Conversion Bath Research Articles

Investigations on corrosion resistance of steel coated with chromium-free ZTM conversion coating

Chromium–free conversion coatings based on Zr/Ti/Mo (ZTM) compounds via chemical conversion technology were investigated to improve the corrosion performance of SPCC-JISG 31 steels. A preliminary study was carried out to evaluate the influences of critical parameters, including the concentrations of active species Zr, Ti and Mo, and the conversion bath's pH, on the protective efficiency using a Statistical design of experiments (DoE) methodology. Passivation layer with compositional Mo, Ti and Zr elements has been employed on the steels by dipping in mixed solutions containing 17 g/l Na2MoO4, 1 g/l K2TiF6 and 7 g/l K2ZrF6 under pH = 5. The morphology and elemental analysis of Zr/Ti/Mo on surface of the steels was studied by the employment of scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). Pull-off adhesion test was performed, indicating that the ZTM coatings enabled to increase the adhesion strength (6.0 MPa) of mild steel to the organic coating than that of traditional phosphate (ZrP) coating. The results of electrochemical impedance spectroscopy and the salt spray test (529 h) evidenced the higher corrosion resistance of the Zr/Ti/Mo coating compared with theconventional phosphate coating.

Complementary corrosion protection of cast AlSi7Mg0.3 alloy using Zr-Cr conversion and polyacrylic/siloxane-silica multilayer coatings

Aluminium cast alloy AlSi7Mg0.3 is a lightweight metal commonly used in automotive, aeronautical and mechanical applications. It has good corrosion resistance but, under harsh operative conditions, would benefit from additional protection. In this study, a corrosion-protective multilayer coating system for AlSi7Mg0.3 based on hexafluoro-zirconated trivalent chromium coating (Zr-CrCC) and polyacrylic/siloxane-silica (PEHA-SS) coating was developed. The Zr-CrCC was formed by immersion of the substrate in a commercial conversion bath (SurTec® 650). PEHA-SS synthesis was based on organic precursors (2-ethylhexyl acrylate and [3-(methacryloyloxy)propyl]trimethoxysilane) and an inorganic precursor, tetraethyl orthosilicate. After deposition on AlSi7Mg0.3, each coating was first characterised individually, followed by the analysis of the multilayer using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The adhesion of the coatings was evaluated with a cross-hatch cut test. The corrosion studies in sodium chloride solution using electrochemical impedance spectroscopy and salt spray testing showed that the multilayer system is superior to individual Zr-CrCC and PEHA-SS coatings. After 4 months in 0.1 M NaCl, the multilayer-coated samples exhibited the impedance at 10 mHz in the range of GΩ cm2, while scribed samples withstood the corrosion attack in a salt spray chamber for one week. Thus, albeit only about 100 nm thick, the Zr-CrCC deposited between the substrate and a 9-micrometre thick barrier sol-gel PEHA-SS coating acts as an active corrosion protection interlayer and contributes to the overall protectiveness of the multilayer system.

Predict the surface free energy of Ti/Zr/V chemical conversion coating on nickel by machine learning approach: Trees vs. neurons

The surface properties of chemical conversion treated materials have received increasing attention and surface free energy (SFE) is an important indicator of it. In this study, an environment-friendly Ti/Zr/V chemical conversion coating was deposited on the nickel and Artificial Neural Network (ANN), Random Forest (RF), and Extra Trees (ET) models were applied to further investigate the relationship between the SFE and the compositions of the conversion bath. The accuracy of SFE prediction of three machine learning models was quantified, and the obtained results reveal that the correlation coefficients for the ANN, RF, and ET models are 0.958, 0.979, and 0.963 for the prediction of dispersion components and 0.964, 0.942 and 0.921 for the prediction of polar components, respectively, and the ANN model shows more outstanding performance in general. The errors in the prediction of the ANN model for the polar and dispersion components of SFE are 0.336 and 0.421, respectively, which are smaller than the extended uncertainties obtained from conventional experimental methods. The obtained machine learning models can accurately predict the variations of SFE for modest changes in the conversion bath components, thus linking the formation mechanism of coating with the variations of SFE.

Effect of Al3+ on the surface microstructure and film formation mechanism of Zr-based conversion coatings with improved corrosion resistance performance on cold rolled steel

Most Zr conversion coatings are prepared on Al alloys, with Al3+ from the substrate contributing significantly to the film growth process. Thus, the effect of Al3+ on the electrochemical characteristics, surface microstructure and film-formation mechanism of Zr-based conversion coatings on old- rolled steel (CRS) substrates was investigated by growing the coatings with and without Al3+ as an additive. Electrochemical tests indicated that the anticorrosion performance of the Zr-Al coatings prepared in the presence of Al3+ was greatly superior to that of Zr conversion coatings obtained in the absence of Al3+. Field-emission scanning electron microscopy and X-ray photoelectron spectroscopy analysis indicated that adding of Al3+ in the conversion bath changed the surface microstructure of the Zr coating, greatly promoted the deposition of Zr and F on the CRS substrate and increased the film thickness and density. Both coatings were embedded into the etched CRS substrates. The presence of Al3+ additive induced the formation of different coating components. The Zr coating mainly consisted of ZrO2 and some FeFx, whereas the Zr-Al coatings comprised of ZrO2, ZrOxFy, Al2O3, AlFx and some FeFx. The present study results confirm the key role of Al3+ as an additive in the film formation and growth process of Zr coatings and its effect on enhancing the anticorrosion performance of CRS substrates in NaCl solutions.

Mn oxide conversion coating on AZ31B Mg alloy with hexafluorozirconate as an inhibitor in the permanganate conversion bath

Surface modification is essential for Mg alloys, including conversion coating, plasma electrolytic oxidation, organic coating, and plasma spraying. Permanganate conversion coating is a potential alternative to chromate coating for Mg alloys. A continuous MnO2 coating on commercial Mg alloys, which generally contain micrometric secondary phase particles, is still challenging. A thin, continuous MnO2 conversion coating layer on AZ31B Mg alloy was successfully prepared by adding hexafluorozirconate anions (ZrF62−) to an acidic permanganate conversion bath. AZ31B Mg alloy comprised α-Mg matrix and micrometric Al8Mn5 particles. In addition to MnO2 and MgO/Mg(OH)2, MgF2 was present in the coating formed in the presence of ZrF62−, but the amount of ZrO2 was scarce. The average thickness of the coating was reduced from 665 to 66 nm when 0.02 M ZrF62− was added. As a result, the contact resistance was reduced from 367 to 8.59 mΩ. Meanwhile, the corrosion resistance was improved. The uniformity of MnO2 coating was discussed, focusing on the Al8Mn5/Mg galvanic coupling effect.

The effect of hexafluorozirconic acid concentration on the formation and corrosion resistance of trivalent chromium conversion coatings on AlSi12Cu1(Fe) cast alloy

AlSi12Cu1(Fe) alloy was subjected to the conversion bath containing: Cr2(SO4)3, HF and 200–800 ppm Zr (as H2ZrF6). Coatings composed mostly of ZrO2 and to a smaller extent by Cr(OH)3 with granular morphology were obtained. Concentration of Zr in the conversion bath did not affect the coatings thickness, which was non-uniform and ranged from 35 to 90 nm. Cracking and detachment were observed, which are due to the presence of possible phases like: Si, Cu0.01Al0.97Si0.02, FeAl3Si2 and Al2Cu, except the α–Al phase. These Cu- and Fe-rich intermetallic particles serve as the cathodic areas that are responsible for the formation of micrometric sizes numerous spheroids in the coating due to intensive hydrogen evolution. Longer protective effect, up to 240 hrs in a neutral salt spray and the lowest corrosion currents after 5 days in 0.05 M NaCl were obtained at 600 and 800 ppm Zr in the bath, respectively. It was due to higher content of oxidized chromium and aluminum compounds in these TCC coatings.

The Importance of Current Density in Electrolytic Phosphate Coatings

Phosphate coatings have been a popular choice for the treatment of steels and other ferrous alloys for a number of decades. In the most common embodiment of the process, phosphate coatings are produced by a conversion reaction between the steel surface and an acidic conversion bath. The resulting coatings are composed of phosphate crystals which act as a physical barrier between the environment and the underlying metal, protecting it from corrosion and wear [1]. Despite being a cheap and well-affirmed technology, conversion phosphating has recently been scrutinized due to its high environmental impact. In fact, one of the main byproducts of the process is a sludge containing phosphates and heavy metals from the deposition bath.A potential solution to the environmental burden carried by the conversion process is represented by the electrolytic deposition method. In this case, the increase in interfacial pH necessary for phosphate precipitation is obtained by applying a cathodic polarization to the samples to coat in an electrolytic setup. As a consequence of the potential shift, substrate dissolution is prevented and so is sludge formation [2].In this work, we demonstrate how the choice of current density influences the resulting properties of zinc phosphate coatings obtained by electrolytic deposition on low carbon steel substrates. Aside from the deposition rate, the applied current density is found to be a determinant factor in the morphology of the coatings as it can be seen in Figure 1. In turn, this affects other properties such as wear and corrosion resistance.[1] B. Díaz, L. Freire, M. Mojío, and X. R. Nóvoa, “Optimization of conversion coatings based on zinc phosphate on high strength steels, with enhanced barrier properties,” J. Electroanal. Chem., vol. 737, pp. 174–183, Jan. 2015.[2] S. Jegannathan, T. S. N. Sankara Narayanan, K. Ravichandran, and S. Rajeswari, “Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests,” Electrochim. Acta, vol. 51, no. 2, pp. 247–256, Oct. 2005. Figure 1

Effect of ultrasonic shot peening and intermediate cerium salt conversion bath treatment controlling corrosion of Mg-Y-Zn-based alloy in salt water

Ultrasonic shot peening (USP) is often regarded as beneficial in mitigating corrosion of Mg alloys. However, increasing surface roughness induced by USP promotes localized corrosion pitting. Herein, we show that an intermediate phosphate bath treatment (CePT) before USP can minimize the surface roughness and ameliorate corrosion resistance of Mg-4.0Y-4.0Zn-0.5Zr-0.2Ca alloy (wt.%) in 0.1 M NaClsolution. In this regard, WZ44 alloy was subjected to Ce(NO3)3 + NaH2PO4bath treatment followed by USP for 30 s using AISI52100 steel balls and vice versa. The sub-surface features ofspecimens analyzed using XRD, FESEM, surface topography, and Vickers hardness reveal that CePT coating before USP increases microhardness (∼142–149 HV0.05 vs ∼ 125–135 HV0.05) and decreases surface roughness (Ra = 1.91–2.38 μm vs Ra = 2.58–2.76 μm). Potentiodynamic polarizationand electrochemical impedance spectroscopy demonstrate that corrosion current decreased (Icorr ∼ 9.81–13.69 µA/cm2 vs. ∼ 23.82–26.22 µA/cm2), coating (R1), and polarization resistance (R2) increased to ∼ 1704–1907 Ω.cm2,and ∼ 1834–1902 Ω.cm2, respectively,for specimens subjected to USP + CePT treatment together indicating superior corrosion resistance.Overall, an intermediate CePT before USP can effectively minimize corrosion pitting phenomena of WZ44 alloy in chlorinated solution by developing a ceramic coating interlocked with surface acting as a barrier between the metal/solution interface.

Investigation of the effect of conversion bath aging time on V/Zr-based conversion coating formation and corrosion resistance ability

Investigation of the effect of conversion bath aging time on V/Zr-based conversion coating formation and corrosion resistance ability

Microstructure control of Ti-based conversion coatings for optimizing organic coating properties in A6063 and A3003 alloys

Ti-/Zr-based conversion coatings are environmentally friendly alternatives to conventional chromate conversion coatings. A prerequisite for their widespread use on aluminum alloys is an efficient treatment process applicable to multiple alloys. In this study, Ti-based conversion coatings with an ideal microstructure were successfully formed on both A6063 and A3003 under the same conversion bath conditions. These coatings exhibited satisfactory impact adhesion with organic coatings and salt spray corrosion resistance for practical applications. In-depth analyses of the deposition process of Ti-based conversion coatings by scanning and transmission electron microscopy showed that Ti-oxide nanoparticles were initially deposited on the matrix, which grew into a continuous layer on A6063, whereas a continuous Ti-oxide layer was formed on A3003 from the initial immersion stage. Lowering the pH of the conversion bath prevented the continuous growth of the Ti-oxide layer on A6063, thereby reducing the adhesion and corrosion resistance. Increasing the bath concentration accelerated the growth rate of Ti oxides in A3003 and A6063, and the formation of Ti oxides with excessive thickness reduced the adhesion. By optimizing the concentration and pH, a continuous Ti-oxide layer of 20–40 nm thickness was formed with a reasonable growth rate on both A6063 and A3003, resulting in satisfactory impact adhesion and salt spray corrosion resistance.

Microstructural evolution and corrosion behavior of constituent particles of AA2024-T3 Al alloy during zirconium conversion coating

The constituent particles of AA2024-T3 Al alloys were identified and their microstructural change after alkaline degreasing, acidic desmutting, and zirconium (Zr) conversion coating was detailed. The constituent particles of AA2024-T3 included Al-Cu-Fe-Mn, θ-Al2Cu, and S-Al2CuMg, and were several micrometers in size. All these three particles were cathodic to the Al matrix after alkaline degreasing and acidic desmutting pretreatment. The OCP evolution in the Zr conversion bath revealed that the conversion coating reactivity of the as-polished AA2024-T3 was promoted after the pretreatment. The polarization test in 0.1 M Na2SO4 + 0.05 M NaCl solution showed that the cathodic kinetics were enhanced after the pretreatment. Subsequent Zr conversion coating inhibited the anodic kinetics but did not alter the cathodic kinetics in chloride solutions. The defects of ZrO2 coating were responsible for the insignificant change in the cathodic activities.

The dual role of Mg2+in the conversion bath during the treatment of magnesium alloys: The completing effect between heterogeneous nucleation and crystal growth

The accumulation of dissolved Mg in the conversion bath during the treatment of Mg alloys is inevitable, which consequently changes the microstructure and corrosion resistance of conversion coating. But this effect has long been ignored. In this work, the dual role of dissolved Mg on the corrosion resistance of coatings were revealed: the corrosion resistance could be gradually improved when Mg2+ concentration retains less than 0.04 M; the further increase of Mg2+ leads to a gradual deterioration of corrosion resistance. A mechanistic explanation focusing on the completing effect between the heterogenous nucleation and the crystal growth was proposed.

Designing for the cerium‐based conversion coating with excellent corrosion resistance on Mg–4Y–2Al magnesium alloy

AbstractThe cerium salt chemical conversion baths containing KMnO4 are applied to prepare protective coatings on the WA42 alloy surface, and the effect of the concentration of KMnO4 on the microstructure and corrosion properties of the coatings is investigated by scanning electron microscopy, X‐ray photoelectron spectroscopy, and electrochemical tests. The results indicate that with the addition of KMnO4 to the conversion bath, the microstructure of the coating is more uniform and denser, and the coating with the KMnO4 concentration of 4 g/L (4M coating) has the most uniform microstructure with the least microcracks. The 4M coating exhibits a two‐layered structure, and it is mainly composed of MgO, Mg(OH)2, CeO2, Ce2O3, Ce(OH)3, MnO, and MnO2. In addition, as the KMnO4 concentration increases from 0 to 6 g/L, the Icorr of the coatings in 3.5% NaCl solution decreases first and then increases, and the 4M coating shows the best corrosion resistance, which should attribute to the uniform and dense microstructure.

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.

New design principles for the bath towards chromate- and crack-free conversion coatings on magnesium alloys

Cracking and low thickness are major obstacles to the high corrosion performance of conversion coating on magnesium alloy. In this work, the ratio of total acidity to pH (TA/pH) was applied as an indicator, and new principles regarding the design of conversion bath were proposed. The treatment bath should be composed of species that can be categorized into two groups: the first group of species that react with Mg substrate to increase the local pH at the interface; the second group that precipitate and contributes to the growth of coating. The species belong to second group exists in a supersaturated state and its precipitation process is almost independent on the reactions of the species in first group. By this way, a thick and crack-free two-layered conversion coating is obtained. Moreover, the nature of the adjustment of TA/pH and the roles of the oxidizing agent and catalyst were discussed.

Nucleation and growth mechanisms of trivalent chromium conversion coatings on 2024-T3 aluminium alloy

Nucleation and growth mechanisms of trivalent chromium conversion coatings on 2024 aluminium alloy (AA 2024) were studied. Nucleation of 25 nm diameter nodules was observed on the ridges of the scalloped structure of degreased and desmutted AA 2024 after very short time of conversion treatment corresponding to the formation of a 12 nm thick precursor layer. Then, the composition of this layer evolved and concomitantly a chromium and zirconium outer layer deposited on top of it. Rather long-lasting anticorrosive properties were measured even for conversion coatings formed after short exposure to the conversion bath, except for the precursor layer.

Electrochemical Behavior During the Zirconium Conversion Coating Formation on AISI 1006 Steel

This study investigated a conversion coating based on hexafluorozirconic acid solution on cold-rolled (AISI 1006) steel sheets, in situ, at different pH values and immersion times. Electrochemical characterization was carried out by electrochemical impedance spectroscopy (EIS) with hexafluorozirconic acid bath as the electrolyte. Laser scanning confocal microscopy (LSCM) was used to evaluate the morphology of the coating in situ, i.e., during the coating formation. According to the results, the polarization resistance (Rp) of the conversion coating is affected by the immersion time and by the pH of the conversion bath. The electrochemical analyzes point out conversion baths at pH between 3.5 and 4.5 and immersion times from 600 to 900 s as optimal operating conditions, achieving an average polarization resistance of 713 ± 107 Ω cm2.

Corrosion Resistance and Formation Analysis of a Molybdate Conversion Coating Prepared by Alkaline Treatment on Aluminum Alloy 6063

A molybdate-based conversion coating (MBCC) was prepared on aluminum alloy 6063 (AA6063) by treatment in an alkaline conversion bath. The obtained MBCC is a gray and uniform conversion coating. Scanning electron microscope and electrochemical measurements were used to analyze the basic properties of the coating. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the composition and formation process of the coating. The surface morphology and cross-sectional observation show that the coating grows rapidly under given conditions, its thickness can reach more than 30 μm in 10 min, and the MBCC prepared with the alkaline method is a non-cracked coating. The potentiodynamic polarization test indicates that the corrosion current of MBCC is more than one order of magnitude lower than the corrosion current of an untreated surface. The XPS measurement illustrates that the coating is made up of Al, Mo, O and F. According to the high-resolution XPS spectral analysis, the elements detected in the coating are in the form of Al2O3, AlF3, MoO3 and Al2(MoO4)3. Based on the XPS results, the formation process of the coating under alkaline conditions was discussed.

Preparation and characterization of photocatalytic TiO2/WO3 films on functionalized stainless steel

In this study, both TiO2 and WO3 were synthetized using controlled hydrolysis followed by dialysis. The produced transparent emulsions were dried into powders. Then, TiO2–WO3 photocatalyst films were deposited by pulsed electrophoretic deposition. The substrates were made of stainless steel pre-functionalized in a conversion bath. The conversion layer played an important role in the adhesion of the composite film into the substrate. Film surface morphology, crystallinity, roughness, thickness, and elemental composition were determined from XRD, optical profilometry and SEM–EDX measurements. The mechanical properties were resolute by nano-indentation test and adhesion was investigated using a scratch test. The photocatalytic decolorization of the Amido Black-10B dye was studied over TiO2–WO3 coatings under UV irradiation. TiO2 film doped 5% WO3 film exhibited the best photocatalytic activity due to its crystalline size and three-phase structure as well as a synergetic effect of TiO2–Fe2O3 and TiO2–WO3. This film showed also good adhesion properties.

Designing for the chemical conversion coating with high corrosion resistance and low electrical contact resistance on AZ91D magnesium alloy

The designing idea for the chemical conversion coating with high corrosion resistance (CR) and low electrical contact resistance (ECR) on AZ91D Mg alloy was presented. The main characteristics of the conversion bath include acidic solution, strong oxidizing agent, proper pH value and cation concentration (determined by thermodynamic calculation). The desired performances are attributed to the unique microstructure of conversion coating, i.e., β phase protruded from the surface and covered by a thinner passive film about 30 nm, contributing to low ECR, meanwhile, a thicker coating of 200–300 nm without cracks was deposited on α-phase, providing a good corrosion resistance.