A Classification Approach for Corrosion Rating of Soil to Buried Water Pipelines: A Case Study in Budhanilkantha-Maharajganj Roadway Areas of Nepal

The present study was focused to estimate six characteristics of soil specimens taken from 0.3 to 1.5 m sampling depth of six sampling sites of Budhanilkantha-Maharajganj roadsides using standard methods, and was assessed their corrosive nature to the buried-metallic pipelines using an empirical corrosion rating model. The estimated soil pH, moisture, resistivity, redox potential (ORP), chloride and sulfate ions were 6.4-7.9, 7-45%, 4.5 × 103-45.5 × 103 Ohm. cm, 317-514 mV (SHE), 12-86 ppm, and 40-294 ppm, respectively, in all the soil sample specimens. The experimental results indicated that the soils could be rated as mildly corrosive to less corrosive groups to the buried galvanized-steel and cast iron pipes in the study areas. A good positive or negative correlation coefficient between resistivity, moisture, chloride and sulfate contents implies that these soil parameters have an equal contribution to the rating of soil corrosivity. A polyethylene-sheet wrapping (i.e., encasement) around the galvanized-steel and cast iron water pipelines or the use of non-conducting materials of gravel/sand around the burying ground could be sufficient for the extension of their life up to 50 years or more. The empirical model is successfully applied for the corrosion rating of soil samples and could be progressive in the future for soil corrosion rating of soils to the underground waterworks. Present findings would be insightful and suggestive in making the corrosive land maps of the studied areas which would be helpful for the potable water pipeline works in other urban areas of Nepal.


Introduction
A notable impact and consequences in the maintenance and operations of any buried-pipelines from soil corrosion is noticed world widely, mostly by parameters of disturbed-soils [1]. It is presumed that the undisturbed-soil is a less corrosive nature than the disturbed ones [2]. Some of the key factors which affect such corrosion by the disturbed-soils to the buried-metallic pipelines to supply city water are moisture-holding capacity, acidity/alkalinity, electrical conductivity/resistivity and potential, salt ions concentration, aeration [3][4][5][6][7][8][9][10][11], and other factors like soil depth, topography, etc [12,13]. However, an estimation of soil corrosivity extent is often more difficult to classify as a concern to adjoining soil properties and materials of the pipelines, mostly due to its exceptionally high heterogeneity and localized complexity [14]. About a 27-30% increase in water pipeline damaged by aggressive soil factors is reported in the USA and Canada alone [15]. Moreover, about 10% of the annual corrosion cost in Australia is covered by buried-metallic pipelines corrosion [16]. The pipeline failures, mostly due to corrosive soil environments, are still growing at an alarming rate even with the applications of different corrosion protection techniques [17][18][19].
Drinking water is mostly supplied through the galvanizedsteel or/and cast iron pipelines in Nepal for a long period [20][21][22], even though a very few are practiced to use of high-density plastic (HDP) pipelines in recent years. A damage of such buried-metallic pipelines causes a high degree of economic and environmental consequences, especially in urban areas of Kathmandu valley. Therefore, it is significant to understand A Case Study in Budhanilkantha-Maharajganj Roadway Areas of Nepal accurately the corrosive nature of each soil factors causing the corrosion of such buried pipes and hence possible to apply an appropriate corrosion control techniques before installation of new water supply metallic-pipelines which can safely operate for a long time, i.e., a reliable 50-year service life.
Some of the widely accepted and have been used classification methods to evaluate the corrosive nature of the soil to buried-metallic pipelines are ASTM [23][24][25], NACE [26,27] and ANSI/AWWA [28,29]. The ASTM and NACE systems are practiced previously to assess the degree of soil corrosivity to the buried-metallic pipelines [30][31][32][33][34][35][36]. In both the ASTM and NACE systems, the electrical conductivity/resistivity of soils is believed as the most important parameter for the evaluation of the buried-pipe corrosion, although the effects of other physicochemical soil parameters take account of less significance. However, the level of soil corrosivity is highly dependent on the experimental conditions, the topography, and other physico-chemical characteristics including the conductivity/resistivity of soil samples.
On the other hand, the 10-point system for the evaluation of the soil corrosion is considered in the AWWA system in which the electrical conductivity/resistivity again appears relevant, although other factors may be pertinent where the difference in corrosion rate is experience. The main drawbacks of the 10-point scoring system do not deal with nonlinear relationships among the soil factors. Apart from, scoring methods neglect some factors, i.e., the corrosive effect of chloride and sulfate ions that are critical to the deteriorating rate of the metallic pipelines in soils. The soil corrosion does not quantify by the 10-points method, and it classifies only two groupings of soil corrosivity, i.e., corrosive and non-corrosive only, not other intermediate levels of soil corrosivity. The soils less than 10 points consider as non-corrosive to ferrous/iron pipes, while the soil with 10 points or more consider as corrosive in the 10-point method [28,29]. Therefore, it does not sufficiently consider different soil corrosivity ratings to different types of buriedmetallic materials.
To address such drawbacks of the conventionally used systems, a new method based on the theory of probability is proposed herein for assessing more accurately the level of soil corrosion to the buried-metallic pipelines. In this comprehensive probabilistic model-based method, all important soil parameters are firstly estimated and then their cumulative effects are equally considered to evaluate the soil corrosion rating to the buried pipelines. The points assigned for each soil parameter summed to classify different corrosion levels and then to decide whether a special protective action should be applied for long term warranty of the buried-pipelines. The detail of the proposed method is described below in the experimental section.
In this context, the main objectives of the work are to estimate six parameters (i.e., soil moisture, pH, resistivity, ORP, chloride, and sulfate contents) of thirty soil sample specimens collected from different sampling depths of each six sites in the vicinity of the Budanilkantha-Maharajganj roadway areas of Kathmandu district, to study the effects of depths on the soil corrosion behavior and to develop a predictive model for assessing of soil corrosion rating which could be addressed the viability of the corrosion protection strategy for the buried-galvanic steel and cast iron pipelines in the study areas. First of all, six sampling sites were selected in the roadways of Budanilkantha-Maharajganj of the Kathmandu district ( Figure 1) and then total thirty soil specimens were collected from each selected six sampling sites with the aid of soil auger from five depths (i.e., 0.3, 0.6, 1.0, 1.2 and 1.5 m) below the ground level in February to May. Each soil sample specimens were taken in an airtight polyvinyl bag so that the moisture remained the same until their laboratory analysis. The six sampling sites are located within the 27°45'24.47"N-27°46'35.03"N latitude and 85°20'57.81"E− 85°21'42.23"E longitude as shown in Table 1.

Experimental Methodology
Six soil parameters of the collected specimens were analyzed using standard methods as; the soil moisture was estimated using ASTM D4959-16 standard [37] in which the gravimetric technique is suggested. The pH and electrical conductivity of the 1:2 soil-water extract of each soil specimens were recorded using a digital pH meter and conductivity bridge, respectively, following by the ASTM standards [23,38]. The soil resistivity was calculated from the recorded electrical conductance value [23].
The oxidation-reduction or redox potential (ORP) of the soil specimens was measured with the help of a digital potential-meter as described elsewhere [39]. A platinum wire mesh and saturated calomel electrode (SCE) were used as working and reference electrodes, respectively, for the recording of the ORP in mV with reference to SCE, and the recorded potential value was converted to a saturated hydrogen electrode (SHE) using equation (1), where, E SHE and E SCE are ORP with reference to saturated hydrogen electrode and saturated calomel electrode, respectively.
The argentometric (Mohr) titration based on the AASHTO T 291-94 standard [40] and a gravimetric titration based on the AASHTO T 290-95 standard [41] were followed to estimate the soluble ions of chlorides and sulfates, respectively, in the soil specimens. A 1:2 soil-water extract was titrated against the standard AgNO 3 solution using potassium chromate as an indicator in the argentometric titration for the estimation of chloride ions in the samples. A new predictive model for the assessment of the corrosion rating of soil samples to the buried-metallic pipelines was applied. In this model, all the analyzed six soil characteristics were estimated and they were classified into essentially less corrosive (ElC), mildly corrosive (MiC), moderately corrosive (MoC) and severely corrosive (SevC) corresponds to I, II, III, and IV corrosion classes, respectively, as summarized in (Table 2). Then, their cumulative point (CP) for each four groups was taken into consideration for assessing ten sub-corrosive groups for rating of soil corrosion to the buried-galvanized steel and cast iron pipelines. Ten sub-corrosive groups are the essentially less corrosive (ElC), essentially less corrosive plus (ElC plus ), mildly corrosive minus (MiC minus ), mildly corrosive (MiC), mildly corrosive plus (MiC plus ), moderately corrosive minus (MoC minus ), moderately corrosive (MoC), moderately corrosive plus (MoC plus ), severely corrosive minus (SevC minus ), and severely corrosive (SevC) in this empirical model.

Estimation of Soil Properties
All the thirty soil specimens from different depths of each six sites show the pH values in the range of 7.1-8.6, as depicted in Figure 2 (a), which categorized the neutral and slightly alkaline soils according to the USDA standard [42].
It is reported that the acidic soil with pH 5 or less is very corrosive to the buried-metallic materials [46]. In such acidic soils, especially the grey cast iron corroded remarkably higher than the galvanized-steel [16]. In general, the soils having 6.5-7.5 pH are considered as neutral and are essentially less corrosive (ElC) nature to the buriedgalvanized steel and cast iron pipelines [16]. On the other hand, the slightly alkaline nature of soil specimens having a A Case Study in Budhanilkantha-Maharajganj Roadway Areas of Nepal pH between 7.6 and 8.5 is classified as mildly corrosive (MiC) according to the ASTM G51-18 standard [38]. Therefore, most of the soil specimens (about 80%) among the thirty soil specimens are rated as MiC, while remaining six soil specimens (i.e., 20%) except one could be rated as ElC to the buried-galvanized steel and cast iron pipelines, as shown in Figure 2 (a). Moreover, there is not a clear relationship between the changes in soil pH values with sampling depth, and much the same results reported in the literature previously [11]. Such information provides us the information for predicting the buried-pipeline failures in the study areas and would help apply the appropriate corrosion controlling techniques in time. The effect of sampling depth on the moisture in soil specimens was studied, and the result is presented in Figure 2 (b). All the thirty soil specimens, collected from different depths of each sampling site contained 14.15-49.71% moisture. The quantity of moisture in seven soil specimens (i.e., 23.3%) among thirty is estimated less than 25% which could be rated as the ElC, while remaining soil specimens except three have 25-40% moisture, and thence they could be rated as MiC to the buried-galvanized steel and cast iron pipelines according to the ASTM D4959-16 standard [37]. These results disclosed the fact that major soil samples are assumed to be mildly corrosive and essentially less corrosive nature around the Budanilkantha-Maharajganj roadway areas.
The moisture in soil samples is generally increased with increasing the sampling depth in all six sites, as shown in Figure 2 (b). The maximum water-retaining capacity of the soil specimens is found in 1.5 m depth from the ground level, mostly due to the water table fluctuations. The water table fluctuation in soils is one of the important factors in assessing the soil corrosion rating which depends mainly on three factors; water flow patterns; ground topography, soil profiles, and soil type and water saturation limits [47]. In general, three types of waters affect the moisture-holding capacity of soil samples; gravitational, free ground and capillary waters [48]. The gravitational water generally reaches to the free groundwater table, especially in coarse-grained soils, although such coarse-grained sandy and coarse soils have less chance of capillary action. These facts support that the increase of soil moisture with sampling depth is mostly due to the free groundwater which is considered to be highly aggressive to the grounded metallic pipelines [49]. Moreover, the soil samples with high water holding capacity are the most corrosive to the buried-metallic materials, while a welldrained soil is less destructive [50][51][52]. The sandy soils with good drainage nature show very high resistivity and hence they could be rated as less corrosive for the buried-metallic materials. Figure 3 (a) shows the results of soil resistivity of all analyzed soil specimens which are found in the range of 4,000-14,500 Ohm. cm. Among these thirty soil specimens, twenty-two specimens have 5,000-10,000 Ohm. cm resistivity which is considered as MiC, only four sample specimens are considered as ElC with the resistivity values more than 10,000 Ohm. cm and remaining four soil specimens are rated as MoC having the soil resistivity of 4000-5000 Ohm. cm. Such a rating of soil corrosion is based on the resistivity value according to the ASTM standard [23,43]. Literature reported that the ductile iron pipe with polyethylene encasement and cathodic protection is not likely to provide a reliable 50-year service life in severally corrosive (SevC) soils with <2,000 Ohm. cm resistivity [53]. These results revealed that most of the soil samples (i.e., 73.4%) collected from the sampling sites to be MiC, while half-half of the remaining 26.6% of the soil specimens could be rated as MoC and ElC. Consequently, it might be said that polyethylene encasement of the buried-galvanized steel and cast iron could be sufficient to save such pipelines for fifty years or more in the study areas. Besides, there is no clear correlation between the resistivity changes with sampling depth, although resistivity value is deceased with depth in most of the soil sampling sites. These results revealed that only variation of the soil sampling depth does not affect the corrosivity of soils to the buried-pipelines, which might be affected by other soil factors like moisture content, salt ions, etc. Also, figure 3 (b) shows the variation of ORP values with five sampling depths of six sites. Fifteen specimens among thirty samples specimens could be rated as ElC with more than +400 mV (SHE) ORP, while it recorded between 315 and 395 mV (SHE) in the remaining 15 soil sample specimens which are considered as MiC based on the ASTM G200-09 standard [39]. A similar relationship was firstly reported between ORP and soil corrosiveness by Starkey and Wight [44]. The ORP value generally affects the types of microbiologically induced corrosion (MIC) that occurs in soils [54,55]. Sulfate is readily converted into highly corrosive sulfides by anaerobic sulfate-reducing bacteria-SRB [56,57]. It reported in the literature that the soil ORP value less than +400 mV (SHE) indicates a contributory environment for soil microbes with the reorganization of a substantial amount of sulfatereducing bacteria (SRB) which causes the formation of bio-film and thus increased the corrosion rate of the buried-metallic pipelines [6]. Besides, there is no correlation observed between the ORP values and the soil sampling depths. The estimated chloride and sulfate ions in all thirty soil specimens are in the range of about 25-63 ppm and  ppm, respectively, as depicted in Figure 4. Most of the analyzed soil samples except three contained less than 50 ppm chloride ions, as illustrated in Figure 4 (a), which could be suggested as essentially less corrosive (ElC) to the buriedgalvanized steel and cast iron pipelines. Only 10% of the analyzed soil sample specimens are considered to be mildly corrosive (MiC) based on the estimated amount of chloride ions in soil samples [36]. Furthermore, there is no regular correlation between the changes of chloride content and soil sampling depth which revealed that the distribution of chloride ions in soil samples does not depend on the depths of the buried pipelines, which might be controlled by other physical or environmental factors.
On the other hand, about 2/3 rd of the thirty soil specimens contained <100 ppm of sulfate ions that are rated as EIC, while remaining 1/3 rd sample specimens rated as MiC to the metallic pipelines in soils having the sulfate ions within the range of 100-200 ppm, as shown in Figure 4 (b). The rating of soil samples as ElC or MiC is based on the ASTM international publications [24,45]. Besides, the effect of the sampling depth on the sulfate amount in the soil samples is observed. The concentration of sulfate ions is generally increased with increasing sampling depth and hence a maximum amount of sulfate ions is estimated in the soil specimens of 1.5 m depth of all six sampling sites.

Correlation Between Soil Properties
Soil moisture has a profound correlation with soil resistivity. Water retaining capacity of a dry or sandy soil is generally very low and hence is typically less-corrosive, while clayey soil that holds more amounts of water shows low resistivity and hence more corrosive to the buried-A Case Study in Budhanilkantha-Maharajganj Roadway Areas of Nepal galvanized steel and cast iron pipelines. An inverse correlation between the soil resistivity and the moisture content in all thirty sample specimens is observed from Figure 5 (a). The soil resistivity is exponentially increased with a decrease of the amount of moisture up to 50%. The empirical relationship between the resistivity (y) and the moisture (x) in soil samples is found to be as; y= 3.37×10 4 -1.78×10 4 log(x) with the regression coefficient (=R 2 ) of 0.677. It was observed a good correlation between the soil corrosivity towards the buried-metallic materials and the nature of decreasing the soil resistivity with increasing the moisture content in it was reported in the literature [31,50].
A fairly good relationship between the soil resistivity and the sum of both chloride and sulfate ions is found, as shown in Figure 5 (b). These results revealed that moisture, chloride and sulfate contents in soil contributed to decrease the soil resistivity or to increase the soil corrosivity to the buriedgalvanized steel and cast iron pipes in the study areas of Kathmandu. Moreover, the correlation between six parameters of all the soil specimens were also studied using a correlation matrix, as shown in Table 3. It exhibits the correlation among the analyzed soil parameters at 0.05 significant levels. The highlighted values are considered for the relationship study consisting of good and strong correlation.  A good negative and positive correlation between moisture and resistivity, and moisture and sulfate ions, respectively, with matric coefficient values of −0.7429 and +0.7390 indicates that the high resistivity of the soil samples is mostly due to the low amount of sulfate in the soil retained-water. Such type of the negative relationship between the resistivity and the moisture content was reported in a uniform soil texture [58] which could also cause a good correlation between soil resistivity and corrosion damages, while a very weak correlation between soil resistivity and moisture content for non-uniform soil texture reported in previously published literature [47]. Similarly, a fairly good negative correlation between the soil resistivity and chloride or sulfate ions is observed from the correlation matrix table. Also, a fairly good positive matrix correlation coefficient is seen between chloride content and moisture or sulfate content in soil samples. Very weak or no-correlation exhibits between remaining other soil characteristics analyzed in the study.

Empirical Model for Soil Corrosion Rating
The empirical classification model was used to designate the ten sub-corrosive groups for the soil corrosion rating to the buried-metallic substances like galvanized-steel and cast iron as already described the aforementioned experimental procedures. These ten sub-corrosive groups are arranged as; ElC<ElC plus <MiC minus <MiC<MiC plus <MoC minus <MoC<MoC pl us <SevC minus <SevC. For the identification of the subcorrosive groups of soil samples, the presence of cumulative points up to 4 (i.e. CP = 4) in any one group is enough to assign a specific corrosive group (i.e., ElC, MiC, MoC or SevC) distinctly in this empirical model. The specific corrosive groups again could be classified into its subcorrosive groups, indicated by the superscripted plus ( plus ) and superscripted minus ( minus ) in the respective specific group if such a group has more factors.
For example, the CP for the II (mildly corrosive-MiC) group is 4 for the soil specimens collected from 1.2 or 1.5 m depth of the MB-1 sampling site (i.e., 2-4-0-0) is sufficient to classify it to the MiC corrosive group firstly. Again, if we check the remaining CP=2 which belongs to I (essentially less corrosion) group, and hence this soil specimen could be classified as MiC minus sub-corrosion group because it is considered that this sample specimen should be more corrosive than that of the soil sample belonging to the MiC sub-corrosive group. In some soil samples, equal numbers of the CP (i.e., CP=3) in tow corrosion groups might be assigned of both specific corrosion groups as illustrated in Table 4. In a likewise manner, all thirty soil sample specimens are classified into different sub-corrosive groups, as summarized in Table 4. Despite this, the proposed empirical model cannot exactly assign any of these ten corrosive sub-groups if each corrosion group has 2 CP values (for example, 2-2-2-0, 0-2-2-2; 2-0-2-2 or 2-2-0-2) despite of the fact that all the analyzed samples specimens do not show such situation in the present study. In such a circumstance, it could be considered a corrosion-prone area in the future [4].
The results shown in Table 4 outlined that the corrosiveness of the soil samples is generally found to be increased with sampling depth, most probably due to the increase of both soil moisture and sulfate ions and decrease of soil resistivity with depth. This implies the role of burying depth on the galvanized-steel and cast iron water pipelines lifetime. Besides, all the analyzed soil sample specimens have belonged to only two specific corrosive groups, i.e., essentially less corrosive and mildly corrosive, with their sub-corrosive groups in the study areas of Budhanilkantha-Maharajganj roadways. Consequently, it makes oneself useful for making a soil corrosion risk assessment map of the study areas that can ultimately be advantageous to identify specific sites that would be susceptible to a given corrosion scenario. Results also indicate that galvanized-steel and cast iron pipes with polyethylene encasement, consisting of a sheet of polyethylene wrapped over the pipe or the use of gravel/sand around the pipelines at the time of installation could be recommended for their protective measures as recommended by various standards [29,53,59,60].  I  II  III  IV  Sub-corrosive groups  0.3  I  I  I  II  I  I  5  1  0  0  ElC  0.6  I  II  II  II  I  I  3  3  0  0  MiC minus ≡ElC plus  BM-1  1.0  I  II  II  II  I  I  3  3  0  0  MiC minus ≡ElC plus  1.2  II  II  II  II  I  I  2  4  0  0  MiC minus  1.5  II  II  II  II  I  I  2  4  0  0  MiC minus  0.3  I  II  II  II  I  I  3  3  0  0  MiC minus ≡ElC plus  0.6  II  II  II  II  I  II  1  5  0  0  MiC  BM-2  1.0  II  II  III  I  I  II  2  3  1  0  MiC  1.2  II  II  II  I  II  II  1  5  0  0  MiC  1.5  II  II  II  I  I  II  2  4  0  0  MiC minus  0.3  I  I  II  I  I  I  5  1  0  0  ElC  0.6  II  II  II  I  I  I  3  3  0  0  MiC minus ≡ElC plus  BM-3  1.0  II  I  I  I  I  I  5  1  0  0  ElC  1.2  II  II  II  I  I  I  3  3  0  0 MiC minus ≡ ElC plus 1. 5  I  II  II  I  I  I  4  2  0  0  ElC plus  0.3  II  I  I  II  I  I  4  2  0  0  ElC plus  0.6  II  II  II  II  I  I  2  4  0  0  MiC minus  BM-4  1.0  II  II  II  I  I  I  3  3  0  0  MiC minus ≡ElC plus  1.2  II  II  II  II  I  I  2  4  0  0  MiC minus  1.5  II  III  III  II  II  II  0  4  2  0  MiC plus  0.3  II  I  I  II  I  I  4  2  0  0  ElC plus  0.6  II  I  II  I  I  I  4  2  0  0  ElC plus  BM-5  1.0  III  II  II  I  I  I  3  2  1  0  ElC plus  1.2  II  II  II  II  I  I  2  4  0  0  MiC minus  1.5  II  II  III  II  I  I  2  3  1  0  MiC minus  0.3  II  I  II  I  I  II  3  3  0  0  MiC minus ≡ElC plus  0.6  II  II  II  I  I  II  2  4  0  0  MiC minus  BM-6  1.0  II  II  II  I  I  II  2  4  0  0  MiC minus  1.2  II  III  II  II  II  II  0  5  1  0  MiC  1.5  II  III  III  II  I  II  1  3  2  0 MiC plus

Conclusion
Six parameters of thirty soil specimens, collected from 0.3 to 1.5 m sampling depths of six sites around the Budanilkantha-Maharajganj roadways (Kathmandu valley), were estimated to investigate the effects of depths on the soil corrosivity to the buried-galvanic steel and cast iron pipelines using an empirical model herein. Experimental outcomes indicated that all most all the soil specimens rated as mildly corrosive and essentially less corrosive, even though a very few specimens belong to the moderately corrosive group as regards the high moisture concentration and conductivity. A fairly to good correlation is observed between the soil resistivity and other soil parameters of moisture, chloride and sulfate content. It assumes that the rate of corrosion of soils to the underground metallic pipes is generally increased with sampling depth based on the empirical model analysis. The implementation of the polyethylene encasement process or the use of gravel and/or sand around the underground pipes before their installation in the study areas seems to be effective to increase such pipelines' life periods.

Funding
This work is partially supported by the Nepal Academy of Science and Technology (NAST) for providing the NAST PhD scholarship to KP Dahal and the NAST Research Grant to J Bhattarai.