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• Air-formed (AF) film is characterized under assistance of ultramicrotomy. • AF film is negligible on α but thick on β after prolonged air exposure. • AF film on β has a MgO/Mg(OH) 2 inner layer and a Li 2 CO 3 outer layer. • AF film on β elevates surface potential of the phase in air. • AF film on β retards filiform corrosion in LZ91 and LA141 alloys. It is recently suggested that air-formed film plays an important role in controlling corrosion resistance of Mg-Li alloys. However, the structure of the air-formed film and its effect on corrosion resistance of Mg-Li alloys has not been fully understood. Firstly, the air-formed films formed on α and β phases in a dual-phase LZ91 Mg-Li alloy after exposure to laboratory air for up to 48 h have been examined by SEM under the assistance of ultramicrotomy. Then, the effect of the air-formed film on surface potential and, consequently, corrosion/oxidation behavior of the alloy has been investigated. Finally, in order to exclude the influence from α phase, the structure of the air-formed film on β phase and its effect on corrosion/oxidation behavior of Mg-Li alloys have been studied based on a single-phase LA141 Mg-Li alloy. The results show that the air-formed film is thin and negligible on α phase but thick on β phase after prolonged exposure to laboratory air. The thick air-formed film on β phase has a multilayer structure with an inner layer consisting of MgO/Mg(OH) 2 and outer layer consisting of Li 2 CO 3 , which greatly elevates the surface potential of β phase in air. Both LZ91 and LA141 Mg-Li alloys firstly undergo uniform corrosion and then filiform corrosion when immersed in NaCl solution and the pre-existed air-formed film on β-Li phase can retard the occurrence of filiform corrosion in the alloys. [ABSTRACT FROM AUTHOR]

The direct industrial importance of the corrosion resistance in WE43 is best emphasized by its extensive use in the automotive, aerospace and electronic industries where weight reduction is a necessary requirement. In this work, the corrosion especially the filiform corrosion in a 3.5 wt.% NaCl solution and their dependence on the Zr distribution for WE43 were studied by weight loss tests, hydrogen evolution tests, electrochemical measurements and microscopic analyses. The Zr distribution significantly influenced the initiation and propagation of the filiform corrosion, and accordingly significantly influenced the corrosion rate. WE43 with a cluster Zr distribution displayed filiform corrosion throughout the entire surface with an irregular network distribution of filaments. WE43 with a uniform line Zr distribution exhibited only a few examples of linear filiform corrosion. WE43 with a dispersive Zr distribution exhibited no filiform corrosion. The stability of the corrosion film was responsible for the filiform corrosion. Anodic polarisation promoted the initiation and progression of the filiform corrosion, while cathodic polarisation had an inhibiting effect on the filiform corrosion. The serrated interface of the filiform corrosion increased the contact area between the substrate and the corrosive medium, and hence increased the corrosion rate. [ABSTRACT FROM AUTHOR]

Synergistical passive barrier enhancement and active self-healing has been an emerging tactic for developing prominent anti-corrosion coatings. Herein, a cerium/2-methylimidazole metal-organic framework (Ce-MOF) was that is in-situ grown on ultrathin α-ZrP nanosheets with outstanding dispersibility. The parallel arrangement of α-ZrP-based nanosheets within the epoxy matrix was driven by shear force from blade-coating technology optimizing the coating's physical barrier and self-repairing capability. This advancement may herald a new era in designing smart coatings and offering potent protection against diverse corrosion types such as filiform corrosion. • Ce-MOF was grown in-situ on ultrathin α-ZrP nanosheets using a one-pot co-precipitation method. • The compatibility and dispersity of CeMOF@PDA-ZrP provided strong adhesion and toughness for the epoxy coating. • The coating offers long-term physical barrier protection and effectively inhibits filiform corrosion of carbon steel. • Local electrochemical impedance spectroscopy showed the strong barrier of the nanosheets and pH-sensitive release of inhibitors. [ABSTRACT FROM AUTHOR]

Organic coatings delamination and corrosion pose an ongoing challenge in materials science. This review explores the knowledge of corrosion mechanisms in painted steel structures up to date, focusing on filiform corrosion (FFC) investigating its mechanisms, and discussing available prevention strategies alongside their suitability. First reported in the 1940s, FFC garnered significant attention in the subsequent decades, particularly in the study of coated aluminum, and very recently magnesium alloys. However, its consideration as a component of steel degradation remained comparatively limited during this period. Despite advancements, comprehensive analysis of FFC has been infrequent, prompting a call for an updated critical analysis. This survey emphasizes the necessity for organized FFC mitigation strategies, discussing alternatives and emerging trends. It underscores FFC's potential role as a precursor to general corrosion, emphasizing the economic implications of aesthetic damage. This review aims to fill gaps, assess current knowledge, and pave the way for future research in the realm of organic-coated steel corrosion. [Display omitted] • Filiform corrosion is a dramatic failure mechanism in organic-coated steel. • The coating delamination is verified to be driven by the anodic corrosion reactions. • The metal-paint interface is subjected to a harsh chemical environment discernible between anodic and cathodic sites. [ABSTRACT FROM AUTHOR]

This study investigates the effectiveness of conversion coatings (CCs) in mitigating filiform corrosion (FFC) on acrylic-coated steel substrates, with a specific focus on iron phosphatizing and trivalent chromium conversion coatings. Aiming to evaluate the effectiveness of a simulated electrochemical approach introduced in a previous publication, several parameters are determined through an approach involving potentiodynamic polarization curves and zero resistance ammeter on bare CCs. These experiments are conducted in test solutions designed to replicate the electrolytic conditions the substrate encounters undergoing FFC. Similar measurements are collected in neutral testing solutions and the outcomes of the two approaches are discussed regarding the performances of the organic coated sample. The electrochemical evaluation of CCs highlights their potential to reduce the driving forces and nucleation tendency. However, for the steel coated with acrylic paint, while CCs reduce the initiation sites for delamination, they have limited impact on filament propagation. • Employing conversion coatings shows promise as an effective strategy to mitigate filiform corrosion on painted steel. • An electrochemical simulation approach can be applied to anticipate the performance of inorganic coatings. • Simulated quantitative data are deliberated in reference to the actual corrosion resistance. [ABSTRACT FROM AUTHOR]

Zirconium oxide-based conversion coatings (CCs) were prepared from hexafluorozirconic acid, H 2 ZrF 6 , with and without CuII as an additive. The CCs were prepared on primary and recycled AA6060. The deposition mechanism, structure, electrochemical properties and effectiveness in protection against filiform corrosion (FFC) have been explored. In presence of CuII additives, a two step deposition mechanism is observed, evidenced by open circuit potential measurements and in situ dynamic electrochemical impedance spectroscopy. Copper deposits predominantly as copper(II) hydroxyfluoride on the top surface, and metallic Cu 0 at the metal/CC interface, as evidenced by x-ray photoelectron spectroscopy and glow-discharge optical emission spectroscopy. The presence of a CuII additive induces a different distribution of the ZrO 2 -based products. After deposition of a weak organic model coating, aluminium pretreated with the CuII-containing system shows a 10%–30% lower maximum filament length compared to the CuII-free system in FFC tests, despite a ca. one order of magnitude higher cathodic activity of the former. [Display omitted] • Deposition of ZrO 2 based conversion coatings is assisted by CuII additives. • Copper hydroxide deposits in the top regions of such conversion coatings. • Presence of CuII in the conversion bath influences the distribution of ZrO 2 • Higher cathodic activity does not induce faster filiform corrosion. • Comparable filiform corrosion performance between primary and recycled alloys. [ABSTRACT FROM AUTHOR]

Abstract The effect of Sn additions on the microstructure and corrosion behavior of new wrought Mg-5Al-xSn (x = 0.71, 1.47, 2.44, and 3.09 wt%) was studied through optical and electron microscopy, XRD, hydrogen evolution test, and electrochemical measurements. Increasing of the Sn content was followed an increase in the quantity of Sn dissolved in the Mg matrix and precipitation of Mg 2 Sn intermetallics when Sn content exceeded 1.47 wt%. The corrosion mode in a short immersion period was pitting corrosion; presence of cathodic sites (i.e., Mg 2 Sn intermetallics) deteriorated the alloys' corrosion resistance. However, with a long immersion period the corrosion mode of the alloys with Sn content above 0.71 wt% was mixed pitting and filiform corrosion, indicating that a passivation layer was formed on the alloys' surface and increased their corrosion resistance. Highlights • Increasing Sn above 1.5 wt% results in precipitation of Mg 2 Sn intermetallics. • Sn content above 0.7 wt% changes the corrosion mode from pitting to pitting + filiform corrosion. • High Sn content (>1.5 wt%) promotes formation of corrosion products and improves long-period corrosion resistance. [ABSTRACT FROM AUTHOR]

In this work, two rare earth carboxylate compounds, lanthanum 4-hydroxycinnamate (La(4-OHcin) 3) and yttrium 3-(4-methylbenzoyl)propanoate (Y(mbp) 3), were incorporated into bisphenol-based epoxy resin to investigate their effectiveness in coating barrier properties and active corrosion inhibition. EIS results showed that the incorporation of rare earth carboxylate inhibitors significantly improved corrosion resistance compared to the inhibitor free coating, with the global impedance modulus remaining at a level higher than 1 GΩ cm2 after 219 days immersion. Following EIS experiments, cross-sectional views of the coatings exhibited a pore-plugging behavior by rare earth containing precipitates, which reinforced the coating barrier properties and delayed the electrolyte diffusion process. These effects were also reflected from the electrochemical parameters extracted from breakpoint frequency analysis and equivalent circuit modelling. Filiform corrosion experiments for artificially scratched coatings suggest that the addition of rare earth carboxylates effectively suppressed the initiation and growth of filaments as well as the development of the coating delamination front. The active corrosion inhibition is possibly related to the formation of a surface protective film consisting of bimetallic complexes and rare earth metal rich precipitates. The electrochemical measurements and surface analyses evidence the dual function of rare earth carboxylate species for enhancing coating barrier properties against electrolyte penetration and providing active corrosion inhibition for the underlying AS1020 mild steel. • Rare earth carboxylates in coating enhanced the corrosion protection of mild steel. • The pore-plugging effect was observed to improve the coating barrier properties. • Active corrosion inhibition was confirmed using artificially scratched coatings. • Inhibition mechanism of rare earth carboxylates in coatings has been discussed. [ABSTRACT FROM AUTHOR]

This study investigated the rust creep propagation of two different coating systems, an epoxy primed (EP/PUR) and a zinc-rich epoxy primed (ZnEP/PUR) coating system exposed to the field and the accelerated cyclic ageing test. Non-destructive scanning acoustic microscopy (SAM) was used to assess the coating samples and detect rust creep propagation. The results showed a linear increase in rust creep propagation over time in all coating systems exposed to the field and the cyclic ageing test. The EP/PUR coating system exhibited a rust creep propagation rate of 0.228 mm/week under field exposure and 0.380 mm/week in the cyclic ageing test, while the ZnEP/PUR coating system showed rates of 0.024 mm/week under field exposure and 0.292 mm/week in the cyclic ageing test. Notably, the rust creep propagated through filiform corrosion mechanism under field exposure for both coating systems and that was not observed in the cyclic ageing test. The reliability of evaluating the anti-corrosive performance and rust creep growth around a scribe of different anti-corrosive coating systems exposed to the accelerated cyclic ageing test, as defined in ISO 12944-9, is questionable. The study highlights the benefits of applying non-destructive scanning acoustic microscopy for rust creep propagation investigation capturing information that may be missed by the traditional standardised method. • Rust creep of coating systems exposed to field and cyclic ageing test was assessed. • Scanning acoustic microscopy is effective in revealing rust creep propagation. • Rust creep propagates via a linear kinetic function over exposure time. [ABSTRACT FROM AUTHOR]

Abstract Electroless nickel/nickel-boron (Ni/Ni-B), Ni/alkaline Ni-P and Ni/acidic Ni-P double-layer coatings were deposited on AZ31 magnesium (Mg) alloy prepared by environmental fluoride-free solution under low temperatures. The morphologies, compositions, and crystal structures of three different double-layer coatings were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion resistance of the different double-layer coatings on Mg alloy surface was studied by electrochemical polarization and electrochemical impedance spectroscopic (EIS) tests, which indicated that the Ni/Ni-B coating presented the more superior corrosion resistance than the Ni/alkaline Ni-P and Ni/acidic Ni-P coatings. Meanwhile, the Ni/Ni-B coating exhibited good repeatability through the electrochemical polarization tests for three Ni/Ni-B samples that prepared under the same processing conditions. In addition, the optimal plating time for the Ni-B coating was further determined by electrochemical polarization tests of samples with different plating times. Highlights • The double-layer Ni/Ni-B coating on AZ31 Mg alloy is prepared by electroless method under relatively lower temperatures. • The pretreatment, activation and plating solutions completely remove fluoride. • The Ni/Ni-B coating exhibits good repeatability. • The optimal plating time of the outer Ni-B coating is obtained. [ABSTRACT FROM AUTHOR]

The filiform corrosion mechanism on acrylic-coated steel is investigated through potentiodynamic curves collecting anodic and cathodic branches in two different electrolytes simulating the liquid environments at the metal-paint interface. Zero resistance amperometry was chosen as an additional tool to achieve numerical results. The potential difference and the current density between the anodic and cathodic portions of the filament tip were estimated using a simplified setup. The role of oxygen availability on the corrosion mechanisms was also investigated by modifying the amount of dissolved oxygen in the simulating electrolytes and the humidostatic chamber employed for FFC propagation. [Display omitted] • Acrylic-coated carbon steel is susceptible to filiform corrosion. • The leading head is characterized by a typical "V-shape". • The morphology and the corrosion rate are determined by the oxygen available. • Electrochemical analyses are able to assess the driving force of the mechanism. • Anode and Cathode reactive sites have comparable areas. [ABSTRACT FROM AUTHOR]

This work aims at studying by electrochemical impedance spectroscopy (EIS) the susceptibility to filiform corrosion of low carbon steel covered by cataphoretic coating. The determination of the exposed metallic area variations of scratched samples during ageing test is an estimation of the disbonding of the coating and/or the filiform corrosion. This area can be evaluated by electrochemical impedance spectroscopy (EIS). A simplified electrical equivalent model used to estimate the exposed metallic area is valid if the corrosion products are correctly dissolved before characterization. Furthermore the steel is a very complex substrate and thus many parameters must be optimized in order to remove the corrosion products before EIS measurements. [ABSTRACT FROM AUTHOR]

Surface treatment of aluminium alloys using steam with oxidative chemistries, namely KMnO 4 and HNO 3 resulted in accelerated growth of oxide on aluminium alloys. Detailed investigation of the corrosion performance of the treated surfaces was carried out using potentiodynamic polarisation and standard industrial test methods such as acetic acid salt spray (AASS) and filiform corrosion on commercial AA6060 alloy. Barrier properties of the film including adhesion were evaluated using tape test under wet and dry conditions. Electrochemical results showed reduced cathodic and anodic activity, while the protection provided by steam treatment with HNO 3 was a function of the concentration of NO 3 − ions. The coating generated by inclusion of KMnO 4 showed highest resistance to filiform corrosion. Overall, the performance of the steam treated surfaces under filiform corrosion and AASS test was a result of the local coverage of the alloy microstructure resulting from steam containing with KMnO 4 and HNO 3 . [ABSTRACT FROM AUTHOR]

The steam treatment of aluminium alloys with varying vapour pressure of steam resulted in the growth of aluminium oxyhydroxide films of thickness range between 450 and 825 nm. The surface composition, corrosion resistance, and adhesion of the produced films were characterised by XPS, potentiodynamic polarisation, acetic acid salt spray, filiform corrosion test, and tape test. The oxide films formed by steam treatment showed good corrosion resistance in NaCl solution by significantly reducing anodic and cathodic activities. The pitting potential of the surface treated with steam was a function of the vapour pressure of the steam. The accelerated corrosion and adhesion tests on steam generated oxide films with commercial powder coating verified that the performance of the oxide coating is highly dependent on the vapour pressure of the steam. [ABSTRACT FROM AUTHOR]

The filiform corrosion (FFC) of organic coated magnesium alloys is investigated using in-situ scanning Kelvin probe and time-lapse photography. FFC is initiated by injecting MgCl 2 , HCl and FeCl 2 into a coating defect and ensuing FFC propagation rates are shown to increase as a logarithmic function of the chloride ion concentration and are strongly dependent on relative humidity. Post-corrosion surface analysis shows chloride abundance near the filament leading edge and evidence of sequestration within corroded regions behind. The FFC mechanism is consistent with chloride-induced anodic dissolution at the front coupling with water reduction on a cathodically-activated corroded surface behind. • Organic coated Mg alloys (E717, AZ31 & AZ91) are susceptible to filiform corrosion. • The filiform corrosion propagation rate is dependent on the Cl- concentration. • Increasing relative humidity produces higher rates of filiform corrosion propagation. • Chloride-induced anodic dissolution at the filament leading edge. • Progressive depletion of Cl- via entrapment within the corrosion product. [ABSTRACT FROM AUTHOR]

• Development of smart composite coatings combining nanocontainers with fluorescent dye and inhibitor. • Detection by fluorescence of the initial stage of the degradation in FFC. • Inhibition of cathodic delamination during the FFC. Filiform corrosion (FFC) is a degradation affecting coated steel in atmospheric conditions. For industrial application the corrosion detection at the early stage is important to apply the reliable protection. Investigation on the use of nanocontainers for the detection and protection of organic layer is described. Fluorescent dye was used to detect the active corrosion and to evaluate the efficiency of inhibitors encapsulated inside PVB model organic coating. Active dissolution is located at the filament head whereas the cathodic reaction occurs along the tail where the oxygen concentration is higher. The fluorescent dye use is a relevant technique for the detection of corrosion early stages and inhibition efficiency. [ABSTRACT FROM AUTHOR]

The in situ formation of I-phase improves the corrosion resistance of the duplex structured Mg–6% Li alloy. Corrosion attack on the surfaces of Mg–6% Li alloy is very inhomogeneous and severe pits and filiform corrosion clearly occur on the surfaces after immersion in 0.1M NaCl solution. For the I-phase-containing Mg–6% Li–6% Zn–1.2% Y alloy, the corrosion attack is homogeneous and no severe pits and filiform corrosion can be observed. [ABSTRACT FROM AUTHOR]

Abstract: Recently thin film metallic glass represents a class of promising engineering materials for structural applications. In this work, the Zr-based thin film metallic glass (TFMG) was fabricated on the Si and AISI 420 substrates using a Zr–Cu–Ni–Al alloy and pure Zr metal targets by a pulsed DC magnetron sputtering system. The chemical compositions, crystalline structures, microstructures and corrosion behavior in hydrochloric (HCl) aqueous solutions of Zr-based TFMGs were investigated. The results showed that the surface morphologies of Zr-based TFMG were very smooth. A compact and dense structure without columnar structure was observed. The amorphous structure of Zr-based TFMG was characterized by the X-ray diffractometer and transmission electron microscopy analyses. After the potentiodynamic polarization test, the better corrosion resistance was achieved for the Zr-based TFMG coated AISI 420 in 1mM HCl aqueous solution. Based on the surface morphologies and chemical analysis results of the corroded surfaces, the pitting, crevice corrosion and filiform corrosion were found. The corrosion mechanisms of the Zr-based TFMG were discussed in this work. [Copyright &y& Elsevier]

Abstract: In-coating cation and anion exchange pigments are studied with respect to their ability to inhibit chloride-induced filiform corrosion (FFC) on organic-coated AA2024-T3 aluminium alloy substrates. In-situ scanning Kelvin probe potentiometry is used to quantify both underfilm potentials associated with populations of propagating corrosion filaments and the kinetics of coating disbondment. Smart-release bentonite pigments containing exchangeable cerium (III) and yttrium (III) cations are shown to be largely ineffective in reducing rates of FFC propagation. The reasons for this are discussed in terms of the chemistry of the electrolyte-filled corrosion filament head. In contrast, anion-exchange hydrotalcite (HT) based pigments are highly effective inhibitors of FFC. A comparison of the extent of FFC observed for various inorganic exchangeable anions is made with as-received HT comprising carbonate anions. Of the anions evaluated, exchangeable chromate unsurprisingly provides the highest FFC inhibition efficiency. It is also demonstrated that exchanging the native carbonate ions for certain organic species which act as complexing agents for copper ions, gives rise to an equivalent level of FFC inhibition. The implication of these findings with respect to the mechanism of FFC on copper containing aluminium alloys is considered. [Copyright &y& Elsevier]

Abstract: The aim of this study is to develop a newly silane sol–gel pretreatment on the barrier properties and filiform corrosion resistance of 6016 aluminium alloy covered by cataphoretic coating. The sol–gel coatings are used as coupling agent between aluminium substrate and cataphoretic paint. The pretreatment is an aqueous solution of three different silane compounds (glycidyloxypropyltrimethoxysilane (GPS), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES)) hydrolysed at two different pH (2 and 3.5). A system without pretreatment was studied as reference. The electrocoatings were cured between 155°C and 195°C in order to modify their mechanical properties. Polarisation curves, EIS and FT-IR measurements were used in order to characterize the silane layers. EIS measurements were used to follow the barrier properties and the water uptake evolution on intact coatings. The filiform corrosion protection of the coating was also evaluated by a normalized filiform corrosion test. [Copyright &y& Elsevier]

Abstract: This article presents for the first time an in situ high-resolution study of the interaction between the active head in filiform corrosion (FFC) and intermetallic particles within an aluminium alloy. For the first time direct evidence will be provided that the intermetallic particles directly determine the so far seemingly random course of the filaments. Both the segments of active filaments and the intermetallic particles (IMPs) were successfully imaged in a humid air (ca. 85% RH) environment by scanning Kelvin probe force microscopy (SKPFM) through a plasma polymer coating of about 340nm thickness. In order to be able to do that, the experimental parameters need to be adjusted in such a way, that the width of the filaments is small enough to be well within the scan window of SKPFM (100μm×100μm). Also it is important that the small IMPs can still be mapped by SKPFM through the coating. This was successfully achieved by use of a HDMSO plasma polymer film. Surface potential values in the head region of the propagating filaments were found to be 200mV lower than the interface between intact plasma polymer and the aluminium alloy, indicating the active region. On the other hand, the surface potential values in the trailing filament tail are found to be about 250mV higher than background, pointing out the cathodic site and superpassivation due to the accumulated corrosion products in this region. It was found that the direction of the filament is determined by the location of the IMPs nearest to the active head. [Copyright &y& Elsevier]

Abstract: Highly corrosion-resistant nanocrystalline Mg–Zn–Y–Al multi-phase alloys have been prepared by consolidation of rapidly solidified (RS) ribbons. The relation between corrosion behavior and microstructure evolution of Mg–Zn–Y–Al alloys with a long period stacking ordered phase has been investigated. In order to clarify the influence of rapid solidification on the occurrence of localized corrosion such as filiform corrosion, several Mg96.75Zn0.75Y2Al0.5 (at.%) alloys with different cooling rates are fabricated by the gravity casting, copper mould injection casting and melt-spinning techniques and their corrosion behavior and microstructures are examined by the salt water immersion test, electrochemical measurements, GDOES, XRD, SEM and TEM. To clarify the effect of aluminium addition on the improvement in corrosion resistance of the alloys, several Mg97.25−x Zn0.75Y2Al x alloys with different aluminium contents are fabricated by consolidating RS ribbons and the formation of corroded films on the Mg–Zn–Y–Al alloys have been investigated. Rapid solidification brings about the grain refinement and an increase in the solid solubility of zinc, yttrium and aluminium into the magnesium matrix, enhancing microstructural and electrochemical homogeneity, which in turn enhanced corrosion resistance. The addition of aluminium to magnesium can modify the structure and chemical composition of surface films and improves the resistance to local breakdown of the films. [Copyright &y& Elsevier]

Abstract: The effect of curing temperature on initiation and propagation of filiform corrosion of an epoxy cataphoretic electrocoating deposited on aluminum alloy substrate was studied. EIS measurements were used to follow the evolution of the water uptake of the intact coatings. In addition, EIS was applied to determine the performances of the systems as a result of exposure to HCl vapours. Cantilever method (stressmeter) was used to determine the stresses generated in an organic coating during a humidity cycle. Scanning Kelvin Probe (SKP) was applied in order to investigate the initiation and propagation of filiform corrosion of painted aluminum alloy. The mechanism of filiform corrosion is discussed according to the paint properties. [Copyright &y& Elsevier]

Abstract: Dispersions of the polyaniline emeraldine salt (ES) of paratoluene sulphonic acid (PAni-pTS) effectively inhibit filiform corrosion (FFC) affecting polyvinyl butyral (PVB) coated AA2024-T3 aluminium alloy. An in-situ scanning Kelvin probe (SKP) technique is used to study the effect of systematically varying PAni-pTS volume fraction (ϕ pa) on FFC initiation and propagation. For ϕ pa <0.15, there is no evidence of FFC inhibition and E corr values recorded for the intact coated aluminium (E intact) remain similar to those measured for unpigmented PVB. At ϕ pa ≥0.15, a marked rise in E intact is observed, FFC propagation rates decrease and significant oxide growth is observed at the coating-metal interface. For emeraldine base (PAni-EB)-containing coatings, there is no evidence of interfacial oxide film formation, no ennoblement of E intact and minimal inhibition of FFC. Conversely, when a PAni-pTS induced oxide covered surface is re-coated using unpigmented PVB and FFC is initiated as per normal, a substantial reduction in the rate of FFC propagation is observed. It is therefore proposed that inhibition of FFC by PAni-pTS arises principally as a result of the protective nature of the oxide film formed at the metal-coating interface. [Copyright &y& Elsevier]

Abstract: The corrosion behaviour of the Mg65Cu7.5Ni7.5Ag5Zn5Gd5Y5 bulk metallic glass prepared by copper mould casting with 3mm thickness was studied in borate solutions with pH 5–8.4 and in sodium hydroxide solution with pH 13. A superior corrosion resistance and passivation ability compared to that of Mg and the conventional AZ31 alloy was detected in weakly acidic to weakly alkaline solutions. This is mainly attributed to the beneficial effect of the alloying components, which are homogeneously mixed in the amorphous phase. They cause the establishment of a more noble alloy state and possess a higher tendency for surface passivation than the main component Mg. A high sensibility to chloride-induced corrosion exists, which is at low chloride concentration strongly pH value dependent. For the first time filiform corrosion was detected for Mg-based metallic glasses with filaments propagating randomly on the passivated amorphous surface. Y-rich crystalline defects in the bulk glassy sample seem to have no evident effect on the corrosion processes. [Copyright &y& Elsevier]

Abstract: In this work, the filiform corrosion behaviour of powder painted aluminium profiles was studied, and the coating barrier properties together with adhesion to the substrate were analysed. Samples coated with a traditional painting procedure (one powder layer application followed by curing), and other samples coated using a special cycle to obtain wood grain effect were compared using accelerated filiform corrosion tests. Moreover, in order to better understand the degradation mechanisms of painted metal substrates, thermal stresses were applied to accelerate the natural weathering. The effects of the thermal aging were analysed by electrochemical impedance spectroscopy. Wood grain effect coatings showed to better resist, with respect to a traditional coating, to filiform corrosion when testing was performed following the DIN 65472 standard. On the contrary, electrochemical impedance measurements showed better barrier properties against water uptake for the traditional coating after thermal ageing. Different barrier properties were found to depend on the microstructure of the two coatings after curing. The traditional one has a homogeneous microstructure, with completely fused powder grains and very fine size pigments. On the contrary, wood grain effect coating possesses a “defective” microstructure with a widespread distribution of partially fused powders grains determining a dense network of grain boundaries, moreover coarser size pigments are present. This microstructure decreases the coating barrier properties, but also can determine a loss of plasticity and the coating under mechanical stress can crack modifying the filiform corrosion behaviour. [Copyright &y& Elsevier]

Abstract: Thermo-mechanical processing of rolled aluminium alloys result in the formation of a deformed near-surface region with different filiform corrosion (FFC) behavior than the underlying bulk. This paper tries to correlate the microstructural, morphological and electrochemical changes occurring on the surface of aluminium sheet, during hot rolling, with the FFC behavior. The alloy under investigation is recycled AA5050. The electrochemical changes taking place in the roll-bite has been profiled and the result has been supported by electron microscopic and optical characterization for better understanding. Factors which may or may not be responsible for FFC susceptibility of rolled AA5050 have been discussed and importance of surface finish has been emphasized. [Copyright &y& Elsevier]

Abstract: The effect of pre- and post-extrusion heat treatments on the filiform corrosion behaviour of a well-known aluminium extrusion alloy AA6063 is studied by a combination of accelerated filiform corrosion exposure tests and potentiodynamic polarisation measurements for four different surface treatments. It is shown that the post-extrusion heat treatment of this AlMgSi alloy can influence the filiform corrosion properties significantly, in particular for the milder surface treatments. In contrast, the relative effect of the pre-extrusion treatment on the filiform corrosion properties is minimal for all treatments. The alloys are most susceptible to filiform corrosion in the β′ condition. The susceptibility decreases with coarsening of the Mg2Si particle distribution. For the post-extrusion heat and surface treated AA6063 material a clear correlation between the polarisation characteristics and the two principal filiform corrosion characteristics, i.e. the propagation rate and total area of attack after accelerated exposure, is observed. The observed correlation is attributed to a pitting corrosion mechanism with a rate depending on the (coupled) Mg2Si precipitate size and fraction. [Copyright &y& Elsevier]

Abstract: As a basis for extended studies into the evolution, structure and corrosion properties of the subsurface layers in hot rolled aluminium alloys, the evolution of the subsurface layers in industrially rolled AA3104 transfer bar has been simulated in a laboratory mill. Test samples taken from the centre of industrially rolled AA3104 transfer bar were highly polished, reheated and homogenised under carefully determined conditions and rolled in the two-high Robertson mill at the University of Sheffield. The stock surfaces and subsurface layers in industrially rolled and laboratory rolled material were then characterised using a combination of optical microscopy, quantification of stock surface morphology, glow discharge optical emission spectrometry (GDOES), focussed ion beam (FIB) microscopy and filiform corrosion testing. The results from the experimental programme have demonstrated that the near-surface metallic element distributions, the microstructures and depths of the subsurface layers and the filiform corrosion susceptibility of the rolled surfaces in the industrially rolled and laboratory rolled materials were all very similar. [Copyright &y& Elsevier]

Abstract: Filiform corrosion is mainly considered as a cosmetic attack and is undesirable in most applications. The initiation and propagation of the filaments are related to different parameters such as the presence of defects, the permeability of the coating to water and oxygen, the adherence of the paint system and the presence of salts. The aim of this work is to study the behaviour of painted aluminium samples towards filiform corrosion or delamination. The 6082 Aluminium alloy was selected and the samples were covered with a cataphoretic epoxy primer without lead (PPG Industries France). Prior to the application of the electrocoat, the samples were pre-treated by a commercial Zr/Ti or chromate conversion treatment or simply etched with a commercial acid etching product. Filiform corrosion was studied by the normalized test (ISO/DIS 4623): painted and scratched samples were inoculated in HCl and exposed in a constant humidity chamber at 40°C and 82% RH for 3 weeks. After exposure the samples were subjected to a visual and optical microscopic examination. Electrochemical impedance spectroscopy was used to study the sensitivity to filiform corrosion. The operating mode of this test is similar to the normalized one. The samples were scratched before inoculation for 1h in HCl and then exposed to the humidity chamber for a maximum of 4 days. The samples were tested by EIS in an acidified 0.1M Na2SO4 electrolyte solution. The resulting impedance spectra were analyzed with an appropriate equivalent electrical circuit which allows the evaluation of the exposed metallic surface area, directly related to the extent of filiform corrosion or delamination. Different parameters were varied: the exposure time in the humidity chamber; the immersion time in the electrolyte sulphate solution as well as its pH and the coating thickness. [Copyright &y& Elsevier]

Abstract: This study investigates the filiform corrosion resistance of chromate-free coating systems fabricated on aluminum alloys by low-temperature plasma interface engineering. The aluminum alloys investigated include AA2024-T3 ([2B]), AA7075-T6 ([7B]), Alclad AA2024-T3 ([2A]), plate stock AA2124-T851([2P]), plate stock AA7050-T7451 ([7P]), and ion vapor deposition (IVD) Al-coated AA2024-T3 ([2I]), IVD Al-coated plate stock AA2124-T851([2PI]), IVD Al-coated plate stock AA7050-T7451 ([7PI]). Direct current (DC) cathodic plasma processes including plasma treatment and plasma deposition were used to create the water-insensitive adhesion of organic coatings to aluminum alloys. With appropriate application to aluminum alloys, DC cathodic plasma coatings of trimethylsilane (TMS) combined with argon plasma post-treatment provided tenacious and water-insensitive adhesion between the water-borne, chromate-free spray primers and aluminum substrates. These chromate-free coating systems, when tested by filiform corrosion test, showed superb resistance to filiform corrosion. Filiform corrosion test was found to be a good measure of water sensitivity of the coating interfaces. [Copyright &y& Elsevier]

Filiform corrosion of AA 5005 H14 aluminium alloy sheet has been investigated. Painted and scribed panels, with different surface treatments, were inoculated in HCl and exposed in a constant humidity cabinet maintained at 40 °C and 75–85% RH for 1000 h. After exposure, the panels were examined by optical and electron microscopy. It is evident that filiform corrosion susceptibility is determined largely by the near-surface microstructure. Heavily deformed layers, comprising oxide-decorated fine grains and dispersoids on as-rolled and mechanically ground alloy surfaces, are readily susceptible to filiform corrosion. Removal of these deformed layers, by caustic etching and acid cleaning, results in a high filiform corrosion resistance. On such surfaces, underfilm corrosion progresses by localized corrosion of the substrate; with comparatively slowly growing filaments propagating by repeated blistering of the overlying lacquer. [Copyright &y& Elsevier]

The effect of the metal substrate microstructure on filiform corrosion (FFC) susceptibility was investigated for super purity based model alloys with compositions based on the specifications of AA3005. Variations in alloying levels of the elements iron, silicon and copper were investigated. Alloys with high silicon content were more susceptible to FFC than alloys with low silicon content. The iron content, at the levels investigated, did not strongly affect FFC properties. The apparent detrimental effect of a high silicon content is attributed to the influence of silicon on secondary intermetallic particle precipitation. Given the same thermo-mechanical treatment, alloys with high silicon content underwent more extensive secondary precipitation of manganese containing intermetallic particles than those alloys with a low silicon content. The resulting microstructure is characterised by a higher density of finely dispersed intermetallic particles and a lower content of manganese in the adjacent supersaturated solid solution. These conditions provide a large number of potential corrosion initiation sites on the surface and also enhance microgalvanic coupling between intermetallic particles and the surrounding aluminium rich matrix, thus promoting the propagation of filamental corrosion attacks. Additions of copper had a detrimental effect on the FFC resistance. The role of copper in promoting FFC is attributed to preferential dissolution phenomena during the corrosion process, whereby copper is locally enriched on the corroding surface. This copper enrichment provides additional area for cathodic reaction, thus enhancing the corrosion process. [ABSTRACT FROM AUTHOR]

Painted aluminium alloys are sometimes affected by filiform corrosion. Such a phenomenon which was intensively studied in the last few years is mainly a cosmetic type of attack. Usually it starts at coating defects or at cut edges following an anodic undermining mechanism. As described in the literature, water permeability and elasticity of the coating, oxygen diffusion paths, and presence of salts are the main controlling parameters. However, the metal coating interface is the critical point; then the substrate chemical composition, as well as the type of chemical cleaning or the type of applied chemical conversion coating proved to be fundamental aspects to improve resistance to filiform corrosion. The aim of this work is to study the filiform corrosion behaviour of aluminium alloys surface cleaned using chemically different products (alkaline, acid, combination of the two) and then treated using environmentally friendly pretreatment baths and chromates baths. The selected aluminium alloy was AA6060 type. After powder painting, aluminium samples were tested following the DIN 65472 standard. At the same time electrochemical tests were carried out following a particular procedure on samples exposed for different times to the environment promoting filiform corrosion. Under the experimental conditions used in our tests, electrochemical impedance spectroscopy was shown to be useful and allowed to obtain more quantitative data in a shorter testing time with respect to traditional testing. It was observed that resistance to filiform corrosion is greatly affected by the chemical composition of the aluminium substrate and that chemical cleaning modifies the surface chemical composition of the alloy so modifying its corrosion behaviour. [Copyright &y& Elsevier]

Aluminium alloys used extensively in aircraft, ships and land transport vehicles are typically protected by a thin conversion coating based on chromium compounds followed by a surface protection layer of polymer paint. Breeching of the protection layer and exposure to a salt spray induces the growth of filiforms from the breech across the aluminium surface under the protective layers. The growth of the filiform is promoted by the formation of a galvanic cell based on chlorine chemistry. In this paper we study the elemental composition of the filiforms using a nuclear microprobe with 3 MeV proton beams. The deep penetration of this beam allows the composition of the intact filiform to be probed in situ through the surface layers. We present elemental maps of the intact filiforms that clearly highlight the presence of Cl in the growing end of the filiform, where the Cl concentration exceeds 20 wt.%, and the peculiar role of potassium in the trail of oxide left behind the growing filiform head. [Copyright &y& Elsevier]

In situ observations of the filiform corrosion process of die-cast AZ91D magnesium alloy were carried out in 0.1 and 0.01 M NaCl solutions (pH 8.0) through a microscale polarization technique. The changes in morphology were observed in real-time using an optical microscope. Filiform corrosion proceeded in lamellar α + β structures adjacent to β phases and sometimes in the α phase grains. The β phases remained intact after filiform corrosion. • Filiform corrosion process of die-cast AZ91D was observed in real-time. • Filiform corrosion proceeded in lamellar α + β structures adjacent to β phases. • Corrosion resistance of coarse lamellar α + β structures may be better than that of dense structures. • Two corrosion morphologies were found for the α phase: spreading in grain and proceeding linearly in grain. [ABSTRACT FROM AUTHOR]

• Flash PEO ensures low energy consumption of the coating synthesis. • 4-Methylsalycilate-loaded PEO coating ensures active corrosion protection of Mg. • Inhibitor-loaded PEO coatings are more protective than chromate conversion coatings. • Excellent full system performance in paint adhesion and neutral salt spray tests. • Flash-PEO/inhibitor/Li-leaching primer system outperforms CCC/chromated primer. In the present work, a flash-PEO coating is developed on AZ31B alloy in a combination of silicate, phosphate, and fluoride based electrolyte in order to offer a green alternative to chromate conversion coatings. Multilevel active protection is achieved through synergetic combination of self-sealing effect of PEO coating itself and active inhibition provided by an organic inhibitor impregnated in PEO pores in a post-treatment step. The results indicate that flash-PEO coatings, loaded with organic corrosion inhibitors, can be recommended for exploitation on industrial level as an equally effective corrosion protection system alternative to CCC for paint-bearing and paint-free applications. [ABSTRACT FROM AUTHOR]

The microstructure and corrosion behavior of as-homogenized Mg- x Li-3Al-2Zn-0.2Zr alloys (x = 5, 8, 11 wt%) were investigated. As the Li content increases from 5 wt% to 11 wt%, the alloys undergo a phase transition from α-Mg single-phase to α-Mg + β-Li dual-phase and then to β-Li single-phase. All alloys contain AlLi phase, filamentous AlLi phase exists in α-Mg matrix in eutectic form while round-like AlLi phase exists in β-Li matrix in granular form. The corrosion of as-homogenized LAZ532–0.2Zr and LAZ1132–0.2Zr alloys is mainly filiform corrosion and pitting corrosion, while that of as-homogenized LAZ832–0.2Zr alloy is mainly galvanic corrosion. As-homogenized LAZ832–0.2Zr alloy exhibits the best corrosion resistance attributed to the finer filamentous AlLi phase in α-Mg matrix, the smaller round-like AlLi particles distributed in β-Li matrix and along the α-Mg/β-Li interface, and the denser oxide film containing a small amount of Li 2 CO 3. • Filamentous AlLi and round-like AlLi exist in α-Mg and β-Li, respectively. • Filiform and pitting corrosion occur in LAZ532-0.2Zr and LAZ1132-0.2Zr alloys. • Galvanic corrosion mainly occurs in LAZ832-0.2Zr alloy. • As-homogenized LAZ832-0.2Zr alloy exhibits the best corrosion resistance. [ABSTRACT FROM AUTHOR]