Internal Corrosion In Pipelines Research Articles

A pipeline corrosion detecting method using percussion and residual neural network

Corrosion of pipeline walls can lead to serious safety accidents such as leaks, fires and even explosions. This paper proposes a corrosion detection method using deep learning based on percussion sound for pipelines. The percussion induced acoustic signals are processed by wavelet threshold noise reduction and double threshold endpoint detection to generate the Mel spectrograms, and then an 18-layer residual network (ResNet18) is used to mine the depth information and classify the degree of pipeline corrosion. We conducted experiments to verify the validity of the approach. Seven working conditions are generated by electrochemical corrosion of a pipe specimen, and percussions are applied at five different positions under the same working conditions to collect the impact acoustic signals. The test results show that the method can quickly, efficiently and accurately detect the degree of pipeline corrosion, classify the degree of pipe corrosion without being affected by the striking position Therefore, the model has great potential for application in detecting the internal corrosion of pipelines based on percussion sounds.

Failure analysis of local effusion corrosion in small diameter gas pipeline: Experiment and numerical

In the later stage of gas well production, water–gas ratio increases, liquid carrying capacity decreases, gas field water accumulates in pipes for a long time, and the corrosion environment of pipelines becomes more and more severe. On the basis of carbon dioxide corrosion, microbial corrosion in gas field water accelerates catalytic pitting and finally leads to serious internal corrosion of pipelines. Therefore, accurate prediction of fluid distribution and flow characteristics in pipes is of great significance for engineering applications. In this work, the location and morphology of the corrosion pit were determined by physicochemical analysis. A computational fluid dynamics (CFD) model was established by using VOF and RNG k-ε models to predict fluid distribution characteristics and fluid accumulation locations in pipelines. The results showed that: 1) Corrosion and perforation occurred in the position of pipeline fluid accumulation. 2) The main elements of the corrosion pit on the inner surface of the pipe sample are O, Fe, C, S and Cl. The main corrosive substances in the pipeline corrosion pit are FeCO3 and Fe2O3. 3) The low-lying areas and uphill sections are prone to corrosion. A decrease in the fluid velocity in the pipe and a corresponding increase in the gas–liquid ratio will result in a greater accumulation of liquid in the pipe. 4) Magnetic flux leakage (MFL) detection method can be used to detect the characteristics and distribution of pipeline corrosion defects and determine the corrosion area. The research results are of great significance to the safety evaluation of small diameter pipelines for gathering and transporting in oil fields.

Temperature Dependence of Adsorption and Effectiveness for a Pyrimidinium-Type Corrosion Inhibitor on Mild Steel

In the oil and gas industry, produced water with a high dissolved salt content is a common byproduct of hydrocarbon extraction from conventional and unconventional wells. Other than salts, corrosive gases such as CO2 are abundant in the production stream, which dissolve and acidify the solution, posing a risk of internal pipeline corrosion. To mitigate this issue, injection of corrosion inhibitors has emerged as a cost-effective approach. In various aggressive conditions, heterocyclic molecules that contain nitrogen atoms have proven to be highly effective corrosion inhibitors for many alloys. In this study, tetrahydropyrimidinium (THP-C14) inhibition efficiencies were investigated at temperatures of 25°C, 55°C, and 80°C using electrochemical methods, including linear polarization resistance and potentiodynamic sweeps. Corrosion inhibition data were then correlated with THP-C14 concentration, using the five adsorption isotherms: Langmuir, Temkin, Frumkin, Flory-Huggins, and Dhar-Flory-Huggins models. These isotherms utilize different assumptions to establish the correlation between coverage and inhibitor concentration. The suitability of these five isotherm models for describing the corrosion inhibition behavior of THP-C14 was examined. In addition, the thermodynamic parameters (Kad, ΔadGo) of adsorption for THP-C14 at 25°C, 55°C, and 80°C were calculated and compared using the aforementioned adsorption isotherm models. Finally, a mechanism was proposed for the adsorption behavior of the THP-C14 corrosion inhibitor model compound. Chloride ions were important for inhibitor adsorption.

A study on the microbial biocorrosion behavior of API 5 L X65 carbon steel exposed to seawater

AbstractDespite much progress achieved, corrosion in oil and gas pipelines remains a concern due to industrial failures, and environmental and economic damages. Oil and gas pipeline failures are very common as a result of the natural degradation of carbon steel exposed to the marine environment. It has been reported that the presence of microorganisms in seawater contributes to the intensification of corrosion and subsequent biofilm formation on metal surfaces. Few scientific publications have investigated the effects of seawater containing natural microorganisms on the internal corrosion of pipelines, which is the motivation for this study. The present study aims to report the corrosion behavior of carbon steel American Petroleum Institute (API) 5 L X65 exposed to seawater collected from the Persian Gulf containing microorganisms such as sulfate‐reducing bacteria (SRB), iron‐reducing bacteria (IRB), and acid‐producing bacteria (APB). Microorganisms were detected, segregated, colonized, and finally injected into autoclaved seawater to investigate their effect on the corrosion behavior of carbon steel. Nondestructive electrochemical techniques were performed to study the corrosion behavior and field emission electron microscope images and energy dispersive x‐ray analysis (EDS) were also utilized to characterize the corrosion products. The results confirmed the amounts of oxygen and iron presented of FeO as the main corrosion product in abiotic conditions. Corrosion products and electrochemical properties of steel have been influenced by microbial activity. The variation of the open circuit potentials (OCP) and Electrochemical Impedance Spectroscopy (EIS) demonstrated that SRB and APB could not only significantly harm an alloy, but also could alter its electrochemical behavior from uniform to local damage. The outcomes proved that biofilm formation and bacterial activity can cause serious degradation.

Mitigation of top‐ and bottom‐of‐the‐line CO2 corrosion in the presence of acetic acid (II): Inhibition using azole derivatives

AbstractCarbon steel flowlines transporting hydrocarbon fluids are susceptible to internal corrosion in the aqueous phase at the base (6 o'clock position) and in aqueous condensed droplets at the top (12 o'clock position). Respectively, these issues are known as “bottom‐of‐the‐line” corrosion and “top‐of‐the‐line” corrosion and inhibitors that are used to control internal pipeline corrosion need to be effective at both locations. Here, we explore whether 2‐mercaptobenzimidazole (2‐MBI), 2‐phenyl‐2‐imidazoline (2‐PI), 2‐amino‐5‐ethyl‐1,3,4‐thiodiazole (2‐AETD), are able to control CO2 corrosion simultaneously at both the top‐ and the bottom‐of‐the‐line in the presence and absence of acetic acid, a common minor constituent of produced hydrocarbon fluids. The performance of the species varied between highly effective (2‐MBI), moderately effective (2‐AETD) to ineffective (2‐PI). Inhibition was effective at both bottom‐ and top‐of‐the‐line, and with acetic acid present. Given that the vapor pressure of these species is negligible, it is suggested that they are carried from the bulk phase to the top‐of‐the‐line dissolved in aerosol droplets rather than in the vapor phase.

A three-dimensional electrode array probe designed for visualising complex and dynamically changing internal pipeline corrosion

This short communication describes a corrosion probe design concept based on a three-dimensional electrochemically integrated electrode array. A representative probe was made for probing complex localised corrosion inside a liquid-containing pipe. Results demonstrate that the probe has sufficient temporal and spatial resolutions required for in-situ monitoring of atmospheric corrosion, channelling corrosion and flow-affected corrosion occurring simultaneously inside the pipe, and for investigating the effects of corrosion influencing factors including differential aeration, liquid flowing, pH and inhibitors. This work suggests that the three-dimensional electrode array probe is a promising tool for probing multi-phase and multi-mechanism localised corrosion in complex industrial environment.

Ultrasonic thickness measuring in-pipe robot for real-time non-destructive evaluation of polymeric spray linings in drinking water pipe infrastructure

Drinking water pipelines are mainly underground and exposed to corrosion. Polymeric spray liners are used as an economical renewal method to mitigate the consequences of internal corrosion of pipelines and prolong their useful service life by many water utilities around the globe. The quality assurance of such applications is currently limited to closed-circuit television (CCTV), which only provides visual information. As part of the Australian government-funded, thirty-four partner multi-million dollar project (CRC-P: Smart Linings for Pipes and Infrastructure project), this work focuses on measuring the post-application thickness of spray liners in pipes. In this article, we report the innovative development of in-pipe robotic sensing suite that leverages ultrasonic sensing to continuously inspect the thickness of the spray linings through non-destructive measurements. The ultrasonic sensor was calibrated by fabricating laboratory test sample made from polymeric spray linings. Continuous uninterrupted coupling was achieved through a custom designed mechanism which was proven to be reliable through field deployments. Extensive lab tests were conducted to validate the sensor measurements with the benchmark measurements producing results with sub-millimeter accuracy. Following lab validations, field trials in buried pipe assets of Sydney Water were conducted for pipe lengths over 35 m, and the field test results show the robotic system is capable of reliably providing accurate spray liner thickness along the pipeline in real-time through non-destructive evaluation.

Effect of Aloe Vera Extract as Green Corrosion Inhibitor on Medium Carbon Steel in Sulphuric Acid Environment

Medium carbon steel is widely consumed by various industrial sectors due to its attractive set of mechanical properties and low cost, but it experiences deterioration when exposed to a corrosive environment. In the present study, Aloe Vera plant extract was studied as a green corrosion inhibitor for medium carbon steel in an acidic medium. The presence of inhibitive compounds in Aloe Vera plant extract was determined by FTIR. Moreover, the inhibition efficiency was determined through gravimetric analysis and electrochemical analysis. The results show that the Aloe Vera plant extract provided inhibition efficiency of more than 90% in both gravimetric and electrochemical analyses. Furthermore, the shift in polarization curves depicts that this plant extract is a mixed type inhibitor acting as an anodic and cathodic inhibitor. Overall, Aloe Vera plant extract provides excellent corrosion inhibition to medium carbon steel in the H2SO4 environment and can be used as a green corrosion inhibitor for mitigating internal corrosion of pipelines and storage tanks.

Assessment of corrosivity of emulsions from a petroleum sample on X80 steel

PurposeThe purpose of this paper is to study the corrosion process by examining the deterioration of X80 steel exposed to a real petroleum sample containing condensed hydrocarbon plus oilfield-produced water, which were subjected to stimulated emulsions in flowing media at 50°C.Design/methodology/approachThe impedance and polarization spectra were used to assess the aggressiveness of the petroleum sample and tried to find a washing process using condensed hydrocarbon with deionized water. Mössbauer technique was used to identify the phases in precipitated ions obtained during an oven-drying procedure of the oilfield produced water.FindingsThe emulsion, chloride, sulphur compounds, heavy metals and the use of a double hydrodynamic system were the most important factors affecting the corrosion of X80 steel. The corrosion rate of this steel increased when oilfield-produced water was stimulated by a double hydrodynamic system (4.56 mm/year). It was determined to be 7.66 mm/year and 4.01 mm/year when steel was exposed to a stimulated emulsion using the petroleum sample and condensed hydrocarbon with deionized water at 24 h, respectively, suggesting that a significant process of hydrocarbon washing could occur and a more corrosive solution was highlighted. Mössbauer results showed that the ions precipitates included the following phases at Wt.%: magnetite (20.0), greigeite (22.8), siderite (3.2), pyrite (2.9), marcasite (26.7) and mackinawite (24.4).Originality/valueA stimulated hydrocarbon/water emulsions with a more homogeneous solution containing high concentrations of saline compounds and heavy metals were used to simulate the susceptibility to corrosion on the internal pipeline steels exposed to any type of immiscible liquids such as condensed hydrocarbon, or crude oil, containing water. A practical application of the presented research could provide a novel framework for understanding the internal corrosion in pipelines from the simulation of washed hydrocarbons after the stimulated emulsions that can be found in the field. Because more susceptibility to corrosion for pipeline steels would be expected at the end of the transportation of the fluid. It is possible to investigate the possible corrosion mechanisms by using a dried oilfield-produced water sample interacting with the pipeline steels.

Mitigating CO2 Corrosion of Natural Gas Steel Pipelines by Thermal Spray Aluminum Coatings

Internal pipeline corrosion due to carbon dioxide (CO2) is a major challenge facing the oil and gas industry. The objective of this study was to investigate the corrosion behavior of aluminum (Al)-based alloys as sacrificial coatings to protect pipelines in a CO2-saturated aqueous electrolyte (3.5 wt% NaCl) at 4 bar CO2 partial pressure (3 bar) and 40°C. The corrosion resistance of Al-based alloys and thermal spray coatings was evaluated in an electrochemical reaction autoclave using electrochemical methods (potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy). Post-corrosion surface characterization was performed by scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy. The obtained data show Al-based alloys demonstrated promising protection against CO2 corrosion with no breakaway degradation issues.

Analysis of acoustic field characteristics to detect internal pipeline corrosion based on ultrasonic full-focus

Seawater pipelines are an important part of ship structures. After a certain period of service, corrosion becomes a serious problem. Ultrasonic corrosion detection has good applicability in seawater pipeline corrosion detection. Aiming at the focusing problem of ultrasonic arrays in pipeline inspection, the sound pressure characteristics and acoustic field directivity in the propagation process were analyzed. A mathematical model of the acoustic field propagation characteristics was established, and the influence of deflection, focusing of the sound beam, and geometric characteristics of the array element on the characteristics of the ultrasonic acoustic field were analyzed through simulation calculations. An ultrasonic array focusing transducer was used to test the corrosion of a Φ168 pipeline. The results show that the number, size, and focal length of the working array significantly affect the focused acoustic field characteristics of the ultrasonic array. The ultrasonic full-focus method can effectively detect local corrosion of pipelines, and the defect detection accuracy has a nonlinear relationship with the number of working array elements and the focal length of the focus. Within a certain range, with an increase in the number of working elements, the overall tendency of the average error of defect detection gradually decreases. With the increase in focal length, the overall tendency of defect detection error first decreases and then increases, and there is a focal length range with the best detection accuracy. In pipeline inspection, the best working parameters can be selected according to the actual size of the pipeline.

Corrosion monitoring in pipelines with a computerized system

This study aims to combine the smart pigs as a non-destructive test (NDT) inspection technique with software developed for the assessment of pipeline corrosion defects to ensure fitness for the surface. The software uses decision support systems, connected through the correlated linkage technique, which is coded using Microsoft Access and Visual C#. This software measures general internal pipeline corrosion forms to identify locations with potential corrosion features and predict corrosion conditions in the future. Computer-aided corrosion management program (CACM) examined maximum corroded depth of internal corrosion, maximum allowable axial corrosion defect length, failure pressure, the corrosion rate, and the remaining pipeline life. This work introduces a wide-ranging review of computer-aided corrosion management programs. The proposed method of assisting and detecting corrosion internal defects and defects data should be available. This software is easy to use without complicated analysis. It helps to reduce unplanned shutdowns in the oil and gas production industry.

Mechanical behavior analysis of gas pipeline with defects under lateral landslide

Landslide is the main factor threatening the operation safety of long-distance gas pipeline, and the internal corrosion of pipeline will also seriously affect its reliability. Using LS-DYNA software, considering the interaction between pipeline and soil, a model of pipeline with defects crossing landslide is established based on the coupling of smoothed particle hydrodynamics and finite element method (SPH-FEM). The effect of the depth, number and spacing of pipeline defects and gas pressure on the mechanical behavior of pipeline is analyzed. The results show that the corrosion defects and gas pressure have little effect on the deformation of the pipeline. It is also found that when the gas pressure of the pipeline increases gradually from zero, the residual strength of the pipeline has a maximum value. Additionally, for the single corrosion defect, the maximum plastic deformation appears in the center of the corrosion defect, but for the double corrosion defect, it appears in junction of the corrosion defects. Furthermore, with the increase of landslide displacement, the plastic strain zone gradually extends along the circumference of the pipeline in these two kinds of defective pipelines. At the same time, the interaction between adjacent corrosion defects is found. The interaction is related to the defect spacing: within a certain range, the interaction increases with the increase of the defect spacing, the maximum equivalent stress appears at the junction of defects, and the stress concentration area expands along the circumferential direction. With the further increase of the spacing, the interaction disappears.

The inhibition performance of mono-ethylene glycol on corrosion rate of x-80 grade carbon steel in saturated brine environment.

The formation/deposition of hydrate and scale in gas production and transportation pipeline has continue to be a major challenge in the oil and gas industry. Pipeline transport is one of the most efficient, reliable and safer means of transporting petroleum products from the well sites to either the refineries or to the final destinations. Acetic acid (HAc), is formed in the formation water which also present in oil and gas production and transportation processes. Acetic acid aids corrosion in pipelines and in turn aids the formation and deposition of scales which may eventually choke off flow. Most times, Monethylene Glycol (MEG) is added into the pipeline as an antifreeze and anticorrosion agent. Some laboratory experiments have shown that the MEG needs to be separated from unwanted substance such as HAc that are present in the formation water to avoid critical conditions in the pipeline. Internal pipeline corrosion slows and decreases the production of oil and gas when associated with free water and reacts with CO2 and organic acid by lowering the integrity of the pipe. In this study, the effect of Mono-Ethylene Glycol (MEG) and Acetic acid (HAc) on the corrosion rate of X-80 grade carbon steel in CO2 saturated brine were evaluated at 25oC and 80oC using 3.5% NaCl solution in a semi-circulation flow loop set up. Weight loss and electrochemical measurements using the linear polarization resistance (LPR) and electrochemical impedance spectroscope (EIS) were used in measuring the corrosion rate as a function of HAc and MEG concentrations. The results obtained so far shows an average corrosion rate increases from 0.5 to 1.8 mm/yr at 25oC, and from 1.2 to 3.5 mm/yr at 80oC in the presence of HAc. However, there are decrease in corrosion rate from 1.8 to 0.95 mm/yr and from 3.5 to 1.6mm/yr respectively at 25oC and 80oC on addition of 20% and 80% MEG concentrations to the solution. It is also noted that the charge transfer with the electrochemical measurements (EIS) results is the main corrosion controlling mechanism under the test conditions. The higher temperature led to faster film dissolution and higher corrosion rate in the presence of HAc. The EIS results also indicate that the charge transfer controlled behaviour was as a result of iron carbonate layer accelerated by the addition of different concentrations of MEG to the system. Key words: CO2 corrosion, Carbon steel, MEG, HAc, Inhibition, Environment.

Extreme value modeling of localized internal corrosion in unpiggable pipelines

Localized internal corrosion is one of the main failure mechanisms of energy pipelines. Microbiologically influenced corrosion (MIC) is a major contributor to internal corrosion in pipelines. Existing research uses extreme value analysis to fit extreme value distributions from data collected through either inspection or experiment to predict localized corrosion. The paper introduces an extreme value model to estimate the depth of localized internal corrosion considering MIC for unpiggable pipelines where inspection data is not available. The depth of material loss is estimated though a combination of corrosion analysis and extreme value modeling. A case study is provided to illustrate the proposed model. The application of a probabilistic approach supports risk-based inspection and maintenance planning for pipelines subject to internal corrosion.

ON APPLICATION OF PIPES FROM POLYMER MATERIALS FOR FIELD PIPELINES

According to the Federal State Budgetary Institution «Central Dispatch Office of the Fuel and Energy Complex» for 2018, in Russia 92 % of incidents and accidents with oil spills occurred due to corrosion. At the same time, about 45 % of pipelines are operated in aggressive soils, 30 % - in slightly aggressive environments, and only 10 % do not need active anti-corrosion protection. According to Rostechnadzor, the total length of the field pipelines reaches 300 thousand km. At the same time, the deterioration of the main stock of production pipes reaches 70 %. Since the most used material for the construction of pipelines is steel, the main problem in the transportation of hydrocarbons is external and internal corrosion of pipelines. In this regard, one of the key points is the replacement of metal pipelines with reinforced polymer pipes that combine corrosion resistance, low moisture absorption, and high dielectric performance. The aim of this work is to analyze the use of pipes made of polymeric materials for field pipelines used to transport hydrocarbons in Russia and abroad. Tasks: - analysis of the operating experience of reinforced polyethylene pipes used for the transportation of hydrocarbons in Russia and abroad; - consideration of the main problems arising during the operation of composite pipelines; - comparison of the characteristics of pipes used in different countries. In the course of the study, the regulatory documents on foreign and domestic polymer pipelines were studied and a generalized analysis of these pipes was carried out. The problems faced by the oil and gas industry in the transportation of hydrocarbons are identified. The history of Western and domestic use of polymer pipelines is considered, on the basis of which the relevance of the use of composite pipelines based on polyethylene is substantiated.

Evaluation the Effect of Velocity and Temperature on the Corrosion Rate of Crude Oil Pipeline in the Presence of CO2/H2S Dissolved Gases

In this paper investigate the influences of dissolved CO2/H2S gases, crude oil velocity and temperature on the rate of corrosion of crude oil transmission pipelines of Maysan oil fields southern Iraq. The Potentiostatic corrosion test technique was conducted into two types of carbon steel pipeline (materials API 5L X60 and API 5L X80). The computer software ECE electronic corrosion engineer was used to predict the influences of CO2 partial pressure, the composition of crude oil, flow velocity of crude oil and percentage of material elements of carbon steel on the rate of corrosion. As a result, the carbon steel API 5L X80 indicates good and appropriate resistance to corrosion compared to carbon steel API 5L X60. The rate of corrosion acquired from the test in flow conditions is most significant than that in static conditions. The crude oil from Noor field has the largest value of corrosion rate, while the crude oil from Halfaya field has the lower value; other crude oils have moderate values. The dissolved CO2/H2S gases contribute by a low degree in internal pipeline corrosion because of the small concentrations.

Performance and Critical Issues of Polymer Liners in Pipeline Industry: A review

Abstract Recently, the material that industry usually used for pipelines are of carbon steel. The pipeline function is to transport crude oil from production sources to process plants. Unfortunately, crude oil consist water and can cause corrosion to the internal pipeline and subsequently result to pipeline damage. The repair process of the corroded pipelines is not economical to oil industry, as such, to protect the pipelines against the internal abrasion and destruction, cost effective measures are essential. Nowadays, the advances of this technology employed the use of special polymers as protective liners against internal pipelines corrosion. However, despite improved properties (chemical resistivity), critical issues related to these polymers arise which include the thermal, aging and friction effects with slurry and have attracted the attentions of research stakeholders from both academia and oil industry. Consequently, this review reports recent advances of the performance of these special polymeric materials with emphasis on their structures and properties. polymers have demonstrated certain important properties to serve as good materials for lining in the oil and gas internal pipelines industry. They have the ability to prevent or reduce the internal pipeline corrosion. The limitation is the critical issues that need further research to improve the thermochemical and mechanical properties.

A numerical simulation of water distribution associated with internal corrosion induced by water wetting in upward inclined oil pipes

Internal corrosion of pipeline is often observed at the pipe bottom, especially at the pipe elbow, which may bring great threat to personnel safety and cause serious environmental pollution once the pipeline perforation happens. The pipe wall will get thinned by corrosion when it is wetted by produced water dissolved with various corrosive media. Therefore, it is essential to predict the water distribution in pipelines and determine the continuous phase that wets the pipe wall when analyzing the internal corrosion of crude oil pipelines. This paper aims to study key parameters, such as water cut, mixture velocity, oil viscosity and inclination angle of pipes, that dominate water distribution and the phase that wets the wall of pipe bottom by applying computational fluid dynamics. The results of numerical simulation are proved to coincide well with the available experimental results.

Effect of Imidazoline Corrosion Inhibitors on Erosion/Corrosion Processes - an Electrochemical Investigation

Erosion-corrosion is a concern for oil and gas pipelines [1]–[3]. The simultaneous influence of fluid parameters such as velocity, turbulence, fluid regime and the characteristics of the corrosion scales formed on the pipe surface can accelerate erosion-corrosion inside oil and gas pipelines [4]. Corrosion inhibitors, made of imidazoline derivatives, are widely used in oil industry to prevent internal corrosion of pipelines. In this work, the influence of an imidazoline corrosion inhibitor on erosion-corrosion rates, in flowing fluid condition, is electrochemically investigated. The test electrolyte is synthesized to simulate condition of a multiphase gas pipeline. Electrochemical Impedance Spectroscopy (EIS) was used to measure the corrosion rate in presence of varying concentration of corrosion inhibitors. A cylindrical working electrode with a rotational speed of 1200 (rpm) was employed to simulate the dynamic behavior of the flowing fluid inside wet gas pipelines. The results revealed that corrosion inhibitor films could be readily removed from the steel surface by the erosional behavior of the flowing fluid if the adhesion factor of the surface films are not sufficient. This would expose the bare metal to the corrosive medium. The consequence of such phenomena is the lifespan reduction of pipelines and thus occurrence of immature erosion-corrosion failures. This study demonstrated that the adhesion force of the corrosion inhibitor films are directly related to the thickness of the formed film on the pipe surface and the shear stress generated by flow at the steel surface.