Ask the Experts

Matt Stauffer
Charlotte, N.C.

Matt Stauffer

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

Q: I'm doing a acid-copper plating process on zinc die caste material ( MAZAK ). The finish obtained from acid-copper plating is excellent, but after that, when I apply lacquer to it, the material tarnishes in few days. What's the problem? Also , can you suggest me treatments to be followed after removing from acid-copper tank.? Thanks.

Copper and copper alloys are prone to tarnishing on exposure to the atmosphere. It does require a suitable lacquer with sufficient thickness to prevent exposure to oxidation. A neutralizing step is recommended to remove the acid from the surface, which in itself can cause issues when coming directly from acid copper into a lacquer. A 1% ammonia dip or a heavily silicated brass cleaner followed by a thorough, clean rinse would be suitable. Extra care should be taken here to ensure adequate neutralization and rinsing of porous or rough surfaces. This will result in a "bleed out" type of tarnish pattern. Proper lacquers for your application are specifically designed for copper alloys. For extra protection, a hex chrome passivation or a non-chrome passivate for copper is available to provide additional protection as the lacquer cures.

Date asked: 02 May 2013

Q: I m working with cyanide copper barrel plating. Parts are zip sliders made of zinc die casting. The problem I'm facing is controlling this solution to get bright copper parts. Can I use any other alloy in this solution? If yes, then which metal and how much? Thanx.

There are various brightener systems used in cyanide copper. Some are metallic, some are organic, and some are both. I would suggest you contact a local finishing supplier to get information on common additives available in your area.

Something to consider regarding cyanide material sources: Sulfur is a common contaminant found in certain sources of sodium and potassium cyanide. This can cause a dark/dull red low current density area. This is easily fixed by a small addition of zinc cyanide, (1-2 g/l) as the zinc reacts with the sulfur compound. The small amount of zinc will give a very low codeposition of zinc, which is not a problem when plating zippers.
 

Date asked: 15 March 2013

Q: I am getting excess sulphate in my chrome tank. Due to this excess, I'm getting dull plating. Can you please tell me to resolve this problem? Also, in my cyanide copper tank the solution becomes dark after 4-5 plating rounds. Can you offer some advice? thanks in advance

You mention a chrome bath and a copper bath, so I'll go out on a limb and assume your plating copper-nickel-chrome. If so, the usual source for excess sulfate is insufficient rinsing after nickel plating. A typical Watts nickel bath will contain upwards of 35 opg (263 g/l) of nickel sulfate. Even a simple sulfate catalyzed hex chrome has only 0.32 opg (2.4 g/l) sulfate. Based on those relative quantities, it is pretty easy to get a fair amount of sulfate from a nickel bath into your chrome if rinsing is insufficient. Use barium carbonate to remove excess sulfate and consider additional rinse tanks (counterflowed) after nickel plating, and/or use of spray rinsing for blind holes, etc.

As far as the dark copper, we need a little more information to go on, but I will throw out the general advice that more cleaning is always good. You may be contaminating your copper with oils or buffing compound due to insufficient cleaning. Carbon treatment of your cyanide copper is an effective way to remove contaminants. Make sure your parts have a water break free surface before you attempt to plate them.
 

Date asked: 18 September 2012

Q: Our nickel metal is high in our bright nickel bath. [My vendor] tells us the only way to bring it down is to decant. Is there a way to bring it down slowly without wasting solution?

The increase in nickel metal growth in most acidic plating processes is caused by the difference in anode and cathode plating efficiencies. In the case of Watts nickel baths, you have 100% anode efficiency and approximately 93% cathode efficiency. The remaining 7% is directed towards the reduction of hydrogen into hydrogen gas. This hydrogen is a common source of gas pitting. So basically, you are dissolving more metal than you are plating out. Most platers dilute their baths to correct them. Some platers look towards insoluble anodes. Care must be taken to avoid the creation of harmful oxidation products when using insolubles.

There are products on the market that utilize a membrane system to prevent this reaction. These can be expensive and do require maintenance.

The other options to consider involve nickel recycling. This can be done by working with a company that recycles nickel solutions (plating or stripping) into nickel metal or by finding another plater, usually a barrel plater, who has a regular need for nickel salts in his process. Usually, an arrangement can be made to be beneficial to both parties.

The final option would be what I would call "home-grown" recycling. Set up a small plate out tank with insoluble carbon anodes. A good alternative for cathodes is nickel anode chips, which can be readily barrel plated.
Occasional adjustment of the pH may be required to raise the pH due to the use of insoluble anodes. Fresh solution can be added as the nickel is depleted from the plate out cell. The plated anodes can now be reused in the anode baskets.
 

Date asked: 12 September 2012

Q: Matt: I am plating semi-brilliant nickel bath over steel; my customer is heating parts (after nickel plating) up to 1250 degress celsius, and they are having blistering problems. What can I do to solve this problem?

This situation may be caused by either base metal preparation or conditions (i.e., stress) in your nickel plate. The problem needs to be isolated in order to solve it.

I would recommend that you plate a zinc-coated steel hull cell panel in the tank using the same semi-bright nickel solution. Strip the zinc with fresh hydrochloric acid, then remove the panel from the acid immediately after stripping is complete to avoid over pickling. Ensure that there is no water break film before you plate in your nickel.

Heat treat the plated panel and check for blistering. If you see blistering, it would appear you have an issue with the semi-bright nickel. If you do not see blistering, chances are your problem is related to surface preparation. You can repeat the plating test for confirmation in the lab by running hull cell panels of your nickel and a newly made nickel, and heat treating both panels. If you confirm that the existing solution blisters and the new solution does not, you will then need to investigate several potential factors, using the hull cell to confirm appropriate corrective action.

1. Excess semi-bright brightener additive. Semi-bright nickel does use leveling agents. Excess class 2 nickel brightener will impact stress levels. This can be removed by electrolysis.
2. Organic contaminant caused by brightener breakdown or soils/oil from poor cleaning. Look at peroxide/carbon treatment for improvement.
3. Stress can be monitored through stress tabs or spiral contractometer.This will allow a more direct evaluation of treatments.
4. Certain metals can codeposit and cause stress. Look for low current density darkness in the hull cell. These metals can be dummy plated to remove.
5. Check iron levels and peroxide treat if necessary. Iron can cause HCD defects. Keep below 20 ppm for your application.
6. Always make sure basic chemistry is correct. Low nickel, low boric, high pH can all cause high current density issues. Start here.

If you find there is no blistering on a test panel, then it is likely that your issue is related to preparation. Investigate to ensure parts are free of water breaks. Check for sufficient oxide removal as well as excess pickling. As Yogi Berra used to say about plating, "90 percent of the plating game is half preparation." 
 

Date asked: 01 August 2012

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