Metal Cleaning and
Surface Pretreatment Processes
The metal cleaning and pretreatment industry is engaged in constant innovation in order to meet new challenges, including:
- Increasingly stringent quality requirements, especially related to residual contamination
- A larger proportion of parts made from multiple materials and having more complex surfaces (holes, channels, grooves, etc.) that require special processes or equipment
- Demand for greater efficiency, consistency, and reliability, at lower cost
- Regulatory and societal demand for “greener” products and processes
Nearly all metal parts – and many plastic parts, too – have some sort of finish coating applied to improve appearance and to increase resistance to corrosion and wear.
It has been estimated that 7 of 10 coating failures are caused by poor surface preparation; these failures cost millions of dollars in wasted time and resources. Coatings applied over properly prepared surfaces cost less per square foot per year than the same coatings applied over a poorly-prepared surface.
When paints or finishes on metal surfaces flake, peel, delaminate, blister, bubble, or wrinkle, the problem can often be traced back to the pretreatment process, such as failure to remove all contaminants, failure to rinse thoroughly, or the uneven or inadequate application of necessary chemical treatments.
Thus, cleaning and pretreatment must be sufficient to ensure high quality in the finished parts. The cleaning and treatment process must be carefully monitored to ensure quality and cost-efficiency.
ASSESSING CLEANING: HOW CLEAN IS “CLEAN”?
The surface is determined to be “clean” when it is free of unwanted contaminants that could interfere with later uses of the part in question.
The degree of cleanliness desired can vary depending on how the part is to be processed and used, how long it will remain in the facility, and how it might be handled in subsequent manufacturing stages. Is the part in question a component or subassembly, or is it destined to be a finished product?
A component might be cleaned more than once during the manufacturing process, with a different standard of cleanliness applied to each stage. If it goes into storage for any length of time, it may be at risk for tarnishing or corrosion, and will require additional cleaning.
A standard of cleanliness required for parts to be used in food or medical applications might be different than a standard of cleanliness for parts to be used in industrial machinery.
Parts may be cleaned more than once before they leave the plant as part of a finished product.
Several quick informal tests can check for cleanliness:
- A water break-free surface indicates that all organic contaminants have been removed. After the parts have been through the last rinse or pretreatment stage, but before they are dried, the clean rinse water should form a uniform, unbroken sheet over the surface of the cleaner part. If the washed part has beads of moisture such as appear on a clean, waxed automobile as it emerges from a car wash, this means that the surface has not been thoroughly cleaned and that some organic contaminants remain. This test is reliable only when the rinse water is fresh and free of any additives, such as detergents or rinsing aids. Rinse water that has been contaminated by additives or by overflow may form unbroken sheets that may mask remaining contaminants. (NOTE: This is not an effective test for removal of inorganic compounds.)
- The white-towel or white-glove test is done by wiping a clean white towel, white cotton glove, cotton swab, or lens tissue over cleaned surfaces (wet or dry). If examination of the white surface reveals gray, black, or off-white residues, then inorganic (and possibly some organic) contaminants are still present. This test is helpful on flat surfaces as well as those that do not receive direct spray.
- A “pull tape” or “Scotch ™ tape” test will also reveal inorganic contaminants. Apply a piece of transparent tape to a cleaned and dried surface, then remove it to a piece of clean white paper, where the high contrast will make it easy to see any remaining contaminants.
- Flash rusting – Ferrous parts that are absolutely clean will rust quickly.
- Ultraviolet light – On parts that had been previously coated with ultraviolet (UV) fluorescing oils, a scan with an ultraviolet light will reveal any remaining traces of UV fluorescence.
More rigorous physical and chemical analyses can also be conducted to check for remaining contamination on cleaned surfaces. For example:
- Sample parts that have been cleaned and dried can be immersed in a solvent solution and agitated, and then the solvent can be analyzed to detect contaminants.
- Surfaces of cleaned parts can be sampled and analyzed to identify and quantify the presence of oxides, organics, and particulate contaminants.
- Salt spray resistance tests evaluate the corrosion resistance of materials exposed to a 5% salt spray, mist, or fog.
- In critical cleaning operations, highly analytical techniques such as infrared micro-profiling, X-ray photoelectron spectroscopy, light reflective technology, and ultraviolet detection can be used to detect residual contaminants.
It is important that these scientific tests are reproducible and that they are done consistently, preferably by the same technician and the same laboratory each time.
ASSESSING PRETREATMENT COATINGS
The quality of pretreatment can be assessed by visual observation of the conversion coating:
- Is the coating uniform appearance and color? Many conversion coatings change the color of the metal substrate when the chemical process has been completed. If the color is inconsistent, streaked, spotted, etc., this can be an indication of a problem with the chemistry or delivery of the solutions. (Variations in color are acceptable in phosphate coatings on mixed metal sub-assemblies.)
- Are there any bare areas, or areas that have too much or too little application?
- Are there shiny spots? In phosphate coatings, shiny spots are indications of inhibition, a condition that exists when the phosphate coating could not form because of surface contamination.
- Is there indication of mapping, that is, visible patterns in the coating? Mapping is caused by an uneven chemical reaction between the metal surface and the conversion coating. These can be caused by oily contaminants that react with the metal and form a permanent stain or bond on the metal surface; this is generally the result of an inadequate cleaning process. Mapping often shows up only after the final application of paint or other finish; this makes repairs very costly.
- Is there any indication of “lace curtaining,” that is, streaking or other faint patterns? These can be caused by misaligned spray nozzles, drying positions, or air flow imbalances. Slight or occasional patterns are not normally a problem, but if they occur frequently, they are an indication of poor system design or poor operator handling.
For any types of substrate and treatment, the coating can be evaluated (within the specifications for each application) by assessing the coating weight, the crystal size or morphology (shape), and the chemical composition.
The coating weight is defined as the amount of coating deposited to a surface within a given area. This can be expressed as grams per square meter (g/sq m) or milligrams per square foot (mg/sq ft).
Pretreatment technologies are designed to apply specific conversion coatings in particular thicknesses (weights) on particular substrates; thus, measurement of the coating weight is a good means of testing how well the coating process is working, including its chemical balance. If the coating weight tests outside the specified range (either too much or too little), the process must be reviewed and corrected. NOTE: This test information is used primarily as an indicator of how well the cleaning and pretreatment process is working, but is not necessarily an indicator of the actual quality of the coating itself.
Many conversion coatings consist of a combination of crystalline and other microstructures that form when the pretreatment is deposited on, and chemically bonds with, the substrate. Though visual observation can detect many imperfections in the conversion coatings, only through microscopic examination can the size and shape of the crystal structure be examined and measured to see if it meets the relevant specifications. The findings are used to resolve chemistry or process errors. This microscopic analysis is generally performed at magnifications of 100 to 1,000 times; for newer coating technologies, higher magnifications up to 30,000 are necessary.
As the pretreatment process is essentially a chemical process, chemical analysis of the coating is a good means of determining if the process is producing the desired result. The analysis is generally done in a laboratory, either by simple analysis or by use of scanning electron microscopes. In the plant, the cleaning and treatment equipment can be equipped with monitors and alarms to alert operators when the solution is too weak or strong, when solution supplies have been exhausted, or when temperature are not within desired limits.
Ultimately, the performance of the paint or coating is the best indicator of the quality of the pretreatment. Does the paint or coating adhere evenly and smoothly? How does the paint or coating perform under adverse conditions, such as impact, pressure, humidity, corrosive elements?
The quality of the water used in cleaning and pretreatment is an essential factor in the quality of the final product. In most aqueous cleaning operations, the chemical baths and solutions are 95%-97% water, and the rinses are usually 100% water. Rinse stages serve to neutralize and dilute, and most importantly, to prevent or minimize contamination between treatment stages.
Depending on the chemistry required for a particular cleaning and treatment process, and the quality and relative hardness of the local water supply, the use of untreated tap water may or may not be appropriate. Impurities in the water can make cleaning solutions and rinses less effective and can affect the quality of the finish coats.
Variations in water chemistry can cause chemical reactions in the cleaning solutions that could damage equipment and products and affect the quality of the finish coats. High levels of salts in hard water can precipitate on some treated surfaces, causing corrosion and blistering in humid environments.
The best water for metal cleaning and treatment processes is very soft water (calcium carbonate not to exceed 200 ppm) with dissolved solids not exceeding 150 ppm, chlorides at or below 15 ppm, and sulfates at or below 25 ppm. It may be necessary to treat tap water before it can be used in the cleaning and pretreatment system, or to use deionized water for final rinses if the incoming water cannot be treated to meet standards.
RECHARGING THE SYSTEM
At some point, regardless of the volume of the operation, the quality of housekeeping practices, and the presence of recycling and recovery programs, the cleaning and treatment system will need to be recharged; that is, it will need to be drained, cleaned, and replenished with fresh fluids.
Work with your supplier to understand how to monitor the chemical health of your system and to know when to re-charge. Monitoring will involve measuring and recording pH, volumes of emulsified oils, etc.
If the quality of the cleaning or conversion coat is not acceptable:
- Check water quality throughout the system.
- Check spray nozzles and other inflow devices to ensure free flow of water and solutions
- Check positioning of racks, baskets, etc., to ensure that maximum impingement (exposure) is achieved.
- Check timing of every stage to allow chemicals time to work.
- Check the chemistry, concentrations, and temperatures of all solutions.
- Clean filters, screens, etc., as often as needed to keep them clear and free-flowing.
- De-scale boilers, washers, and other equipment as often as needed to keep them scale-free.
PLANNING FOR QUALITY
Aqueous cleaning and treatment systems are highly sophisticated chemical operations, subject to error and variation related to water quality, operator judgement, and contamination inherent to the process. Frequent quality checks are essential and are normally carried out on every shift, checking for temperature, pressure, pH, and concentration.
Consider arranging with your supplier for regular audits of the treatment and cleaning and treatment operation to ensure that it is serving your needs. Some high-volume operations undergo audits as frequently as every few weeks.