Civil Engineering Handbook

Acid Baths

The continuing development of acid zinc plating baths based on zinc chloride has radically altered the technology of zinc plating since the early 1970s. Acid zinc plating baths now constitute 40 to 50% of all zinc baths in most developed nations and are the fastest growing baths throughout the world. Acid zinc formulas and operating limits are given in Table 5. Bright acid zinc baths have a number of intrinsic advantages over the other zinc baths:

• They are the only zinc baths possessing any leveling ability, which, combined with their superb out-of-bath brightness, produces the most brilliant zinc deposits available.

• They can readily plate cast iron, malleable iron, and carbonitrided parts, which are difficult or impossible to plate from alkaline baths.

• They have much higher conductivity than alkaline baths, which produces substantial energy savings.

• Current efficiencies are 95 to 98%, normally much higher than in cyanide or alkaline processes, especially at higher current densities, as shown in Fig. 5.

• Minimal hydrogen embrittlement is produced than in other zinc baths because of the high current efficiency.

• Waste disposal procedures are minimal, consisting only of neutralization, at pH 8.5 to 9, and precipitation of zinc metal, when required.

The negative aspects of the acid chloride bath are that:

• The acid chloride electrolyte is corrosive. All equipment in contact with the bath, such as tanks and superstructures, must be coated with corrosion-resistant materials.

• Bleedout of entrapped plating solution occurs to some extent with every plating process. It can become a serious and limiting factor, prohibiting the use of acid chloride baths on some fabricated, stamped, or spot welded parts that entrap solution. Bleedout may occur months after plating, and the corrosive electrolyte can ruin the part. This potential problem should be carefully considered when complex assemblies are plated in acid chloride electrolytes.

(a) Carrier and primary brighteners for acid chloride are proprietary, and exact recommendations of manufacturer should be followed. Values given are representative.

(a) Carrier and primary brighteners for acid chloride are proprietary, and exact recommendations of manufacturer should be followed. Values given are representative.

Current ctensily, A/drri3

Current ctensily, A/drri3

Fig. 5 Comparison of cathode current efficiencies of bright zinc plating electrolytes

Acid chloride zinc baths currently in use are principally of two types: those based on ammonium chloride and those based on potassium chloride. The ammonium-based baths, the first to be developed, can be operated at higher current densities than potassium baths. Both systems depend on a rather high concentration of wetting agents, 4 to 6 vol%, to solubilize the primary brighteners. This is more readily accomplished in the ammonia systems, which makes bath control somewhat easier. Ammonium ions, however, act as a complexing agent in waste streams containing nickel and copper effluents, and in many localities they must be disposed of by expensive chlorination. This was the essential reason for the development of the potassium chloride bath.

All bright acid chloride processes are proprietary, and some degree of incompatibility may be encountered between them. Conversion from an existing process should be done only after a Hull Cell plating test evaluation. Preplate cleaning, filtration, and rack designs for acid chloride baths should be equivalent to those required for nickel plating.

The latest acid chloride zinc baths to become available to the industry are those based on salt (sodium chloride) rather than the more expensive potassium chloride. In many of these baths, salt is substituted for a portion of either ammonium or potassium chloride, producing a mixed bath. Sodium acid chloride baths at present are generally restricted to barrel operation, because burning occurs much more readily in these baths at higher current densities. However, with the continuing development of additive technology, sodium acid chloride baths may challenge the widely used nonammoniated potassium bath in the near future.

Acid chloride zinc baths are now being explored as the basis of zinc alloy plating incorporating metals such as nickel and cobalt, to improve corrosion for specific applications and possibly eliminate standard chromate treating.

A number of zinc baths based on zinc sulfate and zinc fluoborate have been developed, but these have very limited applications. They are used principally for high-speed, continuous plating of wire and strip and are not commercially used for plating fabricated parts. Table 6 shows the compositions and operating conditions for some typical fluoborate and sulfate baths.

(a) At room temperature; 3.5 to 4 pH; at 20 to 60 A/dm2 (200 to 600 A/ft2).

(b) At 30 to 52 °C (85 to 125 °F); 3 to 4 pH; at 10 to 60 A/dm2 (100 to 600 A/ft2).