Nickel Plating

The improved performance of multilayer nickel coatings is due to the combination of layers of nickel having different electrochemical reactivities. If one measures the corrosion potentials of various nickel deposits in the same electrolyte, one finds that the bright nickel Deposits display more active dissolution potentials than do the semi-bright nickels. If bright and semi-bright nickel deposits (for example, in the form of foils separated from the substrate) are electrically connected in the electrolyte, electrons will flow from the bright nickel to the semi -bright nickel. The result is that the rate of corrosion of the bright nickel is increased, whereas the rate of corrosion of the semi -bright nickel is decreased. In a composite coating consisting of bright nickel over semi -bright nickel, this is manifested by enhanced lateral corrosion of the bright nickel layer and delayed penetration of the semibright nickel layer. The extent to which bright nickel protects the underlying semibright nickel layer by sacrificial action is dependent on the difference between the corrosion potentials of the semi – bright and bright nickel. The difference should be at least 100 millivolts (as measured by the STEP Test described below), and there is evidence that larger differen ces in potential are beneficial, especially in low current density areas of complicated parts. If the difference becomes too great, appearance suffers because of the accelerated corrosion of the bright nickel layer; that is, there is an optimum value that represents a compromise between preventing basis metal attack and controlling superficial corrosion. The result is that penetration of the coating and exposure of the underlying substrate occur slowly. Multilayer nickel coatings are, thus, more protective than single – layer bright nickel coatings of equal thickness.

Two coating weights are commonly used to meet the automobile industry’s requirements :
Grade A : coating weight > 24 g/m², average thickness 5 to 7 µm
Grade B : coating weight > 36 g/m², average thickness 8 to 10 µm
Greater coating weights may be applied to increase the duration of the corrosion protection

Although discussion of alloy plating is beyond the scope of this article, nickel alloy plating processes of commercial importance include nickel-iron (without brighteners), nickel-cobalt, nickel -palladium and tin-nickel. An alloy plating process that is growing in importance is zinc-nickel containing 8 to 12 per cent nickel. In addition, the incorporation of inert particles within a nickel matrix is possible and coatings that incorporate silicon carbide, diamonds, mica, PTFE and other materials are being applied for engineering purposes

The pH of the nickel plating solution will rise during normal operation of the bath necessitating regular additions of acid to maintain the pH within the prescribed limits. (A decrease in pH accompanied by a decrease in nickel ion concentration indicates the process is not functioning properly.) The operating temperature may have a significant effect on the properties of the deposits and should be maintained within specified limits (plus or minus 20 C) of the recommended value. In general, most commercial nickel plating baths are operated between 38 to 60 0 C (100 to 140 0F).The nickel plating process should be operated at specified current densities by estimating the surface area of the parts and calculating the total current required . The practice of operating theprocess at a fixed voltage is not commended. Controlling cathode current density is essential for accurately predicting average nickel thickness, for achieving uniform coating thickness on complicated shapes, and for producing deposits with consistent and predictable properties. Since current density determines the rate of deposition, it must be as uniform as possible to achieve uniformly thick nickel deposits. The nickel plating solution has an electrical resistance and almost all components to be plated have prominent surfaces that are nearer the anode than recessed areas. The current density is greater at the prominences because the anode-to -cathode distance is shorter and therefore has less electrical resistance. The apportioning of the current in this way is called “current distribution”. This means that the recessed areas receive a thinner nickel deposit than the prominent ones. Current distribution is controlled by proper rack design and proper placement of components on those racks; by the use of nonconducting shields and baffles; and by the use of auxiliary anodes, when necessary. With care, relatively good thickness distribution can be achieved. The quality of the water used in making up the bath and in replacing water lost by evaporation is important. Demineralized water should be used, especially if the local tap water has a high calcium content (greater than 200 ppm). Filtering the water before it is added to the plating tank is a useful precaution to eliminate particles that can cause rough deposits

Electroless nickel plating intended for engineering purposes is used for wear resistance, abrasion resistance and such corrosion protection of parts as the specified thickness of the nickel plating provides. Heavy deposits of the electroless nickel plating may be used for build up of worn or undersized parts, or for salvage purposes, and to provide protection against corrosive chemical environments.

Watts nickel is an electrolytic system that, as a component of our SuperBright ™ process, provides very bright, decorative finishes. Watts nickel also provides corrosion resistance according to thickness, good abrasion resistance, and a low coefficient of thermal expansion. Watts nickel has a relatively low tensile strength and hardness and relatively high internal stress and therefor is not recommended in engineering applications where part deformation or flexure may occur.

(Medium & High Phosphorus Content)

Professional Plating provides both mid-phos and high-phos electroless nickel plating (EN). Electroless systems are applied to the surface of base metals by a chemical reaction mechanism that does not require electrical current. The benefit of this process is that electrical flux density fields do not govern metal deposition thickness. Flux fields tend to be non-uniform and therefore can result in non-uniform thickness of the electrodeposits. Electroless systems have superior hole filling and leveling characteristics, which make them ideal as a first layer for almost all other electroplated, finishes. Depending of thickness and phosphorous content, electroless nickel plating is an excellent corrosion inhibitor. High-phos electroless nickel plating system specifications were developed by the NATO as a result of their experience with aggressive corrosion conditions. Electroless nickel plating systems have hardness and internal stress levels at the mid-range of all nickel systems and therefore should be specified carefully when subsequent part deformation or flexure will occur.

Sulfamate nickel provides the lowest hardness, lowest internal stress and highest ductility of all the nickel plating systems. The finish is dull; this is an engineering finish and not a decorative finish. Sulfamate nickel has excellent solderability good corrosion resistance. The high ductility of sulfamate nickel makes this product an excellent candidate for applications where part flexure or deformation, such as crimping, will occur.

Matte Sulfamate Nickel plating produces a 99.9% pure deposit without any organic brighteners or levelers. The high purity of the deposit affords temperature resistance up to the 1400°C+ melting point of pure nickel. These deposits have a full matte appearance with a slight yellow or golden cast not known for decorative appeal. They also have extremely low deposit stress that can be in the compressive range. The low stress of sulfamate nickel plating yields nickel deposits with ductility, elongation and machinability far superior to other nickel deposits.
Matte Sulfamate Nickel plating excels in joining applications including brazing, soldering, over molding, epoxy bonding, and welding. The high purity and lack of co-deposited organics improves the wetting of the nickel deposit when soldering and makes sulfamate nickel plating the preferred underplate and diffusion barrier when tin plating, silver plating or gold plating for solderability or brazing. In addition, the unleveled matte surface makes sulfamate nickel plating the choice for adhesion in over-molding or epoxy bonding applications.
Semi-Bright Sulfamate Nickel plate is used in places where a bright surface gives a decorative appearance. Semi-Bright Sulfamate is primarily used in areas where handling would discolor Matte Sulfamate nickel. Semi-Bright sulfamate nickel has excellent corrosion resistance and slightly lower solderability than Matte Sulfamate nickel. However, it is harder and has excellent corrosion resistance.
Coastline Metal Finishing has extensive experience in Sulfamate Nickel Plating in uses extending from shuttle turbo-pumps to photo machined sheet stock.
Bright Nickel plating is often used as an undercoating to improve reflectivity and leveling to smooth out surface defects. Bright Nickel plate also provides an excellent barrier coating to reduce porosity and improve corrosion resistance. If an article is to be flexed, bent or distorted in the application, Sulfamate Nickel should be considered.

Nickel Plating has the ability to offer outstanding corrosion protection, especially when subjected to high temperatures. It is used extensively in automotive under bonnet applications where its temperature and corrosion resistance are vital.