Crevice corrosion monitoring of titanium and its alloys using microelectrodes

Crevice corrosion of titanium and its alloys in 10% sodium chloride was investigated at 100°C with the aid of microelectrodes. Potential, pH and chloride ion concentration inside the crevice were monitored using an Ag/AgCl electrode, a tungsten microelectrode and a Ag/AgCl chloride ion selective microelectrode, respectively. The pH and Cl^? concentrations within the crevice were calculated from the standard potential‐pH and potential‐log[Cl^?] calibration curves. The effect of Mo on the crevice corrosion of titanium was also studied. The passivation behavior on the titanium and Ti‐15%Mo alloy was studied using electrochemical impedance studies. There was no apparent change in pH and Cl^? ion activity inside the crevice for the alloy at 100°C, whereas a marginal decrease in pH and increase in Cl^? ion concentration were observed for pure titanium. Thus pure titanium is susceptible to crevice corrosion in hot 10% NaCl solutions at 100°C. The chloride ion activity was found to be reduced for the alloy so that the pH inside the crevice increased. The corrosion reaction resistance (R_t) was found to increase with the addition of Mo as an alloying element. It also increases with externally applied anodic potential. Hence, Mo is an effective alloying element, which enhances the crevice corrosion resistance of titanium.     

The crevice corrosion behaviour of stainless steel in sodium chloride solution

The crevice corrosion behaviour of 13Cr stainless steel in NaCl solution was investigated mainly by electrochemical noise measurements, considering the influences of the crevice opening dimension (a) and the area ratio of the electrode outside the crevice to the one inside the crevice (r). Results show that the increase of r value prolongs the incubation period of crevice corrosion, but crevice corrosion develops rapidly once the crevice corrosion occurs. The crevice corrosion develops preferentially at the crevice bottom and then spreads to the whole electrode surface. Proton could reduce on the uncorroded area and hydrogen bubbles form inside the crevice.     

Crevice Corrosion of Titanium in High Temperature-Concentrated Chloride Environments

Crevice corrosion of titanium is activated in concentrated chloride media at 100 °C. This was possible only with the tightest gap (0.005 cm) between Ti-Ti surfaces. No crevice corrosion was observed with greater gap dimensions. The design of the crevice led to the occurrence of two concentric circular rings of corroded areas, with many pits on them. After potentiostating in the passive region for 5 h in 25% NaCl (pH = 4.7)—where hydrogen evolution is thermodynamically prohibited—hydrogen gas bubbles were observed to egress out of the crevice mouth during ongoing crevice corrosion. This indicates that hydrogen evolution occurs within the crevice. The results are compatible with the occurrence of gradually increasing ohmic potential shift and localized acidification in the crevice electrolyte as judged by the measured gradual increase of the crevice corrosion current. The high acidity of the bulk electrolyte does not seem to be sufficient or even a necessary condition for crevice corrosion to occur.     

激光烧结石墨烯钛纳米复合材料及其耐腐蚀性能

石墨烯增强钛基纳米复合材料,因其良好的力学性能有望成为轻质高强结构材料。为了研究其硬度和耐腐蚀性能,在AISI 4140 合金结构钢的基板上采用激光烧结的方法制备了石墨烯钛纳米复合材料。采用扫描电子显微镜、X射线衍射仪、显微硬度计、拉曼光谱仪等研究了其微观结构、相组成和显微硬度等,并采用电化学极化法研究了激光烧结石墨烯钛纳米复合材料和纯钛在3.5%NaCl溶液中的腐蚀行为。结果表明：激光烧结后石墨烯存在并均匀分散于钛基纳米复合材料中,石墨烯的添加使得钛基纳米复合材料的显微硬度达到了450 HV0.2,与激光烧结纯钛（180 HV0.2）相比提高了1.5倍。石墨烯钛纳米复合材料的腐蚀电位比激光烧结纯钛的腐蚀电位有明显提高,从-0.64 V提高到-0.59 V;同时,腐蚀电流从1.6×107 A/cm2降低到7×108 A/cm2,说明其耐腐蚀性能优于激光烧结纯钛。     

用丝束电极研究SO_4~(2-)对纯铝缝隙腐蚀的影响

采用动电位极化和丝束电极技术测量了纯铝在2mol/L NaCl和2mol/L NaCl 0.8mol/L Na2SO4溶液中的极化曲线、缝隙内外的自腐蚀电位和电化学阻抗分布,研究了SO42-对铝缝隙腐蚀的影响。结果表明,在NaCl溶液中,缝隙内的铝为阳极、缝隙外为阴极;随浸泡时间增加,腐蚀不均匀性增加。加入Na2SO4后,减小了缝隙内外腐蚀电位差,显著降低了铝的腐蚀速度。Na2SO4是中性溶液中铝的吸附型缓蚀剂,延缓了缝隙腐蚀的发生。     

Electrochemical Behavior and Stress Corrosion Sensitivity of X70 Steel Under Disbonded Coatings in Korla Soil Solution

The corrosion of X70 pipeline steel under a model disbonded coating was studied in a simulated solution of Korla soil by combining in situ electrochemical measurements at different locations in the crevice and stress corrosion cracking (SCC) sensitivity analyses in the corresponding simulated environments. The results from electrochemical impedance spectroscopy showed that the corrosion product resistance R _t and charge transfer resistance R _ct of X70 steel first increased and then decreased with increasing distance from the opening of the crevice in the disbonded coating. Scanning electron micrographs showed that pitting in the crevice became more severe at deeper locations in the crevice. Slow strain rate tests showed that the lowest SCC sensitivity of X70 steel was found at 15 cm away from the opening, and the highest SCC sensitivity was at the end of the crevice.     

Crevice corrosion initiation and propagation on Alloy-22 under galvanically-coupled and galvanostatic conditions

The initiation and propagation of crevice corrosion on the Ni–Cr–Mo Alloy-22 has been studied in concentrated chloride solutions under galvanically-coupled and galvanostatic conditions. Under galvanically-coupled natural corrosion conditions crevice corrosion initiated but propagation was limited by repassivation. This was attributed to the slow kinetics of oxygen reduction on passive surfaces external to the crevice. Under galvanostatic conditions a potential more positive than 200 mV_Ag/AgCl and an applied current greater than 5 μA were required to stabilize propagation. A minimum critical current density to establish active sites within the crevice was estimated to be around 250 μA cm⁻².     

Influence of Cl- and polarization potential on crevice corrosion behavior of X70 pipeline steel北大核心CSCD

The crevice corrosion behavior of X70 pipeline steel in NaHCO3 solution with varying Cl- concentration was investigated by potentiostatic polarization method in terms of the initiation and development of crevice corrosion. Results show that inside the crevice the X70 steel could suffer from localized corrosion in NaHCO3 solution by polarization potential-0.4 V. The acidification initiated firstly at the crevice mouth and then extended gradually to the bottom. The hydrogen evolution could be observed with the development of corrosion and acidification. The cathodic reaction changed from the reduction of the dissolved oxygen to the reduction of hydrogen ions. The presence of Cl- did not change the crevice corrosion mechanism. With the increase of Cl- concentration, however, the crevice corrosion rate increased. The corrosion region moved towards the crevice bottom gradually and then pitting corrosion occurred with the increasing polarization potential. The initiation of crevice corrosion was determined by the polarization potential. © 2016, Corrosion Science and Protection Technology. All rights reserved.     

The corrosion behavior of carbon steel in CO2‐saturated NaCl crevice solution containing acetic acid

The corrosion behavior of X52 carbon steel electrodes in CO₂‐saturated NaCl crevice solution containing HAc was investigated by electrochemical measurements. Chemical environment measurements by Cl^? and pH microprobes show an enrichment of Cl^? ions and an increase of pH values inside the crevice. Moreover, both increments could accelerate with the decreasing dimension of the crevice mouth due to the high diffusive resistance. When the electrode in the crevice solution is coupled with the electrode in bulk solution, the alkalization and the enrichment of Cl^? ions in the crevice solution can result in a negative shift of potential of the electrode in crevice solution, while the potential of the electrode in bulk solution shifts positively during the corrosion process. Thus, a galvanic corrosion is established with the electrode in the crevice solution acting as anode while another in the bulk solution as cathode, i.e., the corrosion in the crevice solution was enhanced while the corrosion in the bulk solution was retarded. The anodic dissolution and the cathodic reduction processes dominate in the crevice solution and in the bulk solution, respectively.     

Crevice Corrosion Behaviors Between CFRP and Stainless Steel 316L for Automotive Applications

Carbon fiber reinforced plastics(CFRP) are promising lightweight materials for vehicle applications. 316 L is one of the most widely used types of austenite stainless steels and applied in lots of automotive applications. The existence of crevices will result in galvanic corrosion and crevice corrosion when CFRPs and 316 L are directly connected. A crevice former for the galvanic system was therefore designed and applied to evaluate the crevice corrosion behaviors and study the mechanism of galvanic crevice corrosion through several electrochemical techniques in this research. The results showed that the crevice corrosion of galvanic systems grew from crevice mouth to the inside crevice and could be divided into four steps, metastable pitting corrosion at the crevice mouth, initiating step of crevice corrosion, propagating step and ending step of crevice corrosion. Because of the influences of the galvanic system, electrode reaction rates were speeded up and the passivation region was shortened at the initiating stage of crevice corrosion. Corrosion rate was observed to be higher in the galvanic system than that in normal crevice systems.     

合金元素铁对钛缝隙腐蚀性能的影响

用浸泡腐蚀试验及电化学测试技术研究了合金元素铁对钛抗缝隙腐蚀性能的影响。试验结果表明,加入铁使缝隙腐蚀再钝化电位ＥＲ变负,缝隙腐蚀诱导期缩短,腐蚀率增大。通过电子探针分析和ＳＥＭ观察发现,铁以ＴｉＦｅ形式在晶界析出,腐蚀优先在ＴｉＦｅ析出物上发生,并且沿晶界向内部发展。由此得知,缝隙腐蚀起源于ＴｉＦｅ析出物,继而成长并促进了缝内活性溶解。     

Effect of cathodic protection on corrosion of pipeline steel under disbonded coating

In this work, a test rig was developed to study the effect of cathodic protection (CP) on corrosion of X70 pipeline steel in the crevice area under disbonded coating through the measurements of local potential, solution pH and dissolved oxygen concentration. Results demonstrated that, in the early stage of corrosion of steel, CP cannot reach the crevice bottom to protect steel from corrosion due to the geometrical limitation. Corrosion of steel occurs preferentially inside crevice due to a separation of anodic and cathodic reaction with the depletion of dissolved oxygen in the crevice solution. The main role of CP in mitigation of sequential corrosion of steel in crevice under disbonded coating is to enhance the local solution alkalinity. With the increase of distance from the open holiday, a high cathodic polarization is required to achieve appropriate CP level at crevice bottom. A potential difference always exists between the open holiday area and inside crevice, reducing the CP effectiveness.     

Corrosion resistant coatings based on synergistic effects of alkaline etching and molybdate treatment for AZ31D magnesium alloy

Eco-friendly, non-toxic protective coatings deposited from molybdate–based solutions have been developed as undercoat thin-films (for subsequent organic top coats) for AZ31D magnesium alloy. Direct treatment of Mg AZ31D substrates with molybdate–based solutions has no significant effect on the overall surface resistance (charge transfer resistance). Alkaline etching of Mg AZ31D surfaces using KOH solution prior to molybdate conversion coating showed significant enhancement in the corrosion resistances The optimum conditions of alkaline etching and molybdate treatment steps have been determined. The total surface resistance was improved from 2.1?×?10³ Ω.cm² (for as-polished AZ31D) to be 3.2?×?10³ Ω.cm² for the alkaline etched samples followed by 10?g?L^?1 molybdate treatment. The resistance to localised corrosion (pitting and crevice) improved significantly after applying the alkaline etching step. Molybdate–based coatings formed on Mg AZ31D exhibited a network of flower-like and needle-like protective molybdenum oxide structures which are belived to be responsible for the improvement in the pitting and crevice corrosion resistance. They impede the corrosive media from reaching the bare metal and hence improve the pitting and crevice corrosion resistances. This simple eco-friendly, low-toxicity pretreatment approach seems very promising and effective for improving the corrosion resistance of magnesium alloys in chloride containing solutions.     

热钾碱脱碳液中304不锈钢换热器管束的腐蚀失效分析

热钾碱脱碳液304不锈钢管束再沸器投用2年后发生了腐蚀泄漏，检查发现腐蚀发生在管子与管板之间的缝隙中.用电化学方法和能谱分析技术，对304不锈钢换热管及16 Mn管板在热钾碱脱碳液中的极化行为及管子表面腐蚀区的腐蚀产物进行了分析，结果表明：换热器管束的腐蚀是由于缝隙内外五价钒浓差造成的16 Mn管板的活化—钝化短路电池所引起，缝隙内闭塞电池的形成加速了缝隙内16 Mn的溶解速度，并使缝隙内介质酸化及Cl-等阴离子富集，导致304不锈钢管子的腐蚀.      

The nature of crevice corrosion of aluminum in chloride solutions

To study crevice corrosion of pure aluminum, polished specimens partly covered with a glass foil were polarized potentiostatically in 1 N NaCl-solution at potentials negative to the critical potential for stable pitting (pitting potential). For comparison, non-crevice experiments were performed on polycrystalline and singlecrystalline material in neutral as well as acidified 1 N NaCl-solution and in AlCl₃-solutions. Corrosion morphology was examined by scanning electron microscopy. In current-time plots recorded during experiments on crevice corrosion, both an incubation and a propagation stage are discernible. If experiments were interrupted during the induction period, micropits were found inside the crevice. This unstable micropitting is detectable down to 0.30 V below the pitting potential. In contrast, during crevice corrosion propagation, the aluminum surface undergoes general attack. In a range of 0.2 V below the pitting potential, dimpled surfaces are produced. At more negative potentials, metal dissolution occurs crystallographically oriented. An identical behaviour was detected on unshielded samples polarized in the same potential range in both 1 N AlCl₃- and acidified 1 N NaCl-solution. Hence, the build-up of an acidic electrolyte is considered the sufficient requirement for crevice corrosion initiation.     

离子注入对LaFe11.6Si1.4合金的缓蚀及其机理研究

目的通过表面Cr离子注入在LaFe_(11.6)Si_(1.4)合金表面生成一层具有耐蚀作用的保护层,从而提高合金的耐腐蚀性能。方法采用表面离子注入法,分别在注入电压为20、30、40 k V,注入计量为5×10~(16)、10×10~(16)、50×10~(16) ions/cm~2的条件下注入Cr离子。利用扫描电子显微镜及X-射线衍仪对合金的表面形貌、组织结构及成分进行了分析,通过电化学方法对合金表面离子注入后的耐腐蚀性进行了研究。结果当Cr离子的注入电压为40 k V,注入剂量为5×10~(16)、1×10~(17)、5×10~(17) ions/cm~2时,合金的开路电位分别是-0.585、-0.584、-0.57V(vs.SCE)。当Cr离子的注入剂量为5×10~(17) ions/cm2,注入电压为20、30、40 k V时,合金的开路电位分别是-0.63、-0.61、-0.57 V(vs.SCE)。可以看到,随着Cr离子注入计量和注入电压的增加,合金表面的腐蚀电位正向移动,耐腐蚀性提高。结论 Cr离子注入能够显著提高合金的耐腐蚀性,分析认为主要是由于合金表面生成了一层具有耐腐蚀性能的Cr_2O_3钝化层。此外,由于注入离子的轰击导致表面La(Fe,Si)13相分解生成α-Fe,也提高了合金的电极电位,增强了耐腐蚀性。     

阴极保护防止绝缘覆盖层下金属的缝隙腐蚀

对阴极保护防止绝缘覆盖层下金属缝隙腐蚀国内外的研究动态进行了综述,总结了有关阴极保护下缝隙内的电位、电流分布和传质过程及其数学模型等方面的研究成果,分析了阴极保护防止缝隙腐蚀的机理。     

Monitoring pitting‐crevice corrosion using the WBE‐noise signatures method

An electrochemically integrated multi‐electrode array namely the wire beam electrode (WBE) and noise signatures analysis have been applied in novel combinations to study crevice corrosion behaviour in the presence of pits. Characteristic electrochemical noise signatures were found to correlate with characteristic changes in WBE current distribution maps, which indicate corrosion rates distributions, corrosion patterns and the degree of pitting and crevice corrosion. Specifically, two characteristic noise patterns were observed: (i) the characteristic noise pattern of quick potential changes towards more negative direction with no recovery (termed noise signature I) was found to correspond with the initiation and stabilization of the anode inside crevice; and (ii) the characteristic noise pattern of the cyclic potential oscillation at a constant frequency (termed noise signature II) was found to correspond with the stable anodic dissolution in the occluded cavity site in WBE current distribution maps. A new parameter namely the localization parameter (LP) has been proposed to describe the degree of localization. The LP for crevice corrosion was found to be low compared to that for pitting corrosion.     

Modeling pipeline crevice corrosion under a disbonded coating with or without cathodic protection under transient and steady state conditions

Underground steel pipelines are protected by coatings and cathodic protection (CP). The pipeline corrosion occurs when the coating is disbonded away from a defect or holiday to form a crevice and the corrosion rate varies temporally and spatially in the crevice. In the presence of dissolved oxygen (O₂) in soil ground water, a differential O₂ concentration cell may develop in the crevice because O₂ diffuses more readily into the crevice through the holiday than through the disbonded coating. CP can decrease or eliminate the O₂ concentration cell depending on the potential applied at the holiday. Since the coatings are usually non-conductive, CP is unable to protect the steel surface deep inside the crevice. The transport of dissolved O₂, and that of dissolved carbon dioxide (CO₂) if present, into the crevice through holiday can be key to determining the crevice corrosion rate. In this work, the transient and steady state behavior of the corrosion process is investigated. The effect of the cathodic portion of iron vs. ferrous ion redox reaction on the crevice corrosion rate, which is often neglected traditionally, is further studied. At steady state, the effect of dissolved O₂ on the crevice corrosion rate and the added effect of dissolved CO₂ are mathematically modeled.     

Effect of the Crevice Former on the Corrosion Behavior of 316L Stainless Steel in Chloride-Containing Synthetic Tap Water

To restrain the failure of the plate heat exchanger (PHE) in customer boiler working fluid, the effect of crevice former type on the corrosion behavior of the 316L stainless steel plate was investigated using electrochemical methods and surface analyses in chloride-containing synthetic tap water (60 °C). The localized corrosion under metal–metal crevice condition was initiated more easily than that under the metal-gasket crevice condition due to the restricted mass transport at the gasket crevice mouth. However, the anodic current under the metal–metal crevice condition was lower than that under metal-gasket crevice condition at a higher anodic potential, indicating that that the metal dissolution under EPDM crevice would be higher than that under metal crevice under the accelerated corrosion condition. Because narrow crevice gap that was formed under gasket accelerated the anodic dissolution at the crevice mouth, the perforation tendency under metal-gasket crevice condition is much higher than that under metal–metal crevice condition. As a result, the crevice geometry, especially the crevice gap, mainly affected the corrosion behavior of PHE material.