Corrosion properties of Zn-Ni-P alloys in neutral model medium

<jats:title>Abstract</jats:title>                <jats:p>The presented work reports on the peculiarities of the anodic behavior, corrosion resistance and protective ability of electrodeposited Zn-Ni-P alloys with a different composition in a model corrosion medium of 5% NaCl. Three characteristic coating types have been investigated using experimental methods such as potentiodynamic polarization (PD) technique and polarization resistance (Rp) measurements. In addition, X-ray diffraction (XRD) analysis as well as scanning electron microscopy (SEM) coupled with an Energy-dispersive X-ray (EDAX) device were applied to determine the differences in the chemical composition and surface morphology which appeared as a result of the corrosion treatment. The data obtained are compared to those of electrodeposited pure Zn coatings with identical experimental conditions demonstrating the enhanced protective characteristics of the ternary alloys during the test period in the model medium. The influence of the chemical and phase composition of the alloys on its corrosion resistance and protective ability is also commented and discussed.                       </jats:p>

section_content: During the last decade the development and characterization of coatings with enhanced protective properties against corrosion has been demanded by the increased industrial requirements for longer service life combined with reduced thickness of the layers. For example, zinc and its alloys obtained by the electrodeposition method, hot dip technique etc. are practically some of the widely used materials for preventing the damage of iron and steel [1]. In general, the interest concerning zinc alloys is grounded on their very good resistance against corrosion which ensures reliable prolonged protection of the substrate. Another reason is to seek alternatives for some toxic layers like cadmium which has been banned since 2007 in the EU. 

The quality of the coatings depends mainly on the deposition technology, bath composition as well as surface morphology and crystallographic orientation [2] [3] [4] [5] [6]. It is well known that the protective properties of the zinc electrodeposits can be also significantly improved by their additional treatment in tri-valence based (chromite) solutions, obtaining of nanocrystalline deposits, nanocomposite coatings, alloying with other metals [7] [8] [9] [10] [11]. For example, the presence of relatively small amounts of nickel in the zinc electrodeposits increases the corrosion resistance of the newly received alloy. In addition, the mechanical properties like strength, hardness, ductility etc. also show better qualitative indices. Neutral salt spray investigations confirm the better protective ability of the Zn-Ni alloy compared to the pure zinc coatings. 

Therefore, the alloying of Zn with metals from the Fe-group, e. g. Ni, Co, Fe is one possible way to realize enhanced corrosion resistance and improved protective ability toward the substrate [12, 13]. These positive influences are better expressed in the co-deposition of the abovementioned metals with non-metals, e. g. phosphorus [14] [15] [16] [17] [18]. Some investigations clearly show that the presence of phosphorus in the metal or alloy matrix increase the corrosion resistance of the electrodeposited coatings [19] [20] [21] [22] [23]. This element also leads to some changes in the microstructure, residual stresses and adhesion strength between the coating and the substrate.
section_name: Introduction

section_content: Corrosion properties of Zn-Ni-P alloys in neutral model medium Additionally, the electrolytic co-deposition of Zn-Ni alloys isan anomalous process. This means that the deposition of the nickel being the relatively noble metal, is partially impeded while the deposition of the zinc takes place. 

The present work will determine the protective characteristics of electrodeposited Zn-Ni-P alloys in a model test medium containing chloride ions as corrosion activators. Another point of interest is the investigation of the anodic behavior in terms of the electrode processes involved as well as the role of the chemical and phase composition. The role of the newly appeared corrosion products for the enhanced corrosion resistance of the alloys is also discussed.
section_name: Central European Journal of Chemistry

section_name: Experimental procedure

section_name: Galvanic coatings

section_content: The composition of the starting electrolyte (SE) differs from the other electrolytes reported in the literature. It contains the following components: NiSO 4 •7H 2 O (100 g L -1 ), NiCl 2 •6H 2 O (100 g L -1 ), ZnCl 2 (30 g L -1 ), beta(β)-alanine (10 g L -1 ) as well as two phosphorus containing additives: NaH 2 PO 2 •H 2 O and H 3 PO 2 (50%). Both latter compounds simultaneously act as very strong buffers at the operating pH values of the electrolyte. The total phosphorous amount in the bath is 4. 4 g L -1. The electrolytic bath is corrected at regular intervals relating to the zinc content and is targeted at a constant concentration of this metal in the solution as well as at a constant correlation between Zn 2+ and Ni 2+ ions in the electrolyte. Some preliminary investigations [20] showed that Zn-Ni-P alloys can be obtained with different elemental and phase composition as well as with specific surface morphology. The alloys with high amounts of Ni and P have predominantly an amorphous structure. It has been determined that they could not successfully fulfill the role of sacrificial coating for the iron or steel substrate. These alloys were not sufficiently of interest from a corrosion point of view. 

As a result, alloys with relative low Ni and P contents have been selected for these investigations. The coatings are electrodeposited galvanostatically at cathodic current density of 2 or 5 A dm -2 respectively, an operating temperature of 40 o C, рН values 3 or 4, respectively and non-soluble Ti-Pt meshes as anodes. 

The time for the electrodeposition of the samples varies between 8 and 30 minutes and to coat with almost equal thickness since this parameter is very important for the characterization of their protective properties. The thermostatic double-chamber electrolytic cell with a volume of 400 mL and a circulation of the bath solution (700 rpm) are applied for the electrodeposition of the Zn-Ni-Р alloys [20]. 

Alloy samples with following composition (in wt. %) have been selected for the following investigation:

-Zn 86 Ni 11 P 3 -Zn 86. 3, Ni 10. 7, P 3. 0; -Zn 76 Ni 18 P 6 -Zn 76. 3, Ni 17. 7, P 6. 0; -Zn 90 Ni 10 P 0 -Zn 89. 6, Ni 10. 4, P 0. 0. This coating is a binary Zn-Ni alloy which is electrodeposited from SE described above but no traces of the phosphorous have been detected via EDX analysis after the electrodeposition process. However, the presence of this element in the electrolyte leads to layers with a tighter and more compact surface morphology compared to the usual Zn-Ni alloys obtained from electrolytic bath which does not contain P-based compounds or additives [20].
section_name: Zn-Ni-P alloy coatings

section_content: Zinc galvanic coatings are obtained from a slightly acidic electrolyte containing ZnSO 4 •7H 2 O (150 g L -1 ); NH 4 Cl (30 g L -1 ); H 3 BO 3 (30 g L -1 ). The deposition conditions applied are: current density of 2 A dm -2, pH value in the range 4. 5-5. 0, room temperature, metallurgical zinc anodes. The coatings are electrodeposited without stirring or circulation of the electrolyte.
section_name: Zinc coatings

section_content: Both sides of the steel plates with dimensions of 20×10×1 mm (total working area of 4 cm 2 ) are electrochemically coated with Zn or alloy galvanic coatings. The thickness of the layers obtained on this substrate is preliminary calculated to be approximately 10 µm.
section_name: Sample preparation

section_content: A model corrosion medium of free aerated 5% NaCl solution with pH ~ 6. 3 at ambient room temperature of about 19 o C is used during the investigations. At least three samples of each coating types (including the pure zinc) have been investigated to ensure reproducibility of the experimental results.
section_name: Corrosion medium

section_name: Sample characterization

section_content: All investigations are carried out in a common threeelectrode experimental cell with a Luggin-capillary for minimizing the ohmic resistance of the corrosion medium. The platinum plate is used as a counter electrode. The corrosion potentials during the test are measured with respect to the saturated calomel electrode (SCE).
section_name: Corrosion investigations

section_content: Potentiodynamic polarization measurements are carried out at a scan rate of 1 mV s -1 using a VersaStat 4 (PAR) device. Prior to the start of the test, the probes were maintained (temporized) for a definite period in the medium under conditions of the open circuit potential (OCP).
section_name: Potentiodynamic (PD) polarization curves

section_content: Polarization resistance is an indicator for the protective ability of the coatings. The measurements are carried out for a prolonged interval of 30 days while the Rp values are taken at selected time intervals (days). The experiments are conducted using an apparature ''Corrovit'' -in the range of ±25 mV relative to the corrosion potential. 

The Stern-Geary equation gives the fundamental reason to apply this method which states that higher Rp value (in Ω cm 2 ) corresponds to higher corrosion resistance and to a lower corrosion current, i. e., Rp ~ 1 / I corr [24, 25].
section_name: Polarization resistance (Rp) measurements

section_content: X-ray diffraction (XRD) analysis is applied to investigate the phase composition and the changes occurring after the corrosion treatment. These are, for example, new corrosion products which can affect the protective ability. The apparature used is X-ray diffractometer DRON-3 (Bragg-Brentano arrangement, CuK a -radiation and scintillation counter). XRD patterns are recorded in a step scanning mode, in steps of 0. 02 o (2Q) and a counting time of 1 s step -1. The PowderCell program [26] is used in data processing.
section_name: Phase composition and surface morphology 2.4.2.1. X-ray diffraction analysis

section_content: analysis Scanning electron microscopy (SEM) is used to observe the surface morphology and to register the changes that appear after the test. The apparature is a scanning electron microscope JEOL JSM-5300, Japan coupled with EDX device.
section_name: Scanning electron microscopy and microprobe

section_name: Results and discussion

section_content: Generally, this method is applied to characterize the cathodic and anodic behavior. The results for the selected alloy samples and pure zinc (given for comparison) are demonstrated in Fig. 1. The corrosion potential of the Zn coating is the highly negative one (-1046 mV), curve 1. Under conditions of external anodic polarization, this electrodeposit demonstrates the highest anodic current density and almost fully disappears at a potential value of about -540 mV leaving the iron substrate practically unprotected. The length of the PD curves shows that the alloy coatings (curves 2-4) last about two times longer compared to the pure zinc under these conditions. 

The Zn 76 Ni 18 P 6 alloy with the highest Ni and P contents -curve 4 -characterizes the lowest anodic current density compared to all other coatings including the pure zinc up to potential values of about -250 mV. This provides proof for its increased corrosion resistance. 

Similar is the behavior of the Zn 86 Ni 11 P 3 alloycurve 3. Both alloy samples consist of the η-phase (Zn-Ni), and their PD curves show lower anodic current densities compared to Zn 90 Ni 10 P 0 during the whole investigated potential zone. 

The alloy Zn 90 Ni 10 P 0 -curve 2 -consists of the η-phase Zn-Ni and the intermetallic compound Ni 5 Zn 21 (see also XRD section below). This sample demonstrates the most active anodic dissolution (highest anodic current density) during polarization. 

The potentiodynamic curve of the substrate (curve 5) is included for more clarity concerning the protective action of the alloys. The PD curves of the latter are placed at more negative potentials (between -1050 and -900 mV) compared to the steel. This shows that the investigated coatings will play the role of over-layers protecting the substrate and dissolving predominantly during the anodic polarization process.
section_name: Potentiodynamic polarization curves

section_content: In contrast to the previous investigations, these data are obtained under OCP conditions during the test. The only period when OCP was not reached is the time when the polarization resistance was measured for about 10-20 minutes. Generally, this method is accepted as more appropriate for the investigation of real corrosion characteristics at a prolonged time interval. 

The results obtained during 30 days of immersion in the model medium are shown in Fig. 2. The data of the pure zinc is given as a comparison -line 1. This coating demonstrates the lowest Rp values, but it must be noted that at the end of the investigation there were practically no visible red spots (appearing of so called "red rust") on the surface of the zinc. This is practical proof that the pure zinc protected the steel substrate although to a lower degree compared to the alloys. 

The polarization resistance of the alloy Zn 76 Ni 18 P 6 -line 4 -demonstrates an increasing tendency in its protective ability (increasing Rp values) up to the end of the first 7-8 days. Thereafter, this parameter decreased by approximately one third and remained almost unchanged until the end of the whole test. Quantitatively very close but with lower protective characteristics was the behavior of the alloy Zn 86 Ni 11 P 3 -line 3. 

The coating with the composition Zn 90 Ni 10 P 0 -line 2 -shows increased Rp values only after the first 5 days and thereafter during the rest of the period. The reason for this result seems to be the alloy composition which consists of the η-phase Zn-Ni and of the intermetallic compound Ni 5 Zn 21 (see EDX and XRD sections). 

It can be assumed that greater amounts of corrosion products appear to have played a significant role in the impeding of the penetration of the aggressive test medium into the depth of the coating.
section_name: Polarization resistance (Rp) measurements

section_content: The differences in the chemical composition before and after the corrosion treatment in the model medium obtained via EDX analysis are summarized in Table 1. The demonstrated data present the results after 10 and 30 days of immersion. 

Additionally, Table 1 shows the phase composition changes during the investigation which are registered by the XRD analysis (see Chapter 3. 4. below). 

Generally, it can be summarized that the newly appeared corrosion products are zinc-based -Zn 5 (OH) 8 Cl 2 •H 2 O, Zn 5 (OH) 6 (CO 3 ) 2 and ZnO. They are characterized with a low product of solubility values [9, 10, [27] [28] [29]. These products seem to be one of the main reasons for the enhanced corrosion resistance of the samples. They ensure a strong barrier effect as well as greater diffusion limitations for the model solution which result in a delay of the corrosion rate of the steel substrate. 

In general, the main observed tendency is that the Zn, Ni and P amounts decrease to different degrees during the test especially after 10 days. Thereafter, their concentration remains almost constant.
section_name: EDX analysis

section_content: Fig. 3 shows the XRD patterns of the alloy Zn 90 Ni 10 P 0. The non-treated (as received) coating consists of the η-phase Zn-Ni and the intermetallic compound Ni 5 Zn 21. In addition, some lines of the iron substrate also appear in the pattern. The Zn/Ni ratio in the alloy before the corrosion treatment is 8. 5. 

After 10 days of the corrosion treatment the lines of Ni 5 Zn 21 gradually disappear, but some traces can be still registered at 2Θ~80 o. Simultaneously, the coating thickness decreases -the lines of the iron substrate become more intensive. The Zn/Ni ratio changes drastically to 51. 2 which indicates that the Ni content is strongly diminished (~ 8. 7 times). In addition, the peaks of some newly appeared Zn-based corrosion products can be observed in the sample -ZnO, Zn 5 (OH) 8 Cl 2 •H 2 O and Zn 5 (OH) 6 (CO 3 ) 2. After 30 days the Zn/Ni ratio is 55. 8 which indicates that the Ni content is additionally decreased although to a smaller degree. 

These results can be compared to the data from the Rp measurements. During the first 3-4 days the polarization resistance value of that sample was very low, close to the pure zinc. After this period, the Rp of the alloy began to increase most probably due to the presence of a mixture of the abovementioned Zn-based products which delay the corrosion process. 

XRD patterns of the ternary Zn 86 Ni 11 P 3 alloy are presented in Fig. 4. Contrary to the previous coating, this one contains only the η-phase Zn-Ni. After 10 days the amount of the η-phase decreased, the coating dissolved steadily and became thinner -the lines of the iron increased. In that case the Zn/Ni ratio did not change significantly -before treatment it was 8. 2; after 10 days -9. 5 at the end of the investigation it was 8. 9. 

During the first 10 days, the Rp values of that alloy were higher compared to the previous case. The Zn/Ni ratio after the 10-th day, increased slightly in relation to the non-treated sample. These observations lead to the conclusion that in such a case Ni dissolves faster than the zinc. The P content decreased about 15 times after a 30 day immersion in the model medium. 

Fig. 5 shows the XRD patterns of the non-treated ternary alloy Zn 76 Ni 18 P 6 as well as the samples treated in the model medium during 10 and 30 days, respectively. The composition the alloy consisted only of the η-phase Zn-Ni. In addition, some lines of the iron substrate could also be observed. The Zn/Ni ratio before the corrosion treatment was 4. 3 most probably due to the Ni content being the highest one compared to the other samples. After a 10 day immersion in 5% NaCl, the coating thickness decreased. The Zn/Ni ratio changed to 9. 3 which indicated that the Ni content was strongly reduced (~3 times). 

This observation can be compared with the data from the Rp measurements. During the first 7-8 days the Rp value of that sample was the highest one from all treated alloys. Thereafter the Rp data decreased with about one third remaining and practically unchanged until the end of the investigation. After their initial decrease, the Ni and Zn contents remained almost constant (~6 wt. % and ~56 wt. % respectively). There were no changes in these values after 30 days of the investigation. Most probably, the relative high Ni and P amounts lead to better protective characteristics at the beginning. The P amount strongly decreased (about 30 times after 30 days test) compared to its initial content in that alloy.
section_name: X-ray diffraction analysis

section_content: The surface morphology of the Zn 90 Ni 10 P 0 alloy before and after 30 days of corrosion treatment is demonstrated in Fig. 6. The non-treated sample is characterized with relative uniform, smooth and compact surface morphology (Fig. 6A ). There are practically no cracks or damages. After 30 days of treatment the amount of the corrosion products was good and it seemed that the latter was agglomerated to a definite degree in a dense layer (Fig. 6B ). This lead to better sealing off the surface and to an improved protective ability, respectively. 

The as-received sample of the ternary Zn 86 Ni 11 P 3 alloy is presented in Fig. 7A. The surface is more uneven compared to the previous case and some relative small holes can be observed. After 30 days immersion in the model medium, the newly appeared corrosion products are almost randomly distributed which has in general a positive influence on the protective characteristics (Fig. 7B ). 

Fig. 8A presents the surface morphology of a nontreated sample of the ternary Zn 86 Ni 11 P 6 alloy. The surface is more disarranged compared to the Zn 90 Ni 10 P 0 alloy and consists of small-sized agglomerates. It seems that their adhesion to the substrate is lower compared to both other coating types since some places look like they are upright. At the end of the corrosion treatment, the newly appeared products were very close in their morphology to these of Zn 86 Ni 11 P 3 alloy (Fig. 8B ).
section_name: Scanning electron microscopy

section_content: The results of the corrosion properties of the investigated alloy coatings can be explained based on the role of their chemical and phase composition. The new zincbased corrosion products as a result of the treatment in the model medium have a low product of solubility values. Thus, it can be concluded that they will impede the permeation of the corrosion medium into the depth acting as a barrier. It is known that zinc is the more electronegative element compared to the Ni. And so, the zinc dissolves predominantly in the model medium under these conditions [9, 10, 28, 29]. 

The corrosion process is accompanied also by the hydrogen evolution process which rate depends on the nature of the compound. For example, the hydrogen overvoltage on the zinc is higher compared to that of the nickel. Since the ternary alloy has a heterogenic structure it can be assumed that the Ni and Ni-P areas are cathodic compared to the rest of the matrix. In these zones the hydrogen overvoltage is lower, and the reduction processes are stronger. This leads to the release of greater hydrogen amounts. Simultaneously, the dissolution rate of the zinc in the surrounding anodic areas rich on zinc (η-phase) increases. However, in this case it would be very difficult to give a precise quantitative evaluation of this effect. 

The alloy Zn 90 Ni 10 P 0 consists of the Zn-Ni η-phase and the intermetallic compound Ni 5 Zn 21 which has a bcc structure. Some investigators have found that the presence of this compound leads to enhanced corrosion resistance [30] [31] [32] [33]. As abovementioned, it was registered that this intermetallic compound disappeared almost fully after 10 days of immersion into the corrosion medium. It could be assumed that the rate of the dissolution process of this alloy is a sum of the dissolution rates of the zinc from the η-phase and zinc from the intermetallic compound. The expected result will accelerate the forming of new corrosion products which will seal up the substrate. However, it seems that under conditions of external anodic polarization (PD curves) the time is insufficient for forming products in sufficient amounts to cover the whole sample surface so the dissolution process remains active. 

There are practically no data concerning the potential value of Ni 5 Zn 21 in the medium. Nevertheless, two possible variants could be discussed that may explain this behavior:

1. The potential value of the intermetallic phase is anodic compared to the Zn-Ni η-phase. It will begin to dissolve as first followed by the dissolution of the Zn from the η-phase. The final result will be the formation of corrosion products. 

2. The potential value of Ni 5 Zn 21 is cathodic compared to the η-phase. This leads first to intensive dissolution of the Zn from the surrounding η-phase. A process of forming Zn-based corrosion products will be registered here as well. It can be expected that some parts of the cathodic phase will tear off during the process followed by intensive dissolution of the already lesser protected underlayer/substrate. 

The alloys Zn 76 Ni 18 P 6 and Zn 86 Ni 11 P 3 consist of the Zn-Ni η-phase which is proved by XRD analysis. From the electrochemical standpoint, it can be presumed that part of the nickel is bound with P in a separate amorphous phase (Ni-P) the latter acting as a strong cathodic zone in the alloy matrix. This will also lead to an accelerated rate of the forming process of corrosion products. The indications for this supposition are as follows:

-Zn does not co-deposit with phosphorous in the absence of nickel ions in the bath [20] ;

-The η-phase is registered via the XRD method but no peaks of a crystalline Ni -P based phase can detected in the XRD patterns;

-The halo of the amorphous Ni -P phase is placed in a zone which is covered by other compounds at 2Q between 35 and 55 degrees. 

In both alloys Ni and P amounts decrease strongly indicating they take part in the dissolution process. The surrounding zinc phase will dissolve faster leading to the separation of some Ni-P containing sections of the coating. 

In all cases a similar behavior influences the formation of new corrosion products with a low product of solubility on the sample surface which impedes the penetration of the model medium. Finally, after a definite period of time, the corrosion products are not sufficient in quantity to cover the damaged zone (or will be with insufficient thickness).
section_name: Discussion

section_content: The obtained Zn-Ni-P alloy coatings demonstrate better protective characteristics in regard to the steel substrate compared to the pure zinc layer in a test medium with chloride ions as corrosion activators. This is based on the phase inhomogeneity and the processes that have occurred. The alloys consist of two phases. In the case of Zn 90 Ni 10 P 0 these are the η-phase Zn-Ni and the intermetallic compound Ni 5 Zn 21. In the case of two other alloys the phases are the η-phase Zn-Ni and the amorphous compound Ni-P. These phases play an important role in the process of the forming of zinc-based corrosion products like Zn 5 (OH) 8 Cl 2 •H 2 O, Zn 5 (OH) 6 (CO 3 ) 2 and ZnO. All three distinguish themselves as having a low product of solubility values. The result is an additional accumulation of products on the surface which impedes the penetration of the aggressive test medium into the depth leading to an improved protection of the steel substrate.
section_name: Conclusion

section_name: Acknowledgement

section_content: The authors gratefully acknowledge the financial support of Project BG 051PO001-3. 3. 06-0038. 

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section_name: Acknowledgement