Have you ever wondered why you need to pretreat before you paint? Consider this.
According to Products Finishing:
A high-quality conversion coating is essential for the durability of painted metal goods. The process of applying an inorganic conversion coating to a metallic surface involves removing any surface contaminants, then chemically converting the clean surface into a non-conductive, inorganic conversion coating. Conversion coatings increase the overall surface area and promote adhesion of the subsequently applied organic film. In addition, conversion coatings change the chemical nature of the surface, which increases corrosion resistance. It is these two functions, increasing surface area and changing the surface chemistry, that serve as a base for preparing the substrate material for paint finishes.
There are a number of driving forces in the pretreatment industry today with quality, cost and the environment being the most predominant. While these aren’t new issues, the pretreatment industry has responded to the needs of finishers by creating technology to address each of these requirements. In understanding the complete manufacturing process, including paint formulations, application equipment and regulatory impacts, it’s possible to address each driver simultaneously.
The conversion coating chemistries predominately used today are either zinc or iron phosphate. There is movement to replace these technologies with new types of phosphate-free or very-low-phosphate metal pretreatments. The new-generation technologies have been commercialized by many vendors over the past several years and are rapidly becoming industry standards. Regardless of the chemistry, conversion coatings are used to promote adhesion and improve corrosion resistance. Depending on the conversion coating and the desired performance, the conversion coating can be applied at a number of points in the process.
To increase the effectiveness of the finish, parts must be clean prior to coating. Aqueous cleaning, vapor degreasing or ultrasonic cleaning are typical cleaning processes, and of the three, aqueous cleaning makes up the majority. For parts that will subsequently be finished with organic coatings, surface pretreatment is required.
Depending on the chemistry, iron phosphate systems can either be a cleaner coater, where the cleaning and coating take place in the same stage, or have a separate cleaning stage. Separate cleaning steps are essential for zinc phosphate systems and the new phosphate-free and low-phosphate conversion coatings. If the cleaner does not fulfill its purpose of removing unwanted soils from the substrate, subsequent processing steps will not produce a uniform conversion coating and therefore adequately protect the metal surface from corrosion.
Typical soils are either organic or inorganic. Rust preventative oils and lubricants, metal forming blends and rolling oils are examples of organic soils. Inorganic soils include mill or heat scale, metallic fines and laser scale.
Three types of cleaners are used in metal finishing: solvent cleaners, acid cleaners and alkaline cleaners. Using the proper cleaner for the application is critical, since the method of cleaning can affect coating characteristics such as coating weight and crystal structure as well as subsequent coating performance.
Solvent cleaners are usually used on small surface areas and offer limited ability to remove difficult oils. Solvents usage is diminishing in favor of more environmentally friendly options. Acid cleaners are chosen for removing inorganic soils such as surface oxides.
Alkaline cleaners deliver optimum results on organic soils. These cleaners are versatile enough to effectively clean the surface by lifting the soil up and dispersing it into the main cleaner bath, where it is held until it is removed mechanically using thermal oil separation, ultrafiltration or by overflowing the cleaner tank to drain off surface oils.
Proper rinsing is a critical, yet often-overlooked step in the pretreatment process. The water rinse process stops chemical reactions from taking place and removes unreacted chemicals from a part’s surface. Effective water rinsing also minimizes the migration of chemicals from one processing stage to the next. For effective rinsing, keeping the rinse water fresh reduces the amount of contamination present on the surface of the parts.
Since the key is controlling the amount of surface contamination on the part, if there is only a single rinse stage, a fresh water halo can be installed in between the chemical and water rinse stage rather than adding fresh water to the main tank. This allows the rinse tank to run at higher levels of contamination while the halo adds fresh water to the tank, but more importantly it floods the part and reduces the surface contamination. In the case of multiple rinse stages, they are counter-flowed and can effectively minimize water used in the rinse stages, requiring only a fraction of the water volume and reducing the amount of effluent produced. You can also reduce water consumption by optimizing your equipment design with proper racking of parts.
Traditionally, the choices for a pretreatment process have been either an iron or zinc phosphate which provided the degree of performance necessary for the operation. Recently, there have been developments to replace this traditional technology with products that address ever-growing concerns with energy and water usage, environmental impact and the general operation of the process.
1) Iron Phosphate Pretreatment Systems. Iron phosphate systems, also known as alkali metal phosphates, are used for parts that require a durable finish but are not exposed to severely corrosive environments. These systems can involve two to six stages, with the shortest sequence being a cleaner-coater stage followed by a tap-water rinse. Short sequence systems are employed if performance requirements are low.
Parts that are more difficult to clean or have higher quality requirements call for a separate cleaning stage, appropriate rinse tanks, iron phosphate, post-treatment rinse and a DI rinse. A post-treatment rinse (chrome or non-chrome) results in improved corrosion performance over the phosphate alone.
Iron phosphates produce an amorphous conversion coating on steel that ranges in color from iridescent blue to gray, depending on operating conditions and product formulation. Mixed metals may be treated with modified formulas that typically contain fluoride.
Iron phosphate processes are much easier to operate and require fewer process stages than zinc phosphating. However, iron phosphates do not provide the degree of corrosion protection imparted by zinc phosphates.
2) Zinc Phosphate Pretreatment Systems. A zinc phosphate system varies from an iron system in two critical areas. First, it requires the use of a surface conditioner stage. Second, a zinc phosphate bath has additional metal ions in the solution which are incorporated into the coating along with the metal ions from the substrate being processed.
Surface conditioning rinses are used in zinc phosphating to refine crystal morphology and control coating weight. State-of-the-art conditioners are liquid products that can be consistently applied using metering pumps.
The surface conditioning rinse takes place just before the zinc phosphate stage and is the only step in the process that is followed by another chemical stage, the zinc phosphate bath. The traditional surface conditioning chemistry is a colloidal suspension of a titanium salt. As these traditional baths age, they become less effective and must be dumped frequently or overflowed to maintain effectiveness.
Recently, zinc phosphate has been used to replace the titanium salt chemistry. This technology improves the refinement of the zinc phosphate coating, yet is not affected by the chemical make-up of the water or bath age.
Zinc phosphating coatings provide exceptional painted part durability in corrosive environments and have the ability to coat mixed metals (steel, zinc-coated steel and aluminum). Several small developments have taken place over the past few years, such as decreasing the environmental impact, improved performance and ease of operation. New zinc phosphate systems operate at lower temperatures, in some cases are free of nitrites and nickel, and offer a reduction in sludge, and some products are internally accelerated. The objectives of the products were to increase quality, operate easily and, in the case of internally accelerated systems, eliminate the need for additional accelerators.
Depending on the metal mix in the system, additives are used to assist in the formation of the conversion coating on the substrate. For example, free fluoride added to the bath optimizes the conversion coating on aluminum and/or zinc. Adding calcium ions to the zinc phosphate bath produces a microcrystalline phosphate coating needed for rubber bonding. Depending on the final application and performance requirements, various other metal ions, organic acids, chelating agents and other chemicals can modify the overall characteristics of the zinc phosphate conversion coating.
Over the years, zinc phosphating systems have evolved from conventional systems containing high levels of zinc and nickel accelerated by sodium nitrite. An additional metal ion, manganese, was incorporated into the base chemistry to create the polycrystalline systems used today. Current polycrystalline systems can be either internally or externally accelerated, and in some processes, nickel was removed to create a nickel-free process.
New-Generation Conversion Coatings
New conversion coating technologies are being introduced that have four significant processing benefits. These coating processes are shorter, simpler and operate at lower temperatures than current zinc or iron phosphate processes. They perform well on all standard substrates of steel, zinc and aluminum. They significantly reduce the environmental impact, while their corrosion performance meets metal finishing specifications for painted metal substrates. All of these benefits provide significant cost savings to manufacturers willing to convert their existing processes.
The new-generation conversion coating process is based on zirconium and additional propriety chemicals. When applied to a metal substrate, these chemicals react to form an amorphous zirconium oxide coating 20-80 nm thick that is significantly different from the iron phosphate and zinc phosphate coatings in use today. The new coating is thinner than traditional iron or zinc phosphate conversion coatings.
The new conversion coating process contains no zinc, nickel, manganese or phosphates; rather, it is based on zirconium containing chemicals. Zirconium is not regulated as a hazardous metal in North America or Europe. The new coating can be applied in fewer total stages than a zinc phosphate process and fewer chemical stages than both zinc and iron phosphate. In its simplest form, the process consists of five stages—two chemical stages and three water-rinse stages. The reduced number of stages will result in a 10- to 30-percent reduction in the overall plant footprint when converting from a standard zinc phosphate to the new-generation conversion coating process. A reduction in water usage also can be realized that is directly related to the reduction in process steps.
After a metal surface receives a conversion coating, the surface is water rinsed to remove unreacted chemicals, and a post-treatment may be applied. The post-treatment can increase corrosion and humidity resistance as compared with conversion coatings without final rinses. In the case of electrocoat applications, final deionized (DI) or reverse osmosis (RO) water rinse is required to minimize drag-in of high-conductivity water on the substrate surface from the post rinse. In these cases, it is imperative to have a reactive final rinse that maintains its properties after the DI or RO rinsing rather than a dry-in-place rinse.
Post-treatments historically have been based on chromic acid. With more stringent effluent guidelines, most finishers have converted to either trivalent-chrome or non-chrome post-treatments. The automotive industry has set standards to virtually eliminate the use of hexavalent chrome in vehicles produced after 2007. Recent advances in dry-in-place (DIP) polymeric post-treatments have shown excellent results when compared with standard non-chrome/DI rinsed systems.
Phosphate Coating Evaluation
There are three characteristics that define a conversion coating: the coating weight, the crystal size or morphology, and the chemical composition. When within the designed specifications for the application and material, all three characteristics contribute to ensuring the proper and expected adhesion and corrosion performance.
Coating weight is defined as the amount of coating deposited within a specific surface area. Typically, coating weight is expressed in either grams per square meter (g/sq m) or milligrams per square foot (mg/sq ft). Each conversion coating technology is designed to deposit a specific coating weight on a substrate. Coating weight is an excellent indicator of whether the conversion coating bath is in proper chemical balance. If the coating weight is low and outside the specified range, something may be wrong within the process, and immediate attention is required.
The crystal structure of the conversion coating is measured through the use of a microscope, either an optical or, most commonly, a scanning electron microscope (SEM) at magnifications ranging from 100 to 1,000 times. In the case of new-generation coatings, this magnification is not sufficient, and other instruments such as an Atomic Force Microscope (AFM) or Field Emission SEM (FE-SEM) are necessary to achieve visible images of the coatings at 30,000 times and greater.
Regardless of the instrument required, the conversion coating is a combination of crystalline and microstructures chemically deposited on the metal surface. Instrumentation is used to determine the size, shape and uniformity of the coating. These instruments are excellent tools to examine any metal or coating imperfections that would go unnoticed with the naked eye. This visual examination of any non-uniform treated surface helps in the troubleshooting the pretreatment process. Through the use of the microscopes, the uniformity of the conversion coating can be monitored to ensure proper coverage and structure size. It is well-documented that structure size plays a very important role on paint adhesion.
In addition to coating weight and crystal structure, the chemical composition of the coating plays a significant role in corrosion performance. Basically, corrosion is alkaline in nature, so the more alkaline resistance a coating delivers, the better the corrosion performance.
Chemical composition can be determined through a simple analysis in the laboratory using more advanced equipment, such as scanning electron microscopes with energy dispersive x-ray or x-ray diffraction. Use of this type of equipment is impractical on site, but can effectively evaluate performance in a laboratory setting. The chemical composition of the coating can help troubleshoot the process in applications where corrosion performance is below expectations.
Because the three coating characteristics take some time for evaluation, a simple visual inspection of the coating at the manufacturing site can look for problems. Phosphate coatings should be uniform in appearance whenever possible. Variations in color are normal on mixed metal sub-assemblies such as automobiles using various zinc-galvanized steel alloys.
Although color may vary, there should not be any visible shiny spots in the coating. Shiny areas indicate a condition known as inhibition. Inhibition is where the phosphate coating has not formed due to surface contamination.
Mapping is a widely used term today that describes various types of patterns that are visible on the conversion coating. These patterns are often most visible after the application of the paint film, making them extremely costly to repair. Mapping is normally caused by an uneven chemical reaction with the metal due to contamination such as oils, compounds, sealers or other materials left on the surface. The contaminants either react with the metal forming a permanent stain, or they are not removed by a chemical stage in process, i.e. poor cleaning, where the contamination is not removed in the cleaning stage and the next chemical stage must remove the surface contaminate. If this is the conversion chemical stage then the time needed to deposit the coating is compromised.
Patterns such as streaking may be a result of drying in drain vestibules, misaligned spray nozzles, or other air and solution flow imbalances within the phosphate system. In most cases, skilled operators can rapidly correct these patterns by realigning nozzles or adjusting pressures. In some instances, additional wetting harnesses are added to systems to address these problems. Some systems are incapable of correcting certain patterns due to original design flaws. Slight patterns are not normally detrimental to ultimate coating quality. The technologies that are available offer the metal finisher several options to choose from to meet a customer’s performance requirements. Currently, iron and zinc phosphate predominate as the products of choice when it comes to applying a conversion coating to a metal surface prior to painting. However, with the ever-changing business climate and the need to reduce costs, systems must become more environmentally friendly while reducing energy and water demands. With the development of the new-generation coatings, the landscape is changing, and the metal finishing industry has an opportunity to meet the cost and environmental constraints while meeting the performance expectations of its customers.
New to industrial lubricants and grease? Consider this.
According to Thomas Net:
“Lubricants are substances applied to reduce friction and wear on surfaces that have relative motion between them. Although this is a lubricant’s primary function, it can also serve as a heat-transfer agent, a corrosion preventative, a sealing agent, and as a means of trapping and expelling contaminants in mechanical systems. While oils and greases are common forms of liquid and semisolid lubricants, they can be found in other forms as well: dry lubricants, gas lubricants (such as air), etc. Specifying lubricants for mechanical systems must take into consideration not only the need to reduce friction and wear but also the need for them to perform some or all of these secondary functions. Equipment manufacturers endeavor to find optimum formulations for their designs and equipment operators are advised to follow manufacturers’ recommendations for their selection and use.
“This article will discuss liquid lubricants, solid lubricants, and grease.
Types of Industrial Lubricants and Grease
“Liquid lubricants are largely produced from petroleum and synthetic fluids. The abundance of petroleum makes its use in petroleum-based oils ubiquitous and economical. Synthetic oils are generally more costly but are used in applications where their improved performance characteristics make the cost tradeoffs worthwhile.
“Among the many characteristics of liquid lubricants, viscosity is a dominant factor. Viscosity is defined as dynamic, or absolute, viscosity, in units of lb-sec/ft2. It is described as the measure of the velocity gradient between stationary and moving parts of a fluid. Kinematic viscosity, v, is defined as dynamic viscosity, or µ, divided by density, ρ, with units of ft2/sec. Kinematic viscosity is also expressed as SSU (or SUS), for Saybolt Seconds Universal, which assigns a number to a lubricant after running it through a capillary-type viscometer under Newtonian flow conditions. A common unit of dynamic velocity in the cgs system is the centipoise. Viscosity can be affected by temperature, shear, and high pressure.
“The Society of Automotive Engineers (SAE) classifies oils by viscosity, with SAE 5W, 10W, and 20W measured at 0°F and SAE 20, 30, 40, and 50 measured at 212°F. Any multigrade oil, SAE 10W-40, for example, will meet the viscosity requirements at both temperatures. Industrial liquid lubricants are classified by ASTM D2422 and ISO 3448. ISO VG (for viscosity grade) 2 through 1500 (in eighteen steps) represents the kinematic viscosity of 2 and 1500 mm2/sec (or, centistoke) measured at 40°C.
“The viscosity index, or VI, assigns a number from 0 to 100 based on an oil’s change in viscosity with change in temperature. A higher number indicates less change in viscosity with change in temperature. The scale was based on comparisons of Pennsylvania and Gulf crudes as the defining limits, but advances in refining have since achieved VIs that exceed both scale endpoints.
“An oil’s pour point defines the temperature at which an oil will flow and is an important consideration for cold starting engines and for gravity lubricators. Pour-point depressants can lower the pour point. A related attribute is the cloud point—the temperature at which any wax in the formulation begins to visibly separate, usually just slightly above the solidification temperature. This is important because wax can clog filters.
“Other attributes of lubricating oils include their flash and fire points, their propensity to foam when used in high-speed rotating applications such as turbines and crankcases, and their ability to withstand high-pressure when used in hypoid gearing and other extreme-pressure situations. A special group of lubricants, dubbed EP lubricants, (for extended pressure), are specifically formulated to inhibit the wear that might result when highly-loaded gears make metal-to-metal contact.
“As noted above, high pressure has an effect on viscosity, tending to increase it as pressures reach higher regions. Designers of highly-loaded machines use this fact and are able to specify relatively low-viscosity fluids that might be unsuitable for use in lower-pressure applications.
“Synthetic oils are formulated generally to increase one characteristic—high VI or thermal stability, for instance—albeit often at the expense of another characteristic such as pour point. Synthetic oils tend to be costlier than mineral-based lubricants and hence are employed in industrial settings only when the performance gains warrant the added expense, as in instruments and heat-transfer systems such as industrial ovens. Synthetics are made from a variety of fluids such as polyglycol for brake fluid, phosphate esters for fire-resistant hydraulic fluid, silicones for use with rubber and plastic, etc.
“The oil used in engines performs many functions besides lubrication: corrosion prevention, cooling, sealing, etc. Engine oil manufactures compound these products with a host of additives, including detergents, VI improvers, EP enhancers, pour-point depressors, and so forth to meet the many functions that engine oils serve.
“You can you the Thomas’ Supplier Discover Platform to find Suppliers of Liquid Lubricants.
“Solid lubricants, sometimes called dry-film lubricants, are chiefly forms of synthetic or natural graphite or molybdenum disulfide, applied loosely to sliding surfaces or mixed with binders. They are used mainly where temperature or pressure extremes or environmental conditions make liquid lubricants impractical. High-vacuum environments are one such setting, where molybdenum disulfide is preferred. Graphite needs the presence of water vapor to act as a lubricant, making it unsuitable for use under vacuum conditions.
“Both graphite and molybdenum disulfide achieve their low coefficients of friction due to the laminar, plate-like structure of their molecules and the relatively weak structure between plates. Some liken their effect as similar to trying to cross a room on which playing cards have been spread over the floor: the individual cards slide easily past each other, minimizing friction between foot and floor.
“Polytetrafluoroethylene (PTFE), another anti-friction material, does not share the same layered structure of graphite and molybdenum disulfide. It is used as an additive in oils and grease and some lubricating sprays. It can be applied as an anti-friction coating or film to a variety of machine parts including compressor pistons, slides, O-rings, etc. It is sometimes combined with aluminum for hard- coat anodizing.
“Solid lubricants can be applied as unbonded powders or granules or mixed with organic or inorganic binders to create curable coatings on friction surfaces. Molybdenum disulfide is sometimes vapor deposited onto compression fittings where it serves as an anti-seize agent.
“Grease is composed of liquid lubricant and a thickener, usually soap, in addition to additives which impart desirable properties to the formulation such as corrosion resistance and tackiness. Normally a semisolid, grease liquifies at a temperature referred to as the dropping point, which can range from 200 to 500°F and higher depending on the thickening agent. Greases thickened with calcium- or lime-soaps tend to have dropping points in the lower ranges while those thickened with clays liquify at temperatures quite a bit higher.
“The NLGI (National Lubricating Grease Institute) rates the consistency of greases from a semifluid (000) to very hard (5) and block type (6) based on penetration tests of the material in a worked state, whereby a standardized object is dropped into the product at a known temperature and time and the depth to which the object sinks is noted. As a point of reference, most grease-lubricated rolling-element bearings use an NLGI 2 grade.
“The consistency rating is not the equivalent of oil viscosity, however. This rating is determined by the viscosity of the base lubricant. Most grease manufactures will publish this data. Greases with identical NLGI ratings can have different performance characteristics. Again, as a point of reference, many grease-lubricated rolling-element bearings will use grease with viscosities similar to SAE 20 or 30 oil. As with oils, grease may be modified with EP agents to protect against damage to precision surfaces from shock loading, severe loads, static loading, or frequent starts/stops. Use of EP agents is recommended only when needed as they can be detrimental to bearing surfaces and the like, especially at elevated temperatures.
Types of Industrial Grease used in Industrial Applications
“Aluminum complex grease is used where high temperatures are expected. With a dropping point of 500°F and a maximum useable temperature of 250-325°F, this smooth grease is often used in food machinery.
“Modified Bentonite clay is used when exposure to very high temperatures is expected. This smooth grease, with a dropping point of 600°F, a maximum useable temperature of 250-325°F, and excellent water resistance, is popular for use in ovens as it has the ability to create its own seal, a plus where bearing seals are exposed to those high temperatures.
“Calcium12 hydroxy stearate is a smooth grease with very good water resistance, albeit lower maximum useable temperature than other greases (250°F) and a dropping point of 290°F.
“Lithium 12 hydroxy stearate is a popular grease for many bearing applications, with good water resistance, smooth texture, a dropping point of 380°F, a maximum useable temperature of 250-325°F, and a capability for long life. It is a very pumpable grease.
“Lithium complex is used in high temperature, high-speed bearings. With a smooth texture and decent water resistance, a dropping point of 550°F, and maximum useable temperature of 250-325°F, it is considered an improvement on Lithium 12, though this still makes up the majority of grease used for general purposes.
Polyurea is good for long life applications. This smooth grease has excellent water resistance, a dropping point of 460°F, and a maximum useable temperature of 250-325°F. It is often used on food machinery.
Sodium tallowate, once the primary grease for wheel bearings, is usually employed only in older, slower bearings and exhibits poor water resistance, a fibrous texture, a dropping point of 390°F, and a maximum useable temperature of 250°F. It is a low-cost grease with good rust preventative properties.
“Hydrodynamic lubrication relies on pressure developed in the lubricating fluid to support the bearing load. For hydrodynamic conditions to exist, there is necessarily a balance that must be achieved between speed, load, and viscosity. These conditions can be met over a fairly broad range. However, high loads, starting and stopping, etc., can all act to disrupt this balance, causing irregularities in the surfaces to come into contact. This so-called boundary-lubrication condition is the reason that soft metals are used in babbitted journal bearings so that the babbitt becomes a sacrificial surface rather than the harder shaft journal. It is the same reason that EP lubricants are used in certain gear applications as additives in the lubricant provide a cushion of sorts during these boundary-lubrication events. So-called µn/P, or Stribeck curves, plot coefficient of friction, f, against the nondimensional group µn/P (viscosity x speed/pressure) for shafts in lubricated journal bearings. As the shaft comes up to speed, the plot moves through zones of boundary lubrication to mixed lubrication to hydrodynamic lubrication.
“When selecting anti-friction metals such as tin- and lead-base babbitts, bronze, copper-lead, and even cast iron, as well as the various forms of sintered bearings, considering the running material (shaft, way, etc.) is equally important. For instance, soft steel will run well in babbitt, cast iron, or soft brass, but not in soft steel or bronze. Hardened steel will run well in soft bronze and many other metals, but not in hardened heat-treated bronze.
Hardened nickel-steel runs poorly in hardened nickel-steel.
“Rolling element bearings have high loads in the contact zones which tends to deform the elements. There is, theoretically, a thin film of lubricant that exists between the rolling elements and the races, known as elastohydrodynamic, or EHD, film.
Handling and Storage
“Oil is commonly shipped in 55-gallon drums and 5-gallon pails, and grease is often supplied in 35 lb. kegs. Oil is also supplied in bulk and stored in tanks. Most lubricants have shelf lives which are often determined by the additives in them. A good practice is to use the oldest stock first. In general, a room that is clean, dry, and free from temperature swings provides the best storage conditions for maximizing shelf life. Drums that are stored outdoors should be positioned on their sides and protected by tarps or shelters.
“During handling, drums may be rolled on their sides but should never be dropped. Drum-handling jaws for forklifts are available that surround the drum perimeters; forklift blades alone should not be used to grasp drum sides.
“It has been shown that oil cleanliness can have an impact on equipment life. The International Standards Organization (ISO) rates oil cleanliness by particles per milliliter based on the number of particles and their sizes. Even new oil can have particle counts higher than would be recommended for some machines, and filtering oil before use can be beneficial for equipment life. Likewise, careful handling of lubricants to avoid mixing different formulations and introducing contaminants is an important part of any lubrication program. Grease should also be carefully controlled to avoid mixing formulations.
“Used lubricants should be recovered or disposed of in accordance with sound environmental practices.”