热网换热站不锈钢换热板点蚀的临界条件

热网二级换热站一般采用板式换热器,在其使役过程中不锈钢换热板一旦出现腐蚀穿孔,不但影响一次水水质,而且干扰换热器稳定运行,甚至影响居民供热,因此阐明换热板发生点蚀的临界条件对于科学设定水质控制标准和防止点蚀发生具有重要意义。为此,通过材料化学分析、XRD、SEM及电化学测试等方法对这一问题进行研究。结果表明:在65℃条件下,304不锈钢点蚀的临界Cl;浓度为125 mg/L,316L不锈钢点蚀的临界Cl;浓度为230 mg/L;不锈钢表面一旦形成垢层,表面会发生局部酸化,此时不锈钢更容易发生点蚀;运行过程中为了防止不锈钢换热板点蚀,不仅要严格控制循环水中Cl;浓度,还应防止换热板表面结垢或附着腐蚀产物。     

Short-term electrochemical corrosion behavior of pipeline steel in saline sandy environments

In this study, we investigate the short-term electrochemical corrosion behavior of X70 pipeline steel in salinized sandy soil using the electrochemical impedance spectroscopy (EIS). The results show that the corrosion behavior of X70 steel in sandy soils with a relatively high water content (~ 18%) is mainly dominated by pitting or localized corrosion, which is controlled by charge-transfer processes. The reason can be attributed to the loose corrosion products (unable to produce an effective protective layer) and discontinuous gradation in sand particles. EIS plots of X70 steel consist of two capacitive loops at the initial stage of corrosion, while a capacitive arc at high-, medium-, and low-frequency is developed after 12 h of corrosion. The capacitive loop formed at the high frequency indicates the resistance and capacitance generated by the penetration of ions into the metal surface through the sand pores and the defects of the corrosion product film.     

Failure analysis of austenitic stainless steel pipes in amine liquid regeneration unit of a desulfurizer

Failure investigation was carried out on the austenitic stainless steel pipes of an amine liquid purification unit. The authors investigated the causes of the pipe failure by analyzing chemical compositions of materials and corrosion products, observing microstructures, and testing the pitting corrosion resistance of materials. The results showed that the failure was caused by pitting corrosion due to the presence of chlorine ions in the working medium. Another important cause of pitting corrosion was the decrease of corrosion resistance of welds owing to the microstructural changes taken place during welding. A large amount of sulfate ions in the working medium accelerated the pits growth. At the same time, proposals were put forward to the user of the failure pipes.     

高铁不锈钢焊接接头在海洋大气环境中的腐蚀行为

为了提高高铁的行车安全,通过现场暴露试验,采用数码显微镜、能量色散谱、拉曼光谱及电化学测量方法研究了高铁不锈钢焊接接头在万宁海洋大气环境中暴露2a后的腐蚀行为规律。结果表明:暴晒2a后不锈钢焊接接头的腐蚀类型以点蚀为主,蚀坑密度较高;腐蚀产物主要有Fe3O4、α-Fe2O3、γ-Fe2O3、α-FeOOH,β-FeOOH和γ-FeOOH,不锈钢焊接试样在3.5%NaCl溶液中阳极行为存在一定钝化特性;各区域点蚀电位高低排序为筋板母材区〉底板母材区〉焊缝区。     

Pitting corrosion failure analysis of a wet gas pipeline

This paper presents corrosion failure analysis of an underground natural gas pipeline. The pipeline material grade is API 5L X65 with 10-in ID. The pipeline transfers multiphase fluid (gas, condensate, and water) from a gas well to a gas gathering plant, located 4200 m away from the well site. 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: the oversized pipeline changed the dominant flow regime to “stratified”. In the stratified flow regime, the accompanying water phase accumulated in the pipelines' low points. Considerable concentration of calcium ions along with high pH in CO₂ media favored precipitation of calcium carbonate. The relatively thick scales adhered to the pipe surface were partially loosened and removed by the regional turbulent flow. This exposed the fresh steel surface to the corrosive media. The uncovered areas acted as the preferential anodic sites coupled with nearby large cathodic sites which were covered by scales and/or corrosion products. Under such conditions, pits emerged on the steel surface until one of them grew faster and failed the gas pipeline.     

Interactions of different types of localized corrosion in surgical implants

Surgical implants often show different types of localized corrosion such as corrosion fatigue cracking, pitting and crevice corrosion on the same part. Interactions of these different corrosion phenomena were investigated. This was done by cyclic loading of electropolished tensile specimens at different constant and changing potentials. Material investigated was a surgical implant steel X2CrNiMo18-15-3 which was immersed in physiological NaCl solution. Pitting and repassivation potentials were determined. Samples with and without artificial cracks as well as masked specimens were tested. Incubation period for first damage, density and size of pits by coulometric and volumetric method were determined. The fracture surfaces were then investigated by SEM. Results show that not in all cases pitting corrosion was the cause for corrosion fatigue cracking. Also pitting is favoured by crack formation. Density of pits increases by a factor of 5 without any change to pitting potential. There are primary pits formed prior to crack initiation and secondary pits formed after crack initiation. At samples without crack there is almost no difference between the optically measured value of total pit volume and the coulometrically determined value. At samples with cracks coulometric volume of pits is much larger than optical one. This proves that there is a significant amount of crevice corrosion in the crack. The corrosion current density in the crack increases by two orders of magnitude when comparing it to electropolished surface of the sample. Results of laboratory experiments are confirmed by failure of a real implant.     

Distribution of corrosion pits on a metal surface

The distribution of corrosion pits on the surface of stainless steel samples treated in a 3% NaCl solution is studied. It is established that the distributions are thermodynamically optimized and can be described by the Gibbs expression for a canonical distribution of the thermodynamic probability of energy fluctuations. A wide occurrence of the pitting corrosion and the broad scatter of pits with respect to size are explained.     

Pits with coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater

Micropits surrounded by coloured halos were observed under incident-light microscope on 1Cr18Ni9Ti stainless steel surface after “ennoblement” in seawater. Ennoblement has been attributed to biofilm formation on stainless steel surface in seawater. In this study, the environment in biofilm for ennoblement was simulated by adding H₂O₂ into seawater at concentrations that were reported to detect in marine biofilm [N. Washizu, Y. Katada, T. Kodama, Corros. Sci.46 (2004)1291.]. H₂O₂ increased the passivity of stainless steel in seawater, but this passivation was not uniform. The probability of pitting corrosion was increased after ennoblement. Equal thickness interference on the deposition film around the pits was believed to be the reason for the coloured fringes. It is conceivable that haloed pits on the stainless steel surface are the characteristic morphological indications for pitting corrosion formed under ennoblement condition in seawater. Examinations on the microbial and structural effects of the biofilm were not included in this study.     

X80钢在不同土壤模拟溶液中的腐蚀行为

采用失重法、扫描电镜(SEM)、能谱分析(EDS)及X射线衍射(XRD)等方法研究了X80钢在3种土壤模拟溶液中的腐蚀行为。结果表明:X80钢在鄯善和新洲模拟溶液中以均匀腐蚀为主;而其在樟树模拟溶液中以点蚀为主。X80钢在3种不同模拟溶液中的腐蚀速率满足:樟树新洲鄯善。在鄯善模拟溶液中,结晶盐CaSO4.2H2O随浸泡时间的增加不断析出并沉积在腐蚀产物层表面,一定程度上抑制了Cl-和溶解O对X80钢的侵蚀作用,减缓了X80钢的均匀腐蚀速率。在樟树模拟溶液中,X80钢表面难以形成保护性产物层,腐蚀随时间不断加剧,最终促进点蚀的发生。     

Monitoring pitting corrosion of AISI 316L austenitic stainless steel by acoustic emission technique: Choice of representative acoustic parameters

This experimental work was aimed at investigating the monitoring of pitting corrosion by the acoustic emission (AE) technique, for pits developed by potentiostatic or galvanostatic polarization on two types of 316L austenitic stainless steels, in a 3% NaCl solution acidified to pH 2. The study of the evolution of AE global activity during the test showed the existence of a time delay before pits became emissive. This time delay and the AE events number rate measured during the propagation step of the pits are closely correlated with the sensitivity of the material towards pitting and with the polarization procedure. Moreover, the evolution of cumulative % of AE signals number versus selected acoustic parameters shows that rise time and counts number of signals appear to be discriminating acoustic parameters for monitoring pitting corrosion of austenitic stainless steels by acoustic emission technique in our experimental conditions, whatever the polarization procedure and the type of tested steel.     

GW93镁合金点蚀过程的原位监测及点蚀机制

镁合金作为最轻的金属结构材料有很多优异性能,但镁自身化学性质活泼,耐蚀性差,尤其易发生点蚀,破坏性和隐患性非常大。若想降低点蚀对镁合金部件安全服役性能的影响,就需要对镁合金点蚀机制有清楚的认识。然而,适用于其他金属材料的经典的点蚀机制是以形成氧浓差电池为基础,阴极发生的是氧还原反应,而镁合金阴极发生的是析氢反应,因此镁合金的点蚀形成过程尚需深入研究。采用扫描振动电极技术(SVET)原位监测了铸态GW93镁合金在3. 5%NaCl(质量分数)溶液中的点蚀过程,采用SEM观察了腐蚀过程镁合金微观形貌变化,采用电流-时间曲线对比了阴阳极电位对点蚀发展的影响。研究结果表明,点蚀坑外是微阴极,发生析氢反应,点蚀坑内是微阳极,发生镁的溶解反应,随着时间增加,点蚀发展过程是动态变化的。镁合金中第二相所导致的微电偶腐蚀加速效应及氯离子在腐蚀坑内的聚集,两者的协同作用驱动了点蚀不断向基体内部生长。     

Super austenitic stainless steels — a promising replacement for the currently used type 316L stainless steel as the construction material for flue-gas desulphurization plant

Potentiodynamic anodic cyclic polarization experiments on type 316L stainless steel and 6Mo super austenitic stainless steels were carried out in simulated flue-gas desulphurization (FGD) environment in order to assess the localized corrosion resistance. The pitting corrosion resistance was higher in the case of the super austenitic stainless steel containing 6Mo and a higher amount of nitrogen. The pit-protection potential of these alloys was more noble than the corrosion potential, indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed that the super austenitic stainless steels have a little tendency for leaching of metal ions such as iron, chromium and nickel at different impressed potentials. This may be due to surface segregation of nitrogen as CrN, which would, in turn, enrich a chromium and molybdenum mixed oxide film and thus impedes the release of metal ions. The present study indicates that the 6Mo super austenitics can be adopted as a promising replacement for the currently used type 316L stainless steel as the construction material for FGD plants.     

Effects of Cl- and SO2-4 Ions on Corrosion Behavior of X70 Steel

Corrosion behaviors of X70 steel were studied by means of electrochemical experiments and morphology observation.First, through potentiodynamic polarization in solution of various Cl- ions concentration, it was found that Epit began to appear in solution of Cl- concentration above 0.1 mol/L, and there was a critical point of Cl- concentration between 0.05 mol/L and 0.1 mol/L, below which the extent of pitting and general corrosion were trivial, while in solution of Cl- concentration above 0.1 mol/L, general and pitting corrosion became greater as the increasing of Cl- concentration. All of them were confirmed by the SEM observations after anodic polarization. Second, via the potentiodynamic polarization curves of X70 steel in 0.5 mol/L Cl- solution with 0, 0.05, 0.5 and 1 mol/L SO42-ions, it was found SO42- ions were able to inhibit corrosion aroused from Cl- ions, accordingly a model was set up to describe the process. In addition, to further explore the inhibited effect of SO42- ions, EIS was used in solutions of different Cl- and SO42- concentrations, the results revealed that the e(ectrochemical resistance has a relation with the [SO42-]/[Cl-], that was, the bigger the value of [SO42-]/[Cl-], the greater the electrochemical resistance.     

几种超级不锈钢在模拟烟气脱硫环境中的点蚀行为

目前,针对904L,254s Mo和2507等几种主要备选超级不锈钢在烟气脱硫环境中的耐点蚀性能缺乏系统研究。在温度分别为20、40、70℃的死亡绿液溶液中,利用循环伏安曲线和扫描电镜(SEM)法,对316不锈钢和超级不锈钢904L、254s Mo及2507的极化行为和点蚀形貌进行了研究。结果表明:在该环境中,升高温度可降低4种不锈钢表面钝化膜稳定性并提高其点蚀敏感性;在不同温度环境下,316不锈钢均有严重的点蚀现象发生,而254s Mo和2507不锈钢表面均无明显点蚀迹象;在20℃时,904L不锈钢表面无明显点蚀迹象,40℃时,其表面出现典型的点蚀形貌,但点蚀坑尺寸较小,在70℃的高温下,其点蚀坑尺寸明显增大,点蚀损伤严重;254s Mo和2507均适合作烟气脱硫设备材料,而316、904L在该环境中需谨慎使用。     

铝电解电容器高压电子箔点蚀机理的研究

采用极化曲线和场发射扫描电镜等方法,研究了铝电解电容器高压电子箔在高温强酸性溶液中的点蚀机理。结果表明:在开路状态下铝光箔在硫酸盐酸发孔溶液中可以产生点蚀,测到自腐蚀电位就是点蚀电位;形成隧道孔后,阳极极化曲线出现点蚀电位,且点蚀过电位与隧道孔长度之间存在线性关系。根据点蚀的微电池模型及其在阳极极化下微电池的腐蚀极化图,提出产生上述现象的原因是阳极极化时带孔铝箔的表面由阴极向阳极转变,其转变的临界点即所测到的点蚀电位。     

Passivity and pitting of austenic stainless steel AISI 316 in the CH3OH-H2SO4 system

The passivity and pitting behaviour of AISI 316 stainless steel in methanol containing different concentrations (0.001 to 1 M) of H₂SO₄ has been investigated by an electrochemical method. Higher concentrations of sulphuric acid have been found to facilitate the passivation process and influence the passivity current and passivation range. The studies reveal the peculiarities of steel pitting and predicts the protective properties of the surface by a process of repassivation in this system. The kinetics of pitting, morphology of the pits and repassivation of the surface have been investigated for this steel. The beneficial effect of molybdenum as an alloying element of steel, on corrosion behaviour has been noticed.     

In-body corrosion fatigue failure of a stainless steel orthopaedic implant with a rare collection of different damage mechanisms

A stainless steel orthopaedic (femoral) implant was examined, which had fractured inside a patient’s thigh. It had been in his leg for almost two years, with no problem in the bone reconstruction process. The plate had apparently fractured during the first few months, when the bone had not been reconstructed completely. Different investigations such as visual assessments, hardness testing, stereoscopy, metallography, quantometry, SEM fractography and EDS microanalysis were performed in order to analyze the failure mechanism and its cause(s). Different damages were observed such as crevice corrosion pitting, initiation of cracks from these pits, intergranular surface cracking inside the crevice, and also SCC-like branched cracks. But, the main failure mechanism was determined to be corrosion fatigue assisted by crevice corrosion.     

Zum Ausscheidungs- und Korrosionsverhalten eines ferritisch-austenitischen Chrom-Nickel-Molybdän-Stahles nach kurzzeitiger Glühbehandlung unter besonderer Berücksichtigung des Schweißens

Structural changes and corrosion behaviour of a ferritic-austenitic chromium-nickel-molybdenum steel after short time heat treatments with special consideration of welding Susceptibility of the ferritic-austenitic steel X 2 CrNiMoN 225 in the solution treated and quenched state to intergranular corrosion and to stress corrosion cracking and pitting corrosion in solutions with chloride ions. Comparison with two austenitic steels (X 5 CrNi 189 and X 3 CrNiMoN 17135). Investigations of the influence of structural changes due to short time isothermal heat treatments and to welding on the mentioned three corrosion types.