Pertinent Facts of Nickel Plating

Plating baths each provide specific benefits that, regardless of cost, have become necessary for various on demand finishes. These include: decorative, functional, wear resistance, and corrosion protection. Nickel encompasses many plating systems, each of which is very important to many aspects of metal finishing. Even though the prices of nickel anodes and related salts have been very volatile, the importance of nickel in plated finishes is still of great significance.

For industrial purposes, almost any metal can be nickel plated. The inherent properties of nickel can be combined with the unique properties of other metals. Some examples of metals commonly plated with nickel are: steel (high strength), brass (easily bent and formed), aluminum (impact extrusion), and die castings (design flexibility). Plastics, which have been treated for conductivity, are also commonly nickel plated. Nickel can be directly plated over several metals. This is typically performed directly over steel, brass, and copper. In some cycles, appropriate immersion treatments or pre plate deposits precede nickel. This is especially prevalent when plating a copper strike and copper plate over zinc parts before nickel. Aluminum, because of it’s unique electropositive nature, must first be conditioned by immersion zincating, before plating either electrolytic or electroless nickel.

The application of a suitable nickel deposit can be of significant benefit to the parts, ultimate quality of the finish, and meet or exceed specifications. These advantages can actually reduce related manufacturing costs, improve marketability, and increase production throughput. Depending on the finishing requirements or service life of parts, nickel contributes the following advantages:
Good electrical conductivity, low coefficient of thermal expansion, magnetic, and good heat conduction. Nickel can be plated as a soft deposit (not far off from copper) or almost as hard as chrome. In fact, nickel can be plated to almost any desired deposit hardness in between this wide range. Aesthetically, nickel can be plated in decorative applications to achieve a wide range of brightness and leveling, still retaining sufficient deposit ductility.

Finished parts can be assembled or mechanically formed into selected commercial products. Along with these benefits, duplex nickel forms an excellent corrosion barrier, especially in the plating of exterior automotive parts. Nickel forms an important barrier to prevent the migration of zinc on tin plated parts. In engineered finishes, nickel decreases contact, resistance and friction, improves solder ability and brazing, and improves resistance to galling and wear. Plating with nickel can salvage worn or miss-machined parts. Almost all nickel deposits can be machined. Incorporating nickel into the deposit, in place of solid metal, reduces some manufacturing costs.

As mentioned previously, nickel can be plated to provide rapid leveling, filling voids, eliminating microscopic “peaks and valleys”, while plating a relatively thin deposit. For this reason the base metal may not have to be mechanically polished, buffed, or mass finished. If this is acceptable, cost savings to prepare the surface for plating may be realized. Nickel can be plated from a variety of specific bath formulations (usually Watts and sulfamate types), to develop deposits that range from flat to dull to semi bright to bright. This affords the finisher the capability to provide nickel deposits that meet engineering requirements, corrosion protection, and aesthetic preferences. The Woods strike effectively activates stainless steel for subsequent nickel plating. Duplex nickel, as mentioned earlier, promotes exceptional corrosion protection, by plating a balanced ratio of a special semi bright and bright nickels. The Step Test is a specific quality control procedure for this application. In recent years Watts baths containing modified organic additives, have successfully replaced cyanide copper strikes over zincated aluminum. Decorative chrome, either trivalent or hexavalent, continues to be a specified finish over nickel. The chrome topcoat enhances overall appearance and maintains an excellent scratch resistant, hard finish. Although we commonly refer to the bright chrome finish, it is primarily nickel (usually bright) with a thin chrome flash. The combination of these deposits give the assembled parts a preferred pleasing appearance, along with exceptional corrosion and wear resistance. For several decades, the combination of bright nickel and flash chrome has been the best selection for plating parts subject to outdoors exposure.
Nickel can be plated as a very ductile deposit. In combination with a proper base metal conditioning and any preplate deposits, the finished items can be stamped, drawn, or formed in a variety of shapes. This is very common in the strip plating of continuous coils that will be used in the manufacture of different types of consumer and industrial goods. In this application, the organic brightener and leveling additives may be kept at lower levels, to achieve the required ductility. Final aesthetic appearance of nickel occurs in a short buffing cycle before optional chrome plating. Parts that require exceptional brightness and leveling may be stamped before the plating cycle.

Nickel can be plated to meet any thickness requirement. Industrial based coatings usually require 0.005 – 0.020 inch. For decorative purposes, nickel thickness may range from 0.0003 – 0.001 inch (or up to one mil). As indicated by the application ranges, there is a minimum that should be plated to meet the intended use of finished parts. As a guide, 1 lb of plated nickel is required for every 22 ft2 of intended parts coverage. Depending on the nickel bath and plating parameters, the deposit tensile strength can range from 50 to 220 thousand lb / inch2.

Nickel anodes, as we are aware, are quite expensive. Be certain a certificate of analysis confirms the quality. Anything less than sufficient purity material could result in severe contamination of the nickel bath. A typical assay is: Nickel (99.950%), Cobalt (0.03%), Copper (0.005%), Carbon (0.001%), Iron (0.001%), Sulfur (0.01). Anodes are provided in various shapes, including: spears, buttons, rounds (sulfur containing S and sulfur free R), and chunks.

There are many applications for plating nickel, using several types of process baths. The demand for nickel plating continues to be relatively strong. Although the prices for nickel anodes and salts have markedly risen, the consumer market for plated finishes keeps this plating service very active. Original and after market automotive finishes have “re-discovered” bright nickel / chrome finishes. The decorative plumbing industry is very positive on nickel / chrome and brushed nickel finishes. Clothing and apparel manufacturers now feature nickel finishes (ex. oxidized, brushed, under flash brass or gold). Nickel anodes and salts may not be cheap (compared to a few years ago), but decorative and industrial finishes for nickel are still in strong demand.

Filtering Cleaners: A Wise Choice

Last month’s blog described several filtration methods, along with equipment, purifying agents, and the overall benefits of filtration. The underlying message was how the subject, as related to plating baths, is so important. Most of us, through experience, can relate to the strong link between effective filtration and quality metal finishing of parts. One would be hard pressed to not see some form of filtration in a plating line. By walking the line backwards, how many cleaner tanks have you seen being filtered? Do you think it is a good idea? Is there any real benefit? From practical experience, using equipment and formulating specific cleaners, I know that filtering these baths is a good decision. Let us consider some reasons for filtering cleaners and available methods.

Is It Clean or Dirty
Obviously, the ideal condition for any cleaner is the freshly prepared working solution, just before immersion of any parts. After the initial dunk of parts, the solution becomes soiled; as should any good soak cleaner behave. As production use proceeds, the bath loads up with contaminants typically consisting of oils, grease, and fine particles. Perhaps you have noticed the tell tale signs: solution turns either a dark or tea brown, milky color, oils tend to separate and float (especially as the cleaner cools), grease rings form on the walls of the tank, sludge and particles build up on the bottom of the tank. Just how soiled does the cleaner become before it starts working against you? Why not try a couple of quick tests that might tip off trouble before it actually strikes.

• -Specific Gravity. Measure the cleaner bath specific gravity when first made up (no parts as yet immersed). On a scheduled basis, measure and record the specific gravity as the bath ages. During this time maintain the cleaner concentration at the initial make up. There will be a point at which the data coincides with a drop in quality cleaning. The specific gravity will have increased to a point that indicates how much oil, grease, and particles have built up in the bath. Corrective action may include making additions of the cleaner concentrate, cut the bath and replenish, or dump and replace with a new make up.



• -Performance Test. Immerse a clean panel (ex. Steel hull cell panel) in the cleaner bath for the same time as the parts. Rinse in cold running water for 60 seconds and examine for water breaks. Next, immerse in dilute acid (5% Hydrochloric or Sulfuric Acid) for 15 seconds, followed by rinsing in cold running water. Examine for water breaks. A positive observation of water breaks at either step would indicate deposition of soils from the cleaner bath on to the panel. Once again, corrective action may include making additions of the cleaner concentrate, cut the bath and replenish, or dump and replace with a new make up.



• -Oil Displacement. As the bath ages, the concentration of emulsified oily soils becomes more concentrated. Take a 50 milliliter sample of the hot cleaner and using care, slowly add to it 50 milliliters of 10% sulfuric acid. Mix the solution well for about 15 minutes. Pour the solution into a clean 100-milliliter graduate cylinder; adjust volume, if necessary, with water to 100 milliliters. Observe as the oils separate. Record the volume and multiply by 2 to obtain the %-displaced oils. As with the two previous tests, corrective action may include making additions of the cleaner concentrate, cut the bath and replenish, or dump and replace with a new make up.

These control examples confirm or predict at what point the cleaner, even with proper maintenance additions, and correct operating temperature, will approach its maximum service life. In most instances, adding more cleaner concentrate may restore quality cleaning, but perhaps only for a short time. We have only considered how to determine to what extent the cleaner is contaminated, with the same type of corrective action alternatives. In no instance has any consideration been given to removing the contaminants or minimizing their buildup. Can this be done? Yes, quite effectively. By filtering the cleaner, the following realistic benefits are readily obtained:

• -Extend cleaner bath service life. Less down time means longer periods of uninterrupted productivity.



• -Less bath dumps reduce the workload in waste treatment.



• -Minimizing contaminants in the cleaner helps to maintain the solution consistency closer to the new make up.



• -Quality cleaning results in satisfactory surface preparation, leading to quality finishing and post treatments.

The cleaner can be filtered using some different options.

Cartridge Filter. These are enclosed canister types that have a polypropylene center around which similar fiber material is tightly wound. The porosity of the filter medium can range from 100 microns to below 5 microns, based on the specific filtering requirement. Particles are retained in the media pattern. The polypro material absorbs oily solutions. The cleaner is continually pumped through the filter cartridge. It is a relatively simple, yet effective system to remove the typical contaminants found in the cleaner. The unit does not take up much floor space. When spent, the supplier can dispose of cartridges sometimes directly to a certified destruct facility.

Oil Absorbing Filter. This unit consists of an enclosed housing that contains polypropylene baskets containing special oil absorbing plastic type media. The cleaner is pumped through the enclosed system, where the media absorbs oils and grease. The saturated media is replaced as needed.

Bag and Indexing Fabric Filters. The cleaner is pumped through a large filter chamber, where oil, grease, and particles are retained. Takes up large floor space. It is a decent filtration system, but not applicable to systems cleaning large volumes of very oily parts.

Ultrafiltration. This is an interesting technology, using a somewhat permeable membrane system. The soiled cleaner is pumped through the (ceramic) membrane tubes. Molecules of sizes larger than water are blocked from passing through, diverted to a discharge. The aqueous cleaner solution passes through and returns to the process tank. Ultrafiltration provides a rapid, very dramatic filtering action. Of the examples given, ultrafiltration is by far the more expensive (approx. $20K and up). Considering a flexible or mobile unit that can be used to treat several cleaner tanks can offset the application, or rental as required.

Filtration can be supplemented by the application of mechanical oil removal devises. These units are quite cost effective and can be used in-tank. An overflow weir or side tank can collect cleaner solution, which cools down about 10-20 degF below the temperature of the cleaner, while oils separate to the top. The oils can be skimmed off using a disk or belt. A coalescer is another oil removing device. It channels the flow of cleaner, separating the aqueous from oily solution.

A final consideration to assist in the filtering of cleaners would be to consider the type of cleaners to be used. Displacement cleaners remove and release the oils for quick removal by the filter or separator. Another type of cleaner is what I refer to as the “mini emulsion”. Oils are kept emulsified as long as the cleaner is agitated (such as barrel soak cleaning). When the solution settles in dead zones or in a side tank, oils are released for suitable removal. Another choice is the emulsion cleaner, that releases significant quantities of oils by simple cooling (ex. from 160 degF down to 120 degF). A chemical additive can also be used for certain cleaner formulations. The mixture of agents selectively emulsifies the oils in favor of the heated cleaner, splitting out in mass with the oils.

Filtering cleaners offers the metal finisher several benefits. They include: quality, economics, productivity, compliance, and safety. The available filtration equipment provides the degree of treatment or sophistication that is preferred. Cleaning is the first step; in fact the most important step in a finishing cycle. By effectively filtering the cleaner, buildup of contaminating soils is kept at a minimum or controlled. Subsequently rinses and process tanks down the line are kept relatively free of contaminated cleaner solution drag in.

Filtering cleaners. Make the wise choice, your choice.