Pretreatment Chemistries and Wastewater Considerations

Posted on Wednesday, January 29, 2020

Metal pretreatment is an essential part of the powder coating process. Without proper pretreatment, the painted layer of a part might fail in a variety of ways and cause the metal substrate underneath to corrode, leading to visual degradation and structural failure. Pretreatment refers to the process that metal substrates undergo prior to paint.

The pretreatment process gives the painted metal increased corrosion resistance and improved paint adhesion to help prolong the life of the part and the painted surface. Metal pretreatment employs a variety of chemicals that must be handled and disposed of appropriately and responsibly in a manufacturing operation.

Any time a pretreatment method is being considered, the following questions need to be answered: 

What specifications does the coating need to meet?
Knowing which specifications a part is required to meet gives a goal that both the line manager and chemical supplier can design for and strive to attain.

What substrates will be coated?
Some products are meant for steel, some for aluminum and galvanized substrates, and some for both.

How will the pretreatment chemicals be applied?
There are some chemicals that should not be sprayed because they will foam.

  • Immersion systems. Can pretreat parts with many nooks and crannies but have a large footprint.
  • Spray systems. Good for simple parts and usually have a smaller footprint than immersion.
  • Manual spraywand systems. Use a lot of chemistry but have a small footprint. Parts need to be relatively clean before entering these systems.
  • Cleaner/coaters. Do cleaning and conversion coating in one step which decreases the number of stages. Parts need to be relatively clean before entering the system.

What sort of contamination will be present in the pretreatment system?
Maybe the city water used on the line contains high amounts of sodium, magnesium, or calcium that could contaminate a chemical stage on the line, or a certain brush lube has a surfactant that will cause excessive foam in the bath.

Are there environmental constraints that will limit use of certain chemistries?
There may be federal, state, and local regulations that constrain how a line can operate. In some locales, there are restrictions on VOCs, tri-chrome, phosphates, or certain metals in waste streams.

Four main factors that influence the performance of any chemistry are:

  • Concentration. The amount of chemical that is in the solution. Having too little will not get the job done, and having too much chemical may cause excess carryover, powdery coatings, and paint adhesion failure.
  • Temperature. The higher the temperature of a system, the more active the chemistry. If the chemistry is not active enough, it will not perform the proper function. Parts can be etched if the temperature is too high and some solutions can even break down.
  • Time. Chemicals need enough time to have the desired effect, but if a part is in an acid cleaner too long, it may be etched, which can cause issues.
  • Agitation. Aids in cleaning and fresh chemicals getting to the parts. Proper agitation is critical to the application of the chemistry.

The Pretreatment Process

The pretreatment process can be broken down into three main steps:

  • Cleaning
  • Conversion Coating
  • Post-Treatment

The cleaning step involves treating the metal substrates to provide a clean surface for the conversion coating step. The less contamination there is on the surface of the metal as it is treated with the conversion coating, the better the conversion coating step will be, and the better the post-treatment and paint will perform. Cleaning chemistries tend to be grouped in the following classes:

  • Alkaline Cleaners. These chemistries have a pH that is higher than 8 and are useful when trying to remove any organic soils from a metal surface. These soils include lubricants, oils, rust preventatives, and dirt that has collected on the metal during fabrication. Non-etching alkaline cleaners are available for aluminum and other light metal substrates where surface appearance is critical. Factors to consider generally include temperature, free alkalinity, and total alkalinity. Some alkaline cleaners have phosphates which are sometimes locally regulated because they can cause eutrophication in bodies of water. Phosphates show up in many pretreatment chemistries, and because of this, pretreatment chemical suppliers have come up with phosphate-free alternatives to their products. Additionally, some oils that are removed from the part surface may contain volatile organic compounds (VOCs) which can also be regulated locally.
  • Acid Cleaners/Pickles. These are chemistries such as sulfuric acid, phosphoric acid, and gluconic acid with pHs under 5. Acid cleaners will perform light duty inorganic contamination removal such as light amounts of rust. Acid pickles will remove mill scale, weld scale, heavy rust, and laser scale from metal surfaces. These chemistries can etch the surface of the metal in preparation for the conversion coating step. Inhibitors are used in the formulations to prevent attack of the base metal. Phosphoric acid contains phosphates, which again can cause eutrophication. When acid cleaners are used in immersion applications, an iron removal system may be necessary to remove excess iron that has been etched from the stage. Typically, temperature, free acid, and iron content are monitored in acid cleaners/ pickles.
  • Neutral Cleaners. Chemistries that have a typical pH in the 6-8 range. These are used when mild degreasing or descaling is necessary and the increased cleaning capabilities of the alkaline or acidic cleaners are not required for the part. The concentration, pH, and temperature of neutral cleaners should be controlled in operation.

Conversion Coating
After the metal is cleaned it is ready for the conversion coating step. Conversion coating refers to the surface of the metal being chemically converted to a completely different substance that has increased corrosion resistance and a greater surface area which will promote enhanced paint adhesion. Most pretreatments are acidic in nature. The acidic solution first etches the metal surface and then deposits a substance which is chemically bonded to metal substrate. Typical conversion coatings are:

  • Zinc Phosphates. These acidic chemistries grow a crystal on the metal substrate. Zinc phosphates tend to outperform iron phosphate and zirconium products in salt spray tests. The zinc phosphate bath should be preceded by a conditioner that creates nucleation sites where the zinc phosphate crystals will begin to grow. Zinc phosphates generate sludge and need to have a sludge removal system. It should come as no surprise that zinc and iron phosphates all contain phosphates. Free acid, total acid, accelerator level, and temperature all need to be monitored when performing zinc phosphating.
  • Nanoceramics. The nanoceramic conversion coatings require little to no heating, do not generate significant sludge, and generally have no phosphates. These are an environmentally friendly and cost-effective alternative to zinc phosphate, and they typically outperform iron phosphates on salt spray tests. Concentration and pH are critical when using nanoceramic conversion coatings.
  • Iron Phosphates. These chemistries create an amorphous layer on the metal substrate rather than a crystalline structure like that of the zinc phosphates. Iron phosphates produce less sludge than zinc phosphates, but still contain phosphates. Temperature, concentration, and pH are critical when using iron phosphates.
  • Chromes/Chromates. These include hex-chrome, tri- chrome, chromates, and chrome phosphate conversion coatings. These are toxic chemistries, and hex-chrome is being regulated out of the pretreatment industry. The military still utilizes these due to their outstanding performance.

Finally, after the conversion coating has been applied, a post- treatment may be applied to seal in the conversion coating and provide improved corrosion resistance and paint adhesion. The post treatment will seal any voids in the conversion coating. Post- treatments tend to be categorized as follows:

  • Organic post-treatments. These post treatments are made of carbon-based chemistry. They will deposit a polymeric layer on the surface of the conversion coating. With all post- treatments, concentration and pH are key to performance.
  • Inorganic post-treatments. Inorganic chemistries are those that do not rely on carbon-based chemicals. These chemicals react with the conversion coating to give enhanced corrosion protection and paint adhesion.
  • Organo-metallic post treatments. Organo-metallics have organic chemistries complexed with metals. These post treatments leave a metal and polymer combination on the surface of the conversion coating. These are used particularly on lines that require improved performance with weaker paint systems and react with the conversion coating rather than just depositing a layer on the surface.
  • Chrome pretreatments. These include hex-chrome and tri-chrome products.

Rinse Stages
The rinse stages need to be accounted for in addition to all the chemistry stages. The rinses collect chemistry due to carryover and dragout. The amount of chemistry that is transferred into the rinses will contribute to overall phosphate and heavy metal levels in the wastewater. The rinses not only remove contaminants from the part surface after it exits a chemistry stage, but they also serve to shut down most chemical reactions that are taking place. This is especially important as parts are exiting phosphate stages as they can over-phosphate, which will give the phosphate a powdery surface and lead to paint adhesion failure. As a part exits an acid cleaner stage, there is a chance that the surface will be too active and possibly cause flash rust. To avoid flash rusting, the rinse after an acid cleaner can contain a chemical to neutralize the acid cleaner carryover. 

The acid cleaner process includes four stages:

  • Acid cleaner.
  • Rinse. This rinse prolongs the life of the neutralizing rinse by decreasing acid carryover as well as preventing salting out of acid/alkali residues.
  • Neutralizing rinse. Typically, a high pH rinse to stop the acid reaction, neutralize the surface, and re-dissolve any flash rust on steel parts.
  • Rinse.

Every rinse stage should be overflowed to remove contaminants by introducing fresh water into the stage. Whenever possible, rinses should be counterflowed from one stage into another to save water use. Whenever there are two rinse stages one after another, the downstream rinse stage with lower conductivity can be counterflowed into the upstream stage to decrease the amount of water needed to overflow the rinses. 

For further information on wastewater treatment, please consult your wastewater system provider.

Zach Benekos is application engineer for Henkel Functional Coatings.