Filiform Corrosion Test Research Articles

Effects of mechanical surface pre-treatment on integrity and corrosion of bare and coated AA6082 substrates

In this study, two classes of surface pre-treatment methods, namely three blast-cleaning methods with different abrasive materials (for initial preparation) and three mechanical methods (for repair applications) were applied to study their effects on surface morphology, composition, and corrosion of AA6082 substrates in artificial seawater. Coating adhesion and coating corrosion protection ability were investigated by applying a single-layer epoxy coating (250 μm dry film thickness) onto the substrates.The mechanical methods delivered cleaner surfaces with more regular surface morphologies, whereas the blast-cleaning methods revealed less clean surfaces and more irregular surface morphologies. The latter methods promoted the detrimental embedment of abrasive particles into the substrates. The corrosion performance of the pre-treated substrates was worse after blast-cleaning compared to mechanical treatments. However, all methods met the adhesion requirements for offshore applications. Regarding coating degradation, only the filiform corrosion test delivered meaningful results, whereas the duration of the cyclic corrosion test (3000 h) was too short to distinguish between the different methods because of neglectable degradation. Mechanical interlocking and isotropic surface morphologies are found to be more important for coating performance than a cleaner surface, and the remains of blasting material were neglectable too.

Study of ZnAl Hydroxides-Based Thin Films to Enhance the Filiform Corrosion Resistance of Acrylic-Coated AA5005

In recent years, the imperative to adopt a sustainable approach to aluminum coating pre-treatments due to environmental concerns has engaged researchers in corrosion studies. This work investigates Zn-Al hydroxides-based conversion coatings developed on AA5005 to enhance filiform corrosion (FFC) resistance. The conversion coatings were applied as a pre-treatment (200 nm–800 nm thick, depending on synthesis conditions) prior to coating with acrylic paint. The synthesis of the Zn-Al hydroxides conversion coating involves a rapid hydrothermal method at near-neutral pH. Electrochemical tests performed on the ZnAl hydroxides conversion coatings onto AA5005 revealed a substantial improvement in localized corrosion resistance. FFC tests in a climatic chamber exhibit a significant decrease (about 50%) in rust creep filament propagation compared to bare AA5005 after 1000 h. Those results were obtained employing Zn and Al salts at neutral pH for ten minutes at 95 °C. The findings obtained suggest that the synthesis process is of potential interest within the context of chemical surface conversion treatments for aluminum. In fact, eco-friendly metallic salts contribute to a more economical and environmentally friendly waste disposal process, making this treatment a potential solution for industrial applications, considering its simplicity, time efficiency, and use of non-toxic products.

Effect of Cu2+ on deposition mechanism and structure of ZrO2-based conversion coatings on AA6060 aluminium alloys and their susceptibility to filiform corrosion

Zirconium oxide-based conversion coatings (CCs) were prepared from hexafluorozirconic acid, H2ZrF6, 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 Cu0 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 ZrO2-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.

Dynamic Electrochemical Impedance Spectroscopy for the Evaluation of Localised Corrosion on Aluminium Alloys

AbstractDynamic electrochemical impedance spectroscopy (dEIS), an extension of conventional EIS, was used for rapid measurements of initiation of localised corrosion on 3005 aluminium alloys with different Mn (1.08–1.39 wt. %) and Cu (0.15–0.22 wt. %) content by evaluating the onset and evolution of a Faradaic process observed in 0.5 M Na2SO4+0.1 M NaCl. Differences between samples were detected with highest sensitivity from the charge transfer resistance during chronoamperometry at ca. 120 mV above the corrosion potential. These differences were caused by differences in pitting initiation, and correlated well with filament density from a filiform corrosion (FFC) test of samples coated with a 20 μm epoxy topcoat. The observation can be explained by the role of pitting in FFC initiation and the similarity in FFC propagation with pit growth. The results suggest that dEIS is an efficient method for screening alloys for their FFC susceptibility.

Role of graphene oxide and functionalized graphene oxide in protective hybrid coatings

This work evaluates the performance of hybrid coatings of polyurethane/polysiloxane filled with graphene oxide as replacement of chromate conversion coatings for protection of 2024-T3 aluminum alloys. The polysiloxanes in the hybrid coating form strong covalent bonds with the metal surfaces and inhibit corrosion, polyurethanes provide mechanical and optical properties, and graphene oxide and isocyanate-treated graphene oxide particles reduce oxygen and moisture diffusion through the coatings. The corrosion performance evaluated using electrochemical impedance spectroscopy and filiform corrosion test indicates the following trends. The hybrid coatings filled with isocyanate-treated graphene oxide particles show lower resistance to corrosion compared to hybrid coatings without graphene oxide, although oxygen diffusion and water vapor transmission are reduced by respectively 50% and 10% in the presence of only 0.67 wt.% graphene oxide. It is revealed that graphene oxide and isocyanate-treated graphene oxide particles promote polysiloxane condensation on particle surfaces, thereby effectively reducing a part of polysiloxanes that form continuous corrosion-protective layers on the aluminum alloy surface.

Steam initiated hydrotalcite conversion coatings: Application to environmental Al alloy surface treatment

A facile process of exploiting high-temperature steam to deposit environmentally friendly hydrotalcite (HT) coatings on Al alloy 6060 was developed in a spray system. Scanning electron microscopy showed the formation of a continuous and conformal coating comprised of a compact mass of crystallites. A range of coating processes based on the formation of HT surface layers has been developed to examine its effect on the coating's thickness and corrosion resistance properties. These varieties include pre-coating cleaning (grid blasting vs. chemical etching), metal species in HT compounds (Al-Zn HT coating vs. Al-Li HT coating), oxidizer additives (K2S2O8, Na2SO4, NH4NO3, KNO3), and post-coating treatment (Mg(CH3COO)2, Mg(CH3COO)2 + Ce(NO3)3 + H2O2, Mg(CH3COO)2 + La(NO3)3). Results showed that grid blasting can increase the coating surface area, while chemical etching improves the chemical bonding connection between the coating layer and substrate, and therefore leads to a higher corrosion resistance in the filiform corrosion test. Al-Li HT coatings on the Al alloy are much thicker than the Al-Zn HT coatings that formed from the same procedures. This variation is likely caused by the pH condition of coating solution, which is essential to the co-precipitation of two or more cations in HT structure. The formation of Al-Zn HT coating was enhanced by oxidizer and ammonium salt additions. The thickness of Al-Zn HT coating decreases patterning after the oxidizing and complexing capability of additives in the following order: K2S2O8 > Na2SO4 > NH4NO3 > KNO3. Post-coating rinsing by Mg(CH3COO)2, in addition to Ce- and La- based salts, is capable of sealing the porous Al-Li HT coatings and increasing the corrosion resistance ability. The Ce modified Al-Li HT coatings show better corrosion protection than the La modified coatings, due to a behaviour of “active corrosion protection” that enables precipitation of insoluble Ce(IV) oxides to reinforce the existed HT coatings.

Steam based conversion coating on AA6060 alloy: Effect of sodium silicate chemistry and corrosion performance

Surface treatment of aluminium alloy AA6060 using an industrially applicable pilot steam jet system with and without silicate chemistry has been investigated. Treatment using steam alone and steam with silicate, resulted in an oxide layer formation with thickness ∼425nm and ∼160nm, respectively. Moreover, the use of sodium silicate resulted in the formation of distinct microstructure and incorporation of silicate into the oxide film. These oxide films reduced the anodic activity 4 times, while the corrosion protection by silicate containing oxide was the function of its concentration. Further, in acid salt spray and filiform corrosion tests, oxide layer containing silicate exhibited two times higher corrosion resistance.

Effect of sol–gel layers obtained from GLYMO/MTES mixtures on the delamination of a cataphoretic paint on AA1050

In this study, the performance of different sol–gel layers produced from different mixtures of silicon alkoxides precursors (γ-glycidoxypropyltrimethoxysilane, GLYMO, and methyltriethoxysilane, MTES) applied on AA1050 aluminum alloy as a pretreatment prior to depositing an electrophoretic paint (polyether–polyamine polymeric blend) is investigated. The structure of the sol–gel coatings was characterized by means of ATR FTIR, while the corrosion protection properties were assessed by polarization curves. Accelerated laboratory tests, such as exposure in both neutral and acetic salt spray chambers and filiform corrosion tests, were employed to investigate the corrosion protection properties of the cataphoretic paint-coated samples. The failure mechanisms of the different coatings were investigated. The experiments revealed that the presence of GLYMO in the sol–gel coating seems not to be suitable for acting as a coupling agent between the paint and the aluminum substrate. On a positive note, this work shows that the filiform corrosion test seems to be the most appropriate test to evaluate the efficiency of sol–gel layers employed as pretreatment to enhance the corrosion protection provided by a paint on an aluminum substrate.

The formation of a hydrotalcite coating on the aluminium alloy 6060 using a spray system

AbstractCoatings with the composition of Li-Al-NO3 hydrotalcite were formed on the Al alloy 6060 using a spray system. The coatings consist of crystals with a typical hydrotalcite structure. Dense, uniform and blade-like flakes cover completely the surface of the Al substrate. The coatings display a multi-layer structure with average thickness of ∼1000 nm. The hydrotalcite-coated samples performed better than those without coatings in salt-spray and filiform-corrosion tests, and further treatment involving sealing with a Mg acetate solution and dipping in a H2O2 + Ce-based solution improved the corrosion resistance ability.

Role of acidic chemistries in steam treatment of aluminium alloys

The effect of acidic chemistry on the accelerated growth of oxide on aluminium alloys Peraluman 706™ and AA6060 under exposure to high temperature steam was investigated. Studied chemistries were based on citrates and phosphates. Results showed that the presence of citrate and phosphate anions initiated oxide growth at the intermetallic particles while growth and corrosion performance of oxide was found to be a function of anions type and their concentration. Further, steam treatment with phosphates exhibited better performance under acetic acid salt spray and filiform corrosion test whereas delay in powder coating resulted in 1.5–3 times inferior performance.

Steam assisted oxide growth on aluminium alloys using oxidative chemistries: Part II corrosion performance

Surface treatment of aluminium alloys using steam with oxidative chemistries, namely KMnO4 and HNO3 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 HNO3 was a function of the concentration of NO3− ions. The coating generated by inclusion of KMnO4 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 KMnO4 and HNO3.

Accelerated growth of oxide film on aluminium alloys under steam: Part II: Effects of alloy chemistry and steam vapour pressure on corrosion and adhesion performance

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 825nm. 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.

Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte nanocapsules

Abstract The self-healing polymer coatings containing organic corrosion inhibitors are intensively investigated as an alternative for highly toxic Cr(VI)-based systems. Protective self-healing coatings are realized by embedding “smart” containers, able to release a corrosion inhibitor under some specific conditions occurring when the corrosion process starts (e.g. on pH change) or upon a mechanical damage. In this study a system with the corrosion inhibitors (2-methylbenzothiazole (BT) and 2-mercaptobenzothiazole (MBT)) encapsulated inside the polyelectrolyte nanocapsules embedded in the water-based epoxy coatings is tested for its self-healing performance. The nanocontainers were prepared by the electrostatic adsorption of polyelectrolytes directly on the oil phase drops containing the inhibiting agent. The results for BT emulsion droplets and the mixture of BT and MBT encapsulated by docusate sodium salt/poly(diallyldimethylammonium chloride) (AOT/PDADMAC) and docusate sodium salt/poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (AOT/PDADMAC/PSS) surface complexes are presented. The X-ray Photoelectron Spectroscopy (XPS) was used to confirm the release of the inhibitor from the scratched coating. The influence of the nanocapsules on the barrier properties and self-healing performance of the epoxy coatings were tested by electrochemical impedance spectroscopy (EIS) in NaCl solution, the salt spray test (SST) according to ISO9227 and filiform corrosion test (FFT) according to EN ISO 3665. Potential blistering was rated according to EN ISO 4628-2.

Filiform corrosion of electrocoated aluminium alloy: Role of surface pretreatment

Commercial Oxsilan, 3-aminopropyltriethoxysilane and sol–gel siloxane coatings were applied on AA6016 aluminium alloy before electrocoating. Standard filiform corrosion test demonstrated the pre-treatment role on the under-paint corrosion rate. All pre-treatments increase the coating adhesion in water electrolyte (EIS). Anodic undermining is the mechanism of the coatings de-adhesion (SKP). Electromotive force (difference in potentials of defect-surrounding interface) and interface ability to promote the chloride migration are rate-determining factors of the filiform corrosion. Contrary to other pre-treatments, the aminosiloxane decreases the interfacial potential, facilitates the chlorides migration and leads to anodic de-adhesion of large coated areas (Tof-SIMS and EDXS).

Silane sol–gel film as pretreatment for improvement of barrier properties and filiform corrosion resistance of 6016 aluminium alloy covered by cataphoretic coating

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.

Structure and stability of adhesion promoting aminopropyl phosphonate layers at polymer/aluminium oxide interfaces

Aminopropylphosphonic acid was investigated as a short-chain bi-functional adhesion promoter between an aluminium alloy and an epoxy amine adhesive. Benefits of the short-chain adhesion promoter are its high solubility in water and epoxy resins, and the formation of densely packed monolayers on the aluminium substrate during the adsorption from both media. Application of the adhesion promoter as an adhesion promoting monolayer adsorbed from aqueous solution and as an additive to the adhesive bulk was investigated. By means of XPS and SAM analysis, monolayer formation on the aluminium passive layer and, at lower coverage, on intermetallic particles was demonstrated. Formation of ionic bonds to the aluminium oxihydroxide surface via acid/base interactions in bi-dentate conformation was proven by means of FTIR-RAS. Filiform corrosion tests, utilized as probe for interfacial stabilization, showed a strong inhibition of this anodic de-adhesion process for both variations of application.

Filiform corrosion attack on pretreated aluminum alloy with tailored surface of epoxy coating

In the study, on the basis of material analysis of 6016 aluminum alloy widely used in Europe automotive industry, the influences of surface pretreatment on filiform corrosion and adhesion of epoxy coating/aluminum alloy interface were investigated, and the effect of rolling direction and coating property on filiform corrosion was also examined. The alloy surfaces were pretreated with etching solution, and then, respectively, with Aminopropyl silane solution, Aminopropyl phosphonate solution, and hexamethyldisiloxane plasma.The results showed that the susceptibility of pretreated surface to filiform corrosion was basically consistent with the distribution of intermetallic particles, and that water permeation and the tensility of coating were necessary for filament to grow. HMDSO plasma pretreatment could not only efficiently cover the matrix of 6016 aluminum alloy and cathodic intermetallic particles containing Al, Si, Mn, and Fe, but also apparently promote the adhesion of epoxy coating/aluminum alloy, as a result, the specimens with plasma pretreatment exhibited the superior performance in the filiform corrosion and peeling tests, so it was suitable to apply for the pretreatment of aluminum alloy in automotive industry.

Influence of the curing temperature of a cataphoretic coating on the development of filiform corrosion of aluminium

Aluminium alloys are known to be particularly sensitive to filiform corrosion. The initiation of this particular type of corrosion is 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. In this work, the filiform corrosion resistance of Al 6016 substrates coated with a cataphoretic paint was studied. The curing of the coating was performed at different temperatures (185, 175, 165, 155 and 135 °C) in order to modify its mechanical properties and its permeability to oxygen. The paint properties were studied by different techniques allowing the estimation of the degradation of the metal–primer system and giving some information about intrinsic paint properties. The corrosion protection of the coating was evaluated by a normalized filiform corrosion test and by electrochemical impedance spectroscopy on scratched samples. The glass transition value and the internal stresses of the cataphoretic coatings obtained for different curing temperatures were determined by a stressmeter equipment. This study enabled us to underline the influence of the curing temperature on the intrinsic properties of the coatings such as the glass transition temperature, the internal stresses, the adherence, the permeability and the corrosion protection properties.

Underfilm corrosion on polyurethane-coated aluminum alloy 2024-T3 containing dissimilar metal fasteners

Panels of AA2024-T3 were fastened using Cu and Cu−Ni fasteners, pretreated, and coated with around 25 μm of polyurethane film. The pretreatments were based on typical conversion coating processes. Samples included alkalinecleaned, deoxidized, and conversion-coated. The conversion coatings included chromate and cerium-based conversion coatings. Samples were tested by scribing around the fastener and on the AA2024-T3 (matrix), exposing them to HCl vapor for 15 min, and subjecting them to a filiform corrosion test by exposure to 82% relative humidity at 40±1°C for up to 1000 hr. Inspection showed that the filiform corrosion that developed from a scribe around the fastener was an order of magnitude larger than from a scribe on the matrix (scribe on AA2024-T3). With respect to the surface treatments, the amount of filiform corrosion for scribes on the matrix was greatest on the alkaline-cleaned samples, followed by the deoxidized samples, with the conversion-coated samples performing the best. The amount of filiform corrosion for scribes around the fasteners was highest for the deoxidized sample.

Effect of powder painting procedures on the filiform corrosion of aluminium profiles

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.