Metal Pre-treatment in Multi-Stage and Single-Stage Systems, Vapor Degreasers

Metal Pre-treatment in Multi-Stage and Single-Stage Systems, Vapor Degreasers

Pretreatment Systems Comparison

Recirculating spray or immersion pre-treatment systems are highly effective in high-volume production environments. These systems can have single or multiple stages.

The choice of a single-stage or multi-stage system depends on the number and size of parts to be treated, the types of contaminants to be removed, and whether or not the treatment must be integrated into the manufacturing process.

Multi-Stage Systems

Multi-stage systems typically consist of three or five process stages, sometimes more. The number and types of stages depend on the characteristics of soils to be removed, the degree of cleanliness required, and the type of pretreatment required to be compatible with the end coat or finish. In general, five-stage systems are more flexible and “forgiving” when dealing with changing types and concentrations of contaminants, especially in adjusting the timing of the various cleaning and treatment stages.

A typical three-stage system consists of the following:

  • Cleaning – The process begins with the removal of obvious soil and contaminants. Depending on the material involved and the nature of the contamination, this can be done by mechanical means (such as blasting or scraping) or through the use of solvents, acids, or alkaline cleaning agents, applied manually or in machines designed for the purpose. This process cleans the part but generally does not add any protective material or process. The intent is to clean effectively without damaging the surface.
  • Rinsing – Effective rinsing is a critical element in the cleaning stage. The rinse removes the cleaning solution and the contaminants that remain on the surface. Some rinse stages also include treatments such as conditioners or corrosion inhibitors. If the wash stage uses very hot solution, or if flash drying might be a problem between stages, a spray system may be installed between stages to prevent residual contaminants from drying onto the surface and to prevent flash rusting.
  • Conversion Coating – Conversion coating strengthens corrosion resistance and prepares the surface to better accept the paint or other finish coating and promote long-term adherence.

 

A typical five-stage system consists of the following:
 

  • Post-Treatment Rinse – After the conversion coating is applied, the surface is rinsed with water (sometimes deionized or treated with reverse osmosis) to remove any conversion agents that have not reacted with the substrate to form the coating. Rinsing also prevents the “migration” of chemicals from one stage to the next. Metal parts that undergo post-treatment rinse that includes a rust inhibitor demonstrate improved resistance to humidity and corrosion (up to ten times better).
  • Drying – Treated parts must be absolutely and completely dried and cooled before paints or coatings can be applied. Drying or dry-off ovens are often used, with drying temperatures typically ranging from 250°F to 400°F. Parts that contain ferrous metals are vulnerable to flash rusting and must be dried quickly and thoroughly. Time, temperature, and air flow must be considered to ensure adequate drying and to prevent the formation of streaks and spots. Drying can also be done by evaporation, compressed air blow-off, air knife, infrared lamps, or cloth wiping. Drying and cooling are usually the most time-consuming steps in the cleaning and treatment process, so it’s important to have enough equipment to prevent bottlenecks.
  • Curing – During drying, water and solvents evaporate from the coating. During curing, a chemical reaction occurs to render the coating hard, inert, and relatively resistant to abrasion and corrosion. Air-dry coatings cure at ambient temperatures; baked coatings must be cured in ovens, generally at temperatures above 250°F.

These automated systems use powered belts or conveyors to move large numbers of parts through connected cleaning and treatment stages. These systems are typically used in high-volume operations and almost always use aqueous cleaners, though some use solvent cleaners. Benefits of multi-stage systems include high operational efficiency, reliability and consistency, and minimal handling by workers.

This table summarizes typical pre-treatment processes using different phosphate conversion coatings:

Three-Stage
Iron Phosphate
Five-Stage
Iron Phosphate
Zinc Phosphate Dry-in-Place
Coating
Low-Phosphate or
Phosphate-Free
Coating
Clean/Phosphate
Water Rinse
Post Treatment
RO/DI Rinse* •
Clean
Water Rinse
Iron Phosphate
Water Rinse
Post Treatment
RO/DI Rinse* •
Clean
Water Rinse
Surface Conditioner
Zinc Phosphate
Water Rinse
Post Treatment
RO/DI Rinse* •
Clean
Water Rinse
RO/DI Rinse* •
Clean
Water Rinse
RO/DI Rinse*
Coating
RO/DI Rinse*

* RO/DI = reverse osmosis/deionized water; • = optional in some systems
SOURCE: Products Finishing, February 18, 2011.

This table compares typical multi-stage pretreatment processes using different low-phosphate or non-phosphate conversion coatings:

Three-Stage

Five-Stage

Ex. 1

Ex. 2

Ex. 1

Ex. 2

Ex. 3

Ex. 4

Clean
Rinse
Coating
[for a dry-in-place coating]

Clean/Coating
Rinse
Rinse/Seal
[for a “clean-coat” coating]

Clean
Rinse
Coating
Rinse
Seal

Clean
Rinse
Rinse
Coating
Rinse/Seal

Clean/Coating
Clean/Coating
Rinse
Seal
Rinse/Seal

Clean
Water Rinse
RO/DI Rinse*
Coating
RO/DI Rinse*

SOURCE: 81st Universal Metal Finishing Guidebook, 2013 and Products Finishing, February 18, 2011.

Single-Stage Systems

Single-stage systems are often preferred for their lower operating costs and reduced environmental impact. They are appropriate for odd-shaped large items, or for large items that have cavities or intricate parts. High-pressure cleaning (e.g., 1,000 psi) can be accommodated.

As an example, the process for a single-stage organic phosphating system is as follows:

The part is immersed in (or sprayed by) a special combination of cleaning and treatment solution for 60-90 seconds. During the brief treatment time, three things happen.

  • First, solvents wash off and solubilize the oils and other particulate contaminants; a special resin in the bath absorbs and contains these solubilized oils and makes use of them as described below.
  • Second, phosphoric acids react with the metal surface to create an inorganic phosphate layer on the substrate metal. (On steel parts, the newly-formed layer is iron phosphate; on galvanized metal, it is zinc phosphate. No phosphate layer develops on aluminum, but the phosphate treatment enhances adhesion of? the paint or coating and inhibits corrosion.)
  • Third, the resin forms a coating on the surface of the cleaned and treated part. The resin incorporates the oils as plasticizers; these make the coating more flexible and less resistant to cracking and chipping. The oils are chemically trapped in the resin and cannot come to the surface and thus will not interfere with adhesion of paints or coatings.
  • After the cleaning stage, the parts are removed from the bath without rinsing and are dried by air drying, oven drying, or blow drying. The resin, which remains on the surface, forms a continuous seal over the surface of the part that can prevent flash rusting for weeks.

Single-stage organic phosphating systems offer several “green” benefits:

Whether they drip dry or have the solution blown off, the excess solution is recovered to the tank.

  • Because the system is essentially self-cleaning (oils are continuously trapped in the resin, and particulates are filtered out), it is easy to maintain. The components of the solution are used in proportion and are always in proper balance. The tank never needs to be discharged or dumped; it can simply be topped off with fresh solution. Bath analysis can be scheduled every one or two months, as opposed to hourly or daily, as must be done with conventional pretreatment systems.
  • The system operates at ambient plant temperatures (60°F-100°F) and does not require extra heating, thus saving energy and reducing risk of injury to workers. Because it uses fewer and smaller pumps and fans that a conventional multi-stage system, it requires less energy to operate.
  • The system uses no water and does not include a rinse stage, so that nothing enters the wastewater stream, and no special hookups or permits are necessary. Unlike conventional pretreatment systems, these single-state organic phosphating systems do not create sludge or other hazardous byproducts, with the exception of the particulate wastes that were on the parts to begin with.
  • In conventional metal pretreatment systems, oils and organic contaminants are wastes that must be collected and disposed of properly. In the organic phosphating systems, these materials are recovered and used to create a protective coating for the cleaned and treated parts.

In addition to improved operational efficiency and reduced environmental impact, single-stage organic phosphating systems can offer comparable results to conventional multi-stage treatment processes and can also yield significant cost savings, depending on the particular operations and processes.

Vapor Degreasing

The single- and multi-stage cleaning systems described above typically use aqueous (water-based) cleaning solutions. When solvents are used for cleaning, they are typically applied with wiping cloths, in flow-over devices, or, most commonly, in vapor degreasers.

Vapor degreasing is extremely effective in precision cleaning of intricate parts. In a typical vapor degreaser, liquid solvent is heated to its boiling point so that it generates a vapor. The vapor rises through the machine where the soiled parts are suspended on racks or in baskets and exposed to the vapor, which loosens surface contaminants on the parts. When the vapor eventually reaches condensing coils located at the top of the device, the cool-temperature coils chill the vapors and condense them back to their liquid state, so that they drip over the soiled parts, washing away the contaminants that have been loosened by the hot vapors. The soiled liquids are drained back into the machine, where they are cleaned and re-routed for re-use. The parts can also be immersed directly into the hot liquid solvent. Vapor degreasing uses less energy and water than aqueous cleaning systems.

Most degreasers are of these four types:

  • Traditional open-top tank
  • Enclosed tank (low-emissions, high energy efficiency)
  • In-line (conveyor) system
  • Airless or vacuum system

The selection of the proper cleaning solvent is critical to a good outcome. The solvent must be appropriate for the contaminants, the substrate, the equipment, and the process. Factors to consider:

  • Effectiveness – The solvent must be effective against the specific contaminant(s).
  • Compatibility – The solvent must not harm the parts that are to be cleaned.
  • Toxicity – The solvent must be as safe as possible for the workers who will be handling it.
  • Environmental factors – The solvent must not pollute the air and should be recyclable.

Physical properties – The solvent must be formulated for use in the specific degreasing machine, and must support the efficiency of the process. Factors to consider include evaporation rate, vapor pressure, boiling point, latent heat, vapor density, surface tension, viscosity, and flammability,

Commonly-used solvents include trichloroethylene, perchloroethylene, and methylene chloride. These are effective against organic soils, and are chemically compatible with a wide variety of materials (metals, glass, plastics, etc.). They are essentially nonflammable, have low latent heat of vaporization, and require little energy. They are non-corrosive and have relatively high stability, and in fact many contain stabilizing additives that convey corrosion protections onto the parts they are cleaning. These solvent vapors are heavier than air, meaning that they can be easily contained. Most modern vapor degreasers are fully enclosed, reducing or eliminating the escape of solvent vapors to the environment. These solvents are easy to recycle, a fact that reduces waste and environmental impact and makes them very cost-effective, as they can be re-used.

Manual Cleaning

Though most cleaning and pretreatment is done with machines, manual cleaning may be done in special circumstances, such as with highly-valuable unique objects, or with pieces whose characteristics make machine cleaning impractical or risky.

Manual cleaning can be done by simple wiping with a clean cloth and a cleaning solution (typically, a solvent) or, as appropriate, with a spray wand. The object can also be dipped into an immersion tank or cleaned in a cabinet washer.

Care must be taken to clean and rinse the surface uniformly and to remove all traces of cleaners, solvents, and fibers deposited by cleaning cloths. The act of wiping often redeposits soils on the surface.

Because simple wiping by hand is labor intensive and is considered not fully effective in preparing the surface for the coating application, and because it can put workers at risk, more efficient and safer machine methods are generally used.

The Solution is Clear with Riveer.

The Solution is Clear with Riveer.

The Solution is Clear with Riveer.