In-House Passivation: Questions Asked and Answered

As Jayco Cleaning Technologies can attest, more machine shops are looking to bring passivation of stainless steel parts in-house for better quality control, lower costs and shorter lead times. But, they have questions given that a finishing process such as this, which removes free iron from the surface of stainless steel parts, often isn’t their core competency. Machining is.

Conversely, the manufacturers of these systems — systems which could be designed to perform nitric acid passivation, citric acid passivation or both — will have questions for them, too.

Per the latter, passivation line manufacturers such as Jayco Cleaning Technologies will ask to what standards is a shop passivating. This is largely driven by the shop’s customers. One common standard is ASTM A967, and there are different requirements for nitric and citric processes within that standard.

It’s also important for passivation line suppliers to understand how parts will be delivered to the system. Parts might be delivered after a cleaning step or straight off a machine tool. An initial wash station will likely be required prior to passivation for parts coming straight off a machine as they will be covered in coolant and/or chips and swarf.

Size of parts to be passivated must also be considered. Most passivation lines are designed to accommodate the largest part requiring passivation. That said, if a shop has only one big part of many requiring passivation, it might make sense to continue outsourcing passivation of that part rather than paying more to size the in-house line to accommodate it. Size wise, shops should go by the 80/20 rule and not let one large part drive the design (and cost) of an entire passivation line.

Perhaps, most importantly, passivation system manufacturers will need to know if a shop is considering nitric, citric or both. Again, this is largely driven by customer requirements.

Nitric acid is much more corrosive than citric acid, the latter having acidity akin to orange juice. Jayco Cleaning Technologies says studies have shown that both processes produce the same results, but industries such as aerospace often still require nitric passivation. There have been instances in which shops have gotten approval from customers to switch from nitric to citric. Medical and general industrial segments are becoming more accepting of citric passivation.

For new customers, Jayco Cleaning Technologies suggests that shops push for citric passivation as it is a safer process. Plus, citric passivation can be a manual process for shops with budget constraints, but the company still recommends automation. Automation is certainly required for nitric passivation.

What Shops Want to Know

Shops considering bringing passivation in-house often have more granular questions. Examples include:

1. Can one system perform both nitric and citric passivation? It is possible to include both nitric and citric in one line? However, this does make the line’s footprint larger, because there needs to be a dedicated rinse station for nitric and another for citric. They can both share the final rinse station, which is commonly monitored using a conductivity probe that will signal fresh water to be injected into it if the bath conductivity exceeds maximum limits. A vacuum drying station is recommended for faster drying.
2. Is external ventilation for fume extraction needed? Nitric tanks do require ventilation, possibly followed by an air scrubbing process to remove nitric gas. Shops must consult with their local municipalities to determine if an air scrubber is necessary.
3. How do you monitor acid usability? Nitric acid bath concentration is determined by measuring its specific gravity, adding more nitric acid if the level is out of range or draining completely. Similarly, citric acid bath concentration is measured using a conductivity meter and, per its reading, more water or more citric acid should be added. Depending on how tight process parameters are, the citric bath might require draining after a week or month of use. These procedures are both akin to monitoring/maintaining a machine tool’s coolant concentrate levels.
4. What is the standard for removing waste acid? For nitric acid, the waste acid must be pumped out of its tank and stored in a drum to be removed by a company certified to handle it. For citric acid, it can be as simple as performing a pH balance and disposing of it as if it is common waste water.
5. How long does it take to bring a passivation system online? This varies depending on the application. Jayco Cleaning Technologies says that for conventional passivation lines, it might take 26 weeks for equipment delivery and then another week for setup and testing. The wild card is process validation, which can be time consuming depending on customer requirements.

Any Other Questions?

If passivation is a process you’ve been considering adding, are there any other questions you might have? Or, if you’ve recently brought passivation in-house, can you tell us about your experiences doing that? Email me at dkorn@productionmachining.com.