Phosphating Bath Research Articles

Environmental phosphate coating for corrosion prevention in CO2 pipelines

The pipelines transporting carbon dioxide (CO2) rich fluids in the CO2 capture and storage (CCS) process require an appropriate internal corrosion protection in cases where the stream is acidic due to the formation of carbonic acid. In this study, vanadate-based MnMgZn phosphate coating is proposed as effective coating to reduce the corrosion in pipelines. The vanadate in the phosphate bath refines the microstructure of the phosphate coating and alters the corrosion behavior significantly. Electrochemical impedance spectroscopy (EIS) and polarization measurements were performed to investigate the corrosion behavior of phosphate coatings in diluted sulfuric acid at different pH.

The Effect of pH on Zinc Phosphate Coating Morphology and its Corrosion Resistance on Mild Steel

Zinc phosphate coating is commonly used for corrosion protection of metallic materials, mainly mild steel. In this study, influence of the pH of phosphating bath on the surface morphology and corrosion resistance of zinc phosphate coatings on mild steel was investigated. The phosphate layers were deposited on steel from phosphating bath at different pH values (1.75 ~ 2.75). The surface morphology and composition of phosphate coatings were investigated via scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The corrosion resistance of the coating was evaluated by polarization curves (anodic and cathodic) in an aerated 3.5% NaCl solution. The results showed that better surface coverage and corrosion resistance for the steel phosphated at pH 2.75.

The Influence of Deposition Temperature on the Morphology and Corrosion Resistance of Zinc Phosphate Coating on Mild Steel

The influence of phosphating temperature on the surface morphology and corrosion resistance of zinc phosphate coatings on mild steel was investigated. The phosphate layers were deposited on steel from phosphating bath at different temperatures (45 ~ 75 C). The surface morphology and composition of phosphate coatings were investigated via scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The corrosion resistance of the coatings was evaluated by polarization curves (anodic and cathodic) in an aerated 3.5% NaCl solution. The results showed that the increase in temperature of the phosphating bath up to 55 C caused an increase in surface coverage and in turn resulted in better corrosion resistance. At high temperature (65 °C and 75 °C) the deposition coverage decreased indicating that the best coverage for the phosphate layer on the metal surface was achieved at 55 °C

A green MnMgZn phosphate coating for steel pipelines transporting CO2 rich fluids

The transport of CO2 rich fluids via steel pipelines in the CO2 capture and storage (CCS) process requires a strategy for internal corrosion protection in cases where the stream is acidic and hence corrosive due to the formation of carbonic acid. Such protection strategies have been scarcely studied and will need significant research. In this study, one possibility to protect the steel pipelines from internal corrosion during CO2 transport is presented based on treatment of the internal surface with a vanadate-based MnMgZn phosphate coating. The addition of sodium orthovanadate (Na3VO4) in a MnMgZn phosphate bath leads to a significant change in the microstructure of the phosphate coating, which will alter corrosion behaviour significantly. Herein, the effect of vanadate concentration (0.0625, 0.125, 0.25mM) in the phosphate bath is studied with respect to the resultant microstructure and corrosion behaviour of the phosphate coatings. Electrochemical studies such as polarization curves and electrochemical impedance spectroscopy (EIS) were performed in diluted HCl solution at pHs2–4 to investigate the corrosion behaviour and distinguish the dielectric and electric properties of the phosphate coating.

Biodegradation behavior of micro-arc oxidized AZ31 magnesium alloys formed in two different electrolytes

Degradation behavior of coated magnesium alloys is among most prominent factors for their biomedical applications. In this study, bio-corrosion behavior of micro-arc oxidized magnesium AZ31 alloys formed in silicate and phosphate baths was investigated in r-SBF medium. For this purpose polarization behavior and open circuit profile of the coated samples were achieved by electrochemical and immersion tests, respectively. Moreover, the morphology and composition of the coatings were evaluated before and after immersion test using scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. The results showed that the phosphate film had better corrosion resistance and greater thickness than silicate film and, in turn, the lesser degradability in SBF solution, so that Ca2+ and PO43− containing compounds were more abundant on silicate film than phosphate film. Moreover phosphate film had greater surface roughness and lesser hydrophilic nature.

Microstructural characterization and surface properties of zinc phosphated medium carbon low alloy steel

The present study concerns an in-depth investigation of the influence of process parameters on zinc phosphating of medium carbon steel to have an understanding of nucleation and growth mechanism and the microstructures developed thereafter. Furthermore, the wear and corrosion behavior of phosphated steel were evaluated in details. Phosphating was carried out in zinc phosphate bath (6–8% ZnPO4) using 0.43wt.% Fe as catalyst at 90°C with variable time ranging from 60s to 1800s followed by water rinsing and oil sealing. The microstructural evolution with time was recorded to understand the nucleation and growth phenomena. The morphology of coating was both plate and needle shaped. The microhardness of the coated layer is significantly lower than that of as-received steel. Fretting wear behavior against hardened steel ball showed that phosphating reduced the wear resistance which was possibly attributed to the presence of phosphate needles causing an increased coefficient of friction and the reduced hardness of the phosphate layer. Corrosion potential of the phosphated steel shifted towards noble direction as compared to the as-received one. The study of the corrosion rate also shows that the rate is considerably reduced by phosphating. Pitting corrosion resistance was also found to be increased (in terms of increased critical potential for pit formation) due to phosphating as compared to as-received steel.

Aqueous deposition of calcium phosphates and silicate substituted calcium phosphates on magnesium alloys

Abstract Attempts were made to deposit homogeneous films of calcium phosphates (CaPs) on two magnesium alloy systems, AZ31 and Mg–4Y, through an aqueous phosphating bath method. The deposition of silicate substituted CaPs by this aqueous method was also explored as silicate substitution is believed to increase the bioactivity of CaPs. The effect of doped and undoped coatings on the in vitro degradation and bioactivity of both alloy systems was studied. FTIR and EDX confirmed the deposition of Ca, P, and Si on both alloys and the coatings appeared to consist primarily biphasic mixtures of hydroxyapatite and β-TCP. These largely inhomogeneous coatings, as observed by SEM, were not shown to have any significant effect on maintaining the physiological pH of the culture medium in comparison to the uncoated samples, as the pH remained approximately in the 8.4–8.7 range. Interestingly, despite similar pH profiles between the coated and uncoated samples, CaP coatings affected the degradation of both alloys. These doped and undoped calcium phosphate coatings were observed to decrease the degradation of AZ31 whereas they increased the degradation of Mg–4Y. In vitro studies on cell attachment using MC3T3-E1 mouse osteoblasts showed that between the uncoated alloys, Mg–4Y appeared to be the more biocompatible of the two. Silicate substituted CaP coatings were observed to increase the cell attachment on AZ31 compared to bare and undoped CaPs coated samples, but did not have as great of an effect on increasing cell attachment on Mg–4Y.

The effect of deposition temperature on the surface coverage and morphology of iron-phosphate coatings on low carbon steel

The influence of deposition temperature and concentration of NaNO 2 in the phosphating bath on the surface morphology and coverage of iron-phosphate coatings on low carbon steel was investigated. The phosphate coatings were chemically deposited on steel from phosphate bath at different temperatures (30–70 °C) and with the addition of different amounts of accelerator, NaNO 2 (0.1, 0.5 and 1.0 g dm −3). The morphology of phosphate coatings was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of iron-phosphate coatings was determined using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique. It was shown that the increase in temperature of the NaNO 2-free phosphating bath up to 70 °C caused an increase in surface coverage. The addition of NaNO 2 in the phosphating bath significantly increased the surface coverage of phosphate coatings deposited at temperatures lower than 50 °C. The phosphate crystals were of laminated and needle-like structures for deposits obtained at temperatures lower than 50 °C, while at higher temperatures needle-like structure was transformed to laminated structure. The increase in NaNO 2 concentration in the phosphating bath from 0.1 to 1.0 g dm −3 did not significantly increase the surface coverage, but decreased the crystals size, consequently favouring the phosphate nucleation and better packing of the crystals.

Study of Modified Properties on Electrolytic Phosphate Coatings with Nano-Al<sub>2</sub>O<sub>3</sub>

The properties of nano Al2O3modified electrolytic phosphating film have been studied through using the accelerated corrosion test, electrochemical testing techniques, scanning electron microscope (SEM) and X-ray diffraction (XRD) .The results show that, after adding nano- Al2O3 in electrolytic phosphating bath , morphology of phosphating film surface turned dense flake crystallization from needle crystal morphology of free-adding nano- Al2O3powder, and the value of impedance of the Al2O3incorporated phosphate coating was increased obviously.When the nano- Al2O3concentration was 15mg / L in electrolytic phosphating bath, the value of impedance of the coating is approximately two times larger than the phosphating coating directly formed on the low carbon steel.The corrosion resisting time of the coating was more than 420 s by CuSO4dripping test, and the corrosion current density of the phosphating sample was 1.164μA/cm2，in 3.5% NaCl solution, the corrosion inhibition efficiency reached 96.7%.

Influence of Additive on Structure and Corrosion Resistance of Manganese Phosphate Film on AZ91 Magnesium Alloy

Manganese phosphate conversion coating was prepared by adding addition agent towards phosphate bath. The structure was investigated using SEM/EDX, and the anti-corrosion performance was evaluated by electrochemical technique. It was found that the film presented in crystal structure, and the crystalloid grain transformed from large grain toward small particles upon the addition. The corrosion resistance was studied by polarization curves and EIS in 3.5%NaCl. Polarization curves showed that the corrosion potential of the coating shifted remarkably towards positive direction comparing with that of AZ91 substrate. EIS results indicated that the radius of the capacity impedance for phosphate film increased distinctly, which validated the excellent anti-corrosion performance of the film.

The EIS investigation of powder polyester coatings on phosphated low carbon steel: The effect of NaNO 2 in the phosphating bath

The effect of different type of iron-phosphate coatings on corrosion stability and adhesion characteristic of top powder polyester coating on steel was investigated. Iron-phosphate coatings were deposited on steel in the novel phosphating bath with or without NaNO 2 as an accelerator. The corrosion stability of the powder polyester coating was evaluated by electrochemical impedance spectroscopy (EIS), adhesion by pull-off and NMP test, while surface morphology of phosphate coatings were investigated by atomic force microscopy (AFM). The adhesion and corrosion stability of powder polyester coatings were improved with pretreatment based on iron-phosphate coating deposited from NaNO 2-free bath.

The corrosion properties of phosphate coating on AZ31 magnesium alloy: The effect of sodium dodecyl sulfate (SDS) as an eco-friendly accelerating agent

Sodium nitrite has been used as an accelerating agent in phosphating bath to improve its properties. However, it is well known that sodium nitrite is a carcinogenic component in phosphating sludge. In this study, it has been aimed to replace sodium nitrite by an environmentally friendly accelerating agent. To this end, sodium dodecyl sulfate (SDS) was used in phosphating bath to improve the phosphate coating formation on an AZ31 magnesium alloy. The effect of SDS/sodium nitrite ratio on the phosphated samples properties was also studied. Using field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), direct current (DC) polarization and electrochemical impedance spectroscopy (EIS) the properties of phosphated magnesium samples were studied. Results showed uniform phosphate coating formation on the magnesium sample mostly in hopeite phase composition. In addition, a denser and less permeable coating can be obtained at these conditions. The corrosion resistance of the phosphated samples was superiorly improved using higher SDS concentration in the phosphating bath.

A dipping E-coating for Mg alloys

Mg alloys are becoming more and more important structural materials in automotive industry. However, most commercial Mg alloys require coating protection because of their poor corrosion performance. In this paper, a self-deposited “electroless” E-coating process for Mg alloys is presented. By utilizing the unique electrochemistry of Mg, a thin film is rapidly formed on the surface of Mg alloys by simply dipping in an E-coating bath. The new developed dipping technique does not need a current or potential that is essentially required in a normal E-coating process. It is believed that the surface alkalization effect of Mg is responsible for the deposition of the “electroless” E-coating pre-film while the diffusion of hydroxyls in the porous film is governing the film growth. The rapidly formed pre-film can offer sufficient corrosion protection for Mg alloys in a chloride-containing environment or a phosphating bath in a paint line. The stability of the film can be significantly improved by the curing process. It is found that the coating performance is influenced by the type of Mg alloy and the surface pretreatment. The dipping coating can also act as a base for further powder-coating.

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.

Development of nano TiO2-incorporated phosphate coatings on hot dip zinc surface for good paintability and corrosion resistance

Phosphating is one of the most important chemical conversion processes for the purpose of corrosion protection and primer for painting. In the present work, nano TiO2 incorporated phosphate coating was developed on hot dip galvanized zinc surface for achieving good paintability and corrosion resistance. Based on the results from preliminary studies, the amount of nano TiO2 incorporated into the phosphating bath was optimized as 0.1g. TiO2 incorporation effectively reduced the extent of zinc dissolution during phosphating and activated the process to achieve the expected coating weight faster. Also it yielded coating with greater thickness than the normal phosphate coating. The TiO2 incorporation resulted in a well crystallized phosphate coating with large crystal size and greater surface coverage. Results from the electrochemical analysis revealed the better barrier protection characteristics and enhanced corrosion resistance of TiO2 incorporated phosphate coatings over the normal phosphate coatings.

Study of an alternative phosphate sealer for replacement of hexavalent chromium

This study evaluates the possibility of replacing the hexavalent chromium passivation treatment used as a sealer after phosphating of carbon steel (SAE 1010) by a treatment with niobium ammonium oxalate (Ox). Samples of carbon steel (SAE 1010) after being phosphated in a zinc phosphate bath (PZn+Ni) were immersed in solution of niobium ammonium oxalate (250mg L−1 of Nb) either at pH 3.0 or pH 8.0. A passivation treatment with a solution with CrO3 (200mg L−1 of Cr6+) was also used for reference.The corrosion resistance of the phosphated samples after passivation treatments was analyzed in a NaCl 0.5mol L−1 solution using electrochemical impedance spectroscopy (EIS) and anodic polarization curves. Salt spray tests were also performed to evaluate their corrosion resistance. The results showed that the highest corrosion resistance was obtained by passivation in a solution with (250mg L−1 of Nb) at pH 8.0.

PEO layers obtained from mixed aluminate–phosphate baths on Ti–6Al–4V: Dry sliding behaviour and influence of a PTFE topcoat

Abstract The present study focuses on the influence of PEO treatments (performed in the AC regime using mixed aluminate–phosphate baths) on the tribological behaviour of Ti–6Al–4V, dry sliding against EN100Cr6 steel. Three different sets of coatings were produced: (set 1) in an aluminate-rich electrolyte; (set 2) in a phosphate-rich electrolyte; and (set 3) identical to set 2, but with the addition of a spray-deposited PTFE topcoat. The topography, microstructure, phase constitution and surface microhardness of each set were characterized and dry sliding tests were carried out in the load range 5–120 N using a block-on-ring tribometer. For each set, a transition load which corresponds to the end of coating life has been identified. The highest transition loads were observed for coatings produced using the aluminate-rich electrolyte (set 1). Intermediate values of transition load were measured for duplex PTFE/PEO coatings (set 3). Single-layer PEO coatings produced in the phosphate-rich electrolyte (set 2) showed the lowest values of transition load, due to the low coating thickness and poor uniformity. The deposition of the PTFE topcoat proved to be beneficial in terms of both friction and wear resistance, particularly in an intermediate (30–50 N) load range.

Phosphate-silicate composite coating formed on AM60 magnesium alloy

A phosphate–silicate composite coating was investigated for AM60 magnesium alloy. The coating was achieved in a phosphate–permanganate solution which involved sealing in sodium silicate solution. The morphology, composition and corrosion resistance on the coatings were analysed. The phosphate–permanganate conversion coating was shown to exhibit network feature and plenty of cracks. The coating containing increased Mn element was produced using acetic acid to adjust the pH of the phosphating bath. The corrosion resistance of the coating was improved. The phosphate–silicate composite coating became more compact. The cracks on the phosphate–permanganate conversion coating were filled in by silica gel through sealing. The main elemental compositions of the phosphate–silicate composite coating were Mg, O, K, P, Mn and Si. The electrochemical polarisation curves demonstrated that the phosphate–silicate composite coating provided more effective protection against corrosion than that of phosphate–permanganate conversion coating and traditional chromate conversion coating.

Characteristics of a fast low-temperature zinc phosphating coating accelerated by an ECO-friendly hydroxylamine sulfate

A fast low-temperature phosphating processing accelerated by an ECO-friendly hydroxylamine sulfate (HAS) is developed. The zinc phosphate coating was fast formed on high-carbon steel in a low-temperature phosphating bath. Growth stages and characteristics of the phosphate coating were investigated by open circuit potential (OCP), SEM, EDS and XRD techniques. The phosphating process can be divided into three stages, namely amorphous precipitation, anodic depolarization and growth of phosphate coating. The phosphate coating consists of Zn 3(PO 4) 2 · 4H 2O and Zn 2Fe(PO 4) 2 · 4H 2O phases. The addition of HAS makes the three stages' time shorten to 53%, 31% and 50%, respectively, while markedly reduces the size of phosphate crystals from 100 µm to about 50 µm, and increases the Zn 2Fe(PO 4) 2 · 4H 2O content from 30% to 44% in the coating. HAS would be widely used as a low-temperature phosphating accelerator to replace the traditional nitrite, due to its less pollutant, higher phosphating rate and better quality of the coating.

“Electroless” deposition of a pre-film of electrophoresis coating and its corrosion resistance on a Mg alloy

Magnesium has unique electrochemical performance, which can be utilized in its coating or surface treatment. In this study, a new self-deposited coating process is explored for magnesium alloys. It is found that a thin film can be rapidly formed on a Mg alloy AZ91D through simply dipping the alloy coupon in an E-coating bath solution without applying a current or potential that is essentially required in a normal E-coating process. The “electroless” deposition mechanism and the film growth kinetics are investigated and the formed pre-film of E-coating is evaluated for its stability and corrosion protection performance in a phosphating acidic electrolyte and a NaCl corrosive solution. It is believed that the surface alkalization effect of magnesium is responsible for the “electroless” deposition of the pre-film. The diffusion of hydroxyls in the porous film is controlling the growth of the pre-film. The rapidly formed pre-film can offer sufficient corrosion protection for the magnesium alloy in a chloride-containing environment and it is also stable enough to enable a magnesium alloy part to go through a phosphating bath in a paint line.