Materials Chemistry and Physics 219 (2018) 175–181 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Barrier ability and durability of NieCoeP coatings in accordance to the content of H3PO3 and NaH2PO2 as phosphorus sources T Katya Ignatova∗, Stephan Kozhukharov, Momchil Alakushev University of Chemical Technology and Metallurgy; 8, Kl. Ohridsky blvd., 1756, Soﬁa, Bulgaria H I GH L IG H T S G R A P H I C A L A B S T R A C T Galvanostatically deposited NieCoeP • on copper substrates coatings are investigated. The impact of H PO and NaH PO as • P-providers on the barrier ability is 3 3 2 2 assessed. and LVA are applied during ex• EIS tended coatings exposure to 3.5% NaCl. of amorphous phase and higher • Both Co:Ni ratios improve the coatings properties. coincidence of both P-providers • The improves the resulting coating performance. A R T I C LE I N FO A B S T R A C T Keywords: Metal alloys NickeleCobaltePhosphorus Electrodeposition Corrosion resistance Impedance spectroscopy The eﬀect of P-providing components (phosphorous acid, H3PO3 and sodium hypophosphite, NaH2PO2) in modiﬁed Watts electrolyte (pH = 2; 80 °C) on the barrier ability and durability of the deposited NieCoeP and NieCo (as a reference) coatings during extended exposure in 3.5% NaCl until 672 h was studied. The data acquired for the charge transfer and polarization resistances (Rct and Rp) by electrochemical impedance spectroscopy, EIS and linear voltammetry, LVA, undoubtedly reveal that the best corrosion protective properties belong to the NieCoeP coating deposited at simultaneous occurrence of H3PO3 and NaH2PO2. It was followed subsequently by those obtained from electrolyte with only NaH2PO2, then, deposited from electrolyte with only H3PO3 and ﬁnally by the reference NieCo coating. The occurrence of an amorphous phase in the structure of NieCoeP coatings, combined with the increase of cobalt content and reducing that of nickel, predetermines the best protective properties of the investigated alloys. The established anomalous Rct and Rp increment during the exposure of NieCoeP coatings could be related to corrosion products accumulation, most probably of metal phosphates, and it forms a barrier layer between the coating and the corrosive medium. 1. Introduction NieCoeP coatings are a new type of alloys [1,2], which combine the high corrosion resistance and hardness, typical for the NieP compositions [3–5] with the superplastic extensibility, wear resistance, high ∗ saturation magnetization and gоod thermal stability owed by the Со-Р [6–8] and by the cobalt enriched NieCo coatings . At the beginning, the triple alloy was investigated as an alternative to solid chromium coatings obtained from solutions involving carcinogenic Cr6+ ions [1,2,10,11]. Currently, it is used as a catalytic material for the Corresponding author. E-mail address: email@example.com (K. Ignatova). https://doi.org/10.1016/j.matchemphys.2018.08.025 Received 17 February 2018; Received in revised form 21 June 2018; Accepted 10 August 2018 Available online 11 August 2018 0254-0584/ © 2018 Elsevier B.V. All rights reserved. Materials Chemistry and Physics 219 (2018) 175–181 K. Ignatova et al. (NaHCO3) at 60°С. Afterwards, the samples were washed thoroughly with water and subsequently etched in solution with concentrated nitric and sulfuric acids in the ratio 2:1 v/v with adding of 10 g L−1 sodium chloride, NaCl (all p.a. Merk), followed by a ﬁnal washing with distilled water and drying. The coating thickness was measured using a handportable, caliper (0÷1250 μm) with combined magnetic-induction and eddy current sensor of type BB20 of the company BioEviBul. The surface morphology of the samples was analyzed by scanning electron microscopy (SEM) using an Oxford Instruments, JSM-6390Jeol. Chemical composition of the electrodeposits was determined with energy dispersive X-ray (EDX) spectroscopy by analyzer, attached to the SEM device. The structure of the NieCoeP coatings was assessed by XRay diﬀraction technique (XRD) using Philips PW 1050 vertical automatic diﬀractometer with secondary graphite monochromator, operating with CuKα radiation and scintillation counter. The diﬀraction curves were recorded in angular interval from 10 to 100° 2θ with step 0.04° and exposure 1 s. The barrier ability and durability of the obtained coatings were determined by two independent methods: electrochemical impedance spectroscopy (EIS) and linear voltammetry (LVA). Autolab 30 universal Galvanostat/Potentiostat, equipped with FRA-2 impedance analyzer, was used. The experiments were implemented in three-electrode cell, ISO16773-2, which provided 2 cm2 working area, exposed in the model corrosion medium (MCM) consisting of 3.5% NaCl in distilled water. The counter electrode was a platinum net and Ag/AgCl/3M KCl (E = 0.194 V) of Metrohm served as reference. The impedance spectra were acquired after certain periods: 24, 168, 336, 505 and 672 h of exposure to MCM at room temperature. The spectra were recorded in frequency interval from 10 kHz tо 10 mHz, distributed in 50 measurement points, at excitation signal of 10 mV, according to the Open Circuit Potential (OCP). Anodic polarization dependencies in logarithmic coordinates were recorded under the same conditions as impedance spectra, i.e. after diﬀerent times of their exposure to the MCM. Potential range was between −270 mV tо 350 mV, in respect to the Ag/AgCl electrode at potential sweep rate of 5 mV s−1. This potential range was selected in order to include the corrosion potential (Ecorr) value. Hydrogen-Evolution Reaction (HER) for water electrolysis and in the hydrogen fuel cells [12,13] and also in magnetic data recorders [14–16]. Recently, the transition metal phosphide nanomaterials have been used in Li-ion batteries, Na-ion batteries, supercapacitors and solar cells [17–21]. Many authors point out that with increasing of phosphorus content over 10–12 mass.% in NieP [3,5,22], CoeP [6–8] and NieCoeP [1–3,12,23] coatings, its structure becomes amorphous and therefore their protective capability increases. This fact can be explained assuming lower interatomic distances of the obtained amorphous structures, resulting in lower density of grain boundaries, dislocations and other surface defects [24,25]. There is contradictive information regarding the nature of the anode dissolution, the ability to passivation and the sensitivity to pitting of NieP, CoeP and NieCoeP coatings [2,3,13,26–28]. Some authors [5,25,26,29] indicate that the NieP coatings do not form any distinguishable passivation layers in acidic media, showing active anodic dissolution. On the basis of both XPS analyses and anodic behavior of NieP alloys, M. Crobu et al.  have concluded that the active anodic dissolution leads to phosphorus accumulation on the metallic surface. It promotes preferential Ni-selective dissolution during the polarization. According to other authors, this process leads to threshold current density in the course of dissolution of NieP alloys due either to formation of Ni3(PO4)2 ﬁlm [5,25] or adsorption of hypophosphite ions . The accumulation of such products on the surface serves as a barrier against further dissolution. The phenomenon is known as “chemical passivity” . The surface layer enrichment by phosphorus is the most probable reason for the superior corrosion strength of the CoeP alloy, in respect to the Со one [8,30]. Other authors  highlight that during the anodic dissolution of amorphous NieP alloys with P content exceeding 12 at% a passive ﬁlm is being formed in chloride and sulphate neutral solutions. Nowadays, no suﬃcient literature data exist about the comparative evaluation among the P-providing additives, regarding the improvement the protective capability of electrodeposited NieCoeP alloys. This article highlights on the comparison of the barrier ability and durability of NieCoeP coatings, deposited at presence of either NaH2PO2 or H3PO3, as well as in their combination. The analysis was based on various advanced analytical techniques, including SEM, EDX, X-Ray and also two independent electroanalytical methods (EIS, LVA). 3. Results and discussion 3.1. EIS measurements 2. Experimental For easier data interpretation, the investigated coatings have the same signs as the respective electrolytes in Table 1. (i.e. the coatings A, B, C, D). The impedance spectra acquired for the entire exposure period from 24 to 672 h for all the investigated coatings are summarized in Fig. 1. The spectra are represented in Bode (Fig. 1, a-d) and in Nyquist (Fig. 1, a’- d’) plots. The comparison of the spectra in Bode plots reveals that at the initial 24 h of exposure, the NieCoeP coatings, deposited from electrolytes А, B and С (Fig. 1, a-с), possess slightly lower impedance modulus values at 0.01 Hz, i.e. |Z| = 105 Ω cm2 (Fig. 1, a-c), compared to these of the referent NieCo coatings, which impedance modulus is |Z| = 105.5 Ω cm2 (Fig. 1, d). This fact imposes the inference that the barrier ability, registered for the initial 24 h for the NieCo reference coatings looks higher than this of the NieCoeP alloyed coatings (А-С). Nevertheless, the comparison of the spectra, recorded for the entire 672 h exposure period, in both types of Bode (Fig. 1, а-d) and Nyquist (Fig. 1, a’- d’) plots shows that the impedance modulus and the corresponding capacitance for coatings А, B and С, undergo negligible change (excluding coating A, which |Z| decreases from 105.3 to 104.6 Ω cm2), whereas for the NieCo composition (i.e. coating D) the |Z| suﬀers remarkable decay, reaching complete barrier properties lose (Fig. 1, d’), corresponding to complete layer failure. The comparison of the acquired impedance spectra with spectra of suitable model equivalent circuits has enabled to deﬁne the numeric NieCoeP coatings were galvanonostatically deposited from three electrolyte compositions (assigned as A, B and C in Table 1) with presence of either H3PO3 or NaH2PO2, (A and B resp.) as well as in their combination (electrolyte C). NieCo alloy coating was deposited in electrolyte D from Table 1. The electrolytes were prepared using analytical grade reagents (trademark p.a. Merck). The pH value 2 was adjusted by adding of solutions NaOH and H2SO4, p.a. Merck. The coatings were deposited for 40 min on copper electrodes (Merck, 99.97%) with surface area 4 cm2 (2 × 2 cm) at j = 62.5 mA cm−2 for 40 min at 80 °C. The Pt foil anode (of 36 cm2) was concentrically placed around the working (copper) electrode. Before the experiments, the copper electrode was degreased in a dilute detergent solution with the addition of 20 g L−1 sodium bicarbonate Table 1 Basic electrolyte composition in [mol L−1], рН = 2. Electrolyte→ Components↓ А B C D NiSO4 6H2O NiCl2 CoSO4 7H2O NaH2PO2 H3PO3 0.66 0.20 0.35 – 0.60 0.66 0.20 0.35 0.60 – 0.66 0.20 0.35 0.60 0.60 0.66 0.20 0.35 – – 176 Materials Chemistry and Physics 219 (2018) 175–181 K. Ignatova et al. Fig. 1. EIS data, represented in both types of Bode (a–d) and Nyquist (a’- d’), for NieCoeP coatings, deposited from electrolytes A (a,a’), B (b,b’), and С (c,c’), as well as for NieCo coating from electrolyte D (d,d’) at diﬀerent times of exposure. investigated compositions. Тhis fact reveals the purely capacitive character of the electric double layer on the surface for all NieCoeP and NieCo coatings, being supplemental, obvious indication for their uniformity. The „n“-exponential factor tends to 100% for all the investigated specimens (Table 2). The quantitative analysis of the impedance spectra has allowed the assessment of the barrier ability of the obtained coatings, on the basis of the Rct values (Таble 2) at the initial 24 h of exposure. Their durability was evaluated using the Rct values detected after the completion of the exposure experiments (i.e. after 672 h of exposure). On the basis of the data represented in Table 2, it could be inferred that the highest barrier ability at the initial 24 h of exposure belongs to the NieCoeP composition A, (Rct = 430 kΩ cm2), followed values of all оhmic and capacitive resistance elements of the electrolyte/coating/substrate system (Table 2). In the present case, the appropriate equivalent circuit (Fig. 2) was composed by the 3.5% NaCl (MCM) resistance Rel, subsequently connected to a parallel connection between the charge transfer resistance Rct and non-ideal capacitance CPEedl of the electric double layer occurring between the electrolyte and the electrode surface (represented as constant phase element, CPE). The insigniﬁcant deviations established between the spectra acquired by measurements of the corroding coatings and the model equivalent circuit evince the uniformity and the nano-size distribution of the investigated NieCoeP coatings. Тhis result coincides with the data obtained by the XRD analysis. Besides, the phase shift between the input potential sinusoid and the resulting current density output sinusoid tends to φ = 90°, corresponding to almost ideal capacitance for all the 177 Materials Chemistry and Physics 219 (2018) 175–181 K. Ignatova et al. Fig. 2. Model equivalent circuit of the EIS method. composition B (shown in Table 2). Thus, the Rct of coating С decreases from 271.4 to 246.0 kΩ cm2, for the entire 672 h of exposure, whereas for composition B, these values increased from 224.0 to 250.0 kΩ cm2. The high phosphorous content possessed by coatings А-С is a possible reason for their superior barrier ability and extended durability, compared to these of the referent NieCo coating (D), because the coating dissolution results in the phosphorus accumulation on the electrode surface, conﬁrmed by other authors [8,26,30], as well. Comparing the data obtained for the barrier ability of the coatings А-D shown in Table 2, with their elemental compositions represented in Table 3, it could be concluded that the high phosphorus content is not the unique consecutively by the NieCo coating D (Rct = 295 kΩ cm2), the NieCoeP coating С (Rct = 271.4 kΩ cm2) and ﬁnally the NieCoeP coating В (Rct = 224 kΩ cm2). Although the rather high initial values of the barrier ability recorded for composition A, it loses its superior properties much faster than the rest NieCoeP layers. The charge transfer Rct of this coating undergoes 4 folds decrement from its initial value. For the referent NieCo coating D, this decrement achieves 15 folds, coincided by complete ﬁlm failure, established after 504 h of exposure. The compositions B and C reveal very high average Rct values during the entire exposure period, especially for coating C. Furthermore, even slight Rct increment was registered particularly for Table 2 Results from the impedance modeling for the NieCoeP (А-С) and NieCo (D) coatings regarding the MCM (3.5% NaCl): electrolyte resistance, Rel; Constant phase element, СРЕedl; n - exponential factor and the charge transfer resistance, Rct. Hours of exposure SAMPLE 24 h 168 h 336 h 504 h 672 h SAMPLE 24 h 168 h 336 h 504 h 672 h SAMPLE 24 h 168 h 336 h 504 h 672 h SAMPLE 24 h 168 h 336 h 504 h 672 h СРЕedl (×10−6) Rel n Rct [Ω.cm2] E, % [sn.Ω−1.cm−2] E, % [−] E, % [kΩ.cm2] E,% 21.00 15.46 13.76 15.96 12.98 0.82 0.60 2.42 0.96 0.59 23.5 23.6 38.9 60.3 91.9 0.60 0.42 1.71 0.78 0.49 0.96 0.97 0.91 0.94 0.95 0.14 0.96 0.41 0.20 0.13 430.00 539.00 180.40 152.40 113.6 1.81 0.96 4.44 2.26 1.54 26.70 23.86 24.00 31.34 38.16 0.52 0.47 0.30 0.36 0.62 43.45 46.43 46.58 47.02 51.13 0.43 0.38 0.30 0.30 0.53 0.95 0.95 0.95 0.95 0.93 0.12 0.10 0.11 0.08 0.15 224.00 278.40 333.20 318.20 250.40 1.31 1.39 1.29 1.24 1.99 17.16 14.28 15.96 21.80 18.74 0.91 0.82 0.73 0.87 1.28 31.34 30.71 31.45 32.84 37.47 0.69 0.57 0.53 0.67 0.97 0.95 0.95 0.95 0.95 0.93 0.16 0.13 0.13 0.17 0.24 271.40 390.00 346.00 323.60 246.80 1.87 1.97 1.69 2.11 2.85 0.56 0.85 1.07 0.83 – 33.60 33.97 40.24 74.65 – 0.13 0.20 0.25 0.21 – 295.00 230.40 86.60 19.36 – 1.65 2.18 1.81 0.94 – A B C D 16.78 13.32 14.10 19.90 – 0.73 1.15 1.27 0.75 – 178 0.96 0.95 0.93 0.87 – Materials Chemistry and Physics 219 (2018) 175–181 K. Ignatova et al. (Таble 2). The ﬁrst probable reason is related with its higher thickness compared to those of the rest coatings. Consequently, for coating B, it is possible with the highest probability to the other coatings, at longer residence times in MCM to reach phosphorus enriched sublayers with enhanced barrier ability due to their distinguishable structures and compositions. The other probable reason is that these sublayers are capable to form corrosion products, such as Ni3(PO4)2 and Co3(PO4)2, which act as a barrier against the dissolution, as proposed by other authors [5,25]. Table 3 Elemental compositions of the investigated coatings А-D (in mass.%). Probe/electrolyte А В С D %P 17.67 (18.99) 14.84 (15.63) 31.20 (38.4) – [a] % Co % Ni 11.67 (12.16) 22.89 (21.01) 35.19 (33,34) 27.71 70.66 (68.85) 62.27 (63.36) 33.61 (28.26) 72.29 a The brackets show the coating composition on the most protrusive part of their surface. reason for the superior barrier ability of coatings A-C. The data in Table 3 reveal that there is an increase of the cobalt amounts, in direction from sample A to sample C, which is another possible reason for the increase of the barrier ability in this direction. In coating C, the phosphorus content reaches 31.2%, which is comparable to both the Co (35.19%) and Ni (33.61%) concentrations. In this sense, our data are concordant to the results reported in Ref. . The lowest thickness is established for coating A (2.5 μm) in comparison with these of coatings В (7.8 μm) and С (5.6 μm). This is a possible reason for the quick loss of barrier ability of coating A. Coatings A and B possess typically amorphous structures, whereas the XRD pattern for coating С, deposited at j = 62.5 mA cm−2 reveals mixed amorphous/crystalline structure, comprising nano-dimensional amorphous phase and crystal hexagonal phase of solid solutions Ni2P and Ni2P-Со2Р. The dimensions of the individual domains for the amorphous phase are below 3 nm. The occurrence of hexagonal crystals (Fig. 3a) with elevated P-content (Table 3, content in brackets) is explained assuming recrystallization processes, after its deposition. Undoubtedly, it could be inferred that the combination among the highest phosphorus content, registered for coating C, the relatively similar Со and Ni contents and its speciﬁc microstructure is the reason for the superior barrier properties, compared to all the rest investigated compositions. These distinguishable structural and compositional features of coating C are consequence of the fact that it is deposited at presence of both P-providing electrolyte ingredients. The Rct increment during the exposure of coatings А, B and С (Таble 2), could be related to corrosion products accumulation, which forms a barrier layer between the coating and the corrosive medium. The values in Table 3, enclosed in brackets, correspond to the coating composition data from the most protrusive part of the coating's surface, acquired shortly before the corrosion tests. The most considerable data dissipations of entire 8% are observed for the P-content in coating C. Although this result does not give an idea for the composition of the inner layers of the coating, it suggests that there is a diﬀerentiation of the composition and structure of diﬀerent layers of the NieCoeP coatings. From the statements mentioned above, it could be concluded that there are two possible reasons for the observed Rct increment for coating B until the highest values among the investigated coatings 3.2. LVA measurements Linear Voltammetry (LVA) was applied as another electrochemical analytical method, in order to conﬁrm the inferences done above, regarding the durability of the investigated coatings А-D. It allows to register the Tafel anodic plots (in logarithmic coordinates), shown in Fig. 4. The LVA curve analysis has enabled to calculate the data for the corrosion potential Ecorr and the polarization resistance Rp for each one of the investigated coatings after diﬀerent periods of exposure to МСМ (Таble 4). The weakest current density increase within the anodic polarization was registered for the coatings B and C. This weak current increment was preserved for the entire exposure period (Fig. 4b and c). For comparison, coating A, deposited from electrolyte with only H3PO3 containing solution suﬀers gradual active dissolution until the 168th hour of exposure (Fig. 4, a). Afterwards, its dissolution rate sharply rises becoming similar to this of the reference NieCo coating D, at the initial exposure period (Fig. 4, d). Current arrest is observed for all investigated NieCoeP coatings, when suﬃciently positive anodic potentials are reached. This eﬀect is observed until the 504th. hour of exposure for coatings B and C (Fig. 4b and c) and until the 168th hour (Fig. 4, a) of exposure for the coating A, respectively. These conditions promote metallic ions and phosphorus heaping, as well as acidiﬁcation of the electrolyte near the surface layer, as a result of the hydroxyl ions oxidation, following reaction (1) : 4OH− - 4e− → O2 + 2H2O; E0 = 0.817 V (SHE) for pH = 7 (1) These conditions predetermine corrosion products formation (most probably metal phosphates), which acts as a barrier against the further ﬁlm dissolution, similar to that established for NieP coating [5,25]. Any current retention was not registered in the case of coating D, in the entire anodic potentials range. These data correlate very well with the EIS results, conﬁrming the lowest durability of coating D and the inferior protective properties possessed by coating A, in comparison with compositions B and C. The anodic curves for coating A after 336 h of exposure and for compositions B and C after 504 h of exposure respectively, achieve a current density maximum at about −0.04 V (Ag/AgCl), followed by Fig. 3. SEM image (a) and X-Ray diﬀractogram (b) of NieCoeP coatings, deposited from electrolyte with composition C from Table 1; j = 62.5 mA cm−2. 179 Materials Chemistry and Physics 219 (2018) 175–181 K. Ignatova et al. Fig. 4. Аnodic polarization curves, acquired for NieCoeP coatings А-С (a-с) and for the reference NieCo coating D (d) acquired after diﬀerent times of exposure to the MCM (3.5% NaCl). weak decrease and subsequent plateau (Fig. 4, а-с). For the referent NieCo coating (D), such current density maximum was registered even at the initial 24 h of exposure, at almost the same potential as for the coatings А-С. (Fig. 4, d). This fact gives the reason to suppose that for all coatings, the observed maxima are related to the formation of the same type of corrosion products. Furthermore, these products are not metallic phosphates, because in the case of the combined NieCo coating there is not any reason for their existence. The potentials of all possible anodic reactions with the MCM components were calculated, in respect to Ag/AgCl reference electrode, in order to determine the nature of the anodic processes, related to the appearance of this peak, and to deﬁne whether the corrosion products are metallic oxides, or consequence of the interactions between the copper substrate and the 3.5% NaCl, (pH = 7), corrosive solution. The standard potential values in respect to standard hydrogen electrode (SHE) are taken from literature data [31,32]. Among all the possible processes on the corroding metallic surface, the reactions represented below (equations (2) and (3)) possess the most closed values to those of observed maxima: CuCl + e− → Cu° + Cl−, E° = 0.137 V CuCl2 + 2e − → Cu° + 2Cl , E° = 0.190 V - value determined after the initial 24 h of exposure is owed by the reference NieCo coating D (Rp = 372.0 kΩ cm2). However, it sharply decays reaching the minimal 16.3 kΩ cm2 value after the 504th hour of exposure (Тable 4). A weaker Rp decrease was registered for coating A. For comparison, coating C possesses rather high Rp values and keeps them during the entire exposure period (Rp = 231 kΩ cm2 after 24 h; 601.0 kΩ cm2 after 336 h, and 267.8 kΩ cm2 after 672 h of exposure, respectively). High and even further increasing Rp values after the 168th and the 504th hour of exposure were registered for the composition B. Similar tendency has been observed for the Rct values determined by the EIS measurements, due to the described above reasons. Because the Rp values comprise both the Rct and the resistance owed by the oxide or salt ﬁlm deposits, the anomalous Rp increment for coatings B and C, evinces the appearance of protective superﬁcial layers composed by corrosion products accumulated during the exposure tests. These products are probably metal phosphates, and this assumption is in agreement with the reports of other authors [5–7,25]. The numerical data obtained from the LVA curve analysis completely correlate with the statements done for EIS measurements about the corrosion protective properties of the investigated coatings. On the basis of both the electrochemical analytical methods, it can be concluded that regarding their corrosion protective abilities, the investigated coatings can be ordered as follows: from the best one C, followed subsequently by B, and A, to the worst one-composition D. (2) (3) The potentials change their values to −0.070 V and −0.020 V, after recalculations in respect to the used Ag/AgCl reference electrode (EAg/ AgCl = 0.194 V, according to SHE). These values are rather similar to the potential values of the observed anodic peaks. Therefore, a conclusion could be done that these peaks are namely result of the substrate metallic surface uncovering. Consequently, copper dissolution appears and it causes a sharp current rise, followed by subsequent current decrease, due to formation of insoluble compounds like CuCl and CuCl2. The processes of formation of these compounds serve as an indication for the initiation of barrier properties loss due to the coating dissolution. The peak and plateau appearance happens at diﬀerent periods of exposure to MCM, which completely correlate with the EIS data, appointing exactly the same periods necessary for coating failure. The LVA curves were submitted to Tafel plot analysis and the resulting corrosion potential Ecorr and polarization resistance Rp are shown in Table 4. The polarization resistance Rp values determined for the investigated coatings (Таble 4) reveal identical to the Rct values evolution trends deﬁned by the EIS analysis (Table 2). The highest average Rp 4. Conclusions The following conclusions can be drown from this study: The highest barrier ability and durability for the entire 672 h period of exposure in 3.5% NaCl was registered for the NieCoeP coating, obtained from the electrolyte with simultaneous occurrence of content of both H3PO3 and NaH2PO2. The high phosphorus content (14÷31%) is not the unique reason for the superior ability of NieCoeP coatings in comparison with NieCo coating. The occurrence of an amorphous phase in the structure of NieCoeP coatings, combined with the increase of cobalt content and reducing that of nickel, predetermines the best registered protective properties of the alloys. It was established anomalous Rct and Rp increment during the exposure of the NieCoeP coatings, wich could be related to corrosion products accumulation, most probably metal phosphates. The resulting 180 Materials Chemistry and Physics 219 (2018) 175–181 K. Ignatova et al. Table 4 Data regarding the corrosion potential Еcorr, mV (versus Ag/AgCl) and polarization resistance, Rp, kΩ cm2 from the Tafel anodic polarization curves, acquired after diﬀerent times of exposure to the MCM (3.5% NaCl). Electrolyte→ (coating) A B C Hours of exposure↓ Еcorr, mV Rp, kΩ cm2 Еcorr, mV Rp, kΩ cm2 < Еcorr, mV Rp, kΩ cm2 Еcorr, mV Rp, kΩ cm2 24 168 336 504 672 −241 −202 – −221 −244 205.6 148.8 – 135.2 125.9 −0.238 −0.231 −0.217 −0.177 −0.208 200.60 352.60 160.60 309.40 220.40 −246 −321 −213 −186 −173 231.00 378.80 601.00 223.20 267.80 −0.211 −0.177 −0.175 −0.165 −0.168 120.64 16.10 9.58 7.58 4.20 NieCoeP alloys can be applied as protective coatings and as electrodes in electrochemical energy storage devices.  Acknowledgements  The authors acknowledge the Scientiﬁc Research Section to UCTM –Soﬁa for the ﬁnancial support via Project 2FHN_К_I – 2017 (internal №11642).   References   C. Ma, S.C. Wang, F.C. Walsh, The electrodeposition of nanocrystalline CobaltNickel-Phosphorus alloy coatings: review, Trans. IME, Int. J. Surf. Eng. Coat. 93 (5) (2015) 275–282.  C. Ma, S.C. Wang, L.P. Wang, F.C. Walsh, R.J.K. 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