Cerium-based Conversion Coatings Research Articles

Influence of step-by-step cerium-based conversion and polydopamine duplex coating on stir cast Al[sbnd]Cu[sbnd]Mg alloys mitigating saltwater corrosion

Intergranular corrosion (IGC) originates from secondary phases along the grain boundaries in AlCuMg-based alloys can dramatically compromises the tensile properties for structural applications. Insofar cerium-based conversion coating (CeCC) has been exploited to provide a cathodic barrier and mitigate intergranular crack propagation. However, CeCC-based coating deposited on secondary phases in AlCuMg alloys limits its potential use to improve corrosion resistance in the chlorinated environment. Herein, mussel-inspired polydopamine (PDA) coating has been explored on CeCC-treated AlCuMg alloy, providing both anodic and cathodic protection and significantly improving the corrosion resistance in 0.1 M NaCl solution. The micro-morphology of CeCC and CeCC/PDA duplex coating was investigated using X-ray diffraction, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), two and three-dimensional surface topography, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) reveal the formation of aluminum and/or cerium oxide/hydroxide-based complex coating [Ce2O3/CeO2/Al2O3/Al(OH)3] on the Cu-rich secondary phases [Al2Cu, Al2Cu(Mg), and Al5Si(CuMg)] providing cathodic protection. However, additional PDA treatment for 12 h provided uniform surface coverage of PDA particles along the grain boundaries as well as α-Al matrix owing to the formation of hydrogen bonding with hydroxy species [Al(OH)3/Ce(OH)4] and catechol and amine functional groups present in PDA. Potentiodynamic polarization (PDP) illustrates that the CeCC/PDA specimens can successfully retard the corrosion of AlCuMg alloy. In particular, CeCC-60 s/PDA specimen demonstrated the lowest corrosion current density (Icorr ∼ 0.16 μA/cm2), and the highest polarization resistance (Rp ∼ 119 kΩ·cm2) during electrochemical corrosion in 0.1 M NaCl solution. The corroded surface discloses accelerated IGC was evident on bare and CeCC specimens. On the contrary, CeCC-60 s/PDA specimen demonstrated a significant reduction in IGC and drastically improved corrosion rate.

The study on the corrosion resistance enhancement of cerium-based conversion coatings by adipic acid and phytic acid post-treatment

This study investigates cerium-based conversion coatings enhanced with phytic and adipic acid treatments to improve the corrosion protection of galvanized steel. Optimal coating parameters were identified, including 15 min immersion in a 5 g/L cerium nitrate bath at pH= 3.5. The two-bath phytic acid post-treatment delivered unprecedented corrosion resistance, with an impedance of 11761 Ω.cm2 that markedly exceeds existing literature values. Surface characterization reveals phytic acid forms a more compact, homogeneous coating layer compared to adipic acid. Pull-off tests confirm improved adhesion strength of an epoxy topcoat applied to the phytic acid modified conversion coating. The substantial performance gains quantify the considerable advance this optimized coating system represents for sustainable corrosion inhibition.

Electrochemical corrosion resistance of aluminum alloy 6101 with cerium-based coatings in an alkaline environment.

Chromium-free materials as eco-friendly coatings with higher corrosion resistance are crucial in various industrial processes. Herein, we report the deposition of cerium-based conversion, a chromium-free, eco-friendly chemical conversion coating for aluminum alloy 6101, by the dip coating method. Immersion in cerium salt precursors assisted with hydrogen peroxide was performed for the deposition of cerium-based conversion coatings on aluminum alloy 6101 at different bathing temperatures. The electrochemical corrosion behavior was assessed in an alkaline solution of sodium hydroxide (pH 11), including mass loss measurements, free corrosion risk, polarization, and electrochemical impedance spectroscopy. X-ray diffraction and photoelectron spectroscopy analysis showed that the coatings were composed of Ce (III) and Ce (IV) oxides. Surface modifications and surface degradation of the coating and substrate after immersion in corrosive media were analyzed by scanning electron microscopy. Additionally, energy dispersive scanning analysis demonstrated the elemental composition before and after corrosion of the cerium salt conversion-based coating. The results demonstrated that selectively deposited cerium-based conversion coatings improved the corrosion resistance by up to 96% in a strong corrosive alkaline media.

Surface Pretreatments of AA5083 Aluminum Alloy with Enhanced Corrosion Protection for Cerium-Based Conversion Coatings Application: Combined Experimental and Computational Analysis.

The effects of surface pretreatments on the cerium-based conversion coating applied on an AA5083 aluminum alloy were investigated using a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), polarization testing, and electrochemical impedance spectroscopy. Two steps of pretreatments containing acidic or alkaline solutions were applied to the surface to study the effects of surface pretreatments. Among the pretreated samples, the sample prepared by the pretreatment of the alkaline solution then acid washing presented higher corrosion protection (~3 orders of magnitude higher than the sample without pretreatment). This pretreatment provided a more active surface for the deposition of the cerium layer and provided a more suitable substrate for film formation, and made a more uniform film. The surface morphology of samples confirmed that the best surface coverage was presented by alkaline solution then acid washing pretreatment. The presence of cerium in the (EDS) analysis demonstrated that pretreatment with the alkaline solution then acid washing resulted in a higher deposition of the cerium layer on the aluminum surface. After selecting the best surface pretreatment, various deposition times of cerium baths were investigated. The best deposition time was achieved at 10 min, and after this critical time, a cracked film formed on the surface that could not be protective. The corrosion resistance of cerium-based conversion coatings obtained by electrochemical tests were used for training three computational techniques (artificial neural network (ANN), adaptive neuro-fuzzy inference system (ANFIS), and support vector machine regression (SVMR)) based on Pretreatment-1 (acidic or alkaline cleaning: pH (1)), Pretreatment-2 (acidic or alkaline cleaning: pH (2)), and deposition time in the cerium bath as an input. Various statistical criteria showed that the ANFIS model (R2 = 0.99, MSE = 48.83, and MAE = 3.49) could forecast the corrosion behavior of a cerium-based conversion coating more accurately than other models. Finally, due to the robust performance of ANFIS in modeling, the effect of each parameter was studied.

Effect of cerium-based conversion coating on corrosion behavior of squeeze cast Mg-4 wt% Y alloy in 0.1 M NaCl solution

Cerium-based conversion coating (CeCC) has been utilized to improve the corrosion resistance of squeeze cast Mg-4wt%Y alloy in 0.1 M NaCl solution. The morphology and composition of the coating analyzed using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), glancing angle X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS) demonstrate the formation of few microns thick pseudo-crystalline phase(s) composed of mainly Mg, O, C and Y species, assigned to hydrated Y(III) (Y(OH)3/Y2O3) CeO2, Mg(OH)2, and lower amount of PO43− ions with mud-crack morphology. The duration of the treatment in cerium nitrate salt bath has a pronounced effect on the evolution of the coating with 30 s treatment demonstrated the formation of uniform coating with dehydrated cracks that becomes loose and porous with nodular CeO2 precipitates for longer duration (1800 s). Potentiodynamic polarization (PDP) and mass loss test reveals that CeCC effectively reduces the corrosion rate by ~50–70% compared to base alloy, with the pitting corrosion around the second phase being the dominant mechanism due to the formation of the micro-galvanic couple between α-Mg phase and Mg24Y5 precipitates. The deep pit formation was severe around Y-rich cathodic sites due to the formation of Y(OH)3/Y2O3 as the corrosion products confirmed by EDX and elemental mapping. Electrochemical impedance spectroscopy revealed that corrosion protection was provided mainly by the barrier layer at the coating/Mg interface with a progressive increase in diameter of the capacitive loop up to 24 h commensurate with the surge in double-layer film resistance (RP) and free corrosion potential (Ecorr) shift towards noble value. Overall, cerium nitrate salt bath treatment in the presence of hydrogen peroxide for short duration is effective to retard the severe corrosion of Mg-4wt%Y alloy in 0.1 M NaCl solution.

Real-Time AFM and Impedance Corrosion Monitoring of Environmentally Friendly Ceria Films on AA7075

Cerium-based conversion coatings have emerged as promising green alternatives to the harmful chromium-based ones, but the mechanism of corrosive protection still remains a subject of academic and industrial research. This study focuses at small scale phenomena of corrosion inhibition imparted by ceria (CeO2) to AA7075. Ceria nanoparticles were deposited from diluted and concentrated CeO2 sols by immersion. A multi-analytical approach, combining Atomic Force microscopy (AFM), Scanning Kelvin Probe Force Microscopy, Glow Discharge Optical Emission Spectroscopy, open circuit potential and electrochemical impedance spectroscopy was employed. Deposition of ceria films led to deactivation of cathodic sites, i.e. decreased Volta potential difference, resulting in increased corrosion inhibition. In situ AFM real-time monitoring revealed that during exposure to NaCl electrolyte, the changes in size of deposited ceria aggregates occurred: nanoparticles disintegrated/desorbed and re-deposited at the coating surface. The process was found to be dynamic in nature. Small particles size and inherent reactivity are believed to accelerate this phenomenon. Due to the greater CeO2 reservoir, this phenomenon was more pronounced with a thicker film, imparting longer term protection.

Influence of the Treatment Time on the Surface Chemistry and Corrosion Behavior of Cerium-Based Conversion Coatings on the AZ91D Magnesium Alloy

The aim of the present work was to investigate the effect of the treatment time on the surface chemistry and corrosion behavior of cerium-based chemical conversion coatings on the AZ91D magnesium alloy. The conversion coating was prepared by the immersion technique from a bath consisting of 0.05 mol.L-1 Ce(NO3)3.6H2O and 0.254 mol.L-1 H2O2 (30 wt.%) for times ranging from 20 s to 120 s. The surface chemistry was examined by X-ray photoelectron spectroscopy (XPS). The corrosion behavior was assessed by electrochemical impedance spectroscopy and potentiodynamic polarization. XPS analysis detected the presence of cerium oxides (Ce2O3 and CeO2) and cerium/magnesium hydroxides. The best corrosion behavior was observed for the treatment conducted for 60 s. The results are discussed with respect to coating morphology and composition.

Self-healing conversion coating with gelatin–chitosan microcapsules containing inhibitor on AZ91D alloy

ABSTRACTThe microstructure of cerium-based conversion coatings with and without microcapsules containing Ce(NO3)3 or La(NO3)3 on magnesium alloys were investigated and the corrosion resistance was evaluated as well. The gelatin–chitosan microcapsules were prepared by complex coacervation. Microcapsules can effectively avoid the traditional porous state, namely, by releasing active and repairing material immediately after changes in coating integrity. Microcapsules distribute in the tiny cracks of coating deposited on magnesium alloys. The electron probe micro-analyser results characterise the element distribution of coating. The potentiodynamic polarisation curve and impedance spectra show that the conversion coating with microcapsules can enhance the corrosion resistance of magnesium alloys. The microcapsules containing La(NO3)3 show a better corrosion resistance so as to promote a self-healing performance.

Study of selective deposition mechanism of cerium-based conversion coating on Rheo-HPDC aluminium-silicon alloys

Cerium-based conversion coatings were deposited on Rheo-High Pressure Die Cast (HPDC) Al-Si alloys by immersion in cerium nitrate aqueous solutions. Rheocast Al-Si alloys have a heterogeneous microstructure and present a challenge for the conversion treatment. Different parameters were studied to optimize the conversion coating, and NaCl or H2O2 were also added to the solution to modify or accelerate the deposition process. The mechanism of the coating formation was studied by means of focused ion beam milling (FIB) assisted SEM. The results show that applying cerium-based conversion coating to Al-Si alloys, is possible and a preferential deposition is obtained due to the presence of iron-rich intermetallic particles inside the eutectic region. The formation mechanism of selectively deposited cerium-based conversion coating includes dissolution of aluminium matrix, selective dissolution of aluminium from the noble intermetallic particles, oxidation of iron from the intermetallic particles, and the deposition of cerium hydroxide/oxide layer. The results reveal that the improvement in corrosion resistance in the presence of selectively deposited cerium-based conversion coating is more significant compared to the homogenous coating deposited from the conversion solution containing H2O2.

Effects of combined organic and inorganic corrosion inhibitors on the nanostructure cerium based conversion coating performance on AZ31 magnesium alloy: Morphological and corrosion studies

Magnesium (Mg) AZ31 samples were chemically treated by a series of room temperature nanostructure cerium based conversion coatings containing Mn(NO3)2·4H2O, Co(NO3)2·6H2O, and polyvinyl alcohol (PVA). The microstructure and corrosion protection properties of different samples were studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and polarization test in 3.5wt.% NaCl solution. Results demonstrated that the AZ31 Mg alloy sample treated by Ce-Mn-PVA showed the highest corrosion resistance. A denser Ce film with lower crack was precipitated on the sample treated by Ce-Mn-PVA conversion coating.

Deposition and Characterization of Cerium-Based Conversion Coating on HPDC Low Si Content Aluminum Alloy

Cerium-based conversion coatings were deposited on high pressure die cast (HPDC) Al-Si alloys using an immersion method. Hydrogen peroxide and sodium chloride were added to the conversion solution to accelerate the coating formation and to understand its formation mechanism. These studies showed that the deposition of cerium hydroxide/oxide conversion layer starts from iron-rich intermetallic particles, which are located inside the eutectic region and then the coating growth continues to cover the entire alloy surface. This phenomenon passivates the active interfaces between iron-rich intermetallic particles and/or the eutectic silicon phase and the aluminum matrix, which are prone to localized corrosion in chloride ions containing environments. Accordingly, values of the total impedance in EIS measurements significantly increased for the treated substrates. Morphologies of the conversion coatings and the oxidation state of cerium compounds were found to be dependent on the composition of the solution and the presence of chloride ions and/or hydrogen peroxide. Aluminum alloy with higher silicon content showed a more active surface during immersion in the conversion solution. This makes it more difficult to be treated using aggressive conversion solutions.

Formation of a cerium conversion coating on magnesium alloy using ascorbic acid as additive. Characterisation and anticorrosive properties of the formed films

Cerium-based conversion coatings were formed on AZ91D magnesium alloy by immersion of the substrate in solutions containing Ce(NO3)3, H2O2 and ascorbic acid (HAsc). The characterisation of the films was performed by electrochemical and surface analysis techniques such as SEM, EDS, X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The degree of corrosion protection achieved was evaluated in simulated physiological solution by the open circuit potential monitoring, polarisation techniques and electrochemical impedance spectroscopy (EIS). The presence of HAsc in the conversion solution causes changes in the morphology, adherence and anticorrosive performance of the films. The improvement in the corrosion resistance is closely associated with the corrosion inhibition properties of HAsc.

Cerium-based modification treatment of Mg-Al hydrotalcite film on AZ91D Mg alloy assisted with alternating electric field

An environment-friendly cerium-based conversion coating (CeCC) was prepared by alternating electric field assisted treatment for the modification of magnesium-aluminum (Mg-Al) hydrotalcite (HT) film on AZ91D Mg alloy. The influence of cerium nitrate (Ce(NO3)3) concentration was evaluated in terms of morphology, composition, and corrosion resistance. The results showed that deposition behavior (including deposition amount and sealing effect) of CeCC was controlled by Ce(NO3)3 concentration. The HT film treated with 10 g/L Ce(NO3)3 was better sealed; the HT film immersed in 30 g/L Ce(NO3)3 displayed larger deposition amount of CeCC. With the increase in concentration from 5 to 40 g/L, the corrosion resistance exhibited a waving trend. The treated HT film with better sealing effect or larger deposition amount revealed superior corrosion resistance.

Formation of Cerium Conversion Coatings on AZ31 Magnesium Alloy

This review deals with one of the surface modification techniques, chemical conversion coating and particularly cerium-based conversion coatings (CeCC) as a promising substitute for chromium and phosphate conversion coating on magnesium and its alloys. The CeCCs are commonly considered environmentally friendly. The effects of surface preparation, coating thickness, bath composition, and e-paint on the corrosion behavior of CeCCs have been studied on the AZ31 magnesium alloy. This review also correlates the coating microstructural, morphological, and chemical characteristics with the processing parameters and corrosion protection. Results showed that the as-deposited coating system consists of a three layer structure (1) a nanocrystalline MgO transition layer in contact with the Mg substrate, (2) a nanocrystalline CeCC layer, and (3) an outer amorphous CeCC layer. The nanocrystalline CeCC layer thickness is a function of immersion time and cerium salt used. The overall corrosion protection was crucially dependent on the presence of coating defects. The corrosion resistance of AZ31 magnesium alloy was better for thinner CeCCs, which can be explained by the presence of fewer and smaller cracks. On the other hand, maximum corrosion protection was achieved when AZ31 magnesium samples with thin CeCCs are e-painted. The e-paint layer further restricts and hinders the movement of chloride and other aggressive ions present in the environment from reaching the magnesium surface.

The corrosion performance and adhesion properties of the epoxy coating applied on the steel substrates treated by cerium-based conversion coatings

The surface morphology and composition of the cerium-based conversion coatings (Ce) were studied for the St-37 steel substrate which was post-treated in an ambient temperature zinc phosphate chemical treatment bath (Ce–Zn). Then, the epoxy/polyamide coating was applied on the substrates treated with Ce, Zn and Ce–Zn chemical treatments. The surface characterization and electrochemical investigations were made by scanning electron microscope (SEM), energy dispersive X-ray photoelectron spectroscopy (XPS), contact angle measuring device, pull-off test and electrochemical impedance spectroscopy (EIS). Results revealed that the epoxy coating applied on the Ce–Zn treated sample showed the highest adhesion and corrosion protection properties.

Influence of buffering on the spontaneous deposition of cerium conversion coatings for corrosion protection of AA2024-T3 aluminum alloy

Cerium-based conversion coatings were spontaneously deposited on AA2024-T3 alloy at 60 °C using buffered and non-buffered CeCl3 solutions in the presence of H2O2. Malonic acid or amino-acetic acid (glycine) was used as buffering additives. The deposition process and the properties of the coatings obtained were followed by linear voltammetry and electrochemical impedance spectroscopy. The surface morphology was studied by scanning electron microscopy. It was found that buffering complicates the conversion process and hampers the deposition rate. The coatings deposited using buffered baths had lower barrier ability and corrosion durability in 3.5 % NaCl corrosive medium compared to those deposited in the absence of buffers.

Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6

Cerium-based conversion coatings were deposited on aluminium alloy 6061-T6 by immersion in two cerium salt sources (chloride- and nitrate-based) assisted with hydrogen peroxide (H2O2). The morphology and composition of the coatings were analysed using scanning electron microscopy and energy dispersive X-ray spectroscopy. Electrochemical measurements to assess corrosion behaviour were performed using free corrosion potential, polarisation and electrochemical impedance spectroscopy with a 3% NaCl solution. The influence of H2O2 on the generation of the coating was studied by cyclic voltammetry tests. The protective properties of the coating generated are heavily dependent upon the chelating effect, chaotropic anion, the pH and H2O2 content.

Preparation and corrosion resistance of cerium conversion coatings on AZ91D magnesium alloy by a cathodic electrochemical treatment

Cerium conversion coatings were prepared on AZ91D magnesium alloy by a cathodic electrochemical treatment. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), electrochemistry workstation and X-ray photoelectron spectroscopy (XPS) were used to characterize the compositions, micro-morphology, corrosion resistance and the oxidation state of cerium and oxygen elements of the coatings. The results showed that the conversion coatings presented an intact and uniform morphology when the cathodic current density was 0.20Adm−2, the temperature was 35°C and the treatment time was 2min. When the applied current densities were higher than 0.25Adm−2, the coatings showed a micro-cracked morphology. The better corrosion resistance of the cerium-based conversion coatings could be obtained when the current density was 0.20Adm−2 when tested in 3.5% NaCl (wt.) solution. There are Ce(IV) and Ce(III) oxidation states in the coating.

Microstructural evolution of cerium-based coatings on AZ31 magnesium alloys

The evolution of microstructure and chemistry was studied for AZ31 Mg alloy substrates after grinding, acid cleaning, alkaline cleaning, cerium-based conversion coating (CeCC) deposition, and phosphate post-treatment. Grinding provided a homogeneous surface with comparable amounts of oxides and hydroxides species. After acid treatment, this layer was ~90nm thick, predominantly composed of oxide species (~85at.%), and Mg deficient compared to the alloy chemistry. Treatment in an alkaline solution selectively removed Al species and produced a porous hydroxide layer. Immersion of the substrate in a cerium solution resulted in spontaneous deposition of a CeCC. Analysis revealed that the as-deposited CeCC contained more than 60at.% Ce(IV) species with nodular CeO2 nanocrystals embedded in an amorphous Ce(III)-rich matrix. After post-treatment in a phosphate solution, the coating was transformed into a dense, homogenous layer with fewer cracks than the as-deposited CeCC and the content of Ce(IV) species decreased to ~50at.%. The post-treated CeCC had a nodular morphology and contained a mixture of CeO2/CePO4•H2O nanocrystal species embedded in an amorphous matrix. Electrochemical results of as-deposited and post-treated CeCCs indicated an increase of ~4× in the corrosion resistance compared to ground uncoated AZ31 Mg alloys in a 0.05M NaCl electrolyte. However, the impedance spectra of the CeCCs at low frequencies showed that post-treated coatings not only have higher impedance but may also act as a barrier for active corrosion species.In general, each of the five processing steps had functionalized the surface of the AZ31 Mg alloy by reducing active cathodic sites, modifying the chemistry, changing the structure or forming protective layers. Understanding the coating evolution has provided insights on the surface preparation of Mg alloys and a basis for studying the response and evolution of these coatings after exposure to corrosive and ambient environments.

Corrosion study of ceria coatings on AA6060 aluminum alloy obtained by cathodic electrodeposition: Effect of deposition potential

Among potential replacements for chromates, cerium-based conversion coatings (CeCCs) have been identified as leading candidates, showing a good promise as environmentally compliant corrosion inhibitors on Al alloys. In this work, CeCCs were synthesized on AA6060 by electrolytic deposition (ELD), a well-suited method for the fabrication of films with controllable properties, at low processing temperature and cost. The CeCCs were deposited at room temperature from Ce(NO3)3 solution, applying different cathodic potentials (−1.1, −1.4 and −1.6V vs. SCE) for different deposition times between 5 and 20min. Structural, morphological and compositional properties of the films were characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy coupled with energy dispersive spectroscopy. Ceria films were nanostructured, exhibiting a cubic fluorite structure. From corrosion perspective, the quality of CeCCs was evaluated in 0.5M NaCl solution via electrochemical impedance spectroscopy and linear sweep voltammetry. As shown, the coating corrosion quality is the interplay between deposition potential and deposition time. The highest potential led to highly cracked films of poor adherence, even at the shortest deposition time, which disqualified them from any corrosion protection application. The lowest potential (−1.1V) produced crack-free coatings, but longer times (≥10min) were needed to provide the protective layer formation. The best corrosion protection offered CeCCs prepared at −1.4V. The highest Rp (58.2kΩcm2) and the smallest CPE (12.6·10−6snΩ−1cm) were obtained at 20min deposition time. However, from economical point of view, 10min deposition is a promising choice. These CeCCs also improved pitting corrosion resistance owing to homogeneous microstructure and sufficient thickness.