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Treatment of chromates and black oxide
preserve bondability or solderability is probably a good rule of thumb. At ACSIST, we suggest to customers who are unsure what they want 200 p-in. minimum nickel (and 30-p-in. minimum gold) to maintain bondability. Charlie Hayes, ACSIST Associates, Minn.
STEAM OR HOT WATER HEAT? Do you use/prefer steam heat or hot water? Steam is noisier, and the boilers seem quite a bit more expensive. Hot water heat requires significantly bigger coils, which can be a big expense when titanium or exotic materials are required. Piping cost is probably equal, efficiency pretty nearly the same I’d guess. How about safety? I would think that if you keep the temperatures low and make sure the piping is big so that high pressures aren’t required, then hot water heat ought to be pretty safe-probably a tossup with low pressure steam. Any other thoughts? Ted Mooney, Finishing Technology, N.J. I’ve looked at hot water versus steam. Hot water is okay for 150” F or less. If you’re running electroless nickel or a nickel acetate anodize seal, however, the coils become incredibly large and the pumps have to be huge. Another concern with hot water is this: If you have a leak with a steam coil, you can have bath contamination, but it will be a small amount of contamination and a low pressure leak. With hot water heating, however, water will enter your bath under high pressure. If it happens on a weekend (most people with hot water boilers keep their tanks hot all weekend), you can easily overflow your tank and the boiler’s automatic water feed will keep the water coming. Ken Rosenblum, Co-operative Plating, Mimi. Thanks, good points. I’ve usually used steam myself. Hot water seems to be gaining popularity around here though. Ted Mooney, Finishing Technology, N.J.
TREATMENT OF CHROMATES AND BLACK OXIDE
Circle 157 on reader information card
114
The City of Albuquerque’s waste minimization program is a free nonregulatory program that offers technical assistance to businesses that request it. Our concerns are primarily with liquid waste that may be discharged into the collection system and harm the wastewater treatment plant. The waste minimization unit has been subscribing to Metal Finishing magazine for the last two years and has found the “Hotline” column extremely valuable. Recently, a plating shop requested our services. Their problem is that they were using a black oxide bath to raise the pH of their chromate bath and convert the hexavalent chrome to trivalent chrome. The result was a cloud and an “offensive odor.” The plating shop uses the black oxide plating bath as a super-aggressive copper oxidizer and has several military contracts. We have a few questions we hope you might be able to answer for us. When the black oxide bath was used to convert the chromium bath, a cloud was emitted. If it was a gas, what METAL FINISHING
??
JUNE 1995
was its composition? We have information about alternative methods of treatment for hexavalent chrome or substitutions for hexavalent chrome, but we do not have any information on how to treat black oxide or any substitutions for black oxide. We would appreciate any assistance you can give us with this problem. Brynda Gutierrez, City of Albuquerque, N.M. I believe that chromating baths usually contain from about l-5% chromic acid, plus sulfuric acid sufficient to hold the pH in the desired range, and a little bit of complexed cyanide as an accelerator. There may be proprietary ingredients in small amounts, but in my experience, there is nothing in a chromate bath that causes a cloud or an “offensive odor” when it is treated. So my theory is that the black oxide bath contains an ingredient that releases this gas and forms this cloud when the pH goes acidic from the addition of the chromate solution or acids that may have been premixed with it before the mixture meets the black oxide. There are many formulations of blackening baths for copper (see the Me& Finishing Guidebook). For example, some are based on ammonia, and some are based on sulfides. If the product is proprietary, I think the supplier would be pleased to tell you at least the general class of the bath. If the product was ammonia-based, I doubt that it would be an effective reducing agent for hexavalent chromium. Further, I think that people would have identified the “offensive odor” as “ammonialike” because this smell is well known. Since sulfides can be an effective agent for reducing hexavalent chromium to trivalent, my guess is that the plating shop is using a sulfide-based black oxide bath, and that the component released was sulfur dioxide gas. Sulfur dioxide is highly toxic, with a sharp odor that the uninitiated probably cannot ascribe. Note that this gas will also be released from the more conventional chromium treatments, such as sodium bisulfite, if the pH is low (about 2.5 or below). The lower the pH goes, the faster it will evolve. Obviously though, hard facts are far superior to wild guesses-so keep trying to learn what you can about the incident! I cannot suggest a treatment strategy for the black oxide without knowing what class of chemical it is; but, in general, bringing the pH to a satisfactory range for discharge to the sewer system and separating out the solids is sufficient for baths that do not contain cyanide, hexavalent chrome, or powerful chelating/complexing agents. In the case of chelating/complexing agents, it is not usually them per se that cause the problem, but their capacity to keep metals such as copper from precipitating. For safety, keep the pH of anything that contains sulfides above pH 2.5, higher if possible, and try to keep it dilute. Ted Mooney, Finishing Technology, N.J. Ted Mooney is an independent consulting engineer based in Kinnelon, N.J.
ME ‘TAL FINISHING
. JUNE 1995
STEAM OR HOT WATER HEAT? Do you use/prefer steam heat or hot water? Steam is noisier, and the boilers seem quite a bit more expensive. Hot water heat requires significantly bigger coils, which can be a big expense when titanium or exotic materials are required. Piping cost is probably equal, efficiency pretty nearly the same I’d guess. How about safety? I would think that if you keep the temperatures low and make sure the piping is big so that high pressures aren’t required, then hot water heat ought to be pretty safe-probably a tossup with low pressure steam. Any other thoughts? Ted Mooney, Finishing Technology, N.J. I’ve looked at hot water versus steam. Hot water is okay for 150” F or less. If you’re running electroless nickel or a nickel acetate anodize seal, however, the coils become incredibly large and the pumps have to be huge. Another concern with hot water is this: If you have a leak with a steam coil, you can have bath contamination, but it will be a small amount of contamination and a low pressure leak. With hot water heating, however, water will enter your bath under high pressure. If it happens on a weekend (most people with hot water boilers keep their tanks hot all weekend), you can easily overflow your tank and the boiler’s automatic water feed will keep the water coming. Ken Rosenblum, Co-operative Plating, Mimi. Thanks, good points. I’ve usually used steam myself. Hot water seems to be gaining popularity around here though. Ted Mooney, Finishing Technology, N.J.
TREATMENT OF CHROMATES AND BLACK OXIDE
Circle 157 on reader information card
114
The City of Albuquerque’s waste minimization program is a free nonregulatory program that offers technical assistance to businesses that request it. Our concerns are primarily with liquid waste that may be discharged into the collection system and harm the wastewater treatment plant. The waste minimization unit has been subscribing to Metal Finishing magazine for the last two years and has found the “Hotline” column extremely valuable. Recently, a plating shop requested our services. Their problem is that they were using a black oxide bath to raise the pH of their chromate bath and convert the hexavalent chrome to trivalent chrome. The result was a cloud and an “offensive odor.” The plating shop uses the black oxide plating bath as a super-aggressive copper oxidizer and has several military contracts. We have a few questions we hope you might be able to answer for us. When the black oxide bath was used to convert the chromium bath, a cloud was emitted. If it was a gas, what METAL FINISHING
??
JUNE 1995
was its composition? We have information about alternative methods of treatment for hexavalent chrome or substitutions for hexavalent chrome, but we do not have any information on how to treat black oxide or any substitutions for black oxide. We would appreciate any assistance you can give us with this problem. Brynda Gutierrez, City of Albuquerque, N.M. I believe that chromating baths usually contain from about l-5% chromic acid, plus sulfuric acid sufficient to hold the pH in the desired range, and a little bit of complexed cyanide as an accelerator. There may be proprietary ingredients in small amounts, but in my experience, there is nothing in a chromate bath that causes a cloud or an “offensive odor” when it is treated. So my theory is that the black oxide bath contains an ingredient that releases this gas and forms this cloud when the pH goes acidic from the addition of the chromate solution or acids that may have been premixed with it before the mixture meets the black oxide. There are many formulations of blackening baths for copper (see the Me& Finishing Guidebook). For example, some are based on ammonia, and some are based on sulfides. If the product is proprietary, I think the supplier would be pleased to tell you at least the general class of the bath. If the product was ammonia-based, I doubt that it would be an effective reducing agent for hexavalent chromium. Further, I think that people would have identified the “offensive odor” as “ammonialike” because this smell is well known. Since sulfides can be an effective agent for reducing hexavalent chromium to trivalent, my guess is that the plating shop is using a sulfide-based black oxide bath, and that the component released was sulfur dioxide gas. Sulfur dioxide is highly toxic, with a sharp odor that the uninitiated probably cannot ascribe. Note that this gas will also be released from the more conventional chromium treatments, such as sodium bisulfite, if the pH is low (about 2.5 or below). The lower the pH goes, the faster it will evolve. Obviously though, hard facts are far superior to wild guesses-so keep trying to learn what you can about the incident! I cannot suggest a treatment strategy for the black oxide without knowing what class of chemical it is; but, in general, bringing the pH to a satisfactory range for discharge to the sewer system and separating out the solids is sufficient for baths that do not contain cyanide, hexavalent chrome, or powerful chelating/complexing agents. In the case of chelating/complexing agents, it is not usually them per se that cause the problem, but their capacity to keep metals such as copper from precipitating. For safety, keep the pH of anything that contains sulfides above pH 2.5, higher if possible, and try to keep it dilute. Ted Mooney, Finishing Technology, N.J. Ted Mooney is an independent consulting engineer based in Kinnelon, N.J.
ME ‘TAL FINISHING
. JUNE 1995