中国腐蚀与防护学报, 2017, 37(3): 267-372
doi: 10.11902/1005.4537.2016.041
Cr对Q420钢在高盐度大气环境下耐蚀性的影响

Effect of Cr on Corrosion Resistance of Q420 Steel in Atmosphere with High Salinity
谭何灵1, 周成1,, 刘希辉2, 曹国明1, 张菁1

摘要:

通过腐蚀失重比较了4种不同Cr含量的Q420钢在模拟的高盐度工业大气环境下的耐蚀性能,研究了Cr对Q420钢锈层结构与组成的影响及其作用机理。结果表明,含Cr钢的耐蚀性能优于Q420钢,且9% (质量分数) Cr钢的腐蚀速率最低,耐蚀性最好;Q420钢的腐蚀速率保持稳定,含Cr钢的腐蚀速率先增大后减小,这是因为含Cr钢的锈层随着腐蚀的进行会由初期的不稳定状态转变为稳定状态,耐蚀性得到增强;Cr促进了内锈层中稳定相α-FeOOH的生成,使锈层结构更加稳定、致密,对腐蚀性介质的传递过程起到了显著的阻碍作用。

关键词: Q420 ; Cr合金钢 ; 高盐度大气环境 ; 耐蚀性能 ; α-FeOOH

Abstract:

Effect of Cr on the corrosion resistance of Q420 steels with different Cr content (2%, 5% and 9% mass fraction) in atmosphere with high salinity was investigated by means of weight loss test, XRD and SEM. The results show that corrosion resistance of Cr modified Q420 steels is higher than that of simple Q420 steel. Among others the Q420 steel with 9%Cr shows the lowest corrosion rate and the highest corrosion resistance. The curve of corrosion rate versus time of the simple Q420 steel kept stable within a high level. While the corrosion rate of Cr modified steels increases in the initial stage and then increases, which indicates that the rust scale became compact and stable in the later stage. It was revealed that Cr can promote the formation of α-FeOOH, which is a stable phase and makes the rust scale much compact and stable so that to act as an effective barrier for the transfer process of the corrosive species.

Key words: Q420 ; Cr alloyed steel ; high salinity atmospheric environment ; corrosion resistance ; α-FeOOH

耐候钢是在碳钢中添加多种合金元素,通过“以锈止锈”的方式来防止基体发生进一步腐蚀的系列钢种之一。目前国内应用较多的耐候钢多是针对陆地工业大气腐蚀环境而添加Cu和P等元素[1],而在海洋及酸性环境下,主要添加Cr,Ni和Mo等元素[2-6]。Cr作为不锈钢中的主要合金元素,能够使钢表面产生一层致密而稳定的钝化膜,使不锈钢具备优良的耐蚀性[7-9],但是不锈钢中Ni、Ti合金元素的添加导致其成本为普通碳素钢的2~6倍,限制了其作为普通建筑材料的使用。在提高普通碳钢的耐蚀性能方面,Cr相对于其他合金元素具有较高的性价比,得到了大量研究工作者的青睐[10-13]。但是目前对高盐度环境下含Cr钢的耐蚀性机理尚存在争议,Xin等[14]的研究表明,316L不锈钢在高浓度海水中长期浸泡会由钝化态转化为活化点蚀状态。我国海岸线较长,而沿海地区高盐度环境下钢材的腐蚀条件又十分恶劣,所以沿海地区钢材料的腐蚀行为受到了越来越高的重视。因此,有必要对钢铁材料在沿海地区高盐度大气环境下尤其是工业海洋大气环境下的服役状态进行相关研究,并为开发更适用于沿海工业大气用的耐蚀钢材料提供理论依据。

材料的大气腐蚀与环境的温度、湿度有直接的关系。干湿周浸循环实验是目前国内外普遍使用的加速腐蚀实验方法。Pourbaix首先提出用该方法研究大气腐蚀[15],研究人员[16,17]的研究证实了采用干湿周浸循环实验手段可以预测材料长期大气腐蚀行为。本文以Q420钢为基材,对其分别添加2.0% (质量分数)、5.0%和9.0%的Cr制得3种Cr合金钢 (文中分别简称为2Cr、5Cr和9Cr)。通过对4种钢在模拟的高盐度大气环境中的加速腐蚀失重实验、腐蚀形貌观察和表面锈层分析,比较了4种材料的耐蚀性,并探讨了Cr对Q420钢在高盐度大气环境中耐蚀性的影响机理。

1 实验方法

本实验材料为Q420钢及3种分别添加2.0%、5.0%及9.0%Cr合金化后的实验钢。基材Q420的主要化学成分 (质量分数,%) 为:C 0.160,Cr 0.038,Mn 1.46,P 0.010,S 0.001,Si 0.25,Ni 0.03,Cu 0.04,Ti 0.015,Fe余量。

采用1.0% (质量分数) NaCl+0.1%NaHSO3的混合溶液作为实验溶液模拟沿海地区的高盐度工业大气环境。腐蚀失重实验试样尺寸为30 mm×25 mm×3 mm,每组3个平行试样。干湿循环周期为60 min,浸润于混合溶液时间为12 min,溶液温度为45 ℃,干燥时间为48 min,试样表面温度为 (70±10) ℃,溶液每3 d更换一次。加速腐蚀实验周期分为4组,分别为72,144,240和360 h。

利用扫描电镜 (SEM,Hitachi S-3400N)、X射线衍射仪 (XRD,D8 Advance) 对试样表面锈层的形貌与组成进行观察和分析。用于微观形貌观察的试样尺寸为20 mm×15 mm×3 mm,实验周期取72和360 h。用于锈层检测的试样尺寸为15 mm×15 mm×3 mm,实验周期取360 h。

2 结果与讨论
2.1 腐蚀失重

根据标准GB/T19292.4-2003,单位面积平均腐蚀速率W的计算公式为:

W = ( w 0 - w t ) × 10 6 2 t ( ab + bc + ac ) (1)

式中,W为失重率,g/(m2h);t为实验时间,h;w0wt分别为试样腐蚀前后的质量,g;w;abc分别为试样的长、宽、高,mm。

单位面积腐蚀减薄量D的计算公式为:

D = 10 - 3 W t ρ (2)

式中,ρ为试样的密度,gcm-3,这里取7.85 gcm-3

综合式 (1) 和 (2) 得到周浸腐蚀试样的单位面积平均腐蚀减薄量计算公式为:

D = 10 3 × ( w 0 - w t ) 2 ρ ( ab + bc + ac ) (3)

通过失重实验计算实验钢在各腐蚀周期的WD,结果如图1所示。

图1a可见,Q420钢的腐蚀速率在实验时间内基本不变;5Cr和2Cr钢腐蚀速率先增大后减小,2Cr钢在腐蚀270 h后腐蚀速率低于Q420钢的,5Cr钢的腐蚀速率一直低于Q420钢的;9Cr钢的腐蚀速率在各个时间段均最低且基本保持不变。由图1b可见,Q420钢的D随时间变化曲线接近直线,而2Cr,5Cr和9Cr 3种钢的腐蚀减薄量曲线斜率逐渐减小。以上分析结果表明,随Cr含量的增大,Q420钢在高盐度大气环境中的耐蚀性提高,这是因为含Cr钢表面形成了更有效的保护性锈层。含Cr钢的腐蚀速率在腐蚀初期有增大的趋势,但在一段时间后腐蚀速率降低,腐蚀过程减缓,这说明含Cr钢在腐蚀初期表面形成的保护性锈层并不稳定,当腐蚀进行到一定阶段后,锈层逐渐稳定,材料的耐蚀性显著增强。

图1 4种不同Cr含量钢的腐蚀速率与腐蚀失厚随时间的变化曲线

Fig.1 Corrosion rate (a) and corrosion thinning (b) of four steels during cyclic immersion test

2.2 锈层形貌观察

图2为4种钢在腐蚀72和360 h后未去除锈层的表面形貌。可见,腐蚀72 h后,Q420钢试样表面呈现深黑色;2Cr钢和5Cr钢表面呈现褐色;9Cr钢表面有少量褐色,大部分区域呈现灰色。在腐蚀360 h后,Q420钢表面呈现褐色和黑色,其他3种钢表面均出现大面积褐色锈层,仅9Cr钢有小面积的灰色区域未被腐蚀产物覆盖。对腐蚀360 h后的试样进行除锈,可见锈层分为外层褐色腐蚀产物以及内层黑色腐蚀产物,且内层与基体结合紧密不易去除。另外可见,含Cr钢在腐蚀72 h后的锈层不完整,部分基体可见;而360 h后产生的锈层比较完整,基体基本全部被锈层所覆盖,这与腐蚀失重得到的含Cr钢锈层在腐蚀过程中由不稳定状态转化为稳定状态的结论相一致。为进一步研究4种材料在腐蚀过程中的锈层结构,对4种钢在腐蚀72和360 h后的锈层截面进行了SEM观察,如图3所示。从图3可见,在腐蚀360 h后,4种钢都形成了一定厚度的锈层,除Q420钢外,其他3种钢的锈层均出现了明显的分层,内层厚度较小且与基体紧密结合,外锈层结构比较疏松,有少量的裂纹。Q420钢的锈层整体疏松多孔,有较多裂纹,与基体结合不紧密;含Cr钢锈层无明显的孔状结构且裂纹较少。以上分析结果表明,随着Cr含量的增大,锈层的致密性提高,能够有效阻碍腐蚀介质中离子的传递和扩散过程,减缓了腐蚀过程。

图2 4种钢腐蚀72和360 h后的表面锈层形貌

Fig.2 SEM images of Q420 (a, e), 2Cr (b, f), 5Cr (c, g) and 9Cr (d, h) steels after corrosion for 72 h (a~d) and 360 h (e~h)

图3 4种钢腐蚀72和360 h后的锈层截面SEM像

Fig.3 Cross sections of rust layers formed on Q420 (a, b), 2Cr (c, d), 5Cr (e, f) and 9Cr (g, h) steels after corrosion for 72 h (a, c, e, g) and 360 h (b, d, f, h)

2.3 锈层XRD分析

为研究内外锈层的组成,对腐蚀360 h后的试样进行物理方法除锈,除锈过程中先除去外层褐色为主的锈层,研磨进行XRD分析;由于内锈层不易去除,将带有内锈层的试样进行XRD分析。4种钢表面的褐色锈层分析结果表明,其组成均为Fe2O3和Fe3O4。去除褐色锈层后试样的XRD谱如图4所示。

图4 4种钢腐蚀360 h去除外锈层后的表面XRD谱

Fig.4 XRD patterns of Q420 (a), 2Cr (b), 5Cr (c) and 9Cr (d) steels after corrosion for 360 h and removing of the outer rust layer

图4可见,Q420钢腐蚀360 h后形成的锈层中内锈层主要为Fe2O3和Fe3O4,而其他3种含Cr钢中同时出现了α-FeOOH。结合Fe2O3和Fe3O4颜色以及腐蚀过程中氧化物生成的情况分析,Q420钢在腐蚀过程中初期腐蚀产物主要为Fe3O4,所以72 h时Q420钢表面锈层呈黑色,在潮湿状态下Fe3O4被继续氧化为Fe2O3,随着腐蚀时间的延长,疏松的Fe2O3容易从基体脱落,使Fe3O4为主的黑色腐蚀产物裸露,所以360 h时Q420钢表面部分区域呈现出黑色。裸露的Fe3O4又继续被氧化为Fe2O3并剥落,同时基体不断被腐蚀产生新的Fe3O4,使Q420的腐蚀速率基本保持不变,因此其腐蚀减薄量呈直线上升。2Cr,5Cr和9Cr钢的锈层组成相同,为α-FeOOH,Fe2O3和Fe3O4,且随着Cr含量的增大,α-FeOOH和Fe3O4组分有增大的趋势,因此内锈层呈现出黑色且厚度较小。2Cr钢在腐蚀144 h后腐蚀速率高于Q420的,可能是由于2Cr钢中Cr含量相对较少,腐蚀初期形成的保护性的腐蚀产物较少,随着腐蚀过程的进行,锈层中Cr含量增加,保护性的腐蚀产物α-FeOOH的含量增加,进而腐蚀速率降低。综合腐蚀失重实验结果可知,含Cr钢的锈层比Q420钢的锈层更具有保护作用,由此推断锈层中起保护作用的主要为腐蚀产物α-FeOOH,且Cr有利于α-FeOOH的形成。Tamura[18]的研究结果表明,尺寸较小的α-FeOOH,特别是纳米级颗粒易于紧密结合在一起,形成附着于基体表面的致密锈层,降低基体的腐蚀速率。以上的分析结果表明,Cr促进了内锈层中α-FeOOH的形成,提高了锈层的致密性和稳定性,阻碍了腐蚀性介质的传输过程,增强了材料的耐蚀性能。

3 结论

(1) Q420钢在模拟的高盐度大气环境中的腐蚀速率基本保持不变,含Cr钢腐蚀速率先增大后减小,且在实验周期内耐蚀性能优于Q420钢。

(2) 含Cr钢腐蚀初期锈层不稳定,随着腐蚀过程的进行,锈层致密性和稳定性提高,能够有效阻碍腐蚀性介质的传输,提高了材料的耐蚀性能。

(3) Q420钢的锈层疏松多孔,主要组成为Fe2O3和Fe3O4,对基体不能起到良好的保护作用;含Cr钢锈层分为内外两层,外锈层组成为Fe2O3和Fe3O4,内锈层组成为α-FeOOH,Fe2O3和Fe3O4。Cr促进了内锈层中α-FeOOH的形成,提高了锈层的稳定性和致密性,材料耐蚀性能增强。

The authors have declared that no competing interests exist.

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目的:研究304不锈钢在模拟深海和浅海中的应力腐蚀开裂( SCC)行为。方法通过控制不同环境因素模拟南海某海域环境,利用动电位扫描、交流阻抗谱、慢应变速率拉伸( SSRT)及SEM表面分析等手段进行研究。结果304不锈钢在模拟海水溶液中呈现钝化状态,出现应力腐蚀敏感性,且裂纹扩展方式为穿晶开裂。在深海中的SCC机制为氢致开裂,浅海中的SCC机制主要为阳极溶解。结论304不锈钢在深海与浅海中的SCC机制不同,但两者的SCC敏感性相近且相对较低,在模拟海水环境中的应用不受海水深度限制。
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ABSTRACT Bulk glassy Fe60-xCrxMo15C15B10 (x = 0, 7.5, 15, 22.5 and 30 at%) alloys with high thermal stability were synthesized and the effects of chromium on the glass formation and corrosion behavior were clarified. The maximum diameter for glass formation is 2-2.5 mm for the 7.5 and 15 at%Cr alloys and 1 mm for the other alloys. In the present glassy alloy system, the temperature interval of the supercooled liquid region (DeltaT(x)) changed with chromium and was enlarged from around 70 K at x = 0, 22.5 and 30 at% to over 80 K at x = 7.5 and 15 at%. Both corrosion rate and anodic current density by potentiodynamic polarization in HCl solutions decreased with an increase of chromium content in the alloys. For the Cr-free alloy, molybdenum was significantly concentrated in the surface film after immersion in 1N HCl solution. The bulk glassy alloys containing chromium was immune to corrosion by the formation of protective passive film enriched with chromium during immersion in the solution.
DOI:10.2320/matertrans.43.2137      URL     [本文引用:1]
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The corrosion behaviour of 316L stainless steel was studied in the concentrated artificial seawater at 7202°C, i.e., the simulated low temperature-multi effect distillation environments, by using electrochemical measurement techniques. The corrosion state changes from spontaneous passivation to pitting after about 115002h of immersion. Pitting corrosion is under mixed control of charge transfer and diffusion processes in the long-term immersion. The salt deposits retard the diffusion of oxygen to the metal surface. The pit depth only reaches about 3802μm after one year of immersion due to the alloying effect of Mo, low dissolved oxygen levels and weak occlusion states.
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AbstractWastage as a result of the combined effects of wear and corrosion occurs in many environments, ranging from offshore to the healthcare industries. In such cases, the degradation is dependent on a wide range of parameters relating to the materials in contact and the nature of the corrosive environments. Defining conditions in which the wastage is minimised is critically important for engineers charged with monitoring such processes. The nature of the tribological contact plays a critical role in determining the effect of corrosion on the wastage rate. In some cases, as in sliding wear, frictional heating may arise at high velocities and applied loads, leading to oxide film formation, even at room temperatures. In other cases, as in solid particle erosion, frictional heating may play a significant role only at very high fluxes of particle impact. The action of a corrosive medium, either in gaseous or in liquid form, thus may have very different effects on such diverse tribological processes. The int...
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The formation of corrosion products during atmospheric corrosion on open and confined surfaces of electrogalvanized steel exposed to periodic wet/dry conditions was studied. The composition of the corrosion products was determined using Fourier transform infrared spectroscopy (FTIR) and extra information about the phases present was obtained by x-ray diffraction (XRD). Corrosion products that formed consisted mainly of different amounts of zinc oxide (ZnO), hydrozincite (Zn
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The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) are modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to elucidate the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, in the rusts, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the rate of air-oxidation at this pH is very fast, and that dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to the dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe and OH ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH ions results in strongly acidic solutions (pH 1.41), causing acid-corrosion. It is proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, delaying the crevice- and acid-corrosion stages. Further, it is argued that iron compounds with negative charges due to the non-stoichiometric proportions of the lattice oxide ions and metal ions (solid oxoanions of iron) exhibit stable cation-selective permeability even with a drop in pH. Rust films including such compounds would stop the passage of aggressive anions and act to protect iron and steel.
DOI:10.1016/j.corsci.2008.03.008      URL     [本文引用:1]
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关键词(key words)
Q420
Cr合金钢
高盐度大气环境
耐蚀性能
α-FeOOH

Q420
Cr alloyed steel
high salinity atmospheric...
corrosion resistance
α-FeOOH

作者
谭何灵
周成
刘希辉
曹国明
张菁

TAN Heling
ZHOU Cheng
LIU Xihui
CAO Guoming
ZHANG Jing