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Original article
13 (
1
); 3301-3318
doi:
10.1016/j.arabjc.2018.11.005

Corrosion performance of mild steel and galvanized iron in clay soil environment

, ,
 Corrosion and Materials Protection Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630 003, Tamil Nadu, India

⁎Corresponding author.

Received: , Accepted: ,
© The Author(s) 2018
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

Clay is a naturally occurring material, and it has been utilized for many industrial processes. In this study, two types of clay namely white clay (WC) and black clay (BC) was used for this investigation. The corrosion performance of the clay samples was carried out using mild steel and galvanized pipes under a buried condition in the field and laboratory exposure studies were carried in the clay extracts. Generally mild steel (MS) and galvanized iron (G.I) are being used as an encasing material for earthing applications. In some places, the soil was enriched with acidic clay, and it causes corrosion of the MS and G.I and it needs periodic replacement of pipes. In this context, a detailed study has been carried out to evaluate the relative corrosion performance of the MS and G.I exposed to the clay environment. Electrochemical corrosion behavior of MS and G.I in clay medium were carried out by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The corrosion product formation was characterized by X-Ray Diffraction and Carbon Hydrogen Nitrogen Spectrum analysis. The surface morphology of MS and G.I after the exposure was characterized by Scanning Electron Microscopy (SEM) studies. The results proved that G.I in clay soil was severely affected by corrosion. The reason may be attributed to the insufficient thickness of the zinc coating and the presence of microbes which enhanced the G.I corrosion.

Keywords

Clay
Mild steel
Galvanized steel
Corrosion
Electrochemical studies
1

1 Introduction

The corrosion of ferrous metals in soil is a major problem in oil and water transported pipeline, storage tank, cooling water pipeline distribution systems (Cole and Marney, 2012). The rate of corrosion depends on external and internal factors. The external factors include temperature, humidity, soil salinity/pH/porosity existing in the soil environment etc. Internal factors include: presence of oxygen content, flow rate, pressure existing inside the pipe, type of the fluid/gas flowing, temperature, type of the material used (Verink and Heidersbach, 1972). Coating controls he external corrosion of oil-carrying pipeline, cathodic protection to avoid the effect of soil corrosion (Javaherdashti et al., 2013). The pipelines buried under soil are subjected to various forms of corrosion such as: localized, pitting, crevice and uniform corrosion (Li et al., 2012; Wu et al., 2013). It has been reported that the metallic materials are subjected to corrosion due to the soil microbial activity, and the corrosion behavior of soil depends on the presence of sulphate reducing bacteria. The bacterial species enhances the corrosion of pipeline material via either through reduction or oxidation of the metal; SRB species generates H2S which accelerates the corrosion of mild steel (Borenstein, 1994; Maslehuddin et al., 2007; El-Shamy et al., 2009; Choi et al., 2011, Tang et al., 2010; Das, 2014). Studies revealed that the presence of sulfur-oxidizing bacteria converts the sulphide to sulphate through sulphideoxidizing enzyme and the iron- and sulfur-oxidizing bacteria of the genus thiobacillus are active only under acidic conditions. It has been reported that the nature of soils is affected by moisture content, pH, resistivity, redox potential, chloride, and sulfate contents (EI-Shamy et al., 2015). The investigation of these soil parameters is considered as very important because the corrosive soil can cause failure of buried pipeline. The effect of moisture content acts as an electrolyte, which allows the ion transfer and reduction in resistivity of the soil by making the soil more corrosive (EI-Shamy et al., 2015). Galvanized products are widely used in transmission towers, telecommunication equipment, road safety and civil construction. In transmission tower footings, galvanized structures are under partially or wholly buried conditions. The corrosion of these structures depends on the aggressiveness of the soil environment (Padilla et al., 2013; Costa Pereira et al., 2015). The transmission tower structures consisting of mild steel and galvanized structures buried under the soil is susceptible to severe corrosion, and it needs periodic replacement of the materials. Hence, the present study focused on the clay soil, mild steel rod and galvanized pipes collected from the site and the evaluation was carried out in the laboratory under simulated conditions.

In the present study, the mild steel (MS) and galvanized iron (G.I) were exposed to two types of clay collected from the site and the performance evaluation was carried out in simulated soil extract medium and under the buried condition in clay soil medium. The relative corrosion performance is monitored through weight loss, open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. Many authors adopted advanced techniques to characterize the materials (Yan et al., 2016a,b, 2018). The clay was characterized by EDAX, and the corrosion products were analyzed through X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and carbon hydrogen nitrogen spectrum (CHNS). All the experiments were carried out at an exposure period of 90 days. The surface morphology before and after the exposure was characterized by scanning electron microscopy (SEM) and the results were discussed.

2

2 Materials and method

2.1

2.1 Materials used

The two clay samples available locally were used and it is named as white clay (WC) and black clay (BC) by the nature of the color. The photographic image of WC and BC samples were given in Fig. 1. The materials such as MS and G.I pipes are considered for this study. The nominal composition of MS and G.I used for the investigation is given in Table 1. The lump of clay samples collected from the site was powdered, and the fine particles passing through 45µ sieve was taken and dried in the oven for 24 h to get rid of moisture content and used for further evaluation studies. The chemical composition and physical properties of the two clay samples are given in Tables 2 and 3, respectively.

Photographic image of the clay (a). White clay and (b). Black clay.
Fig. 1 Photographic image of the clay (a). White clay and (b). Black clay.
Table 1 Nominal chemical composition of MS and G.I.
Material Chemical composition (wt%)
Fe C Mn Si P S V/Al
MS 98.06 0.09 1.53 0.29 0.013 0.002 0.02
G.I (240 g/cm2) Balance 0.40 0.05 0.40 0.045 0.02
Table 2 Chemical composition of the WC and BC.
Clay Chemical composition (wt%)
Al2O3 SiO2 Fe2O3 SO3 TiO2 K2O Na2O
WC 39.17 51.02 1.23 5.31 1.04 0.07 2.18
BC 32.17 54.02 2.42 6.38 0.59 2.90 1.52
Table 3 Physical properties of WC and BC.
S. no Properties White clay (WC) Black clay (BC)
1 Color of clay Yellowish white Brownish black
2 Color of extract Light yellowish Dark yellowish orange
3 pH 2. 30 2.07
4 Free chloride ppm – –
5 Sulphate content (%) 5.813 7.019
6 Moisture content (%) 9.23 21.60
7 LOI % 19.69 47.26
8 Conductivity (mS/cm) 6.58–6.4 7.2–7.4
9 Resistivity (Ohm-m) 1.519–1.506 1.388–1.351

2.2

2.2 Characterization technique

The chemical composition of WC and BC samples were identified by using EDAX - TESCAN Model VEGA-3. The clay samples and corrosion product of the MS and G.I specimens were retrieved from the clay soil after the exposure period and the composition/formation of corrosion products were analyzed using XRD PAN analytical X’PERT PRO system in Bragg-Brentano geometry using Cu Kα1 (1.540 Å) radiation. The powder diffraction covered the 10° < 2θ > 80°range with 0.0170° steps. Identification of peaks were made by employing the “Peak-search” and search match progress built in software (X-pert-High score plus). The CHNS analysis for clay and corrosion products formed on the surface of the MS and G.I was identified by CHNS analyzer and the percentage of sulphur content was calculated. The corrosion products were identified by X-ray photoelectron spectroscopy (XPS, Thermo, ESCALAB 250Xi, USA).

2.3

2.3 Total viable bacteria count

The clay samples are collected from the earth pit, and the bacterial count of the samples are estimated as per the procedure mentioned below: The collected samples were stored in an ice box and transported to the laboratory. The sample preparation and enumeration of bacterial species were done by pour plate technique (Selvam et al., 2012). 1 g of soil sample was dispersed with 99 ml of sterile deionized water. It was subjected to serial dilution and plated by using nutrient agar, Mn agar base, isolation medium for iron bacteria, Thiobacillus agar by pour plate method and for sulphate reducing bacteria (SRB) medium by using most probable number technique. The plates are incubated at 37 °C for 24–48 h and 22 days for SRB. The total viable bacterial counts were enumerated, and the bacterial population is expressed as colony forming units per gram (CFU/g).

2.4

2.4 Corrosion studies

2.4.1

2.4.1 Preparation of the clay extract and metal sample preparation – Laboratory study

MS and G.I specimens of 1 cm2 size was cut and the wire was soldered and sealed with epoxy for electrochemical studies. 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 were cleaned with acetone without damaging the galvanizing coating on the surface. Both WC and BC samples were crushed and sieved through 45µ sieve to remove the coarse particles. The extracts were prepared by the sieved clay powder with distilled water in 1:1 ratio and shaken well in a mechanical shaker for 1 h. Then the extracts were collected by filtration method and used for immersion studies. The pH of the WC and BC extracts were 2.03 and 2.07 respectively.

2.4.2

2.4.2 Earth excavation studies and metal sample preparation – Field study

Earth excavation studies were carried out by using polished MS and surface cleaned G.I specimens packed in the respective clay (WC and BC) and excavated in the earth to a depth of 1 m below ground level. The soil was initially packed under 100% saturated condition. The whole system was subjected to natural weathering conditions for an exposure period of 90 days.

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 excavation studies. A grove is made, and the 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. Both the ends of the MS and GI were also sealed with epoxy. Duplicate specimens were used for this study. The photographic image and the schematic diagram of the earth excavation studies were given in Fig. 2.

Earth excavation studies (a). Photographic image of WC and BC and (b). Schematic diagram.
Fig. 2 Earth excavation studies (a). Photographic image of WC and BC and (b). Schematic diagram.

2.5

2.5 Electrochemical studies

Open circuit potential measurements were made for MS and G.I in clay extract medium and earth excavation medium. But the EIS and potentiodynamic polarization studies were carried out for the MS and G.I in extract medium only.

2.5.1

2.5.1 Open circuit potential (OCP) measurements

The OCP of MS and G.I specimens exposed to WC and BC extracts and earth excavation studies are monitored as per ASTM C 876 at regular intervals and potential vs. time graph was plotted for 90 days exposure period. The saturated calomel electrode (SCE) is used for this measurement.

2.5.2

2.5.2 Electrochemical impedance spectroscopy (EIS)

EIS measurements were carried out for the MS and G.I in WC and BC extracts using Gill A.C. Instruments, UK. The measurement was made using the three-electrode cell assembly at a temperature of 30 ± 2 °C. Three electrode cell assemblies were used for carrying out the experiments. The working electrode was MS/G.I of 1 cm2 area and the rest of the area is covered with lacquer. A platinum foil is used as a counter electrode, SCE as a reference electrode and the both clay extract were used as an electrolyte. Time intervals of 30–45 min. was given for each of the systems to attain a steady state i.e. until it reaches the stable potential and a typical graph between the real part (Zr′) and imaginary part (−Zi″) was obtained for various frequencies ranging from 30 kHz to 10 mHz. This instrument itself is having the provisions for programs to evaluate the impedance parameters values such as (Icorr, Rct, and Cdl) by curve fitting method.

2.5.3

2.5.3 Potentiodynamic polarization

The similar electrochemical setup and instrument mentioned above were used for carrying potentiodynamic polarization measurement, using which anodic and cathodic polarization curves were recorded. This instrument itself is having the provision for programs to evaluate the corrosion kinetic parameters such as Icorr, Ecorr, ba and bc, where ba and bc are the anodic and cathodic slope of the polarization curve. The potentiodynamic condition corresponds to a potential sweep rate of 0.1 mV sec−1 and the potential ranges of −200 mV to +200 mV from the OCP. All the experiments were carried out at a constant temperature of 30 ± 2 °C. Corrosion current density was obtained from Rp and the slope of polarization curve is obtained by using the relation mentioned elsewhere (Saraswathy and Song, 2007a,b).

2.6

2.6 Weight loss measurements

2.6.1

2.6.1 In clay extract-laboratory study

Circular MS and G.I pipe (electrodes) was cut from the bulk specimens. The size of the sample taken was about 30 mm dia. and 25 mm (MS), 38 mm inner dia., 42 mm outer dia., with 25 mm length (G.I) for weight loss measurements. The MS was given mechanical polishing and degreased with acetone before use. G.I. was just degreased with acetone. Then both the MS and G.I specimens were initially weighed and kept immersed in clay extracts. The weight loss experiment was carried out for 30 and 90 days at room temperature. At the end of the exposure period the specimens were taken out and visual observation was made. Then the specimens were washed with distilled water, dried and final weights are taken. From the loss in weight, corrosion rate was calculated using the relationship mentioned elsewhere (Saraswathy and Song, 2007a,b).

2.6.2

2.6.2 Earth excavation – Field study

Both MS and G.I specimens were weighed before excavation for corrosion rate measurements. At the end of the exposure period, the specimens were taken out, and visual observation was made. Then the specimens were washed with distilled water, dried and final weights of the specimens are taken. From the loss in weight corrosion rate was calculated using the relationship mentioned elsewhere (Saraswathy and Song, 2007a,b).

2.7

2.7 Surface examination of MS and G.I

2.7.1

2.7.1 Visual examination

After the exposure period, MS and G.I specimens were examined visually for corrosion products like red rust, pits formed on the surface was identified through a magnifying lens.

2.7.2

2.7.2 Scanning electron microscopy (SEM) analysis

SEM images were obtained using HITACHI Model S-3000H at various magnifications to study the surface morphology of the MS and G.I in both the clay medium after the exposure period.

3

3 Results and discussions

3.1

3.1 Characterization technique

3.1.1

3.1.1 EDAX

Figs. 3 and 4 shows the EDAX spectra of the WC and BC samples. The spectra of both the clay samples contain peaks corresponding to compositional elements such as Ca, C, O, N, Fe, S, Si, Al. From the weight percentage, it is evident that sulphur content of 13.07 and 12.48% are observed in the WC and BC. WC and BC have the Fe content of 12.03 and 23.93% respectively. The iron oxide present in the clay produces certain definite colour dependent on the amount of oxide present. The colouring effect arises due to the mixed presence of carbon and iron found in the clay (Goldberg, 1989). When compared to WC, BC has approximately two times higher percentage of Fe content and the carbon content is also slightly higher (0.7%) than the WC. Due to the presence of high iron content and carbon content the colour of the clay sample is black in colour. In the WC, the presence of alumina and silicon is evidenced from Fig. 3 which gives the white colour of the sample (Goldberg, 1989), whereas Fig. 4 has no proof of silicon. There is a small amount of calcium (1.17% and 1.68%) present in WC and BC and a high degree of sulphate content is evidenced from the EDAX spectrum of both the clay samples, which is a prominent causative factor for accelerated corrosion damage under acidic conditions. This observation was complemented by the CHNS and XRD studies.

EDAX result of WC.
Fig. 3 EDAX result of WC.
EDAX result of BC.
Fig. 4 EDAX result of BC.

3.1.2

3.1.2 Total viable bacteria count

The bacterial counts of the clay samples were present in the range from 102 to 106 CFU/g of soil which is given in Table 4. The higher bacterial counts of acid producing bacteria (sulphur oxidizing) and sulphate reducing bacteria are noticed from the counts. It favoured the production of sulphuric acid by Thiobacillus species and enhanced the sulphate production in both the clay samples. This sulphate ion was utilized by the sulphate reducing bacteria to generate H2S in the clay which made the clay, more acidic.

Table 4 Total viable bacterial counts of WC and BC.
Clay Total viable bacterial counts (CFU/g)
HB IOB MnOB APB SRB
WC 3.2 × 102 3.5 × 104 2.2 × 102 4.3 × 106 1.0 × 102
BC 1.2 × 102 4.5 × 103 2.6 × 104 6.5 × 104 1.2 × 102

*Note: CFU/g-Colony forming units (viable cells), HB - Heterotrophic bacteria, IOB-Iron oxidizing bacteria, MnOB- Manganese oxidizing bacteria, APB-Acid producing bacteria (Thiobacillus sp), SRB- Sulphate reducing bacteria.

3.2

3.2 Electrochemical studies

3.2.1

3.2.1 Open circuit potential

Open circuit potential of MS and G.I in WC and BC extracts (Fig. 5) studies were monitored for an exposure period of 90 days. Initially MS (Fig. 5a) showed an average potential of −450 mV vs. SCE in both the WC and BC extracts. After 15 days of exposure the potential was shifted to the active direction of −570 mV. Further at the 30th day again the potential was shifted to more active potential of −650 mV. After the 30th day the potential of MS gets stabilized and it remains same up to 90 days. It indicates, corrosion was initiated within a week and after that MS was entirely corroded and showed a more negative potential up to the end of the exposure period. Fig. 5b indicates that the potential of G.I is −1050 mV at the initial stage. After an exposure period of 15 days, the potential was shifted to -1000 mV. After 30 days, the potential was shifted to −750 mV. This potential shift was due to the dissolution of the zinc coating in the organic nature of the soil. After the 30th day, the G.I has shown a stabilized potential of −650 mV and it remains saturated until the 90th day. G.I was coated with zinc to protect mild steel and it has shown the potential of −1050 mV vs. SCE during the initial period. After that, the entire zinc layer was dissolved and reached the steel potential of −650 mV in both the clay extracts. This is due to the destruction of the coating layer accompanied by the rapid corrosion of the underlying steel in the organic nature of the clay soil.

Potential-time behavior of MS and G.I in clay extract (a) WC and (b) BC.
Fig. 5 Potential-time behavior of MS and G.I in clay extract (a) WC and (b) BC.

Fig. 6a and b depicts the potential-time behavior of MS and G.I exposed to WC and BC clay earth medium. From the Fig. 6a, it is observed that within 15 days of exposure the potential of M.S was shifted to the more active direction (−670 mV) and it reached the steady state potential and remains stable until the end of the exposure period. G.I has initially shown a potential of −980 mV and suddenly the potential was shifted to −725 mV within 6 days and during the 9th day it reached the potential of −650 mV and after that, the potential remains stable until the end of the exposure period. From Fig. 6b it is evident that M.S has shown an initial potential of −520 mV vs. SCE and after 15 days of exposure the potential is shifted to the active direction of −650 mV and after that it remains stable until the end of the exposure period in the BC clay medium. The same trend was observed for G.I. also. Initially G.I has shown a potential of −990 mV vs. SCE and 2nd day the potential is shifted to −800 mV and from 5th day onwards the potential was shifted to −660 mV and it remains stable until the end of the exposure period. It is observed that the clay medium has shifted the potential of MS and G.I to the active direction at a faster rate than the extract medium. This is due to the presence of microbes present in the organic soil which reacts with both MS and G.I and accelerated the corrosion. The same observation was noticed by Denison and Romanoff (1952). From the results, it is seen that G.I was corroding at a faster rate (by exposing the base metal) than M.S in both the clay environments due to the insufficient zinc coating thickness provided on the G.I. metal surface (Denison and Romanoff, 1952).

Potential-time behavior of MS and G.I in earth medium (a).WC and (b). BC.
Fig. 6 Potential-time behavior of MS and G.I in earth medium (a).WC and (b). BC.

3.2.2

3.2.2 Electrochemical impedance spectroscopy

EIS measurements for the MS and G.I were carried out in WC and BC extract media. Fig. 7a and b depict the Nyquist plots taken for MS and G.I during the 1st day of exposure and Fig. 7a and b after 90 days of exposure respectively. The impedance parameters derived from the curves were presented in Table 5. In WC extract, the Icorr value of MS and G.I during the 1st day of exposure was found to be 0.174 and 0.446 mA/cm2 respectively. After 90 days of exposure Icorr value was increased to 0.500 and 1.121 mA/cm2 for MS and G.I respectively. Rct values observed for MS and G.I. during the initial exposure was 149 and 58 Ω cm2 and after 90 days the Rct value was found to be 52 and 23 Ω cm2 respectively. The corrosion rate measured for MS and G.I was 2.025 and 7.780 mmpy during the initial days of exposure and after 90 days of exposure the corrosion rate values are increased to 5.802 and 12.990 mmpy for MS and G.I respectively.

Nyquist plots for MS and G.I in WC and BC extract medium (a). MS-1st day of exposure; (b). G.I-1st day of exposure; (c).MS- after 90 days of exposure; (d). G.I- after 90 days of exposure.
Fig. 7 Nyquist plots for MS and G.I in WC and BC extract medium (a). MS-1st day of exposure; (b). G.I-1st day of exposure; (c).MS- after 90 days of exposure; (d). G.I- after 90 days of exposure.
Table 5 AC-impedance parameters for MS and G.I in clay extract.
Clay Material Initial exposure (1st day) After 90 days of exposure
Rct (Ω cm2) Icorr (mA cm−2) Corrosion rate (mmpy) Rct (Ω cm2) Icorr (mA cm−2) Corrosion rate (mmpy)
WC MS 149 0.174 2.025 52 0.500 5.802
G.I 58 0.446 7.780 23 1.121 12.990
BC MS 80 0.326 3.779 26 0.985 11.430
G.I 45 0.696 6.601 21 1.220 14.140

Whereas in BC extract the corrosion rate for MS and G.I was observed to be 3.779 and 6.601 mmpy during the initial exposure and it was increased to 11.43 and 14.14 mmpy after 90 days of exposure respectively. Rct values found for the MS and G.I during the initial period of exposure was 80 and 45 Ω cm2 and it was reduced to 26 and 21 Ω cm2 after 90 days of exposure respectively. The Icorr values measured for MS and G.I was 0.326, 0.696 mA/cm2 and 0.985 and 1.22 mA/cm2 during the initial period and after 90 days of exposure respectively. An appreciable decrease in Rct values and increase in Icorr values indicates the severity of corrosion of both the metals in the clay extracts. The capacitive loop formed at the high frequency shows the resistance and capacitance generated by the penetration of ions into the metal surface forming a corrosion product on the surface of the metal (Tang and Davenport, 2007; Okeoma et al., 2012).

3.2.3

3.2.3 Pontentiodynamic polarization

Potentiodynamic polarization measurements carried out for the MS and G.I steel in WC and BC clay extract medium. Fig. 8 depicts the typical polarization curves for MS and G.I in both the clay extracts during the initial exposure (Fig. 8a–b) and (Fig. 8c–d) after 90 days of exposure. The corrosion kinetic parameters obtained from the curves were given in Table 6. It can be seen that the corrosion rate increases with an increase in the exposure period. Initially G.I was showing a potential of −982 and −1008 mV vs. SCE in WC and BC environment respectively. But after 90 days of exposure G.I was showing a potential of −640 and −656 mV vs. SCE in WC and BC environment, which indicated that galvanized coating was dissolved completely in the exposed environment and the bare metal was exposed showing the potential of MS. The same observation was noticed in the OCP measurements also. Further, the corrosion rate obtained was higher than that of the weight loss measurements. This is visually seen in Fig. 9 that; severe corrosion product is observed on the surface of MS and G.I specimens. It indicates that a significant amount of metal is lost at the end of the exposure period. When comparing the performance of MS and G.I, G.I has corroded at a faster rate than MS due to the dissolution of the zinc layer in both (WC and BC) clay extract media (Denison and Romanoff, 1952).

Potentiodynamic polarisation curves for MS and G.I in WC and BC extract medium (a). MS-1st day of exposure; (b). G.I-1st day of exposure (c). MS- after 90 days of exposure; (d). G.I- after 90 days of exposure.
Fig. 8 Potentiodynamic polarisation curves for MS and G.I in WC and BC extract medium (a). MS-1st day of exposure; (b). G.I-1st day of exposure (c). MS- after 90 days of exposure; (d). G.I- after 90 days of exposure.
Table 6 Polarization parameters for MS and G.I in clay extract.
Clay Material Initial exposure (1st day) After 90 days of exposure
Ecorr (mV vs. SCE) Icorr (mA/cm−2) Corrosion rate (mmpy) Ecorr (mV vs. SCE) Icorr (mA/cm2) Corrosion rate (mmpy)
WC MS −534 0.004 0.054 −652 0.122 1.141
G.I −982 0.010 0.117 −640 0.343 3.986
BC MS −624 0.072 0.840 −652 0.213 2.478
G.I −1008 0.105 1.227 −656 0.400 4.637
Photographic image of MS (a&c) and G.I (d&b) specimens after electrochemical studies.
Fig. 9 Photographic image of MS (a&c) and G.I (d&b) specimens after electrochemical studies.

3.3

3.3 Characterization of MS and G.I

3.3.1

3.3.1 XRD

The XRD pattern of both the clay samples and corrosion products of are presented in Figs. 10 and 11. It can be seen that, the XRD peaks for the clay samples are similar, with peaks at 12.03°, 20.18°, 24.8°,26.9°, 34.5°, 35.4° and 38.5° corresponding to the crystal planes of zeolite, SiO2, Fe4(CO)13C, kaolinite, FeSO4, FeS2 and CaCO3 respectively. The presence of FeSO4, FeS2 and CaCO3 crystal phases is a causative factor for accelerated corrosion. Fe2O3, FeO, FeSO4, Fe2S and Fe(OH)3 are noticed on the electrode surface of mild steel exposed to WC and BC medium. In the case of G.I, it is observed that the increase in iron oxide peak at 26.9° and zinc oxide peaks at, 45.7° and 50.2° respectively. G.I pipes are coated with zinc to protect mild steel in the aqueous environment. The zinc coating gets dissolved when it was exposed to the acidic environment over a longer duration. The dissolution of zinc coating increased the corrosion of GI in acidic (organic) soil environment (Denison and Romanoff, 1952).

XRD pattern for: (a) WC; (b) corrosion products of MS; (c) G.I.
Fig. 10 XRD pattern for: (a) WC; (b) corrosion products of MS; (c) G.I.
XRD pattern for: (a) BC; (b) corrosion products of MS; (c) G.I.
Fig. 11 XRD pattern for: (a) BC; (b) corrosion products of MS; (c) G.I.

3.3.2

3.3.2 CHNS analyzer

CHNS analysis is conducted in to compare the sulphate content present in the WC, BC and MS and G.I electrode surface and the data are presented in Fig. 12. It is seen from the figure that the sulphur content found in the corrosion products of MS and G.I were 10.5 and 11.6% and the sulphur content present in the WC was 4.8% respectively. The sulphur content found in MS and G.I in BC is found to be 12.5% and 13.5% respectively. The presence of sulphur content in both MS and G.I are higher than the WC and BC. The sulphate ions reacted with the metal iron and thereby plenty of corrosion products is deposited on MS and G.I pipes. Moreover, the presence of Thiobacillus species enhanced the sulphuric acid and sulphate formation on the electrode surface. Similar results were observed in BC also. When comparing the two clay samples, the sulphate content was higher in BC than WC. When comparing MS and G.I, G.I is having higher sulphate content.

CHNS analysis of WC, BC and corrosion products of MS and G.I.
Fig. 12 CHNS analysis of WC, BC and corrosion products of MS and G.I.

3.4

3.4 Weight loss measurements

3.4.1

3.4.1 In clay extract

The corrosion rate of MS and G.I pipe in WC and BC extracts for the exposure period of 30 and 90 days were given in Table 7. From the Table 7, it was observed that the extent of corrosion is more in the case of G.I when compared to MS. It is noted that an average of 10% increase in corrosion rate is observed in both the clay extracts after 90 days of exposure due to the corrosion process. This was supported by Fig. 13 that, the surface of MS and G.I was damaged due to the severe corrosion and severe rust products are observed on the surface of MS and G.I after 90 days of exposure.

Table 7 Corrosion rate of MS and G.I in WC and BC extracts by weight loss measurements.
Clay Steel Corrosion rate (mmpy)
30 days of exposure 90 days of exposure
WC MS 0.394 0.410
G.I 0.417 0.468
BC MS 0.479 0.487
G.I 0.569 0.579
Photographic image of MS (a-c) and G.I (d-f) before and after exposure in clay extract.
Fig. 13 Photographic image of MS (a-c) and G.I (d-f) before and after exposure in clay extract.

3.4.2

3.4.2 In clay medium – Field exposure studies

Table 8 shows the corrosion rate of MS and G.I specimens buried in clay medium for an exposure period of 90 days. G.I specimen showed more weight loss than MS. The corrosion rate of MS is 0.538 and 0.761 mmpy and the corrosion rate of G.I is 0.5870 and 1.130 for WC and BC respectively.

Table 8 Corrosion rates of MS and G.I in WC and BC exposed to earth excavation studies by weight loss measurements.
Clay Steel Exposure period (90 days)
Weight loss (g) Corrosion rate (mmpy)
WC MS 2.260 0.538
G.I 4.310 0.587
BC MS 3.350 0.761
G.I 4.680 1.130

3.5

3.5 Surface examination studies

3.5.1

3.5.1 Visual observation

Fig. 13 shows the MS and G.I samples before and after 90 days of exposure in WC and BC extract medium. Both the MS and G.I samples are fully covered with severe corrosion products on the exposed surface. When comparing both the clays, the samples exposed to BC was severely attacked and showing reddish brown corrosion products, and in MS samples blistering and severe corrosion products are visible on the exposed surface. In G.I the galvanizing coating is completely disappeared, and the exposed surface is completely changed into brownish red colour due to the reaction with the acidic nature of the extract and the base metal was exposed.

Fig. 14 shows the MS and G.I to WC and BC after 90 days of exposure under buried condition. From the figure, it is observed that, G.I was severely affected by the bulk volume of corrosion products (ZnO) which was well evidenced by the XRD results. The galvanized coating was completely eaten away by the acid present in the clay. In the case of MS also the surface of the samples was entirely covered with blackish brown Fe(OH)3, Fe2O3 and Fe3O4 rust products.

Photographic images of MS (a&b) and G.I (c&d) after exposure in earth excavation medium.
Fig. 14 Photographic images of MS (a&b) and G.I (c&d) after exposure in earth excavation medium.

3.5.2

3.5.2 SEM analysis

The SEM microstructure of bare MS and G.I surface before the exposure is given in Fig. 15a and b. It shows the smooth surface of MS and G.I, indicating there is no corrosion formed on the surface of MS and G.I. The Fig. 16a and c shows the SEM images at various magnifications (500×, 1.0 k and 5.0 k) of MS surface in WC after the exposure period of 90 days. From the microstructure, it is seen that a rough surface is observed with severe corrosion spots in different shapes is exhibited in Fig. 16a. It is due to the acidic nature of the WC which leads to the formation of corrosion products on MS surface. The corrosion products are evidenced from Fig. 16b. A closer view of a corrosion product is magnified and depicted in Fig. 16c. From Fig. 16c it can be observed that the spot was filled with spongy like crystalline layers of corrosion products on the surface of the MS.

SEM micrographs of polished (a). MS and (b). G.I.
Fig. 15 SEM micrographs of polished (a). MS and (b). G.I.
SEM micrographs-after 90 days of exposure in WC medium (a–c). MS and (d–f). G.I.
Fig. 16 SEM micrographs-after 90 days of exposure in WC medium (a–c). MS and (d–f). G.I.

Fig. 16d–f shows the SEM microstructure of G.I surface at various magnifications in WC clay after the exposure period of 90 days. From the SEM image, it is observed that severe deterioration of the smoothness of the G.I surface is noticed. This may be due to the reason that the acidic nature of WC which disintegrated the coating, and the top layer of the coating gets completely damaged forming the corrosion products on the surface of the steel. Fig. 16d shows the deposits of white rust products (ZnO) is spread over the surface due to the aggressive attacks of the corroding medium on the G.I surface. After increasing the magnification (Fig. 16e–f), the zinc metal was partially dissolved to form zinc oxide. The dissolution of zinc oxide created micro voids and micro cracks in the intergranular zones due to the insufficient coating thickness and poor adhesion of the coating (Okeoma et al., 2012).

Fig. 17a–c shows SEM microstructure of MS surface in BC at various magnifications after 90 days exposure period. It is seen that the unusual structure of iron corrosion formation on the MS surface along with many pits with different shapes are observed in Fig. 17a. When increasing the magnification of a pit, irregular and rod-shaped microorganisms, as well as corrosion products, are found in the pit which is clearly evidenced from Fig. 17b–c (De Lima-Neto et al., 2007). Fig. 17 d-f show the SEM microstructure of G.I in BC at various magnifications after the exposure period. From the Fig. 17d, unusual delaminated deposits of corrosion products are observed on the surface of G.I. Severe corrosion product and cracks are visible on the surface of G.I. With increasing magnification, GI surface has shown a cracked zinc layer and symptoms of intergranular corrosion running along the grain boundaries observed from Fig. 17e–f (Urgell et al., 2007; Srivastava and Balasubramaniam, 2005).

SEM micrographs - after 90 days of exposure in BC medium (a–c). MS and (d–f). G.I.
Fig. 17 SEM micrographs - after 90 days of exposure in BC medium (a–c). MS and (d–f). G.I.

3.5.3

3.5.3 XPS analysis

XPS spectra of corrosion products are presented in Fig. 18a–b. From this figure the C1s, Fe2p and O1s were observed in MS and G.I electrode surface, while a new peak of Zn2p (1021 eV) was detected on G.I electrode surface, as shown in Fig. 18b. These results confirmed the rapid dissolution of zinc in a clay medium over a longer duration (Denison and Romanoff, 1952).

XPS survey spectra for corrosion products of (a). MS and (b). G.I.
Fig. 18 XPS survey spectra for corrosion products of (a). MS and (b). G.I.

Figs. 19 and 20 illustrate the high-resolution XPS spectra of C1s, Fe2p and O1s obtained from the corrosion products of MS and G.I electrode surface, respectively. It is seen from Fig. 19a and 20a that the C1s spectra is composed of two peaks at 284.8 eV and 288.97 eV, which is corresponds to C—C and C—O species respectively (Angellier et al., 2005; Rindlav-Westling and Gatenholm, 2003), which may be present in clay. Fe2p spectra of corrosion products of MS and G.I on the electrode surface are shown in Fig. 19b and 20b, respectively. The high-resolution spectra of Fe2p peaks at 711.9 eV (Fe2p3/2) and 724.11 eV (Fe2p1/2) may be associated with FeOOH and Fe2O3, respectively (Zhou et al., 2011; Graat and Somers, 1996). The peaks of Fe2p1/2 was detected at about 727.42 eV indicating the formation of mixed oxides of Fe(II) and Fe(III), such as Fe3O4. The peaks of 714 eV further confirm the formation of Fe3O4 (Changjing et al., 2014). Further, the peaks at 719.1 eV and 732.26 eV were corresponding to shake up satellite features of Fe2p3/2 and Fe2p1/2 respectively (Sharan et al., 2015). Fig. 19c shows the O1s peaks of MS electrode surface, it is composed of the two peaks at 529.78 eV and 532.08 eV; the first peak is corresponding to the binding energy of the O2− and OH− ions, which is in agreement with the presence of the iron oxide/hydroxide layer as detected in the Fe2p3/2 spectrum (Ochoa et al., 2013). The signals at 532.08 also corresponds to C—O and O—C—O species (Liu et al., 2015), which may be present in the clay.

XPS spectra for corrosion products of MS (a). C1s; (b). Fe2p and (c). O1s.
Fig. 19 XPS spectra for corrosion products of MS (a). C1s; (b). Fe2p and (c). O1s.
XPS spectra for corrosion products of G.I (a). C1s; (b). Fe2p and (c). O1s & Zn2p.
Fig. 20 XPS spectra for corrosion products of G.I (a). C1s; (b). Fe2p and (c). O1s & Zn2p.

Further, in Fig. 20d shows the Zn2p spectra of the corrosion product of G.I surface. The two peaks at about 1021 eV and 1023.08 eV, which is corresponding to the Zn and ZnO respectively (Zhang et al., 2017). These results confirm the fact that dissolution of zinc coating, when exposed to the acidic environment over a longer duration. Fig. 20c shows the O1s spectra for corrosion product of G.I electrode, these three peaks at the binding energy of 529.92 eV, 531.32 eV and 532.43 can be attributed to FeO or Fe2O3 or ZnO, FeOOH or ZnOOH respectively (Zhang et al., 2017). XPS spectra confirmed the oxidation state of corrosion products namely FeO, Fe2O3, FeOOH and ZnO.

3.6

3.6 Mechanism

The following mechanism for the corrosion of MS and GI pipes in buried clay soil environment is as follows:

Fig. 21 shows the graphical representation, which depicts the corrosion damage of MS and GI pipe (zinc coated mild steel) in acidic clay soil. The compounds present in the soil such as FeS2, Fe2SO4 and CaCO3, etc. are converted to sulfuric acid and sulphate by Thiobacillus species such as T. ferrooxidans and T. thiooxidans. thiobacillus is a gram-negative bacterium generally gains energy from the oxidation of reduced sulfur compounds present in the soil environment. T. ferrooxidans is an autotrophic bacteria and it is more active under the pH 1.5–2.5, and temperature 15–25 °C. This type of bacteria derives energy from the oxidation of ferrous to ferric ion and reduces sulphur compounds to sulphuric acid. T. thiooxidans is a mesophilic bacteria which grows in moderate temperature of 20–45 °C, and it is active at a pH of 0.5–6.0. It uses sulphur as its primary energy source, and both the bacteria are abundant in natural environments associated with pyritic ore bodies, commonly found in clay soil, sewer pipes, coal deposits, and their acidified drainages (Al-Judaibi and Al-Moubaraki, 2013, Ozgowicz et al., 2012; Rohwerder et al., 2003; Gonzalez-Toril et al., 2003).

Graphical abstract for corrosion of MS in clay medium.
Fig. 21 Graphical abstract for corrosion of MS in clay medium.

The following reactions take place in MS and G.I pipes during the earth excavation studies. The ferric ion reacts with the FeS2 (pyrite) leading to the dissolution of ferrous iron. This is further oxidized by ferric iron to produce sulphate. The reaction generates more acid (Davis et al., 2000; Schippers and Sand, 1990; Steudel, 1996; Hackl et al., 1995; Hansford and Vargas, 2001).

(1)
F e S 2 + 6 F e 3 + + 3 H 2 O → 7 F e 2 + + S 2 O 3 2 - + 6 H +
(2)
S 2 O 3 2 - + 8 F e 3 + + 5 H 2 O → 8 F e 2 + + 2 S O 4 2 - + 10 H +

The dissolution of pyrite by ferric iron along with oxidation of the ferrous iron constitutes a cycle of dissolution of pyrite. Ferric iron precipitates as hydrated iron oxide as follows:

(3)
F e 3 + + 3 H 2 O ↔ F e ( O H ) 3 + 3 H +
(4)
F e ( O H ) 3 → F e 2 O 3

Fe(OH)3 precipitate leads to the formation of yellow, orange or red deposits on the surface (Figs. 9 and 13) of the MS and GI. It further oxidized to form Fe2O3 (brown rust).

4

4 Conclusions

The following conclusions can be drawn from the present investigation:

  • The relative corrosion performance of MS and G.I was carried out in WC and BC.

  • Chemical analysis confirmed that both the clays are acidic in nature. The acidic clays were often associated with high levels of organic matter.

  • Weight loss measurements indicated that G.I has corroded at a faster rate than MS in both the clay extracts due to the rapid dissolution of the coating which is due to the reduced thickness of the zinc coating layer.

  • Earth excavation studies showed that the presence of Thiobacillus bacteria induced the acidic nature of the soil and enhanced the corrosion of MS and GI pipes.

  • Open circuit potential measurements revealed that the entire zinc coating was dissolved in G.I within 10 days of exposure and showed a mild steel potential of -650 mV in both the clay extracts. The destruction of the coating was accompanied by a rapid corrosion of the underlying steel due to the lesser coating thickness.

  • Polarization measurements indicated severe corrosion products on the surface of MS and G.I specimens at the end of the exposure period showing the significant amount of metal loss.

  • An appreciable decrease in Rct values and increase in Icorr values indicated the severity of corrosion of metals (MS and G.I) in both the clay extracts.

  • The XRD and CHNS results showed that the sulphur content present in corrosion products MS and G.I are found to be higher than the WC and BC.

  • G.I used in this study was having a coating weight of (240 g/cm2). Due to the lesser coating weight G.I corroded at a faster rate than MS in both WC and BC. This is to the insufficient coating thickness given to the G.I. The minimum layer thickness required as per ASTM A 123 is 35µ micron (245 g/cm2) minimum and a maximum of 100 µ (705 g/cm2) is required for underground buried structures.

  • From the study, it is further concluded that, if the structure was exposed to the aggressive environment, some precautionary measures should be undertaken to ensure the coating thickness of the zinc layer.

Acknowledgements

Authors thank Director, CSIR-CECRI, Karaikudi, India for his kind permission to publish this paper.

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