H. N. McMurray

Chromate Inhibition of Corrosion-Driven Organic Coating Delamination Studied Using a Scanning Kelvin Probe Technique

The scanning Kelvin probe is used to study the influence of trivalent cerium cations (Ce 3+ ) on the kinetics and mechanism of corrosion-driven delamination processes affecting polyvinyl butyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanized steel. Placing aerated aqueous sodium chloride onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O 2 reduction at the delamination front is coupled to anodic zinc dissolution at the coating defect by a thin (2.5-5 μm) layer of electrolyte which ingresses beneath the delaminated PVB film. Soluble Ce(III) salts in the external electrolyte reduce coating delamination rates by <25% because Ce 3+ (aq) cations migrating beneath the coating immediately encounter a countercurrent migration of cathodically produced OH anions. Consequently, Ce(OH) 3 precipitation occurs at the coating/ defect boundary, with little or no Ce 3+ (aq) reaching the active sites of cathodic O 2 reduction at the delamination front. In contrast. dispersions of silica and bentonite-based Ce 3+ cation-exchange pigments in the PVB coatings enable Ce 3+ (aq) to exchange directly into the under-film electrolyte and significantly retard coating delamination. Delamination is not halted completely because under-film Ce 3+ (aq) exchange continually lags behind the advancing delamination front so that precipitated Ce(OH) 3 increases under-film resistivity by displacing electrolyte but does not form a coherent cathodic film capable of stifling O 2 reduction.

Kinetic and Mechanistic Studies of Rare Earth-Rich Protective Film Formation Using In Situ Ellipsometry

The deposition of rare earth metal (REM) rich (hydr)oxide films on pure iron and zinc surfaces has been studied using in situ ellipsometry. Iron and zinc surfaces freely corroding in near-neutral 0.86 mol dm(-3) aerated aqueous sodium chloride were found to be covered with 6-15 nm thick native (hydr)oxide films. The kinetics of REM-rich film growth for one redox active [Ce(III)] and two redox inactive [Y(III) and La(IU)] cations were measured. Addition of 2.5 x 10(-3) mol dm-3 REM chloride salts to the electrolyte resulted in rapid growth of REM-rich (hydr)oxide films on both metal substrates. Similar film growth rates were observed for each REM cation. REM-rich film deposition proceeds via the precipitation of REM hydroxides [M(OH)(3)] produced by cation hydrolysis proximal to the substrate-solution interface driven by increased interfacial pH resulting from cathodic oxygen reduction. When iron was cathodically polarized to near the free corrosion potential of zinc (-1.05 V vs. SCE) the pre-existing iron (hydr)oxide film was removed within 400 s and no deposition of REM-(hydr)oxide films was observed. On the basis of these findings it is proposed that REM-(hydr)oxide deposits only nucleate efficiently on the native (hydr)oxide covered metal surfaces

Novel environment friendly corrosion inhibitor pigments based on naturally occurring clay minerals

Novel, ceramic, corrosion inhibitor pigments consisting of cerium (III) and calcium (II) cation exchanged bentonites have been shown to provide effective cut-edge corrosion resistance in organic coated galvanised steel. The bentonite pigments were prepared from a naturally occurring (Wyoming) bentonite with a cation-exchange-capacity of 0.7 milli-equivalents per gram. Cation exchange was carried out by repeated washing with aqueous solutions of cerium (III) chloride and calcium (II) chloride to produce bentonites containing 31 500 ppm exchangeable cerium (III) and 13 500 ppm exchangeable calcium (II) respectively. The resulting bentonite pigments were dispersed in a polyester-resin based primer paint system to give a pigment volume concentration of 19%. For comparison, two similar primer systems were prepared containing a commercial calcium (II) exchanged silica pigment (Shieldex: 60 000 ppm calcium (II)) and a strontium chromate dispersion, both with a 19% pigment volume concentration. All three primer systems were applied (5 μm) to the zinc surface of galvanised 0.7 mm gauge sheet steel and overcoated with an architectural polyester topcoat (18 μm). The performance of the inhibitor pigments was compared by measuring the rate of corrosion-driven organic coating delamination from the cut edge of samples during 1000 h of salt-spray testing. The calcium (II) bentonite pigment exhibited an anti-delamination performance similar to that of strontium chromate but superior to that of Shieldex. However, the cerium (III) bentonite pigment was superior in performance to both strontium chromate and Shieldex. Thus, the bentonite pigments represent promising, environmentally friendly, ion-exchange corrosion inhibitors which exhibit good anti-delamination performance by comparison with current commercial systems. Neue umweltfreundliche Korrosionsinhibitorpigmente auf Basis naturlich vorkommender Tonmineralien Es hat sich gezeigt, dass neue, keramische Korrosionsinhibitorpigmente, die aus Cer(III) und Kalzium(II) Kationen-ausgetauschten Bentoniten bestehen, einen wirksamen Korrosionsschutz an Schnittkanten von organisch beschichtetem, verzinkten Stahl bieten. Die Bentonitpigmente wurden aus einem naturlich vorkommenden (Wyoming) Bentonit mit einer Kationen-Austauschkapazitat von 0,7 Milliaquivalenten pro Gramm hergestellt. Der Kationenaustausch wurde durch wiederholtes Waschen mit wassrigen Losungen aus Cer(III)chlorid und Kalzium(II)chlorid durchgefuhrt, um Bentonite mit 31 5000 ppm austauschbarem Cer(III) bzw. 13 5000 ppm austauschbarem Kalzium(II) herzustellen. Die sich daraus ergebenden Bentonitpigmente wurde in einem auf Polyesterharz basierenden Grundbeschichtungssystem feinverteilt, so dass sich eine Pigmentvolumenkonzentration von 19% ergab. Zum Vergleich wurden zwei ahnliche Grundbeschichtungssysteme hergestellt, die ein handelsubliches Kalzium(II) ausgetauschtes Silikapigment (Shieldex: 60 000 ppm Kalzium(II)) und eine Strontiumchromatdispersion, beide mit einer Pigmentvolumenkonzentration von 19%, enthielten. Alle drei Grundbeschichtungssysteme wurden auf die Zinkoberflache eines verzinkten 0,7 mm dicken Stahlbleches aufgebracht (5 μm) und mit einer Polyester Deckbeschichtung (18 μm) uberschichtet. Das Verhalten der Inhibitorpigmente wurde durch Messung der durch Korrosion ausgelosten Enthaftung der organischen Beschichtung von den Schnittkanten der Proben wahrend 1000 h Salzspruhnebelprufung verglichen. Das Kalzium(II)-Bentonitpigment zeigte ein Anti-Enthaftungsverhalten, das dem des Strontiumchromats vergleichbar aber besser als das des Shieldex war. Das Cer(III)-Bentonitpigment war in seinem Verhalten jedoch sowohl dem Strontiumchromat als auch dem Shieldex uberlegen. Das bedeutet, dass die Bentonitpigmente vielversprechende, umweltfreundliche, Ionen-ausgetauschte Korrosionsinhibitoren darstellen, die im Vergleich mit handelsublichen Systemen ein gutes Anti-Enthaftungsverhalten zeigen.

Polyaniline Inhibition of Corrosion-Driven Organic Coating Cathodic Delamination on Iron

The polyaniline emeraldine salt of paratoluenesulfonic acid (PAni-pTS) is dispersed in polyvinylbutyral coatings adherent to an iron surface. Such dispersions are shown to effectively inhibit corrosion-driven coating delamination (cathodic disbondment) when 5% w/v (0.86 M) aqueous chloride electrolyte contacts a penetrative coating defect. A scanning Kelvin probe is used to quantify the influence of PAni-pTS volume fraction (Φ p a ) on delamination rate and the potential of the intact (undelaminated) coated surface (E i n t a c t ). Secondary ion mass spectroscopy and atomic force microscopy are used to determine the time-dependent thickness of an oxide layer developing at the coating-iron interface. At 20°C, high relative humidity (93%), and Φ p a = 0.3, the iron oxide layer grows at a constant rate of 5.6 X 10 - 3 nm s - 1 . E i n t a c t increases monotonically from 0.2 to 0.56 V vs. SHE, and coating delamination rates decrease by ca. 95% as Φ p a is increased from 0 to 0.25. Addition of 0.01 M Na-pTS to the experimental electrolyte has no effect. An inhibition mechanism is proposed in which through-coating cathodic O 2 reduction is suppressed by the ennoblement of substrate potentials and the OH - product of O 2 reduction is absorbed through PAni-pTS mediated pH buffering.

Inhibition of Filiform Corrosion on Organic-Coated Aluminum Alloy by Hydrotalcite-Like Anion-Exchange Pigments

Abstract Novel anion-exchange anticorrosion pigments consisting of nitrate-, carbonate-, and chromate-exchanged hydrotalcite (HT) are shown to inhibit the propagation of filiform corrosion (FFC) ef...

Use of the Scanning Reference Electrode Technique for the Evaluation of Environmentally Friendly, Nonchromate Corrosion Inhibitors

Abstract The scanning reference electrode technique (SRET) has been used to study the influence of a range of anodic and cathodic corrosion inhibitors on patterns of localized corrosion occurring o...

Dopant Effects in Polyaniline Inhibition of Corrosion-Driven Organic Coating Cathodic Delamination on Iron

Coatings comprising various volume fractions of particulate polyaniline dispersed in a polyvinylbutyral binder are applied to iron substrates. A scanning Kelvin probe is used to measure substrate potentials in humid air and follow corrosion-driven coating delamination (cathodic disbondment) when 5% w/v (0.86 M) aqueous NaCl contacts a coating defect. Emeraldine base has no effect on substrate potential or delamination kinetics. Emeraldine salts (ES) doped using p-toluenesulfonic (HpTS), camphorsulfonic (HCS), phosphoric (H 3 PO 4 ), and phenylphosphonic (H 2 PP) acids increase substrate potentials by up to 0.36 V and inhibit delamination with efficiency order of HpTS < HCS < H 2 PO 4 << H 2 PP. Dopant salts added to the corrosive electrolyte do not inhibit delamination. It is proposed that inhibition arises primarily from cathodic O 2 reduction becoming relocated from the ennobled substrate onto the ES coating. However, Fe 3 (PO 4 ) 2 and FePP salt films formed at the ES-substrate interface also contribute by hindering interfacial electron transfer.

Probe diameter and probe-specimen distance dependence in the lateral resolution of a scanning Kelvin probe

The ability of a scanning Kelvin probe (SKP) to resolve lateral differences in specimen surface potential is investigated by two routes. First, electrostatic calculations are used to obtain analytical expressions for the maximum lateral resolution attainable by a “point probe” of negligible physical dimensions, as a function of probe–specimen distance. Second, lateral resolution is measured experimentally by scanning plane-ended cylindrical probes of varying diameters at varying heights across a linear edge, delimiting two electrically continuous coplanar areas of dissimilar metal. Finally the two approaches are combined to obtain a semiempirical relationship between probe diameter, probe–specimen distance, and lateral resolution for plane-ended cylindrical probes. An expression is also developed for the minimum error to be expected in a surface potential measurement when this is associated with a specimen feature exhibiting a diameter comparable with the probe–specimen distance.

Photoelectrochemical investigation of corrosion using scanning electrochemical techniques

The scanning vibrating electrode technique (SVET) has been used to investigate the influence of pigment catalysed photodegradation on the mechanisms of corrosion occurring at exposed metallic cut edges of organic coated galvanised steel materials. Galvanised sheet steel samples (0.7 mm thick with 20 μm zinc galvanising applied both sides) were coated with symmetric thickness polyester resin layers pigmented with photoactive titanium dioxide. Samples were then irradiated on one side only with UVA light (365 nm at 2×1017 photons s−1 for 24 h), producing asymmetric photodegradation of the pigmented polymer layers. Prior to photodegradation the zinc layers exposed at an edge produced by cutting through the coated sheet behave anodically and the exposed steel acts cathodically when immersed in aqueous aerated 5% NaCl. As immersion time increases there is small amount of through coating anodic activity which leads to the anodic/cathodic current balance in the plane of scan over the cut edge falling to 70% after 12 h immersion. Subsequent to photodegradation, local coating thinning is evident in the UV irradiated coating facilitating greater oxygen transport to the metal surface in this region. The resulting differential aeration cell initially focuses anodic activity on the zinc surface proximal to the non-irradiated coating. Cathodic activity proximal to the photodegraded polymer layer results in its chemical degradation through base catalysed hydrolysis of the ester function. The resulting breakdown of coating integrity in the irradiated coating leads to both anodic and cathodic activity occurring through the photodegraded coating and a subsequent loss in overall SVET current balance in the plane of scan above the cut edge.

Quantitative assessment of localized corrosion occurring on galvanized steel samples using the scanning vibrating electrode technique

Abstract Zinc and zinc alloy galvanized steel is used increasingly for structural cladding, automotive, and domestic appliance applications. In assessing the different galvanizing coatings, it is important to understand the nature of corrosion reactions occurring on the metal surfaces. To this end, the scanning vibrating electrode technique (SVET) has been used to study the effect of variation in metallic coating on the localization and intensity of corrosion reactions occurring on the bare metal surfaces when immersed in aerated 0.1% sodium chloride (NaCl). The samples used comprised pure zinc and galvanized steel substrates, namely electro-zinc (EZ), hot dip galvanized steel (HDG), iron (9%) zinc intermetallic (IZ), 5% aluminum zinc alloy (Galfan), and 55% aluminum zinc alloy (Zalutite). The SVET has the resolution and sensitivity to enable the number and intensity of active anodes to be quantified. Zinc galvanized materials show anodes, which do not deactivate within the 24 h of the test whereas zinc a...