With any plating process, deposition is the most interesting aspect.
However, pretreatment is the most important . . .

Surface finishers are often asked to plate difficult substrates. With any plating process, the most interesting subject is the chemistry and subtleties of the deposition process. Pretreatment does not seem to be so interesting. Even in the plating shop, this aspect is often given little attention. However, pretreatment is often the most important procedure. There are many instances where pretreatment seems to be 99 pct of the job and the deposit is almost an afterthought.

Most of the cleaners used in the electroplating industry are proprietary, although most suppliers have similar products.

Powdered Metal Parts. Many parts are made from powdered steel and steel alloys. The major problem with these parts is derusting and porosity. The surface porosity is partially eliminated using vacuum impregnation. In this process, organic polymers are impregnated into the pores of the part. However, some open pores always remain, resulting in bleed out of pretreatment chemistry after plating. The bleed out often appears as a circular stain, and this may not become evident for some time after plating. The best way to avoid this is to make sure that the pores are filled with clean water before corrosive materials like hydrochloric acid are used in pretreatment. A hot water rinse drives out any solution that may be in the pores.

A mild alkaline cleaner derusts the powdered metals. Some derusting cleaners attack the substrate and should be avoided. The same cleaner can be used as a soak cleaner and electrocleaner. In this case, it is always best to use two tanks, because the soak cleaner will remove most of the oils, allowing the electrocleaner to remove the last traces of soils.

This discussion applies mostly to Zamac 3 and Zamac 7 castings, but other alloys may also be plated this way. The major concern about any metal die castings is the quality of the casting. Flashing and air pockets in the casting can make it impossible to provide a quality coating. Pretreating zinc die castings almost always begins with mechanical surface finishing. In most cases, parts are tumbled in clean media, but various bead-blasting techniques also are used. Again, the quality of the electroless nickel coating will depend strongly on the quality of the part's surface after this mechanical finishing.

The surface is fairly clean after the mechanical steps, but special care must be taken to properly activate the surface. One of the best methods uses a mild phosphoric acid based cleaner. Step one uses the cleaner as provided by the supplier, and step two with the cleaner modified by a bifluoride salt.

The key to plating mechanically finished and cleaned die castings is the nickel strike. The nickel strike is prepared in two steps using an alkaline electroless nickel bath. The first step uses the electroless nickel additive containing the nickel and complexors but not the reducing agent. This step provides a semi-bright nickel immersion layer that protects the zinc part from the more aggressive electroless nickel plating process to follow. The second layer is prepared by the full alkaline electroless nickel process. This process may be a phosphorus- or a boron-type process. Often, the best results are from the boron process.

Porosity and bleed out may still be a problem with zinc die castings. Warm rinses before plating are seldom used, but may be necessary. Ultrasonics in a mild acid cleaner often is effective in removing soils from the larger pores.

An important detail is that no metals can touch the parts. Thus, plastic hooks or racks are absolutely required to fixture the work.

Beryllium.
Beryllium is an expensive and toxic material but has found some use in space applications such as mirrors, because the thermal expansion of this material is low. In most cases, parts must be heat treated after plating. This requirement will be specific, but typically involves heating at 325F for four hours, cooling at no more than 100 to 200F per hr and then air cooling to room temperature.

Aluminum Alloys.
Preparing various wrought and cast alloys of aluminum is fairly standard, (1,2,3,4,5,6) and it would be difficult to do justice to the subject in this space. However, some notes may assist in difficult situations.

There are alloys, like 7075, that respond best to an alkaline electroless nickel strike prior to electroless or electrolytic plating. The strike is operated at a low temperature (100-120F) for sufficient time to produce a complete covering of the surface. A dramatic example (7) of a project using this technique is the plating of a 23-ft mirror honored at the 1993 Electroless Nickel Conference sponsored by PRODUCTS FINISHING.

Most alloys respond well to alkaline zincate treatments, with a single application for cast alloys and a double application for wrought alloys. However, some alloys, like 413 castings, are attacked strongly by alkaline zincate. In these cases acid zinc immersion is best.

The acid zinc immersion may also be recommended for barrel work, since the thin smooth deposit is not damaged by the tumbling as much as the thicker zinc deposit from the alkaline processes. High-silicon alloys also respond best to the acid-zinc immersion process.

For this reason, it is important to use a zinc strip process that does not contain nitric acid. The best choice for the zinc strip when using the acid zinc immersion process is persulfate with an inorganic or an organic acid.

It is noted in ASTM B656 that heat treating aluminum parts 285-302F after electroless nickel plating improves adhesion. The MIL-C-26074E specification suggests that non-heat treatable aluminum alloys (class 3) should be heated at 360-390F after plating. However, heat treatable alloys (class 4) must be heated at 240-260F after plating. This point is especially important if the electroless nickel is to be heated to increase the hardness of the electroless nickel. The electroless nickel treatment is often 350C (662F), which will alter the properties of some aluminum alloys.

Magnesium.
Plating magnesium has been a subject of considerable work over the years. Sakata (8) reviewed some of the published methods and provided an alternative. Successful methods use a cyanide copper strike that covers small pores. The major problem with magnesium is that even the smallest pore will become a corrosion site in an acid medium, typical of normal acidic electroless nickels. It results in a black burn site. The situation is more critical than with zinc die castings or powdered metals. Even water in a pore will cause catastrophic corrosion is just a few days, even if a part has been plated successfully without the burn sites.

Sakata's method provides pretreatment steps, followed by the Dow zincate, followed by an alkaline nickel citrate electroless strike. Parts can be plated with an alkaline electroless nickel to build up thickness, but it is difficult to use an acidic electroless nickel without some attack on the substrate wherever there is a large pore. However, even under the best conditions, bleed out sites sometimes become apparent after a few days. As a result, plating magnesium without a cyanide copper strike remains a challenge that will require much more research.

Engineering Plastics. A variety of engineering plastics has been considered as replacements for aluminum in many applications. Some of these plastics have good high-temperature performance, and all of them are lighter and more corrosion resistant than metals. Some of these plastics include polycarbonate, poly-aryletheretherketone, and poly-etherimide resins. When an application requires conductivity or electrical shielding, the plastic parts must be metallized and electroless nickel is used to accomplish this.

The methods for plating some of these materials have been described (9,10,11,12) in other reports. The key to successfully plating these materials involves several issues: 1) The plastic must be plateable. This means that the polymer is filled with glass or other etchable materials, since the pure polymers often do not etch well. 2) Some polymers have a tenacious skin that must be removed by bead blasting or vapor honing. 3) Many of these plastics must be annealed thermally to relieve stress in the part. The temperature and time of the annealing process depend on the type of plastic and the filler. 4) The choice of pre-etch process depends of the nature of the plastic. 5) Certain conditioners before the catalyst can be used to improve adhesion of the metal deposits.

After these considerations, the catalyst, electroless nickel or copper strike, electroless copper buildup, and electroless nickel buildup processes are standard.

One notable example of an engineering application is the plating of Kapton polyimide film(12,13,14) used in flexible electronic circuits. The cycle includes an alkaline etch for polyimide and an alkaline palladium catalyst. The strike layer is electroless nickel, which is preferred to electroless copper because the copper polyimide bond degrades with time while the nickel polyimide bond does not degrade.

Perhaps the greatest challenge facing the surface finisher is preparing various surfaces for electroless nickel plating. The variety of metal and non-metal surfaces mandates that the finisher be ready with considerable knowledge of pretreatment, since even minor variations of the substrate often require significant changes in the pretreatment.