Time-lapse potentiometric imaging of active filiform corrosion using a scanning Kelvin probe technique

Williams, Geraint; McMurray, H. N.; Hayman, Douglass F.; Morgan, P. C.

doi:10.1039/b100835h

Cited by 22 publications

(35 citation statements)

References 18 publications

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“…1, 3, and 4 are consistent with chloride-induced substrate depassivation 25 and anodic attack occurring through Reaction 2 [4][5][6][7]21 Al…”

Section: Discussionsupporting

confidence: 77%

“…21 Initially, continuous regions of decreased E corr , consistent with local surface depassivation, became evident in the immediate vicinity of the penetrative defect. After a period, which we refer to as the initiation time (t 0 ), these continuous regions of depassivation became fragmented to form individual filament heads observed to move normal to and away from the coating defect, i.e., travelling parallel with the substrate rolling direction.…”

Section: Resultsmentioning

confidence: 93%

“…21 Once initiated, filaments tend to propagate away from the coating defect under the influence of differential aearation. [2][3][4][5] Aluminum cations produced by Reaction 2 at the leading edge of the filament-head migrate toward the trailing edge where OH Ϫ generated by Reaction 3 induces cation hydrolysis and the eventual precipitation of water-insoluble Al(OH) 3 .…”

Section: Discussionmentioning

confidence: 99%

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

Williams

McMurray

2003

J. Electrochem. Soc.

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A scanning Kelvin probe ͑SKP͒ is used to study the kinetics and mechanism of filiform corrosion ͑FFC͒ 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 (E corr ) 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 (N HCl ) introduced during initiation is varied systematically and the resulting E corr 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 ͑population͒ filament-head electrolyte volume to N HCl . 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.Filiform corrosion ͑FFC͒, first accurately described in 1944, 1 is an atmospheric corrosion phenomenon affecting organic coated metals and producing characteristic ''thread-like'' deposits of corrosion product beneath the coating. Extensive studies on organic coated aluminum, steel, and magnesium surfaces, reviewed elsewhere, 2,3 have shown that oxygen, aggressive ions such as chloride (Cl Ϫ ), and a high relative humidity ͑RH͒ must all be present for FFC to occur. Corrosion filaments, each comprising an electrolyte-filled ''head'' and a ''tail'' of dry corrosion product, propagate from penetrative defects in the organic coating and may attain a length of several centimeters. The exact mechanism of FFC remains unclear, but it is generally believed that filament advance involves anodic undercutting of the organic coating driven by differential aeration, which arises in turn from facile mass transport of atmospheric O 2 through the filament tail. 2-7 Aggressive anions (Cl Ϫ ) and water tend to be conserved in the filament-head electrolyte and filaments may continue to propagate for long periods of time ͑years͒. 2,3 Standard tests aimed at quantifying susceptibility to filiform attack specify conditions for FFC initiation at artificial coating defects ͑scribes͒. 3 These include exposure to salt fog, 8 exposure to aqueous electrolyte 9-11 ͑including aqueous NaCl, AlCl 3 , and HCl͒, and exposure to HCl vapor. 12 Following initiation, samples are held under conditions of controlled temperature and humidity for a known period of time to allow FFC development...

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“…1, 3, and 4 are consistent with chloride-induced substrate depassivation 25 and anodic attack occurring through Reaction 2 [4][5][6][7]21 Al…”

Section: Discussionsupporting

confidence: 77%

Section: Resultsmentioning

confidence: 93%

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

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

Williams

McMurray

2003

J. Electrochem. Soc.

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“…With the help of SKP, Williams et al [9] showed that the outward migration of hydroxide ions in the electric field alongside the interface stopped inward migration of cerium cations from the defect, while the cerium cations from the dispersed silica and bentonite in the PVB coating reached the active sites and effectively inhibited coating delamination. Williams et al also successfully applied SKP in the study of filiform corrosion [10].…”

Section: Introductionmentioning

confidence: 99%

RETRACTED ARTICLE: Investigation of coating delamination on steels by surface topography and Volta potential difference

Pan

Wang

2012

J Solid State Electrochem

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Corrosion-induced delamination of an epoxy coating on the AISI/SAE 1045 carbon steel was studied under a humid atmospheric condition (temperature of 25°C, one standard atmospheric pressure, and relative humidity of 90 %) by the technique of scanning Kelvin probe force microscopy (SKPFM). Surface-polished 1045 samples were first cold coated with the epoxy and then subject to the atmospheric corrosion under the humid atmospheric condition. At specified time intervals, surface Volta potential of the samples was measured using the SKPFM over the dry surface of epoxy coating. The map of Volta potentials demonstrated high contrasts among three characteristic zones: intact steel-epoxy interface, delaminated interface, and interface with active corrosion, which based on a rigorous calibration procedure were then linked to the actual corrosion potential of the steel (measured using a potentiostat w.r.t. a saturated calomel electrode). The SKPFM was found to be able to provide a mean of direct and nondestructive detection of early active corrosion and coating delamination of steels at a submicroscopic resolution, which outperformed the conventional electrochemical techniques for such purposes.

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“…The scanning Kelvin probe (SKP), which has a lateral resolution of about 50 µm, has been used to map the spatial and temporal distribution of Volta potential distributions beneath a thin-film electrolyte or underneath a coating. [9][10][11][12][13][14][15][16][17] The delamination front on steel was characterized by a steep change in Volta potential (500 mV). The intact interface had a high potential plateau associated with oxygen reduction, and the delaminated region exhibited lower potentials.…”

Section: Fontana Corrosion Center Department Of Materials Science Anmentioning

confidence: 99%

Investigation of Filiform Corrosion of Epoxy-Coated 1045 Carbon Steel by Scanning Kelvin Probe Force Microscopy

Leblanc

Frankel

2004

J. Electrochem. Soc.

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The mechanism for filiform corrosion (FFC) is thought to involve oxygen diffusion through the tail to the active head. The primary cathodic region is near the back of the head (at the head/tail boundary), where oxygen concentration is higher, and the primary anodic region is at the front edge of the head of the filament. Although there is experimental support for this mechanism, a high-resolution description of the FFC process has not been presented. The aim of this study was to provide detailed information about the mechanism of FFC on coated steel using the high spatial resolution of scanning Kelvin probe force microscopy. Segments of active filaments were successfully investigated through 150 and 300 nm thin epoxy coatings in air of 93% relative humidity. Volta potential and topographic maps showed separation of active anodes and cathodes in the head and revealed the presence of voids associated with delamination of the coating along the edge of the tail. The morphology of filaments and Volta potential distributions were strongly dependent on the film thickness. Differences in growth characteristics were explained by mass transport considerations.Filiform corrosion (FFC) affects many metals underneath organic, inorganic, or metallic coatings and results in networks of thread-like corrosion products propagating across the metal surface. Early studies of FFC focused on steel substrates coated with a variety of varnishes or lacquers. The development of new paint systems and the increased use of lightweight alloys in the aeronautical industry have widened the field of study to aluminum and magnesium alloys and to more modern coatings. FFC of steel, aluminum, and magnesium has been observed in various humidity conditions and under many different types of coatings. The growth pattern, behavior, physical dimension, and velocity of the filaments have been reviewed in detail. [1][2][3][4] The literature reveals that there are many common characteristics of FFC, regardless of the nature of the metal or coating. A filament consists of two visually distinguishable parts: an active head, where dissolution of the metal occurs, and a trailing tail, composed of dry corrosion products. The interface between the dry tail and the head on iron is characterized by a V-shaped boundary. The width of filaments ranges from 0.05 to 3 mm, and the filaments can travel considerable distances and weave intricate patterns. The average growth rate ranges between 0.01 to 0.5 mm/day, but growth is not uniform. Degradation of the metal is superficial, as the depth of attack rarely exceeds 10 µm, but FFC can destroy the protectiveness of a coating system. On steel, filaments never cross each other and are simply deflected without actually touching, or split into two heads, which then reflect or stop growing at ''near-collision.'' Most of the characteristics of FFC are consistent with a differential oxygen concentration cell mechanism, 5 which requires that the oxygen concentration be much lower in the head than in

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Time-lapse potentiometric imaging of active filiform corrosion using a scanning Kelvin probe technique

Cited by 22 publications

References 18 publications

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

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

RETRACTED ARTICLE: Investigation of coating delamination on steels by surface topography and Volta potential difference

Investigation of Filiform Corrosion of Epoxy-Coated 1045 Carbon Steel by Scanning Kelvin Probe Force Microscopy

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