Studying the effects of some additives on the zinc plating process, oriented application for alkaline non - Cyanide zinc plating bath

acity 20 liters,- Electrolytic machine, 12V-30 A - Hulls cell, 250 ml, - Haring-Blum cell, 400 ml - Analytical balance, technical balance SHIMADZU AEG-220G with accuracy 0.1 mg And some other devices 7 2.3. Methods of analysis 2.3.1. Hull method. 2.3.2. Haring-Blum methob 2.3.3. Method of determining cathode current efficiency 2.3.4. Measure cathode polarization curve. 2.3.5. SEM –Scanning Electron Microscope. 2.3.6. Fourier FTIR transform infrared spectroscopy method 2.3.7. Measuring ring polarization CHAPTER 3. RESULTS AND DISCUSSION The polymers that can be used as additives to the non-cyanide alkaline zinc plating system must be soluble in the plating solution, depending on the molecular weight that the solubility in the alkali galvanizing solution changes or does not dissolve. After investigating the solubility of polymers it is found that polymers should only be studied at concentrations from 0.05 g / L to 1.0 g / L to ensure they are completely dissolved in the plating solutions. The insoluble additives in the plating solution can become impurities, causing precipitates to enter the coating, which can affect the coating quality. 3.1. Effects of Polivinyl ancol (PVA) to zinc plating process PVA has the ability to complex with metal ions and adsorb on metal surfaces when there is an electric current by the carbon-oxygen bonding polarization in the molecular structure, so PVA is used by many authors as auxiliary. surface leveling for plating systems. Quite a few publications mention the use of PVA as an additive to alkaline non-cyanide zinc plating bath 3.1.1. Effects of the molecular weight of PVA on cathodic polarization. To determine the additive effect on the plating process, the method of measuring the steel electrode cathode polarization curve at 250C, scanning from -1.2 to -1.8 V with a scanning rate of 2 mV / s in the capacities translation with and without PVA, results in figure 3.1, figure 3.2 and figure 3.5. 8 Fig 3.1. Effect of PVA - 05 on cathodic polarization, from -1.2 to - 1.8 V, sweep speed 2 mV / s , 250C Fig 3.2. Effect of PVA - 16 on cathodic polarization, from -1.2 to - 1.8 V, sweep speed 2 mV / s , 250C These plots were swept from the open current potential towards the negative direction at a scan rate of 2 mV/s. In all cases, the polarization curves were characterized by appearance of the first cathodic peak (I) followed by either rapid rise in current density for plating bath without PVA or the second cathodic peak (II) for plating baths containing PVA. There was little difference between PVA plating baths at concentrations of 1.0 and 1.5 g/L. For the plating bath without PVA, the cathodic polarization plot had a peak I followed by rapid growth in current density. It was assigned to the reduction of Zn2+ to Zn that is corresponding to the reaction below: Zn(OH)4 2- + 2e- ↔ Zn + 4OH- (3.q) The following four step reaction path has been proposed for the deposition of zinc from zincate solution where reaction (iii) as the rate determining step [13]: Zn(OH)4 2- ↔ Zn(OH)3- + OH- (3.2) Zn(OH)3 - + e- → Zn(OH)2- + OH- (3.3) Zn(OH)2 - ↔ ZnOH + OH- (3.4) ZnOH + e- → Zn + OH- (v) Since Zn2+ preferred to exist as a tetra or hexa-coordinate species, the coordinated Zn(OH)3 - is more likely to exist as Zn(OH)3(H2O) -, thus step (3.3) became as (3.4) below: Zn(OH)3(H2O) - + e- → Zn(OH)2- + OH- + H2O (3.6) It was also possible that PVA replaced the presence of H2O in the complex Zn(OH)3(H2O) - as step (vii). Zn(OH)3(H2O) - + PVA ↔ Zn(OH)3(PVA)- + H2O (3.7) Hence, energy is needed to break the PVA complex to deposit zinc on steel surface. It might be the reason of appearance of the peak II. Moreover, PVA was able to adsorb on the peaks of substrate surface, which inhibited zinc deposition at peaks and promoted discharge of zinc ions at valleys, 9 because PVA owed to the polarity of the carbon-oxygen bond. Consequently, leveling effect was formed on surface. However, these hypotheses should be investigated by further studies. 3.1.2. Study the effect of the additives in the plating process by method cyclic voltammogram a. Study the effect of PVA in the plating process by method cyclic voltammogram Polarization curves were measured in PVA-containing and non-PVA zinc- plating solutions to study the effect of PVA on potential values and maximum currents.. Table 3.1. Potential values, excesses and currents at the pips of the plating in a solution with and without PVA ECo VAg/AgCl (V) EI’c VAg/AgCl (V) EI’c VAg/AgCl (V) ∆𝐸’c (V) ∆E”c (V) Ip(I’c) (mA/cm2) Ip(I”c) (mA/cm2) So -1,48 Sp2-4 -1,48 -1,54 -1,65 -0,06 -0,17 -27,35 -30,90 Fig 3.3. Cyclic voltammogram of the steel electrode was measured in alkaline zinc plating solutions without additives from -1.2 to -1.65 V, with a scan rate of 2 mV/s, at 25°C Fig 3.4. Cyclic voltammogram of the steel electrode was measured in alkaline zinc plating solutions with and without PVA-16 from -1.2 to -1.65 V, with a scan rate of 2 mV/s, at 25°C The steel electrode ring polarization curves measured in alkaline galvanized solution give results, the peaks I'c and I ”c are equivalent to the peaks (I) and the peaks (II) of the plating process. The chemical reaction is shown later (Figs. 3.1 and Fig. 3.2), Ia is the anode current corresponding to the dissolution of the coating. b.Study on effects of PVA on diffusion process 10 Fig 3.5. Cyclic voltammogram of the steel electrode was measured in alkaline zinc plating solutions (S0) -1,2 to -1,65 V, tốc canning rate change, 250C Fig 3.6. Graph of the dependence of i on v1/2 scan in alkaline zinc plating solutions (S0) Research results of the study on the effects of PVA on the galvanizing process in non-cyanide alkali galvanizing solutions by the ring polarization scanning method showed that the presence of PVA in the plating solution increases the potential in plating solution at the same time reducing the current density at the adsorption peaks. The results show that the presence of PVA in the plating solution makes the slope a (reflecting diffusion coefficient D) of the line i dependent on v1/2, it can be said that PVA-05 and PVA- 16 both increase the diffusion potential in the plating solution. The ability of the additive to cover surface (Ꝋ) is calculated by the formula: Ꝋ = 𝑖−𝑖𝑠 𝑖 (3.8) Where i is the current density without additives, is the current density without additives. Impact level Ꝋ is determined at the potent at -1,65 for results Table 3.2. Table 3.2. Coverage of PVA Dung dịch Ꝋ Ꝋ1 Ꝋ2 D S0 2,446 Sp1-4 0,53 0,55 0,23 -112,9 Sp2-4 0,67 0,61 0,47 -128,9 The results showed that PVA adsorbs the cathode surface at convex peaks, this adsorption process prevents metal precipitation at the protrusion points, metal precipitates at the convex peaks decrease, the metal will precipitate at adjacent concave positions to level the surface. This adsorption process also reduces the rapid increase in particle size, when the metal precipitates at a point, that point will rise higher, and react (3.9): Zn2+ + 2e- = Zn (3.9) The electron density at that location will decrease compared to the surrounding positions, the precipitated zinc position will have a more positive charge than the 11 surrounding position, PVA molecule has negative polarized OH (OH -) will go to the surface adsorption to the surface to hinder the precipitation process, the particle size does not increase, but more new particles appear in the vicinity, this process produces seeded seeds with size Small smooth, coating surface more even. 3.2.3. Effect of molecular weight PVA on the zinc plating process (Hull method) The Hull method shows that when adding PVA to plating solutions with different concentrations, it has the effect of smoothing crystals compared to coatings in solutions that do not contain PVA. As the PVA concentration increases, the surface of the zinc precipitate becomes smoother, the gloss and gloss are enlarged. It can be explained that in the presence of PVA, the reaction (3.6) was changed. PVA can replace the presence of H2O in Zn (OH)3 (H2O -) and become Zn(OH)3(PVA) - as in reaction (3.7) above. As a result, the reaction (3.6) becomes (3.10) below: Zn(OH)3(PVA) - + e → Zn(OH)2- + OH- + PVA (3.10) Assuming that the reaction (3.10) is much slower than (3.6) due to the energy required to break the PVA complex, will explain PVA's seed crystal smoothing properties in the plating solution [12]. The results of the study on the influence of PVA on the plating process by Hull method, the results are consistent with the polarization curves. If the PVA concentration increases, the Zn(OH)3 (PVA) complex - produces more and therefore the zinc precipitate requires more energy to break down the complex, leading to a decrease in the galvanic current density in the plating sample in the capacitance. solution with high PVA concentration. Fig 3.14. Hull cell pattern obtained from alkaline non-cyanide bath containing PVA-05 with various concentrations Fig 3.15. Hull cell pattern obtained from alkaline non-cyanide bath containing PVA-16 with various concentrations 12 3.1.3. Effect of molecular weight PVA to SEM images of the sample plated in alkaline bath. Fig 3.16. SEM images of the sample plated in alkaline bath PVA – 05, 0,5 A/dm2 The presence of PVA-16 and PVA-05 in the plating solution reduces the particle size, changes SEM images, semi bright. 3.1.4. Effect of molecular weight PVA to throwing power and performance a. Effect of molecular weight PVA to throwing power. Table 3.7. Throwing power TT C (g/L) Throwing power(0,5 A/dm2) Throwing powerở (2 A/dm2) PVA-05 PVA - 16 PVA-05 PVA - 16 1 0 30,2 30,2 25,9 25,9 2 0,05 40,1 42,7 37,8 40,6 3 0,10 47,9 44,1 44,9 49,2 4 0,25 62,3 55,8 56,3 52,1 5 0,50 64 76,2 64,7 64,3 6 1,0 72,2 77,2 70,9 70,3 Table 3.9 results show that adding PVA in plating solution increases the throwing power of the plating process. The distribution increase is highly dependent on the PVA concentration in the plating solution, while less dependent on the working current density. PVA-16 has greater molecular mass than PVA-05, and also has a greater impact on throwing power than PVA-05. a. Effect of molecular weight PVA to plating performance. 13 Table 3.10. Performance of plating system with and without PVA TT C (g/L) performance (0,5 A/dm2) performance (2A/dm2) PVA -05 PVA - 1600 PVA-05 PVA - 1600 1 0 80,7 80,7 79,2 79,2 2 0,05 72,7 73,9 39,91 56,65 3 0,10 67,1 69,3 23,53 41,8 4 0,25 49,08 55,8 11,59 19,89 5 0,50 34,92 36,2 7,56 9,15 6 1,0 15,93 15,2 7,18 7,43 The presence of PVA reduces plating efficiency. 3.2. Effect of BT on the galvanizing process 3.2.1. Effect of BT on cathodic polarization The polarization lines are measured in alkakine on-cyanide zinc plating solution with and without BT variable molecular and concentration to evaluate of BT on plating process. Fig 3.20. Effect of BT on cathodic polarization, -1,2 to -1,8V, 2mV/s, 250C The results showed that, the BT with different molecular weights, added to the plating solution at the same concentration, the BT has a higher molecular weight than BT-200, BT-700, has less effect on polarization. cathode than BT with low molecular weight BT-12, BT-18 (Fig 3.21). Due to the same concentration, the low molecular weight exercises contain more molecules, participating in the reaction at more locations.. 3.2.2. Effect of BT on the galvanizing process cyclic voltammogram 14 Table 3.9. Peak values of galvanizing process in solution with and without BT-18 solution ECo VAg/AgCl (V) EI’c VAg/AgCl (V) EI’c VAg/AgCl (V) ȠI’c (V) ȠI”c (V) Ip(I’c) mA/cm2 Ip(I”c) mA/cm2 S0 -1,48 SB2-1 -1,48 -1,52 -1,59 -0,04 -0,11 27,70 40,7 SB2-2 -1,48 -1,52 -1,56 -0,04 -0,08 18,60 43,18 SB2-3 -1,48 -1,53 -1,59 -0,05 -0,05 32,70 39,76 SB2-4 -1,48 -1,53 -1,6 -0,05 -0,05 30,90 41,58 SB2-5 -1,48 -1,52 -1,65 -0,04 -0,04 18,50 43,10 Fig 3.23. Cyclic voltammogram of the steel electrode was measured in alkaline zinc plating solutions (S0)+ BT-700, -0,5 đến -1,65 V, 2 mV/s, 250C Table 3.10. Peak values of galvanizing process in solution with and without BT-700 Solution ECo VAg/AgCl (V) EI’c VAg/AgCl (V) EI’c VAg/AgCl (V) ȠI’c (V) ȠI”c (V) Ip(I’c) mA/cm2 Ip(I”c) mA/cm2 S0 -1,48 SB4-1 -1,48 -1,54 -1,60 -0,06 -0,12 42,37 53,6 SB4-2 -1,48 -1,53 -1,60 -0,05 -0,12 29,70 55,63 SB4-3 -1,48 -1,53 -1,60 -0,049 -0,12 24,23 38,90 SB4-4 -1,48 -1,51 -1,60 -0,032 -0,12 13,85 29,30 SB4-5 -1,48 -1,51 -1,60 -0,032 -0,12 11,80 28,50 15 Fig 3.26 Cyclic voltammogram of the steel electrode was measured in alkaline zinc plating solutions (S0)+ BT-700 -1,2 to -1,65 V, tốc canning rate change, 250C Fig 3.27. Graph of the dependence of i on v1/2 scan in alkaline zinc plating solutions (S0)+ BT-700 Table 3.11. Values on the graph of the dependence of i on v1/2 scan in alkaline zinc plating solutions (S0)+ BT-700 R2 Coefficient a (reflects diffusion coefficient D) b S0 0,9942 244,6 -6,3 SB2-4 0,9499 -181,3 0,045 SB4-4 0,9678 -77,7 5,63 The results show that the presence of BT in the plating solution makes the slope a (reflecting coefficient D) of the dependent line of i in v1 / 2, it can be said that BT- 18 and BT 700 both increase the diffusion potential in the plating solution. Table 3.12 Coverage of BT Solution Ꝋ Ꝋ1 Ꝋ2 D S0 244,6 SB2-4 0,49 0,37 0,25 -181,3 SB4-4 0,61 0,56 0,43 -77,7 Table 3.12 shows that BT adsorb the cathode surface at convex peaks, this adsorption process prevents metal precipitation at the protruding points, the metal precipitates at the convex peaks is reduced, the metal will precipitate at adjacent concave positions to level the surface. This adsorption process also prevents the rapid increase in particle size, when the metal precipitates at a point, the point will rise higher, and the reaction occurs(3.9): Zn2+ + 2e = Zn 16 At the current density at that location lower than the surrounding locations, the precipitated zinc position will have a more positive charge than the surrounding positions, the BT molecule has the function group -N = has double Free electrons will go to, adsorbed on the surface to hinder precipitation, the particle size does not increase, but more new particles appear in the vicinity, this process creates seed plated with small size smooth, coating surface more even. BT with larger molecular weight has higher coverage than BT with smaller molecular weight. Studying the stability of the plating process in a solution containing polyamide additive Measurement of the 10-sweep polarization curve in the plating solution containing BT, the results show that from round 1 to round 6 the peak height decreases, this shows that in the first scan the coating still has convex peaks, Convex vertices are leveled after sweep rings. After the 5th round of scanning, the rings from 6, 7, 8, 9, 10 have the same peak heights, this shows that the coating surface has become flat after 5 rounds of scanning.. 3.3.4. Effect of molecular weght BT to brightness and bright range (Hull) Fig 3.29. Hull cell pattern obtained from BS containing BT-12 Fig 3.30. Hull cell pattern obtained from BS containing BT-18 17 Table 3.13. Effect of the number of substituents and molecular weight of polyamines on brightness and bright ranges of zinc deposits in non-cyanide alkaline plating bath TT Additive content (g/L) Semi-bright ranges (A/dm2) The highest brightness of samples at 60° BT-700 BT20 BT-18 BT-12 BT-700 BT20 BT-18 BT-12 1 0,00 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 2 0,05 0,0 < 6,0 < 2,0 < 1,0 0,0 2,6 2,1 1,4 3 0,1 0 0,0 < 6,5 < 5,5 < 3,0 0,0 4,3 4,0 3,7 4 0,25 <5 <5,0 < 5,5 < 4,0 11,6 7,2 8,3 5,4 5 0,5 0 0,7÷10 <7,0 < 4,0 < 5,0 51,4 39 15,7 9,5 6 1 ,00 <10 Cả tấm < 3,0 <10,0 56,7 38,0 14,0 6,1 The BT is added to the alkali galvanizing solution at a concentration of 0.05 g / L for a smoother coating, which looks more uniform at variable current densities, a better coating distribution on the Hull plate than coating in alkaline zinc solution without BT. When increasing the BT concentration in the plating solution to 0.1; 0.25; 0.5; 1.0 g / L coating on the Hull plate has a wider semi-gloss, in the gloss semi-gloss area increases. BT with low molecular weight BT-12, BT-18 affects the surface leveling process, the crystal smoothing is less than BT with high molecular weight BT-200, BT-700. BT-200 is added to the plating solution at a concentration of 1 g / L semi-gloss over the entire Hull plate. However, the measured glossiness is reduced compared to the 0.5 g / L sample. Effect of BT on coating glossiness of: BT-700, BT-200> BT-18, BT-12. The influence of BT on coating glossiness of: BT-700, BT-200> BT-18, BT-12. After conducting the research according to Hull method, it is preliminarily assessed the influence of the molecular mass of BT on the plating process (gloss and coating gloss). More studies are needed to evaluate the effect of the BT molecular mass on the plating process. However, further studies only need to be conducted in 18 the area of flow density where these additives are most effective. Select current density of 0.50 A / dm2 and 5.0 A / dm2 for further studies 3.3.5. Effect of BT to SEM images Fig 3.40. SEM images of the sample plated in alkaline bath BT – 700, 0,5 A/dm2 The results show that the high molecular weight polyamide BT-12 and BT-18 affect the surface morphology, the larger, the low molecular weight polymins BT- 200 and BT-700. Image Figs of the coating in an additive-free solution at current densities of 0.5 A / dm2 and 5.0 A / dm2 (M0) show that, when plating in an additive- free solution, the image current density greatly affects the seedling size 3.3.6. Effect of molecular and concentration BT to performance and Throwing power a. Effect of molecular and concentration BT to performance Table 3.14. Effect of molecular and concentration BT to performance TT Additive content (g/L) performance (0,5 A/dm2) performance (5 A/dm2) BT-12 BT-18 BT-200 BT-700 BT-12 BT-18 BT-200 BT-700 1 0,0 80,7 80,7 80,7 80,7 79,2 79,2 79,2 79,2 2 0,05 25,5 23,8 81,8 59,8 28,1 27,8 39,3 70,9 3 0,1 0 19,2 19,1 79,4 57,4 25,3 27,2 35,3 47,1 4 0,25 18,1 18,3 63,6 53,3 21,1 22,4 31,1 31,4 5 0,50 17,2 18,3 58,2 46,3 17,6 17,2 25,9 22,2 6 1,0 14,1 16,7 46,1 36,7 17,1 16,9 22,1 21,9 The presence of BT in the plating solution, it reduces the plating performance compared to the plating sample in a polyamide-free plating solution.. b. Effect of molecular and concentration BT to throwing power(Haring - Blum) 19 Table 3.15. Effect of molecular weight and concentration BT to throwing power TT Additive content (g/L) Throwing power(0, 5A/dm2) (%) Throwing power (5 A/dm2) (%) BT-12 BT-18 BT-200 BT-700 BT-12 BT-18 BT-200 BT- 700 1 0,0 80,7 80,7 30,2 30,2 25,9 25,9 25,9 25,9 2 0,05 38,2 38,9 37,5 48,4 38,6 41 37,2 41,8 3 0,10 39 39,2 38,8 49,5 39,8 42,9 39 42,4 4 0,25 44,9 45 42,5 58,3 45,3 45,6 43,1 43,6 5 0,50 51 50,5 49,6 60,8 53,7 51,2 52,6 46,3 6 1,0 58,7 60,1 60 66,6 62,1 59,3 61,3 49,1 Table 3.15. It was shown that when poliamin was added to plating solution with different concentrations and molecular weights increased distribution compared to plating in non-BT plating solutions. The increase in distribution depends much on the concentration, molecular weight and on the BT working current density in the plating solution. When plating at a high working current density, the distribution is inferior to that of a seedling at a low working current density. 3.4. Effect of natrisilicate and polyamide - natrisilicate system on zinc plating process. 3.4.1. Effect of natrisilicate and polyamide - natrisilicate on cathodic polarization After researching, the effect of molecular weight and polyamide concentration on zinc plating in alkaline plating bath without cyanide, BT-700 concentration 0.5 g / L was selected as the base additive. Poliamin BT-700 crystal smooth, for semi-gloss coating, high measured gloss, about 0.8 to over 10.2 A/dm2 semi-gloss. However, the coating surface is not uniform, the test should be conducted very carefully because it is very sensitive and difficult to use in industry. Plating in the plating solution contains only additive BT-700 for low cathode efficiency, at a current density range <0.8 A /dm2 the dark coating, so it is necessary to combine with a second additive to increase stability. of the base additive in the plating system, increasing the plating efficiency and extending the semi-gloss towards the low current density. The results showed that Poliamin and natrisilicate, added to the plating solution, both increased cathode polarity compared with measurements in polyamide and natrisilicate-free plating solutions. Natrisilicate increases cathode polarity but does 20 not change the zinc precipitation substitution, while the polyamide shifts the precipitation potential of zinc to a more negative side, từ -1.48 lên -1.62 V (Fig 3.43a). Fig 3.43. Effect of natri silicate and polyamide – natri silicate on cathodic polarization -1,2 đến -1,8 V, tốc độ quét 2 mV/s, 250C The results showed that when the concentration of natrisilicate in the plating solution increases, the cathode polarization increases. Measurement in a plating solution with a natrisilicate concentration of maximum 8 g / L for maximum polarity. The addition of 8 g / L natrisilicate to the alkaline galvanizing solution showed that cathode polarity increased as the natrisilicate modulus increased. The polarization lines measured in alkaline zinc solutions with poliamin and natrisilicate of different modules have an adsorption peak, the absorption peak shifts to a more negative direction when the natrisilicate modulus is increased.. 3.4.3 Ảnh hưởng của poliamin và natrisilicat đến độ bóng và khoảng bóng lớp mạ kẽm trong bể mạ kiềm không xyanua theo phương pháp Hull. Fig 3.44. . Hull cell pattern obtained from plating solutions containing natri silicate and polyamide – natri silicate. 21 Row 1: Hull sample plated in corresponding solutions, (b) S0: NaOH solution 14 g / L + ZnO 15 g / L, (a) SB4-4 solution S0 + 0.51 g / L BT- 700, (c) Solution S0 + 4 g / L natrisilicate, Row 2: Hull samples plated in the respective solutions, (d) Mn1-1: S0 + 4 g / L modulus natrisilicate 1 (e) Mn1-2: S0 + 8 g / L modulus natrisilicate 1, (f) Mn1 -3: S0 + 16 g / L modulus natrisilicate 1, Row 3: Hull samples plated in the corresponding solutions, (d) Mn2-1: S0 + 4 g / L modulus natrisilicate 2,5 (e) Mn2-2: S0 + 8 g / L modulus natrisilicate 2,5, (f) Mn2-3: S0 + 16 g / L modulus natrisilicate 2,5, Row 4: Hull samples plated in the corresponding solutions, (d) Mn3-1: S0 + 4 g / L modulus natrisilicate 3 (e) Mn3-2: S0 + 8 g / L modulus natrisilicate 3, (f) Mn3 -3: S0 + 16 g / L modulus natrisilicate 3, The research results show that poliamin acts on all research properties, for the galvanizing process in alkaline plating baths without cyanide. While natrisilicate only affects some properties of the plating process. Poliamin increases cathode polarization and shifts the precipitation potential of zinc to a more negative side from -1.48 to -1.62 V. Natrisilicate affects only cathode polarization but does not change the zinc precipitation potential. The Hull method shows the presence of polyamide in crystal smoothing solution, semi-gloss coating on almost the entire Hull sheet. Adding natrisilicate creates a brighter and more uniform coating. The study of surface morphology showed that adding more poliamin, the particle size decreased sharply and there was no grain. The particle size is about 5 to 7 µm in the plating sample in a solution not contained in polyamide to a flat surface with no visible particles in the plating sample in the polymer addition solution. Adding only natrisilicate did not significantly reduce the size of the particles. However, more uniform particles were found. The addition of natrisilicate and in the plating solution containing polyamide more uniformity is observed on the coating surface. The addition of natrisilicate has little effect on the platin