A Parts Cleaning Q&A

Sometimes, technical information is best communicated through real-life application examples. For 15 years, Dave Peterson has been authoring the bi-monthly Cleaning Clinic column for our sister publication, Products Finishing. In this column, Mr. Peterson provides answers to specific parts cleaning and pretreatment issues that readers face in their facilities. Many of the situations can be common in production machining shops as well and may be worth a look here in PM. Here are some recent questions.

Zinc Phosphate of Brazed Steel Parts

Q. We often use a zinc phosphate treatment on our steel parts (corrosion protection for oilfield parts) and have recently tried doing so on a 4140 steel part that has tungsten carbide inserts brazed to the steel with a copper-based braze alloy. Both the tungsten carbide material and the braze material surfaces are badly etched after this process. Can I assume the copper (braze) and cobalt (in the tungsten carbide) are being attacked? —M.B.

A. The zinc phosphate process is primarily only applicable to ferrous substrates. There are a few exceptions, but this is generally the case. I know that the zinc phosphate process is not applicable to copper-based alloys. I do not have experience with the tungsten carbide material that you had mentioned, but would also expect that it would not be compatible with a zinc phosphate formulation. The acidic nature of the typical bath would probably tend to attack the carbide in this alloy.

Mil-Spec for Compressed Air Quality

Q. We just completed a Nadcap audit, and all went well. However, there was this finding: “Observed: No procedure exists and no point-of-use inspection is being performed to verify the cleanliness of the compressed air supply used in the chemical processing area.” Is there a procedure or military specification (mil-spec) for doing this check so I can create a proper procedure? Also, if we started using nitrogen, would this negate the need for this test? —R.Z.

A. I cannot say that I am familiar with a specific standard for industrial compressed air quality. I did find a mil-spec (BB-A-1034) that specifies compressed air for breathing, but that may not be relevant for your needs. The ISO 8573 standard provides guidance as to the amount of contaminants allowed per cubic meter, based on about 10 quality or classes of compressed air as well as test methods for oil, water, particulate and other contaminants. As with many standards, it will be up to you or your equipment suppliers to provide specific requirements for your needs. I did find one suggestion that specified the following for paint spraying: Level 3 for particulate (maximum 5-micron particle size); Level 4 for water, which requires a maximum dew point of +7°C; and Level 3 for oil, which is a maximum of 1 mg/m3.

Burner Tube Replacement

Q. What is a good project timeline for installing a new burner tube in a pretreatment tank? What steps are required? —M.B.

A. The burner tube is generally a prefabricated metal tube (often stainless steel) that is used to contain the flame from the gas train and ignition, as well as transfer the heat to the liquid bath. First, source the burner tube so it is ready to go for the installation. That could be an item purchased directly from the original supplier of the entire system or a separately fabricated burner tube made to the same dimensions and specifications as the original one. If you are using a contract sheet metal shop to fabricate your burner tube, it could take anywhere from a couple days to a couple weeks or more, depending on the size, materials and backlog at the company you are working with. Once that is complete, I would not expect it to take more than a weekend to get the tank pumped out and cleaned, the old burner tube cut out, and the new one installed.

Titration of Nitric Acid Neutralization

Q. We use soda-ash tanks for neutralization in our nitric passivation process. We need to be able to determine the sodium hydroxide percentage (2–5 percent) in solution. Is a titration using HCl (hydrochloric acid) for the titrant and phenolphthalein as the indicator the proper method? If so, can you give me an example of the standard titration method and an example of the percentage calculation once I determine the amount of HCl required to achieve an endpoint? —D.Q.

A. From your description, I am assuming you are simply neutralizing your nitric acid passivation solution that is spent, and then trying to dispose of it, either through your wastewater treatment system or by off-site shipment. In this case, I am not sure that you need the precision of a titration and that you may be better off with a simpler pH measurement. Typically, the pH measurement is what is used to classify something as hazardous because of corrosivity.

If you really need to perform a titration, it is not clear to me what endpoint you intend to titrate to. You are using the correct chemical, sodium carbonate (a weak base), to neutralize the nitric acid solution (a strong acid). However, I am unclear what the significance is of a 2 to 5 percent sodium hydroxide equivalent, since you are using sodium carbonate for the neutralization, a much weaker base than sodium hydroxide. If you need to follow through with a titration on this bath, it will require you to make a known standard each time you need to perform this neutralization, since I would assume you will not be starting with an identical nitric acid concentration. A standard acid-base titration uses a titrant of known concentration to titrate a tank of unknown concentration. However, if you have two unknowns (the starting nitric acid concentration and the amount of sodium bicarbonate addition) you would need to first determine the concentration of the nitric acid with a standard acid-base titration.