Novel strategies for assessing the pitting corrosion resistance of stainless steel surfaces

Pitting corrosion is one of the most common mechanisms of surface damage on stainless steels. Electrochemical methods have been preferentially applied for the evaluation of the pitting corrosion resistance of stainless steels in the laboratory. Nevertheless, some of them are not reliable enough and in general the application of electrochemical methods in the field becomes difficult because of required deep understanding of corrosive phenomena and measurement technology. Therefore, new approaches for the evaluation of the pitting corrosion susceptibility of stainless steel surfaces in the laboratory as well as in the field are necessary. In the present paper two novel strategies including electrochemical noise measurements under anodic polarization for laboratory testing, and an indicator test to assess the susceptibility of stainless steel surfaces to pitting corrosion in the field are introduced. Experimental results concerning the influence of surface treatments on the pitting corrosion resistance on stainless steels have confirmed that final surface condition has a significant effect on their future pitting corrosion susceptibility. In addition, the pitting corrosion resistance of stainless steel surfaces was observed being specifically dependent on the achieved surface topography and in some cases independent on the roughness parameters of the surface.     

合金元素对耐候熔敷金属力学及耐蚀性能的影响

采用周浸加速腐蚀试验,模拟工业大气腐蚀环境,通过组织分析、锈层微观成分及形貌分析、电化学分析等手段,研究了Ni,Cr,Cu元素对耐候熔敷金属耐蚀性能及力学性能的影响规律. 结果表明,Ni元素有利于增加组织中针状铁素体含量,提高熔敷金属−40 ℃冲击韧性,Cr元素增加,熔敷金属−40 ℃冲击韧性降低. 合金元素有利于提高腐蚀锈层中的α-FeOOH的含量,增加锈层致密度,提高锈层阻抗,降低自腐蚀电流密度. Cu元素有利于提高熔敷金属腐蚀初期和后期的耐蚀性能;腐蚀初期,Cr元素提高耐蚀性能的作用不如Cu和Ni,但是腐蚀后期Cr提高锈层耐蚀性能的作用明显.     

Improvement on the pitting corrosion resistance of 304 stainless steel via duplex passivation treatment

A remarkable improvement in the pitting corrosion resistance of 304 stainless steel was attempted using a novel duplex passivation treatment method. First, chemical passivation in nitric acid followed electrochemical passivation via potential polarization of step cycling in sodium nitrate electrolyte. Compared with traditional chemical passivation, breakdown potential was increased from 0.31 V_SCE to positive than 0.9 V_SCE at 70°C in a solution bearing 0.6 M [Cl^?] concentration. The critical pitting temperature was enhanced from 21.5°C to above 70°C in a solution with 6 M [Cl^?] concentration. Impedance analysis and X‐ray photoelectron spectroscopy results show that a more compact passive film with a higher ratio of chromium oxide on iron oxide was achieved by electrochemical passivation compared with chemical passivation. Morphology observation suggested that the potential polarization of step cycling slightly increased the dissolution of inclusions after being subjected to chemical passivation. The probable reason for the improvement on pitting resistance is discussed in detail based on inclusion dissolution and the protectiveness in passive film.     

固溶温度对2205双相不锈钢组织和点蚀性能的影响

对2205双相不锈钢热轧板进行了不同温度的固溶处理,采用光学显微镜和扫描电镜分析了不同固溶状态下的组织演变规律,通过FeCl₃溶液浸泡法研究了固溶温度对2205双相不锈钢点蚀性能的影响。结果表明,950 ℃固溶处理后,组织中有s相;经1000~1100 ℃固溶处理后,由奥氏体和铁素体两相组成。随固溶温度升高,铁素体含量逐渐增加,奥氏体晶粒度减小,孔蚀数量、孔蚀平均尺寸和腐蚀速率均呈下降趋势。经1100 ℃×20 min水冷固溶处理后,奥氏体和铁素体含量约各占一半,组织均匀,表现出良好的耐点蚀性能。     

热处理工艺对5Cr钢力学性能和耐蚀性能的影响

利用光学显微镜、拉伸及冲击试验机等仪器研究了不同热处理工艺对5Cr钢组织和力学性能的影响,利用高温高压反应釜模拟腐蚀过程,并结合电化学测试对比分析了热轧态和调质态钢的耐腐蚀性能。结果表明:5Cr钢经淬火+回火处理后,组织中碳化物弥散析出,具有较高的强度和良好的塑、韧性。热轧态钢的腐蚀速率为0.188 mm/a,调质态钢为0.158 mm/a。调质态钢中作为阴极的碳化物(Fe₃C)少且均匀弥散分布,组织自腐蚀电位较高,腐蚀电流密度较低,电偶腐蚀程度较浅,耐腐蚀性能较好。     

Influence of microstructure on the corrosion resistance of hyperduplex stainless steel

Hyperduplex UNS S32707 is a newly developed austenitic–ferritic stainless steel. The steel contains about 27%Cr, 7%Ni, 4.5%Mo, and 0.4%N, which results in a pitting resistance equivalent factor (PRE) equal to 49. In this study, the pitting corrosion resistance of this new grade of stainless steel was investigated by varying the microstructure using different thermal processes. The critical pitting temperature measurement and cyclic polarization tests confirm the high corrosion resistance of the hyperduplex steel in the solution treated condition. However, deleterious phases form easily during thermal processing and cause a drastic decrease in the corrosion resistance.     

富Cu相对双相不锈钢堆焊层电化学腐蚀及点蚀性能的影响

采用光学显微镜、扫描电镜、动电位扫描试验及点蚀浸泡试验研究了时效处理对含Cu双相不锈钢堆焊层组织及耐蚀性能的影响.结果 表明,经1170℃固溶后,两种双相不锈钢堆焊层主要由铁索体相(α)、奥氏体相(γ)和少量MnO· Cr2O3氧化物组成.其中4Cu堆焊层试样经580℃时效60 min后,在α相中析出了尺寸约为1μm的...     

固溶温度对2507双相不锈钢力学性能及耐蚀性的影响

利用光学显微镜、扫描电镜、XRD、拉伸试验机和电化学综合测试仪等研究了不同固溶温度对2507超级双相不锈钢组织、力学性能和耐蚀性的影响。采用Thermo-Calc热力学软件计算了2507双相不锈钢的热力学平衡相图,并与测试结果进行了对比。研究结果表明,经1050 ℃及以上温度固溶后,σ相溶解;随着固溶温度的升高,铁素体相含量增加,奥氏体相含量降低,α/γ相体积分数比增加;1050~1100 ℃固溶30 min并水冷时,双相不锈钢具有较好的综合力学性能,屈服强度、抗拉强度和伸长率分别大于600 MPa、840 MPa和35%。1050 ℃固溶30 min时,双相钢可获得较好的耐蚀性能。     

An investigation on the effect of bleaching environment on pitting corrosion and transpassive dissolution of 316 stainless steel

Pitting corrosion and transpassive dissolution of 316 stainless steel in a solution containing five percent of commercial bleaching liquid was investigated by employing potentiodynamic polarization method and recording corrosion potential during immersion. Today commercial bleaching liquids are widely used as a cleaner additive. Therefore those house appliances made from stainless steels are in contact with aqueous solution containing bleaching liquid. This may cause severe localized corrosion and transpassive dissolution. In order to investigate the possibility of tranpassive dissolution of stainless steel by bleaching liquid, potentiodynamic polarization and recording the variation of corrosion potential of specimens were carried out in 0.2 M Na₂SO₄ solution containing 5%wt. commercial bleaching liquid. A 500 mV drop in transpassive potential and also instantaneously ennobled corrosion potential revealed the possibility of transpassive dissolution due to the oxidizing effect of the species such as free chlorine and its derivatives in bleaching liquid. Evaluation of the occurrence of localized corrosion at the presence of Cl^? and bleaching liquid was investigated by similar electrochemical experiments in 0.2 M Na₂SO₄ + 0.4 M NaCl containing 5%wt. bleaching solution. Initiation of stable pitting at potentials lower than the transpassive potential as well as a sharp increase of the corrosion potential in this environment demonstrates the possibility of pitting corrosion.     

Modeling and experimental calibration of the corrosion of RHA steel in immersion and salt-fog environments

ABSTRACT

An investigation into the general and pitting corrosion rates of rolled homogeneous armour (RHA) steel in immersion and salt-fog environments is presented. Although the mechanical properties of RHA steel have been studied, the corrosion effects on RHA steel have not been quantified. An immersion environment of 3.5% NaCl was used to induce corrosion for four immersion samples. A Q-fog corrosion tester was used to simulate a salt-fog/humidity/drying environment for four salt-fog samples. The different mechanisms of corrosion and their rates were quantified through mass loss, laser profilometry and scanning electron microscopy. The immersion samples show a linear rate of corrosion of 1.47?g/year (R²?=?0.994) throughout the entire testing period. The salt-fog samples had a significant mass gain for the first 500?h of corrosion, as the corrosion products are retained. Immersion samples showed no signs of pitting, while the salt-fog samples had sustained pit nucleation, growth, and coalescence.     

Depassivation of steel reinforcement in case of pitting corrosion: detection techniques for laboratory studies

Methods for service life prediction of reinforced concrete structures exposed to chloride‐bearing environments require, amongst other parameters, the knowledge of the chloride threshold for pitting corrosion initiation (Cl_th). Nowadays, although the main factors influencing the chloride threshold are well known, it is often difficult to quantify a value of the chloride threshold, partly because of its intrinsic high variability, and partly because of the different test methods that have been used to measure it. All the experimental tests rely on the detection of steel depassivation and simultaneous measurement of chloride content or steel potential. This paper deals with the methods that can be used to detect steel depassivation in relation with the determination of the chloride threshold. Tests in concrete‐pore‐simulating solutions as well as tests in concrete will be considered, and advantages and limitations will be discussed.     

铬、钼对耐候钢熔敷金属耐蚀性能和力学性能的影响

通过周浸加速腐蚀试验、锈层微观分析、电化学等方法,研究了铬和钼对耐候钢熔敷金属耐蚀性能和力学性能的影响.结果表明,熔敷金属中铬、钼含量增加,其耐蚀性能提高.钼对耐蚀性的提升优于铬.铬、钼分别以Cr₂O₃,Fe_xCr_3-xO₄,MnFe_xCr_2-xO₄,Mn_1-xFe_xCr₂O₄,NiFe_xCr_2-xO₄和MoO₃形式富集于锈层中,提高锈层致密性,抑制阳极溶解,增强锈层对基体的保护作用,从而提高熔敷金属的耐蚀性能.铬、钼含量增加,熔敷金属强度上升,冲击韧性明显下降.同时,熔敷金属中M-A组元和粒状贝氏体含量增加,针状铁素体含量降低.组织差异是造成熔敷金属强度升高、韧性降低的主要原因.     

Influence of microstructure on mechanical properties and corrosion behavior of 3%Cr steel in CO2 environment

Corrosion behavior and mechanical properties of two 3%Cr pipeline steels were studied which have same chemical compositions but different microstructures, bainite‐ferrite and ferrite‐pearlite microstructures. Analysis of chemical compositions and surface morphology of the corrosion scale were performed using X‐ray diffraction and scanning electron microscopy in conjunction with energy dispersive X‐ray spectroscopy. The semi‐passive character of corrosion scale was investigated using polarization curves measurements. The influence of microstructure on the chromium enrichment in the corrosion scale for different test periods was investigated. The microstructure was observed to affect the corrosion behavior even when the steel surface has been covered with corrosion scales and continue to have an effect until the corrosion scale possessing semi‐passive character. Possessing both good strength and corrosion resistance, 3%Cr steel with bainite‐ferrite microstructure was very promising for pipeline steel applications.     

The effect of titanium on pitting corrosion resistance of welded supermartensitic stainless steel

This study evaluated the effect of Ti in the supermartensitic stainless steel (SMSS) on the pitting corrosion in welded joints using gas tungsten arc welding. In the welded SMSS sample, there was martensite grain coarsening in the heat-affected zone (HAZ), and d-ferrite formation at the interface between the base metal and HAZ regions. The presence of Ti in the SMSS prevented grain coarsening and promoted the formation of retained austenite in the HAZ regions. The values of the pitting potentials of SMSS + Ti and SSMS using a cell-pen (designed to reveal the behaviour of localised microstructure in a narrow area) were 0.347 and 0.286 V in the non-welded condition, and 0.374 and 0.183 V in the welded condition, respectively. The corresponding values using the conventional cell were 0.349 and 0.257 V in the non-welded condition and 0.230 and 0.089 V in the welded condition, respectively.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.