Zirconium-based Conversion Coatings Research Articles

A Comparative Study of Zinc Phosphate and Zirconium-Based Conversion Coatings on Galvannealed Steel: Effect on Corrosion Performance of Electrocoated Systems

The efficacy of an electrocoated system that uses a Zr-based conversion coating was assessed to protect galvannealed (GA) steel. The aim was to determine if a Zr-based system has the potential to replace a system with a traditional zinc phosphate (ZnP) conversion coating, where there are concerns for the environmental and economic sustainability. A comparative study between the two conversion coatings, and a control system with no conversion coating, is presented. Data from AC and DC electrochemistry are utilized with surface analysis investigations of pristine specimens, where scanning electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction have been used to determine the mechanism through which protection, or coating failure, occurs. Further investigations study surface morphology and stability of specimens during immersion in a simulated electrocoat bath, and their effect on the performance of fully electrocoated systems. The corrosion current density (icorr) for bare, ZnP, and Zr-based conversion coated specimens in chloride-containing solution with no electrocoat were found to be approximately 1.90 µA/cm2, 0.30 µA/cm2, and <0.01 µA/cm2, respectively. Rp values of 0.05 kΩ, 4.2 kΩ, and 20.7 kΩ were recorded for bare, ZnP and Zr-based specimens, respectively. The corrosion current density (icorr) for bare, ZnP, and Zr-based conversion coated specimens under chloride-containing conditions with electrocoat were found to be 3.82 µA/cm2, 0.53 µA/cm2, and 0.06 µA/cm2, respectively. |Z| values at 0.01 Hz (|Z|0.01 Hz) were measured, yielding 45.5 GΩ, 47.4 GΩ, and 31.6 GΩ for bare, ZnP, and Zr-based pretreatments respectively, demonstrating excellent barrier properties after electrocoating. Hydrogen evolution reaction (HER) rate values of 3.52 × 10−2 mol·m−2·s−1, 3.12 × 10−2 mol·m−2·s−1, and 2.94 × 10−2 mol·m−2·s−1 were recorded for the bare, ZnP, and Zr-based systems, respectively. This study produced a ranking order of HER rate. This was bare > ZnP > Zr-based. These results suggest that the Zr-based system is less prone to HER, interfering with the electrocoat deposition process. A performance ranking order for pretreated and electrocoated systems was established, where Zr-based > ZnP > control. The level of thickness and roughness provided by the conversion coating is speculated to be the controlling factor for coating performance, where the ZnP conversion coating was found to provide a thicker layer with greater surface coverage beneficial for a uniform electrocoat film build.

Study of Zirconium-Based Conversion Coating on Aluminum Counter Electrodes of Dye-Sensitized Solar Cells for Anti-corrosion Resistance and Morphology

Study of Zirconium-Based Conversion Coating on Aluminum Counter Electrodes of Dye-Sensitized Solar Cells for Anti-corrosion Resistance and Morphology

A combined XPS/ToF-SIMS approach for the 3D compositional characterization of Zr-based conversion of galvanized steel

The present study investigates the surface and in-depth characteristics, and deposition mechanism of zirconium-based conversion coatings on hot dip galvanized (HDG) steel substrates for automotive applications, based on zinc galvanizing baths with small additions of aluminium. This contribution provides scientific proof of the chemical heterogeneity both at the surface and along the depth of the Zr-based conversion coatings on galvanized steel. To this purpose, the definition of two derivative model systems – pure zinc and pure aluminium – was indispensable due to Al-enrichment phenomena observed at the galvanized steel surface. A 3D evaluation of the sample chemistry before and after formation of the Zr-based conversion coatings was conducted by a combined approach based on X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) with 1-keV Cs+ sputtering. The experimental results provide direct evidence of the fact that Zr-based conversion coatings on the HDG steel are constituted by a mixed Zn, Al and Zr oxyhydroxide in combination with Zr oxy-/hydroxyfluoride on top of a complex fluoride (hydr)oxide Zn/Al layer, rather than a pure ZrO2 film on top of the galvanized layer mainly made of zinc (and aluminium) as previously reported in the literature.

Increased Filiform Corrosion Resistance Utilizing a Zirconium-Based Conversion Coating on an Al-Zn-Mg-Cu (AA7075-T6) Alloy as well as Selected Surface Treatments

This study investigates the effect of surface treatment on the formation of Zr-based conversion coatings on AA7075-T6 automotive aluminum alloys and their resistance to filiform corrosion (FFC). Two different surface treatments were studied: (i) alkaline-cleaning and (ii) alkaline-cleaning with a subsequent acid deoxidation step. A model poly-vinyl butyral primer coating was used as the topcoat and specimens were studied with and without the application of a Zr-based conversion coating. Comparisons were made against a control that had no surface treatment. The FFC filament initiation time and propagation kinetics were of particular interest. Scanning electron microscopy and x-ray photoelectron spectroscopy were used to examine the conversion coating thickness and composition. A bi-layer conversion coating structure is demonstrated and both surface treatments are shown to produce copper enrichment that promotes the formation of the Zr-rich coating. Specimens prepared by alkaline cleaning-only resulted in a substantially thicker oxide layer of which 97% was ZrO2. These specimens provide superior resistance to FFC where the thick Zr-rich oxide is thought to provide a dense blocking layer that prevents electron transfer at the interface. In contrast, the control specimen, exposed only to the copper additions present in the conversion bath, is shown to produce an Al oxide-rich layer with only a 33% ZrO2 contribution in the outer layer. The findings demonstrate that the redistribution of functional copper species, that is shown to occur during surface treatment processes, is crucial for the formation of a robust Zr film.

The effect of bicarbonate additive on corrosion resistance and microstructure characteristics of the zirconium-based conversion coating on carbon steel

Zirconium-based conversion coating (ZrCC) with modification has been studied for improving the corrosion resistance of carbon steel in this paper. Ammonium bicarbonate (NH4HCO3) and sodium bicarbonate (NaHCO3) were used as additives to investigate their roles on the conversion coating properties. The surface of the modified coating was characterized by scanning electron microscope, X-ray photoelectron spectroscopy, and electron probe micro-analyzer. In addition, the corrosion resistance was studied by Tafel polarization, electrochemical impedance spectroscopy, and the neutral salt spray test (NSST). The results indicated that both NH4HCO3 and NaHCO3 as additives could promote the precipitation of ZrO2, ZrF4, K2ZrF6, Fe2O3, and FeOOH on carbon steel and NaHCO3 as the additive could make the surface thicker and more uniform. According to the electrochemical and NSST results, NH4HCO3- and NaHCO3-modified zirconium-based conversion coating (ZrCC–NH4, ZrCC–Na) exhibited more effective corrosion protection property compared with ZrCC, among which ZrCC–Na sample showed the highest corrosion resistance.

Chemisorption of polyester coatings on zirconium-based conversion coated multi-metal substrates and their stability in aqueous environment

In this work, in-situ ATR-FTIR in the Kretschmann configuration is proposed as an interfacial sensitive technique able to probe molecular processes at the buried interface of an industrial relevant polyester primer. Zinc, aluminium and magnesium oxide were used to represent oxides present at galvanized steel sheets used in coil coating. Two competing interactions with polyester resin and melamine-based crosslinker were shown to take place at metal hydroxide sites. This highlights the increased complexity of interfacial phenomena at metal–paint interfaces. Furthermore, in-situ ATR-FTIR was performed in deuterated water (D2O) to study the evolution of interfacial carboxylate bond degradation, without overlap of dominant water signals. For the first time, interfacial bond formation of paints and its degradation in an aqueous environment is studied in-situ. It is shown that the introduction of D2O at the interface initially increases the amount of interfacial carboxylate bonds, whereas upon longer exposure times bond degradation occurs. Significant delay of interfacial bond degradation on hexafluorozirconic acid treated oxides indicate successful stabilization of the metal-polymer interface by zirconium-based conversion coatings. Consequently, in-situ ATR-FTIR is able to demonstrate improved interfacial stability due to zirconium-based treatment in real-time and on a molecular level.

Formation and characterisation of zirconium-based conversion coating on steel

Zirconium-based conversion coating (ZrCC) was successfully prepared on the samples of cold-rolled steel from the solution of H2ZrF6 and HNO3 at controlled pH. Its surface morphology was analysed by the 3D optical microscope and scanning electron microscope. Its chemical composition and structure were investigated by energy-dispersive X-ray spectroscopy, X-ray diffraction analysis and X-ray photoelectron spectroscopy. The open-circuit potential measurement was used to study the coating formation process, and the electrochemical impedance spectroscopy test was employed to analyze the behaviors of the coatings prepared at different deposition time. According to the analysis results, the deposition mechanism of ZrCC on steel was proposed, and the chemical schematic diagram was also designed. The corrosion resistance performance of ZrCC with cathodic electrodeposition primers was also evaluated by salt spray exposure of 1000 h.

Remanufacture of Zirconium-Based Conversion Coatings on the Surface of Magnesium Alloy

Brush plating provides an effective method for creating a coating on substrates of various shapes. A corroded zirconium-based conversion coating was removed from the surface of a magnesium alloy and then replaced with new coatings prepared via brush plating. The structure and composition of the remanufactured coating were determined via x-ray photoelectron spectroscopy, x-ray diffraction, and Fourier transform infrared spectroscopy. The results revealed that the coatings consist of oxide, fluoride, and tannin-related organics. The composition of the coatings varied with the voltage. Furthermore, as revealed via potentiodynamic polarization spectroscopy, these coatings yielded a significant increase in the corrosion resistance of the magnesium alloy. The friction coefficient remained constant for almost 300s during wear resistance measurements performed under a 1-N load and dry sliding conditions, indicating that the remanufactured coatings provide effective inhibition to corrosion.

Protection of AA5083 by a Zirconium-Based Conversion Coating

A chromium-free zirconium-based conversion coating (ZrCC) was prepared on AA5083. The ZrCC improved the corrosion resistance of AA5083 and the adhesion of epoxy resin significantly, the performance of which was much better than that of the commercial chromium-free treatment and close to that of commercial chromate conversion coating (CCC) treatment. The ZrCC displayed a three-layer structure with a thickness of approximately 1.5 μm, including an inner layer at the alloy/coating interface, a thicker middle layer with nodular particles and an outer layer on the top. Formation of the ZrCC proceeded via the stacking of nodular particles layer upon layer, the composition of which was mainly amorphous Na3AlF6, Zr(HPO4)2·H2O, ZrO2 and AlPO4. The ZrCC with the special three-layer structure provided good protection despite the absence of a “self-healing” ability.

Morphological analysis and corrosion performance of zirconium based conversion coating on the aluminum alloy 1050

An aluminum alloy 1050 was treated by zirconium-based conversion coatings at different pHs, immersion times, temperatures and Zr concentrations. Corrosion resistance and morphological properties of samples were investigated by potentiostatic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), energy dispersive X-ray spectrum (EDS) and atomic force microscope (AFM). The results showed that surface treatment of aluminum by zirconium conversion coating caused decrease in corrosion current density and increase in corrosion resistance even at long immersion times. The Zr conversion layer enhanced the adhesion of the epoxy coating to the aluminum alloy surface.

A comprehensive study of the green hexafluorozirconic acid-based conversion coating

The effect of four practical parameters (deposition temperature, time, bath pH and concentration of Zr) on electrochemical and morphological properties of zirconium based conversion coating (ZrCC) on cold rolled steel (CRS) substrates was investigated. DC polarization and electrochemical impedance spectroscopy (EIS) measurements were used for electrochemical studies. Micro structural characterization was carried out by FE-SEM, AFM and EDS. The optimal conditions were determined as follows: solution temperature 20–30°C, immersion time 60–120s, pH=4 and acid concentration of 4% vol. In such condition, corrosion resistance values were maximum and uniformity was improved. Microscopical observations revealed that ZrCC comprises a two-layered structure. The film formation mechanism of ZrCC was discussed in detail. After achieving the optimum conditions, epoxy nanocomposites were applied on ZrCC treated CRS samples and corrosion performance of fully painted system was investigated. Finally the adhesion strength of subsequently applied organic coating on ZrCC treated substrates was measured by pull-off technique which exhibited considerable high values.

Formation of zirconium-based conversion coatings on aluminium and Al–Cu alloys

The influence of copper addition to aluminium on the formation of a zirconium-based conversion coating is investigated using sputtering-deposited substrates. Coatings formed on aluminium are ∼1.5 times the thickness of the aluminium consumed by oxidation, with an O:Zr atomic ratio of ∼2.5. Copper additions reduce the coating growth rate, especially when added in amounts above a few at.%. In contrast, the copper has relatively little effect on the oxidation rate of the substrate. Copper also promotes the formation of a layer of corrosion product beneath the coating and appears to influence the adherence of the coating to the substrate.