Filiform Corrosion Research Articles

In-situ polymerized nanosilica/acrylic/epoxy hybrid coating: Preparation, microstructure and properties

An in-situ polymerization method was employed to synthesize the nanosilica/acrylic/epoxy (SAE) hybrid coating on AISI 430 stainless steel (430SS), as compared with a traditional blending method. Microstructures of the blending SAE hybrid coating (BC) and in-situ SAE hybrid coating (ISC) were characterized by transmission electron microscopy (TEM). Corrosion resistance of BC and ISC on 430SS was evaluated by the neutral salt spray test and potentiodynamic polarization technique. Failure mechanism of the BC on 430SS was suggested by the microstructures and corrosion behaviors. Serious aggregation of nanosilica particles in the BC impairs its structural uniformity and induces the flaws formation. These flaws in the BC initiates the failures of pitting, filiform corrosion and peeling which are accelerated by the O2 concentration cell and H+ self-catalysis in chlorine-containing moist environments. The ISC-coated 430SS shows a more advantageous corrosion resistance than that of the BC-coated. The ISC-coated 430SS can suffer the salt spray over 1000 h. Besides, it exhibits a high corrosion potential beyond 0.925 V and good passivation characteristics during the potentiodynamic polarization.

A comparative electrochemical study of commercial and model aluminium alloy (AA5050)

AbstractA comparative electrochemical study of commercially recycled AA5050 and model AA5050, in their as‐cast state, is carried out to investigate the effect of recycling and furthermore the effect of trace elements like Pb, Cu, etc on electrochemical activation and filiform corrosion (FFC) susceptibility. A systematic combination of surface analytical techniques like scanning Kelvin probe force microscopy (SKPFM), scanning electron microscopy (SEM) and energy dispersive X‐ray (EDX) is used to study the composition and electrochemical behaviour of second phase particles. These results, together with localized breakdown measurements using the electrochemical micro‐cell, elucidate the effect of recycling and the role played by trace elements in influencing the electrochemical behaviour of the alloy.

Corrosion Propagation Behavior of Magnesium Alloys under Atmospheric Conditions

To investigate corrosion mechanisms of magnesium alloys under atmospheric conditions, field exposure tests were performed. In a marine environment, magnesium alloys experienced localized corrosion, including pitting and filiform corrosion. To reproduce such actual atmospheric corrosion in a laboratory experiment, we employed a wet-dry cyclic test at a constant dew point. From the results of the wet-dry cyclic tests, the propagation mechanism of localized corrosion on magnesium alloys has been discussed. Localized corrosion occurred during the wetting period, and the corroded sites thus formed were repassivated during the drying period. In the next wetting period, new localized corrosion took place, but the repassivated sites were not reactivated. The repassivated sites have higher corrosion resistance than the non-corroded part of the surface because of the barrier effect of the corrosion products. In the case of AZ91D, Al enrichment in the surface region of the metal matrix is responsible for this corrosion resistance.

A versatile soft X-ray transmission system for time resolved in situ microscopy with chemical contrast

We present the design and operation of a versatile soft X-ray transmission system for time resolved in situ microscopy with chemical contrast. The utility of the setup is demonstrated by results from following a corrosion process of iron in saline environment, subjected to a controlled humid atmosphere. The system includes a transmission flow-cell reactor that allows for in situ microscopic probing with soft X-rays. We employ a full field technique by using a nearly collimated X-ray beam that produces an unmagnified projection of the transmitted soft X-rays (below 1.1keV) which is magnified and recorded by an optical CCD camera. Time lapse series with chemical contrast allow us to follow and interpret the chemical processes in detail. The obtainable lateral resolution is a few μm, sufficient to detect filiform corrosion on iron.

Influence of Cooling Rate on Corrosion Resistance of Rapidly Solidified Mg-Zn-Y Alloys with Long Period Stacking Ordered Phase

This paper deals with the influence of cooling rate on the corrosion resistance of rapidly solidified (RS) Mg-Zn-Y alloys with a long period stacking ordered (LPSO) phase. To clarify the effect of microstructure modification by rapid solidification on suppressing localized corrosion such as filiform corrosion, several Mg97.25Zn0.75Y2 (at.%) alloys were prepared with different cooling rates were prepared by the gravity-casting, copper mold injection-casting and melt-spinning techniques. In the alloys solidified at a low cooling rate, filiform corrosion propagated in the early stage of a saltwater immersion test, due to the formation of a massive, block-shaped LPSO phase during casting. In contrast, the alloys solidified at a high cooling rate exhibited excellent corrosion resistance. Rapid solidification brought about grain-refinement and the formation of a supersaturated α-Mg single-phase solid solution, resulting in improved electrochemical homogeneity.

Corrosion characterization of Mg–8Li alloy in NaCl solution

The corrosion mechanism of Mg–8Li alloy in NaCl solution was investigated by electrochemical testing and SEM observation. The electrochemical results indicated that the corrosion resistance of Mg–8Li alloy in 0.1 M NaCl solution gradually deteriorated with increasing of immersion time expect for 2 h immersion, which was consistent with the SEM observation of corrosion morphology. Mg–8Li alloy exhibited filiform type of attack under significant anodic control of magnesium solution reaction. The cathodic reaction was driven by hydrogen evolution reaction. The presence of filiform corrosion also proved a resistant oxide film naturally formed on the surface of Mg–8Li alloy.

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.

Environmental Effects on the Hydrolytic Ageing of Epibond 1590–Aluminium Joints — a Dielectric Study

Dielectric measurements are reported on Epibond 1590–aluminium joints exposed to water at 50 and 75°C. Differences in the ageing behaviour are observed which are not simply explained in terms of the expected Arrhenius dependence of moisture ingress into the joints. Ageing is the result of a combination of the effects of water entering the adhesive, leading to plasticisation and changes in the surface layer of oxide on the aluminium substrate. Surprisingly, the initial rate of corrosion at 50°C is higher than that at 75°C, although the rate of moisture uptake into the resin is higher at the higher temperature. This apparently anomalous behaviour can be attributed to filiform corrosion assisting in the opening of the bondline and aiding subsequent moisture and oxygen ingress. Comparison of joints that were periodically withdrawn for dielectric and gravimetric measurements with those aged continuously without withdrawal indicated that atmospheric oxygen plays a significant role in aiding the corrosion process. The dielectric measurements provide a non-destructive method of probing the processes taking place within the joint as it is aged.

Relation between corrosion behavior and microstructure of Mg–Zn–Y alloys prepared by rapid solidification at various cooling rates

The relation between corrosion resistance and microstructure of Mg–Zn–Y alloys with a long period stacking ordered (LPSO) phase has been investigated. In order to clarify the influence of microstructure evolution by rapid solidification on the occurrence of localized corrosion such as filiform corrosion, several Mg 97.25Zn 0.75Y 2 (at. %) alloys with different cooling rates were fabricated by the gravity casting, copper mould injection casting and melt-spinning techniques and their corrosion behavior and microstructures were examined by the salt immersion tests, electrochemical measurements, XRD and TEM. When the cooling rate was less than 3 × 10 4 K s −1, filiform corrosion propagated in the early stage of salt immersion test, due to formation of a massive block-shaped LPSO phase during casting. On the other hand, when the cooling rate was increased up to 3 × 10 4 K s −1, rapidly solidified (RS) alloys exhibited excellent corrosion resistance because of grain refinement and formation of a supersaturated single-phase solid solution. Large-sized Mg 97.25Zn 0.75Y 2 alloys fabricated by consolidation of the RS ribbons also exhibited excellent corrosion resistance with passivity. Enhancement of microstructural and electrochemical homogeneities in the Mg–Zn–Y alloys by rapid solidification techniques results in the passivity of substrate materials.

Inhibition of Corrosion on AA2024-T3 by New Environmentally Friendly Rare Earth Organophosphate Compounds

Abstract The combination of rare earth metals (REM), such as Ce, La, Pr, or the less-refined mischmetal (consisting of a combination of REM) with dibutyl phosphate or diphenyl phosphate, provides a complex that shows excellent corrosion inhibition for AA2024-T3 (UNS A92024) in aqueous chloride environments. In some instances, it is equal to the inhibition provided by the chromate compounds, which are still used by the aircraft industry. The addition of these compounds as an inhibiting pigment in an epoxy coating also shows efficient suppression of filiform corrosion from scribes in the coating, under both alternate immersion and high-humidity conditions. Surface characterization of AA2024-T3 specimens immersed in solutions of these complex salts suggests that a film containing both the organic and rare earth components deposits on the alloy surface. The electrochemical data indicate that this film is acting as a mixed inhibitor, suppressing both the anodic and cathodic processes on the surface.

Environmental effects on the ageing of epoxy adhesive joints

Adhesive bonded joints are exposed to a range of different environments in aerospace applications. This paper reports dielectric and mechanical analysis of aluminium–epoxy bonded adhesives joints exposed to de-ionized water, aqueous urea solution and salt water at 65 °C. The changes observed are the results of plasticization and corrosion. In the case of the aqueous urea solution, passivation of the oxide by the urea reduced the rate of corrosion. Sea water contains mobile ions and a new feature is detected associated with filiform corrosion. The non-polar media aviation fuel and hydraulic fluid are able to plasticize the adhesive and there is a consequent reduction in the strength of the joint. Propylene glycol, although it is polar solvent, produces limited plasticization and degradation of the joints. Dichloromethane was very aggressive and produced a rapid loss of strength of the joints.

高清浄度 AZ31 マグネシウム合金における糸状腐食

It is generally recognized that impurity atoms affect the phenomena of corrosion. In the present study, we have prepared a low impurity magnesium alloy, and the corrosion behaviors of the alloy was compared with that of a commercial AZ31B magnesium alloy. The low impurity alloy was prepared with using 5N-Al, 4N-Zn and high purity magnesium, the last one was prepared by a vapor deposition technique. Chemical analyses showed that the composition of the alloy was about Mg-3% Al-1% Zn (in mass%). Slices with about 5 mm thickness were cut from an ingot of the alloy, and hot-rolled into 1 mm at 623 K, then solution heat treated at 673 K for 86.4 ks. Lowering the impurities resulted in improving a corrosion resistance. Corrosion test using a 1 mass% NaCl solution showed that the low impurity magnesium alloy had a superior corrosion resistance compared with those of the AZ31B and the AZ31PE alloys. In the AZ31B alloy, filiform corrosion occurred after a duration time of 0.6 ks. Heads of the filiform corrosion propagated with random walking, creating regular irtraces, while in the low impurity magnesium alloy, duration time for the filiform corrosion was prolonged to 7.2 ks. Moreover, morphology of tails were changed by lowering the impurity concentrations, into straightforward.

Plasma Electrolytic Oxidation of AZ91 Mg Alloy in the Sodium Stannate Electrolyte

The effect of sodium stannate on Plasma electrolyte oxidation (PEO) process of AZ91 Mg alloy was systematically investigated. The oxide layer formed on an AZ91 Mg alloy created using an electrolyte containing sodium stannate formed a much thicker layer consisting of: MgO, SnO, Mg 2 SiO 4 , and MgF 2 . The result of EDS analysis shows a tin enriched zone at the oxide layer/substrate interface. Corrosion resistance of the sample coated from the electrolyte containing sodium stannate was determined to be superior to that of the sample coated from the electrolyte without sodium stannate. As a result of salt spray test for 120 hrs, severe filiform corrosion is observed on the thin oxide layer of AZ91 Mg alloy without sodium stannate. However, the sample of the electrolyte containing sodium stannate shows localized pitting corrosion. Tin ions in the electrolyte influence on the growth rate of the oxide layer, especially resulting in a formation of dense barrier layer. The co-existence of dense MgO and SnO in the oxide layer could improve the corrosion resistance of the AZ91 Mg alloy.

Corrosion behaviour of Mg 65Cu 7.5Ni 7.5Ag 5Zn 5Gd 5Y 5 bulk metallic glass in aqueous environments

The corrosion behaviour of the Mg 65Cu 7.5Ni 7.5Ag 5Zn 5Gd 5Y 5 bulk metallic glass prepared by copper mould casting with 3 mm thickness was studied in borate solutions with pH 5–8.4 and in sodium hydroxide solution with pH 13. A superior corrosion resistance and passivation ability compared to that of Mg and the conventional AZ31 alloy was detected in weakly acidic to weakly alkaline solutions. This is mainly attributed to the beneficial effect of the alloying components, which are homogeneously mixed in the amorphous phase. They cause the establishment of a more noble alloy state and possess a higher tendency for surface passivation than the main component Mg. A high sensibility to chloride-induced corrosion exists, which is at low chloride concentration strongly pH value dependent. For the first time filiform corrosion was detected for Mg-based metallic glasses with filaments propagating randomly on the passivated amorphous surface. Y-rich crystalline defects in the bulk glassy sample seem to have no evident effect on the corrosion processes.

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.

Scanning Kelvin Probe Studies of Filiform Corrosion on Automotive Aluminum Alloy AA6016

Scanning Kelvin probe (SKP) potentiometry was used to systematically investigate the effect of surface abrasion and subsequent heat-treatment on the open-circuit potential (OCP) of AA6016 in humid air. SKP is also used to follow the kinetics of filiform corrosion (FFC) and to determine characteristic potentials associated with the electrolyte-filled filiform head and dry filiform tail. It is shown that simply abrading with 180 grit paper produces a surface potential ca. lower than the bulk. When the abraded sample was overcoated with a layer of PVB (polyvinyl butyral) and exposed to a fast, superficial filiform corrosion was observed in which metal loss was limited to the thickness of the surface layer (i.e., ca. ). Filiform head OCP values were similar to those of the surface layer, whereas filiform tail OCP values were similar to the bulk. A mechanism is proposed in which the ultrafine grain structure of the surface layer produces an anodic activation, and the potential difference between the surface layer and the bulk provides an increased thermodynamic driving force for corrosion. For postabrasion heat-treatment temperatures between 180 and 350°C the fast filiform process was followed by a slower, deeper form of FFC.

Characterisation of initial atmospheric corrosion carbon steels by field exposure and laboratory simulation

The early stages of the evolution of atmospheric corrosion of carbon steels exposed in both a laboratory simulated and a natural atmosphere environment in Shenyang have been observed by in situ scanning electron microscopy. In the case of laboratory cyclic wet–dry tests, even though the chloride content level is very low, filiform corrosion is initiated in the early stage. The filiform corrosion grows in random directions, forming a network of ridges. White nodules nucleate and grow on the ridges during continued corrosion and eventually connect with each other to form the initial corrosion scale. Pits were also found on the surface beneath corrosion products. In the case of a natural atmospheric environment, both filiform corrosion and other localized corrosion, such as pitting and inter-granular attack take place in the initial stage. It is obvious that there is variety of localized corrosion in the initial stage of atmospheric corrosion.

Inhibition of Filiform Corrosion on AA6111-T4 Using In-Coating Phenylphosphonic Acid

Phenylphosphonic acid (H 2 PP) dissolved in a polyvinylbutyral coating is shown to profoundly inhibit filiform corrosion (FFC) on coated AA6111-T4. Following FFC initiation by application of aqueous HCl to a penetrative coating defect, the time-dependent extent of coating delamination was determined both optically and by repeated in situ measurements using a scanning Kelvin probe. FFC inhibition is shown to become almost complete when H 2 PP is ≥0.5% w/w. In-coating H 2 PP is also shown to depress Volta potential values associated with the intact (uncorroded) coated surface by up to 0.35 V.

Mg-Al-Zn系マグネシウム合金の冷間圧延による組織変化と不純物元素濃度の影響

It is generally recognized that impurity atoms affect the phenomena of recrystallization. In the present study, we have prepared a low impurity magnesium alloy, then deformation behavior in cold-rolling and corrosion behavior of the alloy were compared with those of a commercial AZ31B magnesium alloy. The low impurity alloy was prepared with using 5N-Al, 4N-Zn and high purity magnesium, the last one was prepared by a vapor deposition technique. Chemical analyses showed that the composition of the alloy was about Mg-3%Al-1%Zn (in mass%). Slices with about 5 mm thickness were cut from an ingot of the alloy, and hot-rolled into 1 mm at 623 K, then solution heat treated at 673 K for 86.4 ks. The specimens were cold-rolled at room temperature with reduction rates of 0~80%. For comparison, a commercial AZ31B alloy with 0.8 mm thickness was used. After cold-rolling, foil specimens for transmission electron microscopy were prepared and microstructures were observed. Recrystallization occurred in the AZ31B alloy by 70% cold-rolling at room temperature. Regions with low dislocation density were frequently observed in heavily cold-rolled specimens, which would show recovery phenomena or early stage of recrystallization. Additionally lowering the impurities resulted in improving a corrosion resistance. Corrosion test using a 1 mass%NaCl solution showed that the low impurity magnesium alloy had a superior corrosion resistance compared with that in the AZ31B alloy. In the AZ31B alloy filiform corrosion occurred after a duration time of 27 minutes, while in the low impurity magnesium alloy, filiform corrosion did not occur for 22 hours but general corrosion occurred.