CHAPTER 2 MATERIALS AND METHODS

Transcription

1 37 CHAPTER 2 MATERIALS AND METHODS This chapter discusses the methodology adopted for the development of zinc phosphate coating utilizing galvanic coupling and to evaluate the effect of cathode materials on the phosphatability and corrosion resistance of mild steel. 2.1 Materials All the chemicals used were of Analar grade and were used as received from the supplier without further purification. The following chemicals were used in this study: 1. Nano zinc oxide (ZnO) (AR) (Aldrich) 2. Nano titania (TiO 2 ) (AR) (Aldrich) 3. Nano silica (SiO 2 )(AR) (Aldrich) 4. Nano zirconia (ZrO 2 ) (AR) (Aldrich) 5. Nano alumina (Al 2 O 3 )(AR) (Aldrich) 6. O-Phosphoric acid (85%) (AR) (E-Merck) 7. Sodium nitrite (AR) (E-Merck) 8. Hydrochloric acid (AR) (E-Merck) 9. Antimony trioxide (AR) (E-Merck) 10. Sulphuric acid (AR) (E-Merck) Deionized water was used for preparing the solutions. Mild steel substrates of 8 cm x 5 cm x 0.2 cm in size were used for the deposition of zinc phosphate coatings. The chemical composition of the mild steel used for the study is given in Table 2.1

2 38 Table 2.1 The chemical composition of the mild steel substrates used for the present study Element Wt. % C Si Mn P S Cr Ni Mo Fe Balance 2.2 Bath formulation for the development of nano zinc phosphate coating on mild steel The phosphating bath formulated for the present study consists of nano zinc oxide, phosphoric acid and sodium nitrite as accelerator, to promote coating formation. The bath formulation was made using only the basic components and the use of special additives are avoided. Several trial experiments were conducted to optimize the bath composition and operating conditions. The composition of the bath and operating conditions used for the development of nano zinc phosphate coatings on mild steel panels are given in Table 2.2. Table 2.2 Chemical composition and control parameters of the bath used for the development of nano zinc phosphate coatings on mild steel Chemical Composition Nano ZnO g/L H 3 PO 4 2.3ml/L NaNO 2 0.4g/L Control Parameters ph 2.70 Free acid value(fa) 3pointage Total Acid Value(TA) 25pointage FA:TA 1: 8.33 Temperature 27 C ± 2 C Time 30 minutes

3 Bath formulation for the development of normal zinc phosphate coating on mild steel For a comparison, normal zinc phosphate coatings were also developed as reported [51, 90]. The composition of the bath and operating conditions used for the development of normal zinc phosphate coatings on mild steel panels are given in Table 2.3. Table 2.3 Chemical composition and control parameters of the bath used for the development of normal zinc phosphate coatings on mild steel Chemical Composition ZnO 5 g/l H 3 PO ml/l NaNO 2 2 g/l Control Parameters ph 2.70 Free acid value(fa) 3pointage Total Acid Value(TA) 25pointage FA:TA 1: 8.33 Temperature 27 C ± 2 C Time 30 minutes 2.4 Preparation of the phosphating bath The phosphating bath formulated for the present study was prepared by dissolving 0.25 to 2 g of nano zinc oxide in water containing 2.3 ml of o-phosphoric acid (85%) and diluted to one litre with deionized water. Vigorous stirring was required for dissolving zinc oxide in water in presence of phosphoric acid. The ph of the phosphating solution was fixed at 2.41 ± g of sodium nitrite per litre was added to the phosphating solution just before the phosphating process. The ph of the bath after nitrite addition was 2.70 ± 0.1.

4 Bath compositions for the study on effect of additives on the zinc phosphating of mild steel Effect of the additives such as nano TiO 2, nano SiO 2, nano ZrO 2 and nano Al 2 O 3 on the zinc phosphating was studied using the chemical compositions of the phosphating bath given in Table 2.4. Table 2.4 The chemical compositions of phosphating baths with different additives Bath No Nano ZnO H 3 PO 4 (ml/l) NaNO 2 Nano TiO 2 Nano SiO 2 Nano ZrO 2 Nano Al 2 O 3 Bath Bath Bath Bath Phosphating was carried out at room temperature (27 C ± 1 C ) for a time of 30 min. and the ph of the bath was adjusted to 3 ± 0.1 by adding NaOH. 2.6 Analytical aspects of bath control below: The control parameters of the formulated bath were determined as detailed (i) Free acid value (FA) This refers to the free H + ions present in the phosphating solution. The free acid value of the formulated bath was determined by titrating a 10ml aliquot of the phosphating bath with 0.1N sodium hydroxide solution using methyl orange as the indicator. The end point is the change in the colour of the solution from red to yellow. The titre value thus obtained, expressed as points, gives the free acid value.

5 41 (ii) Total acid value (TA) This represents the total phosphate content of the phosphating bath. The total acid value of the formulated bath was determined by titrating a 10ml aliquot of the phosphating solution with 0.1N sodium hydroxide solution using phenolphthalein indicator. The end point is the appearance of pale pink colour. The titre value thus obtained, expressed as points, gives the total acid value. 2.7 Methodology of phosphating Conventional phosphating process involves the following seven steps: degreasing, pickling, pre-rinsing, phosphating, post-rinsing, chromic acid sealing and drying [13]. With the exception of chromic acid sealing, all the other six steps followed in the present study are given in the flow chart (Fig.2.1). Degreasing Pickling Pre rinsing Phosphating Post rinsing Drying Fig.2.1 Flow chart for operations of phosphating process for the present study

6 42 (i) Degreasing The oil and the greasy matter present on the substrate material were removed by wiping with cotton soaked in trichloroethylene. (ii) Pickling Degreased panels were pickled in 10% sulphuric acid between 70 and 80ºC for 5-10 minutes to remove the rust and mill scale. (iii) Pre-rinsing Pickled panels were rinsed thoroughly in deionised water to remove the acid residues present on it after pickling. (iv) Phosphating The degreased, pickled and rinsed mild steel substrate is immediately immersed into the phosphating solution which is maintained at the temperature required for phosphating using a constant temperature bath. Coating formation was proceeded for the required period of time after which the panels were removed. (v) Post-rinsing The phosphated substrates were then rinsed with deionised water to remove the acid residues and the soluble salts left after phosphating. (vi) Drying After rinsing, the coated substrates were subjected to forced drying using a stream of compressed air. 2.8 Potential-time measurements Potential-time measurements were carried out during phosphating. These measurements were made using a multimeter (model 435 Systronics Digital Multimeter) against the saturated calomel electrode (SCE) using a Luggin capillary.

7 Evaluation of phosphate coatings The following studies were conducted on phosphated test panels. 1. Examination of physical appearance 2. Determination of coating weight 3. Determination of iron dissolution 4. Determination of coating composition 5. Evaluation of surface morphology 6. Evaluation of corrosion resistance (by chemical and electrochemical methods) Examination of physical appearance Visual inspection of the coated panels was done empirically to estimate various parameters such as the uniformity of coating, quality of coating and to detect any gross defects as bare patches etc Determination of coating weight The phosphated panels were weighed accurately. The coating is then dissolved by immersing them for about five minutes in a solution of concentrated hydrochloric acid (specific gravity 1.18) containing 20g of antimony trioxide per litre of the acid [13]. The panels were removed from the solution, rinsed with deionised water, dried with a stream of compressed air and reweighed. The difference in weight of the panel before and after stripping (after deducting the blank loss) was recorded as the coating weight X - Y Coating weight (g/m 2 ) = 10,000 Area of the specimen (cm 2 ) where X = Weight of the specimen (g) with the phosphate coating and Y = Weight of the specimen (g) after stripping off the coating

8 Determination of iron dissolved during coating The amount of metal dissolved during phosphating is determined by calculating the difference in weights before phosphating and after stripping off the coating (also in dry condition) Amount of iron dissolved A - B during phosphating (g/m 2 ) = 10,000 Area of the specimen (cm 2 ) where A = Weight of the specimen (g) before phosphating and B = Weight of the specimen (g) after stripping off the coating Characterization Techniques used for the analysis of phosphate coatings The chemical composition and morphology of the coatings were analyzed by X-ray diffraction analysis (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy and Transmission electron microscopy (TEM). (i) X- ray diffraction The composition of the phosphated mild steel samples was investigated by X-ray diffraction analysis. The phases in the phosphate coating were analyzed by XRD using Philips X Pert pro diffractometer with Cu Kα (λ= Å) incident radiation. The XRD peaks were recorded in the 2θ range of X-ray diffraction is a convenient method for determining the mean size of nano crystallites in nano crystalline bulk materials. The first scientist, Paul Scherrer, published his results in a paper that included what became known as the Scherrer equation in 1981 [128].From the wellknown Scherrer formula the average crystallite size, L, is: where: = l b. q L = The mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size;

9 45 K = Dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about 0.9, but varies with the actual shape of the crystallite; λ = X-ray wavelength; β = Line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians. θ = Bragg angle. The average crystallite size of the nano zinc phosphate coating was determined using this equation. (ii) Evaluation of surface morphology using SEM The surface morphology of phosphated steel samples were assessed by scanning electron microscope (SEM) (Hitachi Scanning Electron Microscope, model SU1510).The distinct features of coating morphology at suitable magnifications were recorded. (iii) Evaluation of surface morphology using TEM The surface morphology of the phosphated mild steel substrates were assessed by transmission electron microscope (TEM) (Make : FEI / Philips TCNAI G2, Switzerland) Evaluation of corrosion resistance The following laboratory tests, more frequently used for determining the corrosion resistance of phosphate coatings were performed. a. Salt spray test b. Potentiodynamic polarization and electrochemical impedance studies a. Salt spray test Evaluation of the corrosion resistance of phosphated substrates (unpainted) using galvanic coupling was performed by subjecting them to a salt mist of 5% sodium

10 46 chloride solution in a salt spray chamber (ASTM B ) for a specified period of time (96 hours)[13]. The edges of the substrates were sealed with paraffin wax to avoid the excessive corrosion at the edges. The extent of corrosion after 96 hours of exposure was assessed and photographed. b. Potentiodynamic polarization and electrochemical impedance studies Potentiodynamic polarization and electrochemical impedance studies of phosphate mild steel panels were carried out using a Biologic Electrochemical Analyser (model SP 300) at the open circuit potential. A 3.5% sodium chloride solution was used as the electrolyte. The solution temperature was maintained at 27+1ºC. Mild steel specimen of 1 cm 2 area was used as the working electrode. A platinum electrode and saturated calomel electrode were used as counter electrode and reference electrode respectively. The saturated calomel electrode was connected via Luggin capillary, the tip of which was held very close to the surface of the working electrode to minimize the IR drop. Open circuit potential (OCP) measurements were recorded for 30 minutes, the time necessary to reach quasi stationary state for open circuit potential, followed by polarization measurements at a scan rate of 1mV/s for Tafel plots. Biologic Electrochemical analyzer (model SP 300) with EC Lab software was used for data acquisition and analysis. For polarization and impedance studies the period of immersion maintained was 30 minutes. Polarization technique was carried out from a cathodic potential of -250 mv to an anodic potential of +250 mv with respect to OCP at a scan rate of 1 mv/s. The electrochemical parameters including corrosion potential (E corr ), corrosion current density (I corr ) and corrosion rate were calculated from Tafel plots. In EIS technique, a small amplitude AC signal of 10 mv and a frequency spectrum from 10 5 to 10-2 Hz was impressed at the OCP and the impedance data were analysed using Nyquist plots. The impedance data were fit into appropriate equivalent electrical circuit using EC lab software. The parameters obtained from the best fit equivalent circuit were analysed.

11 47 Fig.2.2 Electrochemical workstation and the cell assembly used for potentiodynamic polarization and impedance studies