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.
Key words
high-strength steel /
zirconium conversion coating /
oxide /
charge transfer resistance /
element content /
coating weight
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