Du Cuiwei

EFFECT OF APPLIED POTENTIALS ON STRESS CORROSION CRACKING OF X80 PIPELINE STEEL IN SIMULATED YINGTAN SOIL SOLUTION

Stress corrosion cracking (SCC) of X80 pipeline steel in a simulated solution of the acidic soil environments in Yingtan China was studied by means of potentiodynamic polarization curves, slow strain rate test (SSRT) and corrosion morphologies characterized by SEM. The results show that X80 pipeline steel has high SCC susceptibility in the simulated solution and the failure mode is transgranular cracking. The SCC mechanism would vary with the applied cathodic potential. When the applied potential is positive to about −930 mV, the SCC behavior is controlled by the com- bined effect of anodic dissolution (AD) and hydrogen embrittlement (HE), i.e. the SCC mechanism is AD+HE. However, when the applied potentials are lower than −930 mV, such as −1000 and −1200 mV, the process of hydrogen evolution plays the dominant role in SCC occurrence, meaning that the SCC mechanism is HE under such applied potentials. Moreover, SCC susceptibility increases with decreas- ing applied cathodic potential. Compared with X70 pipeline steel in acidic soil environments, HE plays a more important role in affecting SCC occurrence.

CORROSION BEHAVIOR OF COPPER ALLOYS IN DEEP OCEAN ENVIRONMENT OF SOUTH CHINA SEA

The corrosion in deep ocean environment has been paied more and more attentions to the exploitation of marine resources.Different from shallow marine environments,deep ocean environments are specially characterized by high hydrostatic pressure,low temperature,variable dissolved oxygen content and pH value in deep ocean,etc..So the corrosion behaviour of materials,such as ferrous and nonferrous metal and coatings,in deep ocean environments is different from that in shallow marine environments.A number of researches have been carried out to investigate the corrosion behaviour of metals in natural deep ocean in a few developed countries.Such researches,however,began until 2008 in South China Sea.In this work,the corrosion behavior of H62 brass,QA19-2 and QSn6.5-0.1 bronze in 800 and 1200 m deep ocean environments of South China Sea was studied using field tests.The results indicated that the corrosion rates of copper alloys decreased in the following order:H62(0.042 mm/a) QSn6.5-0.1(0.004-0.007 mm/a) QA19-2(0.003 mm/a).The corrosion rate of H62 brass decreased linearly with the increase in depth.While the corrosion rates of QA19-2 and QSn6.5-0.1 bronze decreased first and then increased with the increase in depth.The minimum value of corrosion rate occurred between 800-1200 m.The morphology and composition of corrosion products were investigated using SEM,EDS and XRD.The results demonstrated that the dezincification corrosion obeying solution-redeposition mechanism in H62 brass occurred.The corrosion products were composed of Cu,ZnO,Zn_5(OH)_8Cl_2H_2O and Cu(OH)_2H_2O.And the dealloying corrosion in QA19-2 and QSn6.5-0.1 bronze occurred.The corrosion products of QA19-2 bronze consistof Cu_2O and CuCl_2,and those of QSn6.5-0.1 bronze Cu_2O,CuCl_2 and Cu_2Cl(OH)_3.

RELATIONSHIP BETWEEN HIGH pH STRESS CORROSION CRACKING MECHANISMS AND APPLIED POTENTIALS OF X80 PIPELINE STEEL

Most of the current researches are only limited to the evaluation of high pH stress corrosion cracking(SCC) susceptibilities of pipeline steels at some certain potentials,but the division of the sensitive potential ranges controlled by different SCC mechanisms is rarely reported.When the results of SCC susceptibilities adopted by using the ultimate fracture strength,the reduction-in-area and the elongation rate separately are not consistent,how to evaluate the SCC susceptibilities taking into account both the loss of fracture strength and toughness also needs solving.In this work,the slow strain rate tensile tests were conducted to evaluate the SCC susceptibilities of X80 pipeline steel in concentrated carbonate/bicarbonate solution at different applied potentials.The immersion tests at several constant potentials were used to analyze the potential sensitivity of electrochemical corrosion behavior and its influence on the SCC controlling mechanisms.With the aid of potentiodynamic polarization tests and the thermodynamical calculation,the typical potential ranges of different SCC controlling mechanisms were divided.Considering both of the ultimate fracture strength and reductionin -area loss,a comprehensive index is for estimating SCC susceptibility was established and verified. The main results showed that at -580 mV,the ferritic intergranular was corroded preferentially,and X80 steel exhibited remarkable intergranular SCC(IGSCC) with anodic dissolution(AD) as the controlling mechanism.At -750 mV,the ferritic grain was prone to be dissolved causing the occurrence of pitting,but the steel showed very low SCC susceptibility,which indicated that high transgranular SCC(TGSCC) susceptibility could not be triggered only by AD.The open circuit potential E_(ocp) could be seen as a transition potential between pitting and TGSCC zone,at which the SCC susceptibility was slightly enhanced.At—880 mV,the steel showed tremendous TGSCC susceptibility due to the synergistic effects of AD and hydrogen embrittlement(HE),although it was protected by the applied cathodic potential.And the SCC behavior mainly controlled by HE at—1200 mV,showing quasicleavage fracture mode.Therefore,the typical potential ranges could be divided into four categories: -600——569 mV for IGSCC,E_(ocp)--600 mV for pitting,hydrogen evolution potential -1046 mV-E_(ocp) for TGSCC,and below—1046 mV for HE,i.e.,quasi-cleavage cracking.By applying the criteria proposed in acidic soil simulated solution to illustrate the SCC susceptibility in this work,the comprehensive index I_Σwas proved to be reasonable,which provided a supplementary method for evaluating the SCC susceptibility.

Effects of Alternating Current (AC) Frequency on Corrosion Behavior of X80 Pipeline Steel in a Simulated Acid Soil Solution

The effect of frequency of the applied alternating current(AC) on corrosion behavior of X80 steel in an artificial liquid aiming to simulate the acid soil medium of Yingtan area was studied by means of electrochemical measurement, immersion test and surface characterization technique.The results showed that with the increasing AC frequency, the corrosion rate and the corrosion degree of X80 steel decreased gradually. The corrosion product of X80 steel under the applied AC was loose with many cracks thus could not offer proper protection to the substrate. The offset of corrosion potential of X80 steel decreased as the AC frequency increases. With the increase of AC frequency, the oscillation amplitude of anode and cathode polarization curve gradually receded. In the range of the test frequency, the applied AC to X80 steel not only induced the increase of the current density for both anode and cathode, but also the change of cathode reaction from mixing control to activation control.

CORROSION BEHAVIOR OF X80 STEEL IN YING-TAN SOIL SIMULATED SOLUTION UNDER DISBONDED COATING

In this work,a rectangular crevice disbanded coating model of buried steel pipeline in Yingtan soil simulated solution was made,and the in situ electrochemical measurement of electrochemical impedance spectroscopy(EIS) was used for characterization of X80 steel under the disbonded coating,at the same time,the corrosion behavior was also investigated.The results showed that the electrochemical characteristics in each position of disbonded area was almost same in the early corrosion time,and the EIS was composed of high-frequency capacitance and low-frequency inductance.After corrosion occurred,the high-frequency capacitance radius increased,and the low-frequency inductance disappeared.The extent of corrosion of X80 steel specimen surface was rather distinct with distance from holiday.The corrosion at holiday and the bottom of disbonded area was most serious because of oxygen corrosion and anodic dissolution.However,in the central,it was weaker.After removing the corrosion product,pits appeared obviously on the specimen surface in the central of disbonded area, indicating that the tendency of pitting occurring increased,and the type of corrosion changed from general corrosion to localized corrosion.According to the EIS and experimental results,the corrosion process under disbanded area could be divided into three steps:oxygen depletion,anion migration and corrosion expansion.

Failure Analysis of Cracking on Side Tooth Gingivae of a Percussive Drill Bit

Failures of percussive drill bits have been attributed to the external mechanical force and structural design by engineers and researchers for a long time. Few attentions have been focused on the material itself. Generally, it is the unqualified material, induced by the improper metallurgy and heat treatment process, which makes the drill bits fail soon after they are put into service. Reasons of the crackings on side tooth gingivae of a drill bit, which failed prematurely at the beginning of the service, were analyzed comparatively. Scanning electron microscope, metallographic observation, mechanical performance test and chemical composition analysis were adoptedfor the failure analysis. Results show that microstructure, chemical composition and mechanic properties of the substrate were excluded from the reasons of the cracking. It is mainly the excessive inclusions and defects in the carburized layer which induced the inhomogeneity of material organization and performance. Those inclusions and defects act as stress concentrators and leading to the crack initiation. In addition, toughness of the carburized layer is also decreased due to its over-carburization. It makescracks more easily initiate from the carburized layer and then propagate into the substrate under the stress of interference fit.

Influence of H2S Concentration and pH Value on Corrosion Behavior of Weld Joint of X65 Subsea Pipeline Steel

With an autoclave the environment of submarine gathering system was simulated, then in which the influence of concentration of H2S and pH value on the corrosion behavior of weld joints of X65 pipeline steel was investigated by electrochemical techniques, immersion test, SEM and XRD analysis technique. The results show that the free corrosion potential of different portion of the X65 pipeline steel weld joint exhibited a tendency of decrease, but the corrosion density of increase, corresponding to the following order as: the weld seam, the heat affected zone and the base material. The average corrosion rate of the steel weld joints is in the range of 0.1~0.25 mm/a, the increase of the concentration of H2S and the reduction of the pH value can both lead to the in- crease of corrosion rate. The corrosion mode of weld seam of X65 steel is mainly the uniform cor- rosion in the simulated environment, and the corrosion of weld seam is slighter than that of the heat

Influence of Water Contents on Corrosion Behavior of Continuous Casting Copper-clad Steel in Dagang Soil

The corrosion behavior of continuous casting copper-clad steel in Dagang soil with 20%~30% water contents was studied by polarization curve measurement and EIS. It is shown that during the initial corrosion stage, the corrosion rates of continuous casting copper-clad steel are al- most equal, which imply the water contents have little influence on the corrosion behavior. The con- trol step for corrosion of continuous casting copper-clad steel in the soil is activation polarization process at the initial stage. During the later stage, the corrosion rates of continuous casting copper- clad steel decrease and then tend to be stable with the increase of water contents, which may be due to the synergistic effect of the two opposite factors i.e. the block effect with reducing oxygen and the stimulation effect with environmental water on the electrode process. However the corrosion be- havior is mainly affected by oxygen diffusion control.

STRESS CORROSION CRACKING AND ITS MECHANISM OF 16Mn STEEL AND HEAT-AFFECTED ZONE IN ALKALINE SULFIDE SOLUTIONS

Stress corrosion cracking(SCC) of 16Mn steel and its heat-affected zone(HAZ) in alkaline solution with sulfide and Cl~- was investigated by electrochemical technology,slow strain rate tensile(SSRT) test and U-bent specimen immersing test.Results show that the original microstructure, the coarse grain structure acquired by air cooling treatment and the hardening microstructure obtained from quenching performed a passivation behavior in alkaline sulfide solution.Correspondingly, the passivation current density of them decreased gradually with the cooling rate increased. The corrosion potential of the quenching microstructure,air-cooling microstructure and original microstructure decreases sequentially,indicating that HAZ is the cathodic area and the fusion line as well as bulk steel anodic area.Based on the results,the corrosion is feasible to happen nearby the fusion line would increase,and thus the residual tensile stress area would be exposed to the electrolyte after long-term service,which results in SCC.The susceptibility of SCC was lowered down gradually among hardening microstructure,coarse grain microstructure and original microstructure.SCC mechanism of 16Mn steel in the alkaline solution containing sulfide was anodic dissolution(AD) in terms of intergranular fracture.