The keys to a good powder coating are two: the pretreatment and the grounding.
With these two elements effectively under control, the results become outstanding coatings. With the pretreatment, you ensure that the piece is ready to receive the powder and adhere easily to the piece. With proper ground, you have a piece that attracts the Powder particles, turning into a regular film that covers it.
Powder coating has three steps or procedures: the first is pretreatment, the second is an application, and the final is the curing, but to ensure a successful start, the pretreatment and the grounding must be under control.
Pretreatment could consist of three to seven stages. The piece must be degreased, debris-free, and clean before moving into the application area. There are some processes where sealant is used, even though the necessity of sealant application depends on the piece's intended use to be coated. It depends on the final use of the piece to coat.
Currently, the first stage of the pretreatment is carried out with alkaline degreasers, then rinsed with plenty of water for the second stage and a cleaner free of chromates for the third stage. Innovations in pretreatments gather nanotechnology for aluminum and other metals. Initially, Iron phosphate or zinc phosphate were used as a surface preparation, but in some countries, they are restricted due to their environmental effects.
The main purpose of the pretreatment is to prepare the substrate by opening the substrate pores and eliminating grease, oil, solid particles, and salts to make the surface corrosion-resistant and the optimization of coating adhesion.
The number of stages or types of pretreatment depends on the exposure of the part, so it is related to the type of corrosion it will suffer. Sales and particles can be a source of corrosion and surface contamination. These cause the substrate to lose its paint after a while or begin to generate corrosion between the substrate and the paint. All metals have a layer of oxide on the outside. This outer layer, upon contact with moisture, turns into rust. The rust is a mixture of ferrous and ferric oxide.
Refer to Table 1 for a summary of corrosion classification according to the ISO 12944 standard.
Table 1. Summary of the ISO 12944 corrosion classification.
|Very low risk of corrosion; for indoors with relative humidity of 60%
|Low risk of corrosion; For unheated interiors; interiors with good ventilation.
|Medium corrosion risk. For interiors with fluctuating temperature and humidity. Exteriors far from the sea and industrial areas.
|High risk of corrosion; for densely populated areas or near industrial areas; near open waters and near the coast.
|Very high risk of corrosion; for high and constant humidity. Near industrial areas or use of chemical substances.
To clean a piece, we have several options; we will analyze the alkaline and the mechanical ones here. We can clean mechanically, but if there is grease and oil, mechanical would not be enough. This is cleaning with an alkaline solution, which prepares the surface to increase the area of protection against corrosion.
For mechanical cleaning, sandblasting is commonly used; in the chemical cleaning phase, there are four types of treatments: iron phosphate, zinc phosphate, chromate, and other chemicals free of phosphate, chromium, and heavy metals. Therefore, an ideal pretreatment would be sandblasting and chemical pretreatment. Iron phosphate is no longer used due to its environmental effects, but let’s use it as an example.
Some companies use iron phosphate, but a water treatment facility must return the chemical to a safe pH level. Iron phosphate operates at a temperature between 100 and 150 Fahrenheit with a pH between 4.0 and 5.0, and zinc phosphate operates between 120 and 150 Fahrenheit with a pH between 2.8 and 3.2. Zirconium can work at room temperature with a pH between 3.0 and 6.0. Another negative aspect of iron phosphate is that it contains heavy metals, while other cleaners do not.
A phosphate or chromium film is used to prepare a rough surface for protection, although new chemical treatments come low or free of phosphate, chromium, and heavy metals. The adhesion of the coating to the part improves due to the change in the surface structure provided by the pretreatment. For example, a rough surface of 1 mm can change to 2 mm after application, making a more prepared and flatter surface so that the coating covers the substrate completely.
This pre-treated surface provides better adhesion and anti-corrosion properties. For example, we can see the physical characteristics of a substrate; the numbers represent the critical zones, one being the simplest and three being the most complicated due to having Faraday cages. Potential problem areas can be visualized for applying both pretreatment and coating. Sharp edges can cause problems in both protection and appearance. The ideal would be to polish the edges to reduce the effect. In the case of welds, these must be of good quality, continuous, non-porous, free of burrs, and free of oxidation spatters.
This cleaning can be done through brushing, sandblasting, and shot blasting, which helps remove weld scale. This process could degrease the piece of mineral oils, greases, salts, surfactant polymers, corrosion, dust and other solid particles, but chemical treatments are more efficient in this area.
Sandblasting is the method that uses compressed air to shoot abrasive particles to the surface of the substrate. This process removes contamination, corrosion, and solder residue, leaving the surface inactive for good adhesion. Shot can involve a wide range of materials such as metal particles, plastic pellets, glass beads, natural materials, and solid carbon dioxide.
It can be applied to metal parts with thick walls or thick materials. It is not recommended for thicknesses less than 1 mm or small objects due to the high-pressure jet that attacks the piece. The term sandblasting is often used to describe where sand and other minerals containing free silica are used to attack the surface.
The alkaline degreaser is the most common water-based degreasing agent. It is a degreasing agent with a high pH value. This degreasing method may be unsuitable for non-ferrous materials, but it is more suitable for ferrous materials. Alkaline degreasing is always followed by one or more rinsing steps to prepare the substrate for the next step. The working principle of alkaline degreasing differs from acid degreasing; during degreasing of steel, no etching will occur, as occurs with the acid process.
What acid degreasing is doing is etching the surface of the steel compared to alkaline degreasing where there is no itching of the material. So, the alkaline solution will attack the surface when degreasing a galvanized, aluminum, or yellow metal material. Generally, manufacturers adapt the alkaline degreaser to be multipurpose and not attack aluminum or other metals.
Alkaline degreaser cleaning is done in three stages: Application of the degreaser, then a rinse, and finally, a corrosion inhibitor. If the corrosion inhibitor is absent, in many cases, the object may corrode before entering the drying oven.
The acid degreaser has a low pH value; the steel is attacked with an acid solution. Corrosion protection is inherent to this process. Ferrous objects receive a modified surface coating. The consequence is better adhesion between the coating and the surface.
Basically, this process is iron phosphating because steel reacts with an acid solution; first, in this acid-degreasing cleaning, the steel reacts, and the iron is removed from the surface. It dissolves in the bath and reacts with the phosphates in the bath solution, accumulating on the etched surface as a coating and developing weak corrosion protection, but has good adhesion.
Surface modification follows degreasing, where a layer of sealant or other chemicals is applied to protect it from corrosion and increase adhesion between the substrate and the powder coating. The last rinsing stage must contain corrosion inhibitors. Zinc phosphate alone has good performance due to the high pH value; the working principle is different from the working principle of the cleaners.
Iron phosphate offers limited corrosion protection but excellent adhesion. Iron phosphate can be used for spray application, which is the most common. It could possibly be used in the dipping process, but it is not recommended because there is a lot of sludge formation in the tank. Rinsing after application of iron phosphate is done with deionized water, possibly with passivation, to increase corrosion protection.
According to the Ferropro de México page, “Passivation is a non-electrolytic process, typically using nitric acid to eliminate free iron from the surface and form a protective, inert oxide layer that strengthens metals against corrosion and oxidation, making it a preferred corrosion control method for many industries. Passivation is commonly recognized as the surface treatment of stainless steels, frequently with acid solutions or pastes, to remove contaminants and promote passive film formation on a newly created surface.”
Zinc phosphate has several advantages over other cleaners. Low zinc processes are most suitable before powder coating. Cleaners with zinc and manganese phosphate are gaining position in the market. It is commonly used in both immersion and spray processes. Preferably, the piece should be treated first with an alkaline degreaser.
The degreaser must be adapted for steel and galvanized surface treatments. Aluminum can also be treated with zinc phosphate, if chemical additives are used to protect the aluminum. For aluminum parts, chromate-free cleaners are used; the supplier must have zinc phosphate treatment with additives to avoid damaging the aluminum parts.
In zinc phosphate treatment, the chemical attacks the surface of the steel first; the iron dissolves in the solution, and the zinc phosphate will deposit on the etched surface and form a crystalline layer. The iron content in the bath should be kept low; in these cases, nitrite is usually added since nitrate will oxidize the iron. In the case of immersion tanks, the iron will go to the bottom with the mud and waste.
What nitrite does is convert iron into oxides that will settle to the bottom of the tank as sludge. Sometimes, chemical suppliers give nitrite as an additive and ask to add it weekly or biweekly to the tank to reduce its iron content. Galvanized materials can also be treated with zinc phosphate; in this process, the zinc bonds to the surface of the substrate to protect it against corrosion. Aluminum can also be treated with zinc phosphate, but a chemical additive is required to remove the oxide film present on the surface of the aluminum. Sometimes chromate is attached to aluminum, but it is dangerous for the operator and the environment so some countries banned it.
In summary, iron phosphate is the most traditional type of conversion coating. It is used in pretreatment applications to create a greater adhesion between the part and the powder or wet coat. It takes the surface of your part and changes the morphology by building crystal structures that increase the surface area and allow your paint to have better adhesion. It could be used along with metals like zinc or manganese. It could be applied in a single batch or multiple stages.
Benefits of Iron Phosphate:
- Many processes and application options.
- Excellent paint adhesion and superior bonding.
- Simple to control and operate.
Zirconium cleaner is phosphate-free, has no heavy metals on it, and operates at a more efficient ambient temperature. Zirconium is not hazardous to the operator. It uses fewer stages than Iron Phosphate. It provides an exceptional bonding for powder and liquid coating applications. i
Benefits of Zirconium:
- Environmental advantages.
- Reduce waste.
- Lower energy use.
Care and Recommendations for Pretreatment
- Hang parts properly to allow proper drainage and prevent chemicals and water from becoming trapped; after treatment, powder coat as soon as possible because the component will tend to accumulate moisture, dust, or other contaminants very quickly.
- Make sure the material is degassed properly; otherwise, it could create pinholes in the piece. Check the pH levels and temperature, and rinse water quality to avoid it.
- Once the pieces come out of the curing oven, please do not touch them with ungloved hands, and clean the products with gloves or cotton cloths. Grease and finger marks will be visible on finished items.
- When there is a delay between pretreatment and coating, provide a storage area and adequate packaging to keep the component clean. dry and free of dust.
- Following the supplier's specifications regarding temperature, dosage, and other measurements, titration is very important to verify the amount of chemicals in the tank, which should be within the limits specified by the chemical supplier.
Some standards for Surface Cleaning are:
Table 2. Surface cleaning standards
|America and Global SSPC
|Hand Tool Cleaning
|8501/4 ST2; ST3
|Power Tool Cleaning
|8501/4 ST2; ST3
|White Metal Blast Cleaning
|BS4232 1 2 3
|Commercial Blast Cleaning
|BS4232 1 2 3
|Brush-off Blast Cleaning
|BS4232 1 2 3
|Near-white Commercial Blast Cleaning
|BS4232 1 2 3
|Power Tool Cleaning to Bare Metal
|8501/4 SA2; SA3
|BS4232 1 2 3
|Industrial Blast Cleaning
|8501/4 SA2; SA3
|BS4232 1 2 3
|Commercial Grade Power Tool Cleaning
|8501/4 SA2; SA3
|BS4232 1 2 3
|Brush-Off Blast Cleaning of Non-ferrous Metals
|BS4232 1 2 3
If you have a good ground connection, you can paint those difficult parts where the powder resists penetrating. For example, a Faraday cage, corners, or welding. There are more benefits of a good grounding than not knowing if you have a good one.
There are several non-conformance results for a bad grounding; your transfer efficiency is going to be affected. One of the most important effects is that the piece loses attraction properties, so you are getting less powder on the part. In consequence, you are going to over-spray and possibly get a mix of back ionization and orange peel, and part of the powder is going to be drawn into the recovery system. The part is going to act as an electrical capacitor due to the electrical circuit principle.
You can also see sparks and arks when the electric circuit that forms the process is trying to connect and look for grounding. According to the PCI Handbook, “the production problems caused by poor grounding are inconsistent coating, poor transfer efficiency, and Inadequate film thickness.”
There are several causes of a bad grounding connection, and we listed here some of them:
- Dirty hooks, the powder already coated, acts as an isolator.
- Poorly grounded conveyor or rack.
- Bad maintenance procedures.
- Nonconductive contacts between the piece all the way to the ground.
Some recommendations are:
- Check the grounding from your piece to the hook, from the hook to the rack or conveyor chain, from the chain to the conveyor rollers from the chain and rollers to the line, and then to the ground.
- Check that the ground resistance value does not exceed 1MOhm required or less resistance to the ground at 500 volts. Use a mega meter to find it out. If you do not have it, start to check the grounding as we described in the previous bullet point.
- Check hooks, racks, rollers, ball bearings, and conveyor chains; they have to be clean without coating on the contact points. Burn-off, blasting, and/or stripping are recommended to clean the hooks or other coated parts that need to be clean.
Summarizing grounding, with good ground, you will ensure good and consistent coverage. Your transfer efficiency will improve and the piece will get as much powder on the part as possible with the first pass. It also reduces the fire risk due to the sparks and arcs between the piece and the hook or in another part of the circuit.
“To minimize the possibility of ignition by static electrical sparks, powder transportation, application, recovery equipment, workpieces, and other conductive objects shall be grounded with a resistance to the ground not exceeding 1 Mega ohm.” States the NFPA Bulletin number 33, Chapter 13, paragraph 13-4c regarding a properly grounded.
- Learn Powder Coating – Electrostatic Deposition Painting. Udemy platform.Gajinder Singh, George, September 2021. https://www.udemy.com/course/learn-powder-coating-electrostatic-deposition-painting Powder Coating for Industrial Finishing Applications, CCAI. 6th Edition.
- Powder Coating for Industrial Finishing Applications, CCAI. 6th Edition.
- Powder Coating: The Complete Finisher's Handbook, PCI. 3th Edition.
Mario Quiceno is a powder coating painter in Winnipeg, Canada. Member of the Chemical Coaters Association. He has a mechanical and manufacturing engineer degree from Universidad Autonoma de Manizales, a certification in Project Management, and an MBA from Universidad del Valle.