1. INTRODUCTION:

Corrosion is a prevailing destructive phenomenon in science and technology. As per literature survey, the cost due to corrosion in many countries is as high as 3-5 % of the GNP i.e. wasteful in terms of economy of any country [1-3]. This represents a huge amount of money which should have been channeled into the provision of basic social amenities in these countries. The exposures can be severe to the properties of the metals as well as age of metals also, thus lead to ritual failure of materials in service.

Copper (Cu) and copper alloys have been used from prehistoric times, and their present day importance is greater than ever before [4]. Copper is one of the most important nonferrous materials [5,6] being widely used in various industries, including water distribution networks and cooling systems [4,6]. During the last decades, copper has been intensively used in microelectronics [7], owing to its combination of excellent workability, high thermal and electrical conductivities, attractive mechanical properties over a wide range of temperatures [4], high electrical conductivity and high resistance to electro-migration, which in turn results in greater circuit reliability and markedly higher clock frequency [8]. The long term operation of Cu equipment in contact with water solutions depends to a large extent on the corrosion resistance provided by Cu passivity. The passive film on Cu is usually characterized by a duplex oxide structure with an inner cuprous oxide and an outer cupric hydroxide [9]. The corrosion resistance of Cu passivation films is strongly depends on the chemical composition of water. Copper is poorly passivated in chloride-ions containing water (fresh, brackish and sea water) [5,6]. Various types of inhibitors are frequently used in water systems to improve Cu corrosion resistance.

Copper is characterized by its high electrical and thermal conductivities and good mechanical workability [10]. Mild steel is one of the major construction materials, which is extensively used in chemical and allied industries for the handling of acid, alkali and salt solutions [11-12]. The main objective of the present study is to investigate the inhibitory effects of the binary inhibitor formulation containing Zn²⁺ with SG in corrosion protection of different metals in potable water using surface analytical techniques were also used to investigate the nature of the surface film.

2. EXPERIMENTAL:

2.1. Materials

The specimens of size 1.0cm×4.0cm×0.2cm were press cut from the copper sheet and mild steel, were machined and abraded with a series of emery papers. This was followed by rinsing in acetone and bidistilled water and finally dried in air. Before any experiment, the substrates were treated as described and freshly used with no further storage. The inhibitors Zn²⁺ and SG were used as received. A stock solution of 1000ppm of SG was prepared in bidistilled water and the desired concentration was obtained by appropriate dilution. All solutions were using potable water (Perambalur, Tamil Nadu, India). The study was carried out at room temperature. The physico-chemical parameters are given in Table 1 and structure of SG is shown in Fig 1.

Fig 1. Structure of SG

Table 1. Physico-chemical parameters of potable water

Parameter	Value
pH	7.84
TDS	251ppm
Chloride	30ppm
Alkalinity	113ppm
Total Hardness	102ppm
Conductivity	358μmhos/cm

2.2. Scanning Electron Microscopy

The surface morphology of the corroded metals sample surface in the presence and absence of the inhibitors was studied using SEM (Model: TESCAN vega3 USA). To study the surface morphology of metals, polished specimens prior to initiation of any corrosion reaction, were examined in optical microscope to find out any surface defect, such as prior noticeable irregularities like cracks, etc.

Only those specimens, who had a smooth pit-free surface, were subjected to immersion. The specimens were immersed for seven days. After completion of the tests specimens were thoroughly washed with bidistilled water and dried and then subjected to SEM examination.

2.3. Energy Dispersive X-ray Analysis (EDAX)

EDX (Model: BRUKER Nano Germany) system attached with Scanning Electron Microscope was used for elemental analysis or chemical characterization of the film formed on the metals surface. As a type of spectroscopy, it relies on the investigation of sample through interaction between electromagnetic radiation and the matter. So that, a detector was used to convert X-ray energy into voltage signals. This information is sent to a pulse processor, which measures the signals and passed them into an analyzer for data display on the analysis.

2.4. Atomic Force Microscopy (AFM)

Atomic force microscopy is a powerful method for the gathering of roughness statistics from a variety of surfaces. This exciting new techniques that allows surface to be imaged at higher resolutions and accuracies than ever before. The protective films are examined for a scanned area. AFM is becoming an accepted technique of roughness investigation [13-16]. AFM provided direct insight into the changes in the surface morphology takes place at several hundred nanometers when topographical changes owing to the initiation of corrosion and formation of protective film onto the metal surface in the with and without addition of inhibitors respectively. All the AFM images were recorded on a Pico SPM2100 AFM instrument operating in contact mode in air. The scan size of all the AFM images are 15μm × 15μm areas at a scan rate of 0.20 Hz lines per second.

2.5. Water contact angle

In order to evaluate the contact angle experimentally, the sessile droplet method was used. The sessile droplet rested on a horizontal substrate by a syringe. The substrate was illuminated by a light source, and then a picture was taken by using a high resolution camera (10.1 Mpixle SONY camera). The image was processed by computer by software made for this reason.

3. RESULTS AND DISCUSSION:

3.1. Scanning electron microscopy

Fig 2. SEM images of mild steel for a) potable water (blank) b) potable water with Zn²⁺ and SG

Scanning electron microscopy was analyzed to understand the nature of the surface film in the absence and presence of inhibitors and the extent of corrosion of mild steel, the SEM micrographs of the surface were examined.

The SEM image of mild steel specimen immersed in potable water for seven days in the absence and presence of inhibitor system are shown in Figure 2 respectively. The SEM micrographs of mild steel surface immersed in potable water in Figure 2a (20µm) shows that the surface is strongly damaged, fault the metallic properties and there is a formation of different forms of corrosion products (iron oxides) on the surface in the absence of the inhibitor formulation. It further shows that the corrosion products appear very uneven and the surface layer is too rough. Figure2b (20µm) shows indicates that in the presence of 10 ppm of Zn²⁺ and 100 ppm of SG mixture in potable water, the surface coverage increases which in turn results in the formation of insoluble complex on the surface of the metal (SG-Zn²⁺ inhibitor complex) and the surface is covered by a thin layer of inhibitor which effectively controls the dissolution of mild steel [17].

Fig 3. SEM images of copper for a) potable water (blank) b) potable water with Zn²⁺ and SG

The SEM image of copper specimen immersed in potable water for seven days in the absence and presence of inhibitor system are shown in Figure 3 respectively. The SEM micrographs of copper surface immersed in potable water in Figure 3a (50µm) shows that the surface is strongly damaged, fault the metallic properties and there is a formation of different forms of corrosion products on the surface in the absence of the inhibitor formulation. It further shows that the corrosion products appear very uneven and the surface layer is too rough. Figure3b (50µm) shows indicates that in the presence of 10 ppm of Zn²⁺ and 100 ppm of SG mixture in potable water, the surface coverage increases which in turn results in the formation of insoluble complex on the surface of the metal (Zn²⁺-SG inhibitor complex) and the surface is covered by a thin layer of inhibitor which effectively controls the dissolution of copper.

3.2. Energy dispersive x-ray analysis

The composition of protective film formed on the mild steel surface was analyzed using EDX. The EDX spectrum of mild steel sample immersed in potable water in the absence of inhibitor molecules was failed because it is severely weakened due to the corrosion as shown in Figure 4a. On adding 10ppm Zn²⁺ + 100ppm SG to blank, the decrease of iron peak and appearance of carbon, sodium, oxygen and zinc peak was observed due to the formation of a strong protective film of the inhibitor molecules on the surface of mild steel sample [18] as shown in Fig 4b. The action of inhibitor is related to adsorption and formation of a barrier film on the electrode surface.

Fig 4. EDX spectra of mild steel for a) potable water (blank) b) potable water with Zn²⁺ and SG

Fig 5. EDX spectra of copper for a) potable water (blank) b) potable water with Zn²⁺ and SG

The composition of protective film formed on the copper surface was analyzed using EDX. The EDX spectrum of polished copper sample immersed in potable water in the absence of inhibitor molecules was failed because it is severely weakened due to the corrosion as shown in Figure 5a. On adding 10ppm Zn²⁺ + 100ppm SG to blank, the decrease of iron peak and appearance of carbon, sodium, oxygen and zinc peak was observed due to the formation of a strong protective film of the inhibitor molecules on the surface of copper sample [18] as shown in Fig 5b. The action of inhibitor is related to adsorption and formation of a barrier film on the electrode surface.

3.3. Atomic force microscopy

Fig 6. AFM images of mild steel in potable water

Fig 7. AFM images of mild steel immersed in potable water in the presence of protector formulation

AFM results of mild steel taken in uninhibited and inhibited conditions in potable water. Fig6 shows in uninhibited AFM images clearly show a rough surface (maximum surface roughness 3.9µm) due to rapid corrosion of mild steel. In the presence of (10ppm Zn²⁺ + 100ppm SG) the mild steel less corroded and a different surface morphology having comparatively smoother surface (maximum surface roughness 1.5µm) is observed in Fig7. A smoother layer with a clearly different morphology is as a result of the formation of a protective layer by the adsorbed inhibitor.

Fig 8. AFM images of copper in potable water

Fig 9. AFM images of copper immersed in potable water in the presence of protector formulation

AFM results of copper taken in the absence and presence of protector in potable water. Fig8 shows in absence of inhibitor AFM images clearly show a rough surface (maximum surface roughness 2.5µm) due to rapid corrosion of copper. In the presence of (10ppm Zn²⁺ + 100ppm SG) the copper less corroded and a different surface morphology having comparatively smoother surface (maximum surface roughness 0.9µm) is observed in Fig9. A smoother layer with a clearly different morphology is as a result of the formation of a protective layer by the adsorbed inhibitor.

3.4. Water contact angle

Fig 10. Water contact angle image of mild steel surface after immersion in potable water

Fig11. Water contact angle image of mild steel surface after immersion in potable water of solution containing 10ppm Zn²⁺+100ppm SG

The contact angle measurement were analyzed the nature of wettability, whether it is a hydrophobic or hydrophilic. Fig 10 shows mild steel surface immersed in potable water, surface highly porous, more roughness (contact angle 55° ± 2°) due to mild steel surface get hydrophilic nature. Fig11 shows mild steel surface immersed in the presence of inhibitor formulation (10ppm Zn²⁺ + 100ppm SG) smoother surface appear (contact angle 123° ± 4°) beside the surface gets hydrophobic nature. This confirms the adsorption of a hydrophobic protective film onto the mild steel surface in the presence of inhibitor.

Fig 12. Water contact angle image of copper surface after immersion in potable water

Fig13. Water contact angle image of copper surface after immersion in potable water of solution containing 10ppm Zn²⁺+100ppm SG

The contact angle measurement were analyzed the nature of wettability, whether it is a hydrophobic or hydrophilic. Fig 12 shows copper surface immersed in potable water, surface highly porous, more roughness (contact angle 84° ± 2°) due to copper surface get hydrophilic nature. Fig13 shows copper surface immersed in the presence of inhibitor formulation (10ppm Zn²⁺ + 100ppm SG) smoother surface appear (contact angle 160° ± 4°) beside the surface gets superhydrophobic nature. This confirms the adsorption of a superhydrophobic protective film onto the copper surface in the presence of inhibitor.

CONCLUSION:

Protector system, Zn²⁺ - SG exhibits effective protective film formation which protect corrosion of copper and mild steel from corrosion in potable water.

Atomic force microscope showed the roughness order as follows copper < mild steel.

Surface examination studies confirm the protective film formation of both copper and mild steel.

Water contact angle revealed that copper surface is superhydrophobic nature and mild steel surface is hydrophobic nature.

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