Companies specializing in products that contain metal parts and components require parts to be cleaned before undergoing powder coating processes. Powder coating is a dry coating applied to metal to provide corrosion resistance, wear resistance, durability, and an aesthetic appearance.
The surface of the metal must be prepped to ensure that the powder successfully adheres to the surface. In addition, pretreatment is another method when prepping for powder coating processes. Pretreatment involves changing the metal's surface chemistry to enhance the metals properties and to ensure the powder coating bonds to the metal.
Metals typically undergo specific cleaning and pretreatment methods based on their type. Specialized pretreatments take into consideration the surface properties of the metals, which may be so strong that they resist powder coating adhesion or may experience changes from chemicals that cause negative characteristics that weaken the metals.
The process itself involves several stages. Metals undergo a blasting process that helps to remove any previous coatings as well as removes rough welds, laser scale, rust, and metal scale. Although this process may clean a lot of the surface and remove surface defects, it won't remove oils, grease, or other contaminants. The blasting will create a surface that is more inclined to accept powder coatings once it is cleaned.
Next, the metal undergoes a washing stage. The equipment used for washing the metal will be specific for the process, such as tongue-type nozzles and flat nozzles used for spray jet cleanings, immersion baths, or steam cleaning systems. Types of washing processes may involve dipping, pressure washing, ultrasonic cleaning, aqueous cleaning, and vapor degreasing. When removing oils and waxes, hot water, detergents, or steam cleaning may be used.
While cleaned metal can accept powder coatings, it may not withstand weathering or provide the satisfactory performance that manufacturers are looking for with metal parts and products. After the cleaning, the pretreatment stage begins. After the initial prepping for powder coating, the pretreatment methods may involve 3 various processes.
- Etching: Etching methods involve using acid-based chemicals. It's commonly reserved for metals that have slick surfaces or when it may be difficult for the powder coating to adhere to the surface.
- Phosphating: Phosphating is a conversion coating process using a type of phosphate to provide corrosion resistance and for better powder coating adhesion.
- Non-phosphating: Non-phosphating relies on a combination of etching processes along with a low-solids acrylic sealer. This process typically uses a 1-3 step spray system.
Once the pretreatment is applied, the metals will undergo rinsing. The rinsing stage is designed to remove any unreactive chemicals off the surface. Deionized water rinses or reverse osmosis rinses may be used for electrocoat applications. Otherwise, a dry-in-place rinsing method is used. A post-treatment may also be applied to the metal to increase the surface's resistance to humidity and moisture. This process may involve trivalent chrome, non-chrome, or dry-in-place polymeric treatments.
For aluminum, pretreatments improve the slick surface for powder coating. Along the surface of the aluminum is a natural aluminum oxide film. This film must be modified or replaced to ensure the cohesion process. Before the pretreatment, three different cleaning methods may be done: acid electrolytic cleaning, acid sprays or immersions, or alkaline immersions or sprays.
The pretreatment method may consist of an electrolytic process or a chemical process. The chemical pretreatment method, called conversion treatments, utilizes a no-rinse chrome chemical as it's applied using rollers on both sides. Then the aluminum passes through a hot air dryer/oven. The electrolytic pretreatment uses hot electrolyte in 2 cells and electrodes. As the aluminum passes through the electrodes, they anodize the electrolyte (part sulphuric acid) that modifies the aluminum oxide film. Then the aluminum dries and undergoes the powder coating.
Stainless Steel Pretreatment
Stainless steel can become difficult to pretreat due to its inert properties, as it becomes resistant to chemicals and modifications. An etching pretreatment process using acid baths is one method. Types of acid-based pretreatments may involve nitric and hydrofluoric acids that are combined. This process may also be called acid pickling.
For some applications, sandblasting may help to roughen the surface for treatment methods. Pressure washers can be used for parts of larger sizes, while a cabinet is utilized for smaller parts. Whichever method is chosen, a wash primer may be applied before any top coating is done. A wash primer involves etching the surface using phosphoric acid and a carrier mixture of inorganic fillers mixed with resin.
Pretreatment for steel will be dependent upon how it will be used in the final application. For some applications, a mere treatment of alkaline solution is applied within an immersion bath or as a spray through nozzles. Then the alkaline solution becomes rinsed off.
If the steel requires another step to ensure better powder coating adhesion, then the conversion coating is applied using either iron phosphate or zinc phosphate. For applications where the part or product will be used in the environment and when there are environmental rules or regulations in place, a non-phosphorus pretreatment is used. Then the parts are rinsed off.
When prepping for powder coating, the types of chemicals used dictate the systems that are needed for the metals. These systems may involve one process stage or multiple stages depending on how much of the metal's surface needs to be modified to accept the powder coating. Some pretreatment chemicals include zinc phosphate, iron phosphate, and zirconium phosphate.
Zinc Phosphate: Zinc phosphate may be applied using immersion-type or spray-type processes. The metal undergoes multiple cleaning stages. Then the parts go through a titanated active rinse. The metal becomes immersed in a zinc phosphate bath that has metal ions. A sealant rinse and a deionized rinse finishes the process. Zinc phosphate chemicals are often used when the metal will be exposed to corrosive environments as the chemical provides superior corrosion resistance.
Iron Phosphate: Iron phosphate pretreatment systems may have anywhere from 2 stages to 6 stages. The metal may go through various cleaning stages before moving on to the rinsing and then the iron phosphate application through spraying or immersion. Iron phosphate by itself provides good powder coating adhesion yet not as much corrosion resistance as zinc phosphate. A post treatment rinse using either chrome or non-chrome offers extra corrosion resistant properties to the metal surface. Afterwards, a deionizing rinse process is performed.
Zirconium Non-Phosphate: Zirconium non-phosphate systems use zirconium fluoride that is placed into a sealer of low solids acrylic. A 1-3 step spray system is utilized for this process.
In addition to these chemicals, activating agents and sealant rinses are used.
Lechler Pre-Treatment Nozzles
Cleaning, pretreatment chemical applications, and spray rinses all require versatile and durable nozzles to direct the spray in the desired location while ensuring even coatings. Here at Lechler, we offer various nozzles for water jet cleaning, eductor nozzles to remove sediment, and multi-stage nozzles for rinsing zones. For more information, reach out to our company today.