Ni-CeO_2纳米镀层在酸性NaCl溶液中的腐蚀行为及电化学阻抗谱特征

CORROSION BEHAVIOR AND EIS STUDY OF NANOCRYSTALLINE Ni-CeO_2 COATINGS IN AN ACID NaCl SOLUTION

借助超声辅助脉冲电沉积的方法在Watts-Ni电解液体系中制备了纳米晶纯Ni和Ni-CeO2复合镀层.采用电化学阻抗谱(EIS)测量和等效拟合电路解析,并结合静态浸泡腐蚀实验及其产物分析,研究了纳米CeO2颗粒对Ni镀层在3.5%NaCl+1.5%HCl(质量分数)混合水溶液(简称酸性NaCl溶液)中腐蚀性能的影响.结果表明:经600℃,4 h空气中的时效处理,可促使被填埋在镀层表面的稀土复合相沿晶界析出,并形成连续致密的弥散相钝化膜,填补并黏性钉扎由于电沉积析氢反应而遗留在镀层表面的微孔或热裂纹等缺陷,有效减少点蚀或晶间腐蚀的发生;动态极化曲线(Tafel)测定表明,纳米晶纯Ni和Ni-CeO2复合镀层在酸性NaCl溶液中均表现出明显的自钝化过渡区,后者自腐蚀电流较前者降低了近1.5个数量级;此外,EIS测试显示,纯Ni镀层的EIS由一个高频容抗弧和一个中低频扩散弧组成,时效处理前后Ni-CeO2镀层的EIS均呈现单一容抗弧特征,且相位角在更宽的频率范围内接近90°,从而表现出优异的抗腐蚀性能.在酸性NaCl溶液长期浸泡过程中,少量的Ce3+从CeO2弥散相中被还原释放出来作为缓蚀剂吸附在活性区域,提高了复合镀层的抗腐蚀性能;腐蚀形貌观察显示,Ni-CeO2镀层表面里现轻微的均匀腐蚀并被胶状不溶解的腐蚀产物所覆盖,而纯Ni镀层表面则出现了严重的点蚀现象;对腐蚀产物进行XRD和XPS测试分析表明,纯Ni试样表层主要积累了NiCl2和Ni(OH)2等腐蚀产物;而Ni-CeO2除含有上述这些主要产物之外,还包含CeCl3和CeO2等微溶相,覆盖并封堵点蚀源,可有效地阻止Cl-的扩散渗透行为.

Pure Ni and Ni-CeO2 nanocrystalline coatings were prepared from a Watts-nickel electrolyte using ultrasonic-assisted pulse electro deposition. Effect of incorporating CeO2 addition on their corrosion behavior in a 3.5%NaCl＋1.5%HCl （mass fraction） solution was evaluated by elec- trochemical impedance spectroscopy （EIS） and equivalent electric circuits （EECs）. Meanwhile, static immersion tests and the analysis of corrosion products were carried out. Experimental results indicated that the existence of CeO2 phase in the Ni coatings promoted an effective dispersion strengthening after aged at 600 ℃ for 4 h in air, and also produced a continuous Ce-rich oxide sale acted as the passive layer covered on the coating surface to reduce pinholes or micro cracks that generated from hydro- gen evolution reactions during electrochemical deposition, hence resulting in the decrease of localized corrosion or intergranular attack by Cl^-. Based on the observed results from potentiodynamic po- larization curves, they exhibited an obvious passivation transition attached with a reducing corrosion current density by lower than 1.5 orders of magnitude of Ni-CeO2 coatings （600 I：） as relative to pure Ni. EIS measurements for pure Ni displayed a high frequency capacitive arc together with a middle-low frequency diffusion arc. While for the Ni-CeO2 coatings, they displayed module of capacitive arc with a wide frequency area of phase angle close to 90° so as to confirm superior anti-corrosion property. During long-term static immersion tests in an acid NaCl solution, a small amount of Ce3＋ released from CeO2 phase were served as the corrosion inhibitors by means of making strong adsorption onto the exposed area for increasing corrosion resistance. With the presence of CeO2 addition, slightly uniform corrosion was observed on the surface of Ni-CeO2 coatings as compared to severely pitting corrosion of pure Ni. According to the determination of corrosion products by XRD and XPS, both of their corrosion products were mainly composed of NiCl2 and Ni（OH）2, while some insoluble products of CeC13 and Ce02 phase generated on the surface of Ni-CeO2 coatings, thereby achieving such insoluble corrosion products covered on pitting holes of localized corrosion to preclude the exposed area from corrosive attack and diffusion behavior by Cl^-.