Technical Q & A

Columbia Chemical Technical Support Representatives can be contacted

at 330/225-3200 or techsupport@columbiachemical.com

 

Staining Problems in Acid Zinc: Low Cloud Point

Question:  Why is a low cloud point causing staining problems in my acid zinc barrel?  What treatment is recommended and how can I prevent this problem in the future?

Answer:  Low cloud point is an indication of a high level of organic material (contamination) that has found its way into the plating solution.  Proprietary additives for acid zinc plating are typically formulated with surfactants and/or wetting agents that have high cloud point properties, which prevent the additives from “oiling out”.  When the cloud point falls below a given standard, (somewhere between 120° to 150°F), there is an indication of organic contamination which can interfere with the proprietary additives.  As your plating solution’s temperature approaches the cloud point, organic contamination becomes insoluble in the plating solution.  This insoluble material can coat the surface of the zinc deposit or even be co-deposited with the zinc, causing a white stain or a cloudy deposit.

A good way to improve/raise cloud point is to treat the plating solution with potassium permanganate.  A slurry of potassium permanganate and water is poured evenly over the top of the plating solution, using two pounds potassium permanganate per 1,000 gallons of plating solution.  The plating solution should be allowed to mix for at least one hour and filtered before plating is resumed.  Be sure to wear goggles, gloves and other protective clothing when handling potassium permanganate.  It will likely be necessary to add small amounts of proprietary additives back to the bath for optimum brightness. 

While permanganate treatments are effective at improving/raising cloud point, it is important to eliminate the source of contamination.  The most common culprit are overworked or spent cleaning solutions which cause oils and other organic contamination to be dragged down the line into the plating tank.   Increasing pretreatment rinse water flow rates can help, but dumping and making up new cleaners and then maintaining an appropriate dump/makeup schedule is the most effective way of preventing cloud point and staining problems.

 

Rack Marks on Tubes in Alkaline Zinc

Question: I manufacture and finish conduit tubes for the electric industry.  The conduit is roughly 10 feet long and will have various diameters depending on the customer requirements.  We use alkaline non-cyanide zinc electrolyte for optimum plate distribution and thickness over the entire 10 foot piece.  Recently we have run into a plating issue on the conduit area where the rack hook meets each individual piece.  We are seeing a very dull/dark mark with no zinc thickness, it looks as though the piece has been burnt by the electrical contact point.  Reducing the current only makes the dull/dark mark larger.  Increasing the current does show improvement, but we are afraid we will burn or over-plate the work.  Any thoughts on what can be done to improve the plating quality.

Answer:  The situation you describe is actually a fairly common problem that can occur in high output alkaline non-cyanide zinc plating lines.  The problem, commonly called “rack marks”, is most likely caused by poor throwing power.   The area where the rack hook meets the conduit tube is typically a very low current density area.  Rack marks will worsen with reduced current and improve when current is increased. 

As always, the first step when troubleshooting a plating problem is to ensure that the part is being sufficiently cleaned prior to plate and to verify that plating bath chemistry (level of zinc and caustic soda in this case) is within the supplier’s specifications.  Levels of zinc and caustic soda affect both plating speed and throwing power.  Assuming that these check out okay, the next step is to examine the addition rates of the proprietary additives.

The most common cause of rack marks is low proprietary brightener additives. Generally, proprietary additives are added to the zinc plating solution based on amp-hours.  These additives control appearance/brightness, ductility and plate distribution of the zinc deposit.   However, in high drag-in/drag-out operations like plating tubes, it is often necessary to increase the addition rate of the additive(s) responsible for covering/throwing power.  Basically, these additives slow the zinc deposition rate in the high current density areas and increase it in the low current density areas – like where the rack meets the tube.    Increasing the addition rate of these additives will eliminate the dark mark and increase deposit thickness at the rack/tube interface, without increasing unwanted build-up of zinc on other areas of the tube.

Contact your supplier for their recommendation.  Often times, simply an increase in the main “brightener” component is all that is needed.  However in some cases, a separate additive developed specifically to improve throwing power is provided. 

 

Dull Alkaline Zinc Deposits: Organic Contamination

Question:  I am an alkaline non-cyanide job shop plater in Michigan.  We are having a plating appearance issue that does not respond to any proprietary additives.  The problem is an overall lack of brightness in the deposit.  All of the work coming out of the line just does not have the snap that we and our customers are accustomed to seeing.  Our chemical supplier has analyzed our plating solution and has indicated that all proprietary additives are within the recommended operating ranges, but that the bath is contaminated with organics.  Their recommendation has been to treat the plating solution with carbon or potassium permanganate.  These treatments are effective but require extensive maintenance on the equipment that can slow down my production.  Are there any other treatments or additives available to help with organic contamination?

Answer:  Alkaline non-cyanide zinc plating solutions that do not respond to additions of proprietary additives usually suffer from one of two things.  They either contain excessive amounts of proprietary brighteners or have organic contamination (dragged in oils and soils from the pretreatment cleaning cycle).  Since an analysis of the plating solution did not show a high concentration of proprietary brighteners, let’s assume that the problem is caused by organic contamination.

Although treatments with potassium permanganate and carbon are extremely effective, they can be harsh on the maintenance crew.  Both are messy propositions with alkaline plating solutions, clogging up filters and taking away from production time.  A great alternative is treating the bath with commercial grade sodium hypochlorite.  Sodium hypochlorite is a strong oxidizing agent and is commonly found in or known as bleach.  Treatments with sodium hypochlorite are very effective on organic contamination and do not require heavy maintenance on the filtration system or long shut-down periods.

A full treatment is considered to be 10 gallons commercial grade sodium hypochlorite (12% active) per 1,000 gallons of plating solution.  Slowly add the sodium hypochlorite over the entire surface of the plating solution.  Good solution movement is recommended to ensure adequate oxidization of the organic contaminant.  Note that sodium hypochlorite is a strong oxidizing chemical and necessary safety precautions must be followed, such as wearing appropriate eye protection, gloves and other personal protection equipment.  A minimum of one hour shut-down time is necessary for this treatment. 

A “hypo” treatment along with small additions of your proprietary additives should bring back the brightness and luster that you expect from your alkaline non-cyanide zinc bath.