In metal finishing operations, conversations about process control often begin with two familiar variables: time and temperature.
Connor CallaisYet, according to Connor Callais, product manager at Hubbard-Hall, a third factor is equally critical to successful conversion coating performance—chemical concentration. While operators and engineers may focus heavily on dwell times and bath temperatures, maintaining proper chemical control and implementing the right post-treatment strategy can make the difference between consistent, high-quality coatings and costly quality failures.
During a recent discussion about conversion coatings and immersion processing, Callais emphasized that successful bath management depends on balancing all three process variables.
“There is time and temperature, but the third variable that is crucial for all of these immersion processes is the actual concentration of the chemistries that we’re using,” he says.
For manufacturers running cleaners, phosphate systems, black oxide processes, or other conversion coating operations, that concentration control is not merely a laboratory exercise. It directly influences coating appearance, coating weight, corrosion resistance, and process consistency.
Chemical Control: The Often-Overlooked Variable
Every immersion process is designed to operate within specific chemical parameters. Whether a facility is running an alkaline cleaner, an iron phosphate line, or a zinc phosphate process, maintaining the correct concentration is fundamental.
“Whether that be making sure our cleaner is at 10% by volume or our phosphate solution, we’re going to be monitoring the total and the free acid for it or the dissolved iron,” says Callais. “All of these chemical applications have parameters for the concentration at which they need to be used.”
Those parameters are typically verified through routine testing. According to Callais, Hubbard-Hall generally recommends that operators check process chemistry at least twice per shift.
“With the actual conversion coating, if we don’t maintain the right parameters, we can either drift in our coating weights if we’re applying a phosphate, or we end up with over-etching of the parts if our free acid gets too high.”
“Every few hours, an operator is going to pull a solution of those chemistries and do titrations on the side, some actual bench work chemistry to make sure that we’re staying within those parameters that we need to be at,” he says.
While titrations may seem like a routine maintenance task, they provide valuable insight into whether process chemistry is drifting out of specification. Without regular testing, operators may not realize a bath has deviated until coating defects begin appearing on finished parts.
Why Concentration Matters
The purpose of chemical monitoring extends far beyond maintaining paperwork compliance. Proper concentration control directly affects both cleaning effectiveness and coating performance.
“One ensures consistency in the coatings and the colorings that we’re going to be actually applying,” Callais says. “We want to maintain our cleaners to make sure we’re properly and effectively cleaning the parts adequately every time.”
Cleaning represents the foundation of virtually every finishing process. If soils remain on the substrate, downstream conversion coatings may develop unevenly or fail to meet performance expectations.
The conversion coating stage itself is equally sensitive to chemical balance.
“With the actual conversion coating, if we don’t maintain the right parameters, we can either drift in our coating weights if we’re applying a phosphate, or we end up with over-etching of the parts if our free acid gets too high,” Callais says.
“Conversely, if we start operating it and it’s starting to boil up into a higher, let’s say 290,° for example, we know our solution’s kind of over-concentrated at this point.”
Those consequences can have significant downstream implications. In phosphate applications, coating weight variations can affect paint adhesion and corrosion performance. Excessive acidity may damage substrates or create inconsistent surfaces that interfere with subsequent operations.
Because of these risks, successful operations develop standardized testing schedules and ensure operators understand both the procedures and the reasons behind them.
Black Oxide: A Different Approach to Bath Control
While titration is common for many conversion coating processes, black oxide presents a unique situation.
“The one caveat is that with more of our metal colorings, we don’t really do a titration chemical control on those because they’re such a concentrated caustic solution,” Callais says.
Instead, black oxide operators commonly monitor bath concentration through boiling-point control.
“For black oxides, we want to maintain a certain boiling point, and that boiling point helps us determine what the actual concentration is,” he says.
The concept is straightforward. Because the chemistry has a predictable relationship between concentration and boiling temperature, operators can use boiling-point measurements as a reliable process-control tool.
“Let’s say we have a boiling point that we need to be set at 286°, for example,” Callais says. “If the solution is boiling at a lower temperature, we know, ‘Hey, we have too much water in our system,’ because water is going to depress that boiling point.”
The reverse is also true.
“Conversely, if we start operating it and it’s starting to boil up into a higher, let’s say 290,° for example, we know our solution’s kind of over-concentrated at this point,” he says. “And we need to carefully introduce more water into that bath to maintain that concentration adequately.”
This approach provides a practical way to verify concentration without relying on traditional titration methods.
Callais also addressed a common misconception among operators.
“A lot of times people will say, ‘Well, can’t you just heat it,’ and it’s not necessarily going to give you a true representation of it,” he said.
The reason lies in the physics of boiling.
“With boiling points, you’re never going to be able to increase the solution’s boiling point,” Callais says. “The more heat you put into it, the more it will be kept at a higher temperature until water is driven off. So that’s how we can guarantee the concentration we’re at based on the boiling temperature.”
Bath Control Beyond Chemistry
Although chemical concentration is essential, Callais emphasized that bath control requires a holistic approach.
“With any chemical application, we always go back to the time, temperature, and the concentration,” he said.
Time is often the easiest variable to manage. In many facilities, operators follow documented procedures, or automated systems handle dwell times through programmed recipes.
“Time tends to be pretty general because either you have an operator that’s going to be following an SOP for a certain part,” Callais says. “Or it could be automated to where you have a recipe or some sort of prescribed amount of time that’s going to be spent in each step.”
Temperature, however, frequently presents challenges.
“Making sure that the temperature that we have the solution set at is kind of what it needs to be at,” he said, is an important aspect of bath management.
Improper temperatures can undermine process performance at multiple stages.
“Particularly with cleaners, they’re set too low, and you’re not able to get the effective cleaning out of them,” Callais said. “Or in the actual conversion step, sometimes that temperature has deviated away.”
Many finishing systems rely on digital temperature displays, but those displays are only as accurate as their sensing equipment.
“Generally, we’re going to find that users or applicators will have some sort of digital temperature readout, but we also need to make sure that the actual thermocouple readout stays in range,” he said.
“You want to make sure that you’re able to augment those properties by sealing in that change appropriately.”
Routine calibration is therefore critical.
“Checking the calibration of those thermocouples at least every week is important, because you’ll be surprised how often we go into a place and everything looks to be in the right temperature range,” Callais said. “Still, when you go back and check it, the readout’s 10° lower than it should be.”
That seemingly small discrepancy can significantly affect chemical reactions, cleaning efficiency, coating formation, and overall production consistency.
The Critical Role of Post-Treatment
Even when conversion coatings are applied correctly, the process is not complete once parts leave the coating bath. Post-treatment selection often plays a major role in determining final corrosion performance.
“Most chemical application processes, particularly with conversion coatings, involve doing the work to either add a crystal onto the surface of that part or convert the surface to achieve some sort of metal coloring,” Callais says.
After creating that protective layer, manufacturers must preserve and enhance it.
“You want to make sure that you’re able to augment those properties by sealing in that change appropriately,” he said.
Many conversion coatings provide inherent corrosion protection, but additional treatments can substantially improve long-term performance.
“Many of these applications will enhance the part’s corrosion protection on their own,” Callais says. “Still, you can really optimize that by applying some sort of corrosion protection after the fact.”
Choosing the Right Post-Treatment Strategy
The appropriate post-treatment depends heavily on the part’s end-use requirements.
For applications requiring enhanced corrosion protection and appearance, water-displacing oils remain a popular option.
Callais described these products as systems “where the oil seeps in and displaces any water on the surface of the parts.” He added that the treatment “both enhances the corrosion protection and improves the visual appearance of the part.”
Water-soluble oils offer another path.
“You could use a water-soluble oil, which leaves more of a dry or matte finish, which is often used in the automotive industry,” he said.
In other cases, the focus may be short-term protection between manufacturing steps.
“Sometimes you just need some short-term protection because that part’s going to go on to a painting process or a more tenacious, hard coating that’s going to be applied on top of it,” Callais explained.
For those situations, water-based sealants can provide the necessary interim protection without interfering with downstream finishing operations.
Maintenance Matters
Selecting the correct post-treatment chemistry is only part of the equation. Maintaining those systems properly is equally important.
“With the water-displacing oils and the water-soluble oils, they tend to require more qualitative maintenance of the product,” Callais said.
One of the most important maintenance practices involves removing displaced water.
“With the water-displacing oils, you want to make sure that we are decanting off the water that’s displaced,” he noted. “Typically, it’s going to be down at the bottom of the tank.”
Ignoring that maintenance step can create serious problems.
“If you let it build up too much, you’re going to run into an issue where it starts to impact the area of that tank where the parts are going to be immersed and basically negates all the properties that you’re trying to use,” Callais warned.
He recalled a real-world example involving a black oxide customer.
“We had a customer who was using a water-displacing oil after their black oxide operation, but they just weren’t paying attention to decanting the water,” he said.
Because the incoming water was highly alkaline, contamination gradually accumulated.
“Over time, they just built up a really caustic aqueous layer that ended up mixing in with the oil,” Callais said.
The consequences were costly.
“At that point, there’s nothing you can do because you just wasted a really expensive oil product,” he says. “They basically have to waste the whole thing because all the waxes that are in that product will actually crash out. You really can’t get them back into solution at that point.”
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