Q. I have a customer processing large, high-speed gears (2–3 ft diameter), by ultrasonic cleaning, wiping, rinsing in a tap water tank, then 30-sec immersion in a nitric acid bath (1.02 specific gravity at ambient temperature), rinsing, then into an HCl bath (1.01 specific gravity), rinsing, then into another alcohol/water rinse to dry, then rust preventative oil. These are stored for 2–3 months before spray cleaning at seven bars and twelve min, followed by a water based rust preventative and blow off.
Question: The nitric acid bath generates a dark smut on the cast iron gear that will wipe off to the touch. How can I remove it in my operation? For example, a highly alkaline/chelated cleaner, or are there new methods that will work? These washers are of a steel composition.
Short of asking to change the etching process, which they use to check for cracks on the gears, how can we supply a smut-free gear? Appreciate your help. J.H.
A. Thanks for the detailed description of your process. It is always easier to troubleshoot with the additional information. It is not uncommon to find residual smut on the surface of steel or cast iron following an acid cleaning process. That is primarily carbon, which is added to steel and cast iron to alloy and strengthen the material. Since the carbon is more inert than the surrounding iron matrix, it will be left behind following the etching process.
Since cast irons have substantially more carbon in them, it also makes sense that they will have that much more smut following an acid cleaning process. In my experience, that residual smut layer can be extremely difficult to remove chemically and generally requires some sort of additional physical force to remove it.
I noticed from your process description above that you are already applying a manual wiping process to the gears following the ultrasonic cleaning process. Could it be as simple as moving that existing operation from its current process step further down the line so it is done following the nitric acid exposure? To me, the most logical place for this would be after the rinse following the nitric acid step.
If that is not an option, I would recommend experimenting with the use of your ultrasonic tank with either the nitric acid tank or the following rinse tank rather than in the cleaning process step. That has the potential to either prevent the deposition of the smut on the surface, or help remove it in the subsequent rinsing step.
If that were not to work, the last step I would suggest would be electrocleaning of the gears. This is a method commonly employed to clean steel components prior to plating. There are two types of electrocleaning, one where the workpiece is the anode in the circuit which generates oxygen gas bubbles at the surface of the part, assisting in the lifting and removal of soil and particulate. The other mode of electrocleaning is where the part is made the cathode in the circuit which generates hydrogen bubbles at the surface of the parts. This method may be considered more efficient for soil removal since twice as much hydrogen is generated at the surface than oxygen in anodic cleaning, however, has a major disadvantage with steel. The hydrogen generated at the metal interface can cause hydrogen embrittlement requiring an additional elevated temperature bake of about 300-400°F for about an hour to effectively remove that condition. For that reason, anodic electrocleaning is the method of choice when cleaning steel.
The methods above are listed in the order of increasing complexity and capital cost, but also of increasing effectiveness.
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.
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