1. No compelling need had developed for the advantages offered by the alloy deposits.
2. The processes were difficult to operate consistently.
3. The cost of operating the alloy baths was too high.
4. There was no general consensus as to which alloy was the most suited and cost effective for any particular application.

These factors were overcome by two developments in the automotive and electronics industries. The first was the push for an environmentally acceptable method to obtain increased corrosion protection. The second was the pursuit of an acceptable replacement for cadmium deposits. The zinc alloy baths available today are capable of satisfying both of these needs. The deposits from zinc alloy baths are capable of not only providing enhanced corrosion protection but also increased lubricity, ductility and hardness.

Commercially Available Zinc Alloy Processes
There are a number of commercially available zinc alloy processes, including zinc/nickel (Zn/Ni), zinc/nickel/iron (Zn/Ni/Fe) zinc/cobalt (Zn/Co), zinc/cobalt/iron (Zn/Co/Fe), zinc/iron (Zn/Fe), tin/zinc (Sn/Zn) and zinc/manganese (Zn/Mn). Of these processes, the nickel, cobalt and iron alloys are the only ones of any real commercial concern. The plating processes for the iron, cobalt and nickel alloys are operated and have additive systems similar to their non-alloy counterparts (see Table I). The zinc/manganese alloy is only of interest where galvanic compatibility with magnesium is required. The tin/zinc alloy deposit usually contains a maximum of about 30% zinc, has properties closer to those of tin and is deposited from baths with no real similarity to normal zinc plating baths or those used for other zinc alloys.

Zinc/Nickel. The operating parameters for the alkaline and chloride Zn/Ni systems are shown in Table II. In general, the zinc/nickel alloy, whether alkaline or chloride, is by far the most expensive to operate of any of the alloys discussed here. But, it does have some advantages that the other alloys don’t have. It provides the best corrosion protection, maintains a majority of its corrosion protection even at elevated temperatures and provides greater wear resistance.

The alkaline plating process gives good plate distribution but has very low efficiency and the complexers that are used can adversely affect waste treatment. It is because of these complexers that a zinc/nickel/iron trialloy is usually plated. This codeposition of iron seems to have no affect on the performance of the deposit. The zinc and nickel are replenished using either zinc anodes or a galvanic zinc generator and nickel salts. If a generator is used, the anodes are nickel, but nickel salts are still required.

Chloride plating systems, unlike the alkaline ones, will easily plate hardened and cast metal parts and operate at almost 100% efficiency. However, the alloy and plate distribution of chloride systems is not as uniform as those of alkaline systems. In most chloride baths, the zinc and nickel are replenished using nickel salts and zinc anodes. An alternate, though less popular, approach is to use zinc and nickel anodes on separate rectifiers.

Zinc/Iron. Unlike the cobalt and nickel alloys, the iron alloy can only be produced using an alkaline non-cyanide process (see Table III). But, it is the most economical and easiest of the zinc alloy systems to operate. The deposit has very good corrosion resistance, ductility and weldability. The iron alloy has only one real negative— it loses a fair amount of its enhanced corrosion protection when exposed to elevated temperatures. It is for this reason that it is not recommended for applications where it will experience temperatures greater than 200F.

Zinc/Cobalt. Alkaline systems for plating zinc/cobalt alloys are easy and economical to operate and produce a deposit with exceptional alloy and thickness uniformity (see Table IV). Like the alkaline nickel alloy process it is not unusual to actually plate a trialloy of zinc/cobalt/iron because of the presence of complexers. The alkaline plating process is preferred but the chloride system can be used where it is necessary to plate hardened or cast metal parts.

Choosing the Right Zinc Alloy Process
There are only two choices to be made. The first is the type of alloy that is required. The second, in the case of nickel and cobalt alloys, is whether to use an alkaline or chloride plating bath. The first choice is easy, since this is usually spelled out in the customer’s specifications. The choice of bath types can be a little more involved. There are several considerations that need to be taken into account:

• Substrate—Some substrates, such as cast iron, hardened or carbonitrided parts, require chloride type baths to plate correctly.
• Equipment—The chloride baths necessitate the use of corrosion-resistant equipment. All of the alkaline alloy baths contain complexers of one kind or another that can also be corrosive to equipment. Depending on the alloy bath and the complexers used, it may be necessary to use more corrosion resistant equipment than would be required for a normal alkaline zinc plating bath.
• Waste Treatment—In the case of nickel alloys, it will be necessary to determine what if any modifications will be necessary to handle the nickel in the effluent. Since the alkaline baths contain complexers, the effect of these materials on the ability to remove metals from the waste stream will need to be addressed as well.

Because of the alloy content of the deposit, special chromate formulations are required for each of the different alloy deposits. Without a chromate conversion coating the corrosion characteristics of the iron and cobalt alloys are not significantly different than those of pure zinc. In the case of nickel alloys, with no chromate, the onset of white salt corrosion is about the same as for pure zinc but the progression of the corrosion is slowed depending on the nickel content.

The only passivation process that is of current commercial interest for the iron alloy is a non-silver black chromate. A yellow chromate is available but is not in much demand. There presently is no clear chromate for the iron alloy since the iron tends to discolor chromate films. Iron alloys seem to attain their increased corrosion protection from a change in the physical characteristics of the chromate conversion coating.

There are clear, yellow and black chromate conversion coatings available for the nickel alloys. The improved corrosion protection of the nickel alloy appears to be achieved by shifting the corrosion potential of the deposit closer to that of the steel substrate while still maintaining its sacrificial nature.

There are clear, yellow and black chromate conversion coatings available for the cobalt alloys. The cobalt alloys appear to obtain their increased corrosion protection from a modification of the physical properties of the chromate conversion coatings.

The yellow chromate over an alloy deposit is darker and more highly iridescent. This modified yellow is not only accepted in the industry, but is used by some as a method of differentiating alloy from non-alloy zinc. When processed properly, the clear chromates on the cobalt and nickel alloys and black chromates on the iron, cobalt and nickel alloys look very similar to the chromate coating on non-alloy zinc deposits.

In addition to the chromate conversion coatings, topcoats are also used to improve corrosion protection and lubricity and to reduce the loss of corrosion protection at elevated temperatures.