EIS evaluation of the filiform corrosion of aluminium coated by a cataphoretic paint

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 1 h in HCl and then exposed to the humidity chamber for a maximum of 4 days. The samples were tested by EIS in an acidified 0.1 M 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.     

Polyaniline inhibition of filiform corrosion on organic coated AA2024-T3

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 (?_pa) on FFC initiation and propagation. For ?_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 ?_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.     

Scanning Kelvin probe force microscopy for the in situ observation of the direct interaction between active head and intermetallic particles in filiform corrosion on aluminium alloy

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 340 nm 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 (100 μm × 100 μm). 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 200 mV 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 250 mV 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.     

The kinetics and mechanisms of filiform corrosion on aluminium alloy AA6111

Scanning Kelvin Probe (SKP) potentiometry is used to investigate the effect of surface abrasion and heat treatment on the open-circuit potential (OCP) of AA6111 in humid air. SKP is also used to follow the kinetics of filiform corrosion (FFC) and to determine characteristic potentials associated with FFC filaments. Simply abrading with 180 grit SiC produces a near surface deformed layer (NSDL) with OCP >0.2 V lower than the bulk alloy. When the abraded sample is overcoated with a 30 μm layer of PVB (polyvinyl butyral) and exposed to HCl a fast, superficial, surface-active FFC is observed in which metal loss is limited to the thickness of the NSDL (approximately 2 μm). Filiform head OCP values are similar to that of the NSDL and filiform tail OCP values are similar to the bulk. A mechanism is proposed in which the ultra-fine grain structure of the NSDL produces an anodic activation, and the potential difference between the NSDL and the bulk provides an increased driving force for corrosion. When the NSDL has been removed through surface-active FFC or by caustic etching, a slower, deeper, successive-pitting form of FFC affects the bulk alloy. The rates of both surface-active and successive-pitting FFC reach a maximum after post-abrasion heat treatment at 180 °C.     

Investigation by electrochemical impedance spectroscopy of filiform corrosion of electrocoated steel substrates

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.     

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.     

The role of chromate in filiform corrosion inhibition

This paper is part of a study on filiform corrosion (FFC) on aluminum alloy 2024-T3 and focuses on the surface characterization of corroded samples. Untreated samples were used as well as samples which had undergone pretreatments including polishing, surface etching and chromated conversion coatings (CCC). These samples were coated with both pigmented and non-pigmented epoxy-based coatings. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis was used to investigate the nature of the surface as well as optical images to gauge the rate of FFC advancement. FFC corrosion rates decreased on samples which had received a surface etching pretreatment and a chromate conversion coating. Pigmented coatings reduced the rate of FFC further and led to two different types of corrosion surface morphology. On pretreated samples, the corrosion appeared deeper and pit-like, possibly due to an enhanced polymer-substrate bond. On untreated samples, widespread FFC developed. SEM and EDX analysis of various intermetallic particles (IMPs) on all samples, inside and outside of corroded regions, revealed that Cr from pigments was found deposited on Cu-containing intermetallics in corroded areas while Fe/Mn-containing particles were free from all pigment traces. These results suggest that the Cr deposition on Cu particles hinders the cathodic reduction of oxygen necessary for FFC advancement. For this reason, the coating pigments proved, under FFC conditions, to be a more effective inhibitor than the Cr originating from a chromate conversion coating.     

Influence of crosslinking density of a cataphoretic coating on initiation and propagation of filiform corrosion of AA6016

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.     

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.     

Adhesive strength of powder coated aluminium substrates

Adhesive strengths of differently pretreated powder coated aluminium substrates were evaluated using the pull-off and the tape tests in the dry and wet conditions. Various accelerated test chambers were employed to evaluate the corrosion behaviour of the samples. Scanning electron microscopy (SEM), with energy dispersive X-ray analysis (EDX) were also employed to the better understanding of the nature and morphology of the differently pretreated aluminium substrates prior to and after the accelerated tests.

Accelerated tests and wet adhesion measurements showed that the degreased samples demonstrated a better performance than Polyacrylic acid (PAA) treated samples. The chromate/phosphate conversion coating (CPCC) treated samples gave the best performance and the Polyacrylic acid/ hexafluorozirconic acid treated samples (PZr) gave marginally second best performance in anti-corrosive behaviour.

Based on the results obtained from the different experiments, it is argued that these studies may provide sufficient evidence to show that hydrolysis may destroy interface bonds between PAA and aluminium substrate. Moreover, SEM/EDX analysis, suggest that, the thin PAA polymeric layer was replaced by a crystalline, thick hydrated oxide layer, when samples were exposed to humid conditions.     

Effect of grain size on corrosion of high purity aluminium

A complete understanding of how grain refinement, grain size, and processing affect the corrosion resistance of different alloys has not yet been fully developed. Determining a definitive ‘grain size–corrosion resistance’ relationship, if one exists, is inherently complex as the processing needed to achieve grain refinement also imparts other changes to the microstructure (such as texture, internal stress, and impurity segregation). This work evaluates how variation in grain size and processing impact the corrosion resistance of high purity aluminium. Aluminium samples with a range of grain sizes, from ∼100 μm to ∼2000 μm, were produced using different processing routes, including cold rolling, cryo rolling, equal channel angular pressing, and surface mechanical attrition treatment. Evaluation of all the samples studied revealed a tendency for corrosion rate to decrease as grain size decreases. This suggests that a Hall–Petch type relationship may exist for corrosion rate and grain size. This phenomenon, discussed in the context of grain refinement and processing, reveals several interesting and fundamental relationships.     

The influence of aluminium surface pretreatment on the corrosion stability and adhesion of powder polyester coating

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.     

Effect of curing on the corrosion protective properties of thin organosilane films on aluminium

Film properties and corrosion protection given by thin organosilane films (bis-1,2-(triethoxysilyl)ethane: BTSE) on aluminium were investigated as a function of curing. The thickness was determined using spectroscopic ellipsometry (SE). Impedance spectra, modelled by an electrical equivalent circuit, show three time constants. The variation of the elements in these three time constants as a function of the immersion time in a NaCl solution indicate the structure of the film after curing. It can be concluded that the barrier properties of the organosilane films are influenced more by the curing temperature than by the curing time.     

钛酸铝-莫来石复合材料热稳定性的研究

通过引入氧化铁和两相复合的方法对钛酸铝 -莫来石复合材料的热稳定性进行了研究。结果表明 :当烧成温度在 15 0 0～ 16 0 0℃时 ,烧成温度的升高对复合材料的热稳定性有利。氧化铁可与钛酸铝晶体形成连续固溶体 ,改变钛酸铝的晶格常数 ,降低钛酸铝晶体的畸变程度 ,促进烧结 ,提高热稳定性。仅加入莫来石时 ,其对钛酸铝热分解的抑制效果不佳 ;如果同时引入氧化铁作为添加剂 ,莫来石对钛酸铝的热分解有较好的抑制作用。     

锌粉粒度对锌铬膜耐腐蚀性能影响的研究

本文通过湿热试验、盐水浸泡试验、盐雾试验和电化学方法全面评价了锌铬膜的耐腐蚀性能,并与镀锌钝化膜进行比较。研究了锌粉粒度对锌铬膜层耐腐蚀性能的影响,锌粉粒度越小,所得锌铬膜耐腐蚀性能越强。     

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.     

Simulation of pH-controlled dissolution of aluminium based on a modified Scanning Electrochemical Microscope experiment to mimic localized trenching on aluminium alloys

Some constituent intermetallic (IMPs) particles at the surface of aluminium alloys are considered as preferential sites for the initiation of structural corrosion resulting in localized trenching around the particles and the surrounding Al matrix. In this work, a modified scanning electrochemical microscope (SECM) experiment was used to induce such phenomena via a local alcalinisation on 200 nm thick aluminium coatings promoting their local dissolution in an aerated 0.1 M NaCl electrolyte. The local alcalinisation was induced by the oxygen reduction reaction on the tip of a SECM which mimics the surface of an isolated IMP. From a phenomenological point of view, reproducible cylindrical damage develops in front of the platinum ultramicroelectrode (UME). Using a 2D finite element modelling to simulate the SECM experiments, the role of the local alcalinisation was validated and the calculated Al dissolution rate was found in agreement with the experimental evaluation.     

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.     

复合添加剂对钛酸铝稳定性的影响

探讨了复合添加剂对Ａｌ２ＴｉＯ５ 烧结性能、抗折强度、热膨胀系数和稳定性的影响,利用ＸＲＤ、ＳＥＭ 等手段分析了材料的物相组成和显微结构。结果表明, 利用复合添加剂在不提高Ａｌ２ＴｉＯ５ 热膨胀系数的情况下,获得了具有较高强度的稳定化Ａｌ２ＴｉＯ５ 。     

Influence of the doping agent on the corrosion protection properties of polypyrrole grown on aluminium alloy 2024-T3

The corrosion protection properties of polypyrrole (PPy) electrodeposited onto aluminium alloy 2024-T3 substrates were investigated as a function of the doping agent. We used camphor sulfonic acid (CSA), para toluene sulfonic acid (p-TSA), phenylphosphonic acid (PPA), oxalic acid (OA) and cerium nitrate salt (Ce(NO₃)₃) as doping agents. The resulting coatings have been evaluated towards corrosion protection of aluminium alloy 2024-T3 using electrochemical impedance spectroscopy (EIS). Complementary, scanning electron microscopy (SEM) provided images on the morphology and the thickness of the coatings. The results showed that coatings formed using Ce(NO₃)₃ solution protect the substrate more efficiently compared to the other coatings.