Studies on the development of activated binary clay and corrosion monitoring using embedded sensor

Han-Seung Lee; Seung-Jun Kwon; Subbiah Karthick; Velu Saraswathy; Srinivasan Muralidharan

doi:10.1016/j.arabjc.2017.07.003

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Original article

13 (

); 595-605

doi:

10.1016/j.arabjc.2017.07.003

Studies on the development of activated binary clay and corrosion monitoring using embedded sensor

Han-Seung Lee^{^a}, Seung-Jun Kwon^{^b}, Subbiah Karthick^{^a,^c}, Velu Saraswathy^{^a,^d,⁎}, Srinivasan Muralidharan^{^d}

a Department of Architectural Engineering, Hanyang University, 1271 Sa 3-dong, Sangrok-gu, Ansan 426-791, Republic of Korea

b Department of Civil Engineering, Hannam University, Daejeon 306-791, Republic of Korea

c PG and Research Department of Chemistry, Alagappa Government Arts College, Karaikudi 630003, Tamil Nadu, India

d Corrosion and Materials Protection Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630003, Tamil Nadu, India

⁎Corresponding author at: Corrosion and Materials Protection Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630003, Tamil Nadu, India. vsaracorr@cecri.res.in (Velu Saraswathy)

Received: 2017-5-13, Accepted: 2017-7-2, Published: 2017-01-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Bentonite and marconite are the low resistance moisture retaining conductive backfill materials used in earthing applications. Both the products contains some drawbacks: bentonite has limited moisture retaining capacity and marconite has 15–20% impurities which will corrode the earth connections resulting in the loss of the system which are found to be very expensive. Taking into consideration of the above drawbacks, the present study aimed at developing a cost effective and highly conductive backfill material for earthing application with improved performance. For this study, commercially available bentonite and metakaolin (binary) clay was activated through physical, chemical and thermal treatments and the corrosion performance of binary clay was evaluated by using mild steel (MS) and galvanized (GI) steel Among the three activation methods, chemical activation method was found beneficial for mild steel in binary clay media. The conductivity of the chemically activated clay was 204.7 mS/cm, pH was 12.58, and the particle size distribution was found to be 40–50 µm indicates the better corrosion resistance and quite suitable for earthing applications. Chemical activation of the clay mainly involves the breaking of bonds and dissolution of the three-dimensional network structure of glass which in turn cause Na⁺ ions move closer to the center point of crystal structure and the solubility of SiO₂ in clay markedly increases. Potential-time studies showed that galvanizing loses its coating property within ten days in all the three type of clays used. Activation process significantly reduced the corrosion rate (44 and 74 times) in the case of thermally activated (TAC) and chemically activated clay (CAC) respectively. Earth excavation studies were conducted to monitor the corrosion of MS and GI using embedded MnO₂ sensor. This investigation opens up a lot of scope for utilizing activated binary clay for all earthing applications.

Keywords

Bentonite

Metakaolin

Activated clay

Corrosion studies

Mild steel

Galvanized iron

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1 Introduction

The natural clay minerals can be modified with a polymeric material in a manner that this significantly improves their capability to remove heavy metals from aqueous solutions (Barakat, 2011). Clay based materials are widely used in several industrial applications, such as foundries, petroleum drilling, civil engineering, catalysis, environmental remediation and underground steel pipelines. The pipeline materials are usually expected to have a long working life in the buried soil. Several authors investigated the interaction between the cathodic protection of pipeline material and buried soil. The soil corrosion depends upon the physicochemical properties such as soil type, moisture content and the position of the water table, soil resistivity, soluble ion content, soil pH, oxidation–reduction (redox) potential and the rates of microbes in soil corrosion (Schaschle and Marsh, 1963; Starkey and Wight, 1983; Levlin, 1992; Adeosun et al., 2011; Pal et al., 2004; El-Shamy et al., 2015 ). The fundamental cause of the deterioration of pipeline buried in the underground is soil corrosion. Soil is a complex material, a porous heterogeneous and discontinuous environment constituted by mineral or organic solid phase, water liquid phase and air and other gas phases. Steel pipeline corrodes in soil by complex electrochemical processes because of the presence of different soil electrolytes. The soil corrosion is minimized by modifying the clay and filling the materials. Earlier reports suggested that usage of modified clay was expected to have a long working life for pipelines buried in soil. This type of filling materials contains fine materials which close the pores between the sand and gravel particles (DIN et al., 1998).

The finer soil particles, owing to the increase in swelling, shrinkage, and plasticity, are considered as the factors favoring corrosive medium for underground pipelines and steel structures. On the other hand, the clay structure and the constituents of the filler materials are very important to investigate the structure effect relationship of clay and at the same time the effect of water content before using it as a filler material (Ismail and El-Shamy, 2009). The clay mineral contents are also the main quality control to the mild steel corrosion. For example, montmorillonite and illite absorb moisture more than kaolinite clay minerals. With the result these materials are found to be highly effective in the deterioration of metals (Cicel and Kranz, 1981) hence it is essential to improve the properties, of clay minerals to avoid corrosive environments around the underground steel pipeline. Activation of clay materials may induce physical and chemical changes in the activated materials that can be exploited either to enhance known properties, or for new application purposes (Liang et al., 2008; Christidis et al., 1997, 2004 ). Activation of clay induces their capacity to absorb coloring matters and other impurities in oils. The term of activity denotes increased chemical and physicochemical reactivity which is usually traceable with an increase in surface area of solids (Dellisanti and Valdre, 2005; Dellisanti et al., 2006). Activation of bentonite by acid is an important step for adsorption of some impurities by activated clay.

Studies revealed that, bentonite clay is used as an earth enhancement material. Bentonite, a naturally occurring clay mostly comprised of the mineral montmorillonite, is hygroscopic and absorbs moisture from the surrounding environment. It may not function well in a dry environment due to the poor conductivity. This will lead to failure of the whole grounding systems. Commercial forms of earthing enhancement materials are available in the form of powders, granules, pellets, gels and cementitious mixtures. All the materials are comprised of carbon-based materials or clays like bentonite (or a mixture of both). Others contain copper sulfate or other copper based compounds, which may not be environmentally friendly. Some earthing enhancement materials also contain cement, which after installation sets up like concrete. Although earthing enhancement materials have been successfully used to reduce ground resistance for decades, a product or performance standard has not existed for these types of products until recently. One requirement for earthing enhancement materials is that they must be chemically and physically stable (IEC 62561-7, 2011; Hanna and Drisko, 1970).

Considering the above drawbacks, the present investigation was aimed at developing a binary clay formulation using bentonite and metakaolin in equal proportions with improved conductivity and corrosion resistance performance. The corrosion resistance property of binary clay was improved with different activation techniques. The corrosion resistance properties of GI and MS were evaluated in binary clay media using electrochemical techniques.

2 Experimental

2.1

2.1 Materials used

Bentonite and metakaolin clay was purchased from the local suppliers and used for this study. The specific gravity of bentonite and metakaolin were found to be 2.65 and 2.50 respectively. The chemical composition of bentonite and metakaolin clay are (the specification provided by the manufacturer) reported in Table 1. GI and MS pipes were used for the corrosion evaluation studies. NaOH used was AR grade procured from Fischer Scientific Inc. Distilled water was used for preparing the alkali solutions.

Table 1 Chemical composition of bentonite clay and metakaolin.

Constituents	Weight (%)
Constituents	Bentonite clay	Metakaolin
Al₂O₃	7.13	20.27
Fe₂O₃	2.43	9.08
TiO₂	0.34	0.78
SiO₂	52.18	54.82
CaO	2.59	2.10
MgO	1.51	3.02
Na₂O	0.39	1.31
K₂O	19.03	7.53
LOI	14.40	1.09

2.2

2.2 Methods of activation

2.2.1

2.2.1 Preparation of binary clay

An equal amount of (1:1 ratio) of bentonite and metakaolin clays were thoroughly mixed and used for further activation.

2.2.2

2.2.2 Physical activation

The binary clay sample was sieved through 90 µm to remove any coarser and foreign particles and then mechanically ground in a ball mill for 72 h to a fine powder. The particle size distribution measured was found to be between 70 and 150 µm (maximum particles retained). Physically activated clay is denoted as PAC.

2.2.3

2.2.3 Thermal activation

PAC was loaded in a graphite pot and kept in a muffle furnace at a temperature of about 600 ± 20 °C for 2 h. After cooling at room temperature (35 ± 1 °C), the finely ground clay obtained was used for further investigation. The carbon, sulfur and other impurities were removed by thermal activation. Thermally activated clay is denoted as TAC.

2.2.4

2.2.4 Chemical activation

TAC samples were then subjected to chemical activation with alkali. The clay was circulated among various PVC molded reactors. Reactors 1, 2 and 3 which contain a slurry of clay, 1 M NaOH, and distilled water respectively. Firstly, a slurry of clay from reactor 1 was passed on to the reactor 2 which contains strong alkali. Here the surface of the clay was etched by strong alkali under vigorous stirring. Then the clay was washed by using distilled water in the reactor 3. The process was continued for several cycles, and finally, the slurry was filtered and dried at 40 °C for 48 h. and a very fine powder was obtained. The particle size distribution through sieve analysis was found to be between 40 and 50 µm (maximum particles retained). Chemically activated clay is denoted as CAC. Fig. 1a–c shows the samples of PAC, TAC, and CAC respectively.

Samples of (a). PAC; (b). TAC; (c). CAC.

2.3

2.3 Characterization of activated clay sample by X-ray diffraction studies and SEM

The activated clay samples were characterized by using PANanlytical X’PERT PRO system in Bragg-Brentano geometry using Cu K_α1 (1.540 Å) radiation. The powder diffraction covered the 10°〈2θ〉90° range with 0.0170° steps. The surface morphology and microstructure of the activated clay samples were analysed using scanning electron microscopy (SEM) HITACHI Model S-82 3000H.

2.4

2.4 Evaluation of corrosion performance of activated clay samples

2.4.1

2.4.1 Preparation of clay extracts

Extracts were prepared by mixing powdered and sieved activated clay with distilled water in 1:1 ratio and shaken in a Microid flask shaker for 1 h. Then the extracts were collected by filtration.

2.4.2

2.4.2 Gravimetric weight loss method

Circular MS and GI pipes were cut from the bulk specimens. The size of the sample taken was 30 mm dia. and 25 mm length (MS), 38 mm inner dia., 42 mm outer dia., with 25 mm length (GI) were used for weight loss measurements. MS was given mechanical polishing to remove the superficial rust and subsequently polished with different grades (0/0, 1/0, 2/0, 3/0, and 4/0) of emery papers and finally degreased with acetone solvent before use. But the G.I specimens was only cleaned with acetone without damaging the galvanizing coating on the surface. After taking the initial weight of the specimens the pipes were completely immersed in PAC, TAC and CAC extract for 60 days at room temperature. At the end of the experiment, specimens were removed, washed with distilled water, dried and reweighed. The weight loss of all the samples was calculated. From the weight loss, the corrosion rate was determined using the relationship.

(1)

Corrosion rate (mmpy) = \frac{(87.6 \times W)}{(D \times A \times T)}

where

W is the weight loss in mg.
D is the density in g/cc.
A is the area of the specimen in cm².
T is the exposure time in an hour

mmpy is millimeter per year.

2.4.3

2.4.3 Potential – time studies

As per ASTM C 876, the open circuit potential (OCP) of MS and GI specimens were monitored periodically with respect to saturated calomel electrode (SCE) in various activated clay extracts for an exposure period of 60 days.

2.4.4

2.4.4 Potentiodynamic polarization technique

Polarization measurements for the MS and GI specimens were carried out potentiodynamically in clay extracts. The polarization measurement was made using a three electrode cell assembly at room temperature. The working electrode was MS/GI of 1 cm² area and the rest of the exposed area was covered by using commercially available lacquer. A larger rectangular platinum foil was used as a counter electrode, and SCE was used as a reference electrode. The potentiodynamic condition corresponds to a potential sweep rate of 0.1 mV s⁻¹ and the potential ranges of −200 mV to +200 mV from the OCP. using ACM Instruments, UK (Field Machine). This instrument itself is having the provision for programs to evaluate the corrosion kinetic parameters such as I_corr, E_corr and corrosion rate.

2.5

2.5 Earth excavation studies using embedded MnO₂ sensor

MS cylindrical rod of 32 mm dia., with 450 mm length and G.I cylindrical pipe of 38 mm inner dia., 42 mm outer dia., with 450 mm length was taken for the earth medium studies. The sample preparation procedure adopted for MS and GI was discussed in Section 2.4.2. A wire was soldered in both MS and G.I and sealed with epoxy and the connection was taken from one end of the specimen for potential measurements. Embedded MnO₂ sensor along with activated clay filled MS and GI specimens were buried below 1 m depth of the soil. The whole system was subjected to natural weathering conditions namely: mostly hot occasionally rainy and some times exposed to mist for an exposure period of 6 months. The potential of both the MS and GI were monitored over the period of six months using embedded MnO₂ sensor and the potential vs. time graph was plotted. Fig. 2 shows the schematic representation of earth excavation studies using embedded sensor.

Schematic representation of earth excavation studies using embedded sensor.

3 Results and discussion

3.1

3.1 Chemical analysis

The chemical characteristics of the activated clay samples were analysed in the laboratory, and the results are given in Table 2. pH was measured using the portable ISTEK pH meter- Model 76 P. The conductivity was measured using Elcometer 138e conductivity meter. The particle size was analysed through sieve analysis. The colour of the PAC extract was light yellowish and the pH was found to be 8.05, the colour of the TAC and CAC was found to be colourless and brownish orange respectively. The pH of both thermally and chemically activated clays were higher than the physically activated clay (PAC) (8.99 and 12.58 with respectively). The conductivity of PAC, TAC and CAC were found to be 1.48 mS/cm, 165.40 mS/cm and 204.70 mS/cm. Here it was observed that both the TAC, and CAC extracts were highly conductive due to the increase in solubility of Na⁺ and OH⁻ ions (Kenza El Hafid et al., 2017). CAC enhanced the conductivity 138 times when compared to PAC. This is highly suitable for all earthing applications when there is any deviation in the voltage and leakage of currents which easily conducts into the soil (Gomes et al., 2014). Further, chemically activated clay showed an alkaline pH of 12.58 which is the favorable environment for mild steel with respect to corrosion resistance.

Table 2 Chemical characteristics of various clay samples.

System	Colour	Particle size (µm)	pH	Conductivity (mS/cm)
PAC	Light yellowish	70–150	8.05	1.48
TAC	Colourless	70–100	8.99	165.40
CAC	Yellowish orange	40–50	12.58	204.70

3.2

3.2 Characterization of activated clay samples

3.2.1

3.2.1 X-ray diffraction studies

Fig. 3 shows the XRD pattern of various activated clays. (All diffraction peaks present in the figure coincides with JCPDFWIN data. Fig. 3a is the XRD pattern of PAC which shows diffraction peaks at 12.05°, 21.03°, 25.06°, 26.06°, 36.08°, 39.07°, 50.70°, 60.01°, 63.02° and 67.08°. The diffraction peaks at 12.05°, 39.07° and 50.70° corresponding to montmorillonite peaks [(Mg₃ (Mg₂.Al) (Si₃.Al) (Si₃.Al) O₁₀(OH)₂O₃]. Silicon di-oxide presence is revealed by peaks at 25.06° and 26.06°. The diffraction peaks at 36.08° and 60.0° is corresponding to aluminium oxide and aluminium silicate hydrate peaks and the diffraction peaks at 21.03°, 63.02° and 67.08° are due to the presence of calcium carbonate. The XRD pattern of TAC is represented in Fig. 3b. It reveals the disappearance of calcium carbonate peaks at 21.03°, 63.02°, 67.08° and the peak intensity is increased when compared to PAC. This is due to the thermal decomposition that occurs as a result of activation at a higher temperature. Further, an increase in the intensity of crystallographic peaks, which may be due to the crystal growth and phase formation on activation by high temperature annealing. Fig. 3c is the XRD pattern of CAC. It clearly reveals that the formation and increase in the crystallographic intensity of montmorillonite peaks at 38.10°, 43.24 °and 50.44° (Mousa Gougazeh and Buhl, 2014) corresponding to Zeolite [Na₈ Mg₂Al₁₂ Si₁₂O₄₈ (H₂O)₇₅] formation. The peaks at 21.06° and 27.05° corresponds to Na₆Si₂O₇, and Ca₂Mg₂ Si₁₂O₃₂ (OH)₆. With the result, an enormous increase in the alkalinity of the system occurs, which renders the formation of metal oxides and metal oxide complexes such as aluminium oxide, silicon dioxide, and calcium oxide.

X-ray diffraction pattern for (a). PAC; (b). TAC; (c). CAC.

3.2.2

3.2.2 Scanning electron microscope (SEM)

Fig. 4 shows the SEM images of different activated clay samples. Fig. 4a represents SEM image of the PAC indicates the heterogenous particles with micron-size agglomerates. (Adel Sharif Hamadi et al., 2015). On the other hand, clear spongy crystal growth with smaller and mixed particles are noticed after annealing at 600 °C in the case of TAC as shown in Fig. 4b. But in the case of CAC (Fig. 4c), the combined influence of temperature and chemical activation results in the formation of homogeneous spherical particles with agglomerates of hydroxysodalite that grew onto the surface of cubic crystals of zeolite. The SEM morphology of CAC obtained from this study similar to those reported in the previous literature (Ogunmodede et al., 2015; Heller-Kalli and Lapide, 2007) and results obtained from SEM is well agreeable with the mineralogical composition of zeolite products which was determined through XRD results.

Scanning electron micrographs for (a). PAC; (b). TAC; (c). CAC.

3.3

3.3 Corrosion studies of activated clay samples

3.3.1

3.3.1 Weight loss method

The corrosion rate of MS and GI in PAC, TAC, and CAC extracts for an exposure period of 60 days were given in Table 3. From the table it was observed that MS showed more corrosion rate than GI (PAC) for the exposure period of 60 days. The PAC system showed corrosion rates of 4.5320 × 10⁻² and 3.9836 × 10⁻² mmpy for MS and GI respectively.

Table 3 Corrosion rate of MS and GI in clay extracts by weight loss measurements.

Clay	Specimen	Corrosion rate (mmpy) × 10⁻²
PAC	MS	4.5320
PAC	GI	3.9836

TAC	MS	2.7864
TAC	GI	2.0146

CAC	MS	0.0570
CAC	GI	45.8300

TAC system showed a lower corrosion rate than PAC. The corrosion rate of MS and GI in thermally activated clay (TAC) were 2.7864 × 10⁻² mmpy and 2.0146 × 10⁻² mmpy respectively.

In the case of CAC, it was interesting to note that negligible corrosion rate was observed for mild steel. On the other hand, GI showed the highest corrosion rate of 45.8300 × 10⁻² mmpy. These data proved that GI is not found suitable for chemically activated clay. Since the pH of the chemically activated clay was 12.58, zinc dissolution was at a faster rate than corrosion process (Chen Ai-liang et al., 2012). The better performance of mild steel was observed due to the complete passivation of mild steel surface at very high alkaline pH of 12.58 (Enrico Volpi et al., 2015). The ferrous and ferric system reacts at alkaline pH as follows:

(2)

In the case of {Fe}^{2 +} : {[Fe {(H_{2} O)}_{6}]}^{2 +} + 2 {OH}^{-} \to [Fe {(H_{2} O)}_{4} {(OH)}_{2}] + 2 H_{2} O

(3)

In the case of {Fe}^{3 +} : {[Fe {(H_{2} O)}_{6}]}^{3 +} + 3 {OH}^{-} \to [Fe {(H_{2} O)}_{3} {(OH)}_{3}] + 3 H_{2} O

3.3.2

3.3.2 Potential – Time behaviour

Potential vs. time behaviour of MS in various clay extracts are given in Fig. 5. In PAC, MS has shown a potential of −300 mV during the initial period and there is a gradual increase in potential in the active direction over the period of exposure and it reached the maximum potential of −700 mV vs. SCE at the final exposure period of 60 days. These ranges indicate the active condition of mild steel in PAC extracts. TAC also showed the same type of active state of behaviour of mild steel at the end of the exposure. On the other hand, MS in CAC showed a potential of −160 mV during the initial days and maximum potential of −260 mV after 60 days of exposure. It indicates that MS maintains its passivity throughout the exposure period of 60 days. Almost the graph obtained was parallel to the axis. It indicates the highly passive condition of MS in chemically activated clay.

Potential – time behaviour of MS in various clay extracts.

Fig. 6 shows the potential-time behaviour of GI in PAC, TAC, and CAC extracts. From the figure it was inferred that GI showed −1079 mV, −1080 mV and −1128 mV in PAC, TAC and CAC clay extracts respectively at the initial stage. After 60 days of exposure GI showed −925 mV, −975 mV and −875 mV in in PAC, TAC and CAC extracts respectively. These results indicated that at higher pH zinc undergoes anodic dissolution as follows:

(4)

Zn \to {Zn}^{2 +} + 2 e^{-}

(5)

{Zn}^{2 +} + 2 {OH}^{-} \to Zn {(OH)}_{2}

Potential – time behaviour of GI in various clay extracts.

According to the mechanism proposed by Cachet et al. (2001) and Huyuan Sun et al. (2013) dissolution of zinc in aqueous solution depends on the adsorbed intermediate species (Zn_ad⁺, Zn_ad²⁺ and Zn(OH)_ad). With the result, the GI showed a shift towards a positive direction.

3.3.3

3.3.3 Potentiodynamic polarization technique

The typical potentiodynamic polarization curves for MS and GI in various clay extracts are shown in Figs. 7 and 8 respectively. The corrosion kinetic parameters derived from the potentiodynamic polarization curves are given in Table 4.

Potentiodynamic polarization curves for MS in various clay extracts.

Potentiodynamic polarization curves for GI in various clay extracts.

Table 4 Corrosion kinetic parameters from Potentiodynamic polarization for MS and GI in various clay extracts.

Clay	Specimen	E_corr (mV vs. SCE)	I_corr (mA/cm²) × 10⁻²	Corrosion rate (mmpy) × 10⁻²
PAC	MS	−705	0.3647	3.8620
PAC	GI	−941	0.2923	3.3880

TAC	MS	−610	0.1202	2.2820
TAC	GI	−972	0.0847	0.3920

CAC	MS	−313	0.0045	0.0522
CAC	GI	−1082	3.793	43.9630

The corrosion potential of the various activated system follows the order: $PAC > TAC > CAC$ $- 705 mV > - 610 mV > - 313 mV$

MS in CAC showed highly passive behaviour when compared to TAC and PAC.

The corrosion rate measured for MS in PAC, TAC, and CAC extracts were found to be 3.8620 × 10⁻², 2.2820 × 10⁻² and 0.0522 × 10⁻² mmpy respectively. These data confirmed that the chemical activation process increased the corrosion resistance of clay for MS (Velu Saraswathy and Song, 2006). The corrosion rate was significantly reduced to 74 and 44 times in TAC and CAC clay respectively.

The corrosion rate measured for GI in PAC, TAC and CAC extracts were found to be 3.3880 × 10⁻², 0.3982 × 10⁻² and 43.9630 × 10⁻² mmpy respectively. As observed in potential -time studies, GI showed severe corrosion potential in activated clay extracts. For example, particularly in CAC, GI showed a 13 times increase in corrosion rate when compared to PAC. These results confirmed the accelerated corrosion of GI when compared to MS in CAC.

3.4

3.4 Earth excavation studies using embedded MnO₂ sensor

Open circuit potential of MS and GI in activated clay medium in soil was monitored for the exposure period of 6 months, and the corresponding graphs are given in Figs. 9 and 10 respectively.

Potential-time behaviour of MS vs. embedded MnO2 sensor.

Potential-time behaviour of GI vs. embedded MnO2 sensor.

It was observed from Fig. 9 that the potential-time behaviour of mild steel in PAC and TAC was shifted to the more cathodic direction indicating the active condition of MS in both PAC and TAC. On the other hand, MS in CAC showed a stable passive potential vs. MnO₂ throughout the exposure period of 6 months. Almost the graph obtained was parallel to x-axis indicating the highly passive condition of MS in chemically activated binary clay.

The potential-time behaviour of GI in activated clay is given in the Fig. 10. From this figure, it was observed that within 10 days of exposure in CAC, GI has shown the potential which is equivalent to the mild steel potential. It indicates the active dissolution of galvanizing (zinc layer) in clay extracts (highly alkaline medium) (Macias and Andrade, 1987).

3.5

3.5 Mechanism of activation of clay and improved corrosion resistance

In general, clay is a non-reacting substance when it is in the native state. The following factors are responsible for clay in the corrosion process (Yahaya et al., 2011):

Environment – industrial, marine or both
Moisture content in the clay
Sulfur content in the clay
pH of the clay
Micro organisms present in the clay

PAC which is used in the present study showing a pH of 8.05 accelerated the corrosion of MS and GI in the presence of moisture. As expected, the rate of corrosion is faster in the case of MS and slower in the case of GI.

To improve the individual performance of primary component namely bentonite clay, attempt has been made to blend equal ratio of metakaolin which is a well-known material used in the construction industries. Blending further improved the workability of the binary clay for encasing the mild steel for earthing applications.

The reason for better performance of various activated clays are as follows. By physical activation, coarser and foreign particles are removed by sieving. Further particle size distribution was maintained constant throughout by grounding.

The need for thermal activation arises due to the presence of unburnt particles such as carbon, sulfur and other organics are removed from the bulk. About 10% of the waste materials are burnt by the thermal activation method. Thermal activation further improved the uniformity of the clay particles and reduced thickness of the top surface layer (Chandrasekhar and Pramada, 1999; Mackenzie, 1970; Ma et al., 1995; Pietersen et al., 1990 ).

The chemical activation, further etching the top surface layer of the clay thereby larger particles are available for reaction which is diagrammatically represented in Fig. 11. The need for chemical activation of clay mainly involves the breaking of bonds and dissolution of the three-dimensional network structure of glass (Carlos Alberto Ríos Reyes et al., 2013) which in turn Na⁺ ion move closer to the center point of the crystal structure is illustrated in Fig. 12. It has also been reported that, when NaOH is present, the solubility of SiO₂ in clay markedly increases (Follett et al., 1965).

Diagrammatic representation of activation mechanism.

Crystal structure of chemically activated clay.

The improved corrosion resistance performance of activated clay was observed due to the combined interactive effect of the chemical and physical characteristics of clay.

4 Conclusions

Corrosion performance of MS and GI in binary clay (bentonite and metakaolin) was studied by electrochemical methods.
Physical, thermal and chemical activation methods were employed to improve the corrosion resistance property of binary clay. Among all, chemical activation method was found to be beneficial for mild steel in binary clay.
The conductivity of CAC was 204.7 mS/cm, pH was 12.58, and the particle size distribution was found to be 40–50 µm. Hence all these properties impart the better corrosion resistance and suitable for earthing applications.
Chemical activation of clay mainly involves the breaking of bonds and dissolution of the three-dimensional network structure of glass which in turn Na⁺ ion move closer to the center point of crystal structure and the solubility of SiO₂ in clay markedly increases.
Chemical activation was found effective in MS with improved corrosion resistance property.
Potential-time studies showed that GI loses its coating property within 10 days in all the three clays used. Hence galvanizing is not found suitable for protection of mild steel in clay environments.
The corrosion rate measured for MS in PAC, TAC and CAC extracts were found to be 3.8620 × 10⁻², 2.2820 × 10⁻² and 0.0522 × 10⁻² mmpy respectively. These data confirmed the fact that the activation process increased the corrosion resistance of clay for MS. The corrosion rate was significantly reduced (74 times and 44 times) in the case of CAC and TAC clay respectively when compared to PAC.
Earth excavation studies were conducted and monitored the corrosion of MS and GI using embedded MnO₂ sensor. This investigation puts forth a lot of scope for utilizing the chemically activated binary clay along with MS in earthing applications.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2015R1A5A1037548). One of the author (V.S) thanks the Director, CECRI, and CSIR for conceding the permission to pursue my fellowship at Hanyang University, Korea.

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[11] Dellisanti F., Valdrè G., . Study of structural properties of ion treated and mechanically deformed commercial bentonite. Appl. Clay Sci.. 2005;28:233-244.
[Google Scholar]

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[Google Scholar]

[21] IEC 62561-7, 2011. Lightning Protection System Components (LPSC) – Part 7: Requirements for Earthing Enhancing Compounds. (1st Ed.). Geneva, Switzerland.

[22] Ismail A.I.M., El-Shamy A.M., . Engineering behaviour of soil materials on the corrosion of mild steel. Appl. Clay Sci.. 2009;42:356-362.
[Google Scholar]

[23] El Hafid Kenza, Hajjaji Mohamed, El Hafid Hassan, . Influence of NaOH concentration on microstructure and properties of cured alkali-activated calcined clay. J. Build. Eng.. 2017;11:158-165.
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[Google Scholar]

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[Google Scholar]

[27] Macias A., Andrade C., . Corrosion of galvanized steel reinforcements in alkaline solutions: Part 1: Electrochemical results. British Corr. J.. 1987;22:113-118.
[Google Scholar]

[28] Mackenzie R.C., . Differential Thermal Analysis. London: Academic Press; 1970.

[29] Gougazeh Mousa, Buhl J.-C.H., . Synthesis and characterization of zeolite A by hydrothermal transformation of natural jordanian kaolin. J. Assoc. Arab Univ. Basic Appl. Sci.. 2014;15:35-42.
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[Google Scholar]

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[33] Saraswathy V., Song H.W., . Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete. Electrochim. Acta. 2006;51:4601-4611.
[Google Scholar]

[34] Schaschle E., Marsh G.A., . Some new views on soil corrosion. Mater. Protection. 1963;2:8-17.
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Studies on the development of activated binary clay and corrosion monitoring using embedded sensor

Abstract

Keywords

Bentonite

Metakaolin

Activated clay

Corrosion studies

Mild steel

Galvanized iron

1 Introduction

2 Experimental

2.1 Materials used

2.2 Methods of activation

2.2.1 Preparation of binary clay

2.2.2 Physical activation

2.2.3 Thermal activation

2.2.4 Chemical activation

2.3 Characterization of activated clay sample by X-ray diffraction studies and SEM

2.4 Evaluation of corrosion performance of activated clay samples

2.4.1 Preparation of clay extracts

2.4.2 Gravimetric weight loss method

2.4.3 Potential – time studies

2.4.4 Potentiodynamic polarization technique

2.5 Earth excavation studies using embedded MnO2 sensor

3 Results and discussion

3.1 Chemical analysis

3.2 Characterization of activated clay samples

3.2.1 X-ray diffraction studies

3.2.2 Scanning electron microscope (SEM)

3.3 Corrosion studies of activated clay samples

3.3.1 Weight loss method

3.3.2 Potential – Time behaviour

3.3.3 Potentiodynamic polarization technique

3.4 Earth excavation studies using embedded MnO2 sensor

3.5 Mechanism of activation of clay and improved corrosion resistance

4 Conclusions

Acknowledgements

References

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