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The low-temperature electrochemical corrosion behavior of aircraft 2024 aluminum alloy was investigated in different concentrations of potassium-acetate-type fluid, and the results showed that the corrosion medium initially impacted some corrosiveness on the 2024 aluminum alloy. However, the corrosion rate of the alloy decreased with prolonged immersion in the corrosion medium, and this was attributed to the protective effect of the corrosion products layer, which impeded the attack by the aggressive solution. Time-domain analysis of electrochemical noise measurements at -5 °C revealed the occurrence of uniform corrosion on the alloy at the initial stages of immersion, general corrosion at the middle stages of immersion, and a pitting-dominated corrosion type from the middle to late stages of immersion.

This study focuses on the tribological characteristics of polytetrafluoroethylene rubbing against 316L stainless steel in the presence of various Al2O3 abrasive particles. The ranges of the contact stresses and abrasive sizes were 2.50–5.01 MPa and 2.0–230 µm, respectively. The evolution of the coefficient of friction, abrasive invasion behavior and damage characteristics on worn surfaces, and the transformation of damage mechanisms under different contact stresses are summarized. Results demonstrate that contact stress exerts a considerable effect on abrasive invasion behavior, the coefficient of friction trends become complicated at low contact stress. With decreasing particle sizes under a contact stress of 2.50 MPa, the wear failure changes from two-body wear to three-body wear, or two wear behaviors exist simultaneously. As contact stress increases, the wear mechanism is gradually simplified. The particle size threshold that causes the change of the wear mechanism is also gradually reduced. The wear rate of polytetrafluoroethylene has no linear relationship with abrasive size, whereas wear increases with contact stress. Under 2.50 MPa conditions, the particles easily invade the tribo-interface in the friction process, thus increasing the wear rate of the metallic counterpart. In addition, with increases contact stress, the polytetrafluoroethylene material transfers to the counterface, and local adhesion occurs. When the contact stress is increased to 5.01 MPa, polytetrafluoroethylene even shows an obvious creep.

PurposeThis paper aims to prepare a residual rust epoxy coating by adding different quantities of phytic acid (PA) on the surface of the rusty steel and investigate the corrosion protection of PA and its action mechanisms.Design/methodology/approachA residual rust epoxy coating by adding different quantities of PA was prepared on the surface of the rusty steel. The influence of PA on the corrosion resistance of epoxy-coated rusty steel was investigated by means of electrochemical impedance spectroscopy and adhesion testing.FindingsResults indicated that PA can substantially improve the corrosion resistance of epoxy-coated rusty steel. This improvement is due to the reaction of PA with residual rust and generation of new compounds with protection properties and increased adhesive strength effects on the coating/metal interface. The coating showed better protection performance when 2 per cent PA was added.Originality/valueConsidering the structure of the active groups, PA has strong chelating capability with many metal ions and can form stable complex compounds on the surface of a metal substrate, thereby improving corrosion resistance. In recent years, PA has been reported to be useful in the conversion of coatings or as green corrosion inhibitor. To the best of the authors’ knowledge, few studies have reported the use of PA as a rust converter or residual rust coating. The present work aims to improve the corrosion resistance of residual rust epoxy coating by adding PA.

Purpose This paper aim to investigate the influence of PA on the corrosion behavior of carbon steel with blast cleaned or pre-rusted treatments, and interpret the inhibition mechanism of PA on the steel with different surface treatments. Design/methodology/approach The influence of PA on the corrosion behavior of blast cleaned or rusty steel was investigated by means of electrochemical impedance spectroscopy (EIS). The EIS data were analyzed using the @ZsimpWin commercial software. The morphology and component of steel after immersion were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Fourier transformation infrared (FTIR) and X-ray diffractometer (XRD). Findings EIS analysis results indicated that PA had good corrosion inhibition for blast cleaned or rusty steel. SEM, EDS, FTIR and XRD further indicated that PA had two main corrosion inhibition processes for the corrosion inhibition of blast cleaned or rusty steel: corrosion dissolution and formation of protective barrier layers. Originality/value Most published works focus the attention only toward the effect of corrosion inhibitor for the clean metal surfaces. However, the surface condition of metal sometimes is unsatisfactory in the practical application of corrosion inhibitor, such as existing residual rust. Some studies also have shown that several corrosion inhibitors could be applied on partially rusted substrates. These inhibitors mainly include tannins and phosphoric acid, but not PA. Therefore, the authors investigated the influence of PA on the corrosion behavior of carbon steel with blast cleaned or pre-rusted treatments in this paper.

Purpose This paper aims to investigate the effects of deposition time on the structure and anti-corrosion properties of a micro-arc oxidation (MAO)/Al coating on AZ31B Mg alloy. Design/methodology/approach The study describes the fabrication of the coating via a combined process of MAO with multi-arc ion plating. The structure, composition and corrosion resistance of the coatings were evaluated using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and electrochemical methods. Findings The Al-layer is tightly deposited with a good mechanical interlock along the rough interface due to the Al diffusion. However, the Al layer reduces the anti-corrosion of MAO-coated Mg alloy because of structural defects such as droplets and cavities, which act as channels for corrosive media infiltration towards the substrate. Fortunately, the Al layer improves the substrate corrosion resistance owing to its passive behaviour, and the corrosion resistance can be enhanced with increasing deposition time. All results indicate that a buffer layer fabricated through the duplex process improves the interfacial compatibility between the hard coating and soft Mg alloys. Originality/value An MAO/Al duplex coating was fabricated via a combined process of MAO and physical vapour deposition. MAO/Al duplex coatings exhibit obviously passive behaviours on AZ31 Mg alloy. The structure and corrosion resistance of MAO/Al coatings were investigated.

A hydrophobic surface was fabricated on a micro-arc oxidation (MAO) treated AZ31 Mg alloys via surface modification with myristic acid. The effects of modification time on the wettability of the coatings were investigated using the contact angle measuring device. The surface morphologies and structure of the coatings were evaluated using SEM, XRD and FT-IR. The corrosion resistance was investigated by potentiodynamic polarization curves and long-term immersion test. The results showed that the water contact angle (CA) increases gradually with modification time from 0 to 5 h, the highest CA reaches 138° after being modified for 5 h, and the number and size of the micro pores are decreased. The modification method hardly alters crystalline structure of the MAO coating, but improves the corrosion resistance based on the much positive potential and low current density. Moreover, the corrosion resistance and hydrophobicity can be enhanced with increasing the alkyl chain. The wetting and spreading for the alkylcarboxylate with low surface energy become easier on the micro-porous surface, and alkylcarboxylate monolayer will be formed through bidentate bonding, which changes the surface micropores to a sealing or semi-sealing structure and makes the MAO coating dense and hydrophobic. All the results demonstrate that the modification process improves the corrosion protection ability of the MAO coating on AZ31B Mg alloy.

Micro-arc oxidation (MAO) coatings were obtained on AZ91D Mg alloy in an alkaline Na2SiO3-NaF base solution with and without K2ZrF6. Then, the surface and cross-sectional morphologies, elemental compositions and phase structure of the resulting coatings were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion behaviour of the coatings was evaluated by potential dynamic polarization (PDP) curves and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results show that the self-sealing MAO coating which consists of MgO, MgF2, t-ZrO2, MgSiO3 and amorphous phosphate related to Mg can be fabricated in a K2ZrF6-containing electrolyte solution. With increasing K2ZrF6 content, the coating exhibits a more dense cross-sectional morphology and less coating defects, along with better anticorrosion properties than the traditional MAO coating. The ZrF62− plays an important role in producing the self-sealing MAO coating, which results in a three-layer structure: MgO-rich inner layer, MgF2-rich sealed pore layer and porous outer layer. The hydrolysed ZrO2 can promote the formation of the seal pores during MAO process due to its high boiling point.

A hydrophobic surface was fabricated through the micro-arc oxidation (MAO) and subsequent stearic acid surface modification of AZ31 Mg alloy, achieving a maximum water contact angle of 151.5° after 10 h of modification. Through surface analyses and investigation into the corrosion behavior, this hydrophobicity is ascribed to a combination of the MAO coating’s rough micro-pore structure and the formation of a stearic acid monolayer through bidentate bonding. The hydrophobic MAO coating effectively inhibits corrosion of the Mg alloy, especially pitting corrosion, and demonstrates that surface modification could further increase or accelerate the application of MAO treatment to Mg alloys.

A duplex-layered phosphate conversion coating was obtained on AZ31 Mg alloy by substituting NaF bath with a citric bath. The morphology, composition and corrosion resistance of the coating were investigated using SEM, EDS, SPM and electrochemical methods. A three-stage mechanism for initial formation of the coating was proposed: Dissolution of the loose oxide film and deposition of Mg3(PO4)2 and AlPO4, formation of a composite intermediate layer of Mg3(PO4)2, AlPO4 and Mg(OH)2, and deposition of manganese phosphate nuclei followed by growth and lamination of the nuclei. The nuclei preferentially deposit at the Al–Mn phase surface and near the grain boundary.

A phosphate solution free of chromate, fluoride and nitrite was prepared and an environment-friendly film was obtained on AZ31 magnesium alloy surface via the chemical deposition method. The morphology, composition, phase structure and its corrosion resistance were studied. The effects of film-forming temperature and free acid on corrosion resistance, microstructure and electrochemical behavior of the film were discussed. The results indicate that the corrosion resistance of AZ31 with the phosphate film was better than blank AZ31 substrate, which was most attributed to the great inhibitive action on the anodic dissolution and cathodic hydrogen evolution of the film.

An anticorrosion phosphate film with free of chromate, fluorides and nitrite, was successfully prepared on the surface of the magnesium alloy AZ31 via chemical deposition method. The effects of film-forming temperature and free acid on the corrosion resistance of the film were investigated through SEM. The analysis results indicate that the compactness of the film increased with the increase of film-forming temperature, but more residues is brought. While increasing FA in the proper range can decrease the residues. The EDS characterizations of the film confirm that the phosphate film contains only three elements of Mn, P and O. The analysis of the polarization curves further prove that the film has great inhibitive action on anodic dissolution and cathodic hydrogen evolution, and the corrosion resistance is improved with the increase of temperature