压缩机套管水冷器腐蚀问题探讨

以合成氨装置原料气压缩机套管水冷器的缝隙腐蚀为例,从金属离子的浓差电池、氧浓差电池和形成闭塞电池的自催化作用等方面探讨了其腐蚀类型和机理,分析了影响腐蚀的环境因素,提出了防治腐蚀的相关措施     

华南酸性土壤对碳钢腐蚀的研究

通过对一年的现场测试工作所取得的大量数据的分析,并综合“华南酸性土壤腐蚀试验站”历年来测试数据,初步探讨了碳钢在华南酸性土壤中的腐蚀机理,认为土壤互相粘结后与碳钢表面接触产生了缝隙,从而形成氧浓差电池。土壤中异养菌等多种微生物的存在也对腐蚀产生综合作用。     

陕北盐渍土壤对X80管线钢电化学腐蚀行为的影响

采用电化学测试、SEM及EDS微观分析等方法,研究了X80管线钢在陕北水饱和盐渍土壤中的电化学腐蚀行为。结果表明,在水饱和盐渍土壤中,随腐蚀时间延长,X80钢腐蚀趋势和腐蚀速率均明显增大,局部腐蚀面积和深度不断增加,钢基体表面由以全面腐蚀为主转为以局部腐蚀为主,腐蚀机理为氧浓差腐蚀电池和局部腐蚀自催化效应,腐蚀速度主要受氧扩散过程控制;EIS图谱具有双容抗弧与Warburg阻抗特征,电荷传递和扩散传质的阻力随时间越来越大,而结合层电阻明显减小,这与钢表面生成腐蚀产物膜的完整性和致密性有关,腐蚀产物主要为铁的氧化物、硫化物和土壤中盐类的混合物。     

CO2环境中醋酸对X65钢湿气管线顶部腐蚀行为的影响

为研究延长气田CO₂环境中醋酸对X65湿气管线顶部腐蚀的影响，通过自制顶部腐蚀模拟实验装置以及动电位极化曲线和电化学阻抗法，采用挂片失重、扫描电镜分析手段研究了CO₂环境中醋酸对X65电化学腐蚀行为的影响。结果表明，腐蚀介质pH与HAc质量浓度呈对数函数变化，HAc质量浓度升高则腐蚀电流密度变大，阴极反应增强，腐蚀产物膜的溶解性增加，易产生孔隙，诱发氧浓差腐蚀。不同HAc质量浓度下，EIS高频容抗弧与界面双电层的行程有关，EIS低频斜线与腐蚀介质扩散有关。内层腐蚀产物膜与基体结合界面凌乱。HAc质量浓度增加，顶部腐蚀速率增加，腐蚀速率与HAc质量浓度呈对数函数关系。     

大庆喇嘛甸油田掺水集输管线腐蚀机理及防护措施

结合大庆喇嘛甸油田埋地管线的腐蚀现状,通过对掺水腐蚀管线的材料、力学性能测试、腐蚀形貌宏观与微观分析、材料金相组织分析及腐蚀产物微区元素成分测试等,研究了掺水管线的腐蚀过程和机理,并对其腐蚀防护提供了一些有效建议。结果表明:掺水管线外表面腐蚀严重、腐蚀坑集中是导致腐蚀泄漏主要原因。管线外防腐层的破坏导致地层水渗透到管体表面,水中溶解氧与管体发生吸氧腐蚀,形成不致密腐蚀产物层(Fe2O3),致使管线发生腐蚀破坏;而水中一定的氯离子渗透后,形成集中点蚀坑,在活态-钝态微电偶腐蚀电池作用下,以大阴极小阳极的结构,短时间内加速了管线穿孔泄漏。在合理施工和有效管理的前提下采用外加阴极电流保护、涂层+阴极保护或优选耐蚀材料等方法,都可用作该段管线防腐的有效措施。     

转化器腐蚀原因分析及改进措施

简要对转化器的腐蚀进行分析，提出列管内物料腐蚀主要是由于混合气中的氯化氢遇水生成浓盐酸腐蚀列管以及循环冷却水中微量的氧对碳钢的腐蚀、由温度差形成的腐蚀电池对列管的腐蚀。并提出相应的防腐措施，用含氟玻璃棉代替有机硅玻璃棉并降低气体流速；控制触媒含水；增设除水装置，降低乙炔气含水率；开车前进行露点分析。     

C8钢在直硝法高压釜反应筒内的工况腐蚀研究

本又利用U8钢管在直硝法高压釜反应筒内连续分布的方式进行腐蚀研究。在工况条件下，真实反映了C8钢在反应筒内自上而下的腐蚀规律。研究的结果为发展和改进浓硝酸用特殊不锈钢提供参考。通过外观、腐蚀率、腐蚀产物成分分析、金相检验等方法，对发生的现象给予了阐述和总结。     

硫化氢腐蚀产物氧化历程的研究

油品中的H2S与铁锈的反应产物自燃性很高。在一定温度下,模拟油品中H2S腐蚀产物的生成方式,利用SO2气体分析仪在线监测氧化尾气中SO2气体含量,分析氧化反应产物中单质S含量及硫酸根离子含量,研究了无氧条件下H2S气体与Fe2O3、Fe3O4和Fe(OH)3反应产物的氧化反应历程。结果表明,不同物质的H2S腐蚀产物的自燃性差异很大,其氧化反应历程也不同。Fe2O3、Fe3O4的H2S腐蚀产物在氧化初期以生成单质S为主;当氧化升温分别达到70℃和80℃左右时开始有SO2生成;随着氧化温度继续升高,更多腐蚀产物氧化生成Fe SO4。Fe(OH)3的H2S腐蚀产物自燃性差,氧化过程没有SO2生成。     

硫化氢腐蚀产物氧化历程的研究

油品中的H2S与铁锈的反应产物自燃性很高。在一定温度下,模拟油品中H2S腐蚀产物的生成方式,利用SO2气体分析仪在线监测氧化尾气中SO2气体含量,分析氧化反应产物中单质S含量及硫酸根离子含量,研究了无氧条件下H2S气体与Fe2O3、Fe3O4和Fe(OH)3反应产物的氧化反应历程。结果表明,不同物质的H2S腐蚀产物的自燃性差异很大,其氧化反应历程也不同。Fe2O3、Fe3O4的H2S腐蚀产物在氧化初期以生成单质S为主;当氧化升温分别达到70℃和80℃左右时开始有SO2生成;随着氧化温度继续升高,更多腐蚀产物氧化生成Fe SO4。Fe(OH)3的H2S腐蚀产物自燃性差,氧化过程没有SO2生成。     

NdFeB磁体在AlCl_3-EMIC离子液体中的初期腐蚀研究

研究了电沉积铝过程中NdFeB磁体在离子液体(AlCl3-EMIC)中浸泡初期的腐蚀行为。结果表明:NdFeB磁体在离子液体中的开路电位约为-0.663V,自腐蚀电流密度为2.399×10-5A/cm2,腐蚀同时受阳极反应和阴极反应过程控制;随着浸泡时间的延长,NdFeB磁体表面呈现较明显的腐蚀点、腐蚀坑,以及出现晶粒脱落现象,腐蚀产物主要为Nd、Fe等的氧化物。NdFeB磁体表面未形成完整的防护性铝膜之前,与离子液体的接触时间应尽量短,不宜超过10min。     

仿古铸铁的局部腐蚀和相转移研究

The corrosion behaviors of simulated archaeological iron in solution （0.06 mol·L^-1 NaCl＋0.03 mol·L^-1 Na2SO4＋0.01 mol·L^-1 NaHCO3） simulating soil water composition was investigated by potentialdynamic polarization, constant potential polarization, and simulated occluded cell （O.C.） galvanostatic tests. X-ray diffraction （XRD）, energy dispersive spectrometry （EDS）, and scanning electron microscope （SEM） were used to study the corrosion morphology and the evolution of corrosion product. The objective was to discover the transformation process of archaeological iron, and determine the distribution of chlorinated corrosion products. The results showed that the presence of crevice, cavities, and channels facilitates the localized corrosion under rusts; the autocatalytic effect increases the concentration of Fe^2＋, Cl^-, and SO4^2- , and promotes local acidification within the crevices and cavities. Meanwhile, the phase transformation of corrosion products is concluded to proceed by means of two ways. One is that the ferrous ions are transformed into different kinds of FeOOH via the intermediate Fe（Ⅱ）-Fe（Ⅲ） hydroxyl-salt （i.e. Green Rusts）; the other is that the Fe^2＋ ions are transformed into FeCl2, FeCl3, and orange powders akaganeite at the crevices and cavities.     

2Cr13不锈钢在含饱和CO_2盐溶液中的缝隙腐蚀行为研究

通过电化学动电位极化方法研究了2Cr13不锈钢在含饱和CO2盐溶液中的缝隙腐蚀行为。考察了温度、NaCl浓度以及HAc浓度对缝隙腐蚀的影响规律,结果表明:2Cr13钢缝隙腐蚀击破电位低于点蚀击破电位,同条件下更易引发缝隙腐蚀,且随温度、NaCl浓度、HAc浓度升高,缝隙腐蚀击破电位逐渐下降,缝隙腐蚀诱发阻力减小,缝隙腐蚀倾向增大,2Cr13钢耐缝隙腐蚀性能降低。在该介质中,腐蚀发生时,缝隙部位作为阳极加速腐蚀,缝隙外部作为阴极而得到保护。     

Hydrogen absorption into Grade-2 titanium during crevice corrosion

Hydrogen absorption into ASTM Grade-2 titanium (Ti-2) under actual crevice corrosion conditions was studied using a galvanic coupling technique. The alloy's iron content and its distribution in Ti-2 had a marked effect on the accumulation of crevice corrosion damage and the maximum depth of penetration. Iron present in intermetallic precipitates appeared to suppress crevice propagation and to limit the maximum depth of penetration by catalyzing the proton reduction reaction inside the crevice. Crevice corrosion initiated more rapidly, and the rate of activation of propagation increased, as the temperature was increased over the range 85–135 °C. However, the extent of overall crevice corrosion decreased with increasing temperature. Both the total amount of damage accumulated and the maximum penetration depth increased with increasing chloride concentration over the range of 0.27–2.5 mol dm⁻³, whereas in 5.0 mol dm⁻³ NaCl solution, the amount of crevice corrosion damage was much less. Additionally, both a high temperature and a high Cl⁻ concentration tended to lead to the localization of crevice propagation at a small number of active sites. A relatively constant hydrogen absorption efficiency of 5–8% was obtained, indicating that hydrogen absorption into Ti-2 under crevice corrosion conditions is insensitive to the duration of propagation, the temperature, and the Cl⁻ concentration.     

Impedance of reinforcing steel with disbonded dual polymer–zinc coating

Dual polymer/zinc-coated concrete steel reinforcing bar (rebar) is a novel material intended to improve corrosion related durability in marine and deicing salt environments. Severe fabrication bending induces coating breaks and separation from the steel substrate, creating a crevice where some zinc corrosion takes place. EIS measurements to characterize that corrosion revealed spontaneous fluctuations that included variations in the size of the high frequency loop and sometimes strong intermediate frequency dispersion. This investigation examined the underlying causes for that behavior. The fluctuations were explained by formation and episodic release of hydrogen gas in the crevice underneath the disbonded coating, as a product of the cathodic reaction in corrosion of the zinc layer. The gas caused partial occlusion of the crevice and the crevice opening at tears in the coating. Gas occlusion increased the effective electrolytic resistance inside the crevice, and contributed to an enhanced transmission line configuration with associated frequency dispersion, especially notable in the intermediate frequency part of the impedance spectrum. A radial transmission line model was adopted as a simplified abstraction of the system. Properties of the radial transmission line impedance behavior are discussed. The model output replicated the key features of the measured impedance spectrum and its fluctuations, and permitted isolating the low frequency response as the most representative to evaluate corrosion conditions inside the crevice in subsequent research.     

Crevice corrosion cathodic reactions and crevice scaling laws

A numerical model that predicts the rates of metal dissolution and electrolyte composition along the length of a metallic crevice was used to simulate the crevice corrosion of AISI 304 stainless steel. The model considers both the forward and reverse electrochemical reactions that might take place during the corrosion process. The environment exterior to the crevice, where a net cathodic current is produced, was simultaneously modeled. It was found that cathodic reactions are likely to occur towards the tip of the crevice. For the case when the hydrogen evolution reaction was considered as a possible cathodic reaction in the crevice, it is shown how the delayed instigation of this reaction may be the cause of an experimentally observed increase in pH at the crevice tip. Two critical crevice scaling laws were examined using model predictions and one scaling law fit the model predictions very well. This scaling law differentiates between crevices that will undergo active corrosion and those that will remain indefinitely passive.     

Sensing electrochemical activity in polymer coated metals during the early stages of coating degradation–Effect of the polarization of the substrate

Eventual limitations in the applicability of the SECM in the SG-TC mode to in situ monitor electrochemical activity in the proximity of a sub-centimeter defect (holiday) through the coating applied to protect a metal against the corrosive environments has been explored. The possible occurrence of chemical interferences between the chemical species involved in the corrosion reactions, as well as the formation of reactive secondary products, have been investigated because they would influence the faradaic signals registered at the SECM tip. In order to separate the contributions of the half-cell processes, polarization of the substrate was performed in this work, both negative and positive with respect to the corrosion potential of the sample in the test environment, and the signals detected at the tip were analyzed. By selecting different values for the potential applied to the tip, the local concentrations of Fe(II) ions, hydrogen peroxide and oxygen could be monitored. It has been observed that the oxygen reduction reaction occurring at the cathodic sites on the corroding steel surface can be accompanied by the formation of hydrogen peroxide as intermediate, and this species will interfere with the detection of dissolved metal at the tip.     

Application of wire beam electrode technique to investigate initiation and propagation of rebar corrosion

Multi-electrode technique named as wire beam electrode (WBE) was used to study pitting corrosion of rebar under concrete cover. When WBE embedded mortar sample was immersed in NaCl solution, uneven distributions of galvanic current and open circuit potential (OCP) on the WBE were observed due to the initiation of pitting corrosion. The following oxygen depletion in mortar facilitated the negative shift of the OCP and the smoothing of the current and potential distributions. Wetting–drying cycle experiments showed that corrosion products instead of oxygen in wet mortar specimen sustained the propagation of pitting corrosion due to Fe (III) taking part in cathodic depolarization during oxygen-deficient wet period, which was confirmed by micro-Raman spectroscopy. In addition, new pitting corrosion occurred mainly near the corrosion products, leading to preferentially horizontal propagation of rust layer on the WBE. A localized corrosion factor was further presented to quantify the localised corrosion based on galvanic current maps.     

Corrosion of similar and dissimilar metal crevices in the engineered barrier system of a potential nuclear waste repository

Crevice corrosion is considered possible if the corrosion potential (E_corr) exceeds the repassivation potential for crevice corrosion (E_rcrev). In this study, potentiodynamic polarization and potentiostatic hold were used to determine the E_rcrev of similar and dissimilar metal crevices in the engineered barrier system of the potential Yucca Mountain repository in 0.5 M NaCl, 4 M NaCl, and 4 M MgCl₂ solutions at 95 °C. The results were compared with data previously obtained using crevices formed between Alloy 22 and polytetrafluoroethylene. It was observed that, except for Type 316L stainless steel, all other metal-to-metal crevices were less susceptible to crevice corrosion than the corresponding metal-to-polytetrafluoroethylene crevices. Measurements of galvanic coupling were used to evaluate the crevice corrosion propagation behavior in 5 M NaCl solution at 95 °C. The crevice specimens were coupled to either an Alloy 22 or a Titanium Grade 7 plate using metal or polytetrafluoroethylene crevice washers. Crevice corrosion of Type 316L stainless steel propagated without repassivation. For all the tests using a polytetrafluoroethylene crevice washer, crevice corrosion of Alloy 22 was initiated at open circuit potential by the addition of CuCl₂ as an oxidant, whereas no crevice corrosion of Alloy 22 was initiated for all the tests using Alloy 22 or Titanium Grade 7 metals as crevice washer. However, crevice corrosion propagation was found to be very limited under such test conditions.     

Surface potential distribution over a zinc/steel galvanic couple corroding under thin layer of electrolyte

In this study, surface potential and surface pH changes over a zinc/steel galvanic couple corroding in artificial seawater (ASW) at 60 and 90% RH have been investigated. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test. The potential difference over the zinc and steel surface in 90% RH was very low (less than 200 mV) showing that whole steel surface was under galvanic protection. On the other hand, in 60% RH, after several days of corrosion the potential difference between the zinc coating and the steel surface was very high (more than 500 mV) and hence the galvanic protection was limited to interface region. The X-ray analysis of the sample corroded in 60% RH has shown that the zinc corrosion products were deposited on the steel surface near the interface, the same region has shown a low pH compared to than in other part of the steel surface. This led to conclude that with the progress of corrosion, the coating surface of zinc coated steel acidifies by the hydrolysis reaction of the dissolved zinc ions, and the iron surface showed the alkalinity by the oxygen reduction reaction. Moreover, the parts of the steel surface covered with zinc corrosion products had developed relatively less noble potential than other parts indicating that zinc corrosion products took a role to protect the base steel against corrosion. It was assumed that this behavior was related to a combination of the water absorbing capability of zinc corrosion products and adsorption of zinc ion on the steel surface due to low pH.     

The influence of nickel-aluminium bronze microstructure and crevice solution on the initiation of crevice corrosion

A new mechanistic model has been established for the chemical and electrochemical mechanisms controlling nickel-aluminium bronze crevice corrosion. The crevice corrosion was initially confined to eutectoid regions with slight attack of the copper rich α-phase within the α+κ_III eutectoid. In the presence of high chloride concentrations, copper and aluminium complexes form and the hydrolysis of these complexes leads to the acidification of the crevice solution chemistry. As the crevice solution becomes increasingly acidic the initial protection of the κ-phases due to their higher aluminium contents is lost and become anodic to the α-phase. The continuous nature of the κ_III-phase makes it vulnerable with an ∼80 μm depth of attack after only the first month which is accompanied by internal copper redeposited at cathodic sites. Crevice corrosion of copper-based alloys is often reported to be controlled by a copper-ion concentration cell, however, for nickel-aluminium bronze it appears that primarily the crevice corrosion propagation results from a combined action of localised acidification and differential aeration between the bold and occluded zones. This mechanistic understanding provides further insights in to the development of crevice corrosion experienced by nickel-aluminium bronze after long-term exposures to natural seawater for up to 3 years.