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Review

Electrodeposition of Nickel onto Polymers: A Short Review of Plating Processes and Structural Properties

by
George W. Thompson
and
Mohammad J. Mahtabi
*
Department of Mechanical Engineering, The University of Tennessee at Chattanooga, Chattanooga, TN 37343, USA
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(15), 8500; https://doi.org/10.3390/app15158500 (registering DOI)
Submission received: 30 May 2025 / Revised: 24 July 2025 / Accepted: 29 July 2025 / Published: 31 July 2025
(This article belongs to the Special Issue Advanced Materials Machining—Recent Advances, Applications and Challenges)
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Abstract

Featured Application

As the electrodeposition of nickel on polymers appears to open new avenues for manufacturing structural parts for mostly short- or medium-life components, this short review facilitates the exploration of the properties for researchers in the field.

Abstract

This paper reviews the fundamental principles and techniques of nickel electrodeposition, with a particular focus on metallizing polymeric substrates. It outlines the electrochemical mechanisms involved in depositing nickel from an acidic Watts bath, detailing the roles of key electrolyte components—i.e., nickel sulfate, nickel chloride, and boric acid—and the influence of process parameters, such as current density, temperature, and pH, on deposit quality (density and surface condition) and mechanical properties. In addressing the unique challenges posed by non-conductive polymers, this review compares emerging methods like silver conductive paint, highlighting differences in deposition time, surface resistivity, and environmental impact. Additionally, this paper examines how process parameters affect the as-deposited microstructure, adhesion, and overall mechanical properties (such as hardness, ductility, and tensile strength), while identifying critical issues such as low deposition density and substrate degradation. These insights provide a structured background for optimizing electroplating processes for applications in electronics, automotive, aerospace, and biomedical sectors, and suggest future research directions to enhance deposition uniformity, sustainability, and process control.

1. Introduction

Electrodeposition is a non-spontaneous electrochemical phenomenon where charged ions in an electrolyte migrate under the influence of an electromotive force (electric field) towards a substrate, typically the cathode. However, spontaneous deposition (cementation) of a metal can also occur when more noble metal ions interact with substrates composed of a less noble metal [1,2]. At the cathode, these ions undergo a reduction reaction, transferring electrons and losing their ionic charge, which allows them to deposit as a solid metallic layer. This process is fundamental to many electrochemical systems, including batteries, fuel cells, and electroplating [3]. However, electrodeposition in these systems serves dissimilar roles. In batteries and fuel cells, the deposition of materials on electrodes is often an undesired byproduct, as it can lead to performance degradation, for instance dendrite formation or electrode passivation [4,5]. In the context of this review, electrodeposition refers to the general process of metal deposition through the electrochemical process. Furthermore, electroplating refers to a subset of electrodeposition that uses the same principles to achieve controlled deposition of metal onto a substrate, aiming to enhance surface properties like corrosion resistance, durability, and aesthetic appeal [4,5].
Electroplating has a history dating back over two centuries, originating in the early 1800s when gold was first deposited onto silver using Volta’s voltaic pile in 1805 [6]. Although this work was initially suppressed, the process was reinvented in the 1830s and gained industrial traction by the 1840s, when the use of cyanide-based gold and silver plating solutions were patented in the United Kingdom [7]. The technique soon expanded to copper, nickel, and zinc, and by the late 19th century, it was widely used for decorative and corrosion-resistant applications. Electroplating became essential in the manufacture of coins, jewelry, and metal tableware, establishing itself as a critical process in emerging industrial economies [8].
Throughout the 20th century, electroplating matured into a cornerstone of modern manufacturing and the advent of modern electroplating began in 1916 with the publication of Watts’s paper [9]. The electrolyte formulated by Watt greatly improved previous solutions by increasing efficiency, throwing power, and deposition rate [10]. Variations in the Watts bath formulation are still the most used electroplating solutions in the world. Other major advances included the introduction of electroless plating in 1946 [11], allowing metal deposition on non-conductive substrates. Post-war industrial demand drove innovation in alloy and multilayer coatings, pulse plating, and environmental improvements such as the replacement of cyanide with sulfate-based electrolytes [12]. Applications of electroplating now spans from automotive and aerospace components to electronics and biomedical devices, and recent developments, such as the use of ionic liquids and nanostructured deposits, continue to expand its relevance in high-performance and sustainable materials engineering [13,14]. As of 2022, over 150,000 tons of nickel were electrodeposited annually, which is five percent of the total annual nickel production [15].
While electrodeposition onto metallic substrates is widely practiced, the electroplating of metals onto non-metallic surfaces, particularly polymers, presents unique challenges and opportunities. Electroplating nickel onto polymers is arduous because chemically aggressive plating solutions can swell, crack, or degrade the polymer surface, especially during multistep treatments. Additionally, achieving uniform deposition thickness on complex polymer geometries is difficult, as trapped solutions and irregular surfaces can cause uneven deposits, voids, or dimensional inaccuracies in the plated part. Unlike traditional metallic substrates, polymers are non-conductive, requiring specialized pre-treatment or primary metallization (PM) methods before electroplating. These metallization processes have seen improvements recently that allow for advancements in combining electrodeposition with 3D-printed mandrels to achieve solid, freeform, metallic parts [16,17]. In this review, the fundamental electrochemical concepts of electrodeposition of nickel are explored, with a focus on the advancements in electroplating techniques used for polymeric substrates. Specifically, this paper delves into the properties of the Watts electrolyte bath due to its foundational role in most modern nickel electrodeposition processes. We present an overview of key mechanical properties (such as hardness, ductility, and tensile strength) of electrodeposited nickel onto non-conductive polymers, due to their ubiquitous nature. We also discuss how these properties may be tailored to be used in advanced structural applications, where traditional manufacturing methods are costly and time-taking. These novel methods open new avenues for rapid prototyping and cost-effective medium run production.

2. Electrochemistry of Nickel Electrodeposition

As mentioned before, there are two types of electrodepositions: thin-film and structural. Thin-film deposition processes, usually referred to as electroplating, are typically used for depositing a layer of material to enhance corrosion resistance, heat resistance and conductivity for applications such as printed circuit boards, semiconductors, engine components, fasteners, implants, and solar panels [18,19,20,21]. Structural plating, also referred to as electrodeposition or electroforming, is less common than thin-film plating and is used for rocket nozzles and thrust chambers, radiation shields, high-precision optical instruments, and heat exchangers [22,23,24,25]. Below, we explain the components of the setup to perform electrodeposition of nickel together with the details of chemical reactions happening during the process.

2.1. Components of an Electroplating Setup

The setup for the electrodeposition of nickel involves four main components, as shown in Figure 1. These components include a source of direct electric current, usually provided by a DC rectifier, the positively charged electrode known as the cathode, and the negatively charged electrode known as the anode [15]. The cathode is the part that is to be deposited on, known as the substrate, and the anode is usually a suitable form of nickel metal to facilitate the deposition process by replacing depleted ions in the electrolyte [26]. The final component is the medium of ion supply and transport known as electrolyte. Most nickel electroplating is carried out in an aqueous bath containing dissolved metal salts (ions). These solutions are favorable for plating on polymers because they operate well below the glass transition temperature of most plastics, thus avoiding thermal distortion of the substrate. Aqueous baths are further subdivided by pH into acidic, neutral, and alkaline systems [4]; the acidic (Watts type) formulation remains the most prevalent because it is simple to prepare and easy to control, and consistently produces high quality nickel deposits.
For specialized applications—particularly when simultaneous deposition of multiple elements is required—ionic melts are employed instead of water-based baths. Operating between about 300 °C and 700 °C, these molten salts offer a much wider electrochemical window and higher metal ion solubility than aqueous solutions, making them ideal for alloy and composite coatings [27,28]. Their elevated temperature precludes their use on heat sensitive polymer substrates and demands more robust equipment and stricter safety measures.

2.2. Electromigration Control Methods

There are two common methods for controlling electromigration: current-controlled or galvanostatic and voltage-controlled or potentiostatic [5,15]. During galvanostatic deposition the current is fixed, and the voltage varies as a function of changes in resistance, deposition thickness, and electrolyte composition. Some of the advantages of this method are its simplicity and higher deposition rate, making it more suitable for structural applications. Some of the drawbacks of galvanostatic deposition include the possibility of voltage instability due to variation in temperature or electrolyte composition, which in turn could affect the quality of the deposited part. In the potentiostatic method, a constant voltage is applied to the soluble or working anode, while the current can vary during the deposition process. The advantages of this method include better control over the deposition quality, making it particularly effective at producing uniform thin layers. Some of the drawbacks of the potentiostatic deposition include a more complex setup requiring an extra insoluble anode to monitor the potential and the current variability caused by the electrolyte composition and surface area of the electrodes [4,13,29].

2.3. Watts Bath Nickel Electroplating Reactions

Figure 2 shows a generic arrangement and the reactions that take place during the electrodeposition of nickel from an additive-free Watts bath. The DC power source drives the deposition process by applying a current to the electrochemical cell. Two main processes occur once there is an effective potential or overpotential in the cell: oxidation, and reduction. These processes are governed by the principles of thermodynamics and dynamic electrochemistry [4].
Oxidation occurs at the anode as nickel atoms dissolve into the electrolyte, forming nickel ions ( N i 2 + ) while releasing two electrons for each ion. This reaction is given by
N i   s → N i 2 + + 2 e − .
This is a thermodynamically favorable process under the influence of applied potential [4]. Simultaneously, at the cathode, a complimentary reaction, i.e., reduction of nickel ions, takes place. Reduction of nickel ions from the electrolyte occurs due to the acceptance of two electrons, depositing the nickel onto the substrate as solid nickel metal. The reduction reaction is
N i 2 + + 2 e − → N i   s .
This process is governed by the availability of electrons supplied by the DC power source. The electrolyte facilitates the migration of ions during these processes. Positively charged ions N i 2 + and H + atoms, also known as cations, move toward the negatively charged cathode, while negatively charged C l − ,   O H − , and S O 4 2 − ions, also known as anions, migrate to the positively charged anode. The exchange of these cations and anions establishes a dynamic equilibrium, where ion fluxes and charge transport maintain overall electrical neutrality. At the electrode-electrolyte interface, the balance between the applied potential, overpotential, and concentration gradients of the elements governs the rate of nickel electrodeposition [4,15,30]. The quantitative relationship between electron flow and material deposition is governed by Faraday’s law given as [31]
m = I · t · M n · F ,
where m is the mass of the deposited metal, I is the applied current, t is the time of deposition, M is the molar mass of the metal, n is the number of electrons involved in the reduction of one atom, and F is Faraday’s constant (96,485 C/mol). The overall effect of these processes is the transfer of nickel from the anode to the cathode, forming a dense, adherent nickel microstructure on the substrate.
The electrodeposition of nickel involves a careful balance of electrochemical principles, electrolyte composition, and current control to ensure efficient ion transport and quality deposition. The decisions made on process parameter selection and deposition types are specific to certain requirements and each needs to be thoroughly tailored. More information on the fundamentals of electrodeposition can be found in [5,15,32].

3. Primary or Polymer Metallization

Since most polymers are electrically nonconductive, their surface should be made conductive for electrodeposition through a process called Polymer metallization (PM), which can be made possible through many ways of depositing a metallic layer on it. The most common methods of PM are physical vapor deposition (PVD), electroless plating, thermal spray methods, and direct polymer-metal bonding methods [33,34,35,36,37]. The advantages and disadvantages of each technique are summarized in Table 1.
Electroless plating followed by electroplating is the most prolifically used PM technique [38]. Electroless deposition is a chemical process similar to electrodeposition but without the need for an external power source. Instead, the process relies on a redox reaction between a reducing agent in the plating solution and metal ions in the electrolyte [39]. This process is autocatalytic, meaning that once the initial layer of metal is deposited, it catalyzes further deposition [40]. Electroless deposition is a multistep, time consuming process [38]. The various steps involved in the electroless deposition are schematically shown in Figure 3. The pre-treatment creates a clean, pH-balanced surface so that later catalytic chemicals can infiltrate the surface uniformly. The catalytic stage deposits nanoscale “seeds” that start the autocatalytic reduction. The overall chemical deposition stage builds the final coating with a thickness controlled only by immersion and bath chemistry with no applied current [39].
Although electroless deposition is the most usual form of PM, many difficulties arise during this process. The chemicals used in surface pre-treatments and the chemical deposition stage can interact adversely with certain polymers, leading to the degradation or deformation of the substrate [41,42]. Achieving a uniform deposit can be difficult due to surface irregularities and the potential for uneven catalyst distribution. Furthermore, many of the chemicals used in electroless deposition, such as chromic acid, formaldehyde, and sodium hypophosphite, among others, are harmful to the environment [43].
Another effect that must be considered is how common PM techniques, including electroless plating, modify the interface of the polymer (as the substrate) and the plated metal [41]. For example, as depicted in Figure 4, the etching process in electroless plating roughens the surface of the polymer to allow for the deposition of a primer metal, e.g., palladium, on the surface and, thus, to facilitate the autocatalytic process. It should be noted that for electrodeposition on 3D-printed polymers, PM techniques, such as electroless plating, alter the as-printed surface finish of the polymeric material and therefore roughen the interior surface of the plated metal if the polymer is removed. This roughening is desirable for some processes because it adds adhesive strength and encourages metal-polymer diffusion [43]. However, for other processes, such as deposition on mandrels, where the substrate is removed from the final part, smooth surface characteristics are desired. There have been advancements in electroless plating processes that do not require surface etching to initialize the plating process, but these methods usually require a polymer that is already premixed with another conductive element. These types of polymers are not easily printable and usually result in uneven conductivity due to inhomogeneous incorporation of metal ions [38].

Alternative Methods of PM

Alternative PM methods have been emerging that address some of the drawbacks of common PM techniques. Equbal et al. [16] proposed a unique solution using a paste made of aluminum powder, charcoal, enamel, and distilled water mixed at a weight ratio of 40:3:36:21 for plating onto fused deposition modeling (FDM) parts. This mixture was then applied to a degreased acrylonitrile butadiene styrene (ABS) FDM part and allowed to dry completely. Electroless deposition was then performed at room temperature for 48 h. After the electroless deposition process, a comparison of surface resistivity was made with a sample that was conventionally etched with chromic acid. There was a noticeable difference in the surface resistivities, with the conventional chromic acid etching resulting in a surface resistivity of 920,000 Ω, while the aluminum–charcoal paste achieved a surface resistivity of 0.09 Ω [16]. The lower surface resistivity of the metallized polymer increases the efficiency of the further electroplating process and increases the durability and uniformity of the electrodeposited metal.
In an extension of this idea of applying a conductive layer over the polymer with only minor changes to the as-printed surface, a silver conductive layer was used by Thompson and Mahtabi [17]. Applying a silver conductive paint (SCP) to the as-printed polymeric substrate yielded all the desired properties. The surface resistivity of the painted samples after two applications was 0.015 Ω [17]. A comparison of surface resistivity values can be seen in Table 2. With a single application of SCP having a thickness of 19 microns, the total thickness of the coating is ~38 μm, which is well within tolerances for most engineering applications [44,45]. SCP can also be applied quickly as opposed to the electroless methods, which take around 48 h to achieve a relatively low surface resistivity [16]. Furthermore, the SCP eliminates the need for environmentally harmful chemicals as used in electroless plating, and does not require specialized equipment used in other forms of PM.
Due to the highly conductive nature of silver, SCP provides a highly conductive seed layer allowing the nickel ions to be uniformly deposited. The paint also provides an adequate level of adhesion of the deposited metal to the substrate, as too much adhesion would cause problems removing the polymer substrate, while too little adhesion would cause delamination during the plating process. Lastly, silver is chemically stable in most nickel-plating solutions and does not react adversely during the electroplating process [46].

4. Common Nickel Electrolyte Compositions and Applications

As previously mentioned, the most widely used electrolyte for nickel electrodeposition onto polymers is an acidic aqueous solution of nickel salts, specifically a Watts bath. This electrolyte composition consists of three main ingredients: nickel sulfate, nickel chloride, and boric acid [29]. The sulfate acts as the main source of plateable nickel, while chloride aids in anode corrosion to replace depleted nickel ions in solution, and boric acid buffers the bath’s pH [29].
Since this formulation by Watts, many other nickel electrolyte solutions have emerged for specific applications. In Table 3, each of these solutions is presented with their most common applications. The Watts bath is the most widely used electrolyte and can be modified with numerous additives to tailor specific properties. By mixing in additives such as a carrier for low stress, a brightener for mirror shine, a surfactant to strip hydrogen bubbles, a leveler for sulfur-free semi-bright deposits, and cobalt for high hardness, the electrodeposited nickel can be modified from a soft, ductile material to high-gloss, high-hardness finishes without changing the core nickel-salt ratios. Each alternative electrolyte in the table can only use certain additives that are compatible to maintain their unique properties, such as high deposition rate, ultra-low stress, and wear resistance.

5. Structural Properties of the Electrodeposited Nickel Parts

The characteristics of the initial surface (i.e., the first layers of deposition) and the porosity of the electrodeposited nickel part are affected by the substrate material and its surface, whether it is a polymer or metal substrate. After the initially deposited layers, which is required to smooth the surface of the substrate, given that the electrolyte has sufficient throwing power and leveling characteristics, the structural properties remain mostly unchanged if all other process parameters are held constant [4,15]. Examples of electrodeposited nickel pieces are shown in Figure 5. Figure 5a shows a scanning electron microscopy (SEM) image of the final surface of a nickel piece, electrodeposited on ABS from a Watts bath [50]. This figure indicates the roughness of final surface as well as formation of some microcracks, which is expected due to the absence of external pressure. Figure 5b depicts an optical microscopy (OM) of the cross-section of a nickel wall, which was initially deposited on PLA substrate, which indicates uniform wall thickness upon deposition, using optimized deposition parameters. Varying these process parameters, even in small degrees, has substantial effects on the mechanical and material properties of the electrodeposited nickel. Below, we present an overview of these effects based on Watts-type electrolyte.

5.1. Hardness

The hardness of an electrodeposited nickel part is strongly influenced by the electrolyte chemistry and operating parameters [51,52,53]. In electroplating processes with a Watts-type electrolyte, the addition of chloride ions, C l − , is essential for maintaining the anode’s efficiency by preventing passivation—i.e., the formation of a non-conductive layer that disrupts the dissolution of nickel ions—thus ensuring a uniform current distribution and consistent deposition quality [54]. Along with chloride ions, various additives such as grain refiners, leveling agents, and brighteners are employed in the literature [50,55,56] to modify the plating chemistry and promote a fine-grained microstructure, which, according to the Hall–Petch effect, increases tensile strength [57], and thus hardness. For example, it has been reported that addition of saccharin in the Watts electrolyte can lead to an increase in the hardness of the deposit from 270 HV to 527 HV [17]. Operating conditions also affect the hardness of the deposited material: it is reported that moderately low deposition temperatures (around 30–40 °C) enhance the nucleation of deposit and yield a higher density, while higher deposition temperatures may coarsen the grains and reduce the hardness [48,58].
Electric current density during the deposition also plays a critical role in the hardness of the deposited material. Moderate current densities (~1–5 A/m2) can generate refined microstructures and a hardness between 200 and 300 HV, whereas excessively high current densities can lead to hydrogen evolution, porosity, and dendritic formations that diminish hardness [29,48,59,60]. Therefore, a stable, ripple-free DC rectifier is vital to supply a consistent current density, as fluctuations can lead to uneven deposition and defects that undermine hardness.

5.2. Strength and Ductility

The tensile strength and ductility of nickel parts fabricated via electrodeposition are affected by an interplay between microstructural refinement and internal stress generation. Deposition temperatures above 30 °C promote faster diffusion of nickel ions, thereby increasing nucleation rates and forming finer, denser grains that enhance tensile strength [59], as one may expect due to the well-known Hall–Petch effect. However, grain refinement may compromise the ductility, and high deposition temperatures can also warp the polymeric substrates, leading to separation issues. The role of electrolyte’s pH on the strength and ductility of the deposited part is equally crucial; an acidic range—between 5.0 and 6.0, with 5.0 to 5.5 offering an optimal compromise—favors high hardness and tensile strength while maintaining sufficient elongation, as it minimizes hydrogen evolution and the risk of embrittlement [61]. Moreover, for a balance of strength and ductility, the current density must be tuned: lower current prevents excessive localized heating and ion depletion, preserving ductility, while very high current density can lead to brittle, dendritic structures [48,61]. The stability of the DC power supply is again critical here, as a consistent, ripple-free current helps avoid localized overpotentials that might otherwise trigger hydrogen evolution and irregular deposition, thereby ensuring a deposit that combines high strength with acceptable ductility [62].

5.3. Modulus of Elasticity

The modulus of elasticity of the electrodeposited nickel is generally reported to remain close to that of bulk nickel, typically ranging from 180 to 210 GPa [48,61,63]. This property is influenced by the uniformity and defect density within the deposited material, affecting the nominal cross-sectional area in the calculations, rather than by the microstructural refinement that predominantly affects hardness and tensile properties. A well-controlled deposition process—with stable current density, appropriate temperature, and optimal additive levels—ensures that the deposition is both uniform and dense, thereby maintaining a high modulus. While the inclusion of co-deposited particles or extremely fine grain structures can cause slight variations, overall, the modulus is less sensitive to changes in plating parameters when compared to hardness or strength [48].

5.4. Internal Stress

Internal, fabrication-induced stresses, i.e., residual stresses, in nickel electrodeposits are a complex function of both electrolyte composition and operating conditions. The presence of C l − ions not only prevents passivation but also aids in achieving a more uniform current distribution, which helps minimize localized residual stress [63]. On the other hand, high current densities and extremely low pH conditions tend to trap hydrogen within the deposit, leading to significant internal tensile stresses that can result in microcracking or embrittlement. Moderately elevated deposition temperatures (around 50 °C) are reported to promote equilibrium crystal growth and allow for the diffusion of hydrogen, thereby reducing residual stress [32].
Organic additives must be carefully balanced to reduce the residual internal stress; in optimal amounts, they refine the grain structure and can lower the internal stress [61,64], yet in excess, they may exacerbate internal residual stress as seen in Figure 6. Saccharin can cause compressive stresses at concentrations greater than 0.00035 mol/L, while other additives such as coumarin induce residual tensile stress which increases sharply with concentrations less than 0.0005 mol/L [59]. Additionally, factors such as the current waveform and the geometry of the electrode setup—specifically the anode-to-cathode distance and surface area ratio—are critical in ensuring that nickel ions are replenished efficiently, thus avoiding conditions that might lead to high internal stresses [65,66].

5.5. Fatigue Failure and Other Properties

Beyond its mechanical characteristics, electrodeposited nickel exhibits a range of material properties that influence its performance. The specific heat capacity of electrodeposited nickel is reported to be around 0.44 J/g·K, and its coefficient of thermal expansion falls between 13.6 and 17.2 × 10−6 K−1, both comparable to those of bulk nickel [48]. The thermal conductivity of the electrodeposited nickel tends to be slightly lower than that of pure nickel, due in part to grain boundary and defect scattering, while electrical resistivity is somewhat elevated—typically in the range of 6.5–8.0 µΩ·cm—because of the refined grain structure and minor impurities introduced by additives [67]. The ferromagnetic properties remain largely intact, with saturation magnetization near 55–60 emu/g, though fine-grained deposits may display increased coercivity [48]. Porosity and corrosion resistance of the electrodeposited nickel parts are highly dependent on the deposition quality; uniform, low-stress deposits achieved through optimal mechanical agitation and a stable DC power supply yield higher corrosion resistance and lower porosity [13]. The anode-to-cathode area ratio is also important, ensuring efficient nickel ion replenishment and preventing passivation, particularly in complex shapes where a higher ratio may be necessary [62].
Toward employing the electrodeposited components for structural applications, it is critical to understand and model the various failure mechanisms of these materials. Among various failure modes, fatigue failure is the most dominant in most of the engineering applications [68,69]. As most of the applications of nickel electrodeposition so far have been limited to electroplating, where a layer of nickel is coated on a base metal, direct evaluations of the fatigue of electrodeposited nickel components have not been realized and require thorough experimentation and analysis. As fatigue failure is dominated by the microstructural features of the part and the internal stresses, as well as surface conditions [70,71], extensive research should be carried out to understand, model and improve the fatigue behavior of these parts, accounting for factors specific for these parts such as their surface condition, internal stresses, and presence of defects. It should be noted that electrodeposited materials, without any post-processing, inherently contain internal and surface defects, which may limit their structural applications to short- and medium-life fatigue regimes. Furthermore, depending on the process parameters and the electrolyte, various phases can form inside the material, which can be the subject of future research. These new microstructural phases can affect the crack formation and growth behavior in structural components.
Structural components fabricated via electrodeposition from a Watts-type bath without additives have tensile internal stresses ranging from 110 to 210 MPa, relatively independent of plating temperature or pH, and this internal stress rises steeply with increased chloride content and current density [15]. The presence of tensile internal stress accelerates the fatigue failure of the part by opening micro-cracks that form around the co-deposited sulfur and trapped hydrogen. A drop of ~45% in fatigue endurance limit when highly tensile Watts nickel was compared with a compressive deposit on identical specimens [72]. The highly tensile internal stresses can be counteracted with additives such as saccharin, thereby increasing the fatigue life [15,72]. Since crack nucleation is a surface-dominated phenomenon, any micro-notches left by brightener-induced growth bands or by hydrogen evolution can dominate the fatigue life [70]. It should be noted that due to the presence of internal voids in the as-deposited material, polishing the material may not be a solution to enhance fatigue life as it may turn sub-surface defects into surface defects. Finally, the location-specific characteristics of deposition may also affect the fatigue failure of an as-electrodeposited component as various locations of the deposited material, depending on the size of the part and specifics of the setup, may result in the formation of various microstructures [73].
Hot isostatic pressing (HIP) is proven to be effective in reducing the size and amount of micro-defects in cast and additively manufactured alloys, including Ni-based alloys, hence boosting low-cycle and high-cycle fatigue strength by up to an order of magnitude; typical cycles are 100–150 MPa, 1050–1150 °C, 2–4 h [74,75]. These measures can push the fatigue strength of Watts-type electroforms toward the 200–250 MPa band reported for wrought Nickel 200, but systematic S–N data for compressive, low-sulfur Watts deposits thicker than ~3 mm is still lacking—highlighting a clear area for future work [76].
Hydrogen embrittlement is the loss of ductility and toughness that occurs when atomic hydrogen diffuses into a metal’s lattice, segregates at defects or grain boundaries, and promotes crack initiation and sudden brittle fracture under stress [77]. It is suggested that the detrimental effects of hydrogen on the strength and ductility of the electrodeposited material can be partially mitigated by immediately “baking” the specimens at 190 °C for 4 h which outgases hydrogen before it stabilizes at lattice sites [62]. However, more research work is required to understand the effects of various electrodeposition conditions, such as type of electrolyte, current density, etc., on the mechanisms of hydrogen embrittlement in electrodeposited nickel parts.

6. Summary and Conclusions

With significant advancements made in both the methods and materials used for electrodeposition, this process forms a crucial technique for metallizing polymeric substrates. While traditional electroplating techniques, such as electroless and electrochemical deposition, have provided valuable insights into polymer-metal adhesion mechanisms, there are still substantial challenges to address, particularly in terms of process efficiency, sustainability, and the quality of metal coatings, their mechanical properties, and failure mechanisms. Recent developments, including the use of environmentally friendly plating methods and novel electroplating additives, offer promising solutions to mitigate the environmental impact and enhance the performance of metal coatings on polymeric substrates.
Nickel electrodeposition from a Watts-type bath onto a 3D-printed polymeric substrate is governed by a balance of electrochemical principles, electrolyte composition, and process control, all of which significantly influence the resulting microstructure and mechanical properties of the metal electrodeposit. Additive chemicals, current density, and temperature determine the key mechanical properties, such as hardness, ductility, and internal stress, which in turn affect structural integrity and fatigue life. High internal tensile stress, which is common in certain electrolyte compositions, can reduce the fatigue endurance limit by up to 45%, while compressive stress, achieved by adding saccharin, improves not only the fatigue endurance limit but also the hardness. Surface preparation methods, including the use of silver conductive paint, provide rapid, environmentally friendly alternatives to traditional electroless plating for primary metallization of polymers. Despite recent progress, challenges such as hydrogen embrittlement, sub-surface defects, and location-specific variations in deposition quality remain.
We should emphasize that this article is a short review that focuses only on a Watts bath electrolyte formulation and does not comprehensively address the variability and compatibility between other polymeric substrates and electrolyte formulations. We do not quantitatively review the impact of long-term environmental exposure or cyclic loading conditions on the durability of the as-deposited metal parts present in real-world applications. Finally, practical challenges associated with scaling laboratory findings to industrial production, such as process repeatability, economic viability, and environmental compliance are not addressed, representing areas requiring future research.
Future research in this field includes closing the gap of knowledge between thin electrodeposited coatings and structural deposits by refining alternative plating methods, improving the uniformity and durability of metal deposits, and exploring new electrolyte formulations that enable high-quality secondary metallization of complex geometries. A greater understanding of the underlying chemical processes at play during electrodeposition, coupled with advancements in real-time monitoring technologies, will be key to furthering the precision and control of the deposition process. By addressing the current limitations and expanding the knowledge base in polymer-electrodeposition interactions, the electroplating industry can leverage new processes to develop advanced structural manufacturing techniques and contribute to the development of more sustainable, high-performance structural applications across automotive, aerospace, and biomedical sectors, while also refining techniques in coating applications.

Author Contributions

Conceptualization, M.J.M. and G.W.T.; methodology, M.J.M. and G.W.T.; formal analysis, G.W.T.; investigation, G.W.T.; resources, M.J.M.; writing—original draft preparation, G.W.T.; writing—review and editing, M.J.M.; visualization, G.W.T.; supervision, M.J.M.; project administration, M.J.M.; funding acquisition, M.J.M. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the State of Tennessee’s RevV program, Task Order No. UTREVV24-001.

Data Availability Statement

No new data were created for this publication.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic of an electroplating setup.
Figure 1. Schematic of an electroplating setup.
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Figure 2. Schematic of nickel electroplating process [11].
Figure 2. Schematic of nickel electroplating process [11].
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Figure 3. A schematic of the various steps involved in the electroless plating process.
Figure 3. A schematic of the various steps involved in the electroless plating process.
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Figure 4. Illustration of the stages of etching on ABS polymer.
Figure 4. Illustration of the stages of etching on ABS polymer.
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Figure 5. (a) SEM image of the final surface of an electrodeposited nickel on ABS [50]; (b) OM image of the cross-section of a deposited nickel wall on a removed PLA mandrel.
Figure 5. (a) SEM image of the final surface of an electrodeposited nickel on ABS [50]; (b) OM image of the cross-section of a deposited nickel wall on a removed PLA mandrel.
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Figure 6. Effect of organic additives to Watts electrolytes on internal stress [59].
Figure 6. Effect of organic additives to Watts electrolytes on internal stress [59].
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Table 1. Common polymer metallization techniques and their advantages and disadvantages.
Table 1. Common polymer metallization techniques and their advantages and disadvantages.
PM MethodAdvantagesDisadvantages
Physical Vapor Deposition (PVD)High precision, thin coatings, uniform coverage, high-quality finishExpensive, slow deposition rates, not suitable for complex geometries
Electroless PlatingUniform coating, no electrical current required, suitable for complex geometriesLimited material compatibility, slow deposition rates, less durable
Thermal Spray MethodsHigh deposition rates, one step process, superior coating adhesionMetals compatibility is limited, special resolution is low, coatings are porous
Direct Polymer-Metal BondingStrong bonding, versatility in method selection, customizable surface finishTemperature limitations on polymers, weak mechanical properties, time intensive
Table 2. Comparison of primary metallization methods.
Table 2. Comparison of primary metallization methods.
Primary Metallization ProcessSurface Resistivity (Ω)Required Time for Similarly Sized Parts
Chromic Acid Etching920,000~48 h
Aluminum–Charcoal Paste0.09~48 h
Silver Conductive Paint0.015~0.5 h
Table 3. Common electrolytes used in nickel electrodeposition and their applications [47,48,49].
Table 3. Common electrolytes used in nickel electrodeposition and their applications [47,48,49].
ElectrolyteChemical NameFormulaConcentration
(g/L)		Applications
WattsNickel sulfate N i S O 4 · 6 H 2 O 200–300Corrosion and wear resistant, decorative, structural
Nickel chloride N i C l 2 · 6 H 2 O 45–150
Boric acid H 3 B O 3 30–52
Nickel sulfamateNickel sulfamate N i S O 3 N H 2 2 300–450Abrasion and corrosion resistant, structural
Nickel chloride N i C l 2 · 6 H 2 O 0–30
Boric acid H 3 B O 3 30–45
Sulfate-chlorideNickel sulfate N i S O 4 · 6 H 2 O 150–225High deposition rate and slightly higher internal stress
Nickel chloride N i C l 2 · 6 H 2 O 150–225
Boric acid H 3 B O 3 30–45
All-sulfateNickel sulfate N i S O 4 · 6 H 2 O 225–410Insoluble anode
Boric acid H 3 B O 3 30–45
Hard nickelNickel sulfate N i S O 4 · 6 H 2 O 180High hardness and tensile strength
Ammonium chloride N H 4 C l 25
Boric acid H 3 B O 3 30
Black nickelNickel ammonium sulfate N i S O 4 · N H 4 2 S O 4 · 6 H 2 O 60Plating on brass, bronze, or steel
Zinc sulfate Z n S O 4 22
Sodium thiocyanate N a C N S 15
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Thompson, G.W.; Mahtabi, M.J. Electrodeposition of Nickel onto Polymers: A Short Review of Plating Processes and Structural Properties. Appl. Sci. 2025, 15, 8500. https://doi.org/10.3390/app15158500

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Thompson GW, Mahtabi MJ. Electrodeposition of Nickel onto Polymers: A Short Review of Plating Processes and Structural Properties. Applied Sciences. 2025; 15(15):8500. https://doi.org/10.3390/app15158500

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Thompson, George W., and Mohammad J. Mahtabi. 2025. "Electrodeposition of Nickel onto Polymers: A Short Review of Plating Processes and Structural Properties" Applied Sciences 15, no. 15: 8500. https://doi.org/10.3390/app15158500

APA Style

Thompson, G. W., & Mahtabi, M. J. (2025). Electrodeposition of Nickel onto Polymers: A Short Review of Plating Processes and Structural Properties. Applied Sciences, 15(15), 8500. https://doi.org/10.3390/app15158500

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