Development of an In-Situ Composite Doped Coating for Corrosion Protection and Mechanical Properties Enhancements in Process Engineering

Process engineering has been seen as one of the vital tools for improving surface coating phenomena for advance application. In an attempt to improve the mechanical, physical and chemical performance of the steel structure for ex-tended application, Zn-CeO2/ZnCeO2-Al2SiO5 thin film composite was fabri-cated on mild steel using direct electrolytic route. Process variation of Al2SiO5 particulate ranges from 5 to 15 g per litre. The embedded coating was charac-terized using Scanning electron microscope (SEM). The chemical effect of the developed alloy was characterized through linear potentiodynamic polarization experiment and the performances of samples were examined in simulated 3.5% sodium chloride. The microhardness verification study proves that there is sig-nificant improvement in hardness trend. The tribological assessment indicated that there is less plastic deformation as a result of the counter body. In all, Zn-CeO2/Zn-CeO2-Al2SiO5 exhibits good stability, with agglomeration and great built up of crystal at the interface.


Introduction
Corrosion is an unwanted phenomenon that destroys the lustre and aesthetics of objects and drastically decreases their lifespan. Corrosion is destructive and detrimental to structural, equipment and component failure [1]. The unrelenting effects of corrosion attack have become a global topic for consideration. Corrosion effect is undesirable causing physical and chemical changes in the property of steel it has attacked [2]. The result of studies in a number of countries in relation to corrosion has attempted to determine the national cost of corrosion. The most comprehensive of these studies was carried out in the United States in 1976 and determined that the total yearly cost of metallic corrosion on the economy was estimated high. Corrosion is still a major problem and slows down the advancement and application of such alloys due to failures which reduce the useful life of the prosthetic and cause more harm in the human body [3] [4].
Electrochemical corrosion occurs when a metal atom changes to a metal ion by the loss of electrons. This is because of the presence of cathodic and anodic areas on a metal surface which reacts with air and moisture in the surroundings. The heterogeneous and impure nature of metals contributes largely to their poor corrosion resistance [5] [6]. Corrosion can be controlled through a range of methods which have varying degrees of success. The methods include the use of inhibitors, coating, protective treatments and the utilization of better designs for structures and equipment, understanding the mechanisms of various forms of corrosion and using materials with high corrosion resistance [7]. This present research examined the performance of mild steel coated Zn-CeO 2 /Zn-CeO 2 -Al 2 SiO 5 in a simulated sodium chloride environment. The morphologies of the test samples were studied using SEM while the hardness and wear test was carried out using the Brinell hardness tester and reciprocating wear testing machine.

Preparation of Substrate
The composition of mild steel used in this study is shown in Table 1. Mild steel of dimension (30 mm × 20 mm × 2 mm) and zinc sheets of (50 mm × 20 mm × 5 mm) were prepared. The zinc is commercially available pure zinc (99.99%). Mild steel was polished with different grades of emery papers.

Bath Formulation
The prepared bath composition with optimized mass concentration is shown in Table 2. The essence of this was to create novel composite plating on mild steel using composite particulates of zinc, aluminium silicate (Al 2 SiO 5 ), cerium (iv) oxide (CeO 2 ) in varying proportions with the aim of determining the most effective coating.

Deposition of Zn-CeO2/Zn-CeO2-Al2SiO5
For the pre-plating process, the surface of the samples was prepared using  voltage and time were kept at 1.5 A/cm 2 , 2.0 V and 10 minutes respectively as indicated in Table 3. The coatings of the samples were carried out in accordance to ASTM A53/A53M and A153 [8]. Finally, the plated samples were rinsed in distilled water to wash off the salt solution immediately after the electroplating process then air dried. Thereafter, series of characterizations such as the study of morphology via SEM, electrochemical using potentiodynamic polarization experiment, Brinell hardness and wear tester were employed to study the performance of the deposited coatings on the substrate.

SEM of Deposited Composite Coating
The SEM structure of Zn-10CeO 2 , Zn-10CeO 2 -5Al 2 SiO 5 , Zn-10CeO 2 -10Al 2 SiO 5 and Zn-10CeO 2 -15Al 2 SiO 5 coated mild steel is shown in Figures 2-5        More so, the slight change in the structure of Figures 3-5 compared to Figure  2 could be as a result of Al 2 SiO 5 inclusion in the matrix of the nanocomposite which results in the precipitation and enhanced reinforcement [13]. One can therefore conclude that the porosity of the composite coatings reduces with the incorporation of Al 2 SiO 5 particles in the coating. This is in accordance to the finding of other authors [11] [14].

Electrochemical Test Result
The values of corrosion current density (Icorr) , corrosion potential (Ecorr), corrosion rate (CR) and polarization resistance (R P ) of the coated and uncoated samples in 3.5% NaCl were obtained from the extrapolation of Tafel plot shown in Figure 6. The results reveal the corrosion resisting characteristics of the particles in the test solution. Zn-CeO 2 /Zn-CeO 2 -Al 2 SiO 5 was able to block the active sites of the metals from corroding [15] [16]. Corrosion rate of Zn-10CeO 2 -5Al 2 SiO 5 coated sample was the lowest when compared with others as indicated in Table  4. This could traceable to the nature and tenacity of the passive film produced by Zn-10CeO 2 -5Al 2 SiO 5 on the surface of the coated steel or chemical stability of the samples [17]. Figure 7 shows the mircohardness of the test samples. Zn-10CeO 2 -10Al 2 SiO coated steel has the highest Brinell hardness value of 203 BHN for the electrodeposited sample. Generally, the coated samples exhibit higher value of hardness compared to the uncoated sample which could be attributed to the strain energy between the particles and the steel [18] [19] (Table 5).

Wear Behaviour of Zn-CeO2/Zn-CeO2-Al2SiO5 Deposition
The wear behaviour of the uncoated and coated samples is shown in Figure 8.    Zn-10CeO 2 -15Al 2 SiO 5 coated sample has the lowest wear loss vale of 0.001 g/min. This could be attributed to coherence interaction between the steel and the coating and the processing parameters [18] [20] [21]. The Zn-10CeO 2 -10Al 2 SiO 5 and Zn-10CeO 2 -15Al 2 SiO 5 displays lower wear loss which justifies the higher hardness values they possess. This also indicates these samples possess higher strain energy that exists between the coating and the metal interface (Table 6).

Conclusion
The inclusion of the CeO 2 /Al 2 SiO 5 particles in the zinc matrix improved the structural properties and proper dispersion of the particles. The electrochemical result showed that Zn-CeO 2 /Zn-CeO 2 -Al 2 SiO 5 nano composite exhibited good corrosion resistance in 3.5% NaCl. The coated samples were also found to exhibit better microhardness and wear properties compared to the uncoated samples which are an indication that the steels mechanical characteristics have been influenced.