EN Plate-Out on Equipment
Q. We run a low-phosphorous electroless nickel bath and recently pulled quite a few pounds of nickel pellets from our tank after draining. How do we prevent this kind of plate-out?
A. “Plate-out” or autocatalytic decomposition of an electroless nickel (EN) bath can happen for a variety of reasons, including insufficient passivation of the plating equipment or tank, excessively high solution temperatures or solution pH, contamination, inadequate filtration, and improper heating devices, to name a few.
Proper passivation is important in order for the plating solution to remain stable, and many facilities passivate their tanks every day or at least after every metal turnover (MTO). Passivation is performed using 40- to 50-percent, by volume, nitric acid, or a mixture of sulfuric acid with hydrogen peroxide if nitrate contamination is a concern. An MTO is reached every time the amount of nickel required at make-up, say 6 grams per liter (gpl), is added back to the tank. For example, if you are using a 1,000-liter tank, one MTO is achieved once you have replenished a total of 6,000 grams of nickel (1,000 liters × 6 gpl).
A typical operation employs two tanks for EN to avoid any interruptions in the operation. One tank is stripping while the other is in production. Tank stripping usually takes about 12 to 24 hours, and strippers should be replaced when the nickel concentration gets too high or the stripping rate greatly reduces. Tank walls made of stainless steel should be anodically protected during plating using rectification to prevent plate-out. A thick drop-in liner can also be installed in these tanks after it’s been properly leached to avoid introducing harmful organic products to the bath or if the tank quality has degraded and no longer passivates adequately.
Temperature and pH. Temperature and pH are critical for driving EN deposition reactions. Baths become overactive and unstable when both temperature and pH are increased beyond their acceptable ranges leading to “plate-out.” Localized overheating is common with poorly agitated solutions, so it is recommended that agitation surrounding any device emitting heat be appropriately vigorous. Over-additions of pH regulating chemistries like ammonium hydroxide can lead to high pH levels.
Particles and Impurities. Metallic flakes or fines often enter the bath via corroding metallic features of the plating line and are not always immediately filtered out, often leading to bath decomposition. Nitrate contamination from residual stripping solution can consume stabilizers especially those with sulfur-based compounds often used in low-phos baths. Sometimes nitrates, and also silicates from cleaners, can act as stabilizers themselves and cease deposition altogether. Copper contamination, often generated from electrocleaning bus bars and nickel strike tanks, can also interfere with stabilizers.
The accumulation of insoluble compounds from impure or hard city water (high levels of calcium and magnesium) can lead to instability of the plating solution. Older plating solutions have accumulated large concentrations of orthophosphite, an inevitable byproduct of all hypophosphite-reduced baths, which will create a decomposition chain reaction. Inhibitors like thiourea, lead acetate, lead sulfide, thiosulfate, molybdic acid, sulfur, and thiocyanate, among others, are all catalytic poisons (stabilizers) that are added to prevent this sort of catastrophic chain reaction.
Filtration and Agitation. Well-filtered solutions are those that run through 1-micron-pore filters 10 times/hour (10 turnovers). Filters are designed to catch any foreign particulate that may cause plate-out. Agitation is important to provide parts with fresh solution and move contaminants to the filtration devices. Too much agitation can increase the activity of the stabilizers, leading to potentially reduced plating speed, however, a sufficient amount is necessary to avoid solution stratification or “hot spots” and thus potential decomposition.
Equipment. Depending on what type of equipment used, the potential for plate-out will differ. Many EN platers use electropolished stainless steel or teflon-coated heaters to reduce this potential. Quartz-sheathed heaters are often prone to plate-out after exposure to nitric, and therefore are rarely recommended. Baths with poor agitation tend to use de-rated immersion heaters to circumvent issues associated with localized heating. Panel coils are also highly prone to plate-out and often create hot spots between the coil and the tank wall, causing decomposition. Teflon tubes almost completely resist plate-out, but they are incredibly inefficient as heaters and tend to take up a lot of valuable tank space.
Any of these issues could have been the reason why the aforementioned applicator experienced such a substantial level of decomposition. The subsequent root cause investigation points to poor and infrequent passivation of their tank and equipment.
We found that the use of a weak nitric-acid solution was the single biggest reason for their plate-out issue.
Adam Blakeley is a technical service representive for MacDermid Enthone. Contact him at firstname.lastname@example.org.
Originally published in the January 2017 issue.
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