18 Electrodeposition and Electroless deposition Technique

S.S. islam

1.   Electrodeposition Technique

Electrodeposition technique is a very old process which involves coating a thin layer of one metal on the top of a different metal, to alter its surface properties, by donating electrons to the ions in a solution. This bottom-up approach is versatile and can be applied to a wide variety of potential applications. Electrodeposition has gained much interest in recent past owing to its capability to fabricate one-dimensional structures such as nanorods, nanowires, and nanotubes.

Electroplating is another technique analogous to the electrodeposition. Apart from producing metal coatings, this technique is also used in electrometallurgy wherein a metal is extracted from its ore. Its another use includes formation of molds to create objects in their final shape, this process is called electroformation. Most of the metal depositions produced by this technique are crystalline, and for this reason, this technique is also termed as electrocrystallization. This term was first used by V. Kistiakovski, a Russian chemist, in the early 20th century.

Owing to the presence of positive as well as negatively charged ions, electrolyte conducts ions. The electrolyte is prepared by dissolving the salt of the desired metal, to be deposited, in a suitable solvent (most commonly in water). The solvent can also be an organic or ionic liquid for specific electroplating process. For electrodeposition, the working electrode or the cathode and the anode (or the counter electrode), are immersed into the electrolyte contained in a vessel or cell. To flow electric current in the circuit, a battery is usually connected to the electrodes such that cathode is connected to the negative terminal and the anode is connected to the positive terminal of the battery.

It is usually utilized to deposit a coating, mostly metallic, onto a surface through reduction at cathode. Substrate to be coated works as cathode immersed in the solution consisting salt of the metal required to deposit. The metal ions get attracted towards cathode where they are reduced to the metallic form. Electrodeposition can be considered as a type of conventional thin film deposition technique called electrolytic deposition. An electric current is used to reduce the cations of a selective material from an electrolyte and deposit on a conductive surface. It is often performed to make selective electrical and corrosion resistance of a material or to enhance heat tolerance, decrease wear and friction. Figure 1 shows the schematics of the electro-deposition process for nickel chloride solution.

Figure 1 Schematics of (a) electro-deposition setup, and (b) electrical double layer formed during the process.

The electrolyte solution possesses the positively charged nickel ions (or cations) along with the negatively charged chloride ions (or the anions). When an external field is applied to system, cations migrate to the cathode and deposit as metallic nickel after reduction.

Ni²⁺ + 2e^– ↔ Ni (metal)

Simultaneously, to maintain the neutrality of electrolyte, Ni from anode gets dissolved in the solution.

Ni ↔ Ni²⁺ + 2e^–

This phenomenon is termed as electrolysis. Usually, Pt mesh acts as anode. Oxidation of water occurs at the anode:

2H₂O ↔ 4H⁺ + O2 + 4e^–

The quantity of chloride ions is unchanged during electrolysis but the content of Ni2+ ions will decrease while concentration of H+ ions increases with time.

Due to the effect of applied voltage, reorganization of ions takes place near the surface of electrode and it leads to deposition of an Helmholtz double layer (an electrical double layer), succeeded by the creation of diffusion layer (see Figure 1 (b)). This process can be summarized as:

a.       Migration: Hydrated metal ions move toward cathode due to the impact of current as well as by diffusion and convection.

b.       Electron Transfer: At the surface of cathode, hydrated metal ions enter the diffused double layer where water molecules of hydrated ions are aligned. Afterwards, metal ions reach Helmholtz double layer wherein their hydrate envelope is removed.

c.       The dehydrated ions are adsorbed at the surface of the cathode after neutralization.

d.       The absorbed atoms then migrate or diffuse to the nucleation point on cathode surface.

Thickness of electroplated layer at substrate can be controlled by plating time duration. By some modification in electrode and electrolyte solution, this method can be successfully used for fabrication of nanomaterials, nanorods and poros structures.

Factors that influence the electrodeposition process are:

–          Current density

–          The nature of the anions and cations present in the solution

–          Composition as well as the temperature of the bath

–          Concentration of the solution

–          Waveform of the supplied current

–          The presence of impurities

–          Physical as well as chemical nature of the surface of the substrate

Applications

Electrodeposition is widely used in:

• Decorations: Where an expensive metal is coated over the surface of a base metal to enhance its appearance. Typical applications include jewelry, furniture fittings, tableware, etc.
• Protection: Corrosion resistance coatings including chromium plating of automobile parts as well as appliances for domestic use, nuts, screws, as well as electrical components
• Electroforming: Producing molds, sieves, screens, dry shaver heads, record stamps, dies, etc.
• Enhancement: Improving the electrical and thermal conductivities, solderability, reflectivity, printed circuitry and electrical contacts, production of micro parts for MEMS, etc.

2.   Electroless Deposition Method

It is the deposition of solid continuous coatings (or films), powder of metals, alloy, or compound from an aqueous or non aqueous solution or melt without any external bias. Thus, electroless deposition is purely a chemical process, involving oxido-reduction reactions. Coatings produced by this process are uniform and continuous, making it an attractive technique for various purposes including – electronics, energy conversion, wear-resistant coatings, biomedical, aerospace as well as automotive industries. This technique can be used for deposition of metals, alloys, salts, oxides, polymers, etc. While metallic electrodeposition including reducing the metallic ions has always been cathodic reaction, deposition of the metal-oxides is an anodic process due to oxidation of metals. Therefore, in electroless deposition of the metals, reducing agents are required and for depositing compounds, oxidizing agents are necessary.

In brief, charge transfer mechanism is at the heart of the technique. Metal ions are reduced to metal chosen to deposit by the chemical reducing agents. The coating material is provided by metal salts instead of anode; refilling is done by addition of either salt or external loop to the anode of a metal which has better efficiency than cathode. For reducing the metal, a substrate is used, which works as cathode and a reducing agent provides the electrons for completing the process. In this technique, experimental process takes place only on catalytic surface and if reaction conditions are not correct then it can cause the reaction in whole solution that may provide undesirable results.

Figure 2 shows the deposition process in electroless plating along with the half reactions of the cell for a metal having ‘z’ valency. The metal ions are reduced by the electrons provided by the reducing agent. The produced metal atoms are deposited on the substrate. The most important characteristic feature of electroless plating is that it can also be used to metalize (or deposit metal) semiconductors as well as insulators,e.g., plastics, glasses, and ceramics.

Figure 2 Mechanism of electroless deposition involving a metal with valency ‘z’.

In electroless or autocatalytic plating, the metal ions are reduced to form metals in the presence of specific catalysts. Thus, selected surfaces can be coated with the desired metal layer by depositing catalysts. The electroless plating bath for nickel consists of:

–          A nickel source, i.e., nickel sulfate (NiSO4[H2O]6) or nickel chloride (NiCl2.6H2O)

–          Reducing agent such as sodium hypophosphite (NaH2PO2.H2O)

–          Buffer such as triammonium citrate [(NH4)3C6H5O7)]

In addition to these components, ammonium hydroxide (NH4OH) is added in small quantities to increase the bath pH. pH determines the bath stability and uniform deposition rates. Typically, pH is kept somewhere between 8 and 10.

Electroless plating from nickel sulfate bath undergoes the following reaction:

NiSO₄ + NaH₂PO₂ + H₂O → Ni Plating + NaHPO₃ + H₂SO₄

Electroless plating is a versatile technique to produce a broad range of coatings. Most popular industrial coatings include:

Low Phosphorous (Hard)

This bath is used to deposit coatings having hardness around 60 Rockwell. The additional advantage of this technique is that it provides a uniform all round coating of complex features. Owing to high uniformity of the deposition, there is no need for post-treatments such as grinding. Electroless low phosphorous nickel plating provides superior corrosion resistance towards alkaline conditions.

Medium Phosphorous (Bright High Speed)

Medium Phosphorous electroless nickel plated There is no build up of electroless nickel on steel parts give performance similar to stainless steel. edges or ends, and it covers both inside and outside uniformly. The hardness can be increased from 45 to 68 Rockwell C by applying some sort of heat treatment.

High Phosphorous

High phosphorous coatings provide maximum resistance towards corrosion and it is industry standard where strong acid conditions are frequent (e.g., oil drilling, coal mining, etc.). However, these coatings suffer from poor solderability, as they are only solderable for a brief time. Nonetheless, this property makes them particularly desirable for electronics parts including connector housings and semiconductor packaging.

Electroless Nickel/Teflon Composite

This is the recent application area of electroless nickel plating technique. Teflon coating yields a very low interfacial friction. In this technique, microscopic beads of Teflon are codeposited (upto 20%) along with the electroless nickel. This coating is highly desirable for solving problems like sticking, galling or dragging with moving parts, or heated seal surfaces. This can be a better alternative to liquid lubricants in some applications.

Electroless Nickel on Zinc Die Cast

In applications which require corrosion resistance along with resistance to chipping and flaking, electroless nickel deposition on zinc die cast is highly desirable as it circumvents the need for copper layer.

Advantages of Electroless Deposition

• If the motive is only to improve corrosion resistance, rather than the beautification, as applicable to jewelry industry, electroless plating is generally preferred because it results in more hard and less porous plated parts. Therefore, electroless plated components are more resistant to corrosion.
• Thus, electroless plating is more popular in industries where parts are vulnerable to wear and corrosion, such as oil fields or marine applications.
• Electroless plating is also preferred on parts like pumps and valves, which are frequently subjected to corrosive agents during their operating life.

    3.   Comparison between Electrodeposition and Electroless Deposition Techniques

The comparative schematic of both electro- and electroless- deposition techniques has been included in Figure 3. The fundamental distinction between the two methods is the use, or not, of electricity. Electroplating makes use of a power source like a battery or rectifier, which delivers electricity to a component in a chemical solution, altering the chemical composition and depositing a hard, durable metal coating onto the surface of the component.

Both the techniques make a component tougher and more corrosion-resistant. Both techniques can also make a part more attractive, although this is usually only an issue when plating jewelry or other parts that consumers are likely to view directly, as opposed to parts that you are more likely to situate within a larger machine or use on a factory floor.

Figure 3 Comparative schematics of (a) electro-deposition, and (b) electroless deposition/plating techniques.

Electroplating is a more complex process than electroless plating, requiring clean conditions, using potentially hazardous equipment and in some cases needing multiple applications to get the desired plating thickness.

Electroless plating is a much less complex process. In electroless plating, we start by cleaning the component with chemical cleansers that remove oils and other corrosive elements, then dip it in the aqueous solution and add anti-oxidation chemicals. The result is a plated component that is highly resistant to friction and corrosion. Also, with electroless nickel plating, there is no complex filtration or other equipment required, and since there is no electricity, there is also no danger of electricity-related accidents.

Electroless deposition also results in highly uniform metal deposits having consistent thickness all around the part. Therefore, parts with complex shapes where uniform plating might be difficult to achieve using conventional electroplating methods, electroless plating may be a much better alternative.

Applications More Suited for Electroless Deposition

Electroless deposition finds applications in industries where parts have complex shapes or may be subjected to heavy corrosive factors are prime candidates for electroless plating. These industries may include:

–          Food Service Industry: For example, molds and food processing machine parts.

–          Oil & Gas Industry: Anything from valves, balls and plugs to barrels, pipe fittings and more.

–          Automotive Industry: Vital car parts like shock absorbers, cylinders, brake pistons and gears.

–          Aerospace Industry: Valves, pistons, pumps, rocket components.

–          Chemical Industry: Pumps, mixing blades, heat exchangers, filter units, etc.

–          Plastics and Textiles: Molds, dies, machine parts, spinnerets, extruders and so on.

References

1. Nur, U.S., et al., ELECTRODEPOSITION: PRINCIPLES, APPLICATIONS AND METHODS, Industrial Technology Division, Malaysian Nuclear Agency, Selangor.
2. Al-Bat’hi, S.A.M. “Electrodeposition of Nanostructure Materials.” Electroplating of Nanostructures. InTech, 2015.
3. http://www.sharrettsplating.com/plating-methods/electroless-plating.
4. https://www.sinotech.com/resources/secondary-metal-processes/electroless-plating/.
5. ur Rehman, Atteq, and Soo Hong Lee. “Crystalline silicon solar cells with nickel/copper contacts.” Solar Cells-New Approaches and Reviews. InTech, 2015.
6. http://corrosion-doctors.org/MetalCoatings/Electroless.htm.

    Review Questions

1. Discuss the various types of electrodeposition technique.
2. What type of material can be used for the bath used in nickel plating?
3. How does temperature of the bath affect the electroplating process?
4. How to prepare nickel bath solution for electroless deposition?