Papers by Keyword: Pitting Corrosion

Abstract: The paper defines galvanostatic polarization parameters (current density and test duration), to simulate self-dissolution conditions; the results are confirmed by potentiostatic tests. It is shown that under the conditions of self-dissolution simulation, increasing the solution concentration reduces the nucleation rate, while prolonging the pitting development time. Spectral analysis of chronopotentiograms reveals low-frequency potential fluctuations that characterize the nucleation and passivation of pits at the onset of pitting corrosion. The paper describes the basic regularities, observed in the alteration of surface electrochemistry by impedance spectroscopy; such alteration corresponds to the transition from the passive area to the pitting area. The researchers propose optimal electric equivalent circuits to reflect the surface conditions in early nucleation of pits. The paper also proposes an additional pitting resistance criterion, that is, cumulative electric-charge density.

Abstract: The paper defines galvanostatic polarization parameters (current density and test duration) to simulate self-dissolution conditions; the results are confirmed by potentiostatic tests. It is shown that under the conditions of self-dissolution simulation, increasing the solution concentration reduces the nucleation rate, while prolonging the pitting development time. Spectral analysis of chronopotentiograpms reveals low-frequency potential fluctuations that characterize the nucleation and passivation of pits at the onset of pitting corrosion. The paper describes the basic regularities observed in the alteration of surface electrochemistry by impedance spectroscopy; such alteration corresponds to the transition from the passive area to the pitting area. The researchers propose optimal electric equivalent circuits to reflect the surface conditions in early nucleation of pits. The paper also proposes an additional pitting resistance criterion, that is, cumulative electric-charge density.

Abstract: This paper presents corrosion failure analysis of 13Cr stainless steel (SS) in gas pipeline ingas pipeline, which was used as thermocouple protecting material (TPM). A portion of TMP faileddue to pitting corrosion under unknown circumstances. Scanning electron microscopy (SEM) andX-ray diffraction (XRD) are employed to characterize the scales and/or corrosion products near thefailed portion. Based on visual and microscopic analyses, reviewing the background information andthe thermodynamic calculation, the following rapid pitting corrosion failure sequences wereidentified: Once the pitting appeared, in addition to the gas leakage and expansion, the temperaturedrop should lead a small amount of water in dry gas to condense on the surface of TPM. On one hand,the high salinity produced water will corrode the thermocouple. On the other hand, the high salinityproduced water will pass into the annular space of TPM through the pitting because of the pressuredrop, and the water will stay on inner surface for more time than that of external surface, whichaccelerated pitting of TPM. More and more pitting appeared, and the surface roughness increased.The film-forming property of condensation water will also increase. So, the TPM will be scrappedsoon.

Abstract: This paper presents corrosion failure analysis of an underground natural gas pipeline. The pipeline material grade is 20# steel. The pipeline transfers multiphase fluid (Crude oil and water) from an oil well to an oil gathering plant. A portion of the line failed due to pitting corrosion under unknown circumstances. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are employed to characterize the scales and/or corrosion products near the failed portion. Based on visual and microscopic analyses and reviewing the background information, the following pitting corrosion sequences were identified: When the water ratio was smaller than 50%, the oil slick could cover the surface of the 20# test samples. Some uncovered surface would be corroded. When the water ratio was more than 70%, the surface of 20# steel contacted with more water. The average corrosion rate increased, and the corrosion products also formed, which would behave as a good diffusion barrier to prevent the underlying steel from further dissolution. Meanwhile, because of the corrosion products, the penetration rate also increased, the trend of local corrosion became weak with the water ratio continued to increase. The pitting corrosion varied with the water ratio because of the protection conferred by the oil slick or the corrosion product layer. Under such conditions, pits emerged on the steel surface until one of them grew faster and failed the oil pipeline.

Abstract: Pitting and crevice corrosion resistance of 321 stainless steel and non-vacuum electron-beam cladded stainless steel have been investigated by using potentio-dynamic polarization curve plotting. Lock-Ringer Salt Solution (LRSS) was used as biological media for corrosion test measurements. Microstructural research of samples after corrosion was carried out by using scanning electron microscopy. The experimental results show that the cladded stainless steel possesses higher pitting and crevice corrosion resistance.

Abstract: Corrosion is a common problem in offshore oil and gas wells, especially in thermal recovery wells, which will greatly affect the residual strength of casing strings. Field data show that the corrosion pattern of casing is mainly pitting, local corrosion and uniform corrosion, of which pitting is the most common. For this reason, based on the engineering practice of thermal recovery well casing corrosion, the dynamic changes of wellbore temperature and pressure during the gas production process of thermal recovery wells are considered, and the numerical model of casing corrosion under transient temperature and pressure coupling is established. The influence of corrosion morphology and corrosion depth on the residual strength of the casing is analyzed. The transient stress variation law of the corroded casing is studied. The results show that: (1) Under the same corrosion depth, both pitting and uniform corrosion will reduce the residual internal pressure and external extrusion strength of the casing string, and the degree of influence is similar. The effect of uniform corrosion on the residual tensile strength of the casing string is larger than pitting and local corrosion; (2) the residual strength of the casing decreases linearly with the increase of the corrosion depth of the casing; (3) the stress of the corrosion part of the casing shows a significant dynamic change, in the initial stage of steam injection, casing stress rises rapidly at the corrosion position, and then the rate at which the casing stress rises is slowed, which tends to cause fatigue damage to the casing. The research results have reference significance for the assessment of the service status of offshore thermal recovery well casings.

Abstract: In the present work, a self-built device called “full scale tubular goods erosion-corrosion test system” was used to test the P110SS tubing with a total length of 10 m and a full scale PFF78-70 flat valve to study their erosion-corrosion behaviors in the acceleration simulation operating condition of a well in Sichuang oil field of China. The erosion-corrosion performance of the specimen was investigated by KH-7700 digital microscope, scanning electron microscopy (SEM), X-ray power diffraction (XRD) and energy-dispersive spectroscopy (EDS). The results indicated that the P110SS tubing suffered catastrophic erosion-corrosion; the maximum pitting corrosion rate of the P110SS in the gas phase fluid was 5.29mm/a; while that in the liquid phase fluid was 11.13mm/a; A double-layer corrosion products scale, inner FeCO₃ layer and outer mixture layer mainly dominated by FeO, Fe₃O₄, Fe₂O₃ and FeO(OH), formed on the P110SS tubing. Erosion-corrosion only occurred in the stress concentration area of the valve plate hole of the PFF78-70 plate valve, and the size of the corrosion pits is about 0.5mm.

Abstract: The welding thermal simulation of 2507 Super Duplex Stainless Steel (SDSS) was investigated using Gleeble-3800 thermo-mechanical simulator. The morphology evolution of austenite and ferrite under different t_8/5 and t_12/8 were observed and compared. The impact tests and pitting corrosion tests under different t_8/5 and t_12/8 were conducted. The results showed that the austenite content increased and the austenitic morphology changed from allotriomorphic structure to strip or coarse-blocky structures with the increase of t_8/5 and t_12/8. The effect of t_12/8 on the microstructure of welding Heat Affect Zone (HAZ) was more distinct than that of t_8/5. The impact toughness of HAZ with the increase of t_12/8 was improved due to higher austenite content, while that with the increase of t_8/5 was slightly decreased due to the formation of intermediate phase, such as σ phase. The corrosion tests showed that the pitting resistance of HAZ was improved with the increase of t_8/5 and t_12/8, while the effect of t_12/8 was especially evident.

Abstract: Pitting corrosion is a known process in metals. Pitting, observed in turbine blades, is caused by corrosion mechanisms in operation environment that may contain aggressive chemical compounds. Environment, material composition and operating conditions determine the corrosion process causing a cavity formation and their critical growth. Crack initiation can be a following mechanism. Thus the critical cavity size is the important parameter and its precise estimation can avoid a turbine blade failure. Cavity shape as well as corrosion products inside cavities should be taken into account. 3D scanning technique performed during turbine shutdowns is shown to be a suitable method for measurement of critical cavity size. In this study, the accuracy of 3D technique was confirmed by standard sample preparation methods followed by microscopy focused on corrosion products observation. Cross-sectioning of blades was performed close to the cavities, sections were grinded to show the maximum cavity dimensions and finally polished. Cavity size was measured in few steps by Light Optical Microscopy during grinding to find the maximum dimensions and compare to 3D measurements. Local corrosion in cavities was evaluated by Scanning Electron Microscopy using Backscattered Electrons and Energy Dispersive X-ray Spectroscopy. Non-uniform Fe, Cr, Si rich oxides with dimensions from tens up to hundreds of micrometres were analysed in cavities. No significant effect on oxide removing was observed after using sandblasting technology except possible effect in larger pits. Cracks and pores were detected close to the vicinity of trailing edge of blade.

Abstract: Pitting corrosion caused by wet-dry cycles under corrosive media droplet is one of the key concerns for passive film of metallic materials, particularly stainless steels and aluminum alloys, exposed to atmosphere during service. In this context, the formation of corrosion can lead to high investment cost dealing with corrosion mitigation strategy, e.g. materials selection, electrochemical corrosion control, etc. Based on materials selection perspectives, it is very necessary to have proper understanding of localized corrosion behaviors of metallic materials under solution droplet. Therefore, the present study aims to develop a methodology for pitting corrosion monitoring that can be suitably used for extending better understanding on corrosion phenomena occurring under wet-dry cycles of droplet. A special liquid handling apparatus controlled by Arduino software was constructed and used for generating NaCl solution droplet at given dimension on the surface of stainless steel specimens based on Pendant drop principle. This was recognized as wet cycle. During dry cycle, such NaCl solution droplet was naturally dried off in various conditions of relative humidity. Pitting initiation was observed through a high-resolution CCD camera. Droplet morphology and evaporation time were evaluated at the temperature of 27°C and relative humidity of 10% and 60%. The research results revealed that pitting corrosion started at 1^st cycle without rust formation. Afterwards, the rust formation was clearly noticed when testing cycles of 15 were exceeded.