L.H. Hihara

The galvanic corrosion of SiC monofilament/ZE41 Mg metal-matrix composite in 0.5 M NaNO3

Abstract The effect of composite constituents and dissolved O 2 on the corrosion behavior of a SiC monofilament/ZE41 Mg metal-matrix composite (MMC) exposed to 0.5 M NaNO 3 was studied. The SiC monofilament (MF) (which consisted of a carbon core, a layer of chemical-vapor deposited SiC, and an outer carbon-rich surface) supported cathodic current densities comparable to that of graphite. The corrosion of pure Mg and a ZE41A Mg alloy was not affected by dissolved O 2 . Corrosion rates of the MMC, however, increased with dissolved O 2 since the MFs were effective O 2 -reduction sites, which enchanced galvanic corrosion.

Localized Corrosion Currents and pH Profile over B4C , SiC, and Al2O3 Reinforced 6092 Aluminum Composites I. In 0.5 M Na 2 S O 4 Solution

Particulate 6092-T6 Al metal matrix composites (MMCs) reinforced with 20 vol % of B 4 C, SiC, and Al 2 O 3 exhibited localized corrosion when immersed in a 0.5 M Na 2 SO 4 solution exposed to air at room temperature. For theMMCs reinforced with B 4 C and SiC, the scanning vibrating electrode technique (SVET) revealed that corrosion initiated at localized anodic regions, which transformed into cathodic regions over time. The scanning ion-selective electrode technique (SIFT) revealed that the localized anodic regions were acidified, and the localized cathodic regions were alkalinized. The observed anodic-cathodic transformation behavior was attributed to aluminum which has amphoteric oxides, the formation of microcrevices by reinforcement particles left in relief, and the possible galvanic action between the 6092-T6 Al matrix and the B 4 C and SiC reinforcement particles. The localized anodic and cathodic regions were many times larger than the individual reinforcement particle size. For the Al 2 O 3 -reinforced MMC, localized corrosion coincided with regions showing some cathodic activity and alkalinity. The extent of corrosion of these three MMCs was found to increase with decreasing reinforcement resistivities p (i.e., ρAl 2 O 3 > ρ S i C > ρ B 4 C ).

Galvanic corrosion between SiC monofilament and magnesium in NaCl, Na2SO4 and NaNO3 solutions for application to metal-matrix composites

Abstract Galvanic corrosion of SiC monofilament (MF) coupled to pure Mg and ZE41A Mg alloy was studied using the potentiodynamic polarization technique in concert with the mixed-potential theory. Corrosion penetration rates of Mg were generally equal to 1 cm year −1 in de-aerated and oxygenated 3.15% NaCl, 0.5 M Na 2 SO 4 and 0.5 M NaNO 3 solutions. Galvanic corrosion penetration rates varied from 0.046 to 7.3 cm year −1 , depending on solution oxygenation and the exposed region of SiC MFs. NaF was very effective in reducing galvanic corrosion rates in de-aerated 0.5 M Na 2 SO 4 and 0.5 M NaNO 3 , modestly effective in oxygenated 0.5 M Na 2 SO 4 and 0.5 M NaNO 3 and ineffective in 3.15% NaCl.

Inhibitive Effect of Seawater on the Corrosion of Particulate-Reinforced Aluminum-Matrix Composites and Monolithic Aluminum Alloy

The corrosion rates of particulate 6092-T6 Al metal-matrix composites (MMCs) reinforced with 20 vol % B 4 C, SiC, and Al 2 O 3 as well as monolithic 6061-T6 Al immersed in American Society for Testing and Materials (ASTM) and real seawaters were lower than those immersed in 3.15 wt % NaCl or 0.5 M Na 2 SO 4 solutions. Scanning electron microscopy revealed that precipitates consisting of agglomerated whiskers formed on the surfaces of the MMCs and the monolithic 6061-T6 Al after prolonged immersion in seawater environments. The precipitates were tentatively identified to be Al-Mg hydrotalcite-like compounds based on energy-dispersive X-ray analyses and X-ray diffraction experiments. The MMCs immersed in ASTM seawater exhibited localized corrosion, however, the scanning ion-selective electrode technique revealed smaller pH fluctuations near the localized corrosion regions compared to the MMCs immersed in NaCl and Na 2 SO 4 solutions. It was postulated that the corrosion of the Al-based materials is inhibited both by the precipitates and by the buffering effect of seawater.

Suppressing galvanic corrosion in graphite/aluminum metal-matrix composites

Abstract It has been found that a 0.1 μm-thick insulation layer having a resistivity of about 10 17 Ω cm will be required to decouple electrically a 1 m long graphite fiber from a 6061-T6 Al matrix to limit the galvanic-corrosion rate to a fraction of the normal corrosion rate of 6061-T6 Al exposed to aerated 3.15 wt% NaCl at 30°C. Galvanic corrosion can also be suppressed by inhibiting the oxygen reduction reaction that occurs on the graphite fibers. Galvanic-corrosion current densities of graphite fiber-6061-T6 Al galvanic couples were reduced by 10–100 times by using Zn 2+ (10 ppm) as a cathodic inhibitor in aerated 3.15 wt% NaCl at 30°C.

Electrochemical Examinations on the Corrosion Behavior of Boron Carbide Reinforced Aluminum-Matrix Composites

Galvanic corrosion of a 6092-T6 Al metal-matrix composite (MMC) reinforced with boron carbide (B 4 C) particulates was examined through electrochemical characterization of the MMC as well as a monolithic B 4 C. The results were interpreted based on a recently proposed electronic model for B 4 C that emphasizes a conventional non-degenerate p-type semiconductor band structure with high density of gap states. The electrochemical behavior of B 4 C in the dark that was characteristic of non-active metal electrodes was attributed to the very high density of gap states of B 4 C; while the phenomenon of the enhanced cathodic currents of B 4 C under illumination was attributed to the presence of a depletion layer at the B 4 C/electrolyte interface, characteristic of non-degenerate p-type semiconductor/electrolyte interface. The results suggested that galvanic corrosion of the MMC in the dark was limited by slow oxygen reduction kinetics at the B 4 C reinforcements and that illumination promotes galvanic corrosion of the MMC as a result of photo-enhanced cathodic activity of the B 4 C reinforcements. Scanning electron microscopy revealed that corrosion of the MMC initiated from carbon particles that are likely introduced into the matrix during the processing of the MMC. A combination of scanning vibration electrode technique and scanning ion-selective electrode technique revealed that localized anodic and cathodic sites co-exist during the corrosion of the MMC in an air-exposed 3.15 wt % NaCl solution. The localized anodic and cathodic sites have diameters of approximately 200 μm.

Initiation of corrosion in metal substrates coated with plasma-deposited hydrogenated amorphous silicon alloy ceramic thin films

Amorphous hydrogenated silicon, silicon nitride, silicon–carbon and boron-doped silicon–carbon films on molybdenum and aluminum substrates were evaluated as corrosion barriers. Specimens were anodically polarized in NaCl and Na2SO4 solutions and also treated with NaCl and Na2SO4 during exposure in an environmental chamber at 85% RH at 30°C. The silicon nitride and silicon–carbon films were inert and rendered substantial protection while the silicon films showed signs of extensive deterioration. Initiation sites for localized corrosion in the silicon nitride and silicon–carbon films were found not only at film pinholes caused by spallation, but also at microcracks and in regions where visible defects could not be detected using scanning electron microscopy.

Sol-Gel Route for the Development of Smart Green Conversion Coatings for Corrosion Protection of Metal Alloys

The development of coatings through sustainable routes is needed for the modern industrial sector. The process parameters and chemical ingredients should be wisely chosen to meet the demand of environmental friendly synthetic technologies. In this chapter, the authors have demonstrated the adaptability of using the sol-gel route for the development of eco-friendly (green) silicone coating compositions containing titanium nanoparticles. The developed coating composition was characterized with Fourier Transform Infrared (FTIR), Raman, Nuclear Magnetic Resonance (NMR), and X-ray Photoelectron spectroscopies. The thermal stability of the material was determined using thermogravimetric analysis (TGA). The surface morphology of the coatings was studied with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The corrosion resistance and impervious barrier properties of the Green Conversion Coating (GCC) were evaluated by a series of laboratory and outdoor corrosive atmosphere exposure experiments.

Galvanic Corrosion in Metal-Matrix Composites Containing Semiconducting Constituents

A modified photochemical-diode (PD) model, designated as a photochemical-corrosion-diode (PCD) model, was proposed for interpreting galvanic corrosion in metal-matrix composites (MMCs) containing semiconducting constituents. The characteristics of the PCD model were introduced through direct comparison with that of the PD model. The PCD model was then used to interpret galvanic corrosion in SiC-reinforced Al MMCs. The PCD model and the photoelectrochemical experiments on both Al/SiC MMCs and monolithic SiC predicted that solar irradiation may affect the corrosion of the Al/SiC MMCs, corroborating well with field experiments.

Effect of Embedded Titanium-Containing Particles on the Corrosion of Particulate Alumina Reinforced Aluminum-Matrix Composite

The microstructure and corrosion initiation sites of a particulate alumina reinforced 6092 Al metal-matrix composite (MMC) were examined with scanning electron microscopy and energy-dispersive X-ray analysis (EDXA). The MMC was embedded with a variety of Ti-containing microparticles that were likely introduced into the Al matrix as impurities of the alumina reinforcements. When the MMC was immersed in either 0.5 M Na 2 SO 4 or 3.15 wt % NaCl solution in open-circuit conditions, microcrevices formed around the Ti-containing particles as well as the Fe-Si-Al intermetallic particles. With the semiquantitative EDXA technique, the Ti-containing particles that induced the microcrevices can be roughly divided into three categories: (i) titanium suboxides with compositions close to that of Ti 6 O, Ti 3 O, Ti 2 O, and TiO, (ii) Ti-Zr-Al oxides, and (iii) TiO 2 . Although little is known about the electrical properties of the titanium suboxides and the Ti-Zr-Al oxides, the results suggested that these compounds were either electrically conducting or semiconducting. Microcrevices also formed around some particles with an oval structure. The core region of the oval structure appeared to be a Ti-Si-Al phase, while the shell region of the oval structure appeared to be an Fe-Si-Al phase.