Young-Tai Kho

Microbiologically Influenced Corrosion of Carbon Steel Exposed to Anaerobic Soil

Abstract Microbiologically influenced corrosion (MIC) of plain carbon steel in anaerobic soil was investigated using field survey, the electrochemical polarization technique, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), a thin-film electrical resistance (ER) probe, and galvanic current measurement. The field survey revealed that the risk of MIC could be predicted by the analysis of environmental parameters such as soil resistivity, water content, the content of total organic carbon, reduction-oxidation potential, and the content of sulfate with the consideration of the effectiveness of cathodic protection (CP). From the results of conventional electrochemical experiments, it is evident that the presence and therefore the activity of sulfate-reducing bacteria (SRB) alter the corrosion mechanism of steel by the production of hydrogen sulfide (H2S) and iron sulfide (FeS) film on the steel surface, which reduces the polariz...

Fracture toughness and fatigue crack growth properties of the base metal and weld metal of a type 304 stainless steel pipeline for LNG transmission

Abstract The fatigue crack growth rate and CTOD tests on type 304 stainless steel and weld metal were studied over the temperature range −162°C to room temperature. The girth weld metal specimens were fabricated using a combination of gas-tungsten-arc-welding and shielded-metal-arc-welding. The seam weld joint was made by submerged arc welding. Fracture toughness was evaluated through CTOD tests with three point bend specimens. The fatigue crack growth rate tests were conducted using compact tension specimens in accordance with ASTM E647. The CTOD values were affected by crack orientation with respect to the rolling direction, but orientation had no influence on the fatigue crack growth rates. The fatigue crack growth rates and the CTOD values decreased with decreasing test temperature.

Statistical Approach to Corrosion under Disbonded Coating on Cathodically Protected Line Pipe Steel

Abstract Quantitative prediction models for the corrosion of cathodically protected line pipe steel under a disbonded coating were presented. The optimization plot for the cathodic protection (CP) of crevices beneath a disbonded sleeve applied at the girth weld joint was presented using the multiple quadratic regression analysis by considering the results of numerical modeling and the overprotection problems. It is possible to predict and evaluate the CP status under a disbonded coating using this plot. A model based on the stepwise multiple regression analysis reflecting the contribution of chemical, microbial factors of the soil environment, and that of the effectiveness of CP was presented to predict the maximum corrosion depth of steel pipes under a disbonded coating where the CP current could not penetrate effectively. The results showed that the predicting equation successfully explained the field corrosion phenomena.

Effect of temperature on the charpy impact and CTOD values of type 304 stainless steel pipeline for LNG transmission

Stainless steel pipe of type 304 the with a wall thickness of 26.9 mm and the outer diameter 406.4 mm is welded by manual arc welding process. Mechanical properties and fracture toughness of type 304 stainless steel are investigated in the temperature ranging from room temperature to — 162°C. The results obtained are summarized as follows. The tensile strength noticeably increases as the temperature becomes lower while the yield strength is relatively insensitive to temperature. The Charpy impact energy and CTOD values become higher in the case that crack propagation direction is aligned to the transverse axis upon the rolling direction than longitudinal direction. The drop of fracture toughness is associated with the noticeable diminution of plastic component as temperature seduces from room temperature to — 162°C.

The effects of hydrogen embrittlement by cathodic protection on the CTOD of buried natural gas pipeline

For the corrosion protection of natural gas transmission pipelines, two methods are used, cathodic protection and a coating technique. In the case of cathodic protection, defects are embrittled by hydrogen occurring at crack tips or surfaces of materials. It is, however, very important to evaluate whether cracks in the embrittled area can grow or not, especially in weld metal. In this work, on the basis of elastic plastic fracture mechanics, we performed CTOD testing under various test conditions, such as potential and current density. The CTOD of the base steel and weld metal showed a strong dependence on the test conditions. The CTOD decreased with increasing cathodic potential and current density. The morphology of the fracture surface showed quasi-cleavage. Cathodic overprotection results in hydrogen embrittlement at the crack tip.

Full Scale Burst Test and Finite Element Analysis on Corroded Gas Pipeline

Pipelines have the highest capacity and are the safest and least environmentally disruptive way for gas or oil transmission. Recently, failures due to corrosion defects became of major concern in maintaining pipeline integrity. A number of solutions have been developed for the assessment of remaining strength of corroded pipelines. However, these solutions are known to be dependent on material properties and pipeline geometries. In this paper, a Fitness-For-Purpose (FFP) type limit load solution for corroded gas pipelines made of X65 steel is proposed based on experimental results and finite element analyses. For this purpose, a series of burst tests with various types of corrosion defects was performed. Finite element simulation was carried out on burst test to derive failure criteria. And then, a series of finite element analyses were performed to obtain a limit load solution for a single corrosion defect on the basis of burst test simulation. As a result, an FFP type limit load solution for corroded X65 gas pipelines was proposed.Copyright

Evaluation of stray current effect on the cathodic protection of underground pipeline

Two types of stray current which affect the cathodic protection of underground pipeline are discussed. One is the stray current by DC transit system and the other is that originated from the nearby cathodic protection system. The popular evaluation methods of the pipe-to-soil potential measurement was undertaken to evaluate the corrosion activity of pipeline. In case when the pipe was affected by the stray current, the measured pipe-to-soil potential was significantly deviated from the mean value resulting in normal distribution. Analysis of such distribution pattern has revealed that the pipe under investigation was in the condition of insufficient protection (noble mean potential and high asymmetry coefficient) due to the considerable effect of stray current (high standard deviation value). It was also confirmed that the efficiency of drainage system was so low (<10%) due to the improper selection of drainage point. Simultaneous measurement of the magnitude and direction of the sheath current resulted in information about the place where stray current entered into pipe and escaped to soil. For the second case of stray current problem, it is shown that it has been caused by the combined interference from nearby CP system.

Fatigue crack growth and fracture toughness properties of 304 stainless steel pipe for LNG transmission

The fatigue crack growth rate and fracture toughness tests of type 304 stainless steel were studied over a temperature range of −162°C to room temperature. Girth weld metal specimens were fabricated using a combination of gas-tungsten-arc-welding and shielded-metal-arc-welding. The seam weld metal was made with submerged arc welding. Fatigue crack growth rate tests were conducted using compact tension specimens in accordance with ASTM E647. Fracture toughness was evaluated through CTOD tests with three point bend specimens. The CTOD values were affected by crack orientation with respect to the rolling direction, but orientation had no influence on fatigue crack growth rates. The fatigue crack growth rates and the CTOD values decreased with decreasing test temperature.

Deformation and Fracture Behaviors of Polyethylene Coatings on the Natural Gas Transmission Line With Ultraviolet Exposure

The effect of carbon black, ultraviolet (UV) stabilizer and color pigment on the mechanical property of polyethylene (PE) has been investigated. The transition of fracture mode from ductile to brittle upon UV exposure is identified through microscopic examination of fracture surface. This transition is responsible for the degradation of mechanical properties.The carbon black doped PE shows excellent UV resistance regardless of the presence of UV stabilizer. The mechanical properties of the PE remained unchanged even after 50-days UV exposure. The ductile fracture mode is also maintained in this case. The addition of red color pigment exhibited a very poor UV resistance.Copyright

Coating integrity survey using DC voltage gradient technique at Korea Gas Corporation

The reliability and applicability of various coating defect detecting techniques are investigated utilizing mock pipe. It is shown that both close interval potential survey and dc voltage gradient methods are impertinent as field techniques: They require considerable cathodic polarization in order to effectively locate the coating defects. DC voltage gradient with current interruption technique is recommended as a viable field method in that it is able to precisely locate the defects irrespective of CP condition. Utilizing the method field survey was undertaken for the KGC`s pipeline of 120 km and 106 assumed defects were located.