The influences of adding B4C particles on corrosion behavior of Al-2wt.% Cu alloy was studied in 3.5 wt.% NaCl solution at room temperature using linear and cyclic polarization, immersion test and Electrochemical Impedance Spectroscopy (EIS).[SM2] Nano-composites reinforced with 2, 4 and 6 wt. % B4C were produced through mechanical milling and tested to explore the B4C contents effects on the corrosion properties. [SM3]Influences of the grain size were also studied comparing the coarse-grained and milled Al matrix. Results revealed that the corrosion resistance of Al matrix decreases by reducing the particle size. Sample with 2wt.% B4C showed best corrosion resistance amongst all.
Key words: Mechanical milling, Nano-composite, Al, B4C, Corrosion
Metal Matrix Composites were remained the focus of attentions in aerospace, automotive and military industries in recent years. These materials offer several advantages including the high strength to weight ratio, excellent wear resistance and high stiffness compared to the original alloys. The commonly used reinforcing materials are; silicon carbide, aluminum oxide and boron carbide. Due to density differences between the reinforcements and the matrix materials, segregation has been found to be a major problem in producing metal matrix composites. Ball milling is considered to be an important technique for producing nano-crystalline composites. Growing interest for this technique is due to preparing materials with unique chemical, physical and mechanical properties. Ball milling process makes uniform distribution of reinforcement particles in the matrix, preventing the segregation which is commonly found in composites fabricated through other methods [1-4].
Effects of B4C particles as reinforcement materials on mechanical properties of aluminum base alloys are existed in the literatures, but studies on corrosion behavior for these composites are rarely reported. Corrosion behavior is a key parameter for assessing the applications of composites in marine environments. All in all, incorporation of the reinforcements into Al alloys increases the corrosion rate of composites in comparison with matrix. Primary corrosion initiation sites in MMCs[SM4] are dependent on electrical conductivity of reinforcement material, reinforcement volume fraction, intermetallic phases and corrosive environment. Grain size has also a major effect on corrosion behavior of the composites [5-9].
Present research aims at studying the corrosion properties of Al-B4C composites. The influence of different B4C contents on corrosion behavior of Al matrix composites was investigated. Coarse-grained Al matrix was also used to explore the effect of grain size on corrosion resistance.
Al–2wt.% Cu and the nano-sized B4C particles were respectively used as matrix and reinforcements in fabricating the specimens. Besides a plain matrix sample, others were synthesized through mechanical alloying of the powder mixtures with 2, 4, and 6 wt.% of B4C. Ball milling was done by a planetary mill, equipped with two tempered steel vials containing Chrome steel balls (?=20mm). The rotational speed and the ball to powder weight ratio were set at 300 rpm and 10:1, respectively. Milling process was performed at room temperature under argon gas (99.999%) atmosphere protection for 20h[SM5] to achieve steady state condition. Mechanically milled powders were then cold pressed and hot extruded with an extrusion ratio of 10:1 at 550a-¦C. Reference Al alloy sample was prepared from unmilled aluminum powder using similar pressing and extrusion processes.
Electrochemical measurements including linear polarization, cyclic polarization, weight loss and electrochemical impedance spectroscopy tests were applied to 3.5wt.% NaCl solution at room temperature. Three electrodes system, including a working electrode, a platinum counter electrode and a silver-silver chloride electrode (Ag/AgCl) as [SM6]reference electrode were used. The exposed area of samples was polished to 1200 emery paper.
Tafel tests were performed at a scan rate of 1 mV/s, from -2000mV to 500mV using a 273A Princeton Applied Research EG&G model potentiostat/Galvanostat. Cyclic polarization measurements were carried out under conditions similar to Tafel test. After reaching to the 500mV point, scan direction was reversed. In order to find out the exact protection potential, scan rate of 0.5 mV/s was applied in reverse direction.
Disc shape specimens (10mm in diameter and 3mm thick) were immersed in 3.5wt.% NaCl solution in atmosphere for 1, 3, 7, 14 and 28 days. Corroded samples were cleaned according to G1 standard, [SM7]dried and weighed before and after the experiments using a balance (H- Z- K 210 model) with an accuracy of 0.00001 g. The mass losses for samples were finally measured by considering their total surface area.
Phase characterization of specimens before and after the immersion were carried out through [SM8]X-ray diffraction (XRD) [SM9]technique on a Phillips X‘Pert Pro diffractometer using monochromatic Cu-K? radiation. Morphology and chemical analysis of samples were also characterized using scanning electron microscopy[SM10] (SEM), SU8040model, equipped with an energy dispersive spectrometer [SM11](EDS).
Electrochemical impedance spectroscopy[SM12] (EIS) measurements were done with a frequency ranging from 10 mHz[SM13] to 100 MHz. The results were analyzed by means of Zview2 software.
Result and Discussion- The milling part
Fig 1 shows the morphology of Al/Cu alloy and the Al/Cu-4wt.% B4C composite after 20 h[SM14] of mechanical milling. By increasing [SM15]milling time, the particle size decreases besides narrower size distributions.[SM16] The nearly equiaxed crystal morphology of particles suggests that, the 20 h[SM17] of milling time was sufficient to reach desired steady-state condition. Results in table 1 demonstrate[SM18] that by increasing the B4C contents, the average particle size decreases.
Table 1 shows the influence of B4C content on the crystallite size and the lattice strain of aluminum matrix, according to Williamson–Hall method. As expected, the mechanical milling induced severe plastic deformation, leading to the formation of nano-crystalline metal matrix.
The crystallite sizes varied with B4[SM19]C contents, showing the effect of hard particles on grain refining performance of metal matrix[SM20]. It is known that the milling stages introduce plastic deformation of ductile matrix, micro-welding, and the fracture of deformed particles in metal matrix [10, 11]. As compared to mechanical milling of soft powders, the presence of hard particles causes an increase in local deformation of matrix around reinforcement particles, which indeed would enhance the work-hardening rate of metal matrix. Additionally, fracture toughness of composite powders is lower than that of the matrix material . On the other hand, an increase in the content of particles results in more frequent interactions between the dislocations and the hard particles , which accelerating the onset of mechanical-milling stage, and contributing to grain-refinement process .[SM21]
Microstructural examination of as-cast composites revealed that the B4C particles were not distributed uniformly in the matrix and the regional clusters of particles existed. Since the wetting by molten matrix was poor, a uniform distribution of particles could not be observed in composites fabricated by stir casting. In addition, other factors like stirring speed, pouring condition, solidification rate, etc. [SM22]have also had a noticeable influence on particles distribution. In extruded samples, a more even distribution of B4[SM23]C can be observed. Fig. 2 shows the back-scattered electron SEM micrographs of extruded composites used in this study. A uniform distribution of ceramic reinforcements is evident in both composites. In addition, there are no traces of voids in the microstructure which in turn suggests that there was full-densification of composite upon extrusion.[SM24]
Result and Discussion- The corrosion part
Potentiodynamic Polarization Tests
The Potentiodynamic Polarization behaviors of different samples in 3.5 wt% NaCl solutions after 1 hour of testing are given in Fig. 3. Their Ecorr, icorr and ipassive values (obtained from Tafel-type fit) are summarized in Table2[SM25].
Data shows that Al-cast has a lower corrosion rate than Al-milled. As the milled alloy has finer grains, it was expected to be less corrosion resistant because of having more grain boundaries, means higher susceptibility to electrochemical reactions and hence to corrosion. [SM26]It can also be seen that the characteristics of polarization curves for B4C composite samples are quite similar to base the alloy[SM27], indicating that the reactions are similar for both.
According to table 2, adding 2wt.% B4C to the base material lowered the corrosion rate slightly, because the ceramic particles may to some extent hindered electrochemical dissolution physically.
On the other hand, adding more B4C particle to the composite increases the corrosion rate. In any Al alloy-B4C composites, forming intermetallic compounds plays an important role in any chemical and electrochemical reactions that take place on composite surface in a corrosive environment. Fig.[SM28] 4 shows the X-ray diffraction pattern for Al 6wt.% B4C composite. It can be seen that other than Al matrix, there would be considerable amounts of Al3BC species which were produced when the Al reacted with B4C particles. As Al is more anodic with respect to intermetallic, having more of B4C in matrix dominates the effect of physical blocking of electrochemical reactions for ceramic particles in the solution and corrosion rate increases. Therefore [SM29]other than general corrosion of the matrix, there will be galvanic corrosion between the matrix and intermetallic resulting localized corrosion (pitting) on composite surface.
In Al 6wt.% B4C sample, the corrosion rate decreased. This can be explained through passivation point of view as shown in polarization curves in which, the passive current density increases by increasing the B4C content. This may be caused by the formation of more porous and unstable passive layers produced by higher intermetallic particles and also leading to more susceptibility to localized corrosion.[SM30]
Weight Loss Measurements
Figure 5 represents the weight losses for different samples at different immersion times. Diagram demonstrates that the Al cast has the lowest weight loss, therefore [SM31]the lowest corrosion rate of all samples. B4C composites show higher corrosion rates than Al-milled suggesting that adding B4C to samples increases the corrosion rate.[SM32]
As mentioned above, adding B4C to the alloy produces Al3BC intermetallic during corrosion.SEM micrographs of the Al 6wt.% B4C before and after the immersion for 28 days [SM33]are shown in Fig[SM34] 6. Al matrix and Al3BC intermetallic are pointed out in Fig. [SM35]6. EDX analysis results of the intermetallic phase from Figure 4-b is also demonstrated in Fig.[SM36] 7. It reveals that, considerable amounts of the compound exist in the matrix [SM37]which agrees with the XRD results discussed before.
Finally, it is observed that the results from immersion and polarization tests are in agreement with each other. It is indicating that besides a general corrosion, there is a galvanic corrosion between the matrix and the particles leading to localized corrosion.
Cyclic Polarization Studies:
Characteristic potential values such as:[SM38] pitting potential (Epit), corrosion potential (Ecor), and re-passivation potential (Erp) were determined through cyclic polarization studies. As it is observed in Fig[SM39] 8, the nature of potentiodynamic polarization curves in the 3.5% NaCl solution reveals typical characteristics of the material undergoing spontaneous passivation. Reverse scan shows a hysteresis cycle, showing the characteristics of pitting. After reaching to a maximum level, the current begins to decay without any oscillation. Following a linear current-potential relationship is suggesting that an ohmic controlled process was taking place [12-15].
Additional electrochemical parameters given in the table [SM40]are: ??›?Epit=Epit-Ecorr, ??›?Erp=Epit-Erp. ??›?Epit is a measure of the width of passive region on polarization curve, indicating the susceptibility to pitting. ??›?Erp is used to assess the repassivation behaviour of propagating pits and hence, the ease with which locally active sites can be eliminated.[SM41]
??›?Eprotection represents Erp-Ecorr and indicates the protected region. Pits are formed in this region, repassivation and larger region means more resistance to pitting for composite. [SM42]According to table[SM43] 3, the largest protection region was belonged to Al cast. Milled sample showed a smaller region and because of having more grain boundaries on the surface, by creating smaller nucleation sites for pits, made the sample more prone to pitting. [SM44]Adding B4C to samples confines the region and lowered[SM45] the resistance to pitting corrosion.
In order to study the corrosion behavior of B4C composites and the base alloy, EIS measurements were carried out for all specimens at their Ecorr in 3.5% NaCl solution. Figure 9 shows results in the form of nyquist[SM46] diagrams. There is a common characteristic for all curves, i.e. a capacitive semicircle in the high frequency ranges. High frequency capacity loop was mainly related to the characteristics of electrical double layer formed at the interface between the adsorption layer on [SM47]metal surface and the electrolyte .
The biggest semicircle was noticed for the Al cast sample, indicating that the alloy has the highest resistance to corrosion. Al-BM sample has an additional semicircle in low frequency range, which may be related to the charge transfer across the metal-electrolyte interface. Another noticeable point is that, inductive loop [SM48]is related to the salt layer formation on the surface. It may also demonstrate that, [SM49]adsorption of an anion like chloride which is presented in electrolyte,[SM50] caused the pitting corrosion. Al-BM also has[SM51] lower resistance to corrosion than Al-Cast. Corrosion resistance for Al 4%B4C sample was higher than the Al-BM. But for the 2% and 6% composites, there were less improvement observed [SM52][16, 17].
Results from electrochemical measurements which were carried out on Cast and Milled alloys and the B4C composites[SM53] showed that,[SM54] adding B4C particles to milled alloys will not [SM55]change the corrosion resistance considerably. From corrosion resistance point of view, it would be fair to say that the best sample was[SM56] the Al 2%B4C.