Phosphate Conversion Coatings Research Articles

PREPARATION AND MODIFICATION OF CALCIUM PHOSPHATE CONVERSION COATING

In order to enhance the corrosion resistance of calcium-based phosphate coating, the molybdenum-modified coating was successfully prepared by adding Na2MoO4 in a phosphate bath. The prepared composite film layers were studied by EDS, XRD and SEM. The corrosion resistance performances of the films were investigated by electrochemical measurements. The results showed the main composition of molybdenum-modified film was MoO2, MoP2, and MoP4, the diameter of cracks on the surface of the coating was significantly reduced, which provided better protection to the substrate. In addition, the potentiodynamic polarization curves of the molybdenum-modified film show that the corrosion potential was positively shifted by 0.068 V compared to the calcium-based phosphate coating and the corrosion current was reduced to 2.556 × 10[Formula: see text]A ⋅ cm[Formula: see text], indicating improved corrosion resistance.

Preparation of Conductive and Corrosion Resistant Phosphate Conversion Coating on AZ91D Magnesium Alloy

The application of magnesium alloys in the 3C industry requires the coexistence of excellent corrosion resistance and good electrical conductivity. In this work, a conductive and corrosion-resistant phosphate conversion coating (PCC) on AZ91D magnesium alloy was investigated. The effects of strong oxidant (KMnO4), additive (Na2MoO4), surface-active agent (OP-10) and their content in phosphating bath on PCCs were studied, and the mechanism of action of strong oxidant was analyzed. The results showed that the optimum content for KmnO4, Na2MoO4 and OP-10 in phosphating bath was 3.0 g/L, 1.5 g/L and 1.0 g/L. The PCC formed at the phosphating bath at the optimum condition was completely covered, the coating on α phases had a bilayer structure and the β phases were protruded. The electrical contact resistance (ECR) of the PCC was as low as 4.91 Ω, the Ecorr positively shifted about 27 mV, and the icorr reduced significantly. The presence of KMnO4 inhibited the formation of phosphate crystals and made the β phases protrude from the surface to form conductive spots, which improved the conductivity of PCCs.

Analysis on the Influence Mechanism of Hot-dip Galvanized Steel uniaxial deformation on phosphate conversion coating formation

The mechanism of residual internal stress on the growth and corrosion resistance of phosphate conversion coating (PCC) on hot-dip galvanized (HDG) steel sheets was investigated by finite element analysis, electrochemical tests, scanning electron microscopy (SEM), and coating weight experiments. The results indicate that residual compressive stress inhibits the electrochemical activity of HDG steel sheets. The formation of PCC is also restrained. Residual tensile stress can effectively enhance the electrochemical activity of the substrate, and as the residual tensile stress increased, the cathodic reaction rate of the substrate was increased. In particular, when the residual tensile stress value was 221.0 MPa, PCC grew completely on the surface of the substrate and showed excellent corrosion resistance. When the residual tensile stress reached 261.8 MPa, the fast micro-cathode reaction rate led to the formation of a large number of PCC crystal nuclei, which covered the active site of the substrate and inhibited the further occurrence of the coating formation reaction. The corrosion resistance of the PCC also decreased at this time.

Novel Phosphate Conversion Coating with Superior Corrosion Resistance on the Mg-Al-RE Alloy Based on the Introduction of Silane.

A novel chemical conversion method based on an aqueous electrolyte solution containing hydrolyzed silane as a crucial film-forming agent has been developed successfully. An organic/inorganic composite coating with superior corrosion protection was fabricated on the Mg-Al-RE alloy. The micro-morphology and chemical composition of the coating were examined using scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. The prepared conversion coating possesses a double-layer structure, with a dense bottom layer and a loose outer layer containing microcracks. In addition, the anti-corrosion capacity of various samples was evaluated using salt spray, hydrogen evolution, and electrochemical tests. The analysis revealed that the samples converted by 3-aminopropyl triethoxysilane (APTES)-phosphate solution possessed a higher corrosion resistance. Compared to the blank sample, a 2 orders of magnitude reduction in icorr (6.678 × 10-7 A/cm2) and a 1.360 V improvement in Ecorr were observed. The MPCC-Si sample exhibited the lowest hydrogen evolution rate, indicating the best corrosion protection. Also, the rust grade could still reach 7 after 72 h of the neutral salt spray test.

Microstructural Analysis of Iron Phosphate Conversion Coatings by FE-SEM and XPS

Microstructural Analysis of Iron Phosphate Conversion Coatings by FE-SEM and XPS

Scanning and transmission electron microscopy examinations of composite hybrid chromate and chromate phosphate conversion coatings exposed in hot 100% relative humidity environments

Conversion coatings are sine qua non in the finishing of aluminium alloys. They may be put into service without coating of paints. However, for aesthetics in architectural applications, paint overcoat is common. Chromate phosphate coatings are relatively less toxic than chromate which gained the pride of place until recently. Thus, aluminium specimens coated with hybrid chromate/phosphate conversion coatings have been subjected to 100% relative humidity for 480 h at 313K in comparison to chromate coatings and bare aluminium. Scanning electron microscopy examination revealed that initial dried river bed morphologies on coated specimens were obliterated. The friable outer coatings collapsed into the cracks/environment leaving a relatively smooth surface after exposure. Additionally, transmission electron microscopy of sections before and after exposure revealed similar texture and morphological striations nearly parallel to each other throughout the length of the coatings to the metal/coating interface. These were not disrupted as well as the metal/coating interface which remained relatively smooth with no visible corrosion products after exposure. Thus, the coatings formed effective barrier between the substrate and the relatively harsh environment. From EDS, elemental compositions were the same albeit, spectral intensities remained relatively constant after humidity exposure regimes. The coatings are made up of either Cr, O and Al for chromate and Cr, P, O and Al for chromate/phosphate. At deliberately breached regions, the coatings prevented spread of corrosion and paint delamination over the substrates. On the other hand, bare aluminium was decorated with mounds of hydrated aluminium oxide/hydroxide after exposure as revealed in the SEM/TEM.

Ca–Zn Phosphate Conversion Coatings Deposited on Ti6Al4V for Medical Applications

This paper aims to study the possibility of improving the chemical and surface characteristics of the Ti6Al4V alloy by depositing phosphate layers on its surface. Accordingly, an innovative phosphating solution was developed and used in a chemical conversion process to obtain Ca–Zn phosphate layers on the base material surface. Moreover, the chemical composition of the phosphate solution was chosen considering the biocompatibility of the chemical elements and their possibility of contributing to the formation of phosphate compounds. The obtained layer was characterized by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and potentiodynamic polarization tests. The wetting of the Ca–Zn sample surface was also investigated using water and two liquids similar to body fluids, namely, Ringer and Dulbecco solutions. According to the surface energy study, the polar component is almost two times larger compared with the dispersive one. The SEM and EDS tests revealed a uniformly coated surface with intercalated crystals leading to a rough surface. Furthermore, the XRD results showed not only the presence of hopeite and scholzite but also of phosphophyllite. By the vibrations of the PO4−3 groups, the FTIR test confirmed the presence of these phases. The potentiodynamic tests revealed that the samples coated with the Ca–Zn phosphate layer present better corrosion resistance and a lower corrosion rate compared with the uncoated ones.

Influence of pH and temperature in the performance of Zn phosphate conversion coatings

The influence of variations in temperature (from 50 °C to 70 °C) and pH (2.4 and 2.9) in the Zn phosphate conversion treatment of a high-strength steel was assessed in this study. The effect of the temperature changes depends on the pH of the bath. The results evidenced an enhanced corrosion resistance with the increased temperature for baths at pH 2.4, whereas no relevant differences were concluded for baths at pH 2.9. This latter pH condition provided a better surface coverage, even with lower coating masses, than the former value. The microstructural analysis showed the development of films composed of smaller crystals for the treatments at pH 2.9. Impedance spectroscopy was established as a powerful tool with great potential for characterising phosphate layers. The accuracy of this methodology greatly depends on the equivalent circuit used for the simulation. A model based on the transmission line allows the reliable estimation of the percentage of exposed area (coating efficiency), the identification of changes in the film conductivity, and the qualitative analysis of its structure.

Phosphate Conversion Coating – A Short Review

Phosphate Conversion Coating – A Short Review

Effects of laser shock peening post-treatment on microstructure and corrosion resistance of phosphate conversion coatings

In this work, to enhance the corrosion resistance of Mg alloys and extend their application fields, phosphate conversion (PC) coatings were deposited by chemical conversion followed by laser shock peening (LSP) post-treatment. The microstructure, phase structure, surface residual stress, and corrosion performance of the PC coatings before and after LSP treatment were studied. The PC coating showed a porous and rough structure. By contrast, after LSP treatment, the phosphate crystals on the coating surface were refined, flattened, and densified under the action of the shock wave, while generating residual compressive stress of −30 MPa. The thickness loss of PC coatings after LSP increases with the laser energy due to the competition between densification and erosion of the PC coating. Electrochemical analyses showed the corrosion current of PC coating after LSP decreased from 1.48 μA/cm2 to 0.25 μA/cm2, which was attributed to the dense and flat coating. Meanwhile, the impedance of the PC coating after LSP was increased by 2–5 times compared with the untreated PC coating. Especially when the laser energy was 160 MJ, the internal layer resistance of the coating reached 30090 Ω cm2, and the area damaged by the corrosive medium was smaller. Overall, the LSP post-treatment process is expected to improve the corrosion resistance of PC coatings deposited on Mg alloys, further broadening the application of magnesium alloys in medicine.

Effect of coating thickness on tribological properties of manganese phosphate conversion coating in different motion conditions

Three different thickness of manganese phosphate conversion coatings are obtained on the surface of low-carbon alloy steel by chemical conversion plating. The mechanical properties of the coatings are characterized by a nanoindentation test to obtain the elastic modulus and nano-hardness. And the tribological properties under different conditions are investigated by MTM and SRV testers. The lubrication state of coatings is discussed by establishing a coating-substrate contact model. The influence of particle size and mechanical properties on tribological properties is also analyzed. The results show that the mechanical properties and friction properties of coatings are affected by the thickness of the coating, which can be changed to obtain an antifriction coating suitable for friction pairs.

Corrosion Protection of Phosphate Conversion Coatings to Carbon Steel in the Presence of Bacterial Biofilms

Abstract Despite its relatively limited corrosion resistance, carbon steel is one of the most widely used engineering materials. An effective technique to improve its corrosion resistance, particularly during transport and storage, is phosphating. On the other hand, it is well known that different environmental bacteria can grow and develop on metal surfaces, forming biofilms. Some of those bacteria are capable of solubilizing phosphate. The objective of this work was to evaluate the stability of iron and manganese phosphate conversion coatings applied on SAE 1010 carbon steel in the presence of biofilms for two environmental bacterial strains: Pseudomonas aeruginosa and Paenebacillus sp. Both strains are biofilm-forming and phosphate solubilizing bacteria. Electrochemical techniques complemented with surface observation by scanning electron microscopy were applied. The electrochemical results showed that, although the presence of a bacterial biofilm in most cases produced a decrease in the corrosion rate, they provoked a shift of the pitting potential to more active values. This shift led to narrower passive zones, compromising the protection provided by the phosphate conversion coating to the 1010 SAE carbon steel by facilitating the onset of localized corrosion. The degree of the compromise of passivity was dependent on the coating and the bacterial strain. P. aeruginosa biofilms were more detrimental for both coatings than the Paenabacillus sp. biofilms. Thus, it is important to point out that to maintain the corrosion protection imparted by phosphate conversion coating to carbon steel, it is necessary to avoid the development of biofilms on the coated surface.

Gallium-Strontium Phosphate Conversion Coatings for Promoting Infection Prevention and Biocompatibility of Magnesium for Orthopedic Applications.

Device-associated infections remain a clinical challenge. The common strategies to prevent bacterial infection are either toxic to healthy mammalian cells and tissue or involve high doses of antibiotics that can prompt long-term negative consequences. An antibiotic-free coating strategy to suppress bacterial growth is presented herein, which concurrently promotes bone cell growth and moderates the dissolution kinetics of resorbable magnesium (Mg) biomaterials. Pure Mg as a model biodegradable material was coated with gallium-doped strontium-phosphate through a chemical conversion process. Gallium was distributed in a gradual manner throughout the strontium-phosphate coating, with a compact structure and a gallium-rich surface. It was demonstrated that the coating protected the underlying Mg parts from significant degradation in minimal essential media at physiological conditions over 9 days. In terms of bacteria culture, the liberated gallium ions from the coatings upon Mg specimens, even though in minute quantities, inhibited the growth of Gram-positiveStaphylococcus aureus, Gram-negative Escherichia coli, andPseudomonas aeruginosa ─ key pathogens causing infection and early failure of the surgical implantations in orthopedics and trauma. More importantly, the gallium dopants displayed minimal interferences with the strontium-phosphate-based coating which boosted osteoblasts and undermined osteoclasts in in vitro co-cultures. This work provides a new strategy to prevent bacterial infection and control the degradation behavior of Mg-based orthopedic implants, while preserving osteogenic features of the devices.

The one-step preparation and corrosion resistance of a Ca–Mn–Zn ternary phosphate conversion coating on magnesium alloy AZ91D

PurposeThe aim of this work was to propose a method to prepare composite phosphate conversion coating (CPCC), including ternary phosphate conversion coating (TPCC) and binary phosphate conversion coatings (BPCC), with one-step chemical conversion and to reveal and compare the corrosion resistance between TPCC and BPCC.Design/methodology/approachIn this work, a calcium–manganese–zinc (Ca–Mn–Zn) TPCC was prepared on the surface of magnesium alloy (MA) AZ91D with one-step chemical conversion method; for Ca-Mn-Zn@TPCC, its microstructure was characterized with scanning electron microscope observation and scanning tunneling microscope detection, and its composition was characterized with energy dispersion spectroscopy and X-ray photoelectron spectroscopy analyses. Particularly, the corrosion resistance of Ca-Mn-Zn@TPCC and its comparison with Ca–Mn, Ca–Zn and Mn–Zn BPCCs were clarified with electrochemical and immersion measurements.FindingsCa-Mn-Zn@TPCC, which was composed of Ca, Mn, Zn, P and O, exhibited a mud-shaped with cracks microstructure, and the average crack width, terrain fluctuation and coating thickness were 0.61 µm, 23.78 nm and 2.47 µm, respectively. Ca-Mn-Zn@TPCC provided good corrosion resistance to MA AZ91D; in NaCl solution, the total degradation of Ca-Mn-Zn@TPCC consumed eight days; corrosion products with poor adhesion peeled out from Ca-Mn-Zn@TPCC-coated MA AZ91D spontaneously. Besides, the corrosion resistance of Ca-Mn-Zn@TPCC was better than that of Ca-Mn@BPCC, Ca-Zn@BPCC or Mn-Zn@BPCC.Originality/valueThe successful preparation of Ca-Mn-Zn@TPCC on MA AZ91D surface confirmed the proposed method to prepare CPCC with one-step chemical conversion was feasible; at the same time, it was further confirmed that for phosphate conversion coating, ternary coating had better corrosion resistance than binary coating did.

Exploration and tuning of Al2O3/Mo composite for enhancement of anti-corrosion and tribological characteristics in zinc phosphate conversion coatings

Recently, phosphate conversion coatings have been modified by transition/non-transition metal oxide with distinguished characteristic features such as corrosion resistance, wear resistance, lubrication and adhesion. The present paper explores the development of Al2O3/Mo composite incorporated zinc phosphate coatings on galvanized steel for achieving higher corrosion-resistant characteristics with an added advantage of tribological performance. Incorporation of Mo leads to the phase transformation from γ to α-Al2O3, which ensures the subsequent crystallization of alumina particles by the anisotropic grain growth of Mo content. Particle size, elemental composition and morphology of the Al2O3/Mo composite are tuned for retaining the stable crystalline alpha phase of the composite. The optimum concentration of the tuned content of Al2O3/Mo composite in the phosphating solution can effectively perform as an active nucleating source for the establishment of more compact, homogeneous and dense zinc phosphate crystal growth and coverage in the galvanized steel surface by significantly reducing the activation energy during the phosphating process. The resultant composite phosphate coating ensures the formation of a large quantity of crystalline hopeite rich phase compared to other coatings, which effectively acts as an insulator during the electrochemical reaction by imparting a corrosion-resistant barrier between the substrate and corrosive media. Uniform distribution of composite particles at the grain boundaries and at the bulk grain composition is also achieved. Lower average surface roughness (Sa), root mean square surface roughness (Sq), maximum height (St), surface skewness (Ssk) and higher surface kurtosis (Sku) values are acquired for better anti-corrosion properties with excellent tribological performance. The higher Rp and Rct values with lower Cdl and corrosion rate values of the tuned composition of Al2O3/Mo composite based zinc phosphate coating confirms the better porosity and thereby enhances the corrosion-resistant performance due to the sealing action of the composite.

Chemically depleting the noble impurities from AZ91-T4 magnesium alloy: A new and efficient pretreatment method to improve the corrosion resistance of phosphate conversion coatings

Alx(Mn,Fe)y phase, as a common derivative of Fe-containing impurities, significantly deteriorates the protectiveness of phosphate conversion coatings (PCC) on magnesium alloys due to the micro-galvanic effect. This work presents a novel pretreatment method, aiming at chemically depleting the noble Alx(Mn,Fe)y phase from Mg alloy surface before PCC treatment. The pretreatment bath contains both oxidant and complexing agent, with the pH value and concentration of different constituents being determined by thermodynamic calculation. The corrosion resistance of PCC being preceded by exploiting the novel pretreatment method was improved, which was attributed to the mitigation of micro-galvanic corrosion.

Comparison of the effects of pre-activators on morphology and corrosion resistance of phosphate conversion coating on magnesium alloy

• The effects of three pre-activators (colloidal Ti, oxalic acid, phosphoric acid) on the performance of a PCC was compared. • The pre-activators promote nucleation by peeling off the original oxide film and increasing the specific surface area. • Different pre-activators did not alter the formation mechanism, but significantly changed the growth rate and morphology. • Phosphoric acid was the overall best choice for pre-activating the Mg alloy considering on the PCC performance and cost. In this study, Mg–6.0Zn–3.0Sn–0.5Mn (ZTM630) magnesium alloy was pre-activated by colloidal Ti, oxalic acid, and phosphoric acid, and a phosphate conversion coating (PCC) was prepared on the alloy surface. The morphology and corrosion resistance of the prepared PCCs were investigated. Surface morphology studies showed that the phosphate crystals that formed the coating were the smallest for the sample pre-activated by phosphoric acid. The coating on the colloidal Ti and the phosphoric acid samples had the largest and the smallest thickness and surface roughness, respectively. The reason for the discrepancy was analyzed by comparing the surface morphologies of alloy samples immediately after the pre-activation treatment and various phosphating treatments. X-ray diffraction analysis revealed that all three PCCs contained the same compounds. The corrosion resistance time from the copper sulfate drop test and the electrochemical data from the potentiodynamic polarization curves showed that the coating pre-activated by phosphoric acid had the best corrosion resistance. Finally, the 1500 h neutral salt spray corrosion test confirmed that the phosphating treated magnesium alloy, which was pre-activated by phosphoric acid, exhibited excellent corrosion resistance and behavior.

Effect of phosphating solution pH value on the formation of phosphate conversion coatings for corrosion behaviors on AZ91D

Effect of phosphating solution pH value on the formation of phosphate conversion coatings for corrosion behaviors on AZ91D

Grain refinement promotes the formation of phosphate conversion coating on Mg alloy AZ91D with high corrosion resistance and low electrical contact resistance

The application of Mg and its alloys in 3C (i.e. Computer, Communication and Consumer Electronic) industries requires a strict combination of corrosion resistance and low electrical contact resistance (ECR). The conventional design of conversion coating relies on a thick and compact layer to offer active protectiveness but compromises the electric conductivity. In this work, phosphate conversion coatings (PCC) with low electrical contact resistance were prepared on the sand-cast and die-cast Mg alloy AZ91D, respectively. The microstructural differences, ECR and corrosion resistance were observed and compared. Results show that the PCCs on die-cast alloy have a thickness of 300-400 nm, and are more compact than those on sand-cast counterparts. Interestingly, PCCs of the die-cast exhibit lower ECR, which indicates a mild impact of the improved compactness on ECR. Meanwhile, corrosion resistance of the PCCs on die-cast alloy is greatly improved, which is attributed to the intensified micro-galvanic effect through the grain refinement of the die-cast alloy. It evidently promotes the electrochemical reactions during the conversion treatment to yield high quality PCC.

Understanding the Stability and Recrystallization Behavior of Amorphous Zinc Phosphate

Zinc phosphate, an important pigment in phosphate conversion coatings, forms protective films on rubbing surfaces. We have simulated the underlying reactions under shear by ball-milling zinc phosphate and monitored the reaction of hopeite (Zn3(PO4)2·4H2O) and the retarded recrystallization of the amorphous reaction product by powder X-ray diffraction (PXRD) and quantitative infrared (IR) spectroscopy. Abrasion of stainless steel was simulated by addition of pure 57Fe. The results provide insight into the chemistry of phosphate conversion coatings or during battery cycling of metal phosphates and give theoretical guidance for the preparation of amorphous phosphates. Thermal analysis revealed that the release of structural water is a key step during the reaction of hopeite under shear to ball-milled amorphous zinc phosphate. The back-reaction and associated recrystallization kinetics of amorphous zinc phosphate show a classical Langmuir behavior. Fe impurities inhibit the recrystallization of ball-milled amorphous zinc phosphate strongly. 57Fe Mössbauer spectroscopy and PXRD revealed that Fe is oxidized to Fe2+ and Fe3+ during ball-milling and incorporated locally at the tetrahedral and octahedral sites of the structure. Ball-milled amorphous zinc phosphate is metastable as γ-Zn3–xFex(PO4)2. EPR studies showed the incorporation of Fe3+ to be coupled with the formation of Zn2+ vacancies. The Fe3+ defect sites bind water because of their higher Pearson hardness (compared to Fe2+ and Zn2+), thereby reducing water mobility and inhibiting further reactions like the recrystallization to hopeite. Our findings reveal the amorphization mechanism of Zn3(PO4)2·4H2O in stainless steel ball mills at the atomic scale and highlight how the reactivity of amorphous products is affected by impurities associated with the processing method.