Cleaning and Pretreatment Solutions

Choosing the right cleaning and pretreatment solutions for your operation depends on many factors. The cleaning treatment must remove all the contaminants without causing any harm to the metal surface. Manufacturers have formulated cleaning and pretreatment solutions to be effective against specific contaminants, under certain conditions, in certain types of equipment, on certain types of metal, and for different purposes. Factors to be considered include:

  • Contaminants and Soils – What soils and contaminants need to be removed? What cleaning solutions are effective for these contaminants? And which will not be effective? For example, organic solvents may not be effective in cleansing water-soluble contaminants such as acid and alkali salts; water or water-based cleaners should be used instead.
  • Part Geometry or Shape – What sort of parts need to be cleaned? What are their sizes, shapes, and weights? Will they tend to nest together, and therefore block the flow of cleaning solutions? Are there blind cavities in which cleaning solution and soils can be trapped? What equipment is best for treating these items, and what solutions are appropriate for the equipment?
  • Part Chemistry or Composition – What are the parts made of? What metals (and other materials) are going to be cleaned and treated, including alloys, plastics, rubber, etc.? The exposed surface material of the part is called the substrate. What cleaning and treatment solutions could damage the substrate? For example, amines will tarnish copper alloys; hydroxides will mar aluminum surfaces; and ferrous parts (iron or steel) will need special treatment to inhibit immediate rusting. Non-metal components can also be affected, particularly plastics and rubbers, which can be damaged by the hydrocarbons found in many cleaning solutions.
  • Equipment and Methods – What equipment and methods are already in place? What sorts of solutions are the machines formulated to use? What is their range of operating temperatures? (Cleaning solutions are formulated to be used at specific temperatures.) Which cleaning solutions are compatible with your existing equipment and processes? What limitations might exist (e.g., available floor space, water quality or supply, etc.) that might influence the choice?
  • Other Treatments – Do other coatings need to be applied during the pretreatment process, such as anti-corrosives, sealants, or treatments that are required for specific finish coatings?
  • Standards for Cleanliness – How clean do the parts need to be? What standard is applied in your operation? Is cleanliness relative to performance or to appearance? Are there rinse requirements? How does the end use factor in, if at all? For example, a standard of cleanliness required for metal 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.
  • Compatibility with Coatings — What paints or coatings will be applied later? What cleaning solutions are compatible with these finishes?

Contaminants can be classed as organic or inorganic:

  • Organic contaminants include any waxy or oily substances such as coolants, lubricants, rust inhibitors, milling and machining oils, drawing compounds, soaps, and waxes. These are best cleaned with alkaline cleaners or solvents.
  • Inorganic contaminants include shop dirts, fluxes, abrasives, rust, tarnish, scale, smut, burned-on soils (formed during quenching operations) and other inorganic particulates. These are best removed with acidic cleaners.

Contaminants can also be considered by how difficult they are to remove:

  • Very difficult to remove: Buffing compounds, honey oils, strong rust-inhibiting compounds, chlorinated lubricants, die cast release agents, heavy-duty stearates, oxidized soils, sulfurized lubricants, soils that have been in place for a long time; soils that have been trapped between metal pieces during welding or fabricating processes.
  • Moderately difficult to remove: Waxy or fatty oils, mill oils, water-displacing rust inhibitors, heavy-duty hydraulic oils, lapping compounds.
  • Relatively easy to remove: Water-soluble rust inhibitors, synthetic cutting fluids, spindle oil, mill oils, lightweight machine oils, soluble oil-cutting fluids, and vanishing oils.

Metal-containing substrates can be classed as follows:

  • Ferrous or iron-bearing – Cold-rolled steel, hot-rolled steel, stainless steel, and ferrous castings.
  • Nonferrous – Aluminum, sheet, coil, castings, extrusions, zinc castings, galvanized, terne plate, and zinc plated.
  • Yellow metals – Copper and brass.
  • Mixed metals – Combinations of the above.
  • Composites – Mixtures of metals with other materials.

Each type of metal (including alloys) requires a unique approach to ensure effective results. Industry resources provide guidance on specific processes and recommendations for different metals.

CLEANING SOLUTIONS

Cleaning solutions are classed as aqueous solutions or solvents:

Aqueous Solutions

Aqueous (water-based) solutions can be alkaline, neutral, or acid:

  • Alkaline (pH higher than 9) – Most cleaning solutions are mild to strong alkaline products; these are used safely on a variety of metals, including ferrous metals, aluminum alloys, brass, and magnesium alloys. Alkaline cleaners are best used on organic contaminants, such as oils and waxes, but are effective against a wide range of soils.
  • Neutral – To make up for their milder chemical profile, neutral cleaning solutions use more surfactants to break down soils. They are safer for workers and the environment, but degrade more readily.
  • Acid (pH lower than 6) – Acid solutions are typically used for cleaning stainless steel alloys, brass alloys, wrought aluminum alloys, and copper. They are effective at removing rust, oxidation, and scale.

Aqueous solutions are generally considered to be more “environmentally friendly” than solvents, but are not as effective in cleaning small or complex parts, and require additional rinsing and drying steps, both of which require more complex machinery, more floor space, and more processing time. Aqueous cleaners are usually supplied as powders or liquid concentrates, to be mixed with water before using. They are compounds of ingredients that dissolve soils, adjust water chemistry, and inhibit corrosion.

Solvents

Though solvent cleaners are very effective against a large range of contaminants, they are being phased out due to environmental, safety, and health concerns. Solvents are highly effective against a wide range of contaminants, including adhesives and other hard-to-remove oils. They are typically used on small surface areas, applied by wiping, flow-over, or vapor degreasing. Solvents are superior for cleaning small or complex parts, and are more cost-effective than aqueous solutions.

However, due to their toxicity and volatility, their use is controlled by a range of federal regulations to protect worker safety and the environment. Risk may be minimized by carefully following relevant regulations and manufacturers’ guidelines for handling, storage, and use. The use of sealed vapor degreasers almost entirely eliminates workers’ exposure to solvent vapors.

Test Before Using

Before choosing any cleaning solutions, and certainly before introducing them in your operation, take time to test several products on a range of sample parts, and analyze the results carefully. Your supplier will assist you in this process, using their facilities and expert staff to test samples from your shop and provide advice on selecting the right cleaning solutions and equipment for your operation.

Evaluate the cleaned samples very carefully:

  • Assess them against your usual cleanliness standards.
  • Apply paints or coatings that would normally be used for these parts.
  • Assess the final finished results against the most rigorous quality control standards of you and your customers.
  • Consider how using the particular solution(s) would affect your processes, routines, equipment, and personnel.

The test results will guide you in:

  • Choosing the best solutions for your products and your operation.
  • Planning for the best equipment and workflow for optimum results (spray? immersion? combination?).
  • Determining appropriate operating temperatures, fluid concentrations and pressures, and length of time required for cleaning.

Most cleaning and pretreatment applications do not need the strongest-available solutions. Choose the least corrosive, least hazardous products that will do the job.

The three cleaning processes, with the advantages and disadvantages are summarized here:

CLEANING PROCESS

METHOD

ADVANTAGES

DISADVANTAGES

Mechanical Cleaning

  • Solid media Impingement: Alumina blasting, steel shot, plastic media blasting, etc.

  • Removes most inorganic contaminants

  • Single-step process

  • Impingement roughs up metal surface, good for paint adhesion

  • Reclaim and reuse lowers cost

  • Not applicable for thorough removal of organic contaminants

  • Imbedded media, dust can interfere with subsequent coatings

  • May damage parts or change dimensions of critical parts

  • Provides no corrosion protection

Solvent Cleaning

  • Wipe cleaning, flow over (sink-on-a-drum), vapor degreasing

  • Excellent removal of organic contaminants

  • Can be used cold or hot

  • Does not require water rinsing to remove process chemistry

  • Uses less energy and water than aqueous cleaning systems

  • Wipe and immersion processes can redeposit soils

  • No ability to remove inorganic contaminants

  • Flammability, worker safety issues

  • Provides no increase in either paint adhesion or corrosion resistance

  • Heavily regulated

Aqueous Cleaning

  • Alkaline, acid, or neutral

  • Single or multi-stage processes

  • Spray, immersion, ultrasonic, barrel, electrolytic, vibratory, cleaning devices

  • Environmental advantage over solvents

  • Numerous washer designs to match application need – spray, immersion, vibratory, etc.

  • Soils can be separated from solution via separation/filtration techniques

  • Cost of chemical, water, energy

  • Cost of equipment

  • Maintenance of equipment – nozzles, clean-outs, etc.

  • Requires control, testing

SOURCE: Patel, Suresh. “Importance of Cleaning and Rinsing in the Pretreatment Processes.” FabTECH Canada, 2014, and other sources.

CONVERSION COATINGS

Pretreatment conversion coatings can be applied at a number of points in the process, depending on the coating and the desired end product.

Phosphate Coatings

For more than a century, iron phosphate and zinc phosphate conversion coatings have been the standard conversion coatings in use.

Iron Phosphate Conversion Coating is well-understood, effective, and widely-used. It is easier to use than the zinc phosphate process, as it uses fewer stages, but its amorphous coating offers less protection against corrosion. The addition of a non-chrome seal rinse product can enhance corrosion resistance. Iron phosphate can be applied by spray wands, in a dip system, in batch washers, or in line spray washers. For most cleaning, the process can be as simple as clean/coat stage plus a tap water rinse; for more complex parts or higher quality requirements, a separate cleaning stage can be added, along with a seal rinse and a final rinse with deionized water. Iron phosphate is a good choice for a range of products that require a durable finish but are not exposed to severe environments. It offers good protection with relatively low cost, low hazard, and good flexibility.

Zinc Phosphate Conversion Coating offers more corrosion protection than iron phosphate. It forms a crystalline structured coating that provides very good durability for paint finishes in corrosive environments, even on mixed metals. It is necessary to apply a surface conditioner to refine the crystal morphology, important in ensuring a consistent coating. Zinc phosphate offers good performance, good quality, easy operations, low operational cost, and relatively low hazard. Recent improvements have been made to lessen its environmental impact, including reformulating to operate with lower operating temperatures and to produce less sludge.

Low-Phosphate or Phosphate-Free Coatings

Due to concerns about the environmental effects of phosphates, low-phosphate or phosphate-free treatments are rapidly becoming more popular and may eventually replace zinc phosphate and iron phosphate as the predominant choices.

Non-phosphate treatments do not require high heat during application, thus saving energy and resources; residual heat from the cleaning stage is typically sufficient to support the temperature range (90°F-120°F) required to support the chemical bonding process. Treatment and disposal of low- or non-phosphate finishing treatment wastes is simpler, easier, and less costly than it is for phosphate treatments.

Non-phosphate treatments can be applied with equal effectiveness by spray, immersion process, or pressure wand. Smaller amounts are needed for a quality coating; where iron phosphate is typically applied at 250-500 nm?  (mm?) thickness and zinc phosphate at 1000 nm, newer non-phosphate coatings may be as thin as 50 nm and still be effective. However, surfaces must be exceedingly clean before application, to ensure that the proper bonding reaction can take place on the metal surface.

Oxides of zirconium, titanium, and vanadium are among the most commonly-used of these newer coatings. These coatings are composed of very small particles that yield an exceptionally smooth, densely formed surface that is highly impervious to air, moisture, and corrosion and supports excellent adhesion of the final paint or other finish coat.

Some low- or non-phosphate conversion coatings:

Alkali Metal Phosphate plus Non-Chrome Coating is a relatively new combination that offers good corrosion protection for steel, galvanized steel, and aluminum alloys. It is applied in spray or immersion processes.

Manganese Phosphate Conversion Coating produces a brittle, heavy crystalline coating suitable for holding lubrication intended for breaking-in of parts.

Chromate Phosphates offer very good resistance to corrosion and humidity and excellent paint adhesion. This process was developed specifically for pretreatment of aluminum parts.

Chromate Oxide Films are commonly used for pretreatment of aluminum. They perform better than chromate phosphate coatings, offering excellent resistance to corrosion; in fact, the chemical structure can actually initiate some repair of corrosion. Chromate oxide films are useful for complex parts or those that are made with multiple metals, providing a very durable coating.

Zirconium Non-Phosphate Deposition Coating requires shorter processing time, uses a simpler process, and requires lower operating temperatures than zinc or iron phosphates. Though it creates a thinner coating than iron or zinc phosphates, it offers good corrosion protection for steel, zinc, and aluminum. Because it uses a simpler process with fewer stages and less water, it is less costly in terms of environmental impact and response.

Phosphonate-based conversion coatings are a relatively-new alternative. These coatings combine fluoacids and phosphonate so that, under the right chemical conditions and in combination with quality paints and powder coatings, they form an amorphous layer that promotes adhesion and inhibits corrosion. The protective bond formed between the substrate and the paint or finish coat is stronger than that formed by traditional iron phosphate treatments. An additional benefit is that this can be achieved without extra sealers, at ambient temperatures (that is, without added heat), and with less waste, offering cost savings over other treatments.

Post-Rinse Treatments

After the conversion coat is applied and the surface rinsed with water, a post-rinse treatment may be applied to further increase resistance to corrosion and moisture. As with all parts of the cleaning and pretreatment process, the choice to use a post-treatment depends on the material and its intended use. Parts that receive post-rinse treatments show better durability and resistance to corrosion and humidity.