金属缝隙腐蚀的成因及实验验证

在制品和工程结构中,缝隙腐蚀是一种很普遍且隐蔽的局部腐蚀,其结果会导致构件强度降低,既降低服役寿命,也存在安全隐患。指出缝隙腐蚀发生的三个必要条件——制品或工程结构中有金属或合金、有缝隙的存在、缝隙内有腐蚀介质滞留,概述了缝隙腐蚀的特征及三个阶段。分析了缝隙腐蚀的影响因素,包括金属性质、环境因素(溶液中氧、氯离子浓度,温度,p H值以及溶液的流速)和缝隙的几何形状。介绍了在实验室用三氯化铁溶液验证不锈钢及其合金的缝隙腐蚀敏感性的基本步骤,提出了在工程实际中如何预防缝隙腐蚀的基本思路。     

人工模拟体液中pH值对离子注N人体医用合金腐蚀行为的影响

采用电化学测试技术研究了在人工模拟体液中pH值变化对离子注N人体用SUS316L不锈钢,Co-Cr合金,工业纯Ti和Ti-6Al-4V合金腐蚀行为的影响。结果表明,随着pH值的降低,试样的腐蚀电位负移,SUS316L不锈钢和Co-Cr合金的点蚀电位与缝隙腐蚀电位降低,使材料发生局部腐蚀的敏感性提高,工业纯Ti和Ti-6Al-4V合金的腐蚀电流密度增大,提高离子释放速度,加大对人体的潜在生理危害。     

Initiation and repassivation of crevice corrosion of type 444 stainless steel in chloride solution

Effects of chloride ion concentrations, solution temperature, and crevice-forming materials on the crevice corrosion of type 444 stainless steel were investigated using a potentiostatic method. Critical crevice potential (E_crev) and repassivation potential (E_r) of the creviced alloy decreased with an increase in chloride concentration [Cl⁻], satisfying the logarithmic relationship between E and [Cl⁻]. In addition, E_crev and E_r of the alloy with silicone crevice former were measured to be higher than those of the alloy with an EPDM (Ethylene Propylene Diene Monomer) crevice former, suggesting that silicone was more effective in preventing water from penetrating crevices between a stainless steel sheet and the crevice former. In electrochemical current transient measurements with an applied potential, the intensity of current transients corresponding to the initiation of metastable pits increased abruptly near the E_r of the alloy, indicating that the stability of crevice corrosion is associated with the initiation of metastable pits.     

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.     

Kühlwasserseitige Korrosionsbeständigkeit von metallischen Werkstoffen zur Handhabung von Schwefelsäure

Cooling water side corrosion resistance of high alloyed materials for handling of process side sulfuric acid The approved materials for use in sulfuric acid alloy 825 (German material No. 2.4858) and alloy 20 (German mater. No. 2.4660) have only a low resistance against localized corrosion in chloride containing water and are unsuitable for handling of sulfuric acid. The newly developed austenitic Cr-base alloy, alloy 33, (X1CrNiMoCuN 33-32-1, German mater. No. 1.4591) with 33 % Cr, 31 % Ni, 0,6 % Mo and 0.4 % N should have an excellent resistance against pitting and crevice corrosion additional to its high sulfuric acid resistance, too, because its Pitting Resistance Equivalent No. calculated according to PREN = %Cr + 3,3 · %Mo + 30%N runs to 50. Pitting and crevice corrosion properties of the alloy 33 are tested in comparison to those of reference materials in high chloride containing solutions (1M NaCl, artificial and modified sea water, 10% FeCl₃ · 6H₂O; 500 g/l CaCl₂ ). Pitting potentials and potentials of repassivation of pitting, critical temperatures of localized corrosion (FeCl₃-test, CaCl₂-test, artificial sea water), potentials of repassivation of crevice corrosion as well as depassivation pH values of crevice corrosion following Crolet have been determined. The results confirm that the localized corrosion behaviour of the alloy 33 corresponds to its PREN. With regard to pitting corrosion alloy 33 is comparable with the special stainless steel alloy 31 (mater. No. 1.4562), with regard to crevice corrosion it is comparable with alloy 926 (German mater. No. 1.4529).     

Die Oberflächenbehandlung von Schweißverbindungen hochkorrosionsbeständiger 6% Mo-Stähle und Nickel-Basis-Legierungen

Surface treatments of high alloy 6 Mo stainless steel and nickel alloy weldments High alloy stainless steels (6% Mo) and a high nickel alloy (alloy 625) weldment have been tested in order to answer the question whether post-treatment of the weldment has an effect on the corrosion resistance, especially on pitting corrosion. Therefore, the critical pitting temperature of weldments was tested in acidic chloride solution (standard tests). As a result grinding with rough emery paper as well as sand blasting lowers the localized corrosion resistance in the weldment area, while pickling has a positive effect, especially after blasting. Pickling can be done either by a solution of nitric + hydrofluoric acid or by a commercial pickling paste. In any event pickling is recommended as a final surface treatment for high alloy stainless steels and nickel alloys, especially in case of prevailing highly corrosive conditions such as pitting and crevice corrosion.     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Effect of nickel alloying on crevice corrosion resistance of stainless steels

The crevice corrosion behaviour of stainless steels containing 25 mass% Cr, 3 mass% Mo and various amounts of Ni was investigated in natural seawater. The results showed that ferritic steels containing nickel were more resistant to corrosion than both ferritic steels without nickel and austenitic steels. The superiority of the Ni bearing ferritic steel over the other steels was in close agreement with the depassivation pH of those steels in acidic chloride solutions. The results showed that the addition of Ni to ferritic steel was effective in decreasing the depassivation pH and the dissolution rate in acidic chloride solutions at crevices.     

Korrosionsverhalten einiger hochlegierter Stahl und Hartgußsorten in Waschwässern von Rauchgas-Entschwefelungs-Anlagen

Corrosion behaviour of some cast stainless steels and high alloy white irons in scrubber solutions of flue gas desulfurization plants Weight loss and electrochemical measurements have been used to determine the ranges of applicability of cast austenitic stainless steel Werkstoff No. 1.4408, of two special cast ferritic-austenitic stainless steels NORIDUR® 9.4460 and NORICLOR® NC 24 6 and of two high alloy Cr and CrMo white irons in scrubber solutions of Flue Gas Desulfurization (FGD) plants. Whereas the Werkstoff No. 1.440 8 cannot be used due to its insufficient resistance to general and localized corrosion, NORIDUR® 9.4460 can be used in scrubber solutions with pH > 2.5 and chloride concentrations up to 80 g/l, NORICLOR® NC 24 6 with 5% Mo even in liquids with pH > 1.5 and chlorides up to 100 g/l. At lower pH-values both duplex stainless steels show active corrosion of either the austenite or the ferrite depending on the contents of hydrochloric acid in the solution. At higher chloride concentrations pitting occurs on the passive materials. The CrMo white iron NORILOY NL 25 2 with 25% Cr and 2% Mo can be used in scrubber liquids with pH > 3.5. As the ferritic matrix is cathodically protected by the precipitated carbides, there is no sensitivity of this alloy to chlorides. In liquids with pH < 3.5 there is selective corrosion of the ferritic matrix. For practical application of all these cast alloys the limits for purely corrosive attack have to be modified to assure. resistance to a superposition of corrosion, erosion/abrasion and cavitation on parts exposed to real flow conditions in FGD scrubbers.     

炼油加工过程中氯离子与硫离子对 316L 不锈钢和Monel 合金腐蚀的影响

目的研究常减压装置高温原油馏分及塔顶水相中氯离子、硫离子含量对316L不锈钢和Monel合金(镍基合金)腐蚀的影响。方法通过腐蚀挂片实验,获得316L不锈钢和Monel合金在含不同浓度氯离子和硫离子的水相、油相中的腐蚀速率变化规律。利用扫描电子显微镜,研究316L和Monel合金表面腐蚀后的微观形貌,探讨两种离子对316L不锈钢和Monel合金腐蚀的影响规律。结果在酸值较高的脱后原油中,316L不锈钢和Monel合金的腐蚀速率分别为0.0091,0.0248 mm/a;在酸值较低的常二段馏分中,316L不锈钢和Monel合金的腐蚀速率分别为0.0078,0.0031 mm/a。在常二段馏分中,加入600mg/L氯离子和30 mg/L硫化钠时,316L不锈钢和Monel合金的腐蚀速率分别为0.1755,0.1707 mm/a。在相同条件的脱后原油中,316L不锈钢的腐蚀速率为0.0545 mm/a,Monel合金的腐蚀速率为0.1281mm/a。结论油相中氯离子含量较低时,环烷酸腐蚀占主导因素;而氯离子含量达到较高水平后,氯离子对腐蚀的影响占主导作用。316L不锈钢和Monel合金的腐蚀速率都随氯离子含量的增加而增加,并且硫离子的存在对腐蚀也有一定的促进作用。在塔顶水相中,氯离子和硫离子均对Monel合金腐蚀的影响不大。     

The enrichment of hydrogen and chloride ions in the crevice corrosion of steels

The changes in concentration of hydrogen and chloride ions during corrosion in an artificial crevice on steel were observed from potential changes of Pd-H and Ag/AgCl micro-electrodes inserted in the crevice. Increasing the bulk chloride ion concentration accelerated the rate of pH change and resulted in more acidic conditions in the crevice. Such pH changes were delayed by the addition of inhibitor such as dialkylamines. Though the free chloride ion concentration did not change appreciably, the total concentration of free and complex ions in the crevice increased with continuous dissolution of the steel. Increasing the hydrogen and total chloride ion concentration in the crevice accelerated the anodic dissolution of steels within the crevice.     

The effect of specimen position on the shape of corrosion pits in an austenitic stainless steel

The effect of the exposure mode of specimens on the morphology and the kinetics of development of covered pits growing in an 18Cr12Ni2MoTi austenitic stainless steel at a constant electrode potential (+ 860 mV_NHE) was investigated. It was found that in the case of an electrode facing upwards the growth of pits into the metal bulk is slower than in the case of the electrode facing downwards. In the first case there is a distinct and permanent increase of the concentration of chloride ions in the pits, and as a result, the diffusive inhibition of the metal dissolution in the pits is greater than in the second case, where the accumulation of Cl^? is less pronounced. The results are compared with those obtained earlier for pits growing under analogous conditions in a vertical electrode.     

Implications of internal cathodic reactions for crevice attack of stainless steels in chloride environments

Abstract

Modelling of the crevice chemistry in stainless steels in chloride environments at ambient temperatures has been conducted in order to evaluate the impact of internal cathodic reactions on the potential and pH within the crevice. Consideration of the internal cathodic reactions is of particular importance in predicting the behaviour of crevices formed between dissimilar metals. The model is based on mass conservation of species and incorporates a range of chemical and electrochemical reaction processes. Hydrogen ion reduction within the crevice has a marked effect on the time evolution of the pH in the crevice and consequently on predictions of the time to passivity breakdown based on the critical pH concept. The impact on the steady state pH depends on the corrosion potential and on crevice dimensions. In the latter context, decreasing the gap or increasing the length does not invariably create more severe conditions for passivity breakdown. There is an optimum magnitude of the dimensions for inducing crevice attack. Coupling the stainless steel to a more noble material within the crevice, when the main cathodic reaction is reduction of hydrogen ions, may be more likely to retard crevice attack by virtue of the higher crevice pH, provided that effective coupling of the materials in the external environment is constrained. A specific bulk conductivity exists below which the critical crevice chemistry does not develop, but this is associated with potential drop in the bulk solution rather than in the crevice.     

不锈钢缝隙腐蚀的微区电化学行为及缝内钝化膜厚度的变化

利用微电极和椭圆偏振仪研究了2Cr13不锈钢在3.5%NaCl 溶液中的缝隙腐蚀历程。实验结果表明,不锈钢的缝隙腐蚀经历了三个阶段一孕育期,发展期和稳定期。缝内 pH 值下降和 Cl~-浓度增加均促进了钝化膜的溶解和加速了缝隙腐蚀。从孕育期向发展期的过渡中,存在着使合金钝化膜破裂的临界缝隙溶液(CCS)。在本试验条件下,其组成大约为 pH=3.5,[Cl~-]:1.2N。在孕育期,缝内钝化膜厚度随时间逐渐减薄,直到钝化膜肢裂,此时,缝隙腐蚀即进入发展期。微电极测试技术和椭圆偏振法可应用于不锈钢缝隙腐蚀过程的原位测试。     

An embedded multi‐parameter corrosion sensor for reinforced concrete structures

A prototype of an embedded corrosion sensor has been developed for assessing the corrosion status of reinforcing steel bar (rebar) in concrete. The integrated sensor unit includes an Ag/AgCl probe, a metallic oxide probe, a multi‐electrode array sensor (MAS), and a four‐pin (Wenner) array stainless steel electrode for chloride content, pH, microcell corrosion current, and localized concrete resistivity measurements, respectively. A stable solid probe was used as the reference probe in this unit to express the potentiometric measurement of chloride content and pH probes. In this study, the chloride and pH probes were calibrated in simulated pore solutions (SPSs) regarding temperature and pH fluctuations. The corrosion current results of the MAS probe in SPSs matched very well with those obtained by the linear polarization resistance technique, which was conducted on companion carbon steel specimens. A sensor prototype was embedded into a paste cylinder for long‐term performance evaluation. Up‐to‐date results show that the sensor probes exhibit excellent sensitivity and reliability through 1 year of monitoring. Continuous monitoring in the laboratory for extended periods is underway.     

Pitting and crevice corrosion behaviour of high alloy stainless steels in chloride‐fluoride solutions

The localised corrosion resistance (pitting and crevice corrosion) of two high alloy stainless steels, namely superduplex (SD) and superaustenitic (SA), has been studied in chloride‐fluoride solutions at pH values ranging from 2 to 6.5. The pitting potential (E_pit) and crevice potential (E_cre) have been calculated for these test media using electrochemical techniques (continuous current). The Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT) are in both materials lower then the room temperature. In spite of this fact and due to the high repassivation rate, the resistance of these materials to localised corrosion is high in the tested media. At the highest tested concentration of aggressive anions and pH 6.5 both materials undergo a generalised attack.     

Characteristics of passivity of extremely corrosion-resistant amorphous iron alloys

The amorphous FeCrPC alloy was compared with the crystallized alloy having the same composition in potentiodynamic polarization curves and with an 18Cr8Ni stainless steel in current decay after abrading the specimens under anodic polarization. Through these results along with the previous ESCA study, the extremely high corrosion-rseistance of the amorphous iron alloys containing 8 at.% or more chromium has been interpreted in terms of the rapid formation of thick, uniform, highly corrosion-resistant passive film due to the characteristics of the amorphous alloys.     

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

Laboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic-austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits-crevices were found under the corrosion crust deposits on the duplex stainless steel.     

Untersuchungen zum Aktiv/Passiv-Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Medien. Teil 3. Resultate der Impedanzmessungen und Passivierungsmodell einer Legierung

Investigations into the active/passive transition of 304 stainless steel in organic media containing water and hydrogen chloride Part 3. Results of the impedance measurements and passivation model of an alloy Steady state polarization curves and electrode impedances were measured during the active/passive transition of type 304 stainless steel in dearated ethanolic solution containing hydrogen chloride and different amounts of water. The passivation potential and the critical current density for passivation strongly depend on the water content of the solution. The impedance measurements in the active/passive transition show the same sequence of diagrams independent of the water content of the solution. They indicate the onset of passivation before the maximum current density and show two time constants related to two different passivating species on the alloy surface. The experimental results were interpreted on the basis of a reaction model with parallel dissolution and passivation mechanism of the iron and the chromium compound of the alloy. The resulting total surface composition (related to the steady-state polarization curves) can be described with a reaction model of iron–the alloy behaviour is that of pure metal. The fundamental passivation reaction is described as a potential dependent equilibrium between adsorbed Me(II)- and passivating Me(III)-hydroxide, water molecules being directly involved in the formation of this primary passivating film. In the case of stainless steel this primary passivating film mainly consists of chromium (III) adsorbates. Finally, a general model for the passivation is proposed: The passivation of a pure metal or of an alloy can be understood as the coupling of the stepwise deprotonation of the water molecules at the interface metal/solution and the formation of a high cation charge density in this adsorbed hydroxide/oxide film to build up the passive layer. The effect of water content, pH, adding of passivating species to the solution or the alloying with chromium on the passivation potential and the critical current density thus can be explained.     

低碳钢芯奥氏体不锈钢代用焊条的研究

采用半固态流变压铸成形工艺，研究了高剪切速率双螺杆机械搅拌制浆机中剪切速率对半固态镁合金浆料质量的影响规律。试验表明，在其它工艺参数相同的条件下，随着螺杆剪切速率的增大，半固态浆料的固相率降低，初生α-Mg相晶粒平均尺寸减少，剪切速率高于2730s^-1时，晶粒的圆整度变化不大；剪切速率低于1365s^-1时，组织形态明显恶化。