troubleshooting, testing & analysis

ZINC & ZINC ALLOY PLATING;

PROBLEMS WITH EN BATHS AND

ACID COPPER

BY MATT STAUFFER, PAVCO, CHARLOTTE, N.C.

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

A: The increase in nickel metal growth in most acidic plating processes is causedby the difference in anode and cathode plating efficiencies. In the case of Wattsnickel baths, you have 100% anode efficiency and approximately 93% cathodeefficiency. The remaining 7% is directed towards the reduction of hydrogen intohydrogen gas. This hydrogen is a common source of gas pitting. So basically,you are dissolving more metal than you are plating out. Most platers dilutetheir baths to correct them. Some platers look towards insoluble anodes. Caremust be taken to avoid the creation of harmful oxidation products when usinginsolubles.There are products on the market that utilize a membrane system to preventthis reaction. These can be expensive and do require maintenance.The other options to consider involve nickel recycling. This can be done byworking with a company that recycles nickel solutions (plating or stripping)into nickel metal or by finding another plater, usually a barrel plater, who has aregular need for nickel salts in his process. Usually, an arrangement can be madeto be beneficial to both parties.The final option would be what I would call “home-grown” recycling. Setup a small plate out tank with insoluble carbon anodes. A good alternative forcathodes is nickel anode chips, which can be readily barrel plated.Occasional adjustment of the pH may be required to raise the pH due to theuse of insoluble anodes. Fresh solution can be added as the nickel is depletedfrom the plate out cell. The plated anodes can now be reused in the anode baskets.

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

A: You mention a chrome bath and a copper bath, so I’ll go out on a limb andassume your plating copper-nickel-chrome. If so, the usual source for excesssulfate is insufficient rinsing after nickel plating. A typical Watts nickel bathwill contain upwards of 35 opg (263 g/l) of nickel sulfate. Even a simple sulfatecatalyzed hex chrome has only 0.32 opg (2.4 g/l) sulfate. Based on those relativequantities, it is pretty easy to get a fair amount of sulfate from a nickel bath intoyour chrome if rinsing is insufficient. Use barium carbonate to remove excesssulfate 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 Iwill throw out the general advice that more cleaning is always good. You may becontaminating your copper with oils or buffing compound due to insufficientcleaning. Carbon treatment of your cyanide copper is an effective way to removecontaminants. Make sure your parts have a water-break-free surface before youattempt to plate them.

Q: I’m doing an acid-copper plating process on zinc die cast material ( MAZAK ). Thefinish obtained from acid-copper plating is excellent, but after that, when I apply lacquerto it, the material tarnishes in few days. What’s the problem?

A: Copper and copper alloys are prone to tarnishing on exposure to the atmosphere.It does require a suitable lacquer with sufficient thickness to preventexposure to oxidation. A neutralizing step is recommended to remove the acidfrom the surface, which in itself can cause issues when coming directly from acidcopper into a lacquer. A 1% ammonia dip or a heavily silicated brass cleaner followedby a thorough, clean rinse would be suitable. Extra care should be takenhere 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 yourapplication are specifically designed for copper alloys. For extra protection, a hexchrome passivation or a non-chrome passivate for copper is available to provideadditional protection as the lacquer cures.

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

A: 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 finishingsupplier to get information on common additives available in your area.Something to consider regarding cyanide material sources: Sulfur is a commoncontaminant found in certain sources of sodium and potassium cyanide.This can cause a dark/dull red low current density area. This is easily fixedby a small addition of zinc cyanide, (1-2 g/l) as the zinc reacts with the sulfurcompound. The small amount of zinc will give a very low co-deposition of zinc,which is not a problem when plating zippers.

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

A: 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 thetank using the same semi-bright nickel solution. Strip the zinc with fresh hydrochloricacid, then remove the panel from the acid immediately after stripping iscomplete to avoid over pickling. Ensure that there is no water break film beforeyou plate in your nickel.Heat treat the plated panel and check for blistering. If you see blistering, itwould appear you have an issue with the semi-bright nickel. If you do not seeblistering, chances are your problem is related to surface preparation. You canrepeat the plating test for confirmation in the lab by running hull cell panels ofyour nickel and a newly made nickel, and heat treating both panels. If you confirmthat the existing solution blisters and the new solution does not, you willthen need to investigate several potential factors, using the hull cell to confirmappropriate corrective action.

1. Excess semi-bright brightener additive. Semi-bright nickel does use levelingagents. Excess class 2 nickel brightener will impact stresslevels. This canbe removed by electrolysis.

2. Organic contaminant caused by brightener breakdown or soils/oil frompoor cleaning. Look at peroxide/carbon treatment for improvement.

3. Stress can be monitored through stress tabs or spiral contractometer. Thiswill allow a more direct evaluation of treatments.

4. Certain metals can co-deposit and cause stress. Look for low current densitydarkness in the hull cell. These metals can be dummy plated to remove.

5. Check iron levels and peroxide treat if necessary. Iron can cause HCDdefects. Keep below 20 ppm for your application.

6. Always make sure basic chemistry is correct. Low nickel, low boric, high pHcan 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 issueis 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 usedto say about plating, “90 percent of the plating game is half preparation.”

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

A: Cooling coils in the plating tank or a heat exchanger connected to an industrialchiller are recommended for alkaline zinc plating. There are zinc brighteners on themarket that work well (enough) at elevated temperatures, but none are as brightacross all current densities as a lower temperature bath operated at 25 degreesCelcius. Different additive levels and bath parameters are required at the highertemperatures. You can expect to use more brightener, and you will find that somelow 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 densitybrightness in the zinc plating. High polishing blue chromates are also availableto further your cause.Your ability to produce acceptably uniform brightness may depend upon, tosome degree, the geometry of the parts. Large flat surfaces tend not to polishnearly as well as round surfaces. Air agitation during polishing and chromatingwill 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 youare doing, (small parts vs. large flat parts) as well as the degree of brightness oruniformity desired.

Q: What is best for cleaning parts with small cracks?, I am using 20 pounds, but it seems tonot be enough. What should I use? I am using 2 stage ( cleaner and rinse) and times around35 sec by stage, normal concentration betwen 3 to 4 % of alkaline cleaner and 80°C oftemperature in both tanks.

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

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

A: The oldest answer to your question is the use of a chromic acid passivationstep after nickel plating. Use 20-40 g/l of chromic acid, preferably hot, 35-40degrees C. This has the added benefit of removing any flash rusting that mayhave 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 hexavalentchrome. There are also many water-based lacquers or topcoats that can be usedover nickel plating to supplement corrosion protection in thin coverage areas.

Q: Is Alkaline Non Cyanide Copper effective & successful on Zinc Die Casted (Zamak 5)components?

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

1. A copper strike is traditionally used directly over diecast to provide optimaladhesion. When using cyanide processes, the proper chemistry is neededin terms of pH, free cyanide, and copper content to ensure optimal adhesion.To replace this without cyanide, there are a few commercial processesavailable. If you google “non cyanide copper” you will find them near thetop. These require more attention than the standard cyanide processes andhave a higher operating cost due to the use of insoluble anodes. There arerecommended conditions that should be followed to optimize adhesionover zinc diecast.

2. The copper strike is followed by a cyanide copper plate that is typically optimizedfor higher efficiency in order to provide a sufficient barrier layer priorto nickel plating. This helps improve the corrosion resistance of the finalcoating system as well as minimize contamination of the nickel processes.The same non-cyanide processes as mentioned above can also be used ascopper plate. Pyrophosphate copper is also a time-tested process that is fullybright, and works as a suitable replacement for a heavy cyanide copper. Itdoes use copper anodes, which keeps operating cost down. It also is superiorto 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 inareas where the copper strike is thin or in unplated internal areas.In general, non-cyanide copper is a more common choice when a facility haslittle or no other sources of cyanide in the facility. This makes the increasedoperating cost easily justified.

Q: We own and operate a small nickel plating facility. We only have a nickel sulfate bath.Would it behoove us to also have a nickel “strike” or copper “strike?” Would there be anybenefits in this? Also, do you see manufacturers that honestly want to go green, even if itcosts more?

A: The need for a strike bath is dictated by the type of substrate you are plating.If you are plating brass, copper, or steel, then a strike bath is not necessary. Ifyou are plating zinc die cast or tin alloys, then a strike bath is considered necessary.It is usually a copper strike.There is one benefit of a strike that is worth considering for substrates thatdo not necessarily require it. That is, it does isolate your main bath from contaminants,especially those that are cleaning related. This could mean the oilsand soils themselves, or even the cleaners, can become contaminants whendealing with difficult-to-rinse part configurations, such as hollow interiors orcup-shaped areas that cause high drag-out.Having a separate strike tank takes the brunt of the contamination, and itacts as the “canary in the coal mine,” showing the effect of contaminants beforethey end up contaminating your larger plating tanks. They are much easier andless expensive to treat or replace due to their size.Regarding green initiatives, the most readily adopted green initiatives arethose that save money or are revenue neutral, but sometimes all costs are notfully considered when making these types of decisions. When making a casefor a green initiative, it is important to include all of the costs associated withboth scenarios. This should include, at aminimum: employee training, healthcare costs, insurance costs, rejection rates, productivity impact, chemical totalcost, waste treatment cost, hazardous waste cost, incoming chemical shippingcost, and outgoing waste shipping cost. Some form of risk assessment can alsobe factored in, but calculating a cost for this is more nebulous, although peaceof mind does have a value for many people. If you can cast a wide net with thesecost factors, it is usually much easier to make the math of the “green” decisionmore palatable.For more information, please visit www.pavco.com.