Propagation Of Filiform Corrosion Research Articles

On the Occurrence of Filiform Corrosion on Organic-Coated Carbon Steel Exposed to Cyclic Aging Test

Filiform corrosion (FFC) is one of the possible failure mechanisms of organic-coated steel. Beyond cathodic delamination, FFC causes paint detachment and the dramatic loss of the protective properties given by the applied paint. As both failure mechanisms occur during in-field exposure (depending on the environment), when assessing the performance of the protective paint by lab scale tests, we must be aware of the failure mechanism we induce with different accelerated aging cabinets. In this study, we investigate the effect of the prohesion test on the initiation and propagation of FFC. We highlight the concurrent development of FFC and cathodic blistering, which includes the change between the cathodic and anodic delamination front, during an accelerated aging procedure that cycles between saturated humidity and dry stages. The role of the presence of aggressive contaminants (Cl− and ) is discussed. According to our findings, cyclic aging tests seem to better stick to the failure mechanism occurring during in-field exposure, particularly due to the wet/dry cycles.

Investigation of the initiation and propagation of filiform corrosion on aluminum alloys by electrochemical techniques

ABSTRACT Filiform corrosion (FFC) is a type of localized corrosion commonly observed on coated aluminum alloys (AAs). Preceding research has focused on qualitative approaches of FFC on pure and binary metals. However, there is a lack of studies clarifying the FFC behaviour of commercial AAs. The present study aimed to investigate the FFC response of different AAs via electrochemical (EC) measurements. To do so, AAs 1100, 2024(Cu), 3003(Mn), 5052(Mg), 6061(Mg, Si), and 7075(Zn), were selected. Polarization measurements were carried out and three EC factors were identified from the polarization diagrams: resistance to FFC initiation (ΔEPR), driving force for propagation (ΔEcorr), and FFC kinetics (iFFC). To validate the EC results, accelerated exposure testing was performed. Coated AAs were placed in an environmental chamber and inspected; two parameters (number of threads (N) and filaments length (L)) were calculated and found to agree well with the EC factors. Results suggested that alloying elements Cu and Mg brought higher resistance to FFC initiation and slower FFC kinetics; that is, AA 2024(Cu) and 5052(Mg) showed the best resistance to FFC. In conclusion, the composition of AAs plays a significant role in their FFC susceptibility and EC measurements are effective for investigating the FFC behaviour of AAs.

In-situ observation on filiform corrosion propagation and its dependence on Zr distribution in Mg alloy WE43

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.

The kinetics and mechanism of filiform corrosion affecting organic coated Mg alloy surfaces

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 MgCl2, HCl and FeCl2 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.

A Review on Adhesively Bonded Aluminium Joints in the Automotive Industry

The introduction of adhesive bonding in the automotive industry is one of the key enabling technologies for the production of aluminium closures and all-aluminium car body structures. One of the main concerns limiting the use of adhesive joints is the durability of these system when exposed to service conditions. The present article primarily focuses on the different research works carried out for studying the effect of water, corrosive ions and external stresses on the performances of adhesively bonded joint structures. Water or moisture can affect the system by both modifying the adhesive properties or, more importantly, by causing failure at the substrate/adhesive interface. Ionic species can lead to the initiation and propagation of filiform corrosion and applied stresses can accelerate the detrimental effect of water or corrosion. Moreover, in this review the steps which the metal undergoes before being joined are described. It is shown how the metal preparation has an important role in the durability of the system, as it modifies the chemistry of the substrate’s top layer. In fact, from the adhesion theories discussed, it is seen how physical and chemical bonding, and in particular acid-base interactions, are fundamental in assuring a good substrate/adhesive adhesion.

The kinetics and mechanism of filiform corrosion occurring on zinc-aluminium-magnesium coated steel

This paper describes a systematic study into the initiation and propagation of filiform corrosion (FFC) on industrially important Zinc-Aluminium-Magnesium (ZAM) alloy coatings for steels. An artificial scribe defect is created in model organic coatings applied to ZAM. Corrosion is initiated using HCl, NaCl, FeCl2 and acetic acid. Only acetic acid is able to reproducibly initiate FFC. The FFC (area) rate is shown to be proportional to the amount of acetic acid introduced. The FFC mechanism is explained in terms of the relative susceptibility of alloy phases to anodic dissolution, determined using scanning electron microscopy and scanning Kelvin probe force microscopy.

Characterization of Filiform Corrosion of Mg–3Zn Mg Alloy

The filiform corrosion of Mg–3Zn Mg alloy in various corrosion environments was investigated by scanning electron microscopy (SEM) observations, scanning vibrating electrode technique (SVET) and electrochemical measurements. The results indicate that corrosion filaments can cross grain boundaries and fine precipitation phases to develop continually. The propagation of filiform corrosion is greatly associated with the existence of Cl− in the solutions. A small amount addition of F− or SO42− to the NaCl solution can delay the initiation of filiform corrosion, while abundant F− or SO42− addition can change the corrosion forms. The micro–anodes and micro–cathodes transfer dynamically with the growth of corrosion filaments. Mg–3Zn alloy exhibits a filiform corrosion mode at the applied anodic potentials but keeps intact at the applied cathodic potentials.

The Kinetics and Mechanism of Atmospheric Corrosion Occurring on Tin and Iron-Tin Intermetallic Coated Steels

This paper describes a systematic study into the initiation and propagation of filiform corrosion (FFC) on pure iron, pure tin and industrially important tin and iron-tin intermetallic (FeSn and FeSn2) coatings for packaging steels, as a function of coating weight. The time-dependent extent of FFC was determined optically. FFC was not observed on pure tin or tin coatings but was found to propagate at significantly reduced rates on the FeSn and FeSn2 intermetallic coatings, when compared to pure iron. An explanation of the findings is given, firstly, in terms of the open circuit potential (OCP) and the extent to which polarity of the galvanic corrosion cell formed between relevant phases permits FFC propagation, and secondly in terms of the relative susceptibility of the various phases to anodic dissolution. It is concluded that even when FFC is feasible thermodynamically, propagation rates may be negligible due to the low activity for anodic dissolution of tin and iron-tin intermetallics.

The effect of oxygen partial pressure on the filiform corrosion of organic coated iron

An in-situ time-lapse optical microscopy study, using a novel dual-compartment cell is used to gain mechanistic understanding of filiform corrosion (FFC) affecting an organic-coated iron surface. The apparatus allows independent control of environments surrounding the filament head and tail regions. When oxygen-containing environments surround the filament head, an anterior cathodic arc is formed, constraining the filament head electrolyte. When oxygen is removed from the vicinity of the head, FFC propagation rates remain unchanged, although the constraining arc is not present. Maximum propagation is observed when oxygen is available for transport through the filament tail to the rear of the head.

The Role of Tin in Suppressing Filiform Corrosion on Tin Coated and Iron-Tin Intermetallic Coated Steels

This paper describes a high resolution optical and electrochemical study into how metallic tin acts to reduce the rate of filiform corrosion (FFC) initiation and propagation in tin and iron-tin intermetallic (Fe-Sn IM) coatings for packaging steels. A greater understanding of the role of tin in suppressing atmospheric corrosion such as FFC is of significant interest due to environmental regulation. FFC is aesthetically undesirable, therefore being of concern to the food packaging industry. (1, 2)Tin and Fe-Sn IM coated steel samples were obtained from Tata Steel. In all cases tin had been electrodeposited onto the steel substrate. In the case of the IM coatings the tin layer had been diffusion annealed in an inert atmosphere.Two sample types were prepared. In the first type the tin or Fe-Sn IM coated surface was continuous. In the second type the tin or Fe-Sn IM coating was removed from half the sample surface as shown in Figure 1. All samples were solvent coated with a 30μm thick film of polyvinyl butyral (PVB) lacquer. A 1cm line penetrative PVB coating defect was created by scribing with a scalpel blade. In all cases FFC was initiated by introduction of 2μL of 2.5 x 10-3 M aqueous FeCl2 to the scribe. Two experiment types were carried out. In the first type FFC was initiated on the tin or Fe-Sn IM. In the second type FFC was initiated on the exposed steel substrate and allowed to propagate over the coated portion of the sample.The time-dependent extent of corrosion was determined optically using time lapse photography and by repeated in-situ scanning using a scanning Kelvin probe (SKP) apparatus.The capability of the SKP to visualise the spatial distribution of localized free corrosion potential variation with time has been demonstrated previously. (3)Initiation and propagation of FFC was observed on the Fe-Sn IM but not the tin coating.The rate of FFC propagation on the Fe-Sn IM was lower than that on the steel substrate. Using the SKP it was found that the FFC cell potential on Fe-Sn IM was approximately a quarter of that on iron as seen in Figure 2. This suggests the driving force and therefore propagation rate of FFC on Fe-Sn IM to be a quarter of that on the bare substrate.It has been stated that tin is associated with a high oxygen overpotential elsewhere. (4) A rotating disk electrode (RDE) was therefore used as a supportive technique. (5) It was verified that free tin acts as a poor electrocatalyst for oxygen reduction when compared to iron present in the Fe-Sn IM.The initiation and propagation of FFC is thus prohibited on tin coated samples.It is consequently suggested that Fe-Sn IM coatings are used in conjunction with a passivating pretreatment when being used industrially.1.) Charbonneau, J.E., Microanalysis Scanning, 19, (1997), 512-518.2.) Morita, J. And Yoshida, M., Corrosion, 50, (1994), 11-19.3.) Schmidt, W. and Stratmann, M., Corrosion Science, 40, (1998),1441-1443.4.) Ammar, I.A., Darwish, S., Khalil, M.W. and Galal, A., Z.Werkstofftech, 16, (1985), 194-203.5.) Jovancicevic,V. And Bockris, J.O’M., J.Electrochem.Soc., 133, (1986), 1797-1807.

Inhibition of filiform corrosion on organic-coated AA2024-T3 by smart-release cation and anion-exchange pigments

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.

Effects of Corrosion Inhibiting Pigment Lanthanum 4-Hydroxy Cinnamate on the Filiform Corrosion of Coated Steel

Corrosion protection by lanthanum hydroxy cinnamate (La(4OH-cin)3) in a polyurethane based varnish coating for mild steel has been investigated. Filiform scribe tests, energy-dispersive X-ray spectroscopy (EDXS) and potentiodynamic polarisation (PP) techniques have been powerful tools to better understand the corrosion process at defects and under the coating. Filiform scribe tests showed that La(4OH-cin)3, as a pigment in a coating, inhibited the initiation and propagation of both delamination and filiform corrosion (FFC) on coated steel. The PP experiments provided an insight into the fundamental mechanism of FFC. The results suggest that La(4OH-cin)3 behaves as a mixed inhibitor and stifles the initiation and propagation of FFC. In this paper, the theory of delamination leading to FFC and the likely mechanism of inhibition by the La(4OH-cin)3 will be discussed.

Critical Factors in the Inhibition of Filiform Corrosion by in-Coating Ion-Exchange Pigments

In-coating cation and anion exchange pigments are studied with respect to their ability to inhibit filiform corrosion (FFC) on AA2024 aluminium alloy substrates. Smart-release cerium (III) and yttrium (III)-exchanged bentonite pigments 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 filiform localised corrosion cell. In contrast, anion-exchange hydrotalcite based pigments are highly effective inhibitors of FFC. The efficiency is greatly improved by exchanging the native carbonate ions with anionic species which act as complexing agents for copper ions. The implication of these findings with respect to the mechanism of FFC on copper containing aluminium alloys is considered.

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. 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.

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 ( 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.

In Situ Studies of the Initiation and Propagation of Filiform Corrosion on Aluminum

We have investigated the initiation and propagation of filiform corrosion on coated aluminum surfaces using a new experimental set-up for in situ Fourier transform infrared (FTIR) microspectroscopy and the scanning Kelvin probe. During the initiation partially hydrolyzed aluminum chloride was formed in a defect on the coated surface. The conditions in the defect resembled the environment that is found in growing pits with low pH and high chloride concentration close to the coating interface. This weakened the adhesion of the organic coating and initiated the formation of a filament. The movement of the active head was followed with in situ FTIR microspectroscopy in humid air using the characteristic IR band around 2500 cm−1 from This band is present in spectra from partially hydrolyzed aluminum chloride, which was found in the head during propagation of the filament. The absorption of water in the hygroscopic corrosion products in the head of the filament was followed by measuring the changes in the intensity of the band during variations in the relative humidity. The hygroscopic properties of the aluminum hydroxy chloride salts formed in the head have important consequences for filiform corrosion, especially concerning the influence of climatic parameters such as the relative humidity and the effect of wet/dry transitions on the propagation of filiform corrosion. © 2004 The Electrochemical Society. All rights reserved.

Effect of Climatic Parameters on Filiform Corrosion of Coated Aluminum Alloys

Abstract The effects of temperature, relative humidity, and wet-dry transition on initiation and propagation of filiform corrosion on coated aluminum alloy 6016 (AA6016 [UNS A96016]) have been studied. Corrosion products formed in the filament were analyzed using Fourier transform infrared (FTIR)-microspectroscopy. The aluminum surfaces were tested in both milled finish and grinded conditions with chromium, phosphate, and titanium-zirconium-based pretreatment. An electrodeposited coat (ED coat) and a full paint system used for automotive applications (ED coat + topcoat) were investigated for the different combinations of mechanical finish, surface pretreatment, and coating system. In the temperature range between 5°C and 50°C, filiform corrosion, or underfilm corrosion in general, increased significantly. The effect of relative humidity and wet-dry cycling, on the other hand, seems to be strongly influenced by parameters like pretreatment, coating system, and temperature. Filiform corrosion was the highes...

The mechanism of group (I) chloride initiated filiform corrosion on iron

The initiation and propagation of filiform corrosion (FFC) on polymer-coated iron following contact with aqueous group (I) chloride salts is investigated using in situ scanning Kelvinprobe (SKP) and ex situ SIMS measurements. Prior to the onset of FFC a circular area of cathodic delamination is observed to grow radially from a penetrative coating defect. The delaminated zone is abundant in group (I) cations, whereas Cl − anions are substantially excluded. As cathodic delamination slows to a halt corrosion filaments initiate at the coating defect and propagate across the delaminated zone at ca. 0.33 μm min −1. Filaments travelling beyond the delaminated zone show only a small (<20%) decrease in propagation rate. It is shown that Cl − is substantially conserved in the filament-head electrolyte, whereas group (I) cations are effectively excluded. When FeCl 2 is used as the initiating salt, a cathodic delamination phase is not observed but FFC occurs as before. The implications of these findings are discussed with respect to the role of cathodic delamination processes in FFC on iron.

The Kinetics of Chloride-Induced Filiform Corrosion on Aluminum Alloy AA2024-T3

A scanning Kelvin probe (SKP) is used to study the kinetics and mechanism of filiform corrosion (FFC) as it occurs on polyvinyl butyral-coated AA2024-T3 aluminum alloy. In order to control the quantity of electrolyte entering the filament-head population, FFC is initiated by injecting a measured quantity of aqueous HCl directly into a penetrative coating defect. Subsequently, time-dependent free corrosion potential maps are obtained by repeated in situ scanning of a fixed sample area at 20°C and 93% relative humidity. The number of moles of HCl introduced during initiation is varied systematically and the resulting distributions analyzed to provide detailed information on filament dimensions and FFC propagation rates. Equations are derived relating the volume of individual filament-head electrolyte droplets to filament-head radius. Equations are also derived relating the total (population) filament-head electrolyte volume to Using these relationships a series of kinetic equations are derived predicting propagation rates for individual filaments and filament populations under circumstances where rates of coating delamination are surface controlled or controlled by underfilm ion migration. Comparison with experimental kinetics shows that for the system studied the rate of coating delamination is surface controlled, i.e., proportional to the interfacial area existing between the filament-head electrolyte droplet and the metal substrate. © 2003 The Electrochemical Society. All rights reserved.