Corrosion Potential of Zinc Alloys

Article From: Products Finishing, from MacDermid, Inc.

Posted on: 3/1/1999

Zinc alloys provide greater sacrificial corrosion protection. . .

Electrochemically, alloys can be designed to produce different corrosion potentials than their alloying elements. Therefore, it is possible to maintain the sacrificial protection of zinc coatings over steel, but at a different potential that is closer to steel by alloying it with another metal, preferably one more noble than zinc. The alloy corrodes more slowly than zinc alone, providing better corrosion protection.

Typically, the initially formed corrosion products are insoluble and form a stable protective layer. Zinc-nickel and zinc-cobalt deposits have shown no whisker growth during humidity testing. This characteristic is desirable in electrical and electronic applications. Some alloys are even excellent replacements for cadmium plating.

Zinc alloy plating technologies were introduced in the mid 1980's in the U.S. Their use began ten years earlier in Japan and Europe. Only recently have the alloys been accepted and used on a commercial basis in the U.S. and Canada.

Several factors may have delayed the adoption of these technologies in the U.S., despite their documented success in Japan and Europe.

  1. U.S. regulatory agencies only recently restricted cadmium as a protective coating; therefore, finding a good substitute became urgent.
  2. The desire for improved quality and product reliability that surpassed zinc plating became a necessity in the last few years.
  3. Simultaneous introductions of several zinc alloys and processes required time to evaluate each before new specifications could be developed.

The various alloy technologies available include zinc-iron, alkaline or acid zinc-nickel, zinc-cobalt and tin-zinc.

TABLE I—Zinc-Nickel Bath Parameters
Caustic 13-19 oz/gal
Zinc 0.8-1.6 oz/gal
Nickel 0.10-0.24 oz/gal
Temperature 74-86F
Anode CD 3-9 amp/sq dm
Cathode CD 3-7 amp/sq dm
Alloy Composition 5-15% Ni; Bal.Zn/Depending on Zn:Ni ratio, Ni additive and DC settings

Zinc-Nickel Alloys

The two types of zinc-nickel plating baths are an alkaline, non-cyanide bath and an acid-type bath.

The nickel content in this alloy ranges from five to 15% by weight of the deposit. The balance of the deposit is zinc. Corrosion resistance studies have shown peak performance after chromating in alloys containing 10 to 12% nickel. At high nickel levels, chromating after plating is extremely difficult. At higher than 25 to 30% nickel, the deposit ceases to be sacrificial to steel, especially deposits with 10 to 15% nickel.

The alkaline bath has an advantage of uniformly maintaining the nickel content in the deposit in both low and high current densities. The acid bath is more efficient, but produces a high nickel content in low current density areas, affecting chromating as well as the sacrificial protection in these areas. Because of this, pitting may occur in corrosive atmospheres.

Another advantage of an alkaline bath is its inherent non-corrosive nature to unplated internal areas such as the ID of tubular parts.

Pilot lab and field tests have shown the alkaline zinc-nickel process is better than the other zinc alloys in terms of corrosion resistance, ease of operation and range of applications, including cadmium replacement.

Subsequently, various industries adopted zinc-nickel into their finishing specifications. The plating installations vary from 200 to 5,000 gal for rack and barrel lines.

TABLE II—Acid Zinc-Nickel Bath Compositions
  Types
  Potassium
Chloride
Ammonium
Chloride
Zinc Chloride 130 g/liter 120 g/liter
Nickel Chloride 130 g/liter 110 g/liter
Potassium Chloride 230 g/liter
Ammonium Chloride 150 g/liter
pH 5-6 5-6
Cathode CD 0.1-4 amps/sq dm 0.5 amp/sq dm
Current Dist. Zn 80% 60%
Current Dist. Ni (optional) 20% 40%
Temperature 24-30C 35-40C
Anodes Zinc or zinc and nickel, connected to two separate rectifiers
Alloy Composition 8-15% Ni; balance Zn

Automotive Industry.

This industry is the prime beneficiary of zinc-nickel technology. A combination of substantial performance-warranty upgrades and the need to replace cadmium plating were behind the decisions to specify zinc-nickel coatings.

Most Japanese and European car manufacturers already have zinc-nickel specifications. U.S. plating shops continue to supply the need for alkaline zinc-nickel in accordance with those specifications, which are mostly for under-the-hood components.

Daimler/Chrysler Corp. released new specifications for zinc-nickel and zinc-cobalt in 1989. Ford Motor Co. issued its Engineering Change Code in 1990 to replace cadmium with alkaline zinc-nickel. This only happened after considerable lab and proving ground testing. This code change gave Ford's main suppliers the go ahead to replace their cadmium baths and remain in compliance with regulations. The end product is found in passenger cars and light truck power steering, air conditioning and hydraulic brake components.

Since most of these are tubular parts, the alkaline plating bath provides corrosion resistance to the unplated ID. This is critical when no contaminants are allowed in the power transmission or brake line.

The extended salt-spray requirement on some Ford components is now 768 hrs to red rust for eight microns of zinc-nickel with iridescent chromate. Other automotive requirements for zinc-nickel with bronze chromates are regularly produced and achieve 1,000 to 1,200 hrs to red rust in neutral salt-spray tests.

Electrical Transmission Industry

Several zinc alloy processes have been evaluated for plating heavy electrical transmission components. Alkaline zinc-nickel was selected to replace alkaline zinc. Anchors, cleats and bolts are exposed to the elements in harsh environments along highways and near seashores. Zinc-nickel coatings with iridescent chromate coatings increased corrosion resistance from 250 to more than 1,000 hrs in salt-spray testing. The coating may be applied directly over steel or pregalvanized steel for the required additional protection.

TABLE III—Typical Zinc-Iron Bath Composition (strip line plating)
Ferric Sulfate 200-300 g/liter
Zinc Sulfate 200-300 g/liter
Sodium Sulfate 30 g/liter
Sodium Acetate 20 g/liter
Organic Additive 5 g/liter

Another application is plating coaxial TV cable connectors that are assembled to painted aluminum housings. The connectors were traditionally cadmium plated for maximum corrosion protection indoors and out. Alkaline zinc-nickel has replaced cadmium as an environmentally safer substitute.

Fastener Industry

This industry also depends heavily on cadmium plating. Alkaline zinc-nickel has out performed zinc or cadmium in corrosion resistance tests before and after crimping and baking for hydrogen embrittlement relief. Just as cadmium parts, zinc-nickel-plated parts are easily chromated after baking or heat treating with minimum activation. Another advantage of chromated zinc-nickel over cadmium and zinc is its ability to maintain a high corrosion resistance following heat treatment. Since the chromate is alloy rich, thermal degradation is not as critical as with conventional chromated zinc or cadmium.

This opens the doors to industries involving aluminum bodies. It is expected that alkaline zinc-nickel performance will exceed that of cadmium-plated fasteners in this field.

Defense Industry

A range of activities is happening in this area, primarily propelled by the need to replace cadmium. Cadmium coatings have been the core of many military plating specifications for years, and replacements must be critically evaluated. Studies have been commissioned to research labs such as Battelle and Ocean City Research; thorough investigations have been conducted at several plating shops as well.

Army

FMC is a primary supplier of tanks and armored personnel carriers to the Army. In 1989, after the Loma Prieta, California earthquake, minor amounts of cyanide-cadmium plating solution splashed out of plating tanks. The solution was safely captured, yet the incident raised concern about future accidental cross contamination with acids or other cadmium solutions.

Management directives were given to accelerate the evaluation process for cadmium substitutes. In 1990, FMC obtained approval to apply alkaline zinc-nickel to most of its components that previously were cadmium plated. The switch was publicized as an exemplary step in the adoption of environmentally safe technologies.

TABLE IV—Zinc-Cobalt Bath Composition
  Types
Typical Acid Baths Boric Acid Ammonium Chloride
Zinc Chloride 80-90 g/liter 80-90 g/liter
Potassium Chloride 150-200 g/liter 50-150 g/liter
Ammonium Chloride 50-70 g/liter
Boric Acid 20-30 g/liter
Cobalt Chloride 1-20 g/liter 1-20 g/liter
pH 5-6
Temperature 24-40C
Cathode CD 1.0 - 4.0 amp/sq dm
Anodes Zinc
Alkaline Baths
Zinc Oxide 10-20 g/liter
Sodium Hydroxide 80-150 g/liter
Cobalt (additive) 1.0-2.0 g/liter
Organic Additives as specified
Temperature 25-40C
Cathode CD 0.1-4 amps/sq dm
Anodes zinc

Other branches within the army, such as the maintenance depots, evaluate the same process for in-house plating shops. Ocean City Research Center did a comprehensive study of environmentally acceptable plating technologies. The initial findings showed the alkaline zinc-nickel as one of the best cadmium replacements. Additional work in this area continues.

Alkaline Zinc-Nickel

This bath is simple to operate and is similar to the alkaline non-cyanide zinc bath. The alloying nickel metal is added in a liquid form on an amp/hr basis along with grain refiner brightening agents.

Conventional zinc anodes are used to supply the zinc metal. The main electrolyte is caustic soda, containing the dissolved zincate. Bath parameters are listed in Table I. The typical bath composition of the acid zinc-nickel bath is in Table II.

Zinc-Iron

 This process produces alloys deposits with 15 to 25% iron. Electroplated strip steel initially used this process to improve corrosion resistance. The deposit has good weldability and ductility, which are needed in subsequent manufacturing.

This alloy can be adjusted to improve adhesion of electrocoating on formed steel components. Black chromating is the most suitable for this type of alloy. A major advantage of the zinc-iron alloy deposit is its suitability to black chromate conversion coatings that are silver-free. This finish is highly desired by automotive engineers and designers.

Although zinc-iron had good corrosion resistance as plated and chromated, exposure to heat deteriorates this resistance rapidly. Because of this, it is unsuitable for under-the-hood automotive components. The typical bath formulation for acid-type, zinc-iron used in strip line plating is shown in Table III.

Zinc-Cobalt alloy plating is becoming more popular because of its relatively lower cost of operation compared to zinc-nickel. It offers somewhat lower levels of corrosion resistance; however, the level is still adequate and an improvement over plain zinc of the same thickness.

TABLE V—Corrosion Resistance to Zinc Alloys vs. Zinc
  Hrs to Red Rust
Before Heat Treat After Heat Treat
    120C, 4 hrs 200C, 4 hrs
Zinc-nickel (10-15% Ni) 1,500+ 1,200+ 800+
(6-9% Ni) 1,000+ 800-900 600-700
Zinc-cobalt 500 200-250 180-240
Zinc-iron 1,000 300-350 180-240
Zinc 300-350 200-250 150-200
(Note: coating thickness of 8 microns, yellow iridescent chromate)

The fastener industry seems to be the main beneficiary of this technology. The deposit contains 0.1 to 1% cobalt, although deposits with higher than 0.4% are difficult to chromate and may lead to stress problems. The deposit may be obtained from either an acid or an alkaline electrolyte. Both are relatively simple to maintain and similar to conventional acid or alkaline zinc plating systems. (Table IV)

Tin-Zinc alloys contain 70 to 90% tin with the balance being zinc. Although tin-zinc alloy deposits are not new, new technologies offer baths that are neutral and cyanide free. The deposit is ductile and maintains good solderability even after aging. Corrosion resistance equals or exceeds that of zinc-nickel alloys. Chromating is usually limited to clear or yellow. Applications of the neutral tin-zinc process are growing in the electronic industry, glass to metal seals and fastener industry as a direct replacement for cadmium. (Table VI)

To supplement the zinc alloy technologies, new conversion coatings are under development. These include trivalent chromates and chromium-free coatings ranging from clear to iridescent and black. Additional topcoats are available to improve the overall deposit performance.

Zinc-cobalt has good corrosion resistance to atmospheres containing sulfur and shows excellent results in Kesterich (SO2) tests. However, overall corrosion resistance to red rust is less than that obtained with zinc-nickel. The process offers intermediate corrosion resistance and a choice of chromates. Typical zinc-cobalt baths are shown in Table IV.

NATO, Scandinavian countries, Japan and other countries worldwide have legislated a ban on cadmium. U.S. manufacturers and suppliers are expected to adhere to the requirements to maintain their export and trade positions. The last few years have seen large scale testing and evaluation of zinc alloys and the installation of many plating lines. Corrosion resistance of the various zinc alloy electrodeposits is compared in Table V.

TABLE VI—Bath Composition Neutral Tin-Zinc
Tin as Sn+2 1-3 oz/gal
Zinc 0.7-2 oz/gal
Temperature 65-75F
Cathode 0.5-2.0 amp/sq dm
Anode Tin-zinc cast alloy

New ASTM specifications are available for zinc alloys.

Zinc-cobalt B840
Zinc-iron B842
Zinc-nickel B841

Zinc alloys can be electrochemically designed to produce different corrosion potentials than their alloys elements. This helps to maintain the sacrificial protection of zinc, but the alloy causes it to corrode at a slower rate. Some of these alloys are excellent replacements for cadmium plating in many applications.

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