目的 探究合金成分与处理时间对冷轧双相高强钢表面锆化膜成膜行为的影响,系统考察膜层的电化学耐蚀性能,明确影响膜层综合性能的关键因素。方法 以DP980和DP1180为基材,通过调控锆化处理时间（5~180 s）制备不同厚度的锆化膜。通过扫描电镜（SEM）、能谱仪（EDS）和辉光光谱仪（GDS）分析基材及膜层的微观形貌与元素分布;利用透射电镜碳萃取复型技术（TEM）表征锆化前基材表面氧化物的空间分布形态,利用X射线光电子能谱仪（XPS）表征基材表面氧化物分布及锆化膜的化学组成;在质量分数为3.5%的NaCl溶液（简称3.5%NaCl溶液）和锆化液中,通过开路电位（OCP）、电化学阻抗谱（EIS）和塔菲尔极化曲线测试,评估基材与膜层的表面活性及耐蚀性能。结果 DP980表面的合金元素含量较低,表面氧化物呈弥散均匀分布,无明显晶界偏聚;DP1180的合金元素含量更高,表面氧化物沿晶界聚集。基材表面均匀沉积锆转化膜,相同处理时间下,DP980表面锆元素质量分数（6.68%）明显高于DP1180（4.94%）。3.5% NaCl溶液中,DP1180的开路电位更低,自腐蚀电流密度更高,表面活性更强;锆化液中,DP980的电荷转移电阻低于DP1180,更利于Zr4+沉积。5~180 s处理时间内,2种基材表面Zr沉积量均与处理时间大致呈线性正相关。DP980和DP1180的最佳锆化时间分别为90 s和180 s,对应最高电荷转移电阻（Rct）分别达到4 901 Ω·cm2和2 407 Ω·cm2。结论 合金成分通过调控相组织特征与表面氧化物分布形态,导致了2种钢在不同介质中不同的表面活性规律。在3.5%NaCl溶液中,DP1180因马氏体体积分数更高、微电偶效应更强,而腐蚀活性更高;在锆化液中,DP980因表面氧化物弥散分布、活性位点更均匀而具有更高的成膜活性。综合电化学测试结果,DP980和DP1180的最佳锆化时间分别为90 s和180 s,DP980可在较短时间内形成耐蚀性能更优的锆化膜。
Abstract
Zirconium (Zr) conversion coatings provide a low-temperature, phosphorus-free alternative to phosphating for corrosion protection of automotive dual-phase (DP) steels, but how substrate alloying composition governs coating formation on different DP grades has not been systematically resolved. The work aims to compare the Zr conversion behavior and corrosion performance of cold-rolled DP980 and DP1180 steels which differ mainly in C, Mn and Si content, to establish the composition-dependent mechanism linking substrate oxide distribution to coating formation activity.
Cold-rolled DP980 and DP1180 sheets were degreased and immersed for 5-180 s at room temperature in a commercial two-component fluorozirconate bath with pH 3.5-4.5 and free fluoride content of 20-100 mg/L. Surface and coating morphology were examined by field-emission SEM. Oxide distribution on bare substrates was characterized by transmission electron microscopy on carbon-extraction replicas, and elemental composition and chemical states were determined by EDS and XPS. Zr depth distribution was measured by glow discharge spectroscopy and quantified by numerical integration of the depth profiles, normalized to the lowest-deposition condition, DP1180 at 5 s. Electrochemical behavior of bare substrates and Zr-coated samples was evaluated in 3.5wt.% NaCl solution and in the Zr bath by open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS) from 100 kHz to 0.01 Hz with a 10 mV perturbation, and Tafel polarization scanning at 0.5 mV/s across a 0.5 V window centered on OCP. Equivalent-circuit fitting of the EIS data extracted solution resistance, double-layer and film capacitance (Qdl, Qfilm), charge-transfer resistance (Rct) and film resistance (Rfilm).
TEM replicas showed oxide particles on DP980 dispersed within grains, whereas the higher-Mn DP1180 developed oxide particles 50-200 nm in size that nucleated and coarsened along grain boundaries, consistent with a higher grain-boundary Mn flux driving heterogeneous nucleation. XPS showed that both substrates carried MnSiO3, MnO and MnO2 together with lattice and adsorbed oxygen species, while both Zr coatings consisted mainly of ZrO2 with a minor fluorine-containing phase, indicating that chemical properties of the coating were essentially identical between substrates and that performance differences originated from Zr deposition amount and distribution rather than composition. Under identical 300 s treatment, DP980 accumulated 6.68wt.% surface Zr against 4.94wt.% for DP1180, and Zr deposition on both steels increased nearly linearly with time from 5 to 180 s, with DP980 exceeding DP1180 at every time point. In 3.5% NaCl solution, DP1180 showed a higher corrosion current density than DP980, 8.68 against 5.68 μA/cm2, and a lower charge-transfer resistance, 584.7 against 716.1 Ω·cm2, which was attributed to a stronger ferrite/martensite micro-galvanic effect at its higher martensite fraction. In the Zr bath, the ranking reversed: DP980 showed a lower charge-transfer resistance, 576.7 against 816.6 Ω·cm2, and a higher CPE dispersion index, 0.79 against 0.69, reflecting more uniform surface activity from its dispersed oxide layer. Combining OCP, EIS and Tafel results gave an optimal conversion time of 90 s for DP980, with charge-transfer resistance reaching 4 901 Ω·cm2 and polarization resistance 4064 Ω·cm2, and 180 s for DP1180, with charge-transfer resistance reaching 2 407 Ω·cm2 and polarization resistance 3 262 Ω·cm2. Beyond these time points, capacitive-arc diameters and charge-transfer resistance declined, indicating that fluoride-driven film dissolution outpaced Zr deposition. DP980 exceeded DP1180 in charge-transfer resistance at every treatment time, and its interfacial dispersion index rose from 0.84 at 5 s to 0.88 at 90 s before falling, showing densification followed by over-treatment degradation that was not seen to the same extent in DP1180.
These results show that alloying composition governs Zr conversion behavior through two mechanisms that act in opposite directions and are both controlled by composition: micro-galvanic corrosion in chloride media, and oxide-mediated surface reactivity in the fluorozirconate bath. This framework explains why a steel with lower activity in NaCl solution can show higher activity and better coating formation in the conversion bath, and offers a basis for selecting grade-specific conversion times for corrosion protection of dual-phase steel components.
关键词
高强钢 /
锆化膜 /
氧化物 /
电荷转移电阻 /
元素含量 /
膜重
Key words
high-strength steel /
zirconium conversion coating /
oxide /
charge transfer resistance /
element content /
coating weight
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