Influence of galvanic coupling on the formation of zinc phosphate coating

Transcription

1 Indian Journal of Chemical Technology Vol. 17, May 2010, pp Influence of galvanic coupling on the formation of zinc phosphate coating M Arthanareeswari 1 *, T S N Sankara Narayanan 2, P Kamaraj 3 & M Tamilselvi 4 1,3 Department of Chemistry, Faculty of Engineering & Technology, SRM University, Chennai , India 2 National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai , India 4 Department of Chemistry, Arignar Anna Government Arts College, Villupuram , India marthanareeswari@yahoo.com Received 17 August 2009; revised 31 March 2010 The influence of galvanic coupling of mild steel (MS) with titanium, copper, brass, nickel and stainless steel (SS) on the phosphatability is elucidated. The galvanic couple accelerates metal dissolution, enables quicker consumption of free phosphoric acid and facilitates an earlier attainment of point of incipient precipitation, resulting in higher amount of coating formation. The surface morphology of the coatings exhibit more uniform coating for the mild steel substrates phosphated under coupled conditions. XRD pattern of the zinc phosphate coating formed under coupled condition confirms the presence of phosphophyllite rich coating. The potential-time measurements are also carried out. The study reveals that galvanic coupling of mild steel with metals that are nobler than steel during phosphating proved to be beneficial in accelerating the coating formation. Keywords: Zinc phosphate, Corrosion resistance, Galvanic couple, Mild steel Phosphating is the most widely used metal pretreatment process for the surface treatment and finishing of ferrous and non-ferrous metals. Due to its economy, speed of operation and ability to afford excellent corrosion resistance, wear resistance, adhesion and lubricative properties, it plays a significant role in the automobile, process and appliance industries 1 4. Majority of the phosphating baths reported in literature require very high operating temperatures ranging from 90 to 98 C. The main drawback associated with high temperature operation is the energy demand, which is a major crisis in the present day scenario. Besides, the use and maintenance of heating coils is difficult due to scale formation, which leads to improper heating of the bath solution and require frequent replacement. Another problem is overheating of the bath solution, which causes an early conversion of the primary phosphate to tertiary phosphate before the metal has been treated that results in increase in the free acidity of the bath and consequently delays the precipitation of the phosphate coating 5. One possible way of meeting the energy demand and eliminating the difficulties encountered due to scaling of heating coils and, over heating of the bath, is through the use of low temperature phosphating baths. Though known to be in use since the 1940s 6, the low temperature phosphating processes have become more significant today due to the escalating energy costs. However, low temperature phosphating processes are very slow and need to be accelerated by some means. Acceleration of the phosphating process could be achieved by chemical, mechanical and electrochemical methods. However, each of them has some limitations and/or detrimental effects. Chemical accelerators are the preferred choice in many instances. The use of nitrites as the accelerator is most common in low temperature operated phosphating baths. However, a higher concentration of nitrite is required to increase the rate of deposition of phosphate coatings at low temperatures. The environmental protection agency (EPA) has classified nitrite as toxic in nature and hence use of nitrite as accelerator could cause disposal problems 7. The utility of the galvanic coupling for accelerating low temperature zinc phosphating processes was established recently The present work aims at to study the utility of galvanic coupling for accelerating the low temperature zinc phosphating and to elucidate the effect of cathode materials such as titanium, copper, brass, nickel and stainless steel on the phosphatability of mild steel.

2 168 INDIAN J. CHEM. TECHNOL., MAY 2010 Experimental Procedure Mild steel specimens (hot rolled; composition conforming to IS 1079 specifications) of dimensions cm were used as the substrate materials for the deposition of zinc phosphate coating. Titanium, copper, brass, nickel and stainless steel (AISI 304 grade) substrates were used to create the galvanic couple with mild steel substrate with varying anodic to cathodic area ratio. The structural characteristic of the zinc phosphate coating was evaluated by X-ray diffraction measurement using Cu Kα radiation. The surface morphology of phosphated steel samples using galvanic coupling was assessed by scanning electron microscope (SEM), Cambridge Instruments (Model: Stereoscan 360). The chemical composition of the zinc phosphating bath and its operating conditions are given in Table 1. Table 1 Chemical composition, control parameters and operating conditions of the bath used for zinc phosphating by galvanic coupling ZnO H 3 PO 4 NaNO 2 Chemical composition 5 g/l 11.3 ml/l 2 g/l Control parameters ph 2.7 Free acid value (FA) 3 pointage Total acid value (TA) 25 pointage FA:TA 18:33 Operating conditions Temperature 27ºC Time 30 Min Same operating conditions and phosphating bath were used for phosphating the uncoupled mild steel for comparison. The chemical compositions of the mild steel and of the cathode materials used are given in Table 2. Phosphating was done by immersion process. The amount of iron dissolved during phosphating and coating weight were determined in accordance with the standard procedures 11. The schematic diagram of the experimental setup used for the phosphating process is given in Fig. 1. The potential time measurements during phosphating were carried out using a multimeter (model 435 Systronics Digital Multimeter) against the saturated calomel electrode (SCE) using a luggin capillary. The oxygen reduction Fig. 1 Schematic diagram of the experimental setup used for the phosphating process Table 2 Chemical composition of (a) Mild steel (b) Stainless steel (c) Nickel (d) Brass (e) Copper and (f) Titanium Element C Si Mn P S Cr Ni Mo Fe Wt% Balance (b) Element C Si Mn P Ni Cr S Fe Wt. % <0.08 < <0.030 Balance (c) Element Ni Wt. % (d) Element Pb Zn Fe Cu Wt. % (e) Element Cu Wt. % (f) Element N C H P Fe O Ti Wt% Balance (a)

3 ARTHANAREESWARI et al.: INFLUENCE OF GALVANIC COUPLING ON FORMATION OF ZINC PHOSPHATE 169 current density was measured using a potentiostat / galvanostat frequency response analyzer of ACM instruments (model: grill AC). Results and Discussion Effect of cathode materials The effect of galvanic coupling of mild steel substrate with titanium, copper, brass, nickel and stainless steel substrates on the amount of iron dissolved during phosphating and coating weight is given in Table 3. The corresponding values obtained for uncoupled mild steel substrate are also included in the same table for an effective comparison. It is evident from the values given in Table 3 that the extent of metal dissolution and of coating formation are higher for mild steel substrates phosphated under galvanically coupled condition than the one coated without coupling. It is understandable that galvanic coupling accelerates the initial metal dissolution reaction and enables an earlier attainment of the point of incipient precipitation (PIP) i.e., the point at which saturation of metal dissolution occurs and higher coating weight results 12. Among the different couples studied, namely mild steel-titanium, mild steel-copper, mild steel-brass, mild steel- nickel and mild steel-stainless steel, the mild steel - titanium couple exerts a greater influence on metal dissolution and coating weight. This is due to higher potential difference between the anode and cathode materials of this couple. The anodic to cathodic area ratio is also a major influencing factor in deciding the extent of metal dissolution and of coating formation. Increase in cathodic area exerts a strong influence on the mild steel anode and increases the extent of metal dissolution, which in turn influences the amount of coating formation. Effect of unaccelerated bath Effect of galvanic coupling of mild steel with stainless steel or titanium on the amount of iron dissolution and phosphate coating formation from unaccelerated bath (without sodium nitrite) is shown in Table 4. Compared to mild steel substrate phosphated under uncoupled condition, the extent of metal dissolution and coating weight are higher for substrates Table 3 Effect of galvanic coupling of mild steel with different cathode materials of varying area ratios (1:1, 1:2, 1:3) on the amount of iron dissolved during phosphating and phosphate coating formation System studied Iron dissolved during phosphating* (g/m 2 ) Coating weight* (g/m 2 ) Uncoupled mild steel Mild steel coupled with stainless steel (area raio-ms:ss-1:1) Mild steel coupled with stainless steel (area ratio-ms:ss-1:2) Mild steel coupled with stainless steel (area ratio-ms:ss-1:3) Mild steel coupled with nickel (area ratio-ms:ni-1:1) Mild steel coupled with nickel (area ratio-ms:ni-1:2) Mild steel coupled with nickel (area ratio-ms:ni-1:3) Mild steel coupled with brass (area ratio-ms:brass-1:1) Mild steel coupled with brass (area ratio-ms:brass-1:2) Mild steel coupled with brass (area ratio-ms:brass-1:3) Mild steel coupled with copper (area ratio-ms:cu-1:1) Mild steel coupled with copper (area ratio-ms:cu-1:2) Mild steel coupled with copper (area ratio-ms:cu-1:3) Mild steel coupled with titanium (area ratio-ms:ti -1:1) Mild steel coupled with titanium (area ratio-ms:ti-1:2) Mild steel coupled with titanium (area ratio-ms:ti-1:3) *Average of five determinations (the standard deviation of the above data is within 0.16 g/m 2 ) Table 4 Effect of unaccelerated bath during phosphating using galvanic coupling of mild steel with stainless steel or titanium System studied Iron dissolved during Coating weight* phosphating* (g/m 2 ) (g/m 2 ) Uncoupled mild steel Mild steel coupled with stainless steel (area ratio of MS to SS-1:3) Mild steel coupled with titanium (area ratio of MS to Ti -1:3) *Average of five determinations (the standard deviation of the above data is within g/m 2 )

4 170 INDIAN J. CHEM. TECHNOL., MAY 2010 phosphated under galvanically coupled condition. It is well established that phosphating reaction from unaccelerated baths tends to be slow owing to the polarization caused by hydrogen evolution at the cathode 13. The very slow rate of recombination of hydrogen atoms to form hydrogen gas causes the formation of a very low coating weight 10. This effect is evident for substrates phosphated both under galvanically coupled and uncoupled conditions. The presence of cathode materials in the phosphating bath initially enhances the iron dissolution, which enables quicker consumption of free phosphoric acid and increases the ph at the mild steel-phosphating solution interface. The increase in ph causes the conversion of soluble primary phosphate to insoluble tertiary phosphate with subsequent deposition of the phosphate coating on mild steel substrate 1-4. Since the surface sites for hydrogen evolution are now shifted from mild steel substrate to cathodic substrates, it is presumed that more surface sites are available on mild steel substrate for coating formation which results in an increased coating weight. Potential - time measurements During phosphating, the potential of the galvanic couple is monitored continuously as a function of time for the entire duration of coating formation. A typical potential-time curve depicting the following classification is shown in Fig. 2. The potential-time curves obtained for mild steel-stainless steel and mild steel-titanium [Fig. 2 (a and b)] could be analysed by the following significant points. Initial potential (A) The initial galvanic potential varies with the nature of cathode material coupled with mild steel substrate. Potential measured at the first minute during coating formation in a phosphating bath having 30 min processing time is indicative of the nature of the metal surface undergoing corrosive attack by the free phosphoric acid present in the bath 14. Galvanic coupling of cathode materials with the mild steel substrate is found to shift the measured potential at the first minute to a less negative value as compared to the initial potential of uncoupled mild steel. Fig. 2a Variation of potential with time during phosphating of uncoupled mild steel and mild steel-stainless steel couple (area ratio of MS to SS 1:1, 1:2 and 1:3) Fig. 2 A typical potential-time curve depicting the classification of different points of the curve to analyze the changes that occur during phosphating using galvanic coupling. A - Initial potential; B -Maximum potential; C - Final potential and ti -Induction time Fig. 2b- Variation of potential with time during phosphating of uncoupled mild steel and mild steel titanium couple (area ratio of MS to Ti 1:1; 1:2 and 1:3).

5 ARTHANAREESWARI et al.: INFLUENCE OF GALVANIC COUPLING ON FORMATION OF ZINC PHOSPHATE 171 Maximum potential (B) The maximum potential represents the onset of conversion of soluble primary phosphate to insoluble tertiary phosphate (point of incipient precipitation), following the rise in interfacial ph. At this point, the potential of the galvanic couple is shifted towards more cathodic direction. This is also observed in conventional phosphating process. It is due to the corrosive attack by the free phosphoric acid present in the bath 15. The extent of shift in potential in conventional phosphating process is moderate ( mv) 15. In zinc phosphating, utilizing galvanic coupling the extent of shift in potential from initial to maximum potential (point of incipient precipitation), following the rise in interfacial ph is similar to the conventional phosphating process. However, the maximum potential obtained at this point is found to shift towards anodic values from mild steel-stainless steel couple to mild steel-titanium couple. Increase in the potential difference between the galvanic couple results in a shift in maximum potential towards anodic direction. Final potential (C) The potential near the coating completion time (30 min) can qualitatively suggest the extent to which coating formation has occurred 16. The potential measured at this stage is more anodic for coupled mild steel substrates than the uncoupled mild steel substrate. Among the couples studied, the final potential is more noble for mild steel titanium couple which implies better coating. From the maximum potential there is a shift in the anodic direction. The anodic shift in potential represents the progressive build up of the phosphate coating formation. Even though metal dissolution and coating formation occur throughout the process, the predominant reaction at this stage is the deposition of zinc phosphate coating. The stabilization in potential value noted at the end of phosphating is due to the decrease in the rate of conversion of primary phosphate to tertiary phosphate and hydrogen evolution. The extent of shift in potential from maximum to final potential observed at this stage is due to the competition between hydrogen evolution and deposition of zinc phosphate. Increase in the potential difference between the galvanically coupled mild steel and the cathode materials results in an increased shift in final potential towards anodic direction. Increase in the area ratio between the mild steel and the cathode materials also results in an increased shift in potential towards anodic direction. Induction time (ti) The time taken for saturation of metal dissolution i.e., the induction time (point at which ennobling of potential occurs) is an important parameter in indicating the rate and the extent of coating formation in a phosphating bath 10. Induction time decreases from mild steel-stainless steel couple to mild steel-titanium couple. The decrease in induction period is one of the significant effects of galvanic coupling. This is because the pronounced metal dissolution due to galvanic coupling enhances the consumption of free phosphoric acid at the metal-solution interface and enables an earlier attainment of the point of incipient precipitation. The time taken for attainment of point of incipient precipitation for mild steel-titanium couple is the lowest out of all the five galvanic couples utilized for coating formation. This is due to the higher potential difference between the anode and cathode of this couple which in turn increases the metal dissolution and accelerates the attainment of PIP which results in an increased coating weight. Mechanism of coating formation Conventional phosphating baths consist of dilute phosphoric acid based solutions of one or more alkali metal/heavy metal ions 1-4. These baths essentially contain free phosphoric acid and primary phosphates of the metal ions. When a mild steel substrate is introduced into the phosphating solution, a topochemical reaction takes place, during which the metal dissolution is initiated at the micro-anodic sites on the substrate by the free phosphoric acid present in the bath. Hydrogen evolution occurs at the microcathodic sites. Fe + 2H 3 PO 4 Fe(H 2 PO 4 ) 2 + H 2 The formation of soluble primary phosphate leads to the subsequent depletion of free phosphoric acid concentration in the bath which results in the rise of ph at the metal-solution interface. This change in ph alters the hydrolytic equilibrium that exists between the soluble primary phosphates and the insoluble tertiary phosphates of the heavy metal ions present in

6 172 INDIAN J. CHEM. TECHNOL., MAY 2010 the phosphating bath resulting in a rapid conversion and deposition of insoluble heavy metal tertiary phosphate 1-4. In a zinc phosphating bath, these equilibria may be represented as follows: Zn(H 2 PO 4 ) 2 ZnHPO 4 + H 3 PO 4 3ZnHPO 4 Zn 3 (PO 4 ) 2 + H 3 PO 4 In galvanically coupled condition both metal dissolution and coating formation occur at the mild steel substrate whereas hydrogen evolution occurs at the cathode. While in uncoupled condition all these reactions occur on the mild steel substrate itself. The decrease in the induction period is one of the significant effects of galvanic coupling. This is because of the pronounced metal dissolution resulting from galvanic coupling which forces quicker consumption of free phosphoric acid at the metalsolution interface and enables an earlier attainment of the point of incipient precipitation. Potential-time measurements suggest the occurrence of iron dissolution as the predominant reaction during the initial period, followed by the deposition of zinc phosphate with a simultaneous metal dissolution through the pores of the coating. The continuous evolution of hydrogen at the cathode enables deposition of zinc phosphate on the entire surface of the anode. The continuous evolution of hydrogen visually observed at the cathode material throughout the entire duration of deposition suggests the availability of metallic sites at the mild steel substrate at any given time. In conventional phosphating, the hydrogen evolution also occurs at the mild steel substrate, whereas in using galvanic coupling for zinc phosphating, the surface sites of hydrogen evolution are shifted from mild steel to stainless steel or titanium substrates. It is presumed that more surface sites are available for phosphate coating formation which results in the increased coating weight. Moreover, another advantage resulting from galvanic coupling of mild steel with more noble metals is the formation of phosphate coatings richer in phosphophyllite [Zn 2 Fe(PO 4 ) 2.4H 2 O] phase. With the advent of cathodic electrophoretic painting, the need for phosphate coatings that are richer in phosphophyllite phase is greatly felt as they offer better chemical stability than phosphate coatings richer in hopeite phase, towards the alkaline conditions created during electrophoretic painting. The formation of a phosphophyllite rich coating is expected when the mild steel substrate is galvanically coupled with metals more nobler than it, as the metal-solution interface is most likely to be populated with relatively more amount of ferrous ions than the one phosphated under uncoupled condition. However, the deleterious effect of accumulation of ferrous ions at the metal solution interface is not reflected on the corrosion performance of phosphate coating. The presence of sufficient concentration of nitrite ions in the bath enables the oxidation of ferrous ions to ferric ions, which are subsequently precipitated as ferric phosphate sludge. Surface morphology & XRD SEM images [Fig. 3(a-f)] reveal that galvanic coupling increases the coating formation and improves the fineness of the coating. Coating on mild steel specimens phosphated under uncoupled condition (Fig. 3a) is found to be little less compact. Introducing galvanic coupling [Figs 3(b-f)] gives smooth and compact deposits with reduced porosity. This is confirmed by the electro chemical method of porosity testing. The formation of needle like crystals confirmed the presence of phosphophyllite phase 15. X-ray diffraction pattern (Fig. 4) of zinc phosphate coating formed under coupled condition has shown the presence of both hopeite and phosphophyllite phases. It is proved from the figure that the coating is richer in phosphophyllite phase. Porosity of the phosphate coating The electrochemical method, which measures the oxygen reduction current density, clearly indicates the amount of porosity involved. This method involves the measurement of the oxygen reduction current density when immersed in airsaturated sodium hydroxide solution (ph 12) The current density values measured at -550 mv versus SCE (Table 5) reveal that the panels coated using galvanic coupling have a low porosity value as compared to the uncoupled specimen. The mild steel panel coated using titanium as the

7 ARTHANAREESWARI et al.: INFLUENCE OF GALVANIC COUPLING ON FORMATION OF ZINC PHOSPHATE 173 Fig. 3 Surface morphology of the zinc phosphate coated mild steel specimens: (a) mild steel(ms) under uncoupled condition (b) MS coupled with SS(1:3) (c) MS coupled with Ni (1:3) (d) MS coupled with brass(1:3) (e) MS coupled with Cu (1:3) (f) MS coupled with Ti (1:3) coupling material (area ratio 1:3) has the lowest porosity value when compared to the other mild steel substrates coated using different cathode materials. Thus, it can be concluded that the galvanic coupling of mild steel substrates with the cathode materials during phosphating results in the formation of uniform, fine grained coatings of reduced porosity.

8 174 INDIAN J. CHEM. TECHNOL., MAY 2010 Table 5-Current densities of phosphated mild steel substrates (under galvanically coupled and uncoupled conditions) System studied Current density at 550 mv versus SCE (µa/cm 2 ) Uncoupled mild steel Mild steel coupled with stainless steel (area ratio of MS to SS - 1:1) Mild steel coupled with stainless steel (area ratio of MS to SS - 1:2) Mild steel coupled with stainless steel (area ratio of MS to SS - 1:3) Mild steel coupled with nickel (area ratio of MS to Ni - 1:1) Mild steel coupled with nickel (area ratio of MS to Ni - 1:2) 8.97 Mild steel coupled with nickel (area ratio of MS to Ni - 1:3) 8.05 Mild steel coupled with brass (area ratio of MS to brass - 1:1) 9.23 Mild steel coupled with brass (area ratio of MS to brass - 1:2) 8.12 Mild steel coupled with brass (area ratio of MS to brass - 1:3) 7.03 Mild steel coupled with copper (area ratio of MS to Cu - 1:1) 8.00 Mild steel coupled with copper (area ratio of MS to Cu - 1:2) 7.31 Mild steel coupled with copper (area ratio of MS to Cu - 1:3) 5.99 Mild steel coupled with titanium (area ratio of MS to Ti - 1:1) 6.05 Mild steel coupled with titanium (area ratio of MS to Ti - 1:2) 5.10 Mild steel coupled with titanium (area ratio of MS to Ti - 1:3) 4.12 Fig. 4 Xray diffraction pattern of zinc phosphate coating developed under coupled condition (mild steel coupled with titanium, area ratio of mild steel to titanium is 1:3) Conclusion The extents of metal dissolution and of coating formation are higher for mild steel substrates phosphated under galvanically coupled condition than for the one coated without coupling. The coating weight is a function of galvanic potential exerted by the couple. The increase in the area ratio of anode to cathode increases the coating weight formation. Among the different couples studied, mild steel titanium couple of area ratio 1:3 exerts a greater influence on metal dissolution and coating weight. The experiments performed using phosphating bath without sodium nitrite (accelerator) showed that galvanic coupling not only promotes the iron dissolution but also favours the phosphate coating formation by shifting the hydrogen evolution reaction to cathode. Effective coating formation by galvanic coupling technique is influenced by the nature of the cathode material, anode to cathode area ratio and processing time. Potential time measurements strongly support the mechanisms proposed to explain the role of cathode materials and their area ratios with respect to mild steel anode. These results are in excellent agreement with the conclusions drawn from coating weight measurements. Thus, the galvanic coupling of mild steel with metals that are nobler than steel during low temperature phosphating proved to be beneficial in accelerating the rate of coating formation and producing uniform, less porous and higher weight coatings. Hence, this methodology proved to be cost effective in accelerating low temperature phosphating. References 1 Freeman D B, Phosphating and Metal Pretreatment A Guide to Modern Processes and Practice (Industrial Press Inc., New York), Rausch W, The Phosphating of Metals (Finishing Publications Ltd., London), Guy Lorin, Phosphating of Metals (Finishing Publications Ltd., London), Rajagopal & Vasu K I, Conversion Coatings: A Reference for Phosphating, Chromating and Anodizing (Tata McGraw- Hill Publishing Company Ltd., New Delhi), Sankara Narayanan T S N, Met Finish, 94(9) (1996) Streicher M A, Met Finish, 46(8) (1948) U.S. Environmental protection agency, EPA Enforcement Alert, 3(3) (2000) 1. 8 Arthanareeswari M, Ravichandran K, Sankara Narayanan T S N & Rajeswari S, Indian Surface Finishing, 1(1) (2004) Arthanareeswari M, Ravichandran K, Sankara Narayanan T S N, Kamaraj P & Rajeswari S, J Curr Sci, 2(2) (2002) 153.

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