Protection of galvanized steel from corrosion in NaCl solution by coverage with phytic acid SAM modified with some cations and thiols

Corrosion Science 55 (2012) 339–350

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Protection of galvanized steel from corrosion in NaCl solution by coverage with phytic acid SAM modified with some cations and thiols Abdel-Rahman El-Sayed a,⇑, Ulrich Harm b, Klaus-Michael Mangold b, Wolfram Fürbeth b a b

Chemistry Dept., Faculty of Science, Sohag University, Sohag 82524, Egypt Karl-Winnacker-Institut, Dechema, Frankfurt, Germany

a r t i c l e

i n f o

Article history: Received 14 July 2011 Accepted 28 October 2011 Available online 7 November 2011 Keywords: A. Zinc A. Organic coatings B. Polarization B. EIS C. Passive films C. Rust

a b s t r a c t Ultrathin films of various compounds were prepared on galvanized steel coated with both phytic acid (PA) and diethylene triamine pentamethane phosphonic acid (DETPMPS) self-assembled monolayers (SAM), and in the presence of small amounts of special salt (MnSO4 or Cr(NO3)3). Phytic acid and phytic acid–Mn2+ were modified with dithio-oxamide (DTOA) and 2,3-dimercapto-1-propanol (DMP). Tafel plots and impedance measurements were used. Most investigated conversion coatings of the samples pretreated only in H2SO4/H2O2-mixture, showed that the protective efficiency values (P%) are greater than those of pretreated in both hot alkaline solution and H2SO4/H2O2-mixture. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Demand on good quality corrosion resistant zinc-coated steel has increased significantly in the past few years. The zinc coating formed on the hot-dip galvanized steel gives excellent protection of the steel sheets. For additional corrosion protection, the chromate treatment was used for a long time. Like hexavalent Cr ions, molybdate and tungstate anions are effective corrosion inhibitors for Zn and Al [1–5]. However, high valence chromium compounds (Cr-VI) are very poisonous to the environment and starting from 2007, they are banned in the European Union [6]. Thus numerous studies are directed to seek environmentally friendly substituents for chromium including soluble salts of rare earth metals (such as cerium), nickel or bismuth [1,7–9] or the use of molybdates [10–12], silanes [13], phosphate [14–17], cerium-based [9,18–20], tungstate-based [21] and molybdate-based treatments [22–25]. Other studies of corrosion prevention include chelation by organic substances forming stable and hardly soluble organometallic compounds [26]. A number of papers reported the adsorption of phosphonic- [27,28], sulfonic- [29], hydroxamic amphiphiles [30], diphosphonate, 1,5diphosphono-pentane [31,32], ultrathin polymer coating of carboxylate [33] and polypyrrole [34] on a variety of metal substrates. It is reported that rare-earth conversion coating is one of the alternatives for chromate-free conversion coating. Especially, cerium is environmentally acceptable and effective corrosion inhibitor ⇑ Corresponding author. Tel.: +20 932308786; fax: +20 934601159. E-mail address: [email protected] (A.-R. El-Sayed). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.10.036

for metals like copper and zinc [35,36]. As a corrosion inhibitor phytic acid exhibits many merits, such as nontoxic, stable, soluble, inert, high affinity and cheap [37,38]. The corrosion resistance of galvanized steel could be improved by applying the sole cerium or phytic acid passivation treatment, however, the corrosion resistance is inferior to that of chromate conversion coating [39]. Thiols can graft and organize on metallic substrates leading to self-assembled monolayers (SAMs) [40]. In the presence of traces of oxygen, the zinc is oxidized and subsequently the thiolate-Zn (II)-SR was formed whatever the adsorption time tads. and ageing conditions. This is attributed to the inability of the thiol to reduce the Zn (II) [41]. Shimura and Aramki [42] showed that the persistence of a hydroxymethylbenzene SAM was superior by far to that of toluenethiol SAM adsorbed on the iron surface by the formation of a coordinate bond between sulfur and iron atoms. Literature reveals that the studied compounds such as 2,3-dimercapto-1-propanol (DMP) and dithio-oxamide (DTOA) have not almost yet been used as self-assembled monolayers on galvanized steel. A few investigators studied the synergistic effect between inorganic and phytic acid (PA) to form conversion coating [43]. The aim of this work is to investigate conversion coating on galvanized steel by synergistic effect of phytic–Mn2+ and DETPMPS–Cr3+. Phytic acid or phytic acid–Mn2+ conversion coating film (first step) followed with dipping in DMP or DTOA as a second step. The corrosion behavior of coated samples in 0.5 M NaCl was studied using Tafel-plots and electrochemical impedance spectroscopy (EIS) techniques. The relationships of corrosion resistance for the conversion coating with different compounds and the effect of

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pretreatment samples (before coating) on the protective efficiency were investigated. The chemical structure of some investigated SAM molecules is shown in Fig. 1. 2. Experimental 2.1. Materials A high grade reagents of phytic acid (PA), diethylene triamine pentamethane phosphonic acid (DETPMPS), 2,3-dimercapto-1-propanol (DMP), dithio-oxamide (DTOA), MgSO4 and Cr(NO3)3 were used for the treatment of galvanized steel. Organic solvents, ethanol and n-propanol were all high grade chemicals, utilized in the preparation and modification of DMP and DTOA, respectively, on galvanized steel. High grade reagents of H2SO4, H2O2 and alkaline solutions were used for the pretreatment of the samples before coating films. An aqueous solution of 0.5 M NaCl was prepared by dissolving an analytical reagent of NaCl in distilled water. The galvanized steel sheets were received from ‘‘Chemetall GmbH (Germany)’’ with the product name: ‘‘Gardobond G’’. The surface is nearly pure zinc (>99% Zn). A series of samples with dimension of 30  40  1 mm were prepared as electrochemical test. However the apparent exposed area in the electrolytic cell is 1 cm2. Each sample was cleaned sequentially with ethanol and distilled water pretreatment, then soaked: (a) n a mixture from H2SO4 (0.05 M) and H2O2 (0.5%) for 10 s, and washed with distilled water and ethanol. Then the sample was dried in a hot air stream.

(b) In a hot alkaline solution (50 °C) for 5 min, and washed with distilled water, dipping again in the above mentioned mixture of H2SO4/H2O2 for 10 s, then washed with distilled water and ethanol, and also dried in a hot air stream. The solutions of phytic acid (PA) and DETPMPS (0.5 mM) were prepared by dissolving the appropriate amount by volume in distilled water. The pH value was adjusted to about 4 by diluted KOH solution. The solutions of DMP and DTOA (1 mM) were prepared by dissolving the appropriate amount by volume in distilled water and 5% ethanol and 5% n-propanol, respectively. Solutions (1 mM) of both MnSO4 and Cr(NO3)3 were prepared by dissolving the appropriate amount by weight in distilled water. The temperature of all investigated baths was 45 °C. 2.2. Preparation of coating films The coating films were conducted to the following treatments in Table 1. 2.3. Electrochemical measurements The corrosion resistance of the coating film was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The polarization curves measurements were performed via a model voltalab Potentiostat, and conducted in 0.5 M NaCl solution using a conventional three electrode cell. The specimen under study with an exposure area of 1 cm2, a platinum sheet of 16 cm2 and a saturated calomel

Fig. 1. Chemical structure of some investigated SAM molecules.

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A.-R. El-Sayed et al. / Corrosion Science 55 (2012) 339–350 Table 1 The composition of coating baths for coating films formed on galvanized steel surfaces. Bath composition for first dipping

Dipping time

Bath composition for second dipping

Dipping time

Coating film

0.5 mM Phytic acid 0.5 mM Phytic acid + 1 mM MnSO4 0.5 mM Phytic acid 0.5 mM Phytic acid + 1 mM MnSO4 1 mM DMP 1 mM DTOA 0.5 mM Phytic acid 0.5 mM Phytic acid + 1 mM MnSO4 0.5 mM DETPMPS 0.5 mM DETPMPS + 1 mM Cr(NO3)3

30 s 30 s 30 s 30 s 60 s 60 s 30 s 30 s 30 s 30 s

– – 1 mM 1 mM – – 1 mM 1 mM – –

– – 60 s 60 s – – 60 s 60 s – –

Phytic acid Phytic acid + Mn2+ Phytic acid + DMP Phytic acid + Mn2+ + DMP DMP DTOA Phytic acid + DTOA Phytic acid + Mn2+ + DTOA DETPMPS DETPMPS + Cr3+

DMP DMP

DTOA DTOA

-2

-3

1 2 3 4 5

Untreated Phytic acid 2+ Phytic acid+ Mn Phytic acid+ DMP 2+ Phytic acid +Mn + DMP

1 2 3 4

-4

5

2

log i, µA/cm

electrode (SCE) were used as the working, counter and reference electrodes, respectively. To avoid contamination, the reference electrode was connected to the working through a bridge filled with the solution under test. The tip of the bridge being pressed against the electrode, to minimize, the IR drop, of the solution. All electrochemical measurements were measured after a steady open circuit potential (OCP) was reached. The polarization curve was measured potentiodynamically by sweeping the potential in the positive direction at a scan rate of 1 mV/s and a sweep range from an initial potential of 0.150 V to a final potential of +0.150 V (both are relative to OCP) [44]. EIS measurements were carried out using AC signals of amplitude 5 mV peak to peak at OCP (Ecorr) in the frequency range from 10 kHz to 5 mHz. Potentiostat model pp241 from Zahner Company (Germany) and electrochemical software of this model (pp-inspector) were used. All experiments were conducted for each sample under the same conditions in order to ensure reproduction of the results at room temperature (20 ± 1 °C).

-5

-6

-7

-8

3. Results and discussion -1.25

3.1. Extrapolation of cathodic and anodic Tafel lines 3.1.1. Polarization measurements of the galvanized steel without and with covered samples in 0.5 M NaCl solution, pretreated using H2SO4/ H2O2-mixture Fig. 2 exhibits polarization curves of galvanized steel uncoated and coated with the films of phytic acid, or phytic acid + Mn2+, in addition to both phytic acid, and phytic acid + Mn2+ modified with DMP in 0.5 M NaCl solution after immersion for 2 h. Corrosion parameters were calculated on the basis of the cathodic and anodic potential vs. current density characteristics in the Tafel potential region [45]. The values of the corrosion current density (icorr) for the coated and the uncoated galvanized steel were determined by the extrapolation of the cathodic and anodic Tafel lines to corrosion potential (Ecorr). As can be seen that a marked shift in both cathodic and anodic branches of the polarization curves towards lower current densities of the coated galvanized steel, in addition to significant positive shift in the corrosion potential (Ecorr) are observed. The results showed that the inhibitive effect of the coated films on the anodic process is greater than that on the cathodic one. The marked suppression of the anodic process was observed with the coating film of the phytic acid modified with DMP on the surface, suggesting stabilization of Zn atoms by adsorption of DMP through phytic acid SAM forming a stable bond between sulfur and Zn atoms [46]. The curves also showed inhibition of the cathodic process by coating the surface with this film, being attributed to blocking diffusion of molecular oxygen through the film. The data in Table 2 show that, the corrosion current density is highly decreased in the case of the coated surface with a multilayer of both phytic acid and phytic acid + Mn2+ modified with DMP compared with the case of phytic acid or phytic acid + Mn2+ (first

-1.20

-1.15

-1.10

-1.05

-1.00

-0.95

-0.90

-0.85

Potential, V vs. SCE Fig. 2. Polarization curves of coating samples in 0.5 M NaCl solution, and retreatment in H2SO4/H2O.

dipping) coated samples. Therefore, the calculation of annual consumptions of zinc runs parallel with icorr/cm2 for uncoated and coated samples. It is observed that, the loss of zinc is significantly decreased with effective protective layer. The data in Table 2 exhibit that the protective efficiency (P%) value decreases in the following sequence:

phytic acid modified with DMP > phytic acid þ Mn2þ modified with DMP > phytic acid þ Mn2þ > phytic acid This trend exhibits that phytic acid (first dipping) modified with DMP (second dipping) has higher P% (97%), and the conversion layer appears thick and compact. Accordingly, a surface completely covered by the conversion layer could prevent the penetration of Cl ions [47]. Fig. 3 shows potentiodynamic polarization curves of coating samples with DTOA and DMP in 0.5 M NaCl solution. The calculated corrosion data for the coated samples, derived from the polarization curves, are shown in Table 2. The corrosion current density (icorr) values of the samples treated by DTOA and DMP are very low (0.96 and 0.19 lA/cm2, respectively) compared with that of the uncoated sample (6.08 lA/cm2). On the other hand, corresponding corrosion potential (Ecorr) values of DTOA and DMP were identified as 1034 and 1024 mV vs. SCE, respectively. This indicates that the corrosion potential of the coating samples by DTOA and DMP shifts to more positive potential compared with

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Table 2 Corrosion parameters obtained from Tafel extrapolation curves for galvanized steel in 0.5 M NaCl solution coated with different compounds, and the pretreatment in H2SO4/H2O2. Coating films

Ecorr mV vs. SCE

icorr (lA/cm2)

P%

Corrosion (lm/y)

Untreated Phytic acid Phytic acid + Mn2+ Phytic acid + DMP Phytic acid + Mn2+ + DMP DMP DTOA Phytic acid + DTOA Phytic acid + Mn2+ + DTOA DETPMPS DETPMPS + Cr3+

1113 1113 1093 1053 1043 1024 1034 1053 1055 1084 1080

6.08 2.65 0.80 0.18 0.33 0.19 0.96 0.67 0.13 2.89 3.42

– 56.4 86.8 97.0 94.5 96.9 84.2 89.0 97.9 52.5 43.8

91.00 39.70 12.07 2.65 4.90 2.90 14.39 10.06 1.92 43.34 51.36

acid + Mn2+ modified with DTOA in 0.5 M NaCl solution after immersion for 2 h. The anodic dissolution of Zn,

-2

Zn ! Zn2þ þ 2e -3

and the cathodic process, reduction of molecular oxygen,

O2 þ 2H2 O þ 4e ! 4OH

-4

ð2Þ

both processes were markedly suppressed by coverage of these electrodes with coating films. The data in Table 2 infer that the protective efficiency (P%) value of the coated films decreases in the order:

2

log i , µA/cm

ð1Þ

-5

phytic acid þ Mn2þ modified with DTOA ð97:9%Þ > phytic acid

-6

modified with DTOA ð89%Þ > DTOA ð84:2%Þ 1

-7

1 2 3

2 3

-8 -1.25

-1.20

-1.15

-1.10

-1.05

-1.00

Untreated DTOA DMP

-0.95

-0.90

-0.85

Potential, V vs. SCE Fig. 3. Polarization curves of coating samples in 0.5 M NaCl solution, and pretreatment in H2SO4/H2O2.

that of the uncoated sample (1113 mV vs. SCE) in 0.5 M NaCl. According to the data in Table 2 and polarization curves in Fig. 3, the coating sample by DMP exhibited lower current density for the corrosion and more positive potential compared with those of DTOA. Therefore, DMP passivated sample exhibits better corrosion resistance (P% = 96.9%) than that of DTOA passivated one (84.2%). This behavior can be attributed to the more stable monolayers formed in the case of DMP than those of DTOA. This stability is correlated with the close packing of the monolayers on the surface. The optimization of both structures using DFT calculations (Table 3) shows that DMP has longer C–S bond length than that of DTOA (C–S = 1.91 Å and 1.70 Å, respectively). This longer bond length enables easier adsorption of SH. On the other hand, the charge is equally distributed on the S atoms of DTOA (0.296) which makes resonance with the amine functionality as shown in the HOMO–LUMO orbitals (Fig. 4), where the electrons are delocalized through the molecule surface, dislike from DMP having HOMO orbital with localized electrons on the S atoms. The effectiveness of the monolayers ageing must be connected to the one which relates to the protection against the observed corrosion under the same conditions [40]. The sensitivity to both corrosion and ageing should be attributed to the initial presence of holes/defects which can be the starting point of chemical reactions which become increasingly fast. Fig. 5 exhibits polarization curves of uncoated and coated galvanized steel electrodes with films of both phytic acid and phytic

This indicates that marked suppression is observed in the case of coating film on the surface with multilayer of phytic acid + Mn2+ modified with DTOA. This behavior is attributed to stabilization of Zn metal at the surface by the formation of a stable bond between sulfur and Zn atoms. Also, many interconnection bonds between molecules anchored on the surface are formed. Consequently, they enhanced hindrance to oxygen diffusion through the films of which the thicknesses increased [42]. In addition, the mentioned reasons contribute suppression of the anodic process. The data in Table 2 show that the values of Ecorr shift to more positive potential for the sample coated with mentioned films further examined by polarization measurements after immersion time of 2 h. The protective efficiency value (P%) of phytic acid + Mn2+ modified with DTOA (second dipping) is above 97% during immersion in 0.5 M NaCl for 2 h. This indicates a high persistence of the film in the protective ability against corrosion. The persistence would be attributed to the strong adsorption of phytic acid + Mn2+ leading to zinc passivation, and the increase of the interconnection with DTOA [48]. Fig. 6 shows the polarization curves of galvanized steel coated with films of both D ETPMPS and DETPMPS–Cr3+ in 0.5 M NaCl solution under the same conditions. It can be seen that the polarization curves are almost in superposition for the samples with DETPMPS and DETPMPS–Cr3+ compared with that of uncoated sample. The electrochemical corrosion data should be obtained based on the polarization curves for the two kinds of coatings, which are shown in Table 2. The data exhibit that the corrosion current density values of DETPMPS and DETPMPS–Cr3+ conversion coating are 2.89 lA/cm2 and 3.42 lA/cm2, respectively, which are lower than that of the uncoated galvanized steel (6.08 lA/cm2). Contrarily the corrosion potentials (Ecorr) of the above mentioned conversion coatings are higher than that value of the uncoated sample. Based on the above results, the corrosion resistance of galvanized steel can be improved through the treatment of chemical conversion in a solution containing DETPMPS and DETPMPS–Cr3+ in appropriate conditions. However, the corrosion resistance of

343

A.-R. El-Sayed et al. / Corrosion Science 55 (2012) 339–350 Table 3 Calculated atomic charge for (a) 2,3-dimercapto-1-propanol (DMP) and (b) dithio-oxamide (DTOA). (a) Atomic charge distribution S1 C3 H5

0.101 0.738 0.250

S2 H1 H6

0.194 0.130 0.245

O H2 H7

0.369 0.266 0.346

C1 H3 H8

0.714 0.256 0.129

C2 H4

0.137 0.357

S1 C2

0.296 0.008

S2 H1

0.296 0.381

N1 H2

0.426 0.348

N2 H3

0.426 0.348

C1 H4

0.008 0.381

(b) Atomic charge distribution

Fig. 4. Electronic structure of HOMO–LUMO orbitals of (a) DMP and, (b) DTOA.

DETPMPS conversion layers is higher than that of DETPMPS–Cr3+ layer. This indicates that the stability of the sample with DETPMPS conversion coating is higher than that of the sample with DETPMPS–Cr3+ coating. It may be the addition of Cr3+ ions to DETPMPS

has the ability to chelate with functional groups of the mentioned compound. Consequently, the number of the coordinate functional groups with the surface decreases compared with that in the case of DETPMPS layer. Therefore, the corrosion resistance of galvanized

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1 2 3 4

-2

-3

Untreated DTOA Phytic acid + DTOA Phytic acid + Mn2+ + DTOA

log i, µA/cm 2

-4

-5

3

-6

1 -7

2

-8

4 -9 -1.25

-1.20

-1.15

-1.10

-1.05

-1.00

-0.95

-0.90

-0.85

Potential, V vs. SCE Fig. 5. Polarization curves of coating samples in 0.5 M NaCl solution, and pretreatment in H2SO4/H2O2.

-2

-3

log i, µA/cm

2

-4

-5

-6

1 -7

-1.25

-1.20

-1.15

-1.10

2

1 2 3

3

-1.05

Untreated DETPMPS 3+ DETPMPS + Cr

-1.00

-0.95

-0.90

in the case of the former pretreatment (8.33 lA/cm2) than that of the latter one (6.08 lA/cm2). This behavior can be attributed to that, the pretreatment in hot alkaline solution dissolves most oxides formed on the sample surface, thus rendering the surface more active. The polarization curves of the samples covered with the films of DMP and phytic acid–Mn2+ modified with DMP demonstrated marked suppression of the anodic and cathodic processes, as shown in Fig. 7. The values of protective efficiency (P%) in Table 4 of the two mentioned films are almost the same (93 ± 0.3%). These protective films are persistent during immersion in aerated 0.5 M NaCl for 2 h. Thus, the persistence of these films is far superior on the galvanized steel. This indicates that the presence of phytic acid–Mn2+ layer (first dipping) with DMP conversion coating (second dipping), exhibited slightly increase in P% value compared with that of DMP layer. The influence of the pretreated samples (before coating) with both, (a) H2SO4/H2O2-mixture and (b) hot alkaline in addition to step (a) on the protective efficiency (P%) of the mentioned coating films is compared. The data exhibit that the P% value of the coated film with DMP on galvanized steel pretreated in H2SO4/H2O2mixture only is higher (96.9%) than that of the galvanized steel pretreated with hot alkaline solution in addition to H2SO4/H2O2 (92.8%). The enhanced P% value of the former electrode may be attributed to the formation of a protective Zn oxide deposit, leading to close packing in the film [42]. On the other hand, the lower P% value in the latter case may be attributed to the absence of the oxide on the surface, leading to less interconnected film for preventing corrosion. On the other hand, the data show that P% values of the phytic acid–Mn2+ (first dipping) modified with DMP (second dipping) exhibit approximately the same trends for the two mentioned pretreatments. This behavior may be attributed to the reaction of DMP occurring inside the film of phytic acid–Mn2+ [49]. Fig. 8 represents polarization curves of the coated samples with DETPMPS and DETPMPS–Cr3+ in 0.5 M NaCl solution. It can be seen that the coated samples with the two investigated films result in appreciable shift in the anodic branches and no shift in the cathodic branches of the polarization curves towards lower current densities. The positive shift in the corrosion potential (Ecorr) indicates that the anodic process is much more affected than the cathodic one. This suggests that the coated films on the galvanized steel by the two investigated films act predominantly as an anodic

-2.0

-0.85

-2.5

Potential, V vs. SCE

-3.0 Fig. 6. Polarization curves of coating samples in 0.5 M NaCl solution, and pretreatment in H2SO4/H2O2.

1 2 3

Untreated DMP 2+ Phytic acid + Mn +DMP

-3.5

3.1.2. Polarization measurements of the galvanized steel without and with covered samples in 0.5 M NaCl solutions, pretreated using both hot alkaline and H2SO4 /H2O2 solutions In order to compare between the pretreated samples in both H2SO4/H2O2-mixture and hot alkaline solution followed with dipping in H2SO4/H2O2-mixture, the effect of hot alkaline solution as pretreatment of the uncoated and coated samples has been studied. Fig. 7 demonstrates polarization curves of galvanized steel samples, uncoated and coated with the films of DMP and phytic acid–Mn2+, modified with DMP. The polarization curves of uncoated sample pretreated in hot alkaline followed with H2SO4/ H2O2 solutions, exhibit that the corrosion current density is higher

log i, µA/cm

steel conversion coating greatly depends on the properties of the surface layer [47].

2

-4.0 -4.5 -5.0 -5.5 -6.0

1

-6.5 -7.0

2

-7.5 -8.0

3

-8.5 -1.25

-1.20

-1.15

-1.10

-1.05

-1.00

-0.95

-0.90

-0.85

Potential, V vs. SCE Fig. 7. Polarization curves of coating samples in 0.5 M NaCl solution, and pretreatment in alkaline and H2SO4/H2O2.

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Table 4 Corrosion parameters obtained from Tafel extrapolation curves for galvanized steel in 0.5 M NaCl solution coated with different compounds and the pretreatment in both alkaline solution and H2SO4/H2O2. Coating films

Ecorr (mV vs. SCE)

icorr (lA/cm2)

P%

Corrosion (lm/y)

Untreated Phytic acid DMP DTOA Phytic acid + Mn2+ + DMP Phytic acid + Mn2+ + DTOA DETPMPS DETPMPS + Cr3+

1115 1115 1031 1051 1031 1046 1088 1082

8.33 4.82 0.60 1.92 0.56 0.69 2.07 3.15

– 42.13 92.8 77.0 93.3 91.7 75.2 62.2

124.90 75.20 9.03 28.89 10.27 8.44 30.95 47.28

-3.0

log i, µA/cm

2

-3.5 -4.0 -4.5 -5.0 -5.5 -6.0

1

-6.5

23

-7.0 -1.25

1 2 3

-1.20

-1.15

-1.10

-1.05

-1.00

Untreated DETPMPS 3+ DETPMPS + Cr

-0.95

-0.90

-0.85

Potential, V vs. SCE Fig. 8. Polarization curves of coating samples in 0.5 M NaCl solution, and pretreatment in alkaline and H2SO4/H2O2.

protection [50]. The P% value in the case of DETPMPS film is higher than that of DETPMPS–Cr3+, and this trend is similar to that observed for the samples pretreated in H2SO4/H2O2-mixture. As mentioned above, this indicates that the film formed on the galvanized steel using DETPMPS is more protective than that of DETPMPS– Cr3+. This behavior can be rationalized to that the direct interaction between the surface and functional groups of DETPMPS molecules is more than that of DETPMPS–Cr3+ as complex. On the other hand, the data exhibited that P% values in the case of the samples pretreated in both hot alkaline solution and H2SO4/H2O2-mixture are higher (75.3% and 62.2% of DETPMPS and DETPMPS–Cr3+, respectively) than those of the samples pretreated in H2SO4/H2O2-mixture only (52.5% and 43.8% of DETPMPS and DETPMPS–Cr3+, respectively) in the case of the two investigated films. These results can be discussed on the basis that the pretreatment in H2SO4/ H2O2-mixture has not completely dissolved the oxide on the samples. Therefore, the oxide of zinc deposited on the surface with DETPMPS or DETPMPS–Cr3+ complex together formed a mixture surface layer. This layer containing oxide would be very loose and could not be an effective corrosion resistant [51,52]. This would be due to a gentle reaction between the surface (pretreated in hot alkaline solution and H2SO4/H2O2-mixture) and the mentioned compounds, which favor the formation of compact surface films. Accordingly, the corrosion resistance of the galvanized steel pretreated in both hot alkaline solution and H2SO4/H2O2-mixture could be improved [47]. Fig. 9 shows polarization curves of galvanized steel bare and covered with DTOA layer and phytic acid–Mn2+ SAM modified with

-2.0 -2.5

1 2 3

Untreated DTOA 2+ Phytic acid + Mn + DTOA

-3.0 -3.5 -4.0 2

-2.5

DTOA. It is observed that the current density decreases in both cathodic and anodic polarization curves by coverage of the galvanized steel samples with both DTOA and phytic acid–Mn2+ modified with DTOA .This indicates that the two investigated coating films act as prominent barrier layer to oxygen diffusion and zinc dissolution. The polarization curves exhibit that the cathodic process is suppressed to a greater extent with coating film of phytic acid–Mn2+ modified with DTOA than that of DTOA film only, but the anodic curves were not changed. This behavior can be ascribed to the coating film with phytic acid–Mn2+ modified with DTOA, which leads to the formation of a more densely packed and interconnected film. Thus, the film suppressed the cathodic process by blocking oxygen diffusion through the film. The data in Table 4 show that the value of P% in the case of phytic acid–Mn2+ modified with DTOA (91.7%) is higher than that of DTOA film (77%). This behavior would support to the combination between phytic acid–Mn2+ SAM and DTOA. Therefore, the thickness of the multilayer seems to be large enough to prevent galvanized steel corrosion sufficiently [53]. By comparing the pretreatment of the galvanized steel in H2SO4/H2O2-mixture before the formation of DTOA film and phytic acid–Mn2+ modified with DTOA film, they exhibited higher protective efficiency values (84.2% and 97.9% for the two investigated films, respectively) than those for the pretreatment in both hot alkaline solution and H2SO4/H2O2-mixture (77% and 91.7%). The higher P% values of the films formed on the surfaces pretreated in H2SO4/H2O2-mixture only can be attributed to the presence of some oxide on the surface which assisted in the protective ability

log i, µA/cm

-2.0

-4.5 -5.0 -5.5 -6.0

1

-6.5 -7.0

2

-7.5 -1.25

-1.20

-1.15

-1.10

-1.05

3 -1.00

-0.95

-0.90

-0.85

Potential, V vs. SCE Fig. 9. Polarization curves of coating samples in 0.5 M NaCl solution, and pretreatment in alkaline and H2SO4/H2O2.

346

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of the investigated films. Finally, one can conclude that the presence of the oxide within the film plays an important role in the protection of galvanized steel from corrosion in 0.5 M NaCl [46]. 3.2. Electrochemical impedance spectroscopy (EIS) measurements 3.2.1. Impedance measurements of the galvanized steel without and with covered samples in 0.5 M NaCl solution, pretreated using H2SO4/ H2O2-mixture Fig. 10 shows the typical Nyquist impedance plots for phytic acid, DTOA and DMP coating in the test solution at 2 h of immersion time. The potential was selected at Ecorr, and an analysis of the impedance at the examined potential was made. The data of charge transfer reaction (Rct), conversion coating resistance (Rf) and the capacity of the double layers (Cdl and Cf) were calculated using both the Nyquist and Bode plots of the impedance spectrum (Table 5) [54]. Fig. 11 represents Bode plots of galvanized steel coated with phytic acid, DTOA and DMP conversion coatings in 0.5 M NaCl solution. Occurrence of two inflexion points in the Bode plots indicated that there are two time constants in the corrosion reaction in the case of DTOA and DMP. However the protective efficiency (P%) was determined using polarization resistance of uncoated (Rp°)and coated samples (Rp), where Rp° or Rp is equal to the sum of Rct and Rf (Rp = Rf + Rct). 

P% ¼

Rp  Rp  100 Rp

ð3Þ

Based on the work of Deslouis et al. [55] the corrosion resistance is determined by the dissolution and diffusion controlled processes. An example of the complex plan plot of treated galvanized steel surface with both DTOA and DMP is presented in Fig. 10. The data exhibited two semicircles. The first one at high frequency (the Rct  Cdl) is attributed to the faradaic charge transfer process (Rct), and the second at intermediate frequency due to the conversion coating resistance (Rf) [43,56]. Inductive behavior at low frequency is observed. The inductive effect is associated to adsorption/desorption occurring inside the film [57]. The equivalent circuit proposed for fitting the experimental data is shown in Fig. 12 as previously reported [43,58]. The circuit is based on the following constitutions: Rct is the charge transfer reaction, Rf the conversion coating resis-4.0 -3.5

1 2 3 4

Untreated phytic acid DTOA DMP

-3.0

4

-2.0 -1.5

//

Z , k ohm cm

2

-2.5

-1.0 3

-0.5 1

0.0

2

0.5 0

1

2

3

4 /

5

6

7

8

2

Z , k ohm cm

Fig. 10. Nyquist plot for coating sample in 0.5 M NaCl, pretreatment in H2SO4/H2O2 and measured at Ecorr, Ac amplitude 5 mV, at frequencies from 10 kHz to 5 mHz, and at 20 °C.

tance, Cdl the capacitance of double layer and Cf the capacitance of the conversion coating and L the inductive element at the low frequency (LF), Fig. 12a could be modeled as Fig. 12b [58]. The measured complex-plane impedance plot is similar to that calculated by the equivalent circuit model. The results showed that DMP coating on the galvanized steel has higher resistance to corrosion in 0.5 M NaCl than that monitored for both phytic acid and DTOA conversion coating (Figs. 10 and 11). The impedance parameters derived from these figures are given in Table 5. It is observed that the protective efficiency (P%) of DMP conversion coating is high (95%) compared with that of both phytic acid (18%) and DTOA (85.9%). The low protective efficiency of phytic acid layer alone may be attributed to the thin layer formed on the surface. This behavior can be interpreted on the basis that, the adsorbed molecules are retarded due to the steric hindrance by many functional groups attached to the ring, whereby reducing interaction with the surface [59]. Jianrui et al. [47] stated that the conversion layer formed on Mg alloy surface with phytic acid was very thin which did not effectively cover its surface. As mentioned before [43] the phytic acid conversion coating on galvanized steel is inferior to that of chromate. Fig. 13 shows a typical Nyquist diagram for coated samples with phytic acid alone, in addition to both phytic acid and phytic acid– Mn2+ modified with DTOA, in 0.5 M NaCl solution. The recorded spectra show one capacitive loop only at high frequency and small inductive behavior at low frequency in the case of uncoated and coated samples with phytic acid. However, the curves in the case of coated samples with both phytic and phytic acid–Mn2+ modified with DTOA exhibited two well-separated capacitive loops in addition to the inductive behavior at low frequency. The diameters of these two capacitive loops are higher, while the capacities of the double layers (Cdl and Cf) are lower in the case of phytic acid– Mn2+ modified with DTOA than those of phytic acid modified with DTOA (Table 5). This process may be argued to the inhibition of corrosion process, i.e., the dissolution of galvanized steel and the cathodic process, the reduction of oxygen (Eqs. (1) and (2)) [60]. Therefore, the decrease in the impedance at low frequency limit is related to the inductive behavior [58]. This inductive behavior may be attributed to the relaxation process obtained by adsorption species like Cl ion on the surface [61]. It may also be attributed to re-dissolution of the coated surface at low frequency [62], particularly in the case of samples coated with phytic acid alone in addition to both phytic acid and phytic acid + Mn2+ modified with DTOA. The data in Table 5 exhibit the values of protective efficiency (P%) for the films in the case of phytic acid, in addition to both phytic acid and phytic acid–Mn2+ modified with DTOA, these are 18%, 90.8% and 95.3%, respectively. The high protective efficiency in the case of both phytic acid and phytic acid–Mn2+ modified with DTOA (multilayer) compared with that of phytic acid (monolayer), may be attributed to the increase in the film thickness and the number of interconnections between adsorbed molecules [42]. This behavior may be attributed to the formation of a coordinate bond between sulfur of DTOA and zinc atoms [49]. In order to evaluate the properties of corrosion resistance for different chemical conversion coatings, the Nequist plots for the samples with different surface films were determined in 0.5 M NaCl solution at room temperature (20 °C), the results are shown in Fig. 14. It can be seen that, the two semicircles diameters in the case of phytic acid–Mn2+ modified with DMP are larger than those of phytic acid–Mn2+ modified with DTOA conversion coatings. This improved corrosion resistance is afforded by the second step (DMP). On the other hand, the inductive behavior in the case of Nyquist 4 (Fig. 14) seems to be small compared with that of Nyquist 3. This behavior can be interpreted on the basis that, the diffusion of aggressive species (Cl ions) through coating film (phytic acid + Mn2+ modified with DMP) to substrate is limited [34].

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A.-R. El-Sayed et al. / Corrosion Science 55 (2012) 339–350 Table 5 Impedance parameters for coating samples in 0.5 M NaCl solution, and pretreatment in H2SO4/H2O2. Coating films

Rs (kX cm2)

Rct (kX cm2)

Rf (kX cm2)

Cdl (mF cm2)

Cf (mF cm2)

P%

Untreated Phytic acid DTOA DMP Phytic acid + DTOA Phytic acid + Mn2+ + DTOA Phytic acid + Mn2+ + DMP

0.030 0.030 0.035 0.040 0.025 0.023 0.031

0.41 0.50 2.39 1.41 2.95 5.51 8.71

– – 0.52 6.80 1.52 3.23 9.05

2.43 2.00 0.41 0.61 0.33 0.18 0.11

– – 1.91 0.13 0.65 0.30 0.11

– 18.0 85.9 95.0 90.8 95.3 97.7

4.0

-4.0

4

-3.0

1 2 3 4

Untreated phytic acid phytic acid+DTOA 2+ phytic acid+ Mn + DTOA

4

-2.5

3 2

3.0

Z , k ohm cm

2 2.5

1

-2.0 -1.5

//

log Z, ohm cm

2

3.5

-3.5

1- Untreated 2- Phytic acid 3- DTOA 4- DMP

f = 3 Hz

-1.0

3

-0.5

2.0

1

0.0 1.5

2

0.5 0 -3

-2

-1

0

1

2

3

4

1

5

2

3

4 /

5

Z , k ohm cm

log f (Hz)

6

7

8

9

2

Fig. 11. Bode plot for coating sample in 0.5 M NaCl, pretreatment in H2SO4/H2O2 and measured at Ecorr, Ac amplitude 5 mV, at frequencies from 10 kHz to 5 mHz, and at 20 °C.

Fig. 13. Nyquist plot for coating sample in 0.5 M NaCl, pretreatment in H2SO4/H2O2 and measured at Ecorr, Ac amplitude 5 mV, at frequencies from 10 kHz to 5 mHz, and at 20 °C.

Therefore, the protective layer can be related to the presence of DMP molecules incorporated in the phytic acid–Mn2+ coating film, leading to an effective corrosion inhibitor. Impedance parameters for the corrosion of galvanized steel in 0.5 M NaCl solution could be obtained based on the impedance analysis for the three kinds of coating, as shown in Table 5. The charge transfer resistance (Rct) and conversion coating resistance (Rf) values are higher in the case of phytic acid–Mn2+ modified with DMP than those of phytic acid–Mn2+ modified with DTOA. Accordingly, the protective efficiency of coating (P%) is high in the former case compared with

that of the latter one. This result may be due to the higher adsorbability of DMP molecules through phytic acid–Mn2+ coating layer as mentioned above. Basic information on the interaction between DMP molecules and surface can be provided by calculated atomic charge (Table 3). Accordingly, the higher protective efficiency of DMP can be attributed to the lower the LUMO energy, the easier the acceptance of electrons from metal surface, as HOMO–LUMO energy gap decreased and the adsorption of DMP is high [63]. Generally, DMP or DTOA molecules incorporate into the phytic acid–Mn2+

Fig. 12. Equivalent circuit of samples in 0.5 M NaCl solution.

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A.-R. El-Sayed et al. / Corrosion Science 55 (2012) 339–350

-7

1 2 3 4

-6

-1.2

Untreated phytic acid 2+ phytic acid+ Mn + DTOA 2+ phytic acid+ Mn + DMP

-1.0

-5

1 2 3 4

Untreated phytic acid DTOA DMP

4

Z , k ohm cm

4

-4

-3

//

Z //, k ohm cm 2

2

-0.8 -0.6 -0.4

3

-0.2

-2

1

2

0.0

3

-1

0.2

2 1

0

0

1

2 /

Z , k ohm cm

1

-2

0

2

4

6

8 /

10

Z , k ohm cm

12

14

16

18

2

Fig. 14. Nyquist plot for coating sample in 0.5 M NaCl, pretreatment in H2SO4/H2O2 and measured at Ecorr, Ac amplitude 5 mV, at frequencies from 10 kHz to 5 mHz, and at 20 °C.

conversion layer and block the pores present in this layer. This incorporation competes with Cl ions into the coating layer and modifies its corrosion resistance, leading to a decrease in the corrosion rate [64]. These data are in a good agreement with the above results obtained by Tafel-plots under the same conditions. 3.2.2. Impedance measurements of the galvanized steel without and with covered samples in 0.5 M NaCl, pretreatment using hot alkaline and H2SO4/H2O2-mixture Fig. 15 represents experimental EIS spectra obtained of uncoated and coated galvanized steel with phytic acid, DTOA, DMP of pretreated samples in both hot alkaline and H2SO4/H2O2-mixture, and measured at Ecorr. Nyquist plots exhibit two semicircles with the coated samples by DTOA and DMP, and one semicircle in the case of uncoated and coated samples with phytic acid. The results also show that the two capacitive loops in the case of DMP conversion coating are higher than those of DTOA. However, the two capacitive loops in the case of DTOA are higher than that (one capacitive loop) of phytic acid. Moreover the charge transfer resistance (Rct) and coating conversion resistance (Rf) values (Table 6) for the samples with surface films formed in solution containing DMP were greater than those of DTOA conversion film. But the two mentioned values of DTOA are higher than the corresponding values of phytic acid conversion coating. The results indicated that the corrosion resistance of galvanized steel could be improved by being treated with a solution containing DMP or DTOA. However, DMP exhibited more protection compared to that of DTOA. This indicates the higher stability of the sample with DMP than that of DTOA conversion coating. This would suggest that the conversion layer of DMP is thick and uniform. Consequently, the surface completely covered by conversion layer would prevent the media from immerging leading to increase the corrosion resistance. The impedance parameters of some investigated coating compounds (Table 6) are compared. It is observed that, the protective efficiency (P%) values of both phytic acid and phytic acid + Mn2+ modified with DMP (as second dipping) are approximately the same (92 ± 0.5%). This indicates that the presence of DMP molecules within conversion coating layer plays an important role in the protection of galvanized steel from corrosion in NaCl solution

3

4

2

Fig. 15. Nyquist plot for coating sample in 0.5 M NaCl, pretreatment in H2SO4/H2O2 and measured at Ecorr, Ac amplitude 5 mV, at frequencies from 10 kHz to 5 mHz, and at 20 °C.

without or with phytic acid or phytic acid–Mn2+. However, the coated sample with DTOA showed low value of P% (77.9%) compared with that of both phytic acid and phytic acid–Mn2+ modified with DTOA (80.5% and 86.6%, respectively). This indicates that the modification with DTOA enhanced P% values. This behavior may be attributed to the increased thickness of the film by combination between phytic acid or phytic acid–Mn2+ layer (first dipping) and DTOA molecules (second dipping) [53]. By comparing the impedance parameters (Tables 5 and 6) of the pretreated samples in both H2SO4/H2O2-mixture only and hot alkaline solution in addition to H2SO4/H2O2-mixture before coating conversion layer, some differences of the estimated values are observed. That is, most investigated conversion coating of the samples pretreated only in H2SO4/H2O2-mixture showed that the protective efficiency (P%) values are greater than those of pretreated in both hot alkaline solution and H2SO4/H2O2-mixture. As mentioned before (Tafel-plots), this behavior may be attributed to the presence of traces of oxide on the surface in the case of samples pretreated in H2SO4/H2O2-mixture. Therefore, this oxide facilitates the adsorption of self-assembling molecules (SAM) on the substrate, in particular, the compounds containing sulfur atoms (DTOA and DMP). Lang and Nogues [41] stated that the presence of traces of oxides on the surface appears to be important, confirming that the thiol is probably grafted onto the ZnO. Finally, the protective efficiency (P%) values obtained from impedance measurements of both the two pretreated surfaces are in agreement with the results obtained by Tafel-plots extrapolation under the same conditions.

4. Conclusions The main conclusions of this study are given below: (1) In order to prepare thick films of the phytic acid SAM and phytic acid–Mn2+ adsorbed on galvanized steel, multistep modification using DMP and DTOA was carried out. On the other hand, a comparative study of self-assembled monolayers of DMP and DTOA was performed for the first time. (2) The protective efficiency (P%) values of films were examined by Tafel-plots and impedance measurements of the coated galvanized steel in 0.5 M NaCl solution after immersion for 2 h.

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A.-R. El-Sayed et al. / Corrosion Science 55 (2012) 339–350 Table 6 Impedance parameters for coating samples in 0.5 M NaCl solution, and pretreatment in alkaline and H2SO4/H2O2. Coating films

Rs (kX cm2)

Rct (kX cm2)

Rf (kX cm2)

Cdl (mF cm2)

Cf (mF cm2)

P%

Untreated Phytic acid DTOA DMP Phytic acid + DTOA Phytic acid + Mn2+ + DTOA Phytic acid + DMP Phytic acid + Mn2+ + DMP

0.029 0.030 0.031 0.034 0.021 0.023 0.026 0.025

0.37 0.41 1.18 3.41 1.20 1.52 3.50 3.60

– – 0.50 1.12 0.70 1.25 1.25 1.35

2.70 2.43 0.63 0.29 0.60 0.44 0.28 0.28

– – 2.15 0.95 1.90 1.65 0.82 –

– 10.8 77.9 91.8 80.5 86.6 92.2 92.5

(3) The values of the protective efficiency for the films of DMP and DTOA were maintained above 96% and 84%, respectively, during immersion in corrosive medium for 2 h. The protective ability of the film against corrosion was remarkably persistent during immersion in the solution. (4) The presence of phytic acid layer (first dipping) with DMP conversion coating (second dipping) exhibited slightly increase in P% value compared with that of pure DMP conversion coating layer. (5) Most investigated conversion coatings of the samples pretreated only in H2SO4/H2O2-mixture, showed that the protective efficiency values (P%) are greater than those of the pretreated samples in both hot alkaline solution and H2SO4/ H2O2-mixture.

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[53]

[54]

[55]

[56]

[57]

[58]

[59] [60]

[61]

[62]

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