Most everyone involved with metal finishing processes is aware of the new pretreatment technologies available. Several names have been used to identify these alternatives to phosphate-based treatments. Within this article I will use the acronym TMC, transitional metal conversion, as it describes what is on the substrate after treatment similar to using the terms iron or zinc phosphate.
There are dozens of companies that have this technology. Anyone who mixes hydrofluorozirconic and/or hydrofluorotitanic acid in water can say they have the new pretreatment technology. However, there is more to the formulation for success. What matters is performing on your line day after day. It is very important that you roll smoothly into this change. Most TMC line conversions are made without incident. However, there are situations that can create issues for a successful TMC implementation. All I am asking you is to know why you’re changing and that it is a sound business decision.
The features and benefits of the new pretreatments are hard to ignore. Who doesn’t want to reduce energy consumption, phosphate usage and washer maintenance? Increasing water discharge regulations and energy costs make TMCs very appealing. Before changing your pretreatment to a TMC, it is prudent to know your current pretreatment cost per unit as well as your first-pass efficiency. If your goal is to reduce cost, then you need to know your current costs to determine if you were successful. The cost to treat or haul away waste is also a factor. If your pretreatment change is motivated by environmental restrictions then perhaps there is no reason to take the time to calculate your operating costs.
Identify Benefits and Savings
Unless your goal is to become a “greener” neighbor or manufacturer, if there are no significant benefits or savings then why change your process? Phosphate restrictions and POTW surcharges could motivate a change in pretreatment technology. If you’re operating a wastewater treatment system, then the change to TMC will reduce and possibly eliminate your wastewater treatment costs. Newly imposed environmental restrictions may require your company to treat your wastewater. The capital investment for a wastewater treatment system might be avoided by changing to a TMC.
Cost savings are really what drive the change to TMC. Unless you are using a zinc phosphate process, the savings from switching to TMCs are usually from process-related savings rather than from the TMC product cost and consumption. Energy savings is heavily marketed and realized from eliminating applied heat to the treatment stage and potentially from reduced heating requirements from the dry off oven. The TMC treatment has a lower surface tension than phosphate surface conversions due to lower TMC coating weights requiring less heat to dry the parts.
Maintenance-related costs are generally lower for a TMC. The absence or extreme reduction of phosphate in the treatment solution significantly reduces the formation of precipitates that accumulate as sludge that makes scale. If you have particulate filtration for your phosphate system, then this cost is virtually eliminated. Washer descaling frequency will by substantially reduced and perhaps eliminated as you won’t have the sludge that forms scale.
Product – Process Research
Start your research by determining potential vendors and products. Get references—good and bad. I wouldn’t be shy to ask the hard questions of where they have had problems and how they were resolved. We’re talking about new technology and manufacturing—stuff happens. You need to find ways to shorten your TMC learning curve. This is one of the reasons I think you should ask for “bad” references. It is helpful to know what has gone wrong and how those issues were resolved. You also want to know how your vendor will respond in times of need. It may or may not relate to your situation. At the very least, the discussion can motivate questions that need addressing prior to implementation. If your risk is substantial, you should consider taking some tours.
Somewhere in the same time frame you should conduct testing of the chosen TMCs. You can use laboratory panels as control panels but should include production substrates to make sure the process is viable. If possible, treat your own parts through an existing TMC production pretreatment system. When you compare lab-prepared TMC panels to a production phosphate panels you are not only comparing the pretreatment technologies but also the washers.
You need to also research the requirements of the chosen TMC. Will the construction of your existing washer be compatible with the TMC’s chemistry? Some products require stainless steel construction and all products have better bath life when used in acid-resistant construction. Water quality is also an issue. Most TMCs will require RO or DI water. Solution control equipment and procedures should also be reviewed to make sure you have, or are capable of conducting, pertinent solution control procedures.
Successful implementation requires vendor support. Prior to implementation you need to define support for the start-up and in the following days or weeks. Generally you don’t need a representative for weeks but a plan for the vendor to check in on the process is warranted.
Once you have a viable product and process, you should conduct some training prior to charge-up. Include wash line operators as well as painters and appropriate management to familiarize them with the TMC operation as well as the treatment’s appearance. The appearance will most likely be different from what they are accustomed. There will probably be several questions that would be better addressed prior to charging up the TMC. Discuss your and your vendor’s concerns. They know their product, requirements and limitations. You know your system, requirements and constraints. These need to be fully discussed to make sure that all the necessary steps are taken to ensure a successful implementation.
Those who have used iron phosphates for a substantial time have probably seen something unusual and perhaps unexplainable despite the long history of iron phosphating. The chance for something unforeseen with TMC is possible. If there is still uncertainty about how the new technology will perform on your line, then conduct a short-term trial. Consider a 5-stage pretreatment system with the traditional alignment of cleaner, rinse, phosphate, rinse, final rinse. With some effort and a garden hose for auxiliary rinsing you can conduct a short-term TMC after a shift or on a weekend by putting the TMC in stage 4. Run for an hour or two or set a number of parts to treat, test and set the rest on the side for eventual use. In other words, try it on for size to see if it fits your operation. This will help reassure that a change over has a chance of success or whether some modifications are required. If you have multiple paint lines then perhaps you can start your testing on the smallest line or the line with the least amount of risk.
WHAT CAN GO WRONG
Water quality is very important for most if not all TMCs. Cations such as calcium, iron, and magnesium can precipitate vital ingredients of TMCs. If the water hardness isn’t too hard then you might not need it. Even if the TMC can tolerate some water hardness, there is concern about its use in the final rinse. The water cations dried on the substrate are hygroscopic and will absorb moisture to form blisters in moist environments, compromising paint adhesion which leads to corrosion. The anions such as sulfate and chloride can initiate corrosion.
Poor rinsing and excessive carry over can also create performance issues. The TMC products operate with very low solids in the treatment solution. In other words, they don’t handle contamination very well especially alkaline carry over as it precipitates the active metals in the TMC solution. Lines with insufficient rinsing from lack of stages, poor riser/nozzle performance, or short drain zones lead to quality or product usage issues. High chemical usage can also occur if the solution control procedures aren’t adequate.
Insufficient cleaning causes most finishing issues. Obviously all organic soils need to be removed. Inorganic soil removal is more important for TMCs than for phosphating. Phosphate-based surface conversions are more forgiving for metal variations simply due to the phosphate reaction etching the substrate to start the phosphate conversion. The TMCs do not etch the substrate to this extent to produce a surface conversion. If there is a thin layer of organic or inorganic soil, the phosphate etching will undermine the residual soil to give you a passivation. Inorganic compounds formed on the substrate from aging or from reaction with certain lubricant components can interfere with the continuity of the surface conversion. The inorganic compounds and transitional metal oxides are electro-chemically different. The electro-chemical difference can create corrosion cells. All that is needed is water or moisture and corrosion initiators such as chloride or sulfate commonly found in untreated water. Depending on the quality of your paint and your product’s service environment this may be an issue.
I’m not going to mention any names of companies or vendors. The TMCs have been around long enough that the stories have made it around our metal finishing world. Vendors are more in tune to this as we are all competing for business with new technology. There is an adage used by pretreatment vendors: every line is unique. As much as suppliers do research, it would be unreasonable to expect a research chemist to anticipate every unique situation. Before I start relating war stories, I want you to understand that I’m telling you about the bad things. My intent is not to make you paranoid but rather prepared for a TMC. Most TMC installations go without incident if there is sufficient planning and resources.
The quality issues aren’t always unique to TMCs as they also happen with iron phosphating. Adhesion issues generally stem from improper cleaning and rinsing. Loose deposits will cause paint adhesion issues. Line stops can create flash rusting that is loosely bonded. Ergo, flash rusting can cause paint adhesion issues. A quick test for TMC adherence can be simply done with tape. If you can pull the coating off exposing silver metal underneath, then you will have a paint adhesion problem. This is pretty obvious but leads into a situation where a metal finisher had over a dozen consecutive line stops due to paint color changes. The question was posed: was there a particular area of the 5-stage washer where line stops create adhesion issues.
Laboratory testing was conducted to determine if there was a particularly bad area for parts during a line stop. Lab variations simulated 3-minute line stops in the process stages with normal transfer times, 3-minute line stops in the drain zones with normal stage exposure, and three minute stops in both the stages and drain zones. A fourth set was prepared with normal sequence times as a control. Used production TMC solution was utilized for the lab treatment. The time increment of 3 minutes was used as this was the time it took to hand spray a rack of parts and change colors. Adhesion testing and salt spray testing was performed to compare the variations.
To my surprise, all of the cross- hatch adhesion (ASTM D3359) results were perfect. This was not expected and was most likely due to the differences between the laboratory and the production washers. The neutral salt spray (ASTM B117/D1654) results at 336 hours demonstrated better variation. The test data suggests that the line stops in between stages have a greater impact on performance than stopping in the stages that continued to spray solution. Spray impingement prevents the formation of loose deposits that can interfere with paint adhesion and corrosion resistance.
Other process-related issues include flash rusting. Figure 1 illustrates a uniform coating on HRPO rims and an appearance that is not uncommon with TMCs. The HRPO substrates were treated in a 5-stage washer with the TMC in stage 4, followed by a reverse osmosis rinse. As the parts continued towards the e-coat tank their color changed. You couldn’t wipe off the TMC coating, although it was evident that the reaction continued. About the same time the e-coat ultra-filters were beginning to plug with an iron containing residue. Drippings were collected to determine how much iron was in the residual water entering the e-coat tank. A high number was 10 ppm and values were typically 2–3 ppm. Assuming 10 ppm of iron was constant, it would have taken 52,000 gallons of iron containing water dragged into the 7,000 gallon e-coat tank to match the amount of iron in the e-coat tank. The metal finisher was conducting TMC trials with multiple vendors. It was unknown how much came from the first or second vendor.
Based on material balance, there was no way that water on parts alone established the iron levels in the e-coat tank. High iron loading had to be related to line stops, and I suspect that the parts were rusty due to line stops in pretreatment and then dissolved in the cathodic e-coat tank. A power and free conveyor would eradicate this situation. Iron entering an e-coat tank is a concern you need to consider. Not all lines have this issue, but I have heard of at least four e-coaters having this issue. In one instance, the e-coat solution had to be replaced. Iron accumulation in rinse tanks has also been seen. In as much as I’ve heard of this problem, one would think that it could be remediated with better rinse overflow. This is not always the case and generally leads to the use of “rinse aids” or rust preventatives.
High operating cost has also been observed. High water usage was seen on a line where the total RO water usage in the stage before and after the TMC was 52 gpm. This metal finisher also had extreme TMC usage as well. There was high carry-over from the parts processed on this line. High carry is a concern not only for product usage but for quality as well. Phosphate discharge wasn’t a concern for this finisher. They were solely interested in energy savings. Once they converted to an ambient iron phosphate they reduced chemical purchases by 58%.
This is the crux of this article. I believe in being green. I reduce, reuse, and recycle whenever I can. But I also believe you need to run your line efficiently to survive and prosper. You need to make good decisions so that you aren’t the guy at yet another company saying the conversion to TMC “was a mistake.”
Ancillary processes associated with your pretreatment line may also be affected by changing to the new technology. If you continue to treat your wastewater after implementing a TMC you need to consider the water treatment chemistry. The implementation of TMC changed how the coagulant worked. Figure 2 shows what happened to the right and how the sludge should look on the left. The result was that the sludge became dense and sticky making a mess in the clarifier and sludge thickening tank.
Transitional metal conversions potentially can save you money while improving quality. I believe in the new technology. However, in the last two years I’ve seen lines converted in many ways from iron or zinc phosphate to TMC and some went back to phosphating. The common denominator for failed implementation is carry-over contamination and in-process rusting from slow transfer times. I have also seen TMCs replace with competitive TMC’s. The reason for this transition is that the original product’s capabilities were oversold. It is these situations that motivate my topic.
In closing, do your research, make a plan and roll with the changes with open eyes.
Table 1. Adhesion Test Results
|Process Time Sequence||Adhesion||Creepage from Scribe|
|Normal Time Sequence||5B||0.1 mm|
|3-Minutes in Stages||5B||0.2 mm|
|3-Minutes in Drain Zones||5B||2.9 mm|
|3-Minutes in Stage & Drain Zones||5B||0.4 mm|
Ken Kaluzny is the General Industrial Product Manager for Coral Chemical Company in Zion, Ill. He received a Bachelor of Arts degree in Chemistry from Knox College in 1982. Since then he has held various positions at Coral Chemical. Ken is a member of the Chemical Coaters Association International, the Electrocoat Association, Powder Coating Institute, and the Porcelain Enamel Institute.