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

Abstract

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...

Effect of Surface Pretreatment on the Underpaint Corrosion of AA2024-T3 at Various Temperatures

“…[31][32][33][34] According to this mechanism the head and tail form the anode and cathode, respectively. These anode/ cathode positions have been confi rmed by scanning Kelvin probe measurements at the sites where scribecreep was observed under polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate coatings on steel [35][36] and commercial polymer coatings on AA2024-T3. 37 Model AlCu alloys were created by depositing a regular array of Cu islands on high-purity Al.…”

“…23,[31][32][33][34][35][36][37][38][39][40][41][43][44]47,50,[57][58][59]63,[65][66][67][68][69][70][71][72][73][74][75][76][77][78] The fi lament has what is considered an anodic head and a cathodic tail. The head has an acidifi ed, high Cl -ion-containing electrolyte present, while the tail consists of semi-dry corrosion product and the pH is alkaline.…”

“…The head has an acidifi ed, high Cl -ion-containing electrolyte present, while the tail consists of semi-dry corrosion product and the pH is alkaline. 35,37,79 The local anode in the leading part of the head of the fi lament is where the anodic dissolution of the metal occurs and undercuts the coating. Farther back from the leading edge is the local cathode where ETR, such as reduction of oxygen, occurs.…”

“…The anodic propagation of the fi laments has been ascribed to the differential in the oxygen concentration between the front and the back of the head of the fi lament, 23,31,79 although, clearly, the pH change and Cl -levels also contribute to the formation of a galvanic couple between the low-pH, high-Cl -head and highpH tail. The FFC rate is promoted by HCl solution 35,75 and elevated temperature. 31,43,75,80 Hoch 43 observed that in cold weather, FFC appeared to become inactive and then reinitiated when exposed to warm weather.…”

“…41,63 The micrographs taken periodically of scribecreep length and area were analyzed in a method similar to that of Bautista 31 and Williams and McMurray. 35 Analysis of the collected data included the quantitative determination of the extent of scribecreep by visual estimation of the average length, l ave , of the fi laments:…”