of oxides and hydroxides was dissolved to expose the bare substrate. Stage II: β phase was preferentially dissolved, accompanying with the deposition of conversion film, and then α phase was deposited with conversion film. Stage III: conversion film was gradually improved until the dynamic balance of film dissolution and formation was established.

In this paper, the control equation is first set up by using the reciprocal theory method when considering the section rotation of deep beam, shear deformation of adjacent cross section and the transverse stress. Then the control equation of deep beams under three kinds of boundary conditions is established. According to the comparison between the numerical value and finite element simulation value, we can know that the control equation and the deflection equation of the forced vibration of deep beams are correct. This is a new computing method of the forced vibration problem of deep beams under distributed harmonic load.