Since aluminum is highly electropositive, plating onto its surface cannot be achieved.
Stephen Rudy, CEFInstead, the cycle step before plating is to zincate, by which a zinc-rich coating is bonded onto the aluminum surface, on which subsequent plating occurs. Excellent adhesion of the zincate to the aluminum surface is required, as is the initial plating adhesion to the zincate surface. This critical, final surface preparation step is very important.
Yet, its success is closely related to the critical surface preparation steps: soak, clean, etch, and desmut. A wide variety of aluminum parts are thus plated. To identify some, industries served include consumer, automotive (especially after-market), electrical, aerospace, medical, construction, machinery, and military.
Besides zinc, zincates may contain additional metals that contribute to modifying the structure formation. First-generation zincates comprised zinc oxide and sodium hydroxide (caustic soda) in a balanced concentrate. These commercial blends were introduced in powder form dissolved to achieve a high operating concentration.
The Need for Sufficient Solution Cooling
The excess heat liberated during bath makeup required 24 hours of sufficient solution cooling before the bath could be used (typically 65°F-85°F). In basic operation, following the desmutting step, caustic soda dissolves the surface layer of aluminum, permitting the galvanic deposition of zinc onto the active aluminum surface.
This zinc coating prevents the formation of an oxide and becomes an excellent base to accept subsequent plated deposits. To simplify the zincate makeup and contribute to user safety, the zincate is also available as a manufactured concentrate, ready to use.
Over the years, concerted research was conducted to improve the zincate reactivity and bonding to a wide range of aluminum alloys and castings. It was found that incorporating iron became very beneficial. This work led to a second generation of zincates, now referred to as alloy zincates, by adding iron to the concentrate. Commercial formulations became available in powder and liquid concentrates. The latter significantly reduced the time required for solution cooling, thereby improving production schedules.
Third Generation Zincates Significantly Improved Characteristics
A third generation of zincates, also of the alloy type, significantly improved the characteristics of the zincated deposit over zinc and iron-based zincates. These newer alloy zincates, composed of four metals: zinc, copper, iron, and nickel, provided significant benefits.
The alloy zincate formed was thinner, with a denser barrier film. This major development significantly improved resistance to lateral corrosion compared to the earlier formulations. Modifications to grain structure and limiting deposit thickness helped to achieve these benefits. Another improvement was the incorporation of a formula concentrate of much lower viscosity.
This improved solution penetration into deep grooves and recesses, forming an overall uniform, highly adherent surface. This was a major improvement over the self-contained, high-viscosity sodium hydroxide (caustic soda) blended concentrates.
Lower Solution Viscosity Reduces Solution Drag-Out
Lower solution viscosity also reduced solution drag-out and facilitated post-rinsing. The four metal alloy zincates are commercially available in cyanide and non-cyanide formulations. It was initially necessary to incorporate a small quantity of cyanide to maintain the solubility of the metals. Subsequent improvements to blending and formulation technology offer cyanide-free versions as well, overcoming any stability and solubility without cyanide in the concentrate.
Operating Parameters
Powder zincates:
- 1 2 lb/gal (454 908 g/L),
- 65 85°F (18 30°C),
- 20 120 sec.
Liquid zincates:
- Full strength concentrate at 50 vol%,
- 65 85°F (18 30°C),
- 20 120 sec.
The operating temperature is important to control the development of the zincate film to an effective, desired thickness. Overheating the working solution promotes thicker, spongy-like (less dense) deposits. Some of the liquid concentrate multi-metal zincates should be stored above 50°F (14°C) to prevent freezing and precipitation of formula constituents. This serves as well for their operating baths.
Producing a Uniform Film Covering
Immersion in the zincate should produce a uniform film covering the entire aluminum surface. Patchy films or pull-off by tape test may indicate insufficient cleaning, rinsing, desmutting, or problems in the zincating solution.
Immersion time in the zincate solution may vary according to alloy composition. Normally, about a two-minute zincate immersion is required for commercially pure aluminum, silicon, and copper-containing alloys. Other alloys, such as those containing magnesium, require shorter zincing times.
In some applications, a double zincate dip is preferred. This would refer to cast alloys, wrought alloys containing little or no magnesium, and unknown alloys. In this modification, the first zincate film is deliberately stripped in either 50% nitric acid or in a commercial, nitric acid-free stripping solution.
What actually occurs is the zincate is stripped, but just down to the surface, where it is directly bonded to aluminum. This very thin layer acts as a “seed or anchor” to which a second, subsequent zincate film deposits. The effect is the formation of a tightly adherent, thin zincate film with a greater tendency for satisfactory adhesion in the plating cycle.
In labor-intensive process cycles, rejects can easily increase the handling and reprocessing cost by three to five times. Therefore, the application and consideration of a single or double zincate is very important.
Analysis
Most commercial zincate baths can be analyzed by the titration method. Usually, the alkalinity is determined in one titration and metals (based on zinc) in a second titration. Where it is available, atomic absorption analysis for other zincate metals, such as copper, iron, and nickel, is of benefit.
Replenishment is based on the analysis results. In standard operations, drag-out losses, coupled with vendor advice on bath depletion per surface area zincated, can fine-tune replenishment additions.
Testing for Quality Adhesion
Post-plate baking at 450°F (232°C) for one hour is followed by a cold water quench. Edge grind and peel back. Saw grind. Standard aluminum Q panels are processed in the cycle as a control.
Problems and Corrections
| Problem | Cause | Correction |
| Streaked uneven zincate | A. Additives out of balance | Analyze with appropriate adds |
| With poor adhesion | B. Bath temp. out-of-range | Adjust |
| A & B not effective | C. Surface preparation | Evaluate each step |
| Parts gas in zincate | Insufficient de-smutting | Adjust or replace desmut |
| Spongy poor adhesion | Excess zincate dip time | Adjust |
The structure of the zincate film is detrimentally affected by the drag of excessive aluminum, chromium, lead, and fluoride. For example, as little as 15 ppm Fluoride could be detrimental.
Some tips for good operation and preventive maintenance
- Make chemical adds to each process bath, as established, on a regular basis.
- Do not exceed the optimum service life of the surface preparation baths.
- Determine square feet of surface area zincated in zincate service life to predict appropriate replenishment or bath replacement with new makeup.
- Where possible, know the alloy designations of aluminum processed.
Stephen F. Rudy, CEF, is president of Chem Analytic and has written extensively about the finishing industry. Visit www.chemanalytic.com or call him at 917-604-5001.
