Lab and field investigations on localized corrosion of casing

The corrosion of casing was investigated in the laboratory and oilfield by series methods of EIS, SEM, EDS, well logging, and field coupon test. Data reveal the middle vertical profile along the casing is greatly threatened by localized form rather than generalized corrosion. The pit initiates in the micro‐pore of the corrosion scale on the surface of the steel. The development of pitting depends on the water type classified by geochemist. The calcium chloride water shows highest acceleration to pitting due to the two orders of magnitude higher concentration of chloride ion by comparing sodium sulfate and sodium bicarbonate water types. The element analysis and microstructure observation were used to further describe the pit propagation. It is proposed to get a better understanding of the corrosion damage of casing in the petroleum industry.     

A novel method to determine the flow accelerated corrosion rate in the elbow

Flow‐accelerated corrosion (FAC) is the most common failure in nuclear power plants. The FAC rate conforms to the experimental results in lab, which are calculated by some other FAC rate prediction models. However, when these models are employed to calculate the FAC rate of the elbow, the result is opposite to actual situation of failure of the elbow. In this paper, a new prediction model is proposed to calculate the flow accelerated corrosion (FAC) rate in the elbow, which combine the steady‐state mass transfer model of electrochemical theory and one‐dimensional galvanic corrosion model. Firstly, the distribution of velocity in the elbow is counted by FLUENT. Secondly, the free corrosion potential and the free corrosion current density are calculated by steady‐state mass transfer model. At last, the novel model is used to obtain the FAC rate of the elbow. The result will show that the FAC rate of the outward bend of the elbow is two‐orders than the inward bend of the elbow. The outward bend of elbow is the defective position, which accords with the statistical result of Kuen Ting et al.     

Microbial consortium influence upon steel corrosion rate,using the electrochemical impedance spectroscopy technique

The Electrochemical Impedance Technique was used to evaluate the influence of a microbial consortium, isolated from a gas pipeline, upon API XL52 steel corrosion rate. The bacteria growth exhibited two different kinetics behavior, one for the plancktonic and the other for the sessile phase. The sessile bacteria were found to be the main responsible for the corrosion rate increment observed during the experiments and no relationship between the plancktonic microorganisms and the corrosion rate increment was found. The diagrams obtained from the electrochemical impedance measurements, indicated a biofilm formation and that the system changed from activation to diffusion control. Although the system was under diffusion control, an increment on the corrosion rate was detected, and a localized corrosion process was induced. The results were complemented with some surface analysis using Scanning Electron Microscopy.     

Influence of testing parameters on the corrosion rate of magnesium alloys

In sodium chloride solutions alloy composition, phases, microstructure and grain size influence the corrosion behaviour of magnesium alloys. Concentration and distribution of the critical impurities iron, nickel and copper affect the corrosion performance strongly. Salt spray tests according to ASTM B 117 or DIN 50021 are used to control quality of magnesium alloys. Results of these tests often estimate alloy subcontractors and are therefore very important to placing. Standards specify test solution, test temperature and position of specimens during test in the salt spray chamber. Standards not prescribe preparation of test specimens, exposure period, handling of the specimens after salt spray test nor the interpretation of the results. Results of salt spray tests can be only compared, provided that test conditions are exactly given. Whether the standards fulfil the above described criteria, will be shown by extensive investigations. Therefore the influence of exposure period, surface condition and microstructure was investigated.     

Stress corrosion cracking of explosion welded steel‐aluminum joints

The article presents the test results of stress corrosion resistance of explosion welded steel‐aluminum elements. The tests were carried out by means of the slow strain rate test (10^?6 s^?1) in air and artificial sea water. The following parameters were measured: tensile strength, time‐to‐failure and destructive energy. The tested explosion welded joints show susceptibility to degradation of properties in the corrosive environment. Galvanic corrosion is the main reason of degradation of the measured values. Another mechanism of explosion welded joints' destruction was found in air and artificial sea water. The tests of the explosion welded elements in the fast flow of sea water showed substantial galvanic corrosion.     

Transformation between localized and general corrosion behavior of die‐cast AZ91D magnesium alloy in 3.5 wt% NaCl aqueous solutions

Transformation behavior between localized and general corrosion of die‐cast AZ91D magnesium alloy in anodizing processes was investigated by galvanostatic polarization. The results revealed that, at electric current density 5, 10, and 20 mA/cm², a localized corrosion state appears on the specimen surface in the initial stage (10 min), and is changed into general corrosion with the galvanostatic measurement time. On the other hand, the increase in electric current density has caused the increase of the corroded area on the specimen surfaces at the same galvanostatic measurement time. In comparison, the appearance of localized corrosion based on general corrosion can be attributed to the localized absorption along with enhanced activity and reactivity of Cl^? at the higher electric current density (10 and 20 mA/cm²). Meanwhile, the penetration of Cl^? throughout the corrosion products has caused the destruction of the integrity of these corrosion products, which promotes the continuous growth of the localized corrosion pits under the galvanic effect.     

Influence of external tensile stress on corrosion and trench formation of low alloy steel in a low H2S content sour corrosion environment

The influence of external tensile stress on corrosion and trench formation of low alloy steel in a low H₂S content sour corrosion environment was investigated. These experiments were conducted with steel for pipelines, and electrochemical methods were used. The results showed that external stress increases the amount of corrosion weight loss and trench depth by promoting the anodic dissolution reaction, and stress concentration was proven to be one of the driving forces for trench formation due to localized corrosion. Based on the experimental findings, the mechanism of trenching was discussed from the viewpoint of promotion of the anodic dissolution reaction by dislocations.     

镁合金大气电偶腐蚀初期规律

研究了AZ91D、AM50、AM60铸造镁合金与A3钢、316L不锈钢、H62黄铜、LY12铝合金组成的电偶对分别在青岛和武汉现场暴晒3个月和6个月后的大气电偶腐蚀行为及规律.结果显示, 镁合金始终是电偶对的阳极; 当其与其它4种材料偶接时, 其腐蚀速率增加.镁合金与A3钢偶合后, 其大气电偶腐蚀效应最大, 而与LY12铝合金组成的电偶对的大气电偶腐蚀效应最小.不同镁合金的大气电偶腐蚀效应存在如下关系: γAZ91D>γAM50>γAM60.暴晒3个月后, 青岛的大气电偶腐蚀效应明显高于武汉的大气电偶腐蚀效应.随着暴晒时间的延长, 青岛和武汉的大气电偶腐蚀效应分别呈降低和升高的趋势.     

Simulation on function mechanism of T1(Al_2CuLi) precipitate in localized corrosion of Al-Cu-Li alloys

1Introduction Al-Li alloys,compared with traditional Al alloys,possess more excellent properties,such as lower density,greater elastic modulus and higher specific strength.In the near future,they would be widely applied in airplane structures.However,they…     

Mechanisms of localized corrosion of carbon steel associated with magnetite/mackinawite layers in a cement grout

Carbon steel electrodes covered with a specific low-pH cement grout (pH ~10.7 at 20°C), designed for nuclear waste management applications, were immersed for 30 days in a 0.01-M NaCl + 0.01-M NaHCO₃ solution (pH 7 measured at 20°C), in aerated conditions, at 80°C. The corrosion processes were studied by voltammetry and linear polarization resistance measurements while the corrosion product layers were analyzed by µ-Raman spectroscopy and X-ray diffraction. Most of the electrodes (75%) suffered from localized corrosion, a phenomenon associated with the formation of a heterogeneous Fe₃O₄/FeS layer. It is proposed that the mechanisms of the particular corrosion process observed here are associated with galvanic effects, the large magnetite-covered zone acting as cathode and the locally mackinawite-covered zones being anodic regions.     

Acidification of the electrolyte during the galvanic corrosion of AA7075: A numerical and experimental study

The electrochemical interactions between aluminum alloy 7075 and low-carbon steels under gelled electrolytes were studied. Such electrolytes provided the opportunity to investigate both thick and thin electrolyte systems. The electrolyte was chemically modified to visually track the acidic fronts during the anodic reaction and the subsequent hydrolysis process. Two mathematical models were validated for both thick and ultrathin electrolytes. The acidification of thick electrolytes was extended some millimeters beyond the aluminum alloy surface, whereas the acidic front was localized next to the metallic joint using ultrathin electrolytes. The combination of both numerical and experimental results allows proving (and explaining why) that the acidification process is more aggressive under dilute than under concentrated electrolytes.     

Simulation on function mechanism of T1(Al2CuLi) precipitate in localized corrosion of Al-Cu-Li alloys

To clarify the corrosion mechanism associated with the precipitate of Tl(Al₂CuLi) in Al-Li alloys, the simulated bulk precipitate of T1 was fabricated through melting and casting. Its electrochemical behavior and coupling behavior with (Al) in 3.5% NaCl solution were investigated. Meanwhile, the simulated Al alloy containing T1 particle was prepared and its corrosion morphology was observed. The results show that there exists a dynamic conversion corrosion mechanism associated with the precipitate of T1. At the beginning, the precipitate of T1 is anodic to the alloy base and corrosion occurs on its surface. However, during its corrosion process, its potential moves to a positive direction with immersion time increasing, due to the preferential dissolution of Li and the enrichment of Cu. As a result, the corroded T1 becomes cathodic to the alloy base at a later stage, leading to the anodic dissolution and corrosion of the alloy base at its adjacent periphery. It is suggested that the localized corrosion associated with the precipitate of T1 in Al-Li alloys is caused by the alternate anodic dissolution of the T1 precipitate and the alloy base at its adjacent periphery.     

Influence of irradiation on water-saturated corrosion of carbon steels at 80°C

Two grades of carbon steel were exposed between 3 and 18 months at 80°C, with and without irradiation (maximum: 20 Gy/hr), in 100% humid atmosphere but some specimens were also half or totally immersed in a caustic solution. The main observations are:

• (1) No significant differences regarding the corrosion behavior between the two carbon steel grades were observed.
• (2) Generalized but nonuniform corrosion was observed in all the test conditions.
• (3) There was a large degree of scattering in the corrosion damage in a 100% humid atmosphere and this is attributed to the heterogeneity of atmospheric corrosion initiation.
• (4) Corroded areas neighboring noncorroded areas were observed on the fully immersed and semi-immersed specimens.
• (5) No major differences between the specimens exposed to irradiation and those that were not irradiated were observed; except for immersed specimens, the maximum corrosion depths were obtained under irradiations.