Effect of limiting current density on corrosion performance of Ni–Mo, Ni–Cd and Ni–Mo–Cd alloy coatings

In this article an attempt was made to increase the corrosion resistance of Ni– Mo alloy coatings with the addition of small quantity of CdCl2 into its bath. The limiting of limiting current density (iL) of Ni in both Ni–Mo and Ni–Cd baths due to inherent induced and normal type of codeposition has been successfully alleviated by addition of 1 g/L of CdCl2. The advent of induced and normal type of codeposition of individual binary baths has been used to optimize the Ni content of the ternary deposit for better corrosion stability. The composition vs. current density plots of all coatings have been studied, and thereby optimal iL of Ni in all baths were assessed. The content of Mo was found to be decreased with the small addition of Cd to the bath. Results revealed that (Ni–Mo–Cd)6.0 Adm−2 coating showed better corrosion resistance by reducing iL of Ni, on addition of Cd+2 ions into the bath and was explained in the light of diffusion limited deposition of Ni+2 ions. The results were supported by SEM (scanning electron microscopy), XRD (X-ray diffraction) and AFM (atomic force microscopy) study of Ni–Mo, Ni–Cd and Ni–Mo–Cd coatings at optimal current densities.


Introduction
Corrosion is the deterioration of materials due to chemical reactions with their environment. It is a significant problem in many industries, as it can cause structural failure and equipment malfunction, leading to costly repairs and potential safety hazards. Additionally, corrosion can also lead to environmental pollution, as corrosion products can leach into soil and water. It is important to prevent and control corrosion in order to maintain the integrity of structures and equipment and to protect the environment. There are several methods used to protect materials from corrosion, such as environmental control, material selection (Uygur et al. 2015;Gerengi et al. 2019Gerengi et al. , 2020 and inhibitors (Rizvi et al. 2021) and coatings. One of the best methods followed is electroplating technology to enhance corrosion resistant properties of the base materials.
Ni-Mo alloy coating is of great interest due to their good corrosion resistance (Yang et al. 2022) and their ability to catalyse the hydrogen evolution reaction in alkaline solutions (Goveas et al. 2018;Li et al. 2020;Huang et al. 2015). Owing to their high corrosion resistance, they are considered to be a potential replacement for hard chrome coating at lesser cost (Hu and Weng 2000;Wasekar et al. 2019). Similarly, Ni-Cd coatings finds its extensive applications in decorative finishing of metals and anticorrosive protection of metals (Rao and Hegde 2014;Sriramana et al. 2021;Brenner 1963). It is well known that Ni-Mo and Ni-Cd alloys follow induced and normal type of codeposition, respectively. In both type of alloy deposition, change in the composition of coatings is negligible, or unpredictable with plating variables. From a theoretical point of view, in Ni-Mo alloy coating pure molybdenum (Mo) having E 0 Mo = − 0.20 V cannot be electrodeposited from their aqueous solution, and it can be codeposited easily with iron group metals such as nickel (Ni) forming an alloy. In Ni-Mo coatings, Ni stimulate the deposition of reluctant metal Mo, which do not deposit by itself. Ni-Cd coatings follows normal type of codeposition, where Ni deposits preferentially than 1 3 Cd, which characterized by the preferential deposition of noble Ni ( E 0 Ni = − 0.25 V) instead of Cd ( E 0 Cd = − 0.40 V). In induced type of electroplating, plating variables, like current density (c.d.), temperature, agitation, pH etc. has vagarious dependency on the composition of the bath compared to other normal types of codeposition (Brenner 1963;Xu et al. 2016;Mousavi et al. 2016).
In preliminary study on electrodeposition of Ni-Mo and Ni-Cd alloy coatings showed two peculiarities, in terms of their Ni content: (i) Change of Ni content in Ni-Mo coating with c.d. is insignificant due to induced type of codeposition, and (ii) Ni content of Ni-Cd alloy increases with increase of c.d., due to normal type of codeposition. Even at higher concentration of Ni in the bath, Ni content of the alloy decreased with increase of current density, pH and temperature, reasoned by the preferential deposition of less noble metal (Cd). In other words, Ni content of the coating is extremely low compared its content in the bath. In general, corrosion protection ability of both Ni-Mo and Ni-Cd alloy coatings depends upon their Ni (noble metal) content (Eliaz andGileadi 2008, Huang et al. 2015;Podlaha et al. 1996;Abd El Rehim et al. (1984); Liu and Anderson (1996;Parthasaradhy 1989) and in alloy plating, the metal content depends on the liming current density (i L ) of the metal. Hence, it is very unlikely to develop coatings of high corrosion protection from both Ni-Mo and Ni-Cd alloy baths, by changing the c.d. Hence, to alleviate the problem associated with limiting c.d. of Ni in baths of Ni-Mo and Ni-Cd to develop coatings of high corrosion resistance, electrodeposition of their ternary alloy coating, i.e. Ni-Mo-Cd coating have been tried. The advent of induced and normal type of codeposition that the individual binary baths followed have been used advantageously to optimize the Ni content of the ternary alloy for better corrosion stability. The experimental results are presented, with an account responsible for improved corrosion resistance of ternary alloy coatings. The optimal Ni content of the alloy coatings have been identified. Experimental observations are compared, in relation to its binary alloy coatings, and results are discussed. The effect of limiting current density on deposition mechanism of Ni-Mo, Ni-Cd and Ni-Mo-Cd was studied.

Experimental
Two binary alloy baths, namely Ni-Mo and Ni-Cd, were optimized using conventional Hull method as described elsewhere (Ganesan et al. 2006). The composition and processing parameters of Ni-Mo, Ni-Cd and Ni-Mo-Cd alloy baths are given in Table 1. Electrolytic baths were prepared using LR grade (Merck, Mumbai, India) chemicals, as they are purchased without further purification, using double distilled water. The desired pH's were maintained using Micro pH Meter (Systronics, 362), by proper addition of either NH 4 OH or HCl depending on the requirement. Table 1-Composition and processing parameters of baths used for electrodeposition of Ni-Mo, Ni-Cd and Ni-Mo-Cd alloy coatings under constant condition of temperature and P H . All depositions reported here were carried out at constant temperature (303 K), under condition of constant agitation on copper substrate (7.5 × 3.0 cm), used as cathode. The copper substrates, polished metallurgically to get mirror finish were used for electroplating. It is degreased (trichloroethylene), and then pickled (1:1 HNO 3 ) before to immersing it into electrolyte.
The depositions were carried out on known active surface area (3.0 × 3.0 cm), leaving the other region covered by cellophane tape. Both binary and ternary alloy electrodepositions were carried out under same geometric conditions of anode and cathode, using 300 mL capacity cubic customized electrochemical cell as shown in Fig. 1. Electrodepositions were made at desired c.d.'s using DC power source (Power Analyzer, N6705A, Agilent Technologies, USA). The pure nickel plate was used as anode. All depositions were carried out for 10 min. The cathode and anode were placed parallel at 5 cm distance, during deposition. After electrodeposition, all coatings were washed using distilled water followed by air drying. The coatings were then analysed for its corrosion behaviors.
The rate of corrosion was studied using controlled Potentiostat/Galvanostat (VersaSTAT3, Princeton Applied Research, USA). The three-electrode set up was used to perform corrosion experiments. The corrosion tests were carried in 5% NaCl solution at room temperature by exposing a 1cm 2 surface area of the alloy coatings. The corrosion behaviours of alloy coatings were studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization methods. The surface morphology, composition and phase structure of Ni-Mo, Ni-Cd and Ni-Mo-Cd alloy coatings were analysed using scanning electron microscopy (SEM), interfaced with energy dispersive X-ray spectroscopy (EDS) (Oxford EDS(Xact), and X-ray diffraction study (XRD) (Rigaku-mini-Flex 600), respectively. The surface roughness of the coatings was measured using Atomic Force Microscopy (AFM, Innova SPM Atomic Force Microscope). Binary and ternary alloy coatings deposited at different c.d.'s are conveniently represented as (Ni-Mo) x and (Ni-Cd) y and (Ni-Mo-Cd) z alloys, with subscription x, y, z on right hand side, indicating the c.d. at which they are deposited.

Corrosion behaviour
The corrosion rates were calculated by Tafel extrapolation methods, by scanning the test electrode in a potential ramp of ± 250 mV, at scan rate of 1 mV/sec. The Nyquist plots were drawn, using data corresponding to AC frequency in the range of 100 kHz-10 mHz, with perturbing sine wave of 10 mV amplitude. The corrosion rate (CR) in millimetre per year (mm y−1) was calculated using Eq. (1).
Here, CR is expressed of milli meter per year (mmy −1 ), Where K = 0.00327, E = Equivalent weight of the alloy, D = density of alloy under study and, i corr = corrosion current density in µAcm −2 . (1)

Electrodeposition of alloy coatings
Electrodeposition of binary (Ni-Mo and Ni-Cd) and ternary Ni-Mo-Cd alloy coatings were done at different current densities (ranging from 1.0 to 6.0 Adm −2 ) using electrolytic baths, shown in Table 1. The composition and corrosion protection efficacy of alloy coatings corresponding to different c.d.'s is given in Table 2. The actual metal contents in Ni-Mo, Ni-Cd and Ni-Mo-Cd baths, calculated from the weight of Ni, Mo and Cd salts, are also shown in Table 2.

Induced codeposition of Ni-Mo alloy
To begin with, of Ni-Mo alloy deposition was carried out at four different c.d.'s (ranging from 1.0 to 6.0 Adm −2 ), using the bath shown in Table 1. From the composition data, shown in Table 2 it may be noted that there is no significant change in the Ni content of alloy with c.d. However, a small decrease of Ni content with c.d. clearly shows that the bath followed the induced type of codeposition. It may be further noted that wt% Mo in the deposit is in the range of ~ 30% at different c.d's. studied as compared to its 80.2% in the bath. At the same time, the deposit is having wt% of  Ni in the range of (~ 70%) and was found to be higher, compared to that in the bath (17.2%). A small variation of Ni and Mo content in Ni-Mo alloy coatings, over the range of c.d. studied was found (Table 2). It is the testimony for induced type of codeposition, enabled due to possible complexation of Mo and Ni ions with citrate ions (Parthasaradhy 1989). Thus, from Table 2, it may be noted that a bright and uniform coating of Ni-Mo alloy coating corresponding to 6.0 Adm −2 , represented as (Ni-Mo) 6.0 Adm −2 coating is the most corrosion resistant, among others.

Normal codeposition of Ni-Cd alloy
Similarly, Ni-Cd alloy coatings depositions were carried out at four c.d.'s, like 1.0, 2.0, 4.0 and 6.0 Adm −2 , using Ni-Cd bath shown in Table 1. The composition of the coatings obtained at different c.d.'s are reported in Table 2, with their corrosion rates. It is important to note that over entire range of c.d. studied, wt% of Cd (less noble metal) in the deposit is much more than that in the bath (Ganesan et al. 2006), compared to only 2.6% in the bath ( Table 2). The composition data of Cd in the bath indicates that the bath follows normal type of codeposition with preferential deposition of noble metal (Ni) with increase c.d (Kanani 2006).
The corrosion data reported in Table 2 reveals that corrosion rate of Ni-Cd alloy coatings decreases with increase of Ni content, affected at higher c.d. In other words, corrosion rate decreased with increase of Ni content. However, at 6.0 Adm −2 , i.e. corresponding to (Ni-Cd) 6.0 Adm −2 CR was found to increase once again. It may be attributed to the fact that applied c.d. has exceeded the limiting c.d. of Ni, for bath under operation. Thus, a bright and uniform Ni-Cd alloy coating corresponding to 4.0 Adm −2 , represented as (Ni-Cd) 4.0 Adm −2 showed better corrosion resistant property as compared to all other coatings.

Anomalous codeposition of Ni-Mo-Cd alloy
Based on the corrosion performance of Ni-Mo and Ni-Cd alloy coatings, and their dependency with c.d., an attempt was made develop ternary alloy coatings of higher corrosion resistance by proper manipulation of its Ni content. In this direction, a ternary Ni-Mo-Cd alloy coating was attempted by adding small quantity (1.0 g/L) of CdCl 2 into optimized Ni-Mo bath. The composition and operating variables used for deposition of Ni-Mo, Ni-Cd and Ni-Mo-Cd alloy coating is as shown in Table 1. Ternary alloy coatings were electroplated at various c.d.'s, like 1.0, 2.0, 4.0 and 6.0 Adm −2 , and their corrosion performance were evaluated. The composition and corrosion rates of ternary Ni-Mo-Cd alloy coatings corresponding to different c.d.'s are reported in Table 2. Hence, from corrosion data reported in Table 2, it may be inferred that Ni-Mo-Cd alloy coating corresponding to 6.0 Adm −2 , represented as (Ni-Mo-Cd) 6.0 Adm −2 is the most corrosion resistant coating. The data from Table 2 indicates that, with increase of current density the content of less noble metal (cadmium) increases and it inhibits the deposition of more noble molybdenum. This proves that (Ni-Mo-Cd) coatings follows anomalous type of electrodeposition.

Surface morphology and XRD study
The microstructure of (Ni-Mo) 6.0 Adm −2 , (Ni-Cd) 4.0 Adm −2 and (Ni-Mo-Cd) 6.0 Adm −2 alloy coatings, showing the highest corrosion resistance of respective baths are shown in Fig. 2. The poor corrosion resistance behavior of (Ni-Mo) 6.0 Adm −2 alloy coating may be due to its rough surface, having a granular structure as may be seen in Fig. 2a. The better corrosion resistance of (Ni-Cd) 4.0 Adm −2 alloy coating, compared to that of (Ni-Mo) 6.0 Adm −2 coating is due to a decrease of surface roughness, seen in Fig. 2b. The least corrosion rate of (Ni-Mo-Cd) 6.0 Adm −2 coating may be attributed to increased smoothness of coating, compared to both binary alloys as seen in Fig. 2c. The surface morphology of the corroded (Ni-Mo) 6.0 Adm −2 sample is given in Fig. 2d. From the composition data (Table 2), it may be noted that the smoothness of coatings has an inverse dependency on Ni content of the alloy. Thus, the high corrosion resistance of (Ni-Mo-Cd) 6.0 Adm −2 alloy coating is not attributed by its Ni content of the alloy.

Atomic force microscopy study
The atomic force microscopy (AFM) technique was used to find the surface roughness of optimal coatings. The AFM images corresponding to optimal (Ni-Mo) 6.0 Adm −2 , (Ni-Cd) 4.0 Adm −2 and (Ni-Mo-Cd) 6.0 Adm −2 coatings as shown in Fig. 4. From Fig. 4, it is clear that the surface roughness of (Ni-Mo-Cd) 6.0 Adm −2 alloy coating is lesser than that of (Ni-Mo) 6.0 Adm −2 , (Ni-Cd) 4.0 Adm −2 . The average surface roughness (R a ) of (Ni-Mo) 6.0 Adm −2 , (Ni-Cd) 4.0 Adm −2 and (Ni-Mo-Cd) 6.0 Adm −2 coatings was found to be 35.6, 27.7 and 23.0 nm, respectively. From the AFM data, it is clear that coating (Ni-Mo-Cd) 6.0 Adm −2 was found to be more resistant having a lower average surface roughness value.

Potentiodynamic polarization study
The corrosion behavior of electroplated binary and ternary alloy coatings were studied by the potentiodynamic polarization method and corresponding plots are as shown in Fig. 5.
The corrosion rates of electrodeposited coatings are reported in Table 2. The polarization behavior of Ni-Mo, Ni-Cd and Ni-Mo-Cd coatings, deposited only under optimal conditions are shown in Fig. 5, for comparison purpose. From the nature of the polarization plot, it may be noted that (Ni-Mo-Cd) 6.0 Adm −2 coatings is more corrosion resistant than binary alloy coatings. This is further supported by its highest E corr values as seen in Fig. 5. The nature of the anodic curves of all three alloy coatings indicates that the corrosion is more an anodic controlled (Elias et al. 2016).

Electrochemical impedance spectroscopy (EIS) study
EIS technique is used for ranking coatings, quantifying coating breakdown, assessing interfacial reactions and for predicting the lifetime of coating and/or metal systems (Yaun et al. 2010). The EIS responses of electrodeposited Ni-Mo, Ni-Cd and Ni-Mo-Cd coatings, corresponding to their optimal conditions are shown in Fig. 6. The equivalent circuit has been simulated, and is shown in inset of Fig. 6. A close agreement between the observed and simulated impedance responses for (Ni-Mo-Cd) 6.0A dm −2 coating was found. The equivalent circuit diagram corresponding to coatings are given in inset of Fig. 6 and values are listed in Table 3. It may be seen confirmed from larger diameter of the (unfinished) semicircle corresponding to (Ni-Mo-Cd) 6.0Adm −2 coatings is more corrosion resistant than binary alloys. Increased diameter of the semicircle suggests its better corrosion resistance. The charge transfer resistance (R ct ) value of Ni-Mo-Cd coating is more as compared to Ni-Mo and Ni-Cd coatings. This confirms that Ni-Mo-Cd coating is found be more corrosion resistant than binary coatings.

Effect of c.d. on the composition of deposited coatings
Variation in wt% of M(metal) content (Ni, Mo and Cd) in binary and ternary alloy coatings over a range of c.d. studied is shown in Fig. 7. It may be noted that in Ni-Mo alloy coating, Ni content of the deposit is slowly decreasing with the increase of c.d., as expected in induced type codeposition. Corrosion data, reported in Table 2 clearly shows that c.d. slightly effects the Ni content, and hence corrosion resistance of the alloy coating as mentioned earlier. But in case of Ni-Cd alloy coatings, Ni and Cd content has large dependency with c.d. A sudden increase of Ni content was found in the range of 2.0-4.0 Adm −2 (Fig. 7). The maximum Ni (noble metal) content corresponding to 4.0 Adm −2 indicates that it is its approximate limiting current density for deposition of Ni in that bath.

Effect of addition of Cd +2
The essence of electroplating is charge transfer across the electrode-electrolyte interface, and most importantly the composition of deposit depends on the i L of ions involved. Therefore, a profound understanding of the structure of diffusion layer is important in electroplating, in terms the effect of c.d. on the diffusion of ions and thickens of the diffusion layer. The composition of deposit formed mainly depends on concentration change of ions near the diffusion layer and be explained by simple diffusion theory (Kanani 2006). Therefore, at i L , the rate at which metal ions in Ni-Mo-Cd alloy can deposit may be determined by diffusion and electrical migration of the ions, and is proportional to their concentrations. Thus, by taking the advantage of diffusion theory based i L of metal ions, it is possible to control the Ni content of the alloy coatings, by proper modulation of charge transfer process at EDL. In this study, the i L of Ni was decreased to large extent by addition of small quantity of Cd +2 ions into Ni-Mo bath. The CdCl 2 added in small quantity (1 g/L) decreased the i L of Ni, and allowed the deposition of ternary alloy coating of high corrosion resistance as explained below.
It may be recalled that i L of a metal is the current density at which rate of its deposition is maximum. At i L the concentration of metal ions near the electrode drops to zero, and it depends only on the bulk concentration. Therefore, i L of a metal in an electrolytic bath may be written in the form of Eq. 2.
where n is the number of electrons per ion being transferred, F the Faraday constant (96,490 C/equivalent). C B is the concentration of metal ions in bulk electrolyte and δ is the thickness of diffusion layer. From the above relation, it may be noted that in a given bath (as C B is constant), i L is changing only by altering the thickness of the EDL. Thus, in the light of above discussion, the inverse dependency of Ni content with c.d. due to addition of Cd +2 ions into Ni-Mo bath may be attributed to its decreased i L of Ni. Thus, Cd +2 ions reduced the i L of Ni greatly in Ni-Mo-Cd bath, in relation to that in (Ni-Mo) bath. In the present study, it may be concluded that wt% of Ni in the deposit has decreased due to decrease of its i L, due to addition of Cd +2 ions, and allowing the bath to undergo anomalous ternary alloy deposition. The decrease of i L of Ni in Ni-Mo-Cd alloy bath is due to change in the charge transfer process at the EDL, affected due to different complexation of process compared to that in binary baths.
Hence, least corrosion rate of ternary Ni-Mo-Cd coating is due to decreased i L of Ni due to addition of Cd into bath. Hence, it may be noted that the corrosion performance of binary and ternary alloy coatings need not always depends more on the i L of noble metal, rather than the wt% of noble metal in the deposit. Therefore, it may be concluded that Cd +2 ions acted as such agency to decrease the i L of Ni by increasing thickness of diffusion layer. Representational diagram showing the decrease of i L of Ni in Ni-Mo-Cd bath affected due to addition Cd +2 ions into Ni-Mo bath is shown in Fig. 8, in relation to its value in Ni-Cd bath, following normal codeposition; and Ni-Mo bath, following induced type of codeposition. It may be seen from Fig. 8 that the diffusion layer thickness (δ) corresponding to Ni-Mo-Cd bath is much larger than that for Ni-Mo and Ni-Cd baths. This increase of δ is due to change in the charge/mass transfer (2) i L = nFDC B ∕ Fig. 7 Effect of current density on the composition of binary Ni-Mo and Ni-Cd and ternary Ni-Mo-Cd alloy coatings, electrodeposited from citrate bath at 303 K (horizontal lines represent the actual content of metal in the bath) process at diffusion layer due to addition of Cd +2 ions into the bath.

Conclusions
The experimental investigation to understand the effect of addition of Cd into Ni-Mo bath on the corrosion protection efficacy of Ni-Mo-Cd alloy coatings, following observations were made as conclusions: (1) Ni-Mo-Cd alloy coatings of better corrosion performance, than both Ni-Mo and Ni-Cd alloy coatings can be developed on addition of small quantity CdCl 2 into Ni-Mo bath.
(2) The limitation of high limiting current density (i L ) of Ni of both Ni-Mo and Ni-Cd baths, due to inherent induced and normal type of codeposition has been successfully alleviated by adding of CdCl 2 . (3) The advent of induced and normal type of codeposition of Ni-Mo and Ni-Cd baths, respectively, have been used advantageously to optimize the Ni content of the ternary alloy for better corrosion stability. (4) The composition versus current density (c.d.) plots of alloy coatings have been studied, and optimal i L of Ni in three baths were assessed. (5) Experimental investigation revealed that (Ni-Mo-Cd) 6.0 Adm −2 coating shows highest corrosion resistance due to decrease of i L of Ni, affected due to addition of Cd +2 ions into the baths. (6) The decrease of i L of is due to change of charge transfer process at the electrical double layer (EDL), affected by the addition of Cd ions into the bath. (7) The corrosion performance of binary and ternary alloy coatings always depends more on the i L of noble metal, rather than the wt% of noble metal in the deposit.