Matt Stauffer
Charlotte, N.C.

Matt Stauffer

Zinc & Zinc Alloys, Bright Nickel, Trivalent Chrome, Acid Copper

Q: I have alkaline free cyanide zinc plating baths; the temperature is now 35°C. How can we cool down the solutions in order to have better conditions?. Is there a product that works at high temperatures, or what kind of equipment should we use?

Cooling coils in the plating tank or a heat exchanger connected to an industrial chiller are recommended for alkaline zinc plating. There are zinc brighteners on the market that work well (enough) at elevated temperatures, but none are as bright across all current densities as a lower temperature bath operated at 25 degrees Celcius. Different additive levels and bath parameters are required at the higher temperatures. You can expect to use more brightener, and you will find that some low current density areas tend not to be as bright as the lower temperature process.

This may not be objectionable once the parts are bright-dipped and chromated, as both steps will tend to polish out dullness in the deposit. A stronger (or longer) bright dip step will help compensate for poor low current density brightness in the zinc plating. High polishing blue chromates are also available to further your cause.

Your ability to produce acceptably uniform brightness may depend upon, to some degree, the geometry of the parts. Large flat surfaces tend not to polish nearly as well as round surfaces. Air agitation during polishing and chromating will tend to help compensate, but areas of the part that remain unagitated (interiors) may remain dull.

In general, much of your success will depend on the nature of the work you are doing, (small parts vs. large flat parts) as well as the degree of brightness or uniformity desired.

Date asked: 15 March 2012

Q: What is best for cleaning parts with small cracks?, I am using 20 pounds, but it seems to not be enough. What should I use? I am using 2 stage ( cleaner and rinse) and times around 35 sec by stage, normal concentration betwen 3 to 4 % of alkaline cleaner and 80°C of temperature in both tanks.

Cracks and other areas where solution exchange are not good are challenging to clean and may require equipment improvements to address these issues. High-pressure spray cleaning or ultrasonic cleaning are good options for these types of applications.

Date asked: 21 February 2012

Q: Hello Matt. Do you know of any solution to prevent oxide after nickel plating? The coating is nickel on steel, and the thickness is 5 microns average. It is a bright nickel solution, and we need to protect uncoated parts of low current density. Thank you so much.

The oldest answer to your question is the use of a chromic acid passivation step after nickel plating. Use 20-40 g/l of chromic acid, preferably hot, 35-40C. This has the added benefit of removing any flash rusting that may have occurred in any of the process steps, such as acid rinse or nickel rinse. There are options on the market for similar processes that are free of hexavalent chrome. There are also many water-based lacquers or topcoats that can be used over nickel plating to supplement corrosion protection in thin coverage areas.
 

Date asked: 14 February 2012

Q: Is Alkaline Non Cyanide Copper effective & successful on Zinc Die Casted (Zamak 5) components? If YES do we need to plate a strike before the main copper ?

When plating over zinc diecast, there are generally two copper processes involved. I will discuss both old and new methods here:
 

1. A copper strike is traditionally used directly over diecast to provide optimal adhesion. When using cyanide processes, the proper chemistry is needed in terms of pH, free cyanide, and copper content to ensure optimal adhesion. To replace this without cyanide, there are a few commercial processes available. If you google "non cyanide copper" you will find them near the top. These require more attention than the standard cyanide processes and have a higher operating cost due to the use of insoluble anodes. There are recommended conditions that should be followed to optimize adhesion over zinc diecast.
2. The copper strike is followed by a cyanide copper plate that is typically optimized for higher efficiency in order to provide a sufficient barrier layer prior to nickel plating. This helps improve the corrosion resistance of the final coating system as well as minimize contamination of the nickel processes. The same noncyanide processes as mentioned above can also be used as copper plate. Pyrophosphate copper is also a well tested process that is fully bright, and works as a suitable replacement for a heavy cyanide copper. It does use copper anodes, which keeps operating cost down. It also is superior to acid copper for use over diecast due to the mildly alkaline operating pH. This prevents attack of the zinc base metal that is seen in acid copper in areas where the copper strike is thin or in unplated internal areas.

In general, non-cyanide copper is a more common choice when a facility has little or no other sources of cyanide in the facility. This makes the increased operating cost easily justified.
 

Date asked: 11 February 2012

Q: We are doing Zn alkaline with trivalent passivation + topcoat for the products and these are packed in cardboard boxes and dispatched to the customers. However, once the customers open these boxes, the plated parts are found rusty. Our quality checks before dispatch: 1. Neutral salt spray test on the plated parts and they answer for resistance as per our customers requirement. 2. Plating thickness is within the specification of 6 to 10 microns Zn. What could be the probable reasons for this corrosion?

Thanks for your question. You do not mention the salt spray requirements nor the details of the current process so I will make general observations on potential causes.

Premature corrosion failures are the result of several factors:

1) Storage and shipping conditions. High humidity and/or temperature during shipping may overwhelm a lower-end chromate. Many low-end blue chromates offer only 12-24 hours of protection. If you are in a high humidity environment or are shipping product over saltwater, consider an upgrade to a higher salt spray chromate that is suitable for your application. Blue trivalent chromates are available that readily exceed 120 hrs to white corrosion.

2) Insufficient plating thickness. While you do mention that thickness is satisfactory, be sure that it is sufficient in the areas where the failures are occurring. Depending on the type of zinc plating technology used, you may have critical areas of the part that are not well covered even though a prominent area tests to the desired thickness specification. This could dictate which zinc plating technology you should specify, as well as thickness requirements. Use of a post chromate sealer helps improve salt spray in low thickness areas, as well as with parts that have issues with base metal porosity.

3) Poor cleaning/preparation causing poor adhesion of deposit. Non-adherent zinc does not offer protection. Poor adhesion may not be immediately apparent. Bake parts at 300^oF for 30 minutes and inspect for blisters to confirm acceptable adhesion.

4) Proper parts packaging after plating can also improve chromate coating performance. This includes proper drying, as parts that are not thoroughly dried are prone to failures.
 

Date asked: 06 February 2012