Correlation Studies between Outdoor Exposure and Accelerated Laboratory Corrosion Tests for Galvanic and Non-Galvanic Ceramic-Aluminum Couples

2008

Galvanic corrosion of an Al-based metal-matrix composite ͑MMC͒ reinforced with 40 wt % Si particulates was examined by considering the semiconductor/electrolyte junction properties of the Si reinforcements. Scanning capacitance microscopy and photoelectrochemical experiments indicated that the Si reinforcements were slightly p-doped. At the open-circuit condition, a wide depletion layer was expected to be present at the slightly p-doped Si surface. The blocking characteristics of the depletion layer for electrochemical reactions would thus limit the galvanic interactions between the Si reinforcements and the Al matrix. The photoelectrochemical experiments also suggested that solar irradiation may promote galvanic corrosion because of the enhanced cathodic activity of the Si reinforcements under illuminated conditions. Corrosion was found to initiate from Fe-containing intermetallics, which served as effective cathodes. A chemical decoration method indicated that, while the bulk region of the Si reinforcements was not effective for cathodic reactions, the exposed Si surfaces at the Al/Si interfaces were effective cathodic sites. It was hypothesized that ͑i͒ during the processing of the Al/Si MMC, interdiffusion caused the Si surfaces at the Al/Si interfaces to be highly p-doped with Al, and ͑ii͒ upon propagation of localized corrosion from the sites adjacent to the Fecontaining intermetallics, the highly p-doped Si surfaces were exposed to solution and served as effective cathodic sites to induce galvanic corrosion. A scanning vibrating electrode technique revealed a net cathodic current over the localized corrosion regions, which were many times larger than the intermetallic particle or the Si reinforcement particle. Accordingly, the solution near the localized corrosion regions was alkalinized, as was revealed by the scanning ion-selective electrode technique.

mentioning

confidence: 99%

Galvanic Corrosion and Localized Degradation of Aluminum-Matrix Composites Reinforced with Silicon Particulates

Ding

2008

“…For 6061-T6 Al in an aerated chloride solution, the corrosion potential is coincident with the pitting potential, E pit , but the anodic curve is otherwise similar to that of the deaerated case above the pitting potential. 35 Hence, both the deaerated and aerated anodic 6061-T6 Al curves predict similar galvanic corrosion rates when coupled to the aerated cathodic CFR PMC curve using the mixed potential theory.…”

Section: Resultsmentioning

confidence: 87%

“…Example of skirt length calculation for outdoor exposure.-To suppress i Galv between 6061-T6 Al and the CFR PMC to a specified acceptable value, the corresponding IR loss must be achieved by obtaining the appropriate skirt length, l s , for a given salt loading, m . For example, suppose that i Galv is to be limited to 6 × 10 −6 A/cm 2 , which is the normal corrosion current density, CD, of uncoupled 6061-T6 Al in an aerated 20,000 ppm chloride solution at 30 • C. 35 For an i Galv value of 6 × 10 −6 A/cm 2 , the corresponding value of l s /m is 190 cm 3 /g based on the 3.88 × 10 −4 to 7.58 × 10 −4 g/cm 2 NaCl salt loading (Figure 14). Hence, if it is assumed that the 6061-T6 Al-CFR PMC couple will be exposed to an aggressive environment such as the MCBH, a realistic salt loading must be approximated to obtain a suitable skirt length.…”

Section: The Log L S /M -Log I Galv Relationship (From Humidity-chamb...mentioning

confidence: 99%

Development of Guidelines to Attenuate Galvanic Corrosion between Mechanically-Coupled Aluminum and Carbon-Fiber Reinforced Epoxy Composites Using Insulation Layers

Srinivasan

Nelson

2015

Self Cite

The use of an insulating skirt to attenuate galvanic action between mechanically-coupled 6061-T6 aluminum (Al) and electrically conductive carbon fiber reinforced (CFR) polymer-matrix composites (PMCs) was assessed, analyzed and modeled. The galvaniccorrosion current as a function of time between the 6061-T6 Al and CFR PMCs for a range of insulative skirt lengths, l s , and sodium chloride (NaCl) salt loadings, m , in a humidity chamber at 30 • C and 90% relative humidity (RH) were monitored. Polarization experiments were conducted on 6061-T6 Al and conductive CFR PMC electrodes to study the mechanisms governing galvanic corrosion in solutions ranging from 0 ppm chloride (Cl − ) to 20,000 ppm Cl − concentrations. Based on the electrochemical characteristics of Al and the CFR PMC and the ohmic loss within the galvanic couple, a model was developed that related the galvanic corrosion rate (i Galv ) to the value of l s /m , which is the ratio of the insulating skirt length to salt loading. Experimental data from the polarization experiments as well as data from monitoring the galvanic couples in the humidity-chamber galvanic experiments were used to generate plots of log l s /m vs log i Galv , which can be used in design to determine proper skirt length for various environments (based on salt-loading levels) to limit the galvanic corrosion rate to a specified value. Today, many structures, machines and components are comprised of a material system rather than a single type of material.1 Polymers and composites are also being used with greater frequency. For example, only 3 kilograms of polymers were used in automobiles in the 1980s: 2 whereas, today, more than 100 kilograms of polymers are used in a typical automobile.3 Polymer-matrix composites (PMCs) are also being used with greater frequency in commercial, defense, aerospace, automotive and civil-infrastructure industries. 4,5 The advantages of PMCs, especially lightweight carbon-fiber reinforced (CFR) PMCs, are their higher specific strength and stiffness over those of conventional metal alloys. 1,6,7 Some specific examples of CFR PMC products are aircraft fittings and bicycle frames. [8][9][10] As PMCs are being incorporated into material systems to reduce weight and improve performance, they are often put in intimate contact with metal alloys such as aluminum (Al), 1,11 which can induce crevice and galvanic corrosion at the aluminum interface. 1,12,13 If the PMC is insulative such as glass-fiber reinforced (GFR), only crevice corrosion can be induced at the aluminum interface. If the PMC is conductive such as CFR, then galvanic action will also be induced at the aluminum interface.14-20 Figure 1 shows corrosion of a CFR PMC/6061-T6 Al couple exposed outdoors at Marine Corps Base Hawaii (MCBH) (a severe marine test site) for 12 months. Notice that galvanic corrosion also extends beyond the CFR perimeter under the salt and corrosionproduct deposits.Carbon fibers are electrically conductive, relatively inert, and have more noble corrosion potentials compared to ...

“…The polarization data of Al and C in solutions of various chloride concentrations and pH levels were obtained. The anodic polarization curves for Al 6061 in deaerated 0.5 M Na 2 SO 4 solution of various Cl − concentrations, 31 and pure Al (99.999%) in deaerated 0.05 M Na 2 SO 4 of pH 2 32 were plotted (Figure 10) with the cathodic polarization curve for C fiber in aerated 0.5 M Na 2 SO 4 solution. 33 In the chloride-free solution (Figure 10), lowering the pH can shift the corrosion potential to a more negative value and can move the corrosion current to a higher value.…”

Section: Resultsmentioning

confidence: 99%

Development of a Corrosion Model to Correlate the Atmospheric Corrosion Rate of a Carbon-Fiber Reinforced Aluminum MMC to Weather and Environmental Parameters

Tiwari

2014

A corrosion model was developed to correlate the atmospheric corrosion rates of 50 volume% nickel (Ni)-coated carbon (C) fiber reinforced pure aluminum (Al (99.99%)/C/50f) metal-matrix composites (MMCs) to weather and environmental parameters such as chloride (Cl − ) deposition rate, aerosol pH, and the percentage of time wet (Q4). The model was developed using polarization data of Al and C in solutions of various pH levels and Cl − concentrations. The polarization data indicated that the galvanic corrosion current (i galv ) between Al and C is a function of Cl − concentration and pH. There was a linear relationship between the logarithm of i galv and the logarithm of Cl − concentrations in neutral solutions with an R 2 value close to 0.95. Using the linear relationship, a corrosion model was developed that estimated the atmospheric corrosion rate of Al (99.99%)/50 MMCs as a function of the Cl − deposition rate and Q4 (i.e., [Cl - Compared to monolithic Al alloys, Al/C MMCs offer lower density and superior properties such as high specific strength and stiffness, good wear resistance, and low to zero coefficient of the thermal expansion. [1][2][3][4] Due to their unique properties, Al MMCs have received increased attention as an attractive choice in the automotive, defense, and aerospace industries. For example, Al/C MMCs have been used for antenna waveguide mast in the Hubble Space Telescope, 1,5 electromagnetic interference shielding in electronics 6 structural radiators in space applications, 5,7 and electronic packaging. 6,8,9 Al/C MMC wires are being explored to replace the steel-core Al-clad wires as the strength carrying members in aluminum electrical transmission lines as they offer higher electrical conductivity and low sag characteristics (due to their low CTE) when temperatures rise due to increased power transmission. 10,11 The high electrical conductivity of C, however, makes Al/C MMCs much more susceptible to corrosion due to the galvanic effect between the C fiber and the Al matrix. Galvanic couplings in A 356 Al alloy and graphite MMC (Al 356/Gr) by Kendall and Dull,12 Al 6061 reinforced with 30% C MMC (Al 6061/C/30f) by Dull and Harrigan, 13 Al (LM 13)/C/3f, AS7G0.6/C by Payen et al., 14 and Al 2024/C MMCs by Saxena, Prasad, and Dan, 15 were investigated in NaCl solutions. The presence of crevice corrosion along the region of high fiber densities was recognized as signs of galvanic coupling between the C fiber and Al matrix. 12,13 Payen et al., showed that the C fiber and Al matrix interface served as preferential sites for corrosion using long focal video microscopy. 14 Galvanic corrosion occurred between Al 2024 and graphite particles not only in NaCl but also in HCl, HNO 3 , H 2 SO 4 , HClO 4 acids. 15 Hihara and Latansion, 16 measured the galvanic corrosion current density and the galvanic potential between the graphite fiber and the Al 6061 matrix, using the zero-resistance ammeter (ZRA) in both 0.5 M Na 2 SO 4 and 3.15% NaCl solutions where the galvanic current in NaCl solution was signifi...