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Corrosion products of zinc-manganese coatings: part I—investigations using microprobe analysis and x-ray diffraction
Corrosion Products of Zinc-Manganese Coatings: Part I---investigations Using Mioroprobe Analyois and X-Ray by N. B o s h k o v , a S. V i t k o v a , a a n d K. P e t r o v b
aInstitute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria; and blnstitute of Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria
inc galvanic alloy coatings are preferred for steel due to their better corrosion resistance and protective ability compared with the individual metals in the alloy. Zinc-manganese galvanic alloy, which has not been studied extensively, possesses the optimal characteristics for a protective coating especially in the cases when the Mn content is up to 15%. 1 The electrodeposition conditions for obtaining galvanic Zn-Mn alloy from a sulfate electrolyte have been experimentally found in a previous study. 1 Corrosion resistance of this coating (chromated and nonchromated) and its protective ability were investigated in a model corrosion medium free aerated 5% NaC1 s o l u t i o n - - u s i n g different methods such as polarization resistance (Rp) m e a s u r e m e n t s , P a a t s c h method, potentiodynamic (PD) polarization curves, and testing in a salt spray chamber (NSS-method). 2'~ These studies revealed one interesting and very important p h e n o m e n o n - - a passivation of the Zn-Mn alloy in presence of C1 ions. The passive region appears in the anodic range of PD curves after the zone of maximal anodic dissolution. 2 Phase composition of galvanic alloys Zn-Mn was also studied. 1 Formation of an intermetallic compound MnZn 7 (known as 81 phase) with hexagonal structure containing 550 --_8 atoms in the elementary cell was observed. This coating can be treated with a chromate solution for a galvanic zinc transparent and iridescent yellow type coatings (Duga), 2'3 which leads to a strong increase of the protective ability of the whole system. Literature about the composition of the corrosion products of these coatings as well as about the mechanism of their protection is rather scarce. That is why the aim of Part I of this three-part article is to study the elemental and phase composition of the corrosion products, formed on the surface of chromated and nonchromated galvanic Zn-Mn coatings with different Zn/Mn ratio after treatment in various conditions.
Z
56
EXPERIMENTAL
Sample Preparation Galvanic Zn-Mn alloys with different manganese content were electrodeposited from a starting electrolyte (SE), having the following composition: 10 g/L ZnSO4.7H20; 100 g/L MnSO4.HeO; 60 gfL (NH4)2.SO 4. The deposition was carried out in a thermostatic double-chamber electrolytic cell (500-ml volume), current density 2 A/dm 2, pH value 5, and room temperature (20°C) under continuous circulation of the solution (150 rpm). Steel plates 20 × 10 × 0.1 mm were used as substrates by doubleside deposition. Metallurgical zinc was taken for the anodes. 1 Galvanic coatings (nonchromated and chromated with iridescent yellow solution type coatings) with the following compositions were prepared: 1 1. Zn-Mn(-6%), obtained by SE with two additives 1 with trade names AZ-1 (contains polyethyleneglycol and benzoic acid) and AZ-2 (contains benzalaceton). This alloy consists generally of a pure zinc phase and smaller amounts of manganese and 81 phases. 2. Z n - M n ( - l l % ) , obtained by SE and AZ-1. The alloy contains mainly the intermetallic phase 81 and very small quantities of pure zinc phase. 3. Zinc, obtained by a slightly acidic sulfate electrolyte, pH value 5, room temperature, and current density 2 A/dm 2. The samples were treated in a model corrosion medium of free aerated 5% NaC1 solution (working volume 150 ml) at 200C as follows: Both galvanic alloys (points "a" and "b") were dissolved using external anodic polarization until reaching the passive zone of the anodic potentiodynamic polarization curve [passivating potential Epa~s = - 6 5 0 mV with respect to a saturated calomel reference electrode (SCE); current density in the passive zone ipas~ = 2.10 -5 A/dm2; Epassand ipa~ values were taken from our previous experiments2]. The treatment was stopped afLer exposure time of 10 minutes for the Metal Finishing
Table I. Results from Investigations with Microprobe Analysis of Corrosion-Treated Galvadic Alloys Zn-Mn (in wt%) Corrosionally Exposure at Exposure at
No 1
4
Samples Columns --~ Zn-Mn (-11%)
Untreated Samples A Zn-89.3 Mn-10.7
Epass During 10 Minutes C Zn-44.5 Mn-5.5
C1-22.3 Zn/Mn = 9.6
C1-50.0 Zn/Mn = 8.1
Zn-88.1
Zn-93.0 Mn-5.6 Cl-l.4 Zn]Mn = 16.6 Zn-90.2
Zn-73.6 Mn-0.7 C1-25.7 Zn/Mn = 105.1 Zn-72.3
Mn-8.2
Mn-7.6
Mn-2.3
Cr-3.7
Cr-2.0 C1 <0.2 Zn/Mn = 11.9 Zn-91.6 Mn-4.7 Cr-3.3 C1-0.4 Zn/Mn -- 19.5
Cr-2.0 C1-23.4 Zn/Mn = 31.4 Zn-72.2 Mn-l.5 Cr--4.3 C1-22.0 Zn/Mn = 48.1
Weight ratio Zn-Mn (-6%)
Zn/Mn = 8.3 Zn-94.2 Mn-5.8
Weight Ratio
Zn/Mn = 16.2
Zn-Mn (-11%) + "Duga"
Weight ratio Zn-Mn (-6%) + "Duga"
Zn/Mn = 10.7 Zn-92.0 Mn-5.0 Cr-3.0
Weight ratio
Zn/Mn = 18.4
samples in the passive zone at the conditions, described above. No investigations were carried out with pure zinc at Ep~s~ because this metal does not passivate in the medium used. Exposure of 6 days at corrosion potential (Ecorr) for all sample types in conditions close to those of natural corrosion processes.
Sample Characterization The elemental composition of the sample surface was determined quantitatively using microprobe analyzer J E O L Superprobe 733, Japan. The phase composition was determined using Xray powder diffractometer DRON-3 (Bragg-Brentano arrangement, C u K ~ - radiation and scintillation counter). EXPERIMENTAL RESULTS AND DISCUSSION
Microprobe Analysis Microprobe analysis data in depth of 2 to 4 ~Lm from the surface of the sample are outlined in Table I and show changes in the elemental composition, which depend on: composition of the initial galvanic coating, type of the corrosion treatment, and presence of a chromating film on the sample surface.
Nonchromated Samples In the case of nonchromated alloy samples 1 and 2, a decrease of Zn and Mn amounts after both corrosion treatments is registered. Due to the active anodic dissolution of the entire surface, the metal contents decrease at a greater rate in anodically treated samples 1 and 2 a t Epass (column C) cornSeptember 2001
E¢o~r During 6 Days B Zn-70.4 Mn-7.3
pared with columns A and B. Zinc and manganese contents decrease almost two times that of the initial amounts in sample 1 (compare A with C). In sample 2 at Epa~s the quantity of Zn decreases about one-forth compared with the initial composition, while that of manganese decreases about 8 times (columns A and C). Manganese dissolves at a lesser rate in sample 2 at Eco~ (columns A and B). After anodic dissolution of sample 2 the manganese dissolves in higher degree compared with zinc (column C). An appearance of chlorine after the corrosion treatments is also registered. It occurs in higher contents in sample 1, which consists generally of the intermetallic compound MnZn7, compared with sample 2 by both treatments (columns B and C). The registered chlorine amounts depend on the type of corrosion treatment. The difference between samples 1 and 2 for this element is almost 2 times at Epa~ (column C) and about 16 times at Eco~ (column B). The higher chlorine quantities for sample 1 compared with sample 2 after 6 days exposure in the medium at E¢o~ (column B) could be explained with the appearance of manganese and zinc chlorides, which partially remain on the surface, included in the corrosion products of the intermetallic phase (it covers almost the whole surface of this sample). For example, ZnC12 is almost five times more soluble in water at 20°C compared with MnC12, 4'5 which means that the higher manganese amount must ensure higher corrosion resistance. Due to the longer exposure time in the medium for samples 1 and 2 an 57
appearance of NaC1 in the sites of corrosion damages could be also expected. The higher chlorine amounts for the same samples in column C compared with column B appear due to the passive oxide film, which forms in the passive zone of the PD curve at external anodic polarization and seals up both corrosion products (chlorides) and some amounts of the model medium. The weight Zn/Mn ratio in sample 1 remains almost constant after different corrosion treatments, but increases strongly (more than six times) for sample 2 after Epass (compare column A with C). This is probably due to the faster manganese dissolution in sample 2 in these conditions (column C). This metal is included here in smaller amounts in the coating compared with sample 1, and its phase is scattered into the zinc phase.
~
"....:"jl
58
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l•
Chromated Samples Smaller Zn and Mn contents as a result of the chromating are registered on the surfaces of the corrosionally untreated samples 3 and 4 compared with samples 1 and 2, respectively (column A). Chromium content is also registered by both these alloys. Zinc and manganese quantities after corrosion treatments (B and C) decrease generally in lower degree compared with samples 1 and 2 due to the protection of the chromating film. A decrease of the chlorine amounts after both corrosion treatments is registered by these samples (3 and 4) compared with the nonchromated ones (1 and 2) (B and C). This difference is more than twice by comparison of the samples 1 and 3, column C, but this phenomenon is much more clearly registered at Eco~ (column B). Chlorine quantities after exposure at Epass are very close for samples 3 and 4 (23.4% and 22.0%, respectively, column C), which leads to the conclusion that chromated samples have similar behavior independent on their different composition. Chlorine contents are much higher for specimens 3 and 4 after anodic t r e a t m e n t compared with the results after exposure at E~orr (C and B). The reason is the same as explaned above in the case of nonchromated samples--sealing up the surface with oxides in the passive zone of the PD curve at external anodic polarization. The weight Zn/Mn ratio for samples 3 and 4 changes weakly after the corrosion t r e a t m e n t at E~o~ (A and B). After anodic treatment these changes are nearly two and a half or three times higher (A and C). Chromium content on the surface changes after the different corrosion t r e a t m e n t s - - i n sample 3 its amount decreases about two times, independent on
+
+~* -'s
C
i~
10
20
~
~
A
~
Figure 1. Diffraction patterns in 5% NaCI solution for (A) nonchromated zinc after exposure for 6 days at Eoor,; (B) nonchromated Zn-Mn (6%) after exposure at Epa.; (C) chromated ZnMn(6%) after exposure at EpN,. * diffraction lines of ZHC; + diffraction lines of NaCI
treatment type (A compared with B and C). Contrary to this result, sample 4 illustrates an increase of the chromium content from one-tenth (A compared with B) until more than one-third (A and C), which means that the surface is enriched with this metal due to the prior dissolution of zinc and manganese. A conclusion can be made that the coating consisting practically in phase 81 is more resistant in this medium and at these conditions. X-Ray
Diffraction
Results obtained by X-ray diffraction analysis for chromated and nonchromated Zn-Mn alloys with different Mn content are represented in Figure 1 and Figure 2. Figure 1 shows diffraction patterns taken after exposure at Eco~ during 6 days for nonchromated pure zinc (1A) and for nonchromated (1B), and chromated (1C) Zn-Mn ( - 6 % ) samples after 10 minutes at Epass, respectively. Figure 2 shows the diffraction patterns for nonchromated and corrosionally untreated Zn-Mn ( - 1 1 % ) (2A); for Metal Finishing
Table II. Observed and Published s XRD Data of Zns(OH)sClz.H20
["
.
ll
It.tl
., z .
•
B
5, Za
10
2o
30
40
A
50
2 [deg.] Figure 2. Diffraction patterns in 5% NaCI solution for (A) nonchromated and corrosionally untreated Zn-Mn (11%); (B) nonchromated Zn-Mn (11%) after exposure for 6 days at E©orr; (C) nonchromated Zn-Mn (6%) after exposure for 6 days at E©o~. * diffraction lines of ZHC; + diffraction lines of NaCI
Zn-Mn(-11%) (2B), and for Zn-Mn ( - 6 % ) (2C), both after 6 days at Ecorr. XRD spectra show lines of the substrate a-Fe, of the coating - Z n and/or of S1 (see Fig. 1 and 2). In addition, diffraction lines of the zinc hydroxide chloride Zns(OH)sC12.H20 (ZHC) appear in the alloy coatings after corrosion treatment (Fig. 1B,C and 2B,C). This compound has a hexagonal lattice and may be regarded as a double salt, 4Zn(OH)2.ZnC12.H20. 6 It crystallizes in the rhombohedral system with ah = 0.634--+0.001 rim, Ca = 2.364--+0.002 nm, S.G. R3m, Z = 3 per threefold primitive hexagonal cell. The basic s t r u c t u r a l unit is a charged layer, which m a y be derived from a hypothetical C6-Zn(OH) 2 of the CDI2 type. A q u a r t e r of the octahedrally coordin a t e d Zn atoms are removed and replaced by pairs of t e t r a h e d r a l l y coordinated Zn atoms, one on each side of the layer, giving a layer of composition [Zns(OH)s] 2+. The fourth bond from the tetrahedrally coordinated Zn is to the C1- ion that balances September 2001
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
dobs., A 7.90 5.37 4.00 3.95 3.57 3.16 2.931 2.880 2.722 2.675 2.600 2.368 2.073 1.899 1.791 1.781 1.771 1.701 1.689 1.578 1.551
dpubl., A 7.87 5.35 4.02 3.94 3.58 3.17 2.940 2.878 2.725 2.672 2.601 2.374 2.067 1.900 1.792 1.782 1.768 1.699 1.693 1.585 1.554
I/Io 100 20 20 8 25 35 30 25 55 65 20 35 8 8 8 6 14 14 12 35 16
hkl 003 101 104 006 105 110 113 107 201 202 108 205 211 215 2.0.10 303 217 218 2.0.11 220 223
the charge and the water molecules are situated between the layers. 7 In Table II the observed interplanar distances of ZHC are compared to those published in J C P D S - P D F data. 6 The XRD pattern of the nonchromated ZnM n ( - 6 % ) sample after anodic treatment contains the diffraction lines ofZHC, ~-Fe and NaC1, lB. This means that due to the active anodic dissolution the alloy coating is totally transformed in ZHC. Sodium chloride penetrates in the depth of the coating due to the relative high rate of the used external anodic polarization, 1 mV/sec. The same is the situation with the sample Zn-Mn (-11%), which is not shown on the figure. ZHC also appears on the chromating sample Zn-Mn (-6%), 1C, where lines of zinc and iron are also registered. In this case ZHC covers only the zones where the chromating film is already dissolved. That is why the lines of ZHC are less intensive in this sample compared with Fig. lB. XRD spectra of the samples at Ecor~ after 6 days exposure (Fig. 2B and 2C) illustrate the presence of the compound ZHC for both nonchromated alloys contrary to the pure zinc (Fig. 1A) and to the corrosionally untreated sample Zn-Mn ( - 1 1 % ) (Fig. 2A), where the lines of ZHC are missing. The phases 51 and Zn are also present in Fig. 2B and 2C. These results mean that the coating is not totally transformed at these conditions. Smaller quantities of ZHC appear on the sample Zn-Mn ( - 6 % ) (Fig. 2C) compared with the Zn-Mn ( - 1 1 % ) (Fig. 2B). The same samples, b u t chromated, show relatively 59
weaker lines of ZHC due to the protective action of the chromating film. It is obvious that the coatings of the phase 51, Zn-Mn (-11%), transform more easy to ZHC than the samples ZnMn (-6%). Probably, the homogeneous distribution of Mn in the intermetallic coating causes the nucleation and growth of uniform ZHC layer over the whole surface. Although no chromium compounds are detected by XRD, the microprobe analysis confirms their presence as an element on the sample surfaces. This leads to the conclusion that the chromium compounds are present in smaller quantities or exist in an amorphous form. s Lower amounts of NaC1 are registered for the samples, treated at Eco~ (Fig. 2B). For the nonchromated alloys this compound is also in small quantities (Fig. 1B, 2B, 2C). The chromating film protects the galvanic coating and impedes the formation of ZHC. Moreover, by the alloys treated at Eco~, during the experimental time the corrosion products are gradually transfering in the solution and partially remain in the depth of corrosion damaged states.
the Zn-Mn (-6%) coatings in similar conditions of corrosion treatment. . Contrary to other zinc alloys, Zn-Mn passivates in 5% NaC1 solution at external anodic polarization due to the formation of ZHC. The newly created passive film consists entirely from this compound, but not from oxides and hydroxides as in other corrosion media. For example, the ZHC appears at a pH value of about 7.0 to 7.5, while the Zn(OH) 2 at pH values higher than 10.0. . The results for this alloy at Eco~r can be compared with our previous results from salt-spray-chamber. 3 Chromated samples of Zn-Mn (~ 11%) resist up to 300 hours till 2.5% of their surface is damaged. Chromated pure zinc specimens from a slightly acidic bath and chromated samples of Zn-Mn (-6%) endure at these conditions almost 180 hours. 3 ACKNOWLEI)6EMEMTS
The authors express their gratitude to the Bulgarian National Fund "Scientific Researches," Sofia, for the financial support of these investigations.
CONCLUSIONS
REFEREI~ICES
The following conclusions, based on the results obtained, can be made:
1. 2. 3. 4.
1. After corrosion treatment of Zn-Mn alloys with different manganese content and phase composition in free aerated neutral model medium with presence of corrosion activators C1 ions the compound ZHC appears on the specimen surface. It has a low product of solubility (10 -14"2 mol]L)9 and ensures higher protective ability in comparison with the pure zinc. 2. ZHC forms readily only in the case when manganese is available. Galvanic coatings of the intermetallic Zn-Mn compound produce more thick and compact ZHC layers than samples containing
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Boschkov, N. et al., Metalloberfl~che, 52(7):514; 1998 Boschkov, N. et al., Metalloberfldche, 53(7):27; 1999 Boschkov, N. et al., Metalloberfl~che, 53(8):32; 1999 Kratkii spravotchnik chimika, Gosudarstvenoe nauchno-technicheskoe izdatelstvo chimicheskoj literaturi, Moskwa; 1954 Spravochnik chimika, t. III, Izdatelstvo "Chimija," Moskwa, Leningrad; 1964 JCPDF-PDS, 7-155 Nowacki, W. and J.H. Silverman, Z. Kristalogr., 115: 21; 1961 Sziraki, L. et al., J. of Appl. Electrochemistry, 29:927; 1999 Rasines, J., Tesis Doctoral, Universidad de Madrid, Facultad de Ciencias, Serie A, No. 111, Seccion de Quimicas, 27, Madrid; 1970
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800-473-9178• 713-462-2500 60 ton $29,418 • ALL MAJOR FAX 713-462-2544 80 ton $36,205 CREDIT E-mail: mmarrone@waterchillerscorn 100 ton $43,759 CARDS www waterchillerscorn • FLEXIBLELEASING ACCEPTED PLANS AVAILABLE FOB Houston,TX Circle 091 on reader card or go to w w w . t h r u . t o / w e b c o n n e c t
60
Metal Finishing
aInstitute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria; and blnstitute of Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria
inc galvanic alloy coatings are preferred for steel due to their better corrosion resistance and protective ability compared with the individual metals in the alloy. Zinc-manganese galvanic alloy, which has not been studied extensively, possesses the optimal characteristics for a protective coating especially in the cases when the Mn content is up to 15%. 1 The electrodeposition conditions for obtaining galvanic Zn-Mn alloy from a sulfate electrolyte have been experimentally found in a previous study. 1 Corrosion resistance of this coating (chromated and nonchromated) and its protective ability were investigated in a model corrosion medium free aerated 5% NaC1 s o l u t i o n - - u s i n g different methods such as polarization resistance (Rp) m e a s u r e m e n t s , P a a t s c h method, potentiodynamic (PD) polarization curves, and testing in a salt spray chamber (NSS-method). 2'~ These studies revealed one interesting and very important p h e n o m e n o n - - a passivation of the Zn-Mn alloy in presence of C1 ions. The passive region appears in the anodic range of PD curves after the zone of maximal anodic dissolution. 2 Phase composition of galvanic alloys Zn-Mn was also studied. 1 Formation of an intermetallic compound MnZn 7 (known as 81 phase) with hexagonal structure containing 550 --_8 atoms in the elementary cell was observed. This coating can be treated with a chromate solution for a galvanic zinc transparent and iridescent yellow type coatings (Duga), 2'3 which leads to a strong increase of the protective ability of the whole system. Literature about the composition of the corrosion products of these coatings as well as about the mechanism of their protection is rather scarce. That is why the aim of Part I of this three-part article is to study the elemental and phase composition of the corrosion products, formed on the surface of chromated and nonchromated galvanic Zn-Mn coatings with different Zn/Mn ratio after treatment in various conditions.
Z
56
EXPERIMENTAL
Sample Preparation Galvanic Zn-Mn alloys with different manganese content were electrodeposited from a starting electrolyte (SE), having the following composition: 10 g/L ZnSO4.7H20; 100 g/L MnSO4.HeO; 60 gfL (NH4)2.SO 4. The deposition was carried out in a thermostatic double-chamber electrolytic cell (500-ml volume), current density 2 A/dm 2, pH value 5, and room temperature (20°C) under continuous circulation of the solution (150 rpm). Steel plates 20 × 10 × 0.1 mm were used as substrates by doubleside deposition. Metallurgical zinc was taken for the anodes. 1 Galvanic coatings (nonchromated and chromated with iridescent yellow solution type coatings) with the following compositions were prepared: 1 1. Zn-Mn(-6%), obtained by SE with two additives 1 with trade names AZ-1 (contains polyethyleneglycol and benzoic acid) and AZ-2 (contains benzalaceton). This alloy consists generally of a pure zinc phase and smaller amounts of manganese and 81 phases. 2. Z n - M n ( - l l % ) , obtained by SE and AZ-1. The alloy contains mainly the intermetallic phase 81 and very small quantities of pure zinc phase. 3. Zinc, obtained by a slightly acidic sulfate electrolyte, pH value 5, room temperature, and current density 2 A/dm 2. The samples were treated in a model corrosion medium of free aerated 5% NaC1 solution (working volume 150 ml) at 200C as follows: Both galvanic alloys (points "a" and "b") were dissolved using external anodic polarization until reaching the passive zone of the anodic potentiodynamic polarization curve [passivating potential Epa~s = - 6 5 0 mV with respect to a saturated calomel reference electrode (SCE); current density in the passive zone ipas~ = 2.10 -5 A/dm2; Epassand ipa~ values were taken from our previous experiments2]. The treatment was stopped afLer exposure time of 10 minutes for the Metal Finishing
Table I. Results from Investigations with Microprobe Analysis of Corrosion-Treated Galvadic Alloys Zn-Mn (in wt%) Corrosionally Exposure at Exposure at
No 1
4
Samples Columns --~ Zn-Mn (-11%)
Untreated Samples A Zn-89.3 Mn-10.7
Epass During 10 Minutes C Zn-44.5 Mn-5.5
C1-22.3 Zn/Mn = 9.6
C1-50.0 Zn/Mn = 8.1
Zn-88.1
Zn-93.0 Mn-5.6 Cl-l.4 Zn]Mn = 16.6 Zn-90.2
Zn-73.6 Mn-0.7 C1-25.7 Zn/Mn = 105.1 Zn-72.3
Mn-8.2
Mn-7.6
Mn-2.3
Cr-3.7
Cr-2.0 C1 <0.2 Zn/Mn = 11.9 Zn-91.6 Mn-4.7 Cr-3.3 C1-0.4 Zn/Mn -- 19.5
Cr-2.0 C1-23.4 Zn/Mn = 31.4 Zn-72.2 Mn-l.5 Cr--4.3 C1-22.0 Zn/Mn = 48.1
Weight ratio Zn-Mn (-6%)
Zn/Mn = 8.3 Zn-94.2 Mn-5.8
Weight Ratio
Zn/Mn = 16.2
Zn-Mn (-11%) + "Duga"
Weight ratio Zn-Mn (-6%) + "Duga"
Zn/Mn = 10.7 Zn-92.0 Mn-5.0 Cr-3.0
Weight ratio
Zn/Mn = 18.4
samples in the passive zone at the conditions, described above. No investigations were carried out with pure zinc at Ep~s~ because this metal does not passivate in the medium used. Exposure of 6 days at corrosion potential (Ecorr) for all sample types in conditions close to those of natural corrosion processes.
Sample Characterization The elemental composition of the sample surface was determined quantitatively using microprobe analyzer J E O L Superprobe 733, Japan. The phase composition was determined using Xray powder diffractometer DRON-3 (Bragg-Brentano arrangement, C u K ~ - radiation and scintillation counter). EXPERIMENTAL RESULTS AND DISCUSSION
Microprobe Analysis Microprobe analysis data in depth of 2 to 4 ~Lm from the surface of the sample are outlined in Table I and show changes in the elemental composition, which depend on: composition of the initial galvanic coating, type of the corrosion treatment, and presence of a chromating film on the sample surface.
Nonchromated Samples In the case of nonchromated alloy samples 1 and 2, a decrease of Zn and Mn amounts after both corrosion treatments is registered. Due to the active anodic dissolution of the entire surface, the metal contents decrease at a greater rate in anodically treated samples 1 and 2 a t Epass (column C) cornSeptember 2001
E¢o~r During 6 Days B Zn-70.4 Mn-7.3
pared with columns A and B. Zinc and manganese contents decrease almost two times that of the initial amounts in sample 1 (compare A with C). In sample 2 at Epa~s the quantity of Zn decreases about one-forth compared with the initial composition, while that of manganese decreases about 8 times (columns A and C). Manganese dissolves at a lesser rate in sample 2 at Eco~ (columns A and B). After anodic dissolution of sample 2 the manganese dissolves in higher degree compared with zinc (column C). An appearance of chlorine after the corrosion treatments is also registered. It occurs in higher contents in sample 1, which consists generally of the intermetallic compound MnZn7, compared with sample 2 by both treatments (columns B and C). The registered chlorine amounts depend on the type of corrosion treatment. The difference between samples 1 and 2 for this element is almost 2 times at Epa~ (column C) and about 16 times at Eco~ (column B). The higher chlorine quantities for sample 1 compared with sample 2 after 6 days exposure in the medium at E¢o~ (column B) could be explained with the appearance of manganese and zinc chlorides, which partially remain on the surface, included in the corrosion products of the intermetallic phase (it covers almost the whole surface of this sample). For example, ZnC12 is almost five times more soluble in water at 20°C compared with MnC12, 4'5 which means that the higher manganese amount must ensure higher corrosion resistance. Due to the longer exposure time in the medium for samples 1 and 2 an 57
appearance of NaC1 in the sites of corrosion damages could be also expected. The higher chlorine amounts for the same samples in column C compared with column B appear due to the passive oxide film, which forms in the passive zone of the PD curve at external anodic polarization and seals up both corrosion products (chlorides) and some amounts of the model medium. The weight Zn/Mn ratio in sample 1 remains almost constant after different corrosion treatments, but increases strongly (more than six times) for sample 2 after Epass (compare column A with C). This is probably due to the faster manganese dissolution in sample 2 in these conditions (column C). This metal is included here in smaller amounts in the coating compared with sample 1, and its phase is scattered into the zinc phase.
~
"....:"jl
58
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S
l•
Chromated Samples Smaller Zn and Mn contents as a result of the chromating are registered on the surfaces of the corrosionally untreated samples 3 and 4 compared with samples 1 and 2, respectively (column A). Chromium content is also registered by both these alloys. Zinc and manganese quantities after corrosion treatments (B and C) decrease generally in lower degree compared with samples 1 and 2 due to the protection of the chromating film. A decrease of the chlorine amounts after both corrosion treatments is registered by these samples (3 and 4) compared with the nonchromated ones (1 and 2) (B and C). This difference is more than twice by comparison of the samples 1 and 3, column C, but this phenomenon is much more clearly registered at Eco~ (column B). Chlorine quantities after exposure at Epass are very close for samples 3 and 4 (23.4% and 22.0%, respectively, column C), which leads to the conclusion that chromated samples have similar behavior independent on their different composition. Chlorine contents are much higher for specimens 3 and 4 after anodic t r e a t m e n t compared with the results after exposure at E~orr (C and B). The reason is the same as explaned above in the case of nonchromated samples--sealing up the surface with oxides in the passive zone of the PD curve at external anodic polarization. The weight Zn/Mn ratio for samples 3 and 4 changes weakly after the corrosion t r e a t m e n t at E~o~ (A and B). After anodic treatment these changes are nearly two and a half or three times higher (A and C). Chromium content on the surface changes after the different corrosion t r e a t m e n t s - - i n sample 3 its amount decreases about two times, independent on
+
+~* -'s
C
i~
10
20
~
~
A
~
Figure 1. Diffraction patterns in 5% NaCI solution for (A) nonchromated zinc after exposure for 6 days at Eoor,; (B) nonchromated Zn-Mn (6%) after exposure at Epa.; (C) chromated ZnMn(6%) after exposure at EpN,. * diffraction lines of ZHC; + diffraction lines of NaCI
treatment type (A compared with B and C). Contrary to this result, sample 4 illustrates an increase of the chromium content from one-tenth (A compared with B) until more than one-third (A and C), which means that the surface is enriched with this metal due to the prior dissolution of zinc and manganese. A conclusion can be made that the coating consisting practically in phase 81 is more resistant in this medium and at these conditions. X-Ray
Diffraction
Results obtained by X-ray diffraction analysis for chromated and nonchromated Zn-Mn alloys with different Mn content are represented in Figure 1 and Figure 2. Figure 1 shows diffraction patterns taken after exposure at Eco~ during 6 days for nonchromated pure zinc (1A) and for nonchromated (1B), and chromated (1C) Zn-Mn ( - 6 % ) samples after 10 minutes at Epass, respectively. Figure 2 shows the diffraction patterns for nonchromated and corrosionally untreated Zn-Mn ( - 1 1 % ) (2A); for Metal Finishing
Table II. Observed and Published s XRD Data of Zns(OH)sClz.H20
["
.
ll
It.tl
., z .
•
B
5, Za
10
2o
30
40
A
50
2 [deg.] Figure 2. Diffraction patterns in 5% NaCI solution for (A) nonchromated and corrosionally untreated Zn-Mn (11%); (B) nonchromated Zn-Mn (11%) after exposure for 6 days at E©orr; (C) nonchromated Zn-Mn (6%) after exposure for 6 days at E©o~. * diffraction lines of ZHC; + diffraction lines of NaCI
Zn-Mn(-11%) (2B), and for Zn-Mn ( - 6 % ) (2C), both after 6 days at Ecorr. XRD spectra show lines of the substrate a-Fe, of the coating - Z n and/or of S1 (see Fig. 1 and 2). In addition, diffraction lines of the zinc hydroxide chloride Zns(OH)sC12.H20 (ZHC) appear in the alloy coatings after corrosion treatment (Fig. 1B,C and 2B,C). This compound has a hexagonal lattice and may be regarded as a double salt, 4Zn(OH)2.ZnC12.H20. 6 It crystallizes in the rhombohedral system with ah = 0.634--+0.001 rim, Ca = 2.364--+0.002 nm, S.G. R3m, Z = 3 per threefold primitive hexagonal cell. The basic s t r u c t u r a l unit is a charged layer, which m a y be derived from a hypothetical C6-Zn(OH) 2 of the CDI2 type. A q u a r t e r of the octahedrally coordin a t e d Zn atoms are removed and replaced by pairs of t e t r a h e d r a l l y coordinated Zn atoms, one on each side of the layer, giving a layer of composition [Zns(OH)s] 2+. The fourth bond from the tetrahedrally coordinated Zn is to the C1- ion that balances September 2001
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
dobs., A 7.90 5.37 4.00 3.95 3.57 3.16 2.931 2.880 2.722 2.675 2.600 2.368 2.073 1.899 1.791 1.781 1.771 1.701 1.689 1.578 1.551
dpubl., A 7.87 5.35 4.02 3.94 3.58 3.17 2.940 2.878 2.725 2.672 2.601 2.374 2.067 1.900 1.792 1.782 1.768 1.699 1.693 1.585 1.554
I/Io 100 20 20 8 25 35 30 25 55 65 20 35 8 8 8 6 14 14 12 35 16
hkl 003 101 104 006 105 110 113 107 201 202 108 205 211 215 2.0.10 303 217 218 2.0.11 220 223
the charge and the water molecules are situated between the layers. 7 In Table II the observed interplanar distances of ZHC are compared to those published in J C P D S - P D F data. 6 The XRD pattern of the nonchromated ZnM n ( - 6 % ) sample after anodic treatment contains the diffraction lines ofZHC, ~-Fe and NaC1, lB. This means that due to the active anodic dissolution the alloy coating is totally transformed in ZHC. Sodium chloride penetrates in the depth of the coating due to the relative high rate of the used external anodic polarization, 1 mV/sec. The same is the situation with the sample Zn-Mn (-11%), which is not shown on the figure. ZHC also appears on the chromating sample Zn-Mn (-6%), 1C, where lines of zinc and iron are also registered. In this case ZHC covers only the zones where the chromating film is already dissolved. That is why the lines of ZHC are less intensive in this sample compared with Fig. lB. XRD spectra of the samples at Ecor~ after 6 days exposure (Fig. 2B and 2C) illustrate the presence of the compound ZHC for both nonchromated alloys contrary to the pure zinc (Fig. 1A) and to the corrosionally untreated sample Zn-Mn ( - 1 1 % ) (Fig. 2A), where the lines of ZHC are missing. The phases 51 and Zn are also present in Fig. 2B and 2C. These results mean that the coating is not totally transformed at these conditions. Smaller quantities of ZHC appear on the sample Zn-Mn ( - 6 % ) (Fig. 2C) compared with the Zn-Mn ( - 1 1 % ) (Fig. 2B). The same samples, b u t chromated, show relatively 59
weaker lines of ZHC due to the protective action of the chromating film. It is obvious that the coatings of the phase 51, Zn-Mn (-11%), transform more easy to ZHC than the samples ZnMn (-6%). Probably, the homogeneous distribution of Mn in the intermetallic coating causes the nucleation and growth of uniform ZHC layer over the whole surface. Although no chromium compounds are detected by XRD, the microprobe analysis confirms their presence as an element on the sample surfaces. This leads to the conclusion that the chromium compounds are present in smaller quantities or exist in an amorphous form. s Lower amounts of NaC1 are registered for the samples, treated at Eco~ (Fig. 2B). For the nonchromated alloys this compound is also in small quantities (Fig. 1B, 2B, 2C). The chromating film protects the galvanic coating and impedes the formation of ZHC. Moreover, by the alloys treated at Eco~, during the experimental time the corrosion products are gradually transfering in the solution and partially remain in the depth of corrosion damaged states.
the Zn-Mn (-6%) coatings in similar conditions of corrosion treatment. . Contrary to other zinc alloys, Zn-Mn passivates in 5% NaC1 solution at external anodic polarization due to the formation of ZHC. The newly created passive film consists entirely from this compound, but not from oxides and hydroxides as in other corrosion media. For example, the ZHC appears at a pH value of about 7.0 to 7.5, while the Zn(OH) 2 at pH values higher than 10.0. . The results for this alloy at Eco~r can be compared with our previous results from salt-spray-chamber. 3 Chromated samples of Zn-Mn (~ 11%) resist up to 300 hours till 2.5% of their surface is damaged. Chromated pure zinc specimens from a slightly acidic bath and chromated samples of Zn-Mn (-6%) endure at these conditions almost 180 hours. 3 ACKNOWLEI)6EMEMTS
The authors express their gratitude to the Bulgarian National Fund "Scientific Researches," Sofia, for the financial support of these investigations.
CONCLUSIONS
REFEREI~ICES
The following conclusions, based on the results obtained, can be made:
1. 2. 3. 4.
1. After corrosion treatment of Zn-Mn alloys with different manganese content and phase composition in free aerated neutral model medium with presence of corrosion activators C1 ions the compound ZHC appears on the specimen surface. It has a low product of solubility (10 -14"2 mol]L)9 and ensures higher protective ability in comparison with the pure zinc. 2. ZHC forms readily only in the case when manganese is available. Galvanic coatings of the intermetallic Zn-Mn compound produce more thick and compact ZHC layers than samples containing
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Boschkov, N. et al., Metalloberfl~che, 52(7):514; 1998 Boschkov, N. et al., Metalloberfldche, 53(7):27; 1999 Boschkov, N. et al., Metalloberfl~che, 53(8):32; 1999 Kratkii spravotchnik chimika, Gosudarstvenoe nauchno-technicheskoe izdatelstvo chimicheskoj literaturi, Moskwa; 1954 Spravochnik chimika, t. III, Izdatelstvo "Chimija," Moskwa, Leningrad; 1964 JCPDF-PDS, 7-155 Nowacki, W. and J.H. Silverman, Z. Kristalogr., 115: 21; 1961 Sziraki, L. et al., J. of Appl. Electrochemistry, 29:927; 1999 Rasines, J., Tesis Doctoral, Universidad de Madrid, Facultad de Ciencias, Serie A, No. 111, Seccion de Quimicas, 27, Madrid; 1970
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Metal Finishing