实海浸泡条件下聚氨酯涂层的失效行为

Failure Behavior of Polyurethane Coating Immersed in Real Seawater

目的在青岛市小麦岛试验站开展实海浸泡试验,探究聚氨酯涂层在实际服役过程中的失效行为。方法选用TS55-80聚氨酯涂层/Q235碳钢体系为试验样品,开展实海浸泡试验。从聚氨酯涂层的表面形貌、失光率、色差、涂层附着力、化学结构及涂层热稳定性等角度对聚氨酯涂层的失效行为进行研究。结果在实海浸泡条件下,聚氨酯涂层表面会出现明显的鼓泡和裂纹等缺陷,在浸泡6个月后的涂层表面可以观察到明显的腐蚀产物。随着浸泡时间的延长,涂层的化学结构发生了明显变化,涂层的热稳定性显著降低。在浸泡12个月后,聚氨酯涂层的失光率为69.9%,属于严重失光;涂层的色差达到3.20,属于轻微变色,涂层的附着力降至0.82 MPa,涂层与金属基体的结合强度大幅下降;聚氨酯涂层的阻抗值降至2.81×10^(3)Ω·cm^(2),说明涂层的防护性能基本丧失。结论在实海浸泡条件下,聚氨酯涂层中颜料颗粒的脱落会造成涂层表面孔隙数量的增加,这会加速海水中水和氧气等腐蚀性介质的渗透过程,使得涂层/金属界面处的电化学反应快速进行,导致涂层的防护性能快速下降。此外,聚氨酯链中氨基甲酸酯键的水解是造成聚氨酯涂层发生降解的主要原因。

Polyurethane coating shows good characteristics of corrosion resistance and water repellency,and it is easy to repair.Thus,it is widely used in corrosion protection of marine engineering equipment.However,the protective performance of the polyurethane coating is often lost for effects of various factors such as dissolved oxygen content,temperature,and seawater scouring during its service in the marine environment.The work aims to conduct real seawater immersion test in Xiaomaidao Test Station of Qingdao to study the failure behavior of the polyurethane coating immersed in real seawater.In this work,the Q235 carbon steel(100 mm×75 mm×2 mm)was used as the metal substrate,and the TS55-80 polyurethane coating was sprayed on the surface of the metal substrate.The test samples of the TS55-80 polyurethane coating/Q235 carbon steel system was immersed in the seawater.The test samples were divided into 6 cycles:0 month,1 month,3 months,6 months,9 months,12 months.The macro morphology and micro morphology of the polyurethane coating surface was observed with an electronic magnifying glass.The appearance changes of the polyurethane coating were evaluated by measuring and calculating the gloss loss and color difference of the coating.A pull-off adhesion tester was used for the adhesion testing following ASTM D4541-02,and the diameter of the dolly was 20 mm.The EIS measurements were conducted within the frequency range of 100kHz-10 mHz at room temperature with an electrochemical test system,the amplitude was 20 mV and the test area was 1 cm^(2).The EIS spectra were analyzed with the ZSimpWin 3.30 software.The chemical structures of the coating samples were analyzed by FT-IR and XPS.The binding energies in the XPS test results were calibrated by reference to the C1s signal at 284.8 eV.Thermal stability of the polyurethane coating was evaluated with a thermal gravimetric analyzer under a nitrogen flow(10cm^(3)/min).It showed that when the polyurethane coating was immersed in seawater for 6 months,obvious bubbles and cracks appeared on the surface of the coating,and the corrosion products were observed.With the immersion time increased,the chemical structure of the coating was changed,and the thermal stability of the coating was significantly decreased.After immersed for 12 months,the gloss loss rate of the polyurethane coating was 69.9%,which was a serious gloss loss.The color difference of the coating was 3.20,which was a slight discoloration.The adhesion of the coating was decreased to 0.82 MPa,which indicated that the bonding strength between the coating and the metal substrate was significantly decreased.And the impedance value of the coating was decreased to 2.81×10^(3)Ω·cm^(2),which meant that the protective performance of the polyurethane coating was basically lost.When the polyurethane coating is immersed in real seawater,the shedding of pigment particles would cause the number of pores on the coating surface to increase,the penetration of corrosive media in seawater would be accelerated.Thus,the electrochemical reaction at the coating/metal interface rapidly developed,resulting in the significant decrease of the protective performance of the coating.In addition,the hydrolysis of urethane bonds in the polyurethane chains is the main reason for degradation of the polyurethane coating.