doi: 10.11902/1005.4537.2016.035

Degradation Behavior of a Modified Epoxy Coating in Simulated Deep-sea Environment

Abstract:

The degradation behavior of a modified epoxy resin coating was comparatively studied in sea water at atmospheric pressure and in a simulated deep-sea environment with high hydrostatic pressure of 6 MPa by means of electrochemical impedance spectroscopy (EIS), 3D optical microscope and scanning electron microscope (SEM). The results showed that the resistance of the coating decreased to 105 Ωcm2 after 30 d immersion under high hydrostatic pressure, while that decreased to 108 Ωcm2 at atmospheric pressure. The deep-sea environment can induce the enlargement of the active area and shorten the water-saturation process of coatings, therewith, the corrosion rate of the substrate was instantly accelerated. SEM showed that the hydrostatic pressure can deteriorate the attachment of pigments with the epoxy and weaken the adhesion between the epoxy coatings and the metal substrate. In this case, the active area of corrosion was enlarged, whilst the degradation of coatings and the corrosion of the steel substrate simultaneously occurred.

Key words: deep sea environment with high pressure ; modified epoxy coating ; electrochemical impedance spectroscopy ; coating degradation

1 实验方法
1.1 涂层制备

1.2 实验室测试海水环境

1.3 电化学测试

1.4 形貌观察

2 结果与讨论
2.1 EIS测试

2.1.1 常压海水环境下的EIS 图1是常压海水环境条件下,改性环氧涂层的EIS谱随浸泡时间的变化。可以看出,在30 d的实验周期中,尽管涂层的阻抗逐渐降低,但涂层的阻抗谱特征没有发生明显的变化,Nyquist图始终为一个容抗弧。从相位角图中可以看出,EIS高频区相位角接近90°,说明该有机涂层相当于一个电阻值很大、电容值很小的隔绝层,对海水起到良好的阻隔作用。然而,涂层在浸泡期间不可避免的会吸水,随着浸泡时间的增加,涂层的低频阻抗模值减小,此时涂层的电容值增大而电阻值减少。基于涂层的EIS特征,可以采用如图2所示等效电路Rs(CcRc) 进行拟合。其中,Rs为溶液电阻,Rc为涂层电阻,Cc为涂层电容。

Fig.1 Nyquist (a, b) and Bode (c, d) plots of the modified epoxy coating immersed in sea water for different time at atmospheric pressure (Scattered points: experimental data, solid lines: fitting results)

2.1.2 模拟深海高压海水环境下的EIS 图3为模拟深海高压海水环境下不同浸泡时间后改性环氧涂层的EIS谱。可以看出,在30 d的浸泡周期内,涂层阻抗谱变化可以分为两个阶段：浸泡初期 (0~9 d),阻抗谱只有一个容抗弧,相位角约等于90°,有机涂层可视作一个良好的屏蔽层;此时,Nyquist图并不是一个规则的半圆,即存在弥散效应,可采用等效电路Rs(QcRc) 进行拟合。9 d之后,阻抗谱出现两个容抗弧,呈现出两个时间常数,这其实是随着涂层吸水逐渐达到饱和,腐蚀性溶液通过扩散逐渐到达涂层/基体界面处,形成腐蚀微电池对金属基体造成腐蚀,此时可以采用等效电路Rs(Qc(Rc(QdlRct))) 进行拟合[18](见图4)。其中,Qc为涂层电容,Qdl为双电层电容,Rct为电荷转移电阻。考虑存在弥散效应,采用常相位角元件近似替代电容。

Fig.2 Equivalent circuit model of Rs(CcRc)

2.1.3 常压和高压两种海水环境下涂层电化学参数的比较 低频阻抗模值可以比较直观地展现涂层性能随浸泡时间的变化,图5给出了0.01 Hz时涂层阻抗模值随时间的变化曲线。可以看出,两种环境下涂层的低频阻抗模值都随浸泡时间逐渐降低,初期下降较快,后期降低较慢。相比于常压海水环境,模拟深海高压海水环境在浸泡9 d后涂层阻抗发生急剧下降,30 d后已降低至约3×105 Ωcm2,这表明涂层已失去防护作用,涂层下的金属已经发生腐蚀。而常压海水环境下,30 d后涂层阻抗模值仍维持在约3×108 Ωcm2,表明涂层仍有较好的保护作用[11]。浸泡初期,在模拟深海高压海水环境中涂层阻抗高于在常压海水环境中的,这可能是涂层试样差异 (如涂层厚度存在一定差异) 造成的。但随着浸泡时间延长,高压海水环境中涂层加速失效,表明该环境条件比常压海水条件更为严苛。

Fig.3 Nyquist (a, b) and Bode (c, d) plots of the modified epoxy coating immersed in sea water for different time at 6 MPa (Scattered points: experimental data, solid lines: fitting results)

Fig.4 Equivalent circuit models of Rs(QcRc) (a) and Rs(Qc(Rc(QdlRct))) (b)

Fig.5 Impendences determined at 0.01 Hz for the epoxy coating after immersion for different periods in sea water at atmospheric pressure and 6 MPa pressure

Cc是衡量涂层性能的一个重要参数,取决于涂层厚度和介电常数,是反映涂层吸水特性的一个重要指标。从图6可以看出,改性环氧涂层在两种测试环境下,Cc都经历了逐渐增加并稳定在一个值附近的过程;但是在高压海水环境条件下,Cc达到稳定值的速度更快,这实际上反映了模拟深海环境条件下,涂层更快地吸水达到饱和的过程。

Fig.6 Capacitance (a) and pore resistance (b) vs time curves of the modified epoxy coating immersed in sea water under atmospheric and 6 MPa pressures

Fig.7 Qdl (a) and Rct (b) vs time curves of the coated steel after immersion in sea water for 9 d under 6 MPa pressure

$X v % = 100 % × lg C c t C c 0 lg 80$ (1)

Fig.8 Evolutions of water absorption rates of the modified epoxy coating immersed in sea water under atmospheric and 6 MPa pressures

2.2 形貌观察

Fig.9 Surface morphologies of the modified epoxy coating before immersion test in sea water at atmo-spheric (a) and 6 MPa (b) pressures

Fig.10 Surface morphologies of the modified epoxy coating after immersion in sea water for 30 d at atmospheric (a) and 6 MPa (b) pressures

Fig.11 SEM images of the modified epoxy coating immersed in sea water for 30 d at atmospheric (a) and 6 MPa (b) pressures after pull-off tests

3 结论

(1) 相比于常压海水环境,改性环氧涂层在模拟深海高压海水环境条件下更容易失效。涂层在常压海水中浸泡30 d后,阻抗仍维持在108 Ωcm2数量级,表明涂层仍处于完好状态;而在高压 (6 MPa) 海水中浸泡30 d后阻抗已降到105 Ωcm2数量级,表明涂层已失去保护作用,金属基体已发生腐蚀。

(2) 涂层吸水率随浸泡时间逐渐增大,并在后期吸水逐渐达到饱和。在模拟深海高压环境条件下,涂层在第9 d吸水达到饱和,而常压海水中这一过程需要16 d。涂层吸水率在模拟深海高压海水环境中的要比在常压海水中的更大,吸水更快。

(3) 深海海水的高压作用会使改性环氧树脂涂层中的颜填料更容易脱附,并且也会使涂层/金属基体界面弱化,腐蚀活性表面积增大,从而促进涂层破损和基体腐蚀。

The authors have declared that no competing interests exist.

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