Zinc Phosphate Conversion Coatings Research Articles

Dry sliding wear behavior of hard-chrome and zinc-phosphate conversion coatings on steel substrates

We have studied tribological properties of conversion coatings, hard chrome on AISI 430 stainless steel, and zinc phosphate on AISI 1044 carbon steel. These substrates were chosen due to their extensive industrial applications. The goal of the present study is to improve the wear resistance of these substrates through the coatings. We performed nano-indentation and ball-on-disc tribometer experiments to study mechanical and tribological behavior of substrates and coatings. We find that when zinc-phosphate coating is applied to AISI 1044 steel, a new phase Zn2Fe(PO4)2·4H2O (phosphophyllite) is formed on the surface that increases the surface hardness. We further find that coefficient of friction decreases approximately by 8% due to hard-chrome coating on AISI 430 steel and by 48% due to zinc-phosphate coating on AISI 1044 steel under load and speed conditions applied in the present study. We attribute our findings to formation of: (1) an adhering layer of chromium in the case of chrome–steel contact, leading to chrome–chrome contact at the interface of test-specimen and counter-surface, and (2) a glossy layer at interface in the case of phosphate-steel contact, owing to the compaction and crushing of phosphate wear debris. Our wear studies show maximum reductions in specific wear rate to be around 80% and 89% for chrome and zinc-phosphate coatings, respectively. We note that adhesion and abrasion are the primary modes of wear for chrome and zinc-phosphate conversion coatings, respectively at the initial stage. However, adhesive wear switches to delamination wear at a later stage for hard-chrome coating.

A comparative study of zinc phosphate and zirconium-based conversion coatings on galvannealed steel: effect on corrosion performance of electrocoated systems

ABSTRACT The efficacy of an electrocoated system that employs a Zr-based conversion coating was assessed for the protection of 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 of the latter. 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 in combination with surface analysis investigations of pristine specimens where scanning electron microscopy, x-ray photoelectron spectroscopy and x-ray diffraction have been employed to determine the mechanism through which protection, or coating failure, occurs. Further investigations study the 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·cm−2, 0.30 μA·cm−2, and <0.01 μA·cm−2, 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·cm−2, 0.53 μA·cm−2, and 0.06 μA·cm−2, 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 2.94·10−2mol·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, which interferes 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 surfce coverage beneficial for a uniform electrocoat film-build.

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.

Corrosion Resistance of Zinc Phosphate Conversion Coatings on AZ91D Surface

Magnesium alloys have been widely applied in many fields, because of their high strength-to-weight ratio. However, magnesium alloys have high chemical activity and are easily corroded. The poor corrosion resistance of magnesium alloys greatly limits its further application. In this paper, the zinc phosphate conversion coatings were prepared on the surface of AZ91D magnesium alloys. Nano-zinc oxide was the source of zinc and the zinc phosphate conversion coatings were prepared by the given process: 1.25 g/L NaNO3, 3 g/L C6H8O7 H2O, 2.5 g/L NaF, 5.5 g/L ZnO, 12.5 mL/L H3PO4, reaction temperature 50°C, reaction for 30 minutes. The full immersion uniform corrosion test was conducted for the fabricated coatings. The morphology and composition of corrosion in different corrosion stages were characterized by XRD, SEM and other microscopies. The results showed that: (1) the corrosion process of the conversion coatings could be divided into three stages: the dissolution of the conversion coatings, the corrosion of the matrix and the deposition of insoluble matter; (2) XRD analysis and other methods found that the pine-needle magnesium oxychloride compounds were formed in the process of immersion firstly, and it was dissolved into Mg(OH)2 over time; (3) With the extension of immersion time, Mg(OH)2 increased continuously and played a major role in corrosion prevention. The deposited Mg(OH)2 was divided into two layers. In the initial deposition stage, it was mainly evenly dispersed on the surface of the alloy to form a tightly arranged inner layer. Afterwards, the crystals of Mg(OH)2 agglomerated and formed a sphere, becoming the outer layers.

Nano ZnO-assisted formation of zinc phosphate conversion coating for improving corrosion protection of AZ91D magnesium alloy

In this work, micron and nano ZnO powders were used to assist in the formation of zinc phosphate coatings to improve the corrosion resistance of AZ91D magnesium alloy, respectively. The results showed that the ZnO particles acted as the nucleation sites to promote the formation of flower-like Zn3(PO4)2 · 4H2O phase, which was the main component of the zinc phosphate conversion coatings. In addition, the micron ZnO-assisted zinc phosphate conversion coatings (MACC) reduced the corrosion rate of AZ91D alloy from 3.36 to 3.20 mm · a−1. Besides, the corrosion resistance nano ZnO-assisted zinc phosphate conversion coating (NACC) was four times as high as that of MACC, due to that the NACC had the finer grains and more compact structure. The more compact structure resulted from that the nano ZnO particles not only increased the number of nucleation sites but also decreased the concentration of Zn2+ in the phosphatization solution to inhibit the growth of Zn3(PO4)2 · 4H2O phase.

EFFECT OF ZNCONCENTRATION IONS ON CORROSION BEHAVIOR OF ZINC PHOSPHATE CONVERSION COATING ON AL-MG-ZN ALLOY

The effect of Zn2+ concentrations on the formation of zinc phosphate conversion coatings(ZPCCs) on Al-Mg-Zn alloy was investigated. The morphology of the coatings was studied byscanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion resistances ofthe coated and uncoated samples were evaluated by open circuit potential and potentiodynamicpolarization techniques. The results revealed that the conversion coating consists of hopeite as asingle phase. Furthermore, the results indicated that the corrosion resistance of the coated Al-Mg-Zn samples was found to increase with increasing Zn2+ concentration.

Influences of Surface Finishes on Properties of Biological Zinc Phosphate Conversion Coating on Titanium

Chemically formed phosphate conversion coatings (PCC) usually have good adhesion strength due to in situ growth and have great potential for biomedical applications. However, the fabrication of PCC on titanium (Ti) metals has seldom been studied because inherent chemical inertia and oxide layer block the dissolving process. Previously, the authors successfully fabricate intact PCC on pure Ti by a novel hydrothermal phosphorization method, and the coating shows good adhesion. However, metallographically polished specimens are used only in order to eliminate the influences of surface topography. In this study, the feasibility of such method to be performed on sandpaper polished and acid‐etched pure Ti are studied. Results show that rough surface benefit the nucleation of PCC and refine the crystal size, whereas, a smooth surface allow sufficient growth of crystal grain resulting in a thicker coating with stronger adhesive strength. In spite of rough surface, PCC formation on acid‐etched Ti is depressed due to the existence of titanium hydride. All the coatings obviously improve corrosion resistance of Ti substrate. The cytotoxicity test suggests that the PCCs present no adverse effects on L‐929 cells and has a cytotoxicity ranking of 0–I grade. The study shows that the hydrothermal zinc phosphorization has a potential to be extensively used for Ti‐based implants with different surface finishes.

Corrosion Protection of Steel with Zinc Phosphate Conversion Coating and Post-Treatment by Hybrid Organic-Inorganic Sol-Gel Based Silane Film

This paper aims to enhance the corrosion protection performance of steel pre-treated by zinc phosphate conversion coating (ZPCC) through post-treatment by a hybrid organic-inorganic sol-gel based silane coating (SC) based on the mixture of trimethoxymethyl-silane and tetraethyl orthosilicate. The microstructure, surface morphology and chemistry of the ZPCC and ZPCC-SC samples were characterized by X-ray diffraction analysis, atomic force microscopy, scanning electron microscopy, energy dispersive spectroscopy and contact angle measuring device. The corrosion protection performance of the bare steel and those treated by ZPCC and ZPCC-SC was evaluated by electrochemical impedance spectroscopy, polarization and salt spray tests. Results showed that deposition of SC on the ZPCC provided a smooth, crack free and dense film with higher water repellency. Results obtained confirmed significant improvement of the ZPCC corrosion resistance after modification by SC. The SC could noticeably enhance the barrier characteristic of ZPCC through filling the porosity of ZPCC and increasing the electrolyte diffusion length.

Effect of pH on the Phase Composition and Corrosion Characteristics of Calcium Zinc Phosphate Conversion Coatings on Titanium

Calcium and/or zinc phosphates (Ca-Zn-P) coatings are adopted to improve the bio-performance of titanium (Ti) using the phosphate chemical conversion (PCC) method. Although solution pH is well known in affecting PCC coating formation rate and properties, little is known about its effect on the phase composition of Ca-Zn-P coatings. In this study, we have prepared different types of Ca-Zn-P PCC coatings on Ti from solutions at various pH levels. The chemical composition, microstructure and elemental distribution of the coatings were examined via X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy equipped with an energy dispersive spectrometer. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy in the simulated body fluid. It is shown that pH can significantly affect the phase composition of the coatings. At pH 2.50–3.25, plate-like hopeite (Zn3(PO4)2·4H2O) coatings were formed on Ti, and at pH 3.50–4.25, flower-like scholzite (CaZn2(PO4)2·2H2O) coatings were deposited. Electrochemical analyses revealed that hopeite coatings enable better corrosion resistance than scholzite coatings. The phase change of Ca-Zn-P coatings can be attributed to the different pH-dependent activities of Ca2+ and Zn2+ ions, and the effect of zinc on the formation of Ca-P phases depends on the solution pH.

Solvent Free Synthesis of Four Cationic Surfactants: Evaluation of Their Corrosion Inhibition Properties on Zinc Phosphate Conversion Coating on Mild Steel

AbstractFour cationic surfactants have been synthesized in the absence of solvent using triethylamine and four different long chain alkyl halides. Using FTIR, 1H NMR spectra and elementary analysis the chemical structure of the synthesized surfactants were confirmed. The physio-chemical properties have been evaluated using standard analytical procedures. The corrosion inhibition properties of these four surfactants as additives were evaluated on zinc phosphate conversion coating on mild steel using nano silica as accelerator. The efficacy of the surfactants as corrosion inhibitor increases with increase in the side chain length and surface activity. With the help of X-ray diffraction (XRD), Atomic force microscope (AFM) and Scanning electron microscope (SEM) the surface morphology have been studied. The protective performance of the coatings was evaluated by salt spray exposure studies.

The Effect of Immersion Time and Immersion Temperature on the Corrosion Behavior of Zinc Phosphate Conversion Coatings on Carbon Steel

The formation and corrosion performance of zinc phosphate coating on carbon steel at different temperatures and immersion times was studied. The corrosion performance of coatings was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. The characterization and composition of coatings were studied by SEM and EDS analysis. The corrosion resistance in 3.5%wt. NaCl solution was increased in the presence of zinc phospoate conversion coatings. The effect of immersion time and coating temperature were studied on anti-corrosion behavior of coatings. Coating temperature showed a significant effect in phosphate conversion coating and higher corrosion resistance was obtained with 45 oC operating temperature. Also the experimental results indicated that the corrosion resistance increased with increasing the immersion time. This behavior can be related to the increase of the phosphate coating continuity which formed on the surface. Surface analysis results indicated that the coating obtained from 20 min immersion time was more uniform and continuous.

Nano Silicon Dioxide Accelerated Zinc Phosphate Conversion Coating on Mild Steel Using Decyltriethylammonium Bromide as an Additive

In this work, the phosphating process was accelerated by ECO-friendly nano silicon dioxide along with decyltriethylammonium bromide as an additive. Accelerator and additive incorporation effectively reduced the extent of zinc dissolution during phosphating and activated the process to achieve the coating weight faster. Also it yielded coatings with greater thickness than the normal phosphate coating. The parameters such as coating weight, surface coverage, roughness and corrosion resistance due to the presence of nano silicon dioxide and decyltriethylammonium bromide were evaluated. X-ray diffraction (XRD),Atomic force microscope (AFM) and Scanning electron microscopy (SEM) were availed to study the chemical composition, modified surface and microstructure of the phosphate coatings. XRD studies revealed the presence of Zn 3(PO 4)2.4H 2O (hopeite) and Zn 2Fe(PO 4)2.4H 2O (phosphophyllite) in the composition of the phosphate coating. The corrosion behaviour of the coated steel was assessed by open circuit potential, potentiodynamic polarization curve and electrochemical impedance spectroscopy.

The Effects of Surface Pre-Treatment Methods on the Structure and Corrosion Resistance of Zinc Phosphate Conversion Coatings on AZ31 Mg Alloy

The Effects of Surface Pre-Treatment Methods on the Structure and Corrosion Resistance of Zinc Phosphate Conversion Coatings on AZ31 Mg Alloy

Corrosion resistance of calcium-modified zinc phosphate conversion coatings on magnesium–aluminium alloys

The influence of the microstructure of the calcium-modified zinc phosphate (Zn–Ca–P) conversion coatings of Mg–Al alloys on corrosion resistance was investigated using OM and SEM and hydrogen evolution tests. The results demonstrated that the Zn–Ca–P coatings markedly enhanced the corrosion resistance of the alloys. The microstructure and chemical compositions of the alloys exerted a significant influence on the corrosion resistance of their coatings. A model was proposed to elucidate the formation mechanism of the porous Zn–Ca–P coating on the AM30 alloy.

Comparative corrosion study of zinc phosphate and magnesium phosphate conversion coatings on AZ31 Mg alloy

In this study, zinc phosphate and magnesium phosphate conversion coatings were formed on AZ31 Mg alloy (AZ31) by immersion treatment in zinc phosphate and magnesium phosphate conversion coating (ZPCC and MPCC) solutions. The coatings were characterized by a scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The corrosion behavior and corrosion resistance of coatings were studied by immersion test, potentiodynamic polarization curves and salt-spray tests (SST) with and without paints coated by electrophoretic deposition. During the immersion test, corrosion initiated much slowly on the MPCC than ZPCC. The ZPCC-coated samples showed pitting corrosion while filiform corrosion was observed on the MPCC-coated samples. Although ZPCC showed thicker coating, more positive corrosion potential and lower corrosion current density than MPCC, but it was very susceptible to corrosion under salt-spray conditions comparing with MPCC. The electropainted MPCC also showed much longer stability under salt-spray conditions than electropainted ZPCC.

Studying corrosion performance, microstructure and adhesion properties of a room temperature zinc phosphate conversion coating containing Mn2 + on mild steel

In this work, a series of low temperature zinc phosphate conversion coatings formed on mild steel. The surface treatment was carried out at different pHs in the phosphate solutions containing Mn2+. An epoxy/polyamide coating was then applied on the phosphated substrates. Potentiostatic polarization test was carried out in order to evaluate the corrosion protection properties of the phosphated samples in 3.5wt.% NaCl solution. The microstructure and morphology of the coatings were studied by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and atomic force microscopic (AFM). The adhesion strengths of the epoxy coatings applied on the surface treated samples were measured before and after 30days of immersion in 3.5wt.% NaCl solution by a pull-off test. Results showed that the corrosion resistance of the phosphate coating was significantly enhanced using Mn2+ (especially at 7g/L). A less porous and more compact phosphate coating formed on the metal surface in the presence of Mn2+ compared with the one without additive. Adhesion properties were enhanced by addition of Mn2+ to the phosphate coating composition.

Effects of Zn2+ concentration and pH on the zinc phosphate conversion coatings on AZ31 magnesium alloy

The effects of various Zn2+ concentrations and pH levels (nine conditions in total) on the formation of zinc phosphate conversion coatings (ZPCCs) on AZ31 magnesium alloy were investigated, and corrosion resistances of the coated samples were evaluated by immersion test and potentiodynamic polarization experiment. The corrosion resistance of the coated AZ31 samples was found to increase with increasing Zn2+ concentration and the lowest corrosion rate was obtained for the samples coated at pH of 3.07, independent of Zn2+ concentration. The best coatings on AZ31 were obtained at [Zn2+]=0.068M and pH 3.07.

Zinc phosphate conversion coatings on magnesium alloys: A review

Phosphating is one of the most widely used surface treatments of steels and aluminum due to its low-cost, easy mass production, good corrosion resistance and good adhesion with paint. Many researchers have tried to expand applications of the phosphating process, especially to magnesium alloys for automobiles and aerospace applications. Recently, the coatings on magnesium alloys by zinc phosphate conversion coatings (Zn3(PO4)2·4H2O) have been intensively studied. This paper reviews the state-of-the-art of phosphate conversion coatings developed for magnesium alloys, in terms of coating properties, phosphate conversion coatings processes and compositions of phosphating bath.

Wastewater treatment for electrocoating

Because it is an immersion process, electrocoating employs the use of a large amount of water. Typically, pigment, resin, and additives make up only 10 to 15% of the contents of an electrocoating paint tank. Electrocoating is also an extremely efficient coating process due to recycling of paint through ultrafiltration, with usage typically ranging from 95 to 99%; however, the small amount of paint, which may at some point elude deposition, must be extracted from the waste stream prior to discharge. Wastewater treatment strategies for electrocoating-like those for pretreatment stages-are based on three main considerations: (1) removing impurities from the process tanks; (2) retaining useful materials in the process tanks; (3) minimizing the impurities for disposal. Waste streams from other sources, i.e., pretreatment stages, may not be initially compatible for treatment with electrocoating paint wastes. Problems arise often enough to warrant conservative strategies-the most basic of which is to separate the different wastes for differing methods of treatment before combining the resultant waste streams for common disposal (Fig. 1). Cleaning wastes are one category. Zinc and iron phosphate conversion coatings and their rinses are another. The chrome seal and the rinse(s) for that stage are a third category. Finally, there are the E-coat paint wastes themselves. Treatment for pretreatment wastes has been examined elsewhere in this publication. Here, we will consider treatment strategies specific to the electrocoating process.

Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc–calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn–Ca phosphate conversion coatings are mainly composed of hopeite (Zn3(PO4)2·4H2O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF2, Zn or ZnO and hopeite. Both of the zinc and zinc–calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn–Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn–Ca coating.