Corrosion Investigation of Stir Cast Zn-27Al Based Hybrid Composites Reinforced with Quarry Dust and Silicon Carbide Particles

Abstract

This paper investigates the corrosion behaviour of Zn-27Al based hybrid composites reinforced with quarry dust (QD) and silicon carbide particles (SiC_p). The Zn-27Al hybrid composites containing 8 wt% and 10 wt% SiC_p-QD particles reinforcement with varied weight percentage of 0, 25, 50, 75 and 100% quarry dust respectively were synthesized using stir casting technique. The corrosion behaviour of the composites was investigated in 0.3M H₂SO₄ and 3.5 wt% NaCl solution at 25ºC using electrochemical methods in accordance with ASTM G59-97(2014) standard. The results show that for 8 wt% reinforcement, the hybrid composites A₂ and A₃ (corresponding to 50% SiC_p: 50% QD and 25% SiC_p: 75% QD respectively) having corrosion rate of 0.006 mmpy displayed superior corrosion resistance. For 10 wt% reinforcement, hybrid grades B₁ (75% SiC_p: 25% QD)_, B₂ (50% SiC_p: 50% QD) and B₃ (25% SiC_p: 75% QD) has superior corrosion rate of 0.0172, 0.0126 and 0.0135 mmpy respectively while B₄ (corresponding to 100% QD) shows the most superior corrosion rate of 0.00315 mmpy when compared with the monolithic alloy having corrosion rate of 0.213 mmpy all in marine environment (3.5 wt% NaCl solution). However, except for composite grade A₂ with 0.055 mmpy corrosion rate and B₄ (corresponding to 100% QD) with superior corrosion rate of 0.0143 mmpy in 0.3M H₂SO₄ medium, all other composite grades performed poorly in the acidic medium.

Keywords:
Zn-27Al alloy based composites; Stir Casting; Corrosion; Silicon carbide; Quarry dust

1. Introduction

Zn-27Al belongs to a family of zinc alloys which are commercially referred to as ZA alloys designated by ZA-8, ZA-12 and ZA-27 corresponding to approximate aluminum content in the alloys. Zn-27Al alloy with a nominal content of 27 wt% aluminum has been reported to offer good strength, low density, excellent bearing capability, wear resistance, favourable combination of physical, mechanical and technological properties as well as better corrosion resistance of all the ZA alloys ^1-2

Corrosion features of the as-cast Zn-27Al alloy has been studied and reported ^3-4 indicating the presence of aluminum to have favourable effect on its corrosion behaviour. Zn-27Al shows high resistance to corrosion in atmospheric exposure, natural waters, soils as well as wide range of chemical environments due to the ability of the zinc to form a passive (protective) layer of corrosion products at the surface comprising mainly of zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂), simonkolleite or their mixtures ^5,3. The most common form of corrosion prevalent in Zn-27Al in a variety of natural environments is uniform corrosion ³. However, there has been a lack of specific corrosion data on zinc-aluminum based metal matrix composites and their corrosion resistance to date, due to the limited application of these alloys as matrix material for metal matrix composites ⁶. Hard reinforcements such as alumina (Al₂O₃), silicon carbide (SiC), zirconia (Zr), and graphite (C_g) have been successfully utilized as a whole or to form hybrid which result into enormous property enhancement to enrich the strength, hardness, elastic modulus, and wear resistance of Zn-27Al while other properties such as damping capacity, fracture toughness, impact resistance, and corrosion resistance have been improved ^7-8,6. The inclusion of the reinforcing particles and fibers as well as the fabrication methods associated with metal matrix composites can influence the corrosion resistance of the metal matrix ⁶.

During the past few decades, several authors have investigated the corrosion behavior of ZA-27 alloy based metal matrix composites ^9-13.

Seah et al⁹ investigated the corrosion characteristics of ZA-27 alloy reinforced with graphite particles in HCl solution and SAE 40 oil respectively which was aimed at exploring the possibility of using the composite in such acidic environments and also to evaluate the composite as a bearing material. The results however showed that the ZA-27/graphite particles composites corrode in HCl solution but not in SAE 40 oil. They also observed that corrosion resistance increases with increasing graphite particles. The effect of zircon particles on the corrosion characteristics of ZA-27/zircon particles composites in HCl was also reported by Sharma et al¹⁰. The results revealed that material loss was due to pit formation which was developed at matrix/particle interface induced by cracked. However, corrosion resistant of the composites increases with increase in zircon content. An attempt was also made by ¹¹ to examine the corrosion behavior of ZA-27 alloy reinforced with glass fibers in HCl solution. The composite was developed using compo casting technique and the corrosion test conducted under room temperature using weight loss method. In order to substantiate the difference between the corrosion behavior of heat-treated and as-cast specimens, some composite specimens were heat treated at 320^oC for 2, 3, and 4 hrs respectively. The results showed that corrosion rate seems to decrease with time of corrosion test which might be attributed to the presence of aluminum in the matrix ZA-27 alloy. The corrosion rate also decreases monotonically with increasing glass fiber contents which might be as a result of the glass fiber inertness. However, the heat-treated samples showed an improvement in corrosion resistance was also. Bobic et al¹² also evaluated the corrosion behavior of as-cast and heat-treated ZA-27 alloy in NaCl solution. The immersion test and polarization resistance measurements were used to assess the corrosion behavior of the ZA-27 alloy as-cast and heat-treated specimens in accordance with ASTM G31 standard. It was observed that T4 heat treatment applied has small beneficial effect on the corrosion resistance of ZA-27 alloy. Investigation was also carried out by ¹³ on the corrosion behavior of compo cast ZA-27/SiC_p composites in NaCl solution. The corrosion rate was determined using the conventional weight loss measurement in accordance with ASTM G31 standard. It was observed that a uniform corrosion occurred at the surface; however, the corrosion resistance of the ZA-27/SiC_p is lower compared to the matrix alloy. According to the results obtained ^9-13, the corrosion resistance of ZA-27 alloy based metal matrix composites increases with the enhancement of reinforcing phase content in the composite material; the corrosion rate was also found to decrease with time.

In this present work, attempt is made to investigate the corrosion behavior of stir cast ZA-27 alloy based hybrid composites reinforced with silicon carbide and quarry dust particles in sodium chloride and sulfuric acid solutions respectively which has not been studied yet. This work is motivated by the prospect of developing low cost-high performance ZA-27 alloy matrix hybrid composites using quarry dust particles as reinforcements with comparable properties with those reinforced with solely SiC_p which is a more expensive material.

2. Material and Methods

2.1 Materials

The materials used for the production of ZA-27 alloy selected as base matrix in accordance with ASTM B669-82 for the produced composites are commercial zinc (with chemical composition presented in Table 1) and aluminum 6063 (with chemical composition presented in Table 2). Chemically pure silicon carbide (SiC_p) of particle size 30µm and stone dust particles (<38µm) were selected as reinforcing materials for the production of the composites. The chemical composition of the stone dust taken into study in accordance with ¹⁴ is SiO₂- 62.48%, Al₂O₃- 18.72%, Fe₂O₃- 6.54%, MgO-2.56%, CaO- 4.83%, K₂O-3.18%, TiO₂- 1.21%.

Table 1
Samples Designation for the produced ZA-27 Composites

Samples Designation	Wt.% (SiCp: QD)
8 wt% Reinforcement
A0	100:0
A1	75:25
A2	50:50
A3	25:75
A4	0:100
10 wt% Reinforcement
B0	100:0
B1	75:25
B2	50:50
B3	25:75
B4	0:100

Table 2
Electrochemical data of the unreinforced and reinforced ZA-27 alloy composites from the tafel extrapolations in 3.5wt% NaCl solution

Sample	Ecorr (mV)	Icorr (µA)	Corrosion rate (mmpy)
Control (unreinforced Zn-27Al)	-860.59	-14.208	0.21289
For 8 wt% reinforcement
A0 (Zn-27Al-100% SiCp)	-800.544	-539.539	0.008084
A1 (Zn-27Al- 75%SiCp: 25% QD)	-761.927	-1.587	0.023789
A2(Zn-27Al- 50%SiCp: 50% QD)	-988.12	-429.087	0.0064291
A3 (Zn-27Al- 25%SiCp: 75% QD)	-988.12	-429.087	0.0064291
A4 (Zn-27Al- 100% QD)	-1.006	-1.313	0.019676
For 10 wt% reinforcement
B0 (Zn-27Al- 100% SiCp)	-785.048	-2.331	0.034938
B1 (Zn-27Al- 75% SiCp: 25% QD	-781.903	-1.147	0.017193
B2 (Zn-27Al- 50% SiCp: 50% QD)	-902.006	-846.159	0.012678
B3 (Zn-27Al- 25% SiCp: 75% QD)	-769.092	-903.748	0.013541
B4 (Zn-27Al- 100%QD)	-908.679	-210.749	0.0031577

2.2 Production of Zn-27Al alloy based Composites

Double stir casting method was utilized in accordance with ¹⁵ and ¹⁶ for the production of the composites in the course of this research. The appropriate amount of silicon carbide and stone dust to prepare 8 wt% and 10 wt% reinforcements in the Zn-Al alloy matrix respectively was determined using charge calculations. Five different stone dust-silicon carbide (SD-SiC_p) reinforcement weight mix ratios were prepared consisting of 0, 25, 50, 75 and 100% stone dust respectively. In order to remove the moisture content and improves wettability between the reinforcing materials and the zinc-aluminum alloy melt, the silicon carbide and stone dust was initially pre-heated respectively at a temperature of 250ºC according to ¹⁷.

The Aluminum (6063) billet was first accurately weighed and charged inside a gas fired lift out crucible with a fitted temperature probe to monitor the temperature, and fired to a temperature of 670ºC until a molten aluminum is obtained. The furnace temperature was later reduced to about 500ºC before weighed zinc was introduced into the crucible and allowed to completely melt. The molten Zn-Al alloy obtained was then allowed to cool inside the furnace to a semi-solid state (at a temperature of about 450ºC); after which it was well stirred with using mechanical stirrer for 5 minutes at 200 rpm to attain homogenization. The pre-heated weighed silicon carbide and stone dust particles were then charged inside the melt and the resulting slurry stirred manually for about 5-10 minutes while a small amount of Borax was introduced as wetting agent to enhance further wettability of the reinforcing materials with the molten alloy as stated by ¹⁸. The resulting composite slurry was then superheated to 530ºC and second stirring conducted with the aid of mechanical stirrer. The stirring operation was carried out for 10 minutes at 400 rpm before pouring into the prepared sand molds to obtain cast composites. The sample designations for the different compositions of the produced ZA-27 alloy based hybrid composites are shown in Table 1.

2.3 Corrosion analysis

Corrosion testing was conducted using electrochemical methods in accordance with ASTM G59-97 (2014) standard. The experiments were carried out using an AutoLAb potentiostat (Versa STAT 400) with versa STUDIO electrochemical software. Corrosion behavior of the samples was investigated in 0.3M H₂SO₄ and 3.5%wt NaCl solution at room temperature (25ºC). The experiments were performed using a three electrode corrosion cell set-up comprising the sample (ZA-27 Alloy) as the working electrode, saturated silver/silver chloride as reference electrode, and platinum as counter electrode. The working electrodes were prepared by attaching an insulated copper wire to one face of the sample using aluminum conducting tape, and cold mounting it with epoxy resin. The working electrode was immersed in test solutions (0.3M H₂SO₄ and 3.5 wt% NaCl) until a stable open circuit potential was obtained. Open-circuit corrosion potential (OCP) measurements were carried out in a separate cell for 120 minutes whereas potentiodynamic polarization measurements were carried out using a scan rate of 0.25 mV/s at a potential initiated at -250mV to +250mV with respect to OCP. After each experiment, the electrolyte and the test samples was replaced. The linear Tafel segments of the anodic and cathodic curves were extrapolated to corrosion current densities (I_corr) and corrosion potential (E_corr). Three repeat tests were carried out for all alloy compositions, and the reproducibility and repeatability were observed to be good as there were no significant differences between results from triplicates.

3. Results and Discussion

3.1 Corrosion behavior in marine (3.5 wt% NaCl) environment

The corrosion results obtained from the tafel plots electrochemical studies in 3.5 wt% NaCl are presented in Figure 1 while the tafel extrapolations of the corrosion current densities (I_corr) and corrosion potentials (E_corr) which are presented in Table 2 indicate clear distinct corrosion behavior between the unreinforced and reinforced ZA-27 composites grades. From Figure 1, for 8wt% reinforcement, it can be observed that the reinforced composites grade A series show better corrosion resistance in the salt environment compared with the unreinforced ZA-27 alloy having corrosion rate of 0.213mmpy, however, hybrid composites grade A₂ and A₃ (corresponding to 50% SiC_p: 50% QD and 25% SiC_p: 75% QD respectively) with corrosion rate of 0.006 mmpy displayed superior corrosion resistance which might be attributed to the corrosion products forming a tenacious protective layers on the samples ¹⁹. The combining effects of QD/SiC particles might also slow down the dissolution of the matrix material as observed in the hybrid samples ²⁰. This is however showing the beneficial effects of quarry dust as partial or sole replacement to SiC_p.

Figure 1
Polarization curves of unreinforced and reinforced ZA-27 alloy composites in 3.5 wt% NaCl solution

For 10 wt% reinforcement, it can be observed that all the composite grade B series also displayed better corrosion resistance. The hybrid grades B₁ (75% SiC_p: 25% SD)_, B₂ (50% SiC_p: 50% QD) and B₃ (25% SiC_p: 75% QD) has superior corrosion rate of 0.0172, 0.0126 and 0.0135 mmpy respectively as compared with B₀ (corresponding to 100% SiC_p reinforcement) while B₄ (corresponding to 100% QD) shows the most superior corrosion rate of 0.00315 mmpy. This good behavior observed might be due to increase in grain boundaries offered by the hybrid particles, thereby resulting to decrease in the depth of corrosion penetration ²⁰ while the superior corrosion resistance observed in B₄ which is solely reinforced by quarry dust might be due to the resistivity or inertness of the quarry dust.

3.2 Corrosion behaviour in acidic (0.3M H₂SO₄) environment

The corrosion results obtained from the tafel plots electrochemical studies in 0.3M H₂SO₄ are presented in Figure 2 while the tafel extrapolations of the corrosion current densities (I_corr) and corrosion potentials (E_corr) which are presented in Table 3 indicate clear distinct corrosion behaviour between the unreinforced and reinforced ZA-27 composites grades. From Figure 2 it can be observed that all the composites grade series A and B did not perform well in the acidic environment except for composite grade A₂ with 0.055 mmpy corrosion rate while the overall superior corrosion rate of 0.0143 mmpy was observed with B₄ (corresponding to 100% QD) showing similar trend observed in salt environment which indicates better resistivity of quarry dust in the corrosion media. This poor behaviour of monolithic and reinforced composites in acid media has generally been observed and reported by several researchers.

Figure 2
Polarization curves of unreinforced and reinforced ZA-27 alloy composites in 0.3M H₂SO₄ solution

Table 3
Electrochemical data of the unreinforced and reinforced ZA-27 alloy composites from the tafel extrapolations in 0.3M H₂SO₄ solution

Sample	Ecorr (mV)	Icorr (µA)	Corrosion rate (mmpy)
Control (unreinforced Zn-27Al)	-704.661	281.212	4.2135
For 8 wt% reinforcement
A0 (Zn-27Al-100% SiCp)	-704.933	-378.23	5.6671
A1 (Zn-27Al- 75%SiCp: 25% SD)	-699.172	-566.503	8.488
A2(Zn-27Al- 50%SiCp: 50% SD)	-618.056	-3.704	0.055506
A3 (Zn-27Al- 25%SiCp: 75% SD)	-705.576	-578.523	8.6681
A4 (Zn-27Al- 100% SD)	-708.555	-523.784	7.848
For 10 wt% reinforcement
B0 (Zn-27Al- 100% SiCp)	-710.388	-528.114	7.9128
B1 (Zn-27Al- 75% SiCp: 25% SD)	-717.992	-432.52	6.4806
B2 (Zn-27Al- 50% SiCp: 50% SD)	-711.518	-487.182	7.2996
B3 (Zn-27Al- 25% SiCp: 75% SD)	-702.269	-1.03	15.564
B4 (Zn-27Al- 100%SD)	-314.474	-985.942	0.014773

4. Conclusions

The corrosion behavior of stir cast ZA-27 based hybrid composites containing 8 and 10 wt% varied proportion of quarry dust (QD) and silicon carbide (SiC_p) respectively was investigated. The results show that:

• The corrosion behavior for both 8 wt% and 10 wt% reinforced composites show that all the composites displayed superior corrosion behavior when compared with monolithic alloy in 3.5 wt% NaCl environment while they all perform poorly in 0.3M H₂SO₄ environment except for composites grade A₂ and B₄.

• Composite grade B₄ (corresponding to 100% QD) displayed overall superior corrosion rate both in 3.5 wt% NaCl and 0.3M H₂SO₄ justifying the potential of stone dust particles to be use as a partial or sole replacement for SiC_p in metal composites development in corrosive media.

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  » https://doi.org/10.1080/02726351.2016.1165321

Publication Dates

• Publication in this collection
  13 May 2019
• Date of issue
  2019

History

• Received
  28 Aug 2017
• Reviewed
  03 Dec 2018
• Accepted
  17 Apr 2019