John R. Scully

Electrochemical Impedance of Organic‐Coated Steel: Correlation of Impedance Parameters with Long‐Term Coating Deterioration

Abstract : Electrochemical impedance results are presented for 550 day exposures of organic coated carbon steel samples. Coatings consisted of translucent pigmented and unpigmented epoxy and conventional opaque epoxy polyamide systems. Coating thicknesses ranged from 20 to 185 microns. Specimens were exposed under freely corroding conditions and at two cathodic polarization levels (-805 mV and -1250 mV versus SCE) in ASTM artificial ocean water. The objective was to identify impedance parameters which measure subcritical coated-metal system property changes at early exposure times that are indicators of significant long term coating deterioration. Impedance data developed at early times, including coating resistance, coating capacitance, the increase in frequency associated with the coatings resistive-capacitive (RC) 45 degree phase angle, and low frequency impedance data, are compared to the coating systems performance evaluated after 550 days exposure. Keywords: Electrochemical impedance, Organic coating, Corrosion, Alternating current impedance, Anti-corrosion coating.

In Situ Confocal Laser Scanning Microscopy of AA 2024-T3 Corrosion Metrology I. Localized Corrosion of Particles

The morphology of attack at and around the intermetallic compounds (IMC) present on bare AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Exposures were conducted in 0.1 M Na 2 SO 4 + 0.005 M NaCI at pH 3, 6, and 10 as well as near-neutral 0.5 M NaCl. The types of attack observed could be categorized as matrix and IMC pitting, trenching adjacent to IMC, and matrix etching. The electrochemical behavior of bulk synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic compounds as well as that of AA 2024-T3 was used to rationalize the observed attack metrology. The galvanic coupling between the AA2024-T3 matrix and the intermetallic particles controlled the attack rates. In Al-Cu-Mg, the strong polarization to the open-circuit potential of the alloy caused rapid dissolution (ca. 10 mA/cm 2 ), whereas for the Al-Cu-Fe-Mn the dissolution rates were on the order of 100 μA/cm 2 . The limited dissolution rates of the Al-Cu-Fe-Mn phase were due to the cathodic polarization of these particles by the matrix under open-circuit conditions. Several pits were initiated at large Al-Cu-Mg particles. These pits were stable within the Al-Cu-Mg phase, but could not form stable pits in the alloy matrix during open-circuit corrosion. Calculation of growth rates and pit stability products for the individual IMC emphasized the role of metastable pitting in the observed corrosion metrology, which developed on AA2024-T3 during open-circuit corrosion.

In Situ Confocal Laser Scanning Microscopy of AA 2024-T3 Corrosion Metrology II. Trench Formation around Particles

The mechanism of trench formation next to cathodic intermetallic compound particles (IMC) on bare polished AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Trenches formed at the interface between the matrix and some IMC of the Al-Cu-Mn-Fe and Al-Cu types in all electrolyte solutions studied, including 0.1 M Na 2 SO 4 + 0.005 M NaCI with pH adjusted to 3, 6, and 10 as well as in near-neutral 0.5 M NaCl (pH 6). The trenches in the acidic solution were narrower, and not every cathodic IMC examined developed trenches. Two models for trench formation are compared in their ability to rationalize the experimental observations. The alkaline model ascribes the trenching completely to the effects of local pH increase and is unable to rationalize trenching at low pH, the dependence of trenching on particle type, and the effect of chloride ion content on trenching rate. An anodic trench model combines the influences of IMC cathodic kinetics, galvanic coupling, and anodic dissolution of the matrix, and particle/matrix metallurgy on the trenching behavior. The latter model is able to rationalize the effects of chloride content, bulk pH, as well as the observation that not all particles of a given type undergo trenching.

Oxygen Reduction Reaction Kinetics on Chromate Conversion Coated Al-Cu, Al-Cu-Mg, and Al-Cu-Mn-Fe Intermetallic Compounds

Oxygen reduction reaction (ORR) kinetics were investigated on hulk synthesized analogues of Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic phases with and without chromate conversion coating (CCC) in 0.1 M Na 2 SO 4 + 0.005 M NaCl (pH 6) with minimal levels of total dissolved chromate. The results were compared to AA 2024-T3 and high purity Al. Cu, Cr, and Au. Net cathodic ORR mass transport-limiting current densities of Al-based materials, lacking large quantities of Fe, Mn, or Cu were lower than the theoretically predicted rates in the mass transport controlled regime of ideal electronic conductors. This suggests a second rate-limiting factor in the case of these Al-based materials that was deduced to he related to Al-rich surface oxides. A second rate-limiting effect was also seen for pure Cr, implying that Cr 2 O 3 inhibits ORR kinetics, CCC inhibited open circuit corrosion, via reduced ORR kinetics. It also serves as a diffusive barrier to O 2 transport. The role(s) of CCC as an enhanced electronic barrier to electron transfer or barrier to O 2 chemisorption remains unclear.

Electrochemical characteristics of the Al2Cu, Al3Ta and Al3Zr intermetallic phases and their relevancy to the localized corrosion of Al alloys

Abstract The electrochemical behaviors of θ-Al 2 Cu phase, β-Al 3 Ta and, to a lesser extent, Al 3 Zr were compared to high purity Al in ambient temperature inert buffer solutions and, in certain cases, dilute halide solutions. The aim of this work is to develop a better understanding of electrochemical characteristics of these intermetallic phases. In particular, information was sought concerning (a) their galvanic couple relationship with respect to Al and (b) the relationship between the passive film formed on each phase and its ability to support both cathodic and anodic electron transfer reactions (ETR). The open circuit potentials (OCPs) of all three intermetallic phases were more positive than that of Al in inert solutions ranging from pH 2 to 12. The Al 2 Cu phase supports the reduction of water reaction at enhanced rates relative to pure Al due to the presence of metallic Cu o in an Al 2 O 3 rich oxide but supports oxygen evolution due to a combination of this effect as well as formation of more electrically conductive copper oxides. A similar effect is observed for Al 3 Ta and is attributed mainly to the formation of a more conductive mixed oxide containing Ta 2 O 5 .

Inhibition of Pitting Corrosion on Aluminum Alloy 2024-T3: Effect of Soluble Chromate Additions vs Chromate Conversion Coating

Abstract Pitting potentials (Epit) of aluminum alloy (AA) 2024-T3 (UNS A92024) were improved with additions of 0.01 M sodium chromate (Na2CrO4, pH 8) and 0.0062 M Na2CrO4 + 0.0038 M chromic acid (H2CrO4, pH 6) additions to 0.1 M sodium sulfate (Na2SO4) + 0.005 M sodium chloride (NaCl, pH 6) after 2-h periods at open-circuit potential (OCP). Chromate conversion coatings (CCC) on AA 2024-T3 also improved Epit in 0.1 M Na2SO4 + 0.005 M NaCl. However, CCC did not improve the Epit determined in fast potentiodynamic scans without periods at OCP. These tests were designed to minimize chromate leaching and long-range transport of chromate. The OCP of CCC AA 2024-T3, Al2Cu, Al2CuMg, 99.999% Cu, and 99.998% Al were not changed significantly in 0.1 M Na2SO4 + 0.005 M NaCl. Moreover, the OCP of AA 2024-T3, Al2Cu, Al2CuMg, and 99.999% Cu were not changed significantly in 0.1 M Na2SO4 + 0.005 M NaCl with the addition of 0.01 M soluble chromate at pH 6 and pH 8. The mass transport limiting current density for O2 reducti...

Corrosion of magnesium alloys: the role of alloying

Abstract The demand for light-weighting in transport and consumer electronics has seen rapid growth in the commercial usage of magnesium (Mg). The major use of Mg is now in cast Mg products, as opposed to the use of Mg as an alloying element in other alloy systems and there is an emerging market of wrought Mg products and biomedical Mg components – such that the past two decades have seen a significant number of new Mg-alloys reported. None-the-less, the corrosion of Mg alloys continues to be a challenge facing engineers seeking weight reductions by deployment of Mg. Herein, authors review the influence of alloying on the corrosion of Mg-alloys, with particular emphasis on the underlying electrochemical kinetics that dictate the ultimate corrosion rate. Such a review focusing on the chemistry–corrosion link, both in depth and in a holistic approach, is lacking. As such the authors do not describe aspects such as high-temperature oxidation or cracking, but focus on delivering the state-of-the-art with regards to alloying influences on corrosion kinetics. It has been demonstrated that Mg itself will not be thermodynamically passive in environments of pH<11, regardless of the extent and type of alloying and hence corrosion kinetics require unique attention. Authors consolidate the presentation to include essentially all commercially available alloys and in excess of 350 custom alloys with wide variations in composition; in addition to reviewing the range of intermetallic compounds and impurities that form in such alloys systems. An update is also given regarding mechanistic advances and the role of grain size on corrosion of Mg. A wider understanding of the role of chemical effects upon corrosion of Mg is both timely and serves to highlight metallurgical approaches towards kinetically retarding the corrosion problem. The latter is of key relevance to next generation lightweight alloys and rational design of wrought Mg and bio-Mg.

Crevice Corrosion Stabilization and Repassivation Behavior of Alloy 625 and Alloy 22

Abstract The crevice corrosion resistance of Alloy 625 (UNS N06625) and Alloy 22 (UNS N06022) was compared based on critical potentials for stabilization and repassivation as well as crevice stabilization rates derived from critical potential data. The effects of temperature and anion composition in 5 M lithium chloride (LiCl) brine-type electrolytes were examined. Repassivation potentials measured at 95°C were similar for both alloys and were not greatly influenced by bulk electrolyte composition, pH, or accumulated anodic charge associated with crevice attack. However, a decrease in temperature increased repassivation potentials to a greater extent for Alloy 22 in comparison to Alloy 625. Crevice corrosion stabilization properties of Alloy 625 and Alloy 22 were influenced by temperature and electrolyte composition but not bulk solution pH ranging from 2.75 to 7.75. Crevice stabilization occurred at more active critical potentials in 5 M LiCl electrolytes with a molar ratio of chloride ions to total oxya...

Cerium Dibutylphosphate as a Corrosion Inhibitor for AA2024-T3 Aluminum Alloys

The suitability of cerium dibutylphosphate [Ce(dbp) 3 ] as a corrosion inhibitor for AA2024-T3 aluminum alloy in sodium chloride aqueous solutions has been investigated. Weight loss tests combined with electrochemical assessment have been used to evaluate the degree of protection and determine the inhibition characteristics of this compound. It was found that Ce(dbp) 3 offers superior protection when compared to CeCl 3 , with no discernable corrosion products, significant pitting, or evidence of replated copper on the surface. Cathodic polarization indicated inhibited oxygen reduction reaction kinetics consistent with reduced Cu replating. Corrosion protection seems to be enhanced at higher Cl - concentrations, suggesting the inhibiting film is more readily deposited when some corrosion takes place. X-ray photoelectron spectroscopy analysis of the surface confirmed the presence of both Ce(III) and Ce(IV). Focused ion beam secondary-ion mass spectroscopy (SIMS) analysis clearly indicated the presence of a 500 nm thick cerium-containing layer on the surface of the alloy after 10 days immersion in the inhibited solution. A strong phosphorus signal was also detected in the SIMS experiment. Toxicity testing using the EC-50 test suggested that cerium dibutylphosphate is able to fulfil the basic requirements for consideration as an environmentally friendly corrosion inhibitor.

Threshold chloride concentrations of selected corrosion-resistant rebar materials compared to carbon steel

Abstract The threshold chloride concentration for solid Type 316LN (UNS S31653) stainless steel, Type 316L (UNS S31603) stainless steel clad, 2101 (UNS S32101), Fe-9%Cr, and carbon steel rebar (ordinary ASTM A 615M) was investigated using potentiodynamic and potentiostatic current monitoring techniques in saturated calcium hydroxide (Ca[OH]2) + sodium chloride (NaCl) solutions. There is general consensus in this study and the literature that the chloride threshold for carbon steel is less than a chloride to hydroxl (Cl−/OH−) molar ratio of 1. Solid Type 316LN stainless steel rebar was found to have a much higher chloride threshold (i.e., threshold Cl−/OH− ratio > 20) than carbon steel (0.25 < Cl−/OH−< 0.34). Type 316L stainless steel clad rebar possessed a chloride threshold expressed as a Cl−/OH− ratio of 4.9 when cladding was intact. However, surface preparation, test method, duration of period exposed to a passivating condition prior to the introduction of chloride, and the presence of cladding defects...