Q: We are a job shop handling electroless nickel and zinc. The rinses from the cleaner and pickle are recirculated. Our system consists of a counterflowing arrangement, which passes into a sump, through a bag filter and pH adjustment, then carbon filtration and ion exchange. This works for some time and then the water becomes gray and cloudy. Can you offer any suggestions'?
A: Cleaners often contain wetting agents that react with acids to form oily compounds, which can form cloudy emulsions. The carbon is probably picking up some of this material. It is suggested that you recirculate the acid and cleaner rinses separately. More frequent maintenance of the carbon filters should also help.
Q: We barrel plate a steel bearing that is copper flashed and has Teflon coated on the inside diameter before it gets to us for tin plating. The bath is an alkaline stannate type in an automated line with low make up because we're only required to put on a quick flash. Star-shaped anodes are used with bath conditions of 170°F, 5 V, 100 A/barrel. The procedure is electrocleaner, double cold water rinse, weak muriatic acid, triple cold water rinse, tin flash. drag out rinse, double cold water rinse. hot water rinse, and spin dry.
This system has worked for many years, but now we're getting a white stannate residue on the Teflon. It's like a powder. We tried shocking the bath with hydrogen peroxide. This oxidizes it and gets rid of the residue, but it's now an almost daily requirement with the peroxide and we are also using a lot of potassium hydroxide. Any suggestions?
A: The formation and maintenance of a proper anode film is critical to the operation of a stannate bath. When the tin anode dissolves during plating two possible reactions are possible: formation of tin (iv) or tin (II). The tin (IV) forms a yellow-green oxide film on the anode, which is desired. The tin (II) forms a black oxide film. Hydrogen peroxide is added to convert tin (II) to the desired tin (IV).
Addition of hydrogen peroxide should be considered a band-aid approach. The fact that hydrogen peroxide addition is solving y our problem, at least on a temporary basis, indicates that tin (II) formation is the real culprit in your case. It is essential to eliminate the cause of the tin (II) formation.
You must check the anodes during operation of the bath to see if the proper film is being formed. When they are covered with a bright yellow or green film, with the current on, then tin (IV) is being dissolved into the bath. If the anode is not the proper color then you will have to film the anodes. If the anodes are light and easily handled, this can be done by placing the anodes in the tank one at a time with the current on. The current will rise rapidly and then slow as the film is formed on each anode in turn. Film formation requires an initially higher current density. Note that the film will dissolve when the bath is idle.
It's possible that your bath has too high an anode area for the film formation to occur normally. Try replacing the star anodes with slabs. The proper anode current density range for different anode materials will be found in the 1996 Metal finishing Guidebook and Dictionary Issue in Table X on page 300.
Q: I'm looking for information regarding water quality for the anodizing process. Are there any guidelines for makeup water. rinses, etc. regarding chloride content'?
A: In their book "The Technology . of Anodizing Aluminum," Brace and Sheasby suggest that the chloride content of sulfuric acid used for anodizing not exceed 30 ppm. They cite a British standard (DEF 151), which limits chloride in the anodizing bath to the equivalent of 0.2 g/L sodium chloride. Slightly above this limit can produce pitting.
Kliment, in a paper entitled "Impurity limits in hot water for sealing anodized aluminum" published in the Transactions of the Institute of Metal Finishing, volume 64, pages 160-162, 1986, set as a critical limit a value of 1,000 ppm of chloride as an impurity for hot water sealing baths. He recommended use of deionization to control impurities.
Q: We are not getting proper deposition in the low current density regions for cyanide zinc barrel plating, particularly at higher temperatures (5060°C). Our bath composition is as follows: l0-12 g/L zinc, 20-25 g/L sodium cyanide, and 40-50 g/L sodium hydroxide. Why does this happen?
A: Higher bath temperatures . breakdown cyanide and addition agents more rapidly, which leads to poor coverage. The effects of higher bath temperature can be compensated to some extent by increasing the ratio of sodium cyanide to zinc metal. Geduld, in his book "Zinc Plating." quotes the following recommendations for a mid-cyanide bath: at 25°C total sodium cyanide to zinc ratio should be '.', while at 46°c the ratio must be increased to 3.0. Temperatures of 50 to 60°C are not recommended as the results are marginal and addition agent cost will be more than four or five times higher than operation at normal temperature ranges.
1. Premature Rusting on Wrought Iron Furniture
Q: Our company strips and repaints wrought iron casual furniture. After stripping (some of the furniture is 30 years old), we conventionally prime and electrostatically paint it. The pieces look beautiful; however, 4 weeks after delivering the furniture to the customer, we have rust in the cracks and crevices where the primer would not seal. We are looking into painting our pieces with a dipping process. We are not sure what to use or how we should use it to prevent rust. What do you suggest without having to refinance the farm?
A: After stripping, it is most important that all the old paint is cleaned from the cracks and crevices. Any old coating, even the slightest amount remaining in a crack, will act as a wick and absorb moisture and cause premature rusting through the new primer and topcoat. It may require the use of a wire brush to remove the old coating totally. Let's assume a piece 30-years old may have several layers of coating; there could be a mixture of alkyd enamel, epoxy or epoxy ester, and maybe a coat of latex house paint. It would require a very powerful paint stripper to remove such a mixture of different kinds of paint.
Rather than spraying the primer with conventional equipment, try using high-volume low-pressure (HVLP) equipment. Combining the equipment and a low viscosity primer with good wetting, low surface tension will provide good flow into cracks and crevices. The HVLP spray gun will minimize presetting the primer causing bridging across the cracks as with high-pressure atomizing air.
Inspect the dried primer coat to be sure it forms a continuous layer over the entire surface. It may be necessary to keep the spray gun or at least a brush to touch up any suspect areas before applying the topcoat. Excessive roughness in the dried primer should be smoothed out with fine sandpaper or a Scotch Brite pad.
Dip coating in my opinion is not a practical process for a refinishing shop unless there is a large number of pieces painted within a few days. Dipping requires a tank large enough and a volume of primer to cover the piece when submerged in the tank. Dip coatings need large volumes of solvent and are sensitive to temperature. Unless the coating has excellent flow, it will leave heavy drips and fat edges; it is also prone to bubbling and short-time tank stability. With the electrostatic topcoat spray, use a crisscross spray pattern; it will assure good overall coverage.
2. Solvent for Electrostatic Spraying
Q: What type of solvent is recommended for electrostatic spray of an acrylic amine baking enamel?
We are finishing metal door frames with a self-etch primer and a universal primer/ surfacer and an acrylic melamine baking enamel topcoat. Presently, we use conventional air spray but intend to change to electrostatic spray. Please suggest the recommended solvent to adjust the product for electrostatic application to assure maximum transfer efficiency.
A: Acrylic-modified melamine baking enamels will usually adapt to good electrostatic spray application. The most important solvent adjustment will be to slow down the solvent evaporation and adjust the conductivity and polarity of the coating. The reduced viscosity recommended for electrostatic spray is between 30 and 60 cp. Reduce the coating with aromatic hydrocarbon SC150/Solvesso 150, which is a slow-evaporating, low-polarity solvent.
The equipment suppliers can provide a range of electrical resistance recommendations for the coating material. If lower electrical resistance is required, add I or 2 fluid ounces per I gallon of coating of diethylene glycol monoethyl ether, which has high polarity and is also slow evaporating.
To maximize transfer efficiency, there are two adjustments with electrostatic spray application: KV power output in the equipment; and electrical resistance of the coating material.
The electrical resistance is measured with an instrument provided by the coating manufacturer.
Q: We are painting galvanized steel fabricated door frames where there is some spot welding and regular welded joints. We cannot use aqueous cleaners and have to clean 150 frames per day for painting.
Can you suggest a low-toxicity. nonflammable organic solvent (chlorinated or aromatic hydrocarbon) for cold cleaning application along with operating instructions?
A: Your request for solvent cleaning using chlorinated and aromatic type solvents is not a practical choice at this time. Chlorinated ozone-depleting solvents will soon be banned from commercial use. If solvent must be used, your choices are limited.
A solvent that is flammable but has a high flash point is aliphatic naphtha. which has good cleaning properties and reasonable evaporation. You stated that aqueous cleaners cannot be used, but how about using high-pressure steam cleaning with a degreasing detergent.