DEVELOPMENT OF ELECTROLESS PROCESS FOR DEPOSITION OF ZN SILICATE COATINGS

Transcription

1 REPORT OF THE FINAL PROJECT ENTITLED: DEVELOPMENT OF ELECTROLESS PROCESS FOR DEPOSITION OF ZN SILICATE COATINGS by Veeraraghavan S Basker Department of Chemical Engineering University of South Carolina Columbia, SC Submitted in Partial fulfillment to ECHE 789B May 6, 2002 Page 1/11

2 DEVELOPMENT OF ELECTROLESS PROCESS FOR DEPOSITION OF ZN SILICATE COATINGS OBJECTIVES The objective of this project was to develop a new electroless process based on Elisha s Mineralization Process for coating galvanized steel and to study the corrosion properties of the final deposit. Another key goal of this project was also to improve the stability and corrosion resistance of deposits obtained by Elisha Mineralization (EM) process. Hence, the objective of the proposed research were: (1) to develop, characterize and optimize the parameters for novel electroless mineralization process of Zn (formation of thin Zn-silicate film) The process was based on interaction of the substrate with the electrolyte containing silica source, reducing agents and other additives/catalysts which will increase the deposition rate of the mineral on the substrate. (2) to improve the electrochemical mineralization of Zn using Elisha s mineralization process and to optimize the deposition parameters for the modified process. Specifically, the goal was to study the effect of the following process parameters on the deposit characteristics obtained from the modified EM process: (i) time of deposition, (ii) applied voltage, (iii) concentration of PQ solution, (iv) posttreatment temperature and, (v) duration of post-treatment heating. In both studies subsequent to mineralization, impedance analysis and linear polarization have been used to electrochemically characterize the final deposit. Material characterization was done using Electron Dispersive Spectroscopy (EDAX) and Scanning Electron Microscopy (SEM). From the electroless studies it is seen that it is possible to duplicate Elisha s Mineralization process through the use of reducing agent (sodium borohydride) in the bath. Increasing concentration of borohydride leads to increase in the average corrosion resistance of the mineralized sample. It is seen that addition of a post-treatment heating step increases the corrosion resistance of the deposit by several orders of magnitude. It is also seen that increasing the concentration of the PQ solution also leads to more uniform deposits. Addition of the posttreatment step to Elisha s mineralization process results in a similar increase in the resistance of the mineralized sample. Operating conditions for obtaining a stable deposit with uniform layers Page 2/11

3 of SiO 2 on the zinc surface have been optimized. A mechanism to explain the increase in resistance due to the addition of the heating step has been derived based on the experimental results obtained so far. EXPERIMENTAL The experimental study is divided into two major parts. In the first part we deal with replication of Elisha s mineralization process, albeit through an electroless technique. In the second part, the goal is to improve the characteristics of the EM process by post-treatment heating. Electroless Deposition Studies: The deposition bath used for the first set of studies was the same as given by Elisha Technologies (800 ml of distilled water ml of PQ N Sodium Silicate solution ml of 10% FeCl 3 solution). Prior to deposition the panels were degreased with acetone and washed with demineralized water. Two different sets of studies were done on these panels. The first set of studies focused on developing a mineralization process based on Elisha s bath. The following studies were done: First, the effect of temperature was studied at 25 o C, 75 o C and 85 o C for 5 minutes. Next, the effect of ph was studied at 75 o C at 10.5 and11. The deposition time was 5 minutes. Finally, the effect of deposition time was studied by doing electroless plating for 5,10, 15 and 20 minutes. Subsequent to mineralization one set of panels was rinsed immediately. Another set of samples was rinsed after 24 hours. In the second set of studies, the effect of post-treatment heating on corrosion resistance was studied. The following studies were done: First, mineralization was done in Elisha bath at 75 o C for 15 minutes. Second, mineralization was done in PQ bath with different concentrations 1:1, 1:2, 1:3, 1:4, 1:8. To prepare a 1:3 PQ bath, 1 part by volume of PQ solution was mixed with three parts by volume of water. Deposition was done at 75 o C for 15 minutes. Finally, mineralization was done in 1:3 PQ bath with different concentrations of Sodium borohydride (5, 10 and 15 g/l of NaBH 4 ). Page 3/11

4 All samples was prepared as described above and was left to dry in air for 24 hours. The samples were rinsed before corrosion testing. The corrosion characteristics of all the panels obtained from the studies described above were tested in 0.5 M Na 2 SO 4 solution at ph 4. A representative panel area of 1 cm 2 was chosen for testing. The rest of the panel was masked with an insulating tape. A three-electrode setup was used to study the corrosion behavior of the mineralized samples. The electrolyte used in this study is 0.5 M sodium sulfate. Ti coated with Pd was used as the counter electrode. Hg/Hg 2 Cl 2 was used as the reference electrode. All potentials in this study are referred with respect to the calomel electrode. Corrosion studies were done using Scribner Associates Corrware Software with EG&G Princeton applied Model 273 potentiostat/galvanostat and a Solartron 1255 frequency analyzer. The electrode was left on open circuit till it s potential stabilized. After the potential stabilized, non-destructive evaluation of the surface was done using linear polarization and impedance analysis. During linear polarization, the potential was varied 10 mv above and below the open circuit potential of the mineralized sample at a scan rate of mv/s. The impedance data generally covered a frequency range of 5 mhz to 10 khz. A sinusoidal ac voltage signal varying by ± 10 mv was applied. The electrode was stable during the experiments and its open circuit potential changed less than 1 mv. Separately, samples were prepared for SEM and EDAX analysis. Surface images of bare and mineralized galvanized panels were obtained with a Hitachi S-2500 Delta SEM. Constitutive elements on the surface of the panels were analyzed using energy dispersive analysis with X-rays (EDAX). RESULTS AND DISCUSSION PART I: ELECTROLESS DEPOSITION STUDIES The bath used was obtained from PQ Corp. and is a N silicate solution with a SiO 2 :Na 2 O ratio of At low ph, silicate polymerizes or gels. Increasing the ph is a prerequisite for maintaining the stability of the bath. The soluble silicate is a complex mixture of silicate anions. Increasing the silica to alkali ratio more than two results in condensing some of the silicate to polymeric (colloidal) silica. Further larger anions which are two or three dimensional condensation products of silica monomer form with increase in SiO 2 :Na 2 O ratio. Table I presents the corrosion resistance data of electroless plated samples prepared under different temperatures. Page 4/11

5 In general increase in temperature from 25 o C to 75 o C leads to an increase in resistance. However, increasing the temperature further to 85 o C results in decrease in resistance. These results indicate that 75 o C is the optimum temperature for mineralization for electroless samples. Next, the effect of ph was studied. Table II presents the corrosion resistance data for electroless plated samples prepared at ph 10.5 and 11. Increasing the ph from 10.5 to 11 leads to an increase in average resistance. It is seen that samples that are rinsed later have better corrosion resistance than samples rinsed immediately. Similar results are also seen for samples prepared at different temperatures. Figure 1 presents a comparison of the SEM and EDAX analysis of samples rinsed immediately and rinsed later. It is seen that the samples that are rinsed immediately have no Si on the surface indicating the absence of any mineralization on the surface. The samples that have been left to dry for one day (rinse later) have approximately 12% Si on the surface. The presence of Si surface films contributes to the increased corrosion resistance seen in Tables I and II. Table III presents the effect of deposition time on the corrosion resistance for samples deposited at 75 o C. From Table III it can be seen that the average resistance does not change significantly with increase in deposition time. In general the resistance remains in the range of Ohm-cm 2. After 5 minutes of mineralization we get an average resistance of Ohm-cm 2. Increasing the deposition time beyond 5 minutes results in an increase in corrosion resistance. Effect of Concentration on Electroless Process: One of the key results obtained from the previous set of studies is the increase in resistance for samples that were left to dry for one day. This indicates that Si left on the surface dries and crystallizes. Since, it is obvious that higher amount of Si lead to better corrosion resistance it was decided to vary the concentration of Elisha bath and study the effect. Hence, the goal of this study was to increase the concentration of PQ solution more than that used by Elisha. In order to achieve a uniform and stable coating of Si on the surface we wanted to study the effect of varying the concentration of PQ solution on the mineral characteristics. This was studied by varying the PQ solution concentration from 1:1 to 1:8 (Elisha bath). A 1:1 solution is prepared by adding 1 part of PQ solution (as received) to 1 part of water. Table IV presents the corrosion resistance of samples mineralized in different PQ solutions without any applied Page 5/11

6 current. The corrosion resistance increases with increase in solution concentration. Since, the SiO 2 concentration in the bath increases from 1:8 to 1:1 solution, this could cause more amount of silica to condense on the surface and thereby increase the corrosion resistance. For 1:8 and 1:4 PQ solution the resistance does not remain constant all over the surface. However, for 1:3 solution and concentrations higher than that resistances in the range of 10 5 Ohm-cm 2 are consistently seen across the surface. Since, for mix ratios lower than 1:3 no uniformity in resistance is seen, 1:3 PQ solution represents the optimum bath concentration to obtain uniform corrosion resistance. However, when these samples are left in water, the corrosion resistance drops quickly. This is due to the entry of water through the cracks present on the surface. To improve this property various additives were introduced to the plating solution and the corrosion resistance was studied. Among different additives studied, sodium borohydride showed the best results. We next present the results of studies done with different concentrations of borohydride. Effect of Borohydride Addition: In this study the goal was to evolve hydrogen on the surface of the galvanized sample during electroless deposition. During electroplating hydrogen is evolved on the surface of the cathode and the rate of hydrogen evolution can be controlled by varying the applied potential or current. However, in case of electroless deposition this can be accomplished only with the use of selected reducing agents. Common reducing agents used in literature are sodium hypophosphite, sodium borohydride, dimethyl amino borane and hydrazine. Among these the last one hydrazine is highly toxic and cannot be used for our studies. Sodium hypophosphite precipitates in PQ solution and leads to instability of the bath itself. Dimethyl amino borane is stable in solution but inhibits the precipitation of silicates on the surface of zinc. Sodium borohydride exhibits none of these characteristics and has been explored in detail for getting better deposits. Deposition was done in 1:3 PQ bath, which was optimized in the previous study. Different amounts of sodium borohydride were added to 1500 ml of this solution. The cathodic process in this case can be written as follows: 2H e H The hydrogen that evolves during the cathodic process gets adsorbed on the surface and does not allow any mineralization to occur. Since, hydrogen evolution occurs at all times with borohydride, the resistance decreases with increase in borohydride concentration. An alternate explanation could be that borohydride adsorbs strongly on the surface of Zn and does not allow 2 Page 6/11

7 SiO 2 precipitation. Figure 2 presents the Si concentration for samples mineralized with borohydride in 1:3 PQ bath. Comparison between samples dried in air and heated at 175 o C is also shown. For samples without any heating, the Si content increases with increase in borohydride concentration. However, the opposite is seen for samples heated at 175 o C. Here the Si content decreases initially with increase in borohydride concentration. However no subsequent decrease in Si concentration beyond 5 g is seen. The samples prepared using NaBH 4 were also subjected to stability studies. The stability of the coatings prepared without any post-treatment temperature has been given in Table V. As seen from the table, the stability of the coatings is improved with the addition of Sodium Borohydride. The average corrosion resistance of the coatings prepared using 10 g NaBH 4 drops from Ohm-cm 2 to Ohm-cm 2. In comparison, the resistance of the samples prepared without any NaBH 4 drops from Ohmcm 2 to 580 Ohm-cm 2. Figure 3 gives these results in the form of a plot. Our next goal was to determine the amount of Si on the surface of the samples with different concentrations of borohydride by cyclic voltammetry. These studies were done on samples mineralized in 1:3 PQ bath with different amounts of NaBH 4. Subsequent to deposition, the samples was left to dry in air for 24 hours. They are then rinsed and the studies were done in a three-electrode setup using calomel reference electrode in 0.5 M Na 2 SO 4. Voltammograms were obtained by recording the current while varying the sample potential from 1.6 V to 0.8 V and back to 1.6 V at a scan rate of 5 mv/s. Figure 4 presents the CVs obtained for samples prepared with different amounts of NaBH 4 and then left to dry in air for 24 hours. The CV obtained from the Bare Zn galvanized sample has been shown for comparison. The currents on shifting the potential from 1.6 V to more positive values than 1.1 V correspond to corrosion of the surface layer. Increasing the potentials to values more positive than 1.1 V leads to stripping of Zn from the substrate. In the reverse scan Zn deposition happens and a peak appears in the current corresponding to the mass transfer limited current. Similar results are seen for all samples. However, the peak reduction current and the maximum in the oxidation current decrease rapidly with the SiO 2 -coated samples. Since, the currents are dependent on the amount of material lost from the surface the CVs can be used to obtain a rough estimate of the inhibiting efficiency of silica on Zn. The inhibiting efficiency can be obtained from the following expression: Page 7/11

8 Peak Current in Elisha sample Inhibiting Efficiency(%)= Peak Current in Baresample X100 Figure 5 shows the inhibiting efficiency of the samples mentioned above. We can see that increasing the amount of NaBH 4 helps in increasing the inhibiting efficiency. Similar to the corrosion stability test, CVs were performed on the samples with and without heating after placing them under water for 1 week. Figure 6 shows the CVs of the samples prepared with different amounts of NaBH 4 and dried in air for 24 hours. We can see that the currents from the samples have increased to the order to 1 ma (in the case of 5 g NaBH 4 ). This observation has been summarized in Figure 7. Figure 7 shows the change in the inhibiting efficiency of the samples before and after corrosion testing. From these results, we can see that the change in the inhibiting efficiency is the lowest for samples prepared with 10 g NaBH 4 for both with and without post-treatment heating. These results agree well with the corrosion studies performed earlier. Next, we studied the change in the surface morphology and Si concentration of the samples due to immersion in water. Figure 8 shows the SEM pictures of the sample prepared with 10 g NaBH 4 before and after corrosion. We can clearly see from the plot that a crack exists on the surface in the order of 2 µm. These cracks facilitate the entry of water through the coating and attack the underlying surface. This has been clearly shown in Figure 8. We can see that the cracks become large to the order of 8-10 µm. Further, appearance of flakes of Zn can be seen on the surface. EDAX analysis of these samples was done to study the change in the surface Si concentration before and after corrosion. Figure 9 shows the Si content of the samples prepared with different amounts of NaBH 4 and left to dry in air for 24 hours. We can see that the surface Si concentration drops for all the samples but the drop is the lowest for sample prepared with 10 g NaBH 4. From these studies, it can be concluded that samples prepared with no current containing 10 g NaBH 4 in 1:3 PQ solution show better stability than samples prepared by other electroless techniques. Further, studies need to be done to evaluate the effect of Sodium Borohydride on the stability of the coatings. SUMMARY We begin by summarizing the significant results obtained from this study. Figure 10 presents a comparison of the corrosion resistance of samples mineralized under different Page 8/11

9 conditions by electroless deposition. It is seen that samples mineralized by electroless deposition with sodium borohydride possess the lowest corrosion rate. Deposition in the presence of sodium borohydride leads to more stable films. We next present the significant results obtained from this study: As a first step we have been able to replicate Elisha s Mineralization Process without applied current. We have also optimized the deposition ph and bath temperature. SEM and EDAX analysis in combination with impedance data shows that formation of an uniform Si rich layer on the Zn surface leads to better corrosion resistance. It has been shown that the final corrosion resistance and Si content on the surface are directly related. Mineralization in a bath of 1:3 PQ solution leads to a uniform corrosion resistance across the surface. Increasing the bath concentration further does not result in any further significant increase in resistance. However, when placed in water, the samples prepared without current in both Elisha and 1:3 PQ bath show significant drop in resistance for samples. Addition of sodium borohydride to Elisha bath leads to an increase in corrosion resistance for samples without heating. Samples mineralized in the presence of sodium borohydride show larger resistance after one week of corrosion testing than samples mineralized in the absence of NaBH 4. Page 9/11

10 LIST OF TABLES Table I Table II Table III Table IV Table V Comparison of corrosion resistance for samples mineralized at different temperatures in Elisha bath without any applied current. Comparison of corrosion resistance for samples mineralized at different ph in Elisha without any applied current. Corrosion resistance for samples mineralized in Elisha Bath at 75oC for different times of deposition without applied current. Mineralize in PQ bath (1:3) without current for 15 minutes and dried at 100 o C for 1 hour. Mineralize in PQ bath (1:3) without current for 15 minutes & with NaBH 4 and dried in air for 24 hours and left in water for 1 week. LIST OF FIGURES Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. SEM & EDAX Analysis of Samples Rinsed Immediately and Rinsed Later Comparison of Si Content for samples mineralized in 1:3 PQ solution with no current with different amounts of Sodium Borohydride Drop in Corrosion Resistance for samples mineralized in 1:3 PQ solution with no current with different amounts of Sodium Borohydride. Samples were dried in air for 24 hours and left in water for 1 week CVs for samples mineralized in 1:3 PQ solution with no current with different amounts of Sodium Borohydride. Samples were dried in air for 24 hours Inhibiting efficiency obtained from CVs for samples mineralized in 1:3 PQ solution with no current with different amounts of Sodium Borohydride. Samples were dried in air for 24 hours CVs for samples mineralized in 1:3 PQ solution with no current with different amounts of Sodium Borohydride. Samples were dried in air for 24 hours and left in water for 1 week Change in the Inhibiting efficiency for samples mineralized in 1:3 PQ solution with no current with different amounts of Sodium Borohydride. Samples were dried in air for 24 hours and left in water for 1 week Page 10/11

11 Figure 8. Figure 9. Figure 10. Change in Morphology for sample mineralized in 1:3 PQ solution with no current with 10 g/l of Sodium Borohydride. Samples were heated at 175 o C for 1 hour Change in Si concentration for samples mineralized in 1:3 PQ solution with no current with different amounts of Sodium Borohydride. Samples were dried in air for 24 hours and left in water for 1 week Comparison of Corrosion Resistances of Various Coatings prepared with No Current with Continued Exposure in Water. The Mineralized Samples shown prepared with no post-treatment heating Page 11/11