Iron Phosphate Coatings Research Articles

Development of Microencapsulation-Hybrid Jig Separation Technique as a Clean Coal Technology

In this study, the microencapsulation-hybrid jig separation technique was developed to improve the separation efficiency of pyrite and coal in the particle size range of 1–4 mm where conventional jig separation becomes inefficient. A hybrid jig is a gravity concentrator combining the concepts of jig separation and flotation to stratify particles based on their apparent specific gravity. Meanwhile, microencapsulation—a technique that encapsulates target materials with a protective coating—was applied to render pyrite hydrophilic and improve its separation from hydrophobic coal. The results showed that the required time for separation in the hybrid jig (0.5 min) was shorter than in conventional jig (2 min). Moreover, the effects of particle size on separation efficiency were reduced when a hybrid jig is used. However, the separation efficiency of hybrid jig separation was lower than that of the conventional jig because attachment of bubbles occurred to both pyrite and coal, which are hydrophobic. Using the microencapsulation-hybrid jig separation technique, the separation of coal and pyrite was significantly improved (~100%) because of the formation of hydrophilic iron phosphate coatings on pyrite that limited bubble attachment. This means that microencapsulation-hybrid jig separation is a promising clean coal technology that not only enhances the separation efficiency of the hybrid jig but also passivates pyrite and limits AMD formation in the tailings/rejects.

Improvement of flotation and suppression of pyrite oxidation using phosphate-enhanced galvanic microencapsulation (GME) in a ball mill with steel ball media

Acid mine drainage (AMD) is one of the biggest and most challenging environmental concerns of the mining and mineral processing industries. In our previous study, we reported a new and promising approach to passivate sulfide minerals and limit AMD formation called galvanic microencapsulation (GME), a technique that forms protective coatings via galvanic interactions between sulfide minerals (e.g., pyrite) and metal powders (e.g., zero-valent iron and aluminum). In this paper, the possible application of GME in a ball mill with steel ball media and phosphate to suppress pyrite floatability during coal cleaning and limit pyrite oxidation after disposal is proposed. The results showed that adding phosphate and maintaining sufficient oxygen supply during GME treatment in the ball mill passivated pyrite and suppressed its oxidation via the formation of protective iron phosphate coatings. Moreover, the iron phosphate coatings on pyrite made the mineral more hydrophilic, which improved the separation efficiency during coal flotation.

Conditions favoring the formation of iron phosphate coatings on the pyrite surface

The feasibility and conditions for the establishment of phosphate coating on a pyrite concentrate material to inhibit acid generation were studied. Batch tests involving the treatment of pyrite samples with H2O2 (control tests) and coating solution (H2O2, KH2PO4) buffered to pH 5.0–6.0 with sodium acetate were performed. The concentration of H2O2 and the liquid to solid ratio were characterized as critical factors for the formation of coating. A coating consisting of Fe–K phosphate phases and mixed Fe–K phosphate/Fe3+ oxyhydroxide phases was effectively developed on the surface of pyrite particles treated with H2O2 0.1 M, KH2PO4 0.4 M, buffered at pH 5.5 with CH3COONa 0.2 M.

Surface coverage determination of iron-phosphate coatings on steel using voltammetric anodic dissolution technique

In this study, the effect of deposition time and concentration of NaNO2 in the phosphate bath on the surface morphology of iron-phosphate coatings on low carbon steel 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), while surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique in the borate solution. The addition of NaNO2 in the phosphate bath significantly increased the surface coverage since better packed crystals of smaller size, which favour the phosphate nucleation, were obtained. It was also shown that prolonged deposition time increased the surface coverage, coating roughness and crystal size in lateral direction, altering also the crystal shape from big platelets nonuniformly distributed on steel surface during initial time, to better packed laminated and needle-like structures during prolonged exposure.

Electrocrystallization mechanism of iron phosphate coatings onto mild steel electrode surfaces

Samples of mild steel AISI-SAE 1018, 1045 and 1080 were electrochemically phosphatised, both potentiodynamically and potentiostatically, in a concentrated H3PO4 aqueous solution. The conversion coating layers formed were characterised by means of scanning electron microscopy and energy-dispersive x-ray analysis. The kinetics and mechanisms of the electrocrystallization process of FePO4·2H2O(c) films were studied using the potential step technique. It was found that the mechanism governing the formation of FePO4·2H2O(c) anodic film was common for all the steel samples, namely, the overall electrodic process comprises four different simultaneous contributions to the overall current (j(t)). The individual contributions were: j ad(t), the current density contribution due to H3PO4 adsorption; j d(t), the current density involved during electrodissolution of iron of the steel electrode in the zones which were not covered yet by the FePO4·2H2O(c) film; j g(t), the current density contribution due to 2D nucleation and growth of an insoluble FePO4·2H2O(c) conducting adlayer; and j f(t), the current due to the growth of the passive layer induced by electrodissolution of iron metal and transport of the resulting Fe(II) ions through the FePO4·2H2O(c) film. However, each stage contributed to the overall process in a different manner, depending upon the carbon contents. The results from voltammetry indicated that it is possible to establish the potential at which the FePO4·2H2O(c) film begins to form and that this was also a function of the carbon contents.

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.

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.

Corrosion stability of polyester coatings on steel pretreated with different iron–phosphate coatings

The influence of steel surface pretreatment with different types of iron–phosphate coatings on the corrosion stability and adhesion characteristics of polyester coatings on steel was investigated. The phosphate coating was chemically deposited either from the simple novel plating bath, or with the addition of NaNO 2, as an accelerator in the plating bath. The morphology of phosphate coatings was investigated using atomic force microscopy (AFM). The corrosion stability of polyester coatings on steel pretreated by iron–phosphate coatings was investigated by electrochemical impedance spectroscopy (EIS) in 3% NaCl solution, while “dry” and “wet” adhesion were measured by a direct pull-off standardized procedure. It was shown that greater values of pore resistance, R p, and smaller values of coating capacitance of polyester coating, C c, on steel pretreated with iron–phosphate coating were obtained, as compared to polyester coating on steel phosphated with accelerator, and on the bare steel. The surface roughness of phosphate coating deposited on steel from the bath without accelerator is favorable in forming stronger bonds with polyester coating. Namely, the dry and wet adhesion measurements are in accordance with EIS measurements in 3% NaCl solution, i.e. lower adhesion values were obtained for polyester coating on steel phosphated with accelerator and on the bare steel, while the iron–phosphate pretreatment from the novel bath enhanced the adhesion of polyester coating on steel.

Improved iron phosphate corrosion resistance by modification with metal ions

ron phosphate coatings have found a wide range of use in the metalI finishing industry as a convenient method for achieving reasonably high levels of corrosion resistance. Flexibility of processing parameters and ease of use are probably the biggest reasons that iron phosphating enjoys its continued popularity. The corrosion resistance of iron phosphate coatings can be dramatically improved by the selective addition of combinations of certain divalent metals. Excellent results have been attained by adding mixtures of zinc, nickel, and calcium at concentrations below 200 ppm to conventional iron phosphating baths. Changes in the iron phosphate coating brought on by these metal additions have promoted increases in corrosion resistance from 10 to 75%.

Iron phosphate coatings on cold-rolled steel: Morphology formation and intrinsic adhesion mechanisms

The surface morphology, composition and performance of iron phosphate (P) coatings obtained in an industrial phosphating process on cold-rolled steel (CRS) base material (S4) at selected service lifetime (SLT) intervals ((1−330) × 10 3 m 2) and under operating conditions of the industrial plant were studied. Scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) were used to image directly and characterize the surface morphology and elemental composition of iron phosphate thin coatings as well as the corresponding bare CRS panels used for phosphating. The performance of the resulting iron phosphate was evaluated by assessing the interface bonding (using a mechanical shear test) and extent of bonding deterioration (using an accelerated electrochemical test) of a fast-cured electrocoated thermosetting powder coating. SEM results indicated that the iron phosphate layers formed on CRS substrate have similar morphology and thickness throughout most of its cycle of service, indicating that the system operates under optimum conditions. Pretreatment of iron phosphate increased the shear strength from 38 N mm -2 for untreated CRS to 45–48 N mm -2, thus enhancing the adhesion bond by about 25% at all S4PL interfaces studied over the control S4L interface. Accelerated corrosion test results show that the percentage of delaminated areas for most SLT stages ((60−280) × 10 3 m 2) was about 27%; for the SLT stages of 10 3 and 330 × 10 3 m 2 the percentages of delaminated areas were about 85% and 75% respectively. A model of the mechanism of formation of iron phosphate on CRS is proposed which accounts for the dependence of the microstructure on the operating conditions. On the basis of the results, a model for the mechanism of the intrinsic adhesion link at the organic-coated iron-phosphated CRS interfaces is presented suggesting that it cannot be totally attributed to a mechanical interlocking bond but rather to the presence of hydration water or crystallization water molecules in the Fe 3(PO 4) 2 · 4H 2O molecular structure.

Iron Phosphate Coatings—Composition and Corrosion Resistance

Abstract The corrosion resistance of three types of iron phosphate coatings on cold-rolled steel was examined. The alkali metal phosphating baths studied were three of the types most commonly encountered in industrial finishing operations. Coating composition was varied by using different accelerators in the phosphating bath. The components of each of the phosphate coatings were deduced by stripping test panels in a solution of 6 M HCl inhibited by Sb2O3. The stripping solution was then analyzed for the concentrations of ferrous iron, ferric iron, and phosphate. In the second part of the study, painted steel test panels were exposed to 5% salt fog as in ASTM B-117. Corrosion resistance was found to be strongly related to coating composition. Corrosion performance suffered as the fraction of ferrous salts in the coating increased.

Determination of differences in tribological behavior and surface morphology between electrodeposited and traditional phosphate coatings

Traditional phosphate coatings are formed by submersing iron or steel in an aqueous solution of phosphoric acid and other chemicals without applying an axternal voltage. In contrast, the newly developed electrocoated iron phosphate films described in this paper are deposited electrochemically from an oil containing mixed organic phosphates. The friction characteristics and surface morphology of the electrodeposited iron phosphate films on piston ring specimens were compared with those of traditional phosphate coatings. The electrodeposited film consists of an amorphous structure and provides lower friction than does either traditional crystalline manganese or iron phosphate coatings. These results indicate that surface morphology has a strong influence on friction-reducing film formation mechanisms.

A Reaction Mechanism for Producing Low-Friction Iron Phosphate Coatings

Engine friction reduction through lubricant modification is an appealing approach for improving vehicle fuel economy because the benefits realized can be readily applied to all vehicles. Since tribological processes are influenced by electrochemical interactions between a lubricant and the lubricated sliding surface, electrochemical techniques should be valuable in reducing engine friction. An iron phosphate reaction film has been coated on cast iron electrodes by means of electrolysis of a lubricant containing 2.5 mass percent dilauryl hydrogen phosphate (DHP or “acid phosphate”). Friction tests showed a 20 percent lower friction coefficient for iron phosphate-coated piston ring specimens in comparison to uncoated ones. Laboratory reaction studies indicated that the electrolysis process accelerated a spontaneous corrosion reaction between the electrode and the acid phosphate. Increasing either the electric field or the agitation of the lubricant also accelerated the reaction rate for iron phosphate film ...

Nonmetallic conducting coatings: Properties and behaviour of iron and iron‐copper phosphate coatings

AbstractSurface conversion of steel by means of conducting iron phosphate coatings protects mechanical parts blanked from steel sheets, even in a very high‐humidity environment, and also allows their assembly through electrical resistance welding. Corrosion tests carried out by exposing the materials to alternate humid/dry cycles, in a humidostatic chamber at 40°C, indicate that the welding process gives rise to corrosion phenomena. These can be strongly mitigated by using conversion coatings of mixed iron/copper phosphates. Conditions for obtaining optimum corrosion resistance for the final conversion coatings are therefore defined.Measurements of the related corrosion potentials made at 25 °C in a sodium hydroxide solution (pH 12, saturated in oxygen) indicate the existence of threshold values above which corrosion resistance of the phosphatized specimens is acceptable, and suggest a method for performing an industrial control of phosphating efficiency.The performance of converted steel surface has been related to the porosity of phosphate coatings.

Corrosion testing of phosphatized steel samples

AbstractA method for corrosion testing of phosphatized steel samples has been described. The specimens, treated with different phosphatizing, solutions and covered by a chemically inert paint film, have been exposed in a sulphur‐dioxide‐polluted atmosphere with 90% relative humidity at 22.0°C. The corrosion rate has been measured as an increase in weight. The corrosion resistance shows a good correlation with the crystal coverage of the zinc phosphate layers. Filiform corrosion can be inhibited by zinc phosphate as well as iron phosphate coatings. However, underfilm corrosion is more frequent on the iron phosphatized samples.

Materials

Jenolite Division of Duckhams Oils, Rusham Road, Egham, Surrey, have made an addition to their range of phosphates. Kleenfos 100 ST is a degreaser/phosphate which is claimed to be capable of producing iron phosphate coatings by spray or immersion. The manufacturers state that it can be employed to phosphate iron and steel, galvanised metal and also aluminium and deposits coating weights of up to 100 mg/sq. ft.