A Comparison of Electroless and Electrolytic Nickel
Gold and silver are truly precious metals and gain the attention of investors around the globe. But if profits were your main objective, you could not have done better than an investment in the nickel metal market a few years ago. After increasing more than 500% over the past three years, nickel has since settled in below $15/lb but is not expected to drop below $10/lb.
This new baseline for nickel metal pricing has resulted in surcharges and other cost increases for platers. Many were slow to pass on the increases, even though it had a significant impact on costs for plated components. The impact of increased nickel prices was especially hard on electroplaters. Electroless nickel is an alloy, so the nickel metal price increase and overall process cost is slightly disconnected.
Electroless nickel is a well-accepted plating process in some application areas, but it has been perceived as too expensive to compete with its electroplating counterparts. As a result it has as been relegated to market segments that focus only on its uniformity, corrosion, wear resistance and other unique properties.
But historically high nickel prices have the plating industry exploring additional opportunities for EN. The process’s higher material costs quickly begin to fade when you consider increased throughput, less nickel metal waste from over-plating and reduced nickel metal dragout. Other factors to consider are efficiency of electrical consumption for process heating versus power supply, waste treatment, emissions and ease of racking or bulk processing.
Processes and Properties
Table 1 compares the processes and deposit properties of EN and electroplated nickel. A quick glance at the table reveals that EN far surpasses Watts nickel in terms of deposit properties. Hardness, wear resistance, stress and corrosion protection all weigh heavily in EN’s favor. Electroplated Watts nickel offers process benefits that include high plating speed and long solution life. Watts nickel also offers better deposit leveling and brightness, although the new lead- and cadmium-free EN processes are not far behind.
For many applications, thinner EN deposits can provide better corrosion protection than electroplated Watts nickel. Steel plated with a high-phosphorus EN deposit has served as an alternative to stainless steel, and EN has enhanced wear resistance due to its higher as-plated hardness. EN deposits, whether bright or semi-bright, are suitable for solder applications. Sulfamate nickel electroplate also has good solder properties, but Watts nickel generally does not due to co-deposited organics and poor thickness distribution.
Many current EN systems are formulated to meet RoHS and ELV requirements. These new chemistries have resulted in processes that have improved microstructure and can provide better leveling than older systems. This new generation of EN also has enhanced brightness over more solution turnovers than the older chemistries.
So modern EN systems compare favorably with nickel electroplate in terms of engineering properties and processing. As pointed out above, the impact of nickel metal pricing cannot be ignored, but other factors must also be considered when comparing overall process costs. These include:
- Material costs
- Energy costs
- Labor costs/throughput
- Process considerations
- Environmental considerations.
|Table 1 Comparison of EN and electroplated nickel processes and deposits|
|Deposit Property or Process Parameter
|Appearance||Very bright||Semi-bright||Bright/very bright|
|Corrosion Resistance (ASTM B 117)*||24||1000||100|
|Taber Wear Index
|Taber Wear Index
|Intrinsic Stress, psi||15,000–70,000||-3,500–2,000||1,000–10,000|
|Nickel Metal Bath||75 g/L||6 g/L||6 g/L|
|Plating Speed (mils/hr)||1–2||0.3–0.5||0.6–1.0|
|Plating Temperature, °C||65||88||88|
|Amperage Required||10–100 asf||None||None|
|*1 mil thick deposit on polished Q panel with waxed edges. Hours to failure.|
Material costs have historically been the anchor that has kept EN at bay. However, rising metal costs have narrowed the gap between EN and electroplating considerably: at historical nickel prices (~$4/lb), EN was 4.5× more expensive than electroplated nickel. At current pricing (~$14/lb), EN is now only 1.6× more costly from a material standpoint. These costs are based on nickel metal only and do not include the cost of brighteners and other additives used in bright nickel plating, which would only further narrow the cost differential.
These data also assume an application will require the same deposit thickness for EN as it does for electroplated nickel. Most studies have found that much less EN is required to deliver the same result attained with the electroplated coating.
A quick overview of energy consumption reveals some interesting facts. Because EN operates in the range of 185–190°F, a typical 200-ga EN bath requires roughly 40kW/hr to heat and maintain the solution. On the other hand, to maintain a 200-gal Watts nickel bath in the range of 135–150°F requires approximately 15 kW/hr.
Of course, the electrolytic process uses additional power for plating. Assuming use of a 100-A rectifier set at 8 V, the electroplating process will consume an additional 48 kW/hr—a total of ~60kW/hr for Watts nickel. Although EN consumes more power for heating, overall power consumption may be similar.
The primary drawback of electrolytic nickel plating is that either large or small parts will suffer from current density effects unavoidable in the process. Large sheet metal parts will need extensive racking/fixturing to obtain any reasonable thickness uniformity. Current density must be kept low to avoid burning or extreme build up on edges. If extensive or complex racking is used, the fixtures will also be plated, resulting in wasted metal. It’s not uncommon to fill barrels to less than half capacity to get acceptable plating, and racking must be done with caution. The nickel plating on these racks or fixtures will be need to be removed or stripped prior to re-use. Recovery of this metal has not proved to be cost-effective.
Barrel work requiring either low-current-density processing or extensive plating media can be used. This media is discarded, re-used or stripped. All the media plated results in metal and power costs that can’t be recouped. This also results in increased process times with increased power consumption.
For EN, all that is needed is a method to hold the part and expose it to fresh solution while avoiding air pockets and other potential problems. There are no limitations on rack or barrel loading, because, in certain cases, EN formulations can be modified to handle work loads in excess of 3 sq ft/gal. Most racking involves use of J-hooks, bolts/cables or wires. When racks are used some EN chemistries can even eliminate or reduce the plating on the rack.
Nesting is always an issue with any type of plating. Electroless plating results in minimal nesting, and even flat parts can be either basket or barrel plated. In many cases, the hydrogen gas evolved during the process can assist in avoiding nesting.
Blind or through holes are virtually impossible to plate efficiently in electrolytic processing. This has always been a strong point for EN. In addition to eliminating coverage issues in these recessed areas, the potential for galvanic corrosion is eliminated.
There are many different EN chemistries to choose from, and the selection process depends upon the desired deposit properties. Most commercial systems operate in a similar manner, but if careful attention is paid to control and operation most EN processes are reliable and relatively simple to operate by reasonably trained personnel.
Many platers believe electrolytic nickel systems are basic and static and do not require significant chemistry modifications for different types of processing. This is not the case. Additives and/or contaminants can have a profound effect on performance, as will operating conditions. Certain deposit properties, most notably hardness and corrosion protection, can be only slightly improved.
High-speed systems often require chemical concentrations which will not allow for barrel plating. The brightener in these systems may not work for normal rack or barrel plating, and the opposite also holds true: Barrel systems operate at different metal contents and contain additives which can negatively impact plating of large parts or high-speed applications. Deposit lamination, cloudiness and poor coverage are defects that commonly can result from a solution which has not been optimized for the specific type of part or processing.
Electrolytic plating tanks are designed for the work to be processed, and as such are limited by rectification, anode/cathode ratio and racking. The primary purpose of an EN tank is to provide a large enough vessel to ensure solution uniformity, provide thorough filtration and meet production requirements. Tanks can range from electropolished stainless steel with sub-micron filtration to 55-gal drums with electric heaters and submersible filters.
Most EN systems operate between 180–190°F, offering a natural evaporator system. Spray rinsing over EN helps keep chelated nickel from the waste stream while minimizing staining. The EN process operates at a comparatively low metal content, which facilitates recapture of the nickel and reduces overall waste generation. Typical electroless nickel systems operate at less than 1 oz/gal nickel metal—less than 10% of a standard Watts bath.
One only needs to see the post-plate rinses in a bright nickel barrel line to get a sense of the enormous dragout loss. If metal is not recovered, precipitation methods are commonly used to remove nickel from solution, and a significant volume of sludge can be generated.
When operated properly and subjected to periodic carbon and dummy treatments, electrolytic nickel baths will last many years. A major drawback of EN plating is the inherent buildup of reaction byproducts that can lead to relatively “short” bath life.
There are numerous methods to treat EN waste internally, and many EN platers do this. There are also options to have spent solutions hauled away for landfill and/or reclamation. All things considered, it is commonly held that waste treatment adds less than 10% to the total cost of an EN process. Factoring this into our earlier cost calculations will have only a marginal effect.
Also, there are now systems available that can regenerate the EN solution and allow its use for extended periods of time. There are numerous facilities running these systems in production with well over 200 metal turnovers and no compromise of the deposit or process characteristics.