R.G. Buchheit

Electrochemical Characteristics of Intermetallic Phases in Aluminum Alloys

This paper presents a survey of corrosion potentials, pitting potentials, and electrochemical characteristics for intermetallic particles commonly present in high-strength aluminum-based alloys. Results from relevant pure metals and solid solutions are also presented. It is seen that corrosion potentials and pitting potentials vary over a wide range for various intermetallics. Elaboration of the results reveals that the electrochemical behavior of intermetallics is more detailed than the simple noble or active classification based upon corrosion potential or estimated from the intermetallic composition. Intermetallics capable of sustaining the largest cathodic current densities are not necessarily those with the most noble Ecorr, similarly those with the least noble Ecorr will not necessarily sustain the largest anodic currents. The data herein was collected via the use of a microcapillary electrochemical cell facilitating electrode investigations upon intermetallic particles in the micrometer-squared range. This survey may be used as a tool for clarification of localized corrosion phenomena in Al alloys.

Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings

Abstract This review covers the aqueous chemistry of chromium and the origins of chromium toxicity. Evidence from older and more recent studies is presented showing that inhibition of Al corrosion is derived from both inhibition of oxygen reduction and inhibition of metal dissolution reactions mainly due to a delay in the onset of pitting. Inhibition of corrosion by chromates appears to be closely linked to their ability to irreversibly adsorb on to metal and oxide surfaces. With respect to chromate conversion coatings (CCCs), the evolution of modern coating formulations is presented with a focus on key advances that led to process simplification and improved coating performance. The current understanding of CCC formation, protection, and aging is presented. In the last section of the paper, processing and properties of selected chromate-free conversion coating chemistries is discussed. This discussion focuses on coatings that offer particularly attractive attributes such as self-healing and excellent adh...

Pulsed electron-beam technology for surface modification of metallic materials

This article concerns the foundations of a new technology for surface modification of metallic materials based on the use of original sources of low-energy, high-current electron beams. The sources contain an electron gun with an explosive-emission cathode and a plasma anode, placed in a guide magnetic field. The acceleration gap and the transportation channel are prefilled with plasma with the use of spark plasma sources or a low-pressure reflected discharge. The electron-beam sources produce electron beams with the parameters as follows: electron energy 10–40 keV; pulse duration 0.5–5 μs; energy density 0.5–40 J/cm2, and beam cross-section area 10–50 cm2. They are simple and reliable in operation. Investigations performed with a variety of constructional and tool materials (steels, aluminum and titanium alloys, hard alloys) have shown that the most pronounced changes of the structure-phase state occur in the near-surface layers quenched from the liquid state, where the crystallization front velocity rea...

Investigation and Discussion of Characteristics for Intermetallic Phases Common to Aluminum Alloys as a Function of Solution pH

This paper presents results for corrosion potentials, pitting potentials, and electrochemical characteristics for intermetallic particles commonly present in high strength aluminum-based alloys, for tests conducted in a 0.1 M NaCl solution of varying pH via the use of a microcapillary electrochemical cell. The intermetallics investigated were Mg2Si, MgZn2 ,A l7Cu2Fe, Al2Cu, Al2CuMg, and Al3Fe. Elaboration of the results reveals that the electrochemical behavior of such compounds varies markedly with pH, with attendant ramifications for localized corrosion and protection in Al alloys. Examples of this are shown for AA7075-T651, where it is shown that the localized corrosion morphology is drastically different upon the bulk alloy depending on the pH of the test environment. A stochastic pitting is observed at an acid pH, near-neutral conditions result in a deterministic-type pitting, and a general corrosion is observed at an alkaline pH.

Active Corrosion Protection in Ce-Modified Hydrotalcite Conversion Coatings

Abstract Chromate conversion coatings (CCC) are noteworthy because of their ability to self-heal if mechanical or chemical damage occurs, provided the damage is not too severe. Self-healing, or act...

The Electrochemical Characteristics of Bulk‐Synthesized Al2CuMg

An S-phase intermetallic compound (Al{sub 2}CuMg) was synthesized in bulk form to characterize its electrochemical behavior by conventional techniques. Large S-phase crystals, suitable for analysis, were formed in an Al-Cu-Mg ingot. The structure of Al{sub 2}CuMg was verified by electron-backscatter Kikuchi patterns and X-ray diffraction. The composition of Al{sub 2}CuMg was verified by electron-probe microchemical analysis. Open-circuit potential and potentiodynamic polarization measurements were conducted in aerated and deaerated 0.5 M NaCl solutions. Bulk S-phase electrodes exhibited steady-state open-circuit potentials ranging from {minus}0.920 to {minus}0.930 V{sub SCE}, which did not vary significantly with changes in solution aeration. In aerated solutions, the phase dissolved vigorously at modest anodic polarizations, and no passive response was detected. Limiting cathodic current densities on the order of 1.0 mA/cm{sup 2} were measured for small cathodic polarizations, probably due to the development of a porous, catalytic surface. Overall, the S phase supports rapid anodic and cathodic reaction kinetics, which contribute to the instability of the compound and subsequent liberation of Cu. These results implicate the presence of S-phase intermetallic particles as a key contributor to the poor corrosion resistance of Al-Cu-Mg alloys.

Electrochemical Properties of Intermetallic Phases and Common Impurity Elements in Magnesium Alloys

The electrochemical properties of the key intermetallic particles that form in commercial Mg alloys are presented. Results were collected via microcapillary electrochemical testing upon bulk intermetallic analogs in dilute chloride solution. The intermetallics investigated were Mg17Al12 ,M g 2Al3 ,M g 2Ca, Mg12Ce, Mg12La, Mg3Nd, Mg2Si, Mg24Y5, and MgZn2. It was found that the intermetallic phases, with the exception of Mg2Ca, were more noble than Mg, supporting increased levels of cathodic kinetics; however, the variation in electrochemical response between intermetallics was large in terms of corrosion potential, presence of a passive window, and currents sustained over a range of potentials.

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 .

Localized Corrosion Behavior of Alloy 2090—The Role of Microstructural Heterogeneity

Abstract The pitting and intergranular corrosion behavior of alloy AA 2090 (Al-2Li-3Cu, UNS A92090) in a 3.5 wt% NaCl solution was investigated. Techniques used included potentiodynamic polarization, galvanic couples, and pH measurements in simulated crevices. Polarization scans were performed on under-aged and peak-aged material to obtain the standard polarization parameters. Corroded specimens were examined with optical microscopy, scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDAX) to distinguish various local corrosion morphologies. Ingots were cast to approximate the subgrain boundary, T1 (Al2CuLi) phase, and Al-Cu-Fe constituent phases inherent in the alloy. These were then galvanically coupled to solution heat treated (SHT) 2090 to identify their role in local corrosion processes. Simulated crevices were produced by inserting pH micro-electrodes into crevices machined in 2090 blocks to measure pH versus time response in occluded environments. Based on the experiments...

Inhibition of AA2024-T3 on a Phase-by-Phase Basis Using an Environmentally Benign Inhibitor, Cerium Dibutyl Phosphate

the oxygen reduction reaction. 8,9,18 Therefore, in this work, we investigated the effectiveness of Cedbp3 as both an anodic and cathodic inhibitor on intermetallic compounds present as particles in 2024-T3, with the aim of characterizing electrochemically the mechanism of inhibitor action, while adopting a novel approach to achieve this. Ultimately, the inhibitor must reveal mixed properties to approach the inhibition efficiency of chromates. We suggest that Cedbp3 representing an optimized inhibitor of the Ce-based family, based on chemical considerations, will serve as good environmentally benign corrosion inhibitor for AA2024-T3.