Microstructural characterization by scanning electron microscopy

Sol-gel

The sol-gel process is a wet-chemical synthesis technique for preparation of oxide gels, glasses, and ceramics at low temperature [17]. The process involves the evolution of inorganic networks from colloidal suspension (sol). In particular, the hydrolysis and condensation of metal alkoxides (e.g. TEOS) result in the gelation of the sol to form a network of a continuous liquid phase called gel [18]. The sol-gel method is regarded as one of the most promising technique due to its various advantages such as low environmental impact, good chemical stability, low-cost and suitability for application on large areas and complex-shaped substrates [19–21]. The simultaneous hydrolysis and condensation reactions, resulting from the sol synthesis process are presented in Table 2.2 where ‘M’ stands for metal, ‘O’ for oxygen, and ‘R’ for reactive groups. The sol-gel process results in the formation of –M–O–M– oxide bridges between metal atoms and by-products of water molecules or alcohol for water or alcohol condensation respectively. The sol-gel method is used to prepare both inorganic coatings and inorganic-organic hybrid coatings on metals. Amongst the inorganic precursor, alkoxysilane, is the most preferred since it is mild and easily controllable [22]. Li et al. [22]. studied the effect of sol-gel hybrid coatings on the corrosion properties of laminated AA6061-T6 aluminum alloy. Using tetraethyl orthosilicate (TEOS) as SiO2 source, ethanol as a solvent, both glycol and sodium dodecyl sulfate (SDS) as molecular controlling agents, and alkaline as a catalyst for the hydrolysis and condensation processes, they were able to prepare homogenous and uniform SiO2 coatings on aluminum substrates with efficient anti-corrosion properties. The corrosion protection performance of SiO2 coated sample using sol-gel hybrid coating (HC) was compared with chromate conversion coating (CCC), and bare aluminum substrate (BS) (Figure 2.5). The corrosion current density values were found to be 1.82, 1.98, and 7.28 μA.cm-2 for HC, CCC, and BS samples, respectively. Generally, the lower the corrosion current density, the better the corrosion protection of the material [23]. These results were attributed to the continuous, homogenous and compact surface of the HC coatings on the aluminum substrates.

Electrodeposition process

The electrodeposition of metals and alloys involve the electrochemical reduction of ions from aqueous, organic, and fused salt electrolysis. The deposition of material species involves the reduction of ions in the solution as, Mn+ + ne- → M. The seemingly simple single reaction needs pre- and post complex steps before contributing to the whole deposition process. The two types of charged particles, an ion and an electron, can cross the interface. Hence, four types of fundamental areas are involved in the due process of deposition: (1) electrode-solution interface as the locus of deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the deposits, and (4) structure and properties of the deposits [24]. Using the electrodeposition technique, a thick coating can easily be deposited on a conducting substrate with good control of thickness exhibiting excellent coating adhesion properties. Wu et al. [25] studied the corrosion protection properties of superhydrophobic silica films on steel by the electrodeposition process. The authors reported that the thickness and the roughness of the fabricated films are related to the electrodeposition potential and time as shown in Figure 2.6 (a) and (b), respectively. The increase in the electrodeposition potential and time leads to the formation of a thicker film on the metallic substrate, which could be due to the increase of the deposition rate on the metallic substrate. Furthermore, the corrosion performance of the coated sample with optimum conditions was investigated by SEM analysis as shown in Figure 2.7. The SEM images of the steel substrate before and after immersion in a corrosive solution of 3.5 wt.% NaCl were presented in Figure 2.7. It can be seen from the images that the substrate presents corrosion-related features after immersion (Figure 2.7 (b)). On the other hand, the SEM image of the superhydrophobic film after immersion in corrosive solution (Figure 2.7 (d)). shows no change compared to before immersion (Figure 2.7 (c)). These results show the good corrosion protection properties of the fabricated films by electrodeposition process.

ENHANCED CORROSION PROTECTION OF ALUMINUM BY ULTRASONICALLY DIP COATED SODIUM SILICATE THIN FILMS

Metals and metal alloys have become an indispensable part of modern society and find applications in medical devices, electrical products, household products, transportations, food packaging, housing and construction, manufacturing, etc. [1–3]. In particularly, aluminum metal and alloys are not only attractive for their characteristic lightweight, high stiffness and good physical properties, but also excellent high strength to weight ratios. However, Al metals degrade and deteriorate by the electrochemical process in aggressive environments, which results in their wear and energy loss [4]. This electrochemical process herein referred as corrosion accounts for 20 % of energy loss annually, constituting to about 4.2 % of the United States’ gross national product (GNP) [5]. Overall, the economic impact of corrosion on the United States economy is estimated to cost more than 1 trillion dollars annually [6]. Given the colossal economic loss due to corrosion, various strategies such as the use of corrosion inhibitors have been attempted to protect metals against corrosion. Traditionally, chromium-based coatings have been used due to their excellent anti-corrosive property [7,8]. However, due to increased environmental and safety concerns, their use have been discontinued in recent times [9,10]. In this regard, alternative anti-corrosion coatings such as silicate-based materials have attracted the attention of the scientific community in recent times [11]. Silicate-based materials do not only offer excellent polymerizability, chemical, and heat stability but also due to their biocompatibility and environmental friendliness, are ideal for the protection of metallic pipelines against corrosion [11,12]. Due to the formation of strong metal-silicate bonding between the thin film silicate coatings and the underlying metal substrate, silicate coatings offer good anticorrosive property. Such coatings have extensively been investigated for protecting metals such as mild steel [13], copper [14], zinc [15], magnesium [16,17] and aluminum against corrosion [18–20]. In particular, sodium silicate, commonly known as ‘water glass’, is used to protect metals against corrosion by coating and inhibition mechanism [17–21]. In the inhibition process, sodium silicate is added in the corrosive solution to reduce the corrosion rate of metals [11,17,20]. On the other hand, in the coating process, metals are coated by sodium silicate thin films to protect against corrosion [15,22,23].

Gao et al. [17] evaluated the corrosion properties of magnesium alloy by an inhibition process utilizing sodium silicate as an inhibitor. In this process, they immersed the magnesium alloy in the corrosive solution of standard ASTM D1384-87 (Na2SO4, 148 mg/L, NaHCO3 138 mg/L, NaCl 165 mg/L, pH 8.2) with different sodium silicate concentrations. The inhibition efficiency was found to be improved with the increase of sodium silicate concentration and reached a maximum of 99.4%. They concluded that this gain is due to the formation of a transparent silicate thin film on the magnesium alloy substrates. Similarly, Garrity et al. [20] studied the corrosion protection of aluminum alloy through the inhibition process by incorporating sodium silicate in the 0.1 M NaCl aqueous solution. In their study, the corrosion protection mechanism is found to be due to the formation of aluminosilicate on the aluminum substrate. Thus, the addition of sodium silicate to domestic or industrial water supply as a corrosion control agent of the metallic pipeline has also ensured zero effects on environment/health issues, which added desirable quality to silicate materials [11]. However, a few studies were focused on the corrosion properties of sodium silicate as a coating on metallic substrates. Salami et al. [16] used micro arc oxidation (MAO) process to fabricate sodium silicate coatings on magnesium substrate and demonstrated a low corrosion rate of 0.12 mm/y compared to 9.8 mm/y for the bare magnesium alloy. A similar study by Zhang et al. [24] used the MAO process for the fabrication of sodium silicate coatings on titanium alloys and investigated their corrosion behaviors. Additionally, using an immersion process, Cerda et al. [14] synthesized a sodium silicate thin film coating on copper substrates that showed high anti-corrosion property 3 orders of magnitude higher than the as-received copper. Similarly, Gaggiano et al. [25] detailed the sodium silicate deposition on porous aluminum oxide by a dip-coating process. They observed that the sodium silicate interacted with the pore on the aluminum surface that led to the formation of aluminosilicate. The authors have not reported any corrosion-related studies of silicate coatings. However, the fabrication of sodium silicate thin films on aluminum metals and their corrosion protection have not been fully studied and therefore require further investigation. In this work, a simple ultrasonic dip-coating process is utilized to fabricate sodium silicate thin films coatings on aluminum alloy substrates. The effect of concentrations of sodium silicate in the solution is envisaged to study the surface morphology, chemical composition, and corrosion properties of the fabricated thin films.