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The localized corrosion of AA2060-T8 and the role of two major types of intermetallic particles (IMPs), heterogeneous Al-Cu-Fe-Mn and S-phase (Al2CuMg), were studied using scanning Kelvin probe force microscopy (SKPFM) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Topography and Volta potential maps were collected to determine the corrosion morphology around, and the relative nobility of, the same IMPs before and after sequential exposure to NaCl solution. Chemical compositions and the correlation to SKPFM maps of the representative IMPs and corroded structure were identified using SEM-EDS. These results provide insight into the electrochemical behavior and corrosion mechanism of the Al-Cu-Fe-Mn and S-phase IMPs and associated effects on localized corrosion of AA2060-T8 compared to the legacy AA2024-T3.

The corrosion of Al alloys is rich in complexity because many different forms of corrosion can occur, the details of corrosion depend strongly on the alloy microstructure, and most Al alloys are heterogeneous with a microstructure that varies with thermomechanical processing. This paper reviews several studies on the corrosion behavior of Al alloys, focusing on how the composition and microstructure influence the forms and rates of corrosion, including pitting corrosion, intergranular corrosion, stress corrosion cracking, and surface layer attack. Thin film pitting, foil penetration, scanning Kelvin probe force microscopy, video imaging, and transmission electron microscopy are a few of the techniques that are described.

The localized corrosion susceptibility of a clean grade of Type 304 (UNS S30400) stainless steel (SS304), which was processed with the intent of having fewer and smaller inclusions, was compared with regular grade SS304. Transmmission electron microscopy (TEM) analysis revealed that inclusions were composed of multiple phases that were either multi-element oxide or manganese sulfide (MnS). Oxide inclusions, which had only multi-oxide phases, were the dominant inclusion type in clean SS304, whereas MnS-containing inclusions, having both oxide and MnS phases together, were more frequent in regular SS304. The average size of inclusions was larger in regular SS304. Cyclic polarization tests showed that pitting potentials of clean SS304 were higher than regular grade SS304. Under exposure to strong oxidizing acid with Cl−, MnS-containing inclusions initiated pitting, whereas oxide inclusions did not, indicating that pit initiation is more frequent in regular SS304 because it has more MnS inclusions. The fewer pit initiation sites in clean SS304 was the reason for its higher pitting corrosion resistance.

The active aluminum-rich primer (AlRP) was invented and developed at NAVAIR to sacrificially protect aluminum alloys and steels from corrosion. The Al pigments (Al-Zn-In) in AlRP were fabricated from a sacrificial anode alloy, which has a lower open-circuit potential than common aluminum alloys. However, initial results indicated that AlRP undergoes severe self-corrosion. Therefore, the Al pigments were pretreated in a trivalent chromium passivation (TCP) bath to reduce the self-corrosion rate. The objectives of this study are to understand the anti-corrosion properties of AlRP on aluminum alloy 2024-T3 substrate and to evaluate the effect of TCP treatment on the Al pigment particles. The polarization curves of AA2024-T3 and active aluminum alloy (Al-Zn-In) show that TCP-treated active aluminum alloy has lower corrosion potential than AA2024-T3 and thus would sacrificially protect it. The AlRPs were exposed in an accelerated exposure test, GMW14872. Exposed samples were then examined using scanning electron microscopy and energy dispersive x-ray spectroscopy to understand the coating degradation process. The AlRP with TCP-treated pigments outperforms a similar coating with untreated pigments.

Hydrogen evolution (HE) on anodically polarized Mg, commonly referred to as negative difference effect, was studied by galvanodynamic measurements coupled with real-time gravimetric H2 volume collection, the scanning vibrating electrode technique, and potentiodynamic polarization experiments. High purity Mg (99.96% Mg) electrodes were studied in chloride-free 0.1 M citric acid solution buffered at pH with the aim of determining the source of anodic HE and the role of the corrosion film on the process. In such conditions of pH, the typical dark corrosion product exhibited in neutral and alkaline solutions was not found, but the HE rate still increased with increasing potential. Evidence that HE on dissolving high purity Mg is associated with the regions dominated by the anodic dissolution reaction is provided. The role of noble impurity enrichment on the electrode surface during anodic polarization and the effect of Fe re-deposition are also discussed.

A very simple electrochemical cell for corrosion experiments has been developed using a syringe as the cell with the syringe plunger replaced by reference and counter electrodes. Because the exposed area is defined by a hanging droplet, no masking is required and thus no crevice corrosion forms. This cell therefore enables measurements of pitting potentials of metals that are susceptible to crevice corrosion. The method was validated by cyclic potentiodynamic polarization experiments on Type 304H stainless steel. Crevice or interfacial corrosion was not evident in multiple replicate specimens tested with the syringe cell method.

The trivalent chrome process (TCP) is one of the leading non-chromate conversion coatings and has been shown to provide excellent corrosion protection for Al alloy substrates. The goal of this work was to assess and quantify the self-healing or active corrosion inhibition properties of TCP on AA2024-T3 (UNS A92024). Samples exposed in the artificial scratch cell were characterized using electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS) for assessment of various aspects of active corrosion inhibition. Analysis of the electrolyte exposed to the TCP surface was conducted by inductively coupled plasma-optical emission spectroscopy (ICP-OES). TCP treatment was found to greatly improve the corrosion resistance of AA2024-T3 in corrosive environments. After exposure in dilute Harrison's solution, chromium species were detected in the exposed electrolyte, indicating that chromium is released from the TCP coating. Chromium was found on the surface of the uncoated sheets in the artificial scratch cell, which provided evidence that chromium species can be transported from a TCP coating to a nearby uncoated area as in a scratch. EIS data showed the polarization resistance of the uncoated surface exposed near a TCP surface in the artificial scratch cell was twice as much as uncoated controls, proving that TCP coating can provide active corrosion inhibition to a nearby uncoated Al alloy surface.

The stress corrosion cracking behavior of sensitized aluminum alloy 5083 (AA5083) was studied by slow strain rate testing in 3.5 wt% NaCl solution with and without the addition of K2CrO4, Na2SiO3, or NaVO3 as corrosion inhibitors. Sensitized AA5083 was very susceptible to intergranular stress corrosion cracking (IGSCC) at the open circuit potential (OCP) in the solution with no inhibitor. However, it became immune to IGSCC in the orientation tested when the added inhibitors shifted the OCP to a potential below the breakdown potential (Eb) of the β phase. Chromate and vanadate inhibited IGSCC in this fashion, whereas silicate did not. Sensitized AA5083 samples exhibited severe IGSCC when polarized above the Eb of the β phase during testing, even in the presence of chromate. When the samples were polarized below the Eb of the β phase, ductility was fully recovered with no evidence of IGSCC. A high rate of β-phase dissolution, which is essential for IGSCC, can be prevented by shifting the OCP below the Eb of the β phase.

The evolution of precipitates on the top surface of a friction stir welded AA2219-T8 joint was characterized using scanning electron microscopy and transmission electron microscopy. The pitting corrosion of different regions in the joint was investigated by a microcell method and the salt spray test. The pitting corrosion was mainly dependent on the nature of precipitates in each region of the joint. The heat affected zone had the worst susceptibility to pitting corrosion as a result of the presence of coarse θ′ phase, the existence of precipitate-free zones, and the decrease in dissolved Cu in the matrix. However, the thermomechanically affected zone had a slightly improved pitting resistance because θ′ phase redissolved into solid solution. In the shoulder affected zone (SAZ), θ′ and θ phases were completely and partially redissolved, respectively, and the content of dissolved Cu in the matrix increased. As a result, the corrosion resistance significantly increased in the SAZ.

A test assembly consisting of a coated and scribed aluminum alloy 7075-T6 panel galvanically connected to uncoated through-hole noble fasteners has recently been utilized to quantify corrosion attack during corrosion testing in laboratory chambers. In this work, the corrosion morphology and extent of attack of Al alloy panels with five different surface pretreatments prior to painting and scribing were compared after exposure to ASTM B117 with galvanic connection to uncoated stainless steel fasteners. The nature of the attack for samples with different surface pretreatment samples was found to exhibit two different trends: they either penetrated deeply into the substrate at the scribes or spread out under coatings. The galvanic currents between the coated Al alloy panel and the bare 316 stainless steel fasteners were monitored during 21 days exposure in an ASTM B117 chamber. The current trends measured for different surface pretreatment panels were consistent with optical profilometry (OP) results. However, the galvanic current and OP techniques both underestimated the extent of corrosion attack because of local H2 evolution and undercut attack, respectively. The results from both techniques were modified to compensate for these deficiencies. Acceleration factors associated with the galvanic interaction for the different surface pretreatment panels were determined.