南极洲拥有得天独厚的自然地理位置和环境，具有丰富的油气矿产资源和极高的科学研究价值。极地资源的合理开发与利用是全世界共同面临的重大问题，而极地装备是科学认知极地、合理开发利用极地资源的基础保障[1]。极地大气条件下材料的腐蚀失效威胁着极地装备的安全使用。低合金钢具有较高的强度以及良好的耐腐蚀性能，被广泛应用于海洋工程建设中，其大气腐蚀特性在世界范围内得到了广泛关注，但目前仍缺乏对南极低温环境下低合金钢大气腐蚀行为的认识，南极极端恶劣的大气环境对低合金钢的安全服役提出了严峻挑战。
尽管极地大气环境中金属表面长时间被冰雪覆盖并处于低温环境，但金属仍有不同程度的腐蚀[8~10]。Rosales和Fernández[11]在南极半岛阿根廷的Jubany站研究了钢、Zn、Cu和Al的腐蚀行为，结果表明，在海盐存在的情况下，冰下的金属表面会形成液态水层，由此腐蚀可以在低于0 ℃时发生。同时，含盐颗粒的沉积在南极是一个重要的问题，如Cl-的存在可以提高电解液的电导率，加速金属的腐蚀。Brass[12]通过2种氯化物盐和水的三元相图讨论了冰点的降低对腐蚀速率的影响，发现低温环境中含盐颗粒的富集能够降低凝固点，使得腐蚀反应在-50 ℃以下发生。Fu等[13]研究发现，完全冻结的NaCl溶液仍然是导电的，而且存在3种导电路径。Ohanian等[9]也发现南极环境下金属表面的冰层之下存在电化学反应，并测得低合金钢的腐蚀电位为-250 mVSCE。White等[14]发现冰层中温度在-20 ℃时依然存在电化学腐蚀反应。而Bartoň等[15]则认为在污染性的海洋大气环境中低于-5 ℃时发生腐蚀，这是由于液膜中较高的盐浓度延迟了结冰。以往研究均表明，南极低温环境冰层、雪层覆盖下电化学腐蚀过程依然存在。
本课题组前期已对Q235碳钢在南极大气环境中的腐蚀行为进行了报道[20]，但是对低合金钢的南极大气环境中的腐蚀行为仍不清楚。本工作分析表征了Q460和Q690低合金高强钢暴露于南极中山站大气环境中1和12个月后的腐蚀速率、腐蚀形貌和腐蚀产物，探讨了低合金钢在南极低温环境中的腐蚀行为与机理。
1 实验方法
1.1 实验材料
本工作使用Q460和Q690低合金钢用于南极室外暴露实验，其主要化学成分如表1所示。试样经打磨抛光后使用4% (体积分数)硝酸酒精刻蚀，并采用Axio-Lab.A1光学显微镜(OM)和Gemini SEM 300扫描电子显微镜(SEM) 进行组织观察。暴露试样尺寸为150 mm × 75 mm × 2 mm，经打磨抛光、除油清洗并干燥后进行称重及尺寸记录。
表1 低合金钢Q460和Q690的化学成分 (mass fraction / %)
Table 1
| Steel | C | P | Si | Cr | Mn | Ni | V | Nb | Ti | Cu | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Q460 | 0.09 | 0.01 | 0.36 | 0.023 | 1.42 | - | 0.066 | 0.034 | 0.012 | 0.014 | Bal. |
| Q690 | 0.06 | 0.005 | 0.21 | 0.51 | 1.76 | 0.134 | - | - | - | 0.3-0.4 | Bal. |
1.2 室外暴露环境
试样准备完毕后在南极中山站大气试验场(69°22ʹ24ʺ S, 76°22ʹ40ʺ E)进行现场暴露，如图1所示，样品正面与水平面成45°进行暴晒，时间为2019年1月~2020年1月，共12个月。南极中山站大气试验场的气象数据如下：年平均温度为-9.5 ℃，年平均相对湿度为61%；在南极暖季(10月~2月)，日最高温度高于0 ℃；在南极寒季(3~9月)，日最低温度可达-36.4 ℃；在2019年1月份下雪的天次只有1 d，按照海水在南极的冻结温度在-1.9 ℃计算，冰雪冻-融循环天次共有7 d[21]。根据King等[22]所提出的标准，在寒冷环境中以温度高于-10 ℃和相对湿度高于50%计算表面润湿时间(TOW)，南极暖季达到672 h/月，南极寒季低至24 h/月。因为没有雨水，盐不会被冲走，导致南极地区含盐颗粒的沉积量很高。此外，中山站有极昼和极夜现象，连续白昼时间54 d，连续黑夜时间58 d。
图1
图1
本实验中的南极中山站暴露地点
Fig.1
Exposure site in Zhongshan station in Antarctic continent
1.3 表征方法
试样取回后，首先使用Canon 6D MarkII数码相机对暴露后的低合金钢的宏观腐蚀形貌进行观察，然后刮取锈层用作成分分析。采用500 mL HCl、3.5 g C6H12N4及去离子水配置成1000 mL的除锈液，将带有腐蚀产物的完整试样浸泡在除锈液中，用超声波清洗机超声除去腐蚀产物，除锈后的试样依次使用去离子水、无水乙醇冲洗，吹风机冷风吹干，并称重记录。
腐蚀速率测试：大气腐蚀研究中，室外暴晒环境中金属材料的腐蚀失重速率计算为：
式中，v表示腐蚀速率(mm/a)，mt 表示除锈后试样的质量(g)，m0表示暴晒前试样的质量(g)，S表示试样的表面积(cm2)，ρ表示低合金钢的密度(约为7.86 g/cm3)，t表示暴露腐蚀实验时间(h)。
腐蚀形貌观察：使用Gemini SEM 300型SEM对除锈前后的试样表面进行观察。截面试样采用环氧树脂封装后用砂纸依次打磨至5000号并抛光，使用去离子水、无水乙醇冲洗，冷风吹干后待用。使用SEM和KEYENCE VK-X250型激光共聚焦显微镜(CLSM)观察除锈后低合金钢的表面腐蚀形貌，并随机选取100个点蚀坑统计深度分布信息。
腐蚀产物分析：刮取暴露12个月后的锈层在研钵中研磨均匀，使用D8 Advance X射线衍射仪(XRD)进行物相分析，并使用Jade软件和RIR (reference intensity ratio)值法对锈层相组成进行定性和定量分析。工作电压设置为50 kV和30 mA，扫描角度为10°~90°，步长为0.03°，扫描速率为3.6°/min。使用Renishaw in Via型Raman光谱仪对腐蚀产物的相组成与分布进行分析，扫描范围100~1700 cm-1。
2 实验结果
2.1 显微组织观察
Q460和Q690低合金钢的微观组织如图2所示。Q460钢主要为晶粒细小的铁素体和珠光体组织，Q690钢主要为板条贝氏体组织。
图2
图2
Q460和Q690低合金钢微观组织的OM和SEM像
Fig.2
OM (a, b) and SEM (c, d) images of microstructures of Q460 (a, c) and Q690 (b, d) low alloy steels
2.2 腐蚀速率测试
Q460和Q690低合金钢暴露1和12个月后的腐蚀速率变化如图3所示。在南极大气环境下，暴露1个月时，Q460和Q690低合金钢的腐蚀速率分别为29.7和77.0 μm/a，腐蚀速率差异较大。暴露12个月时，Q460和Q690低合金钢的腐蚀速率分别为10.7和18.7 μm/a，腐蚀速率均大幅度降低。表明南极低温环境下冰层、雪层覆盖下电化学腐蚀过程依然可以发生。其中，Q460钢表现出更好的耐腐蚀性能。而在暴露初期，2种低合金钢的腐蚀速率均较高，因为实验用钢暴露1个月时处于南极暖季，由于冰雪的冻-融循环导致液膜的长周期存在从而加速了腐蚀的进程。而在南极寒季，冰层持续覆盖金属表面，冰层下电化学腐蚀过程减缓，南极大气环境的大幅季变导致材料腐蚀速率在不同的暴露周期内有较大的差异。
图3
图3
Q460和Q690低合金钢的腐蚀速率随暴露时间的变化
Fig.3
Variation in the corrosion rates of Q460 and Q690 low alloy steels as a function of exposure time
2.3 腐蚀形貌分析
2.3.1 宏观形貌
Q460和Q690低合金钢暴露于大气后形成的腐蚀产物表面形貌如图4所示。锈层的颜色由腐蚀产物的结构和特征所决定，暴露不同时间后其颜色会发生明显的变化。暴露1个月时，Q460钢正面腐蚀产物覆盖并不完整，且不均匀，试样边缘处和固定试样片材的圆孔周围腐蚀较为严重，腐蚀产物呈橘黄色和棕褐色，背面锈层较少，仍能看到未腐蚀的基体。Q690钢相比于Q460钢腐蚀更为严重，腐蚀产物呈棕褐色，背面腐蚀较为轻微，也呈现出均匀腐蚀的趋势。暴露12个月时，Q460和Q690钢已完全被腐蚀产物所覆盖，正面腐蚀产物呈棕褐色，背面出现黑色块状腐蚀产物，而试样正面腐蚀产物更加均匀致密。
图4
图4
南极大气环境下Q460和Q690低合金钢暴露不同时间的正反面宏观形貌
Fig.4
Macromorphologies of the corrosion products on skyward (a, c, e, g) and groundward (b, d, f, h) surfaces of Q460 (a, b, e, f) and Q690 (c, d, g, h) low alloy steels exposed to Antarctic atmosphere for 1 month (a-d) and 12 months (e-h)
2.3.2 表面腐蚀产物微观形貌
图5为南极大气环境下Q460和Q690低合金钢暴露不同时间的微观形貌。暴露1个月后，Q460和Q690钢表面存在多种形态的腐蚀产物，局部仍可见未腐蚀的钢基体。Q460钢较大倍数下观察到相对平整的块状腐蚀产物，同时产生了较大裂纹。局部腐蚀产物放大后的微观形貌如图5b2和b3所示，可见大量细长且有序排列的棱柱状的β-FeOOH，而且棱柱状微观产物之间有较多缝隙，这有利于侵蚀性离子的进入[23]。Q690钢表面出现了片层状的γ-FeOOH[24]，局部有微裂纹出现。通常来说，β-FeOOH和γ-FeOOH作为暴露初期所形成的腐蚀产物还原性较强，能够促进腐蚀的发生[25]。暴露12个月后，Q460和Q690钢表面完全被腐蚀产物所覆盖。如图5c3所示，Q460钢表面出现较多花板状α-FeOOH，且表面裂纹不再明显。Q690钢表面较大倍数下仍显示出大量裂纹(图5d1)，表面腐蚀产物相对平整光滑，在裂纹周围腐蚀产物主要为棉球状和羽毛状的γ-FeOOH。
图5
图5
南极大气环境下Q460和Q690低合金钢暴露不同时间的微观形貌
Fig.5
Micromorphologies of corrosion products formed on Q460 (a1-a3, c1-c3) and Q690 (b1-b3, d1-d3) low alloy steels exposed to Antarctic atmosphere for 1 month (a1-a3, b1-b3) and 12 months (c1-c3, d1-d3) with different magnifications
2.3.3 截面微观形貌
图6为Q460和Q690低合金钢暴露于南极大气环境中所形成的腐蚀产物的截面形貌。可以看出，在暴露初期Q460和Q690钢所形成的锈层厚度在50 μm左右，Q460钢锈层中间部分和Q690钢锈层与钢基体间均产生了明显的裂纹。暴露12个月后，Q460钢上所形成的锈层变得致密连续，裂纹较少，厚度约为150 μm，没有出现明显的分层现象。Q690钢上所形成的锈层出现大量孔洞及裂纹，裂纹沿纵向分布，锈层呈现出脱落的趋势。
图6
图6
南极大气环境下Q460和Q690低合金钢暴露不同时间的截面微观形貌
Fig.6
Cross-sectional morphologies of Q460 (a, b) and Q690 (c, d) low alloy steels exposed to Antarctic atmosphere for 1 month (a, c) and 12 months (b, d)
2.3.4 去除腐蚀产物后腐蚀形貌分析
图7为南极大气环境下Q460和Q690低合金钢暴露不同时间的表面腐蚀形貌。在暴露1个月后，Q460钢和Q690钢出现了不同程度的局部腐蚀现象。2者正面均比背面腐蚀严重，点蚀坑有横向扩展的趋势，背面所形成的点蚀坑深度小于正面，而点蚀坑数量多于正面。对于Q460钢，背面所形成的点蚀坑深度最浅，同时数量也较多。暴露12个月时后，Q460和Q690低合金钢表面整体呈现均匀腐蚀，基本已看不到未腐蚀的钢基体。而Q690钢表面仍存在少量点蚀坑(图7d1~d3)。由于暴露初期出现了明显的局部腐蚀，而在长周期暴露后，低合金钢表面发生均匀腐蚀，因此统计了Q460和Q690低合金钢暴露1个月后的表面点蚀坑深度分布。为了消除表面不平整带来的影响，只统计了深度超过5 μm的点蚀坑。如图8所示，Q460钢点蚀坑深度集中于5~15 μm，深坑更少，而Q690钢深度超过20 μm的深坑相对更多，表明Q460钢在南极低温环境中耐点蚀能力更强。此外，2种低合金钢的背面深坑深度均略小于正面，这可能是由于南极低温环境下冰层雪层的冻-融循环为金属表面提供了持续的Cl-。以上结果表明，低合金钢暴露于南极大气环境中在初期出现了明显的点蚀现象，并随暴露时间的延长，点蚀坑横向扩展，彼此相连，点蚀向均匀腐蚀转变。
图7
图7
南极大气环境下Q460和Q690低合金钢暴露不同时间的表面腐蚀形貌
Fig.7
Surface morphologies and 3D topographies of Q460 (a1-a4, c1-c4) and Q690 (b1-b4, d1-d4) low alloy steels exposed to Antarctic atmosphere for 1 month (a1-a4, b1-b4) and 12 months (c1-c4, d1-d4)
(a1-d1, a2-d2) skyward (a3-d3, d4-d4) groundward
图8
图8
南极大气环境下Q460和Q690低合金钢暴露1个月后的表面点蚀坑深度分布统计
Fig.8
Distribution statistics of surface pits depth of Q460 and Q690 low alloy steels exposed to Antarctic atmosphere for 1 month
2.4 锈层成分分析
2.4.1 XRD
图9a和b分别为Q460和Q690低合金钢暴露于南极大气环境12个月后形成的锈层粉末的XRD谱和所对应物相组成占比。由于XRD难以区分Fe3O4和γ-Fe2O3，而且2者可以相互转化，因此以Fe3O4/γ-Fe2O3表示2者的总量。同时计算了α / γ* (α代表α-FeOOH，γ*代表γ-FeOOH、β-FeOOH与Fe3O4/γ-Fe2O3的总量[26]。α / γ*代表锈层保护性指数，α / γ*的值越大，锈层保护性越好)，如图9c所示。可以看出，低合金钢在南极大气环境中所形成的腐蚀产物主要为α-FeOOH、γ-FeOOH、β-FeOOH以及Fe3O4/γ-Fe2O3。对于Q460钢，γ-FeOOH和β-FeOOH占据了主要的物相组成，Fe3O4/γ-Fe2O3的含量仅有1%左右。对于Q690钢，正面和背面物相成分差距较大，正面γ-FeOOH是腐蚀产物的主要成分，而且背面Fe3O4/γ-Fe2O3的含量更多。相比于Q460钢，α-FeOOH的含量较少，其α / γ*也低于前者。上述结果表明，低合金钢暴露于南极大气环境中所形成的腐蚀产物除了α-FeOOH、γ-FeOOH和Fe3O4/γ-Fe2O3以外，存在大量β-FeOOH，而且Q460钢的锈层的保护性比Q690钢更好，Q690钢背面所形成的锈层的保护性最差。
图9
图9
Q460和Q690低合金钢暴露于南极大气环境形成的腐蚀产物的物相组成
Fig.9
XRD spectra (a), proportion of each phase (b), and the protective ability (c) of the corrosion products formed on Q460 and Q690 low alloy steels (α represents α-FeOOH; γ* represents total of γ-FeOOH, β-FeOOH, and Fe3O4/γ-Fe2O3)
2.4.2 Raman光谱分析
为了确定Q460和Q690低合金钢暴露于南极大气环境后形成的锈层的物相分布，对锈层截面进行检测，检测位置如图6所示黄色标记点1#~10#，检测结果如图10所示，Raman光谱的峰值位置参见文献[27]。检测结果表明，暴露初期，腐蚀产物物相主要为γ-FeOOH和α-Fe2O3。而α-Fe2O3的存在可能归因于Raman测量的激光功率引起的局部加热导致的Fe3O4/γ-Fe2O3的氧化而成[28]，因此暴露初期腐蚀产物物相主要为γ-FeOOH和Fe3O4/γ-Fe2O3。暴露12个月后，随着腐蚀的加深，锈层变厚，不同的相在锈层中有了明显的分布。对于Q460钢，锈层内部如图10a-Point 3#所示，对应的物相为α-FeOOH[29]。锈层中间层如图10a-Point 4#所示，对应的物相为β-FeOOH[30]。锈层外层如图10a-Point 5#所示，对应的物相为γ-FeOOH。对于Q690钢，锈层内部Raman光谱如图10b-Point 8#所示，对应物相为α-FeOOH和Fe3O4/γ-Fe2O3[31]。锈层中间层和外层分别对应β-FeOOH和γ-FeOOH。以上结果表明，暴露初期所形成的γ-FeOOH和Fe3O4/γ-Fe2O3不稳定，随暴露时间的延长，腐蚀产物继续生长并转化，内部形成致密稳定的α-FeOOH，锈层中间层主要为β-FeOOH，而外层主要为γ-FeOOH。对于Q690钢，在锈层与基体界面处存在部分Fe3O4/γ-Fe2O3，同时α-FeOOH的含量少于Q460钢，表明Q460钢暴露于南极大气环境所形成的锈层内层的致密性更大，保护性更好。内外锈层物相的不同可能与合金元素的加入以及腐蚀介质中Cl-的浓度有关[32]。
图10
图10
低合金钢Q460和Q690暴露于南极大气环境形成的腐蚀产物的物相分布
Fig.10
Phase distributions of the corrosion products (see in Fig.6) formed on Q460 (a) and Q690 (b) low alloy steels (A, G, L, and M represent akaganeite (β-FeOOH), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and megnetite/meghemite (Fe3O4/γ-Fe2O3), respectively)
2.5 腐蚀机理分析
南极低温海洋大气环境季节特征明显，由于降水少，含盐颗粒会沉积在材料表面。在南极暖季，温度和相对湿度较高，冰雪的冻-融循环导致薄液膜长周期存在加速了腐蚀的进程。而在南极寒季，材料表面长周期被冰雪所覆盖，尽管在较低的温度和相对湿度下，材料表面仍然可以形成薄液膜，腐蚀反应仍然可以进行。
随着腐蚀的进行，锈层进一步形成并生长。γ-FeOOH将发生相转变进而转化形成
南极含盐颗粒沉积于材料表面，使得大量Cl-持续进入到薄液膜中，Cl-在锈层/金属界面处的富集一方面导致冰点的降低，另一方面使得局部酸化进而导致β-Fe2(OH)3Cl的形成，再经过不稳定的中间产物绿锈I (GRI)的转化，最终形成β-FeOOH[37]。完整的形成过程如式(
一定含量的Mn存在时，锈层具有阳离子选择性，Mn2+会占据Fe3O4反尖晶石结构的一个八面体中心[39]。通常Mn2+可以和PO
3 结论
(1) Q460和Q690低合金钢在南极大气环境下暴露1个月时腐蚀速率分别为29.7和77.0 μm/a，暴露12个月时，2者的腐蚀速率分别降低至约10.7和18.7 μm/a。
(2) Q460和Q690低合金钢大气暴露12个月后形成的腐蚀产物主要为α-FeOOH、γ-FeOOH、β-FeOOH以及Fe3O4/γ-Fe2O3，锈层内层主要为α-FeOOH和β-FeOOH，锈层外层主要为γ-FeOOH。Q460钢的锈层保护性比Q690钢更好。在南极低温环境下，由于Cl-的存在，2种钢的锈层均产生了较多的β-FeOOH以及裂纹。
(3) 暴露1个月时，Q460和Q690低合金钢均发生了严重的局部腐蚀，2者正面均比背面腐蚀严重，Q690钢比Q460钢腐蚀严重。随暴露时间的延长，2者的腐蚀行为由局部腐蚀向均匀腐蚀转变。
(4) 南极低温环境下，暴露初期处于南极暖季，冰雪冻-融过程导致液膜长周期存在促进了腐蚀的进行且加速局部腐蚀。长周期暴露时，金属表面被冰雪所覆盖，冰层以及锈层对溶解氧及侵蚀性离子阻挡使得腐蚀的发生受到了抑制。
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The initial corrosion behavior of carbon steel exposed to the coastal-industrial atmosphere in hongyanhe
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