Zinc Phosphating Research Articles

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.

Treatment of rinse water from zinc phosphate coating by batch and continuous electrocoagulation processes

Treatment of spent final rinse water of zinc phosphating from an automotive assembly plant was investigated in an electrochemical cell equipped with aluminum or iron plate electrodes in a batch mode by electrocoagulation (EC). Effects of the process variables such as pH, current density, electrode material and operating time were explored with respect to phosphate and zinc removal efficiencies, electrical energy and electrode consumptions. The optimum operating conditions for removal of phosphate and zinc were current density of 60.0 A/m(2), pH 5.0 and operating time of 25.0 min with Al electrode and current density of 60.0 A/m(2), pH 3.0 and operating time of 15.0 min with Fe electrode, respectively. The highest phosphate and zinc removal efficiencies at optimum conditions were 97.7% and 97.8% for Fe electrode, and 99.8% and 96.7% for Al electrode. The electrode consumptions increased from 0.01 to 0.35 kg electrode/m(3) for Al electrode and from 0.20 to 0.62 kg electrode/m(3) for Fe electrode with increasing current density from 10.0 to 100.0 A/m(2). The energy consumptions were 0.18-11.29 kWh/m(3) for Al electrode and 0.24-8.47 kWh/m(3) for Fe electrode in the same current density range. Removal efficiencies of phosphate and zinc were found to decrease when flow rate was increased from 50 to 400 mL/min in continuous mode of operation. The morphology and elements present in the sludge was also characterized by using SEM and EDX.

Effect of Zinc Phosphating on Corrosion Control for Carbon Steel Sheets

Effect of Zinc Phosphating on Corrosion Control for Carbon Steel Sheets

Zinc phosphating of 6061-Al alloy using REN as additive

Zinc phosphate coating formed on 6061-Al alloy was studied with the help of electrochemical measurements, Fourier Transform Infrared (FTIR), and Scanning Electron Microscopy (SEM), after dipping it in phosphating solutions containing different concentrations of Rare Earth Nitrate (REN). REN, which acted as an accelerator in the phosphating solution, could catalyze the surface reaction and accelerate the phosphating process. REN mainly enabled the P in the phosphate coating to exist in the form of PO 4 3 and promoted the hydrolysis of phosphatic acid in a liquid layer at the cathodes. This resulted in the evolution of H 2 at the cathodes, which increased the local pH value and in turn drove the precipitation of the phosphate coating. Additionally. REN was adsorbed on the surface of the aluminum substrates to form a gel during the phosphating process. These gel particles were good crystal seeds, which helped to form phosphate crystal nuclei and possess the function of a nucleation agent that could decrease the phosphate crystal size. The corrosion resistance of the formed zinc phosphate coatings was improved.

A Study on Corrosion-Resistant Properties of Unleaded Bi-Containing Automotive Epoxy Primers

The aim of this work is to evaluate the difference in the corrosion resistance between unleaded containing bismuth automotive epoxy primers and leaded one. It was also discussed the effect of zinc phosphate pretreatment. The specimen coated unleaded epoxy primer showed 0.5V higher corrosion potential than that of bare steel. The result of salt spray test did not indicate remarkable difference of corrosion resistance in all specimens above 10 thickness up to 1200 hours. In the cyclic corrosion test, unleaded epoxy primers on phosphated substrate performed good corrosion properties until 800 hours. The lead free primer performed the equivalent corrosion resistance as leaded coating on phosphated steel, but slightly inferior to that of leaded on bare steel. These results show that the pre-treatment of zinc phosphating is effective as well as pigment changing in performing anti-corrosion properties in automotive bodies.

Effect of Mn 2+ additive on the zinc phosphating of 2024-Al alloy

Zinc phosphate (ZPO) conversion coatings formed on 2024-T3 aluminum alloy, after dipping in Mn 2+-containing ZPO coating baths for different times, have been studied by X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning Auger microscopy. An optimal coating (in terms of thickness and morphology) is formed when the alloy is immersed for 3 min in a ZPO coating solution containing 2000 ppm Mn 2+. Comparisons are made with coatings formed from a Ni 2+-containing ZPO bath. Both Ni 2+ and Mn 2+ additives decrease the coating crystal size, with Mn 2+ giving a greater reduction, and the coating formed from the Mn 2+-containing solution is thicker than that formed with Ni 2+. The coatings are heterogeneous in depth with Mn 2+ depositing during the early stages of coating, while precipitation of Ni 2+ occurs mainly later in the coating process. Patterns of deposition across the surface are also affected significantly by the composition of the ZPO coating solution. Mn 2+ in the coating bath gives the most even distribution of coating across the different microstructural regions on the surface.

Investigation on the effect of benzotriazole on the phosphating of carbon steel

Phosphating and corrosion inhibitors are commonly used for corrosion prevention of metallic materials, mainly carbon steels. In this study, the effect of benzotriazole (BTAH) added to a Zinc phosphating (PZn) bath on the corrosion resistance of a carbon steel (1010) was investigated. Anodic polarization curves and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion resistance of the phosphated steel with or without BTAH, in two solutions, 0.1 mol L−1 H2SO4 and 0.5 mol L−1 NaCl. The results showed better corrosion resistance for the steel phosphated with BTAH comparatively to that without BTAH.

Influence of sodium metanitrobenzene sulphonate on structures and surface morphologies of phosphate coating on AZ91D

The gray phosphate coating was formed on AZ91D magnesium alloy from the zinc phosphating bath containing sodium metanitrobenzene sulphonate in about 4 min. The structure, surface morphologies and phase compositions of the phosphate coatings were observed and analyzed by using SEM, XRD and EDS. It is shown that the phosphate coating becomes denser and has less micro holes with increasing the concentration of sodium metanitrobenzene sulphonate in the bath in the range of 2.0 to 6.0 g/L. The addition of sodium metanitrobenzene sulphonate greatly increases the micro cathode sites for the formation of the phosphate coating and decreases the porosity of the coating.

The effect of pH and role of Ni2+ in zinc phosphating of 2024-Al alloy: Part II: Microscopic studies with SEM and SAM

Coatings formed on 2024-T3 aluminum alloy were studied by scanning electron microscopy (SEM) and scanning Auger microscopy (SAM) after dipping in zinc phosphating (ZPO) baths at different acidities, with or without the Ni2+ additive. The objective was to learn more about the ZPO coating mechanism on the different microstructural regions of 2024-T3. When the initial coating solution pH is 4 (optimal acidity), a slower etching rate at the Al–Cu–Fe–Mn intermetallic particle causes significant precipitation of ZnO, which differs from the coating on other regions of the surface where phosphate predominates. The larger crystals (∼μm dimension) on the matrix and the Al–Cu–Mg particle contain more phosphate compared to other areas on the surface. When Ni2+ is added to the coating solution, the Al–Cu–Mg particle is more thickly coated compared to when the Ni2+ is not present. The slower rate of precipitation when Ni2+ is present in the coating solution increases the exposure of the alloy substrate to the acidic environment, so allowing more dissolution of Mg and Al from the Al–Cu–Mg particle. This results in the particle becoming more cathodic in nature, and therefore more coating deposits at this location. Evidence from SAM supports the presence of NiAl2O4, hypothesized in Part I, forming at coating pores later in the process.

The effect of pH and role of Ni2+ in zinc phosphating of 2024-Al alloy: Part I: Macroscopic studies with XPS and SEM

Coatings formed on 2024-T3 aluminum alloy were studied by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) after dipping in zinc phosphating (ZPO) baths at different acidities, for different lengths of time, and with or without Ni2+ additive. The overall objective was to learn more about the role of Ni2+ on the ZPO coating mechanism, particularly since this additive is believed to improve corrosion protection for the Al alloy. Secondary phosphates dominate the coatings when the Ni-containing solution is adjusted to starting pH values of either 3 or 5, while tertiary phosphate is predominant at pH 4. AlF3 precipitates during the early stages of the coating process. Ni2+ has two main roles in the mechanism. First, the rate of increase in local solution pH is retarded by the slower kinetics of reactions involving Ni2+ compared to Zn2+, leading to thinner ZPO coatings when Ni2+ is present in the coating solution. Second, most Ni2+ deposition occurs during the later stages of the coating process, by nickel phosphate deposition and/or by formation of a Ni-rich oxide.

Electroless Ni–P deposition plus zinc phosphate coating on AZ91D magnesium alloy

An electroless Ni–P deposition process has been developed to treat the AZ91D magnesium alloy surfaces against corrosion. Magnesium alloy AZ91D was first phosphatized in a zinc phosphating bath containing molybdate. Then an electroless Ni–P deposition was carried out on the phosphate coating from a sulfate solution. The phases in the phosphate coatings were analyzed by XRD. Microstructures of phosphate coatings and electroless Ni–P depositions were observed by SEM and EDS. It was found that there was metallic zinc in the phosphate coating and the addition of Na 2MoO 4 in the phosphating bath resulted in the increase of zinc in the coating. A lot of disperse metallic zinc particles acted as the catalyst nuclei for the succeeding Ni–P deposition. Consequently, the Ni–P depositions with dense and fine microstructure were obtained on the phosphate coatings gained from the phosphating bath wherein 2.0∼2.5g/L Na 2MoO 4 was added. The Ni–P plus phosphate coatings on the AZ91D magnesium alloy exhibited acceptable corrosion resistance as shown by the results of the Salt Spray Corrosion Test.

The analysis of corrosion performance of car bodies coated by no nickel and low nickel zinc phosphating processes

In today's automotive industry in order to protect car bodies from corrosion, spray or immersion type zinc phosphating processes are applied. In both types, nickel and chromium are widely used though they are harmful to human health and environment. In this study, car body's corrosion performance, coated by no nickel (0 ppm) and low nickel (100, 200, 300 ppm) immersion type zinc phosphating (without chromium passivation) processes, are compared to the bodies that are coated by spray and immersion type processes including nickel (500–700 ppm) and chromium. After analyzing coating weight, composition, morphology of the crystals and salt spray test corrosion performance of car bodies specimens coated by no nickel and low nickel processes are as good as the ones coated by spray and immersion phosphating processes including nickel and chromium. In developing environment consciousness, it is inevitable to favor no nickel or low nickel processes since they give no harm to nature and human health.

Microstructural effects on the initiation of zinc phosphate coatings on 2024-T3 aluminum alloy

The initiation of coatings deposited on to 2024-T3 aluminum alloy from supersaturated zinc phosphating solutions has been studied using scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The alloy microstructure, especially associated with the second-phase particles, strongly affects the formation stages of the coating process, where etching of the substrate has a significant role. At the start, zinc phosphate (ZPO) crystals form on the Al–Cu–Mg second-phase particles, rather than on the matrix or on the Al–Cu–Fe–Mn particles, with the initial nucleation appearing at interfaces between Al–Cu–Mg particles and the matrix. In contrast, the formation of the ZPO coating is delayed on the cathodic Al–Cu–Fe–Mn particles, compared to those of the Al–Cu–Mg composition. When the coating process is completed, the whole sample surface is covered with ZPO although its thickness varies at the different micro-regions.

21世紀の自動車車体用リン酸亜鉛処理技術

21世紀の自動車車体用リン酸亜鉛処理技術

Effect of divalent cations in low zinc ambient temperature phosphating bath

A new zinc phosphating bath, which produces coatings at relatively lower temperatures within a reasonable time by using of chemical accelerators has been devised. Improvement of the bath performance by the addition of divalent cations like calcium, manganese and magnesium has been studied. Bath formulation and operating conditions have been optimized by coating weight determinations. Corrosion resistance property of the resultant coatings has been evaluated in 1,000 ppm Cl− by electrochemical methods such as potentiodynamic polarization and impedance measurements. Results of the electrochemical techniques have been complemented by salt spray, humidity and immersion tests. Porosity and roughness of the coatings have also been studied. Results show that the phosphating bath with manganese addition gives good coatings within 30 min. Studies have shown that the corrosion resistance of the resultant coatings are much superior than the conventional coatings.

Effects of magnetic fields on the phosphating process

The influence of permanent magnetic fields (MF) 0.5 Tesla (T) on the morphology and corrosion protection (‘drop’ method) of phosphate coatings on high purity Zn (99.9) surfaces has been studied. Two types (i) of Zn phosphating and (ii) ZnCa phosphating solutions have been investigated. The weights of the phosphate coatings without MF differ, approximately twice (P Zn phosphate ∼ 8 g/m 2, while P ZnCa phosphate ∼ 4 g/m 2. ZnCa phosphate coatings are better defined and compact. The number of crystallites ranges from 3.3 10 −2/cm 2 to 3.3 10 −3/cm 2. The amount of crystallites increases in the case of ZnCa phosphating. Three variants of MF were applied: (i) phosphating in perpendicular MF, (ii) in MF parallel to the surface; (iii) phosphating solutions were pre-treated by MF for 10 min and then phosphating process was carried out. A zinc phosphating process in Zn phosphate solution is less influenced by MF in comparison with ZnCa phosphating solution. In this case, phosphate coating is fine grained and compacts, and the number of crystallites increases. The weight reduction of ZnCa phosphate coating reaches to 25%. Corrosion protection increases several times. The fact that the influence of MF remains when solutions are pre-treated in MF and then phosphating is carried out (‘memory’ effect), facilitates its industrial use.

Study of the Cryolite Formation during the Zinc Phosphating of Aluminium

SUMMARYTo improve the corrosion resistance and the adhesion properties of aluminium, chemical conversion processes, such as zinc phosphating, are applied. During zinc phosphating of aluminium the co-precipitation of cryolite often occurs. Cryolite needs to be formed in solution to remove Al3 ions, which have a poisoning effect on the phosphating process, away from the surface. Cryolite formation on the surface must be avoided, since it leads to a decrease of the corrosion resistance and of the adhesion properties. In this paper, the influence of mass transport on this co-precipitation competition is reported. The resulting conversion layers are investigated by SEM/EDX. It can be concluded that the amount of mass transfer has a large influence on the formation of both the phosphate and cryolite crystals. Hence, this competition in precipitation is explained in terms of mass transport of the different species involved in the chemical reactions at the surface.

Activation of galvanized steel surfaces before zinc phosphating — XPS and GDOES investigations

The effect of activation in the zinc phosphating has been studied phenomenologically by SEM and AFM. Kinetic investigations show that the activation plays an important role for the phosphating reaction. The activations underlie coagulation and sedimentation due to their colloidal character which again affect the reaction kinetics of phosphating. The adsorption of the titanium phosphate colloids was analyzed by XPS and GDOES. Whereas ESCA is not able to detect the adsorbed titanium phosphate particles using titanium as a central element, it is possible to identify it with GDOES.

Real-time monitoring of zinc phosphating process by quartz crystal impedance system

The zinc phosphating process in manganese-modified low-zinc phosphating baths has been monitored firstly by an in situ quartz crystal impedance system (QCIS) which allows rapid and simultaneous measurements of admittance spectra of the zinc-coated piezoelectric quartz crystal (PQC) resonator. The electrical equivalent circuit parameters are obtained by non-linear least square regression analysis of synchronously acquired conductance and susceptance data. The experimental results show that the kinetic growth process of zinc phosphate coating can be monitored on-line by measuring the equivalent circuit parameters and frequency shifts of the zinc-coated PQC resonator. According to the motional inductance ( L m ) change of zinc-coated PQC during zinc phosphating treatment, the whole process can be divided into four regions: pickling reaction, exponential growth, linear growth and exponential decay growth, respectively. And effects of inorganic additives (Ni 2+ and/or Mn 2+) on the properties and performance of zinc phosphating coatings as well as the variations of equivalent circuit parameters of zinc-coated PQC during the phosphating treatment are also investigated.

Studies on Acceleration of the Low Temperature Zinc Phosphating Processes

SUMMARYThe present work aims to discuss the effective means of accelerating the low temperature zinc phosphating processes by employing two methodologies and to evaluate the possible implications of adopting such methodologies in decreasing the processing time. The first one utilizes the galvanic corrosion principle whereas the second one involves the addition of a powerful oxidizing agent, namely, iodine pentoxide to the bath. It is suggested that both methods would impart a higher rate of metal dissolution, which would be helpful in accelerating the rate of deposition. Phosphate coatings were produced on mild steel substrates using a zinc phosphating bath capable of operating at low temperature (27°C). under both uncoupled and galvanically coupled conditions (with stainless steel or copper substrate) and with and without the addition of iodine pentoxide. The study revealed an enhanced dissolution of mild steel when it is galvanically coupled to more noble metals and also when the bath is modified with iodine pentoxide. It is concluded that the processing time of the low temperature zinc phosphating processes can be considerably decreased by adopting either the galvanic coupling methodology or the addition of iodine pentoxide.