Tech Talk: Functional Zinc Plating
While offering sacrificial corrosion protection based upon its larger negative electro-potential than iron, zinc is not used without subsequent treatment.
Electrodeposited zinc is used for coating iron and steel parts when protection from the corrosive effects of the atmosphere is the primary goal. However, zinc by itself, while offering sacrificial corrosion protection based on the fact that it has larger negative electro-potential than iron, is not used without subsequent treatment. Electroplated zinc becomes dull gray after exposure to air, so to get bright zinc, it is given a subsequent post-treatment in a conversion coating that most often contains either hexavalent or trivalent chrome. Non-chrome-bearing conversion coatings have also been employed.
Up until the 1970s, commercial zinc plating had been traditionally done from a cyanide electrolyte. Because of environmental considerations, other processes were developed. Today, alkaline non-cyanide (alkaline zincate) and acid chloride baths comprise the bulk of commercial zinc plating. These processes are applicable to both rack and barrel zinc processes.
Alkaline Non-Cyanide Zinc Plating
Initial alkaline non-cyanide zinc plating solution processes were associated with a great number of problems and difficulties. This, in part, was due to the lack of experience and knowledge about the chemistry of deposition. Also, the ease with which zinc could be deposited from a cyanide electrolyte really challenged the finisher with a steep learning curve. From general bath control (zinc and caustic concentrations) to a tendency to contaminate easily (particularly organic brightener occlusion, which imparted brittleness and subsequent blistering) to the necessity of enhanced cleaning cycles due to the absence of cyanide, a natural cleaning agent—all presented the plater with a tall order of challenges. In comparison with cyanide systems, non-cyanide alkaline zinc baths have a narrower range of optimum operating zinc concentrations. Bath efficiency was very much dependent upon the concentration of zinc and caustic and their ratios.
Much of the research and development done over the past 30 years and the subsequent product developments have all but eliminated many of the major problems associated with alkaline non-cyanide zinc plating. Today, zincate plating is a highly successful process and commercially viable.
Today’s systems require simple control of both caustic and zinc, and the addition of additive systems that support a significantly wide range of operating parameters. Furthermore processing lines have been enhanced to include the cleaning capabilities necessary for alkaline plating. Additive chemistries, along with the development of chromate conversion coatings (primarily trivalent in nature), have enhanced the appearance of the deposit, rendering it aesthetically pleasing. In most cases, the appearance will compete with acid zinc plating.
Acid Zinc Plating
Acid zinc plating processes and the technology development in this area have substantially changed the face of zinc plating since the 1970s. More than 50 percent of all zinc plating today is done from an acid electrolyte. While we use the term acid zinc, the reality is that, while typically done at an acid pH of 5-6, this is just slightly below the neutral pH of 7 and does not represent an extremely acidic solution.
There are basically three types of chloride zinc baths in commercial application today, and these are based on the electrolyte composition. In the initial baths developed, the electrolyte is zinc chloride and ammonium chloride. These have the advantage of being able to be operated at high current density, but the ammonium ions act as complexing agents for nickel and copper in waste streams, and can require extensive waste-treatment procedures.
The second bath development was the all-potassium-chloride electrolyte, which is used in conjunction with boric acid. This system overcomes the waste-treatment issues, but high-current-density burning is a bit more sensitive in this electrolyte, and dependence on a good organic additive system becomes essential.
The third system is a modification of the all-potassium system that includes a small percentage of ammonium ion, which eliminates the need for boric acid. The lower amount of ammonium chloride (typically 20 percent of the total chloride) minimizes the waste-treatment issues. However, consideration should be give if the plater has nickel and or copper plating effluent, which is treated in a common waste-treatment facility.
In general, acid zinc plating offers the following advantages:
- Waste disposal is minimal, with precipitation of effluent zinc at a pH of 8.5-9.0.
- Current efficiencies are high (in excess of 90 percent), even at high current densities.
- Because of the nature of the organic additives, outstanding specular brightness can be achieved.
- Chromate conversion coatings are readily accepted and provide a wide range of color and corrosion resistance properties.
- Cast iron, malleable iron, heat-treated and carbon-nitrided parts, which are almost impossible to plate in alkaline plating solutions, are readily plated in an acid electrolyte solution.
- Power requirements are reduced because of the overall efficiency.
- Hydrogen embrittlement is reduced.
Roger Sowinski is vice president of technology at Asterion. For information, visit asterionstc.com.