Corrosion Behavior in External Surface of API 5L X52 Underground Pipelines in Buzurgan Region/ Missan province

External corrosion of underground pipeline is one of the most common damage mechanisms associated with the soil environment particularly in ageing pipelines. This study investigates the external corrosion phenomenon for exporting crude oil pipeline (28'') in Buzurgan region / Missan province. Corrosion rate was measured by using weight loss method, and Tafel extrapolation. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were also used in this work. During the field survey, soil sample was taken from Buzurgan region (collected as close as possible to buried pipes at 1.2 m) for tests were carried out for soil that included: pH, moisture content, resistivity and chemical composition (XRF, XRD, and soluble salts). It was found that the environment of the areahas a low soil resistivity value (870) ohm.cm. On other hand, the samples that were immersed in Buzurgan soil showed the that highest damage was by pitting corrosion due to the high soluble salts content particularly chlorides (7388 )ppm and sulfates (2570) ppm.


Table (1) Tendency to Corrode Based on Soil Resistivity [4]
The big challenge that can be exposed for the buried pipeline is transporting the ingredients from the oil exploration site to several miles away from cities and towns and production units like refineries, since they are inflammable, and thus, any degradation due to corrosion such as leakage can cause a severe accident, leading to an explosion. The damage that may occur to the transport system due to corrosion has several risks, including economic, environmental, or maybe procure to accidents or lead to loss life of people [5]. So it was necessary to keep the pipes from the corrosion conditions surrounding it and to maintain the longest life of the pipes as well as taking into account safety and environmental preservation [6]. The objective of the present work is improving the understanding of corrosion behavior through studying the susceptibility to corrosion of low carbon steel pipeline API 5L X52 in soil environment under protection failure. In addition to provide a best understanding of the relationships between soil parameters and the corrosion defects.

Material:
The material sample used in the present study was a piece of pipeline API 5L X52 (is the same as the 28" export pipeline metal used in the MOC field), Table (2) shows the chemical composition analysis of this sample, was carried out by metal analysis SPECTRO at General Company for Inspection and Engineering Rehabilitation-Baghdad. The microstructure of the sample was observed by using an optical microscope, as shown in Figure (   On the other hand, electrochemical experiments (polarization experiment was done using potentiostat instrument) were performed in a conventional three-electrode cell accommodates for 400 mL solution (The electrolytes studied were an aqueous extract of the soil). An Ag/AgCl and a platinum foil were used as the reference and counter electrode, respectively; the process has been programmed in computer to draw potential (mV) and log current (µA) to obtain corrosion current by intersection of the tangents to the two polarization curves. Potential quoted in this paper was referred to the Ag/AgCl. Whereas, the polished API 5L X-52 steel sample with its total surface area of 1 cm 2 was used as the working electrode. Open-Circuit

Surface Analysis
Morphology and micro-analysis experiments were carried out by scanning electron microscopy and Energy dispersive X-ray spectroscopy (SEM/EDS) on metal surface that was exposed for corrosion environment in simple immersion test, the time of expoture was 30 days in soil has local moisture (field condition). This morphologic information together with the elemental composition analyses could shed light on possible metal and soil interaction.

Soil survey
Soil is the environment of corrosion in this study, thereby it is important to study its properties and know the chemical composition for it. Sample was collected from Buzurgan region near the exporting pipeline.

Soil Resistivity Survey
Resistivity measurements indicate the relative ability of a medium to carry electrical currents. The soil resistance can be measured directly by using the four pins-winner method, the soil resistivity measuring device was (EP-TECH) model: (EPT/124F) made in Canada. It has been borrowed from Al-Taakhe Engineering Company. as shown in Figure (2). The resistivity (ρ) is then [9] ρ = π a R Where: ρ is soil resistivity (Ω .cm) a is electrode separation (cm) R is resistance (Ω)

Moisture Test for Soil
The moisture content of the sample of the site nearby transport pipeline in Buzurgan reign was tested in the laboratory, the soil sample was weighed then dried at temperature of (110 °C), a sample allowed to dry for 15 to 16 hours after that dry weight was taken. This test was carried out in Missan Oil Company -Department of Engineering Inspection -Corrosion Laboratory.

pH Test for Soil
This test was used to determine the pH of a soil. The principle use of the test is to evaluating the corrosivity of a soil environment and information on the stability of a metal as a function of pH [11] .

Results and Discussions
Experimental results of the soil sample are given in Table (3). showed that pH is not a prevailing factor in the corrosion mechanism when the soils have a pH in the range of (4-8.5) [12]. Table (3) illustrates the result of pH measurement for soil, this results show that the soil is alkaline. However, of the above it can be considered that pH is not a dominant factor of making the soil more corrosive environment. It is seen from the tables above that the presence of large amounts of aggressive ions such as chlorides accelerates

No.30-(3) 2021 Journal of Petroleum Research & Studies (JPRS)
E44 corrosion in the metal surface, if the concentration of chloride in soil greater than 5000(ppm) the degree of corrosivity is classified as sever [13]. In addition to increasing the likelihood of pitting corrosion, a research study conducted in Al-Faw reign (southern Iraq) showed that the surface of the metal was clearly degraded likewise pitting corrosion is so clear on the steel pipe specimen surface due to the high chloride content in the soil [14] On the other hand, the presence of chloride tends to decrease of the resistivity [15]. This corresponds to the low resistivity values of the studied area. In contrast, high concentrations of calcium and magnesium in soils tend to make it alkaline, this seems clear of pH measurement.
As for the sulfate ions, they have lower risk than chloride relative to corrosive effect. However the risk may increase in case of the sulfate-reducing bacteria (SRB) is presence. The steel corrosion processes started when the steel sample were exposed to natural soil. When the soil moisture content was local, corrosion rate of steel samples decreased as immersion time increased. This behavior is shown in Figure ( showed a direct correlation between the moisture content and the corrosivity of soil and which is considered a relevant parameter in assessing the corrosivity of soils[9] Therefore, a factor such as soil moisture must be taken into account when studying corrosion in buried pipeline. Also, it can be noted that the corrosion potential reaches a level at which the corrosion potential stabilizes. The constant OCP means that there is equilibrium between dissolution and deposition [15].

Fig. (7) Open Circuit Potential Curves for X52 in Aqueous Extract of Buzurgan Soil
The  The value of corrosion rate shown in Table (4) indicates that the highest rate of corrosion. This corresponds with the result of simple immersion test as previously indicated. It was found that the high percentage of soluble salts in the soil, especially chloride and sulfate is the cause of high corrosion rates. As a result, the rich soil by soluble salts; it has highest conductivity that leads to increase the corrosion rate [18].    Table, and C. SEM Image with High

Magnification after Corrosion (Buzurgan soil)
The EDS for the low carbon steel surfaces after immersion test demonstrates that iron and oxygen have been distinguished, which prove the formation of a corrosion product layer.
Sample SEM micrograph images of the surface of corrosion samples are shown in Figure (10

Technical and economic feasibility
Buried pipeline transports the ingredients from oil exploration site to several miles away from cities and towns, the cost of their construction amounts up to millions of dollars, in addition to the difficulties associated with this type of work, like passing through agricultural lands, rivers, seas, mountains, marshy areas, and desert and may be cross other services like paved roads, railways, transmission lines, underground pipes/cables, and so forth. Motivations of the present work is to achieve the aspiring aim of cost-effectively by maintain the existing pipelines, and extending its safe operating life.

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The pH readings of the soil indicate that it is alkaline soil, and is a non-influencing factor in the process of corrosion.