Corrosion Failure Cause Analysis of Buried Pipelines in Oil and Gas Stations

: Objective: To explore the failure cause of buried pipelines in an oil and gas station. Method: The chemical elements and metallographic structure of the failed pipes were analyzed to evaluate whether the pipe body meets the requirements of relevant standards; the morphology and composition of the corrosion products were analyzed to discover the cause of corrosion. Result: The metal surface was rough and full of pitting pits with severe localized corrosion, and no crack of the metallic matrix was found. The corrosion products mainly contain Fe 3 O 4 and a small amount of FeCO 3 , wherein Fe 3 O 4 is the secondary corrosion product formed in the air, and FeCO 3 is the corrosion product of CO 2 in an oil and gas environment. The surface of the corroded product is loose in structure and easy to peel off, leading to further corrosion of the metallic matrix. Conclusion: The pipeline corrosion failure was caused by CO 2 -induced corrosion failure in the medium.


Introduction
Oil and gas pipeline corrosion is concerned with the safe operation of oil and gas production. The corrosion is characterized by multi-phase flow corrosion mediums where the oil, gas and water coexist; CO 2 , H 2 S and acidic water are the main corrosion media. CO 2 , H 2 S and high Cl-are more likely to lead to localized corrosion, and along with higher temperatures of some pipelines, the localized corrosion rate is accelerated, resulting in corrosion failure and bringing hidden hazards to the safe operation of pipelines. It is necessary to carry out sampling analysis on the failed pipelines, explore the cause of corrosion failure, and take control measures to ensure their safe operation.

Sampling
The pipelines at the oil and gas station are the 20# carbon steel pipeline of φ219×7. They are located at a 4 m buried pipeline at the intersection of the production header and the outer oil pipeline.

Working Condition and Medium
The design pressure of the pipeline is 4.0 Mpa, the operating pressure is 2.20 -2.30 MPa, the medium is crude oil, the sulfur content is 0.95%, and hydrogen sulfide content is 0.18 mg/Kg.

Test Content
(1) Material Inspection Material inspection mainly includes component analysis, metallographic analysis, etc. used to check whether the performance of steel materials meets the requirements.
(2) Analysis of Failure Mechanism It mainly includes the macro morphological observation and corrosion product analysis used to analyze the cause of corrosion failure.

Macro Inspection
By analyzing the pipelines' macro morphology, we found obvious localized corrosion on the inner wall of the pipelines, and the most serious corrosion occurs in the liquid deposition area at the bottom of the pipelines, mainly localized platform-shaped pitting corrosion. There are many corrosion pits with deep depths of up to 4.8 mm. Densely distributed along the axial direction, the pits are mostly elliptical and horizontal.

Matrix Analysis of 20# Carbon Steel Pipeline
Component analysis: Component analysis was carried out on the test sample, and the spectrum analyzer was used in the test for analysis. It can be seen from the analysis results shown in Table 1 that all element contents in the sample meet the requirements of GB/T 9711-2011. Hence the sample has qualified chemical composition.

Corrosion Product Analysis
The measured results of the corrosion products on the inner wall are shown in Figure 6. As can be seen from the XRD analysis results, the materials on the inner wall of the pipeline mainly consist of Fe3O4 and a small amount of FeCO 3 , wherein Fe 3 O 4 is the secondary corrosion product formed in the air, and FeCO 3 is the corrosion product of CO 2 in the oil and gas environment.

Analysis of Micro Morphology on the Corroded Surface
The micromorphology and energy spectrum analysis were carried out on the corrosion products. The micromorphology is shown in Figures 7 and Figures 8,  and the energy spectrum is shown in Figures 9 and 10. The component analysis of corrosion products is shown in Tables 2 and 3. As can be seen from the table, the corrosion products mainly contain Fe, O and a small amount of other elements such as calcium chloride.     It can be seen from Figure 7 that the corrosion products are loose in structure with obvious cracks on the surface surrounded by corrosion pits. As shown in Figure 8, the metal surface is rough and full of corrosion pits with severe localized corrosion, and no cracks are found on the metallic matrix. The corrosion products formed are loose in structure, and it is difficult for carbon steel to generate FeCO3 protective film in the medium and low-temperature area (＜60℃). However, a loose product film was formed, which is easy to peel off and cause the matrix to further corrode.

Analysis Result
(1) By cutting and derusting the pipelines, and observing the macro and local morphology of each pipeline's inner wall, we found that the inner wall is severely corroded. The liquid deposit area at the bottom of the pipeline was severed corroded, mainly composed of localized platformshaped pitting corrosion, while other areas are dominated by uniform corrosion, with deeper depths of corrosion pits in severe corrosion area, up to 4.8 mm.
(2) Component analysis showed all element contents in the pipeline meets the standards. According to the metallographic analysis, the metallographic structure mainly consists of ferrite and pearlite, with uniform crystalline grains and a small amount distributed in bands, similar to the structure obtained by normalization.
(3) In terms of corrosion products, the corrosion products of pipeline mainly contain Fe3O4 and a small amount of FeCO 3 , wherein Fe 3 O 4 is the secondary corrosion product formed in the air, and FeCO 3 is the corrosion product of CO 2 in an oil and gas environment.
(4) As shown in the field emission scanning SEM image, the corrosion products are loose in structure and full of corrosion pits with severe localized corrosion, and no cracks on the metallic matrix. The corrosion products formed are loose in structure, and it is difficult for the carbon steel to generate protective film on the surface in the medium and low-temperature area (＜60℃). However, a loose product film was formed, which is easy to peel off and cause the matrix to further corrode.

Conclusion
(1) The surface of the corroded pipeline is rough and full of corrosion pits with severe localized corrosion, and no cracks were found on the metallic matrix.
(2) The test results of the elements and metallographic structure of the steel pipe meet the requirements of the standard.
(3) The pipeline corrosion was caused by CO 2induced corrosion failure in the medium.