Troubleshooting Pretreatment Systems
Your pretreatment system directly affects both the quality of your product and your operating efficiency. To ensure the optimal performance of a pretreatment system, you must have the ability to measure the performance level of each item in the process. The chain of processes all must be completed successfully in order to guarantee a quality product. If one link in the chain fails, the entire system is put at risk. Developing a troubleshooting policy to address the total pretreatment process enables the manufacturer to gain control over the operation.
Troubleshooting often implies a response to a problem that has previously occurred. "It's broke, so now we must fix it." A comprehensive troubleshooting program that examines all aspects of the pretreatment process allows a proactive approach to solving problems. The goal is to identify the problem before it occurs, or at least to anticipate it. We obtain this by knowing as much information as possible about our systems. Knowledge of how the system performs under controlled conditions, and when stressed, permits you to quickly identify problem situations. This sound knowledge base, combined with a thorough quality testing program, excellent record keeping and routine preventive maintenance programs, will achieve the maximum performance level of your pretreatment system and maintain that level indefinitely.
While primarily geared towards high pressure spray washers, the troubleshooting ideas presented here can be applied to any finishing line.
Theories of TroubleshootingThere are three basic theories of troubleshooting, including eliminating problems before they occur (prevention), eliminating problems as they occur (identification/resolution), and eliminating problems from recurring (process control). All three theories rely on first establishing a quality system. This requires proper selection of equipment, substrate, paint and pretreatment chemistries, all fine-tuned to your specific system and application. Once a history of quality performance has been established, one can effectively apply troubleshooting techniques. This article does not cover product selection techniques and assumes that you have the correct system in place.
SubstratesA consistent quality substrate from your supplier is essential. Inspect and document all incoming materials. Control over the substrate will permit consistency in the metalworking process, which in turn leads to consistency in cleaning. A variable substrate puts added pressure on the pretreatment process and raises the potential for problems. Maintenance of the metalworking equipment and fluids will greatly contribute to a dependable substrate.
A "sound" surface is required for pretreatments to be effective. A sound surface is defined as a cleanable part, with compatible soils, that is free of corrosion. Compatible soils are easily removed by your cleaning system and do not contribute to secondary problems. Secondary problems include emulsification of soils, neutralization of the cleaning product, and foam. These phenomena can be verified in beaker studies off-line and it is strongly suggested to do this prior to making any changes in your lubricants.
Corroded or heavily oxidized parts are very problematic. Most pretreatment systems cannot handle rusty parts. These parts are best treated off-line (shot blasting or chemical derusters) prior to being pretreated.
CleaningCleaning is the first and most critical step in the pretreatment process. Without effective cleaning you will not achieve a quality finish. Cleaning removes all surface contaminants that otherwise would inhibit the conversion coating or paint bond. Contaminants include oil, grease, smut, oxides, light corrosion, fingerprints, rust preventatives and shop dirt.
Verification of cleaning can be performed online by the water-break test. If a surface is free of organic soils (oil and grease), clean rinse water will sheet out unbroken over the surface of your part. Detergents in the rinse water, from cleaner drag-out, can lead to a false passing of this test, so flush the parts with fresh water for verification. An inspection door in the space between stages, with good lighting, is critical for this test and for troubleshooting in general.
The water-break test measures only organic soils, so a periodic towel wipe test is also in order. This test will reveal any metal fines or inorganic soils (carbon smut) left on the part. The towel test can only be performed on wet parts. A perfectly clean towel is usually not obtainable, nor is it required for achieving a quality finish. For the towel test, look for relative changes vs. past performance.
More sophisticated cleanliness tests also are available. These include contact angle tests, surface carbon analyzers, Photo Acoustic tests, and electroless copper immersion plating. In most cases, the water-break test and the towel wipe are sufficient.
Washers with four or more stages will almost always utilize an alkaline cleaner. Alkaline cleaners are forgiving on equipment, effective in removing typical metalworking fluids, and able to condition the surface to accept a conversion coating. In an ideal system, the soils are removed from the work but also rejected from the cleaner bath. The result is a floating oil layer that can be removed from the system with an oil skimmer, separator, or by overflowing the bath. By rejecting the oils, you can significantly extend your bath life. Immersion systems, on the other hand, can be hurt by oil rejection if the part is lifted out through the oil layer and the soils are allowed to redeposit.
Filtration to remove solids from the cleaner will help protect your spray pumps and nozzles. Ultrafiltration can be very effective at prolonging bath life, but you must be careful to find a membrane that is compatible with the temperature, pH and chemistry of your system. Also, you must verify that your cleaning detergents (surfactants) are not being removed from the bath during ultrafiltration.
Conversion CoatingOnce a part is thoroughly clean, it will benefit greatly from the formation of a conversion coating. A conversion coating will chemically alter the substrate, resulting in a surface that is highly receptive to paints or coatings and significantly more corrosion resistant. Commonly used conversion coatings include zinc and iron phosphates, chrome phosphates and chromates. Newer non-chrome conversion coatings are also becoming available.
The chemistry of conversion coatings is more complex than cleaning and must be monitored and maintained to ensure consistent performance. When phosphatizing, the reaction is a two step process with metal attack followed by phosphate deposition. Success relies on proper acidity levels along with correct accelerator and product concentrations. Often two or more titrations must be confirmed along with pH checks.
Performance of your conversion process is measured by observation, coating weights and paint performance tests. Controlled test panels, which provide a consistent substrate composition, should be run through the washer periodically and then subjected to coating weight analysis. Coating weights can be determined by X-ray fluorescence or weigh-strip-weigh methods.
While maintaining a certain level of coating weight is important, the actual functional range is quite wide. History has proven that the quality of the conversion coating is far more important in impacting performance that the coating weight. Factors which effect the deposition of a conversion coating include the accelerator type, bath concentration, temperature, acidity, alloy composition and contact times.
Problem characteristics of conversion coatings include non-uniformity, coating voids, light coatings, heavy coatings (powdery), streaky coatings, films on the work and flash rust. You should take the time to study the appearance and performance characteristics of "good" production and test panels so you can readily distinguish poor performance.
RinsingThe rinse stages between processes are critical and too often taken for granted. Rinse stages should receive the same level of attention as the chemical baths. Rinse tanks function to complete the previous stage process, condition the work for subsequent treatments, and minimize the transfer of processing solutions to subsequent stages. After the cleaner stage, the rinse water completes the cleaning process by flushing away the detergent soil complexes. After conversion coating, the rinse water removes unreacted chemicals and potentially harmful accelerators that would otherwise affect the painting process.
Failure to maintain clean rinse tanks results in excessive product consumption rates, excessive sludging, reduction in bath life from contamination, splotchy or non-uniform coatings, and potential flash rust. Rinse water must be neutral and free of contaminants. Simply adding alkali to acidic rinses (or vice versa) is a dangerous idea as the result is heavy salt buildup. Rinse waters should be overflowed and recharged as frequently as possible. Two rinse tanks in a row is ideal as you can overflow the second rinse back to the first. This conserves water while achieving superior rinse performance. You may overflow rinse tanks to the process tank before them. Never overflow a rinse tank after a conversion coating or pickle tank to the rinse stage prior to the conversion coating bath, as you can "pre-coat" the work and inhibit the formation of a quality coating. Fresh water should be added through the final riser of the rinse stage to yield the best rinsing possible.
Rinse tanks should be monitored by pH and total dissolved solids (TDS). A good rule to follow is to keep the TDS of the working rinse under twice the level of your make up water. Check the rinse stage quality at least once per shift.
Seal RinsingA seal rinse compound will react with any exposed substrate (microscopic voids in the phosphate) and reduce the electrochemical potential for corrosion. The result is a substrate that is more compatible with paint. A zinc or iron phosphated part will always benefit from a good seal rinse compound.
While chromium-based products are still the most versatile and optimum performers, the industry has seen the development of some top quality non-chrome seal rinses in recent years. Many studies have demonstrated that under proper conditions, a good non-chrome seal rinse can equal the performance of chrome.
In order to achieve top performance, the seal rinse tank, or any final rinse stage, must be kept clean and free of contaminants. This is achieved by maintaining the rinse prior to the final stage. Final seal products should be tightly controlled according to the manufacturer's procedures, especially the pH of the bath. Care should be taken in selection of water for the final stage.
Do not use soft water to make this stage up as sodium can cause paint adhesion failures. Chlorides and sulfates are also problematic as they will lead to significant reductions in corrosion resistance and potentially flash rust. In general you want to keep the combined total ppm level of chlorides and sulfates below 100 ppm (50 if possible) to ensure good performance. In many areas, the tap water does not meet this criteria. In those areas, it is strongly suggested to use reverse osmosis water or deionized water to make up the final stage.
Problems associated with final stage quality include flash rust, poor adhesion and poor corrosion resistance. The effect of the final rinse on the overall performance is very significant and, therefore, modifications to this stage will have a significant impact on the quality of your system.
TitrationsThe use of titrations continues to be the best method available to control your process baths. Unfortunately, it is also an area that can cause some confusion. Often baths will develop color over time that make certain endpoints hard to read. The addition of small amounts of deionized water to the titrating vessel will often help. Additional indicator added close to the end of the titration will also aid in resolving your endpoint. Clean glassware and a well lit work area is required. A bright white background will make reading the titration much easier. Auto zero burettes are far superior to pipettes for titrant additions. Experience has shown the burette style with a push button gives better control over those with stopcocks. If the above methods do not solve your endpoint resolution problem, you can perform titrations utilizing a pH meter and magnetic stir bar. When more than one operator will be performing bath checks, it is critical that they all titrate to the same color endpoints using the same procedures. Avoid contamination of your reagents and, if in doubt, acquire fresh materials.
A working bath should be titrated at least once per shift. A system with smaller sumps will require more frequent bath checks, as will situations where carryover of process solutions is high.
Analytical TechniquesRoutine analytical studies of the finishing tanks and final seal rinse stage greatly assists in the troubleshooting process. Experience shows that the ability to monitor metals in solution and ionic species provides a wealth of information on the "health" of the system. Metals analysis by Atomic Absorption or Inductively Coupled Plasma Emission (ICP) will help verify the activity of your conversion coatings and greatly aid in the waste treatment/disposal process. In many cases, the instrument can be used to verify coating weights of non-traditional conversion coatings by stripping the coating in acid and analyzing the stripping solution.
Ion chromatography is an excellent tool for testing rinse waters and conversion coatings. It will allow you to monitor essential components like phosphates and fluorides, and also contaminants like chlorides and sulfates. Techniques can be developed with the Ion Chromatograph to analyze your iron phosphate bath for accelerators such as molybdate, chlorate and SNIBS. It is important to have an analysis of a "good" bath in order to be able to compare it to a "problem" bath.
Bath Life-Alkaline CleanersStrategic timing of bath recharges will create efficiency while reducing the risk of defective parts. If you are able to schedule this process at a time convenient to both waste treatment and maintenance departments, the process will go very smoothly. Waiting until the solution is totally spent and non-functional causes production concerns that impact the entire operation.
Cleaner bath life will be due primarily to soil loading. Oil rejecting cleaners will prolong the bath life, but in time all systems require recharging. A couple of simple tests will help you to predict this. Performing a free and total alkalinity titration will reveal the level of contaminants building in the stage. This is achieved by running both a phenolphthalein (free) titration and a methyl orange (total) titration. The ratio is obtained by dividing the total alkalinity by the free alkalinity. The free alkalinity is held constant by your product additions. Over time, the total alkalinity rises as the bath ages. You need to compare the ratio value of a fresh charge to the aged system. When the ratio increases by 100 percent to 200 percent, it is normally time to recharge the bath.
An oil split on your cleaner bath will show the emulsified oil level. This should be performed in a stoppered graduate cylinder or a Babcock bottle. Sulfuric acid or TKPP works very well. This test should be run once or twice a week.
Another technique that will provide useful information is monitoring your product consumption rates. Early on, you should observe a fairly uniform rate of product use to hold your free alkalinity titration constant. As the bath ages, the rate of consumption required to maintain the bath increases remarkably. At some point, you will benefit economically by dumping the bath and recharging, rather than adding excessive volumes of product.
The best system relies on the above techniques and correlates them to past experience in order to develop routine schedules. As long as your production does not change significantly, you can expect the baths to function accordingly.
Avoid the temptation to only partially dump your cleaner bath and recharge a fraction of the system. This practice is not cost effective and usually results in "throwing good money on top of bad". Use the dump period to clean out the tanks with a high pressure sprayer and also to flush the risers.
Bath Life-PhosphatesThe bath life of a phosphate tank is more difficult to predict. With the cleaner bath you can quickly see if performance has been reduced by performing cleanliness tests. On the phosphate tank, it is not as straight forward, as performance of the conversion coating is often measured by long term environmental tests.
The product consumption rate rule outlined above will hold true for the phosphate tank. After that, your visual inspections are the next best tool. Streaky coatings or staining result from contaminated, overworked phosphate baths and are a clear sign that recharging is necessary. Analysis of the conversion bath on a regular basis will verify that active constituents are maintaining the proper balance.
Dry-Off OvensOnce the parts exit the final stage of your washer, they must dry as quickly as possible. Failure to dry the parts in a reasonable time period will result in flash rust. Neither the seal rinse nor the phosphate, by nature, are able to resist corrosion prior to the painting process. This is especially true while the parts remain wet. Heating the final stage and positioning air knives at the exit of the washer will help drive off the liquids. Positioning the dry-off oven as close as possible to the washer is the best solution. Acidic vapors from any other operations must be eliminated from this area. Acidity in the dry-off oven, from sources like Freons, chlorinated solvents, and gas burner exhaust, will contribute to corrosion of the substrate if left unchecked.
ConveyorsConveyors are a very common source of finishing problems. Most often, the problem is liquids dripping from the conveyor or harnesses onto dry or partially dry parts. You will see a drip residue pattern with distinct edges. The edging of the drip is important because you can tell if the liquid fell on a dry surface as opposed to a wet surface where the edges would be diffused.
Conveyor drips are problematic because they often contain conveyor lubricant. In addition, they may also be alkaline from your cleaner stage, especially if the liquid had accumulated in cavities in the racking system. Either way, the drip area is not compatible with paint, and blisters or patterns in the finish result. Prevention is the best way to avoid the problem. Spray washer conveyors that ride above the washer roof are ideal. The harnesses or racks pass through a slit in the roof with brushes that inhibit any liquid from ever contacting the conveyor. An alternative is a shroud around the conveyor with positive pressurized air within to keep it dry. The least desirable solution is to keep your spray patterns aimed down and maximize the ventilation to avoid buildup of mists. Generally the later solution is not effective. Keeping the conveyor isolated from the washer chemicals is by far the best policy.
Product DesignThe engineering of the parts to be processed, along with racking of the parts, will impact their performance. Problems arise in two forms. First is in the shape of the part if it prohibits the solutions from effectively reaching all areas. A spray cleaner requires impingement to be effective. Cascading a cleaning solution in receded areas will not provide sufficient cleaning. These parts are better suited to an immersion cleaning process or they should be precleaned prior to processing. Racking should be designed to maximize contact in the receded areas while not entrapping too much liquid.
The second problem encountered is excessive carryover of process solutions. This occurs when the solutions are unable to drain from the part, due to the configuration. Again, strategic racking will help resolve this, along with design modifications that include drain holes or slits. This problem can be very severe on high speed lines. Excessive drag-out causes contamination of baths (and rinses) and increased consumption rates.
Performance TestingTo assure that the parts being finished meet your requirements, you must develop a series of performance tests geared towards your specific application that will effectively verify that the goals are being met. I have already covered a number of these tests, such as visual inspections and bath chemistry checks. As the part exits the paint cure oven, you need to subject it to a battery of initial tests, including the following:
A.S.T.M. ADHESION TESTS
Conical Mandrel (D522-93A)
Impact Resistance (D3281-84 Reapproved 1989)
T-bend (D4145-83 Reapproved 1990)
A.S.T.M. PAINT CURE TESTS
MEK Rubs (D4752-95)
Pencil Hardness (D3363-92A)
These tests should be performed as quickly as possible, as a problem here indicates a serious concern that must be addressed. Additionally, based on your specific application, you may want to develop a series of long-term performance tests involving accelerated environmental studies. Salt spray tests (B117-94) are still widely used as are humidity exposure tests (D2247-94). While neither test can exactly mimic real world exposure, they are good tools for comparative purposes and to verify ongoing production. A number of cyclical tests, involving periods of salt spray, humidity, UV light and other conditions, have shown better correlation to the real world. Regardless of the technique used, these tests require time to complete and, therefore, are difficult to use as quality assurance techniques. In some applications, water immersion tests (D870-92) and detergent immersion tests (D2248-93) will provide useful information in a timely manner. These tests can be modified to your application to provide more relevant information.
Once you have established the level of acceptable performance, an operating window should be determined. Parts not complying to your finish standards should not be released.
MaintenancePreventive maintenance is the key to achieving consistent performance. A planned program must be developed and adhered to, as corrective maintenance causes downtime, lost production and, in many cases, can easily be avoided. Target areas include the conveyor, spray pumps, pump screens, risers, spray nozzles, float valves, oil skimmers, feed pumps, controllers, ventilation systems and heat exchangers. Verify that each component is functioning normally and schedule overhauls in accordance with manufacturer's instruction.
Verification of spray patterns is required on a daily basis as clogged or misaligned nozzles will reduce performance in all stages and can cause cross contamination of process tanks. Likewise, the pump screens should be cleaned at least once a shift as they provide the only source of protection for the spray pumps, especially if cotton gloves are used to handle the work.
Spray washers by nature are not energy efficient. If scale or debris is allowed to build up on heat exchangers or fire tubes, their efficiency is further reduced. When required, the stages with scale buildup must be chemically descaled. Depending on the system in place, you will want to schedule a descaling process once or twice per year.
The areas around the washer should also be maintained. Good lighting, especially at viewing stations between stages, is very important to verify performance. Gauges and instrumentation must be clean and in working order. Avoid allowing things to "pile up" around the washer as they can block access to safety equipment such as eyewash stations and safety showers. Any spilled chemical must be cleaned up immediately. Shop dirt in the paint area and after the final rinse is a common cause of finishing problems and is best mitigated with good housekeeping practices. Routine maintenance practices should be logged on a regular daily, weekly and monthly basis. Never take preventive maintenance for granted as it is one of your best tools in troubleshooting potential problems.
Record KeepingThe importance of good record keeping cannot be stressed enough. The washer should have comprehensive solution control charts where you can record titration values, temperatures, pH, spray pressure, product additions, feed pump rates, chemical drum/tote volumes, bath appearances, part appearances, spray pattern inspection, pump screen maintenance, etc. I like to dedicate a page to each stage in your finishing line, including rinses, with ample room to make comments. Likewise, you should have log books covering performance tests, maintenance, and troubleshooting histories. Records and thorough testing are mandatory for troubleshooting because we need to have data on process conditions when things are performing well, in order to be able to correct the application when things go wrong. Thorough records greatly assist your supplier's ability to help solve problems. In facilities with multiple shifts, the log books permit communication between line operators so tasks are not duplicated.
Control TechnologyProcess controllers with metering pumps can be very beneficial on finishing lines. Continuous sampling of the activity of the bath and addition of product in very small increments, only when needed, will provide extremely tight control over the process. Correctly used and maintained, a controller will eliminate concentration swings reducing the potential for out-of-specification baths and often reducing chemical consumption. Changes in the bath chemistry are very gradual and less likely to shock the process. Elimination of manual chemical additions provides an added level of safety around the washer. While all processes will benefit with a controller, lines with the greatest fluctuation in chemical concentrations, due to small volume tanks or large amounts of solution drag out, will benefit the most.
Conductivity, pH, redox and specific ion controllers are widely available at a range of costs. Systems utilizing electrodeless conductivity probes are durable and reliable in most metal finishing baths. More than one controller can be used on a single stage when both activity and pH control is required. Programmable controllers are the most effective systems. These units allow the input of a custom concentration and temperature curve based on the specific chemistry you are using. Often an output signal can be monitored by a chart recorder or PC which will provide valuable troubleshooting data and potential for SPC.
Like any instrument, the controller must be maintained and the reading verified. Bad information is worse than no information at all. Probes should be regularly cleaned. Calibration of the unit should be performed once a day or more often as needed. The titration is still the best test procedure and the controller should support the titration, not replace it. In facilities with more than one shift, it is often helpful if only one operator calibrates the controller to assure consistency.
An alternative to a controller is a chemical metering pump in its own. It will allow small continuous additions of product while also reducing the handling of the chemicals. The pump should be tied into the conveyor so it only pumps when the line is running. The flow rate is adjusted as needed to maintain your titrations. This technique is very useful when separate accelerators or bath modifiers are added to conversion coatings at a slow, steady rate.
Troubleshooting ProblemsPreventive maintenance, strategic bath and performance checks, and skillful observations will greatly reduce your finishing line problems, but it won't totally eliminate them. Undoubtedly, you will encounter problems. This is when your thorough understanding of each component in your finishing process comes into play.
By far, the most difficult task is identifying the source of the problem. Using the skills outlined previously (and in the full version of this article at www.paintandpowder.com), you should be able to swiftly realize that a problem does in fact exist. Rectifying the problem is usually quite simple, such as redirecting spray nozzles or recharging a rinse tank. The challenge lies in singling out the variable that is causing the problem.
Pretreatment or paint related? Test panels that have been pretreated but not painted, either commercially purchased or prepared online at a time when good performance was verified, are also very helpful. Commercially purchased, pretreated panels must closely match the system on your line. These must be stored well, as a phosphated part without a paint film does not inhibit corrosion. You should run the part through the dry-off oven to eliminate any moisture on the surface, and allow the part to go right into the paint section. This technique will identify if your problem is pretreatment or paint related. If this test panel passes, you should turn your attention to the phosphate or rinse stages, but if it fails, the problem is most likely paint related. Logical use of panel tests is extremely effective in localizing problems.
Rinse problems are identified by either shutting off the final stage, or by artificially rinsing with a hose or a bucket off line. A contaminated or overcharged seal rinse stage quickly will be identified by this practice.
Viewing stations with good lighting allow you to monitor each stage and permit you to rack or remove parts and panels as needed to answer potential questions. Folding dock lights should be mounted at each station. Temporary lighting is acceptable for maintenance periods but not while production is running. A tunnel washer with no viewing stations is very difficult to troubleshoot.
Communication is essential for troubleshooting. Always communicate potential problems before they occur, such as use of a new lubricant or substrate supplier. Gain the input of your vendors and, if possible, get involved in programs sponsored by your finishing community as these are inexpensive sources of excellent knowledge. As you try to rectify the problem, remember to only change one variable at a time. In this manner, you will easily identify which variable was the true cause of the problem and long-term solutions can be established. Remember that troubleshooting requires a logical, detective-like approach where nothing should be taken for granted or assumed. Learn from past experiences!
If your system is well established with a good performance history, and suddenly a reduction in performance is noted, you must realize that something on your finishing line has changed. You need to ask yourself three questions:
- What has changed?
- How has it changed?
- Why did it change?
Once you can resolve these questions, you are well on the way to rectifying the problem, and more importantly, avoiding the problem in the future.
The goal is to use "tools" to measure performance levels, quickly identify problem situations, localize the source of the problem, and rectify the situation. Troubleshooting begins when the plant and finishing line are first designed and has no endpoint. The entire picture relies on a sound understanding of the operation, both mechanically and chemically, with the ability to predict how changes will impact the overall process. The ability to measure and verify the output of each component of your system is the key to this process. Once established, you can be assured that no matter what your finishing process entails, you will be achieving the best results possible.