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On the mechanism of formation of zinc pack coatings
Journal of Alloys and Compounds 407 (2006) 221–225
On the mechanism of formation of zinc pack coatings N. Pistofidis, G. Vourlias, D. Chaliampalias, K. Chrysafis, G. Stergioudis ∗ , E.K. Polychroniadis Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Received 14 May 2005; received in revised form 20 June 2005; accepted 22 June 2005 Available online 9 August 2005
Abstract The mechanism of the formation of zinc (Zn) pack coatings is studied with scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). DSC showed that the coatings formation takes place in three steps. The initial step (at 193.9 ◦ C) is endothermic and involves the transformation of ␣-NH4 Cl to -NH4 Cl and the NH4 Cl decomposition to NH3 and HCl. During the second step (at 248.6 ◦ C), which is exothermic, Zn2+ salts are formed and especially ZnCl2 . Finally at 264.1 ◦ C Zn is deposited by an endothermic reaction on the ferrous substrate through the decomposition of ZnCl2 . The as-cast Zn diffuses into the iron lattice forming the gamma (-Fe11 Zn40 ) and delta (␦-FeZn10 ) phases. Al2 O3 is not involved in the above-mentioned mechanism and acts only as filler. © 2005 Elsevier B.V. All rights reserved. Keywords: Metals; Surfaces and interfaces; Scanning electron microscopy; X-ray diffraction; Thermal analysis
1. Introduction Zinc has a number of characteristics that make it very well suited for use as a protective coating for ferrous substrates against atmospheric corrosion [1–3]. Their performance results from their ability to react with the atmospheric compounds (O2 , CO2 and H2 O) thus forming dense, adherent films over the coating surface which are know to act as a barrier that protects the ferrous materials against corrosion. Moreover, zinc is anodic to iron and steel and consequently offers cathodic protection. A number of different methods of casting zinc coatings are commercially available. The most widely used is hot-dip galvanizing [1–3]. During this process the ferrous substrate is covered by a layer of zinc with an average thickness of a few tens of micrometers. In spite of the fact that the effectiveness of the method is undisputable [1–3], its high environmental impact imposes the investigation of alternative coating techniques friendlier to the environment. A promising technique that could be used instead of hot-dip galvanizing is chemi∗
Corresponding author. Tel.: +30 2310 99 8085; fax: +30 2310 99 8241. E-mail address: [email protected] (G. Stergioudis).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.06.039
cal vapor deposition (CVD) by pack cementation [4]. This method consists of formation of zinc coatings by heating the substrate up to 400 ◦ C. For this purpose the substrate is covered with a mixture of powders containing zinc and a halide activator. Pack cementation offers high integrity coatings, which are composed by two layers referring to the gamma (-Fe11 Zn40 ) and the delta (␦-FeZn10 ) phase of the Fe-Zn phase diagram. Their corrosion resistance is similar to that of the hot-dip galvanized coatings [5]. The present work examines the steps of the mechanism of the formation of zinc coatings deposited with pack cementation. For such an investigation it is necessary to determine the chemical reactions and physicochemical transformations that take place between the powders of the mixture. Differential scanning calorimetry (DSC) is a useful tool to trace stepwise in real time the variations, which occur during the chemical reaction, while at the same time the temperature is recorded. However, as DSC measurements give only qualitatively information on the phenomenon, it is necessary for the determination of the intermediate products of every reaction of transformation to use XRD and SEM analysis. The whole investigation offers important data for the optimization of pack cementation method which, scaling up,
N. Pistofidis et al. / Journal of Alloys and Compounds 407 (2006) 221–225
222
could be used for industrial applications. The revelation of the intermediate products could assist in the selection of cheaper reactants and in the design of more convenient processes so as to avoid exposure of the personnel to hazardous materials. However the most important contribution of this work is the determination of the optimum reaction temperature so as to minimize any possible energy losses.
2. Experimental The powder mixtures used in the DSC experiment are reported in Table 1. Concerning the mixtures (1) and (3), a small piece of commercial, hot-rolled, low carbon steel sheet SAE 1010 was immersed in the powder in every experiment. The steel substrate was sandblasted prior to the deposition. The thermal behaviour of these mixtures was investigated using a Setaram DSC 141 differential scanning calorimeter. Temperature and energy calibrations of the instrument were performed using the well-known melting temperatures and enthalpies of high purity zinc and indium. The samples weighing about 15 mg were crimped in stainless steel crucibles and an empty stainless steel crucible was used as reference. This study was non-isothermally performed by heating the specimens from room temperature up to 550 ◦ C with a heating rate of 10 K min−1 . In order to determine the intermediate products, powder with the composition of mixture (1) with a steel piece immersed in its mass was placed in three porcelain crucibles. The crucibles were covered with porcelain lids, sealed with high-temperature resistant cement and placed in a tubular argon-purged electric furnace, where they were heated for 60 min up to 210, 255 and 285 ◦ C, respectively. The heating temperatures were determined from the data offered by DSC. After the heating period the crucibles were left in the furnace to cool down to ambient temperature without interrupting the Ar flow. The as-cured powders were examined with XRD with a one-cycle Philips diffractometer, where Cu ˚ was used. K␣ radiation (λ = 1.54406 A) Furthermore the same powder (mixture (1)) was heated up to 400 ◦ C in an open crucible which was connected with a vessel containing Nessler reactant (alkaline solution of K2 (HgI4 )) to determine whether ammonia is formed during pack cementation [6]. This solution reacts exclusively with NH4 + , producing red-brown precipitate and it is very sensitive to this ion. Consequently, it is suitable to determine whether ammonia is formed during pack cementation.
1 2 3
3. Results and discussion A typical scanning electron micrograph of the pack coatings formed using a powder with the composition of mixture (1) is presented in Fig. 1. The coating is composed by two layers, which as the EDS analysis showed (Table 2), refer to the gamma (-Fe11 Zn40 ) and the delta (␦-FeZn10 ) phase of the Fe–Zn phase diagram. Furthermore inclusions of Fe–Zn solid solution containing about 50 wt.% Fe were observed. These observations imply that inward diffusion (e.g. diffusion of Zn in the substrate) takes place, as Al2 O3 or other powder inclusions are observed in the coating [4]. The DSC plot acquired after heating the mixture (1) up to 550 ◦ C is presented in Fig. 2 (plot (a)). The types of transformations, which were recorded, are summarized in Table 3. The first one at 193.9 ◦ C (referring to the peak (A) in Fig. 2) is endothermic and the energy absorbed is relatively low. The next at 248.6 ◦ C (referring to the peak (B) in
Fig. 1. SE micrographs of coatings formed with the mixture (1) at 400 ◦ C and heating time 2 h. The numbers refers to Table 2.
Table 1 Powder mixtures examined with DSC Mixture no.
A similar method was used to determine whether the gaseous products contain Zn2+ . The gaseous stream produced by heating the powder mixture (1) was dissolved in a solution of K3 [Fe(CN)6 ]. This compound is known to react with Zn2+ by forming a yellowish precipitate [6]. Finally, the as-formed coatings were examined with scanning electron microscopy by using a 20 kV JEOL 840 A SEM equipped with an Oxford ISIS 300 EDS analyzer and the necessary software to perform line scan and chemical mapping of the samples. The examination with SEM took place in order to investigate the Zn diffusion, which also involves in the coating formation.
Table 2 Results of the EDS analysis of the phases of Fig. 1
Compound (wt.%) Zn
NH4 Cl
Al2 O3
No.
Fe composition (wt.%)
Phase
Formula
70
5 100 7
25
1 2 3
10–11 24–28 48–50
␦ Solid solution
FeZn10 Fe11 Zn40 –
93
N. Pistofidis et al. / Journal of Alloys and Compounds 407 (2006) 221–225
Fig. 2. DSC plot of mixture (1) composed by 70 wt.% Zn, 5 wt.% NH4 Cl and 25 wt.% Al2 O3 and DSC plot of pure NH4 Cl.
Fig. 2) is exothermic and comparatively wide, which means that the phenomenon takes place at a long temperature range. Therefore, it is likely to be composed by several elementary reactions, which are accomplished at very close temperature intervals. The final at 264.1 ◦ C (referring to the peak (C) in Fig. 2) is endothermic and very steep, while at 419 ◦ C zinc melting is recorded, although this phenomenon is not visible in Fig. 2. Zinc melting does not participate in the coating formation because the deposition temperature is limited at 400 ◦ C, as it has been already mentioned. From a rough analysis of the DSC measurements, it is deduced that the coating formation takes place in three steps. The endothermic phenomenon at 193.9 ◦ C is due to the transformation of ␣-NH4 Cl to -NH4 Cl [7] (see plot (b) in Fig. 2), thus it might not be related to the coating formation. Nevertheless the peak recorded is wider than the peak referring to the transformation of pure NH4 Cl. Hence, it is most probable that additional transformations take place apart from the NH4 Cl allotropy and as a result, even at such low temperature, reactions participating in the coating formation could have occurred. As regards to the exothermic manifestation at 248.6 ◦ C, it could be ascribed to the reactions between the powder compounds and especially to those between Zn and NH4 Cl. These reactions are exothermic and take place in several steps [8]. Consequently the large width of the peak is justified. As far as it concerns the final peak at 264.1 ◦ C, which is endothermic, it is likely to be connected to the decomposition Table 3 Transformations of the DSC plot of powder mixture (1) Peak no.
Temperature (◦ C)
Type of transformation
(A) (B) (C) (D)a
193.9 248.6 264.1 419
Endothermic Exothermic Endothermic Endothermic
a
Not visible in Fig. 2.
223
Fig. 3. DSC plot of mixture (1) composed by 70 wt.% Zn, 5 wt.% NH4 Cl and 25 wt.% Al2 O3 (a) and DSC plot of mixture (3) composed by 93 wt.% Zn, 7 wt.% NH4 Cl (b).
reactions of the compounds formed at lower temperature. These reactions are usually exothermic. Al2 O3 seems to be inert with regard to Zn and NH4 Cl as the comparison of the two plots (a and b) as Fig. 3 shows. Even though no Al2 O3 was present in the DSC crucible, no essential change was observed at the DSC plot. A limited delay in the first endothermic peak (A), as well as the small one, which is developed during the exothermic reaction (C), are of course changes in the initial thermograph due to the absence of the Al2 O3 . But, as it is also observed, these variations do not alter the morphology of the diagram, which corresponds to the coating process. Hence the coating formation is accomplished through reactions where, by taking into account the powder composition, only Zn and NH4 Cl participate. Al2 O3 seem that affects slightly this process and acts only as filler. To summarize the conclusions drawn by DSC plots, it could be stated that the coating formation takes place at least in three steps. The first one includes ␣-NH4 Cl transformation to -NH4 Cl. The second which is exothermic could refer to a series of composition reactions, while the third one which is endothermic includes decomposition reactions. However, to understand the formation mechanism of the coatings it is necessary to detect the intermediate products of the reactions that take place during the heating period of the powder. For this purpose an open crucible with mixture (1) was initially heated up to 400 ◦ C. The surface of the crucible was connected with a polyethylene tube to the bottom of a vessel containing Nessler reactant. As soon as the temperature in the crucible reaches 100 ◦ C, the Nessler reactant was coloured yellow-red. This observation proves that ammonia (NH3 ), which was dissolved in the Nessler reactant was evolved from mixture (1). The same experimental set up was used to determine whether Zn2+ ions are formed. As the temperature of the crucible reached about 200 ◦ C, the K3 [Fe(CN)6 ] solution was colored yellowish.
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the following equations: ␣-NH4 Cl → -NH4 Cl
(1)
-NH4 Cl → NH3 + HCl
(2)
Zn + 2HCl → ZnCl2 + 2H2
(3)
ZnCl2 forms easily coordination compounds with NH3 and NH4 Cl, so by taking into account the large excess of Zn in the reaction system, none of these three compounds should be pure. As the temperature increases, the situation changes in the crucible. Fig. 4b shows that after 255 ◦ C the main compounds in the powder mixture are Zn, ZnCl2 and Zn(NH3 )2 Cl2 . Hence the ZnCl2 formation continues, while NH3 is depleted from the powder mixture. These reactions are exothermic. A further increasing of the temperature at 285 ◦ C leads to the decomposition of ZnCl2 (endothermic reactions) formed during the previous step, as Fig. 4c shows, where the halide peaks are vanished and only reflections of Zn and ZnO are observed. It is likely that the ZnCl2 decomposition takes place as its vapors come in contact with the ferrous substrate, resulting to the Zn deposition. The formation of ZnO after NH3 depletion is expected because NH3 creates a reductive atmosphere, which protects Zn. However, after its depletion the Zn remnants in the powder (which are not negligible, since there is a large excess of Zn in the system) react with the O2 , which is trapped in the crucible prior to scaling.
4. Conclusions
Fig. 4. XRD pattern of the powder after heating at 210 ◦ C (a), 255 ◦ C (b) and 285 ◦ C (c). In pattern (a) at 210 ◦ C peak (1) refers to Zn, (2) to Al2 O3 , (3) to Zn(NH3 )2 Cl2 , (4) to ZnCl2 ·3NH4 Cl and (5) to ZnO. In pattern (b) at 255 ◦ C peak (1) refers to ZnCl2 , (2) to Zn(NH3 )2 Cl2 , (3) to Zn, (4) to Al2 O3 , and (5) to ZnO. In pattern (c) at 285 ◦ C peak (1) refers to ZnO, (2) to Zn and (3) to Al2 O3 .
Furthermore the powder mixture (1) was heated up to 210, 255 and 285 ◦ C. The as-treated powders were examined with XRD. The XRD patterns are presented in Fig. 4. In Fig. 4a the reflections referring to the initial compounds of mixture (1) are observed, e.g. Zn and Al2 O3 . However, there is no peak corresponding to NH4 Cl, although there are many peaks originating from Zn(NH3 )2 Cl2 and ZnCl2 ·3NH4 Cl. Consequently the powder transformations begin with the transformation of ␣-NH4 Cl to -NH4 Cl and follow the decomposition of NH4 Cl and the reaction of the decomposition products (NH3 and HCl) with Zn to the formation of Zn halides. The sequence of the chemical phenomena could be described by
From the above analysis it is obvious that the coating formation of Zn pack coatings takes place in three steps. The initial (at 193.9 ◦ C) is endothermic and involves NH4 Cl decomposition and reaction of the decomposition products with Zn to the formation of ZnCl2 coordinated with NH3 . During the second step (248.6 ◦ C), which is exothermic the ZnCl2 formation continues, while NH3 is depleted. Finally, at 264.1 ◦ C (endothermic transformation) ZnCl2 is also decomposed to form Zn that is deposited, while the ammonia depletion leads to the oxidation of the Zn excess. Al2 O3 is not involved in the above-mentioned mechanism and act only as filler.
Acknowledgments This project was financed by the Greek Ministry of National Education through the program Pythagoras II.
References [1] A.R. Marder, Prog. Mater. Sci. 45 (2000) 191. [2] ASM Handbook, Hot Dip Coatings, vol. 13: Corrosion, ASM, New York, 1999.
N. Pistofidis et al. / Journal of Alloys and Compounds 407 (2006) 221–225 [3] Galvanizers Association, The Engineers and Architects’ Guide to Hot Dip Galvanizing, Galvanizers Association, Sutton, Coldfield, 2000. [4] ASM Handbook, Diffusion coatings, vol. 6: Surface Treatments, ASM, New York, 1999. [5] Y. He, D. Li, D. Wang, Z. Zhang, H. Qi, W. Gao, Mater. Lett. 56 (2002) 554.
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[6] V. Alexeev, Analyse Qualitative, Edition, Mir, Moscow, 1975. [7] M. Olszak-Humienik, Thermochim. Acta 378 (2001) 107– 112. [8] CRC Handbook of Physics and Chemistry, CRC Press, 57th ed., 1976–1977.
On the mechanism of formation of zinc pack coatings N. Pistofidis, G. Vourlias, D. Chaliampalias, K. Chrysafis, G. Stergioudis ∗ , E.K. Polychroniadis Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Received 14 May 2005; received in revised form 20 June 2005; accepted 22 June 2005 Available online 9 August 2005
Abstract The mechanism of the formation of zinc (Zn) pack coatings is studied with scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). DSC showed that the coatings formation takes place in three steps. The initial step (at 193.9 ◦ C) is endothermic and involves the transformation of ␣-NH4 Cl to -NH4 Cl and the NH4 Cl decomposition to NH3 and HCl. During the second step (at 248.6 ◦ C), which is exothermic, Zn2+ salts are formed and especially ZnCl2 . Finally at 264.1 ◦ C Zn is deposited by an endothermic reaction on the ferrous substrate through the decomposition of ZnCl2 . The as-cast Zn diffuses into the iron lattice forming the gamma (-Fe11 Zn40 ) and delta (␦-FeZn10 ) phases. Al2 O3 is not involved in the above-mentioned mechanism and acts only as filler. © 2005 Elsevier B.V. All rights reserved. Keywords: Metals; Surfaces and interfaces; Scanning electron microscopy; X-ray diffraction; Thermal analysis
1. Introduction Zinc has a number of characteristics that make it very well suited for use as a protective coating for ferrous substrates against atmospheric corrosion [1–3]. Their performance results from their ability to react with the atmospheric compounds (O2 , CO2 and H2 O) thus forming dense, adherent films over the coating surface which are know to act as a barrier that protects the ferrous materials against corrosion. Moreover, zinc is anodic to iron and steel and consequently offers cathodic protection. A number of different methods of casting zinc coatings are commercially available. The most widely used is hot-dip galvanizing [1–3]. During this process the ferrous substrate is covered by a layer of zinc with an average thickness of a few tens of micrometers. In spite of the fact that the effectiveness of the method is undisputable [1–3], its high environmental impact imposes the investigation of alternative coating techniques friendlier to the environment. A promising technique that could be used instead of hot-dip galvanizing is chemi∗
Corresponding author. Tel.: +30 2310 99 8085; fax: +30 2310 99 8241. E-mail address: [email protected] (G. Stergioudis).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.06.039
cal vapor deposition (CVD) by pack cementation [4]. This method consists of formation of zinc coatings by heating the substrate up to 400 ◦ C. For this purpose the substrate is covered with a mixture of powders containing zinc and a halide activator. Pack cementation offers high integrity coatings, which are composed by two layers referring to the gamma (-Fe11 Zn40 ) and the delta (␦-FeZn10 ) phase of the Fe-Zn phase diagram. Their corrosion resistance is similar to that of the hot-dip galvanized coatings [5]. The present work examines the steps of the mechanism of the formation of zinc coatings deposited with pack cementation. For such an investigation it is necessary to determine the chemical reactions and physicochemical transformations that take place between the powders of the mixture. Differential scanning calorimetry (DSC) is a useful tool to trace stepwise in real time the variations, which occur during the chemical reaction, while at the same time the temperature is recorded. However, as DSC measurements give only qualitatively information on the phenomenon, it is necessary for the determination of the intermediate products of every reaction of transformation to use XRD and SEM analysis. The whole investigation offers important data for the optimization of pack cementation method which, scaling up,
N. Pistofidis et al. / Journal of Alloys and Compounds 407 (2006) 221–225
222
could be used for industrial applications. The revelation of the intermediate products could assist in the selection of cheaper reactants and in the design of more convenient processes so as to avoid exposure of the personnel to hazardous materials. However the most important contribution of this work is the determination of the optimum reaction temperature so as to minimize any possible energy losses.
2. Experimental The powder mixtures used in the DSC experiment are reported in Table 1. Concerning the mixtures (1) and (3), a small piece of commercial, hot-rolled, low carbon steel sheet SAE 1010 was immersed in the powder in every experiment. The steel substrate was sandblasted prior to the deposition. The thermal behaviour of these mixtures was investigated using a Setaram DSC 141 differential scanning calorimeter. Temperature and energy calibrations of the instrument were performed using the well-known melting temperatures and enthalpies of high purity zinc and indium. The samples weighing about 15 mg were crimped in stainless steel crucibles and an empty stainless steel crucible was used as reference. This study was non-isothermally performed by heating the specimens from room temperature up to 550 ◦ C with a heating rate of 10 K min−1 . In order to determine the intermediate products, powder with the composition of mixture (1) with a steel piece immersed in its mass was placed in three porcelain crucibles. The crucibles were covered with porcelain lids, sealed with high-temperature resistant cement and placed in a tubular argon-purged electric furnace, where they were heated for 60 min up to 210, 255 and 285 ◦ C, respectively. The heating temperatures were determined from the data offered by DSC. After the heating period the crucibles were left in the furnace to cool down to ambient temperature without interrupting the Ar flow. The as-cured powders were examined with XRD with a one-cycle Philips diffractometer, where Cu ˚ was used. K␣ radiation (λ = 1.54406 A) Furthermore the same powder (mixture (1)) was heated up to 400 ◦ C in an open crucible which was connected with a vessel containing Nessler reactant (alkaline solution of K2 (HgI4 )) to determine whether ammonia is formed during pack cementation [6]. This solution reacts exclusively with NH4 + , producing red-brown precipitate and it is very sensitive to this ion. Consequently, it is suitable to determine whether ammonia is formed during pack cementation.
1 2 3
3. Results and discussion A typical scanning electron micrograph of the pack coatings formed using a powder with the composition of mixture (1) is presented in Fig. 1. The coating is composed by two layers, which as the EDS analysis showed (Table 2), refer to the gamma (-Fe11 Zn40 ) and the delta (␦-FeZn10 ) phase of the Fe–Zn phase diagram. Furthermore inclusions of Fe–Zn solid solution containing about 50 wt.% Fe were observed. These observations imply that inward diffusion (e.g. diffusion of Zn in the substrate) takes place, as Al2 O3 or other powder inclusions are observed in the coating [4]. The DSC plot acquired after heating the mixture (1) up to 550 ◦ C is presented in Fig. 2 (plot (a)). The types of transformations, which were recorded, are summarized in Table 3. The first one at 193.9 ◦ C (referring to the peak (A) in Fig. 2) is endothermic and the energy absorbed is relatively low. The next at 248.6 ◦ C (referring to the peak (B) in
Fig. 1. SE micrographs of coatings formed with the mixture (1) at 400 ◦ C and heating time 2 h. The numbers refers to Table 2.
Table 1 Powder mixtures examined with DSC Mixture no.
A similar method was used to determine whether the gaseous products contain Zn2+ . The gaseous stream produced by heating the powder mixture (1) was dissolved in a solution of K3 [Fe(CN)6 ]. This compound is known to react with Zn2+ by forming a yellowish precipitate [6]. Finally, the as-formed coatings were examined with scanning electron microscopy by using a 20 kV JEOL 840 A SEM equipped with an Oxford ISIS 300 EDS analyzer and the necessary software to perform line scan and chemical mapping of the samples. The examination with SEM took place in order to investigate the Zn diffusion, which also involves in the coating formation.
Table 2 Results of the EDS analysis of the phases of Fig. 1
Compound (wt.%) Zn
NH4 Cl
Al2 O3
No.
Fe composition (wt.%)
Phase
Formula
70
5 100 7
25
1 2 3
10–11 24–28 48–50
␦ Solid solution
FeZn10 Fe11 Zn40 –
93
N. Pistofidis et al. / Journal of Alloys and Compounds 407 (2006) 221–225
Fig. 2. DSC plot of mixture (1) composed by 70 wt.% Zn, 5 wt.% NH4 Cl and 25 wt.% Al2 O3 and DSC plot of pure NH4 Cl.
Fig. 2) is exothermic and comparatively wide, which means that the phenomenon takes place at a long temperature range. Therefore, it is likely to be composed by several elementary reactions, which are accomplished at very close temperature intervals. The final at 264.1 ◦ C (referring to the peak (C) in Fig. 2) is endothermic and very steep, while at 419 ◦ C zinc melting is recorded, although this phenomenon is not visible in Fig. 2. Zinc melting does not participate in the coating formation because the deposition temperature is limited at 400 ◦ C, as it has been already mentioned. From a rough analysis of the DSC measurements, it is deduced that the coating formation takes place in three steps. The endothermic phenomenon at 193.9 ◦ C is due to the transformation of ␣-NH4 Cl to -NH4 Cl [7] (see plot (b) in Fig. 2), thus it might not be related to the coating formation. Nevertheless the peak recorded is wider than the peak referring to the transformation of pure NH4 Cl. Hence, it is most probable that additional transformations take place apart from the NH4 Cl allotropy and as a result, even at such low temperature, reactions participating in the coating formation could have occurred. As regards to the exothermic manifestation at 248.6 ◦ C, it could be ascribed to the reactions between the powder compounds and especially to those between Zn and NH4 Cl. These reactions are exothermic and take place in several steps [8]. Consequently the large width of the peak is justified. As far as it concerns the final peak at 264.1 ◦ C, which is endothermic, it is likely to be connected to the decomposition Table 3 Transformations of the DSC plot of powder mixture (1) Peak no.
Temperature (◦ C)
Type of transformation
(A) (B) (C) (D)a
193.9 248.6 264.1 419
Endothermic Exothermic Endothermic Endothermic
a
Not visible in Fig. 2.
223
Fig. 3. DSC plot of mixture (1) composed by 70 wt.% Zn, 5 wt.% NH4 Cl and 25 wt.% Al2 O3 (a) and DSC plot of mixture (3) composed by 93 wt.% Zn, 7 wt.% NH4 Cl (b).
reactions of the compounds formed at lower temperature. These reactions are usually exothermic. Al2 O3 seems to be inert with regard to Zn and NH4 Cl as the comparison of the two plots (a and b) as Fig. 3 shows. Even though no Al2 O3 was present in the DSC crucible, no essential change was observed at the DSC plot. A limited delay in the first endothermic peak (A), as well as the small one, which is developed during the exothermic reaction (C), are of course changes in the initial thermograph due to the absence of the Al2 O3 . But, as it is also observed, these variations do not alter the morphology of the diagram, which corresponds to the coating process. Hence the coating formation is accomplished through reactions where, by taking into account the powder composition, only Zn and NH4 Cl participate. Al2 O3 seem that affects slightly this process and acts only as filler. To summarize the conclusions drawn by DSC plots, it could be stated that the coating formation takes place at least in three steps. The first one includes ␣-NH4 Cl transformation to -NH4 Cl. The second which is exothermic could refer to a series of composition reactions, while the third one which is endothermic includes decomposition reactions. However, to understand the formation mechanism of the coatings it is necessary to detect the intermediate products of the reactions that take place during the heating period of the powder. For this purpose an open crucible with mixture (1) was initially heated up to 400 ◦ C. The surface of the crucible was connected with a polyethylene tube to the bottom of a vessel containing Nessler reactant. As soon as the temperature in the crucible reaches 100 ◦ C, the Nessler reactant was coloured yellow-red. This observation proves that ammonia (NH3 ), which was dissolved in the Nessler reactant was evolved from mixture (1). The same experimental set up was used to determine whether Zn2+ ions are formed. As the temperature of the crucible reached about 200 ◦ C, the K3 [Fe(CN)6 ] solution was colored yellowish.
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the following equations: ␣-NH4 Cl → -NH4 Cl
(1)
-NH4 Cl → NH3 + HCl
(2)
Zn + 2HCl → ZnCl2 + 2H2
(3)
ZnCl2 forms easily coordination compounds with NH3 and NH4 Cl, so by taking into account the large excess of Zn in the reaction system, none of these three compounds should be pure. As the temperature increases, the situation changes in the crucible. Fig. 4b shows that after 255 ◦ C the main compounds in the powder mixture are Zn, ZnCl2 and Zn(NH3 )2 Cl2 . Hence the ZnCl2 formation continues, while NH3 is depleted from the powder mixture. These reactions are exothermic. A further increasing of the temperature at 285 ◦ C leads to the decomposition of ZnCl2 (endothermic reactions) formed during the previous step, as Fig. 4c shows, where the halide peaks are vanished and only reflections of Zn and ZnO are observed. It is likely that the ZnCl2 decomposition takes place as its vapors come in contact with the ferrous substrate, resulting to the Zn deposition. The formation of ZnO after NH3 depletion is expected because NH3 creates a reductive atmosphere, which protects Zn. However, after its depletion the Zn remnants in the powder (which are not negligible, since there is a large excess of Zn in the system) react with the O2 , which is trapped in the crucible prior to scaling.
4. Conclusions
Fig. 4. XRD pattern of the powder after heating at 210 ◦ C (a), 255 ◦ C (b) and 285 ◦ C (c). In pattern (a) at 210 ◦ C peak (1) refers to Zn, (2) to Al2 O3 , (3) to Zn(NH3 )2 Cl2 , (4) to ZnCl2 ·3NH4 Cl and (5) to ZnO. In pattern (b) at 255 ◦ C peak (1) refers to ZnCl2 , (2) to Zn(NH3 )2 Cl2 , (3) to Zn, (4) to Al2 O3 , and (5) to ZnO. In pattern (c) at 285 ◦ C peak (1) refers to ZnO, (2) to Zn and (3) to Al2 O3 .
Furthermore the powder mixture (1) was heated up to 210, 255 and 285 ◦ C. The as-treated powders were examined with XRD. The XRD patterns are presented in Fig. 4. In Fig. 4a the reflections referring to the initial compounds of mixture (1) are observed, e.g. Zn and Al2 O3 . However, there is no peak corresponding to NH4 Cl, although there are many peaks originating from Zn(NH3 )2 Cl2 and ZnCl2 ·3NH4 Cl. Consequently the powder transformations begin with the transformation of ␣-NH4 Cl to -NH4 Cl and follow the decomposition of NH4 Cl and the reaction of the decomposition products (NH3 and HCl) with Zn to the formation of Zn halides. The sequence of the chemical phenomena could be described by
From the above analysis it is obvious that the coating formation of Zn pack coatings takes place in three steps. The initial (at 193.9 ◦ C) is endothermic and involves NH4 Cl decomposition and reaction of the decomposition products with Zn to the formation of ZnCl2 coordinated with NH3 . During the second step (248.6 ◦ C), which is exothermic the ZnCl2 formation continues, while NH3 is depleted. Finally, at 264.1 ◦ C (endothermic transformation) ZnCl2 is also decomposed to form Zn that is deposited, while the ammonia depletion leads to the oxidation of the Zn excess. Al2 O3 is not involved in the above-mentioned mechanism and act only as filler.
Acknowledgments This project was financed by the Greek Ministry of National Education through the program Pythagoras II.
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