When metal is stored, handled, or processed, it often picks up unwanted substances like machine oils, lubricants, and even animal or vegetable oils. These contaminants can seriously affect the coating by reducing its ability to stick to the metal and by lowering the overall quality of the finish. That’s why it’s so important to remove these oily residues before you start coating. There are various ways to do this, including physical, chemical, and electrochemical methods.
Degreasing pretreatment for powder coating refers to the essential process of removing oils and greases from metal surfaces before applying the powder coating. This step ensures that the coating adheres properly and achieves the desired finish. Various techniques, such as chemical solvents or electrochemical treatments, are used to clean the surface thoroughly, providing a clean slate for the coating application.
Curious about the methods and why they’re so important for your coating process? Let’s dive deeper into the specifics of degreasing pretreatment and how it can impact the final result of your powder coating.
Physical Degreasing Method
Physical degreasing is all about using solvents that can dissolve greasy contaminants like animal, vegetable, or mineral oils to wash them away from the surface. While this method works quickly, it’s not always the most thorough at removing all the oil. Plus, many of the solvents used are flammable, explosive, and somewhat toxic, which makes them less safe and more challenging to handle.
This method is a good fit for steel stamping parts, castings, aluminum, copper, and their alloys, as well as die-cast components. It’s especially useful for getting rid of mineral oils that are tough to remove with alkaline solutions and for surfaces with heavy grease buildup. When choosing a solvent, it’s important to pick one that’s strong enough to dissolve the grease, evaporates well, is less toxic, doesn’t catch fire easily, is affordable, and doesn’t corrode the metal.
Common solvents include things like benzene derivatives, ethers, ketones, esters, and chlorinated solvents. Some of the go-to degreasing agents are trichloroethane, trichloroethylene, and perchloroethylene. A popular technique called vapor degreasing works quickly, is efficient, and cleans all kinds of oils and greases thoroughly. Sometimes, emulsifiers are added to these solvents to boost the effectiveness when spraying or soaking parts. However, due to safety concerns, this method is now mostly used for cleaning small parts in the electronics industry.
When it comes to solvent degreasing, there are a few methods you can use: wiping, vapor degreasing, and immersion. To ensure a thorough cleaning, it’s often best to go through the process twice, and it’s important to regularly replace the solvent to maintain quality. Since most organic solvents are toxic and flammable, safety and proper ventilation should always be a priority during use and storage.
Chemical Degreasing Methods
Chemical degreasing, often done with an alkaline solution, is a common approach where sodium hydroxide is the main ingredient. In many cases, additional components like sodium carbonate, sodium metasilicate, sodium silicate, sodium phosphate, and sodium pyrophosphate are added to enhance the cleaning power. This method works well for removing animal and vegetable oils, but it’s not effective for mineral oils.
To boost the effectiveness of chemical degreasing, surfactants such as wetting agents and detergents are often included in the alkaline solution. Surfactants are compounds made up of hydrophilic (water-attracting) and lipophilic (oil-attracting) parts. They work by adsorbing at the interface between oil and water, reducing the surface tension, and helping to emulsify oils that don’t react with the alkaline solution. This emulsification process allows for the effective removal of mineral oils as well. Typically, the amount of surfactant used ranges from 0.3% to 3%.
Formulation and Process Conditions for Alkaline Degreasing of Different Materials
| Formula & Process Conditions | Steel/Iron (Soak 1#) | Steel/Iron (Soak 2#) | Steel/Iron (Spray) | Aluminum & Alloys | Copper & Alloys |
| Sodium Hydroxide (g/L) | 80 | 80-100 | 4 | 3-5 | 25-30 |
| Sodium Carbonate (g/L) | 45 | 20-30 | 8 | — | — |
| Trisodium Phosphate (g/L) | 30 | 30-40 | 4 | 40-50 | 25-30 |
| Sodium Silicate (g/L) | 3-5 | — | — | 15-25 | 5-10 |
| Temperature (°C) | 90-95 | 85-95 | 72 | 50-70 | 60-90 |
| Time (min) | 2-5 | 10-15 | 2 | 2-5 | 10-20 |
There are two main techniques for applying chemical degreasing: the immersion method and the spraying method.
In the immersion method, the workpiece is soaked in a degreasing solution to remove the oil and grease. On the other hand, the spraying method involves spraying the degreasing agent directly onto the surface of the workpiece to clean it. Both methods are effective, but the choice between them depends on the specific requirements of the process and the type of contamination.
Chemical degreasing solutions can also be categorized based on their pH levels into alkaline, neutral, and acidic solutions.
Alkaline degreasing solutions, which have a pH value between 9 and 11.5 or higher, are further divided into weak alkaline (pH 9 to 10.5), moderate alkaline (pH 10.5 to 11.5), and strong alkaline (pH greater than 11.5) solutions. Neutral degreasing solutions have a pH between 6.9 and 9.5, while acidic degreasing solutions range from pH 1.0 to 5.5. Each type is selected based on the specific requirements of the cleaning process and the nature of the contaminants.
1. Low-alkaline degreasing solutions
Low-alkaline degreasing solutions, which include both weak and moderate alkaline solutions, are gentle on the workpiece surface, causing minimal damage. They can be used at low to medium temperatures and are highly effective at removing grease. These solutions are particularly well-suited for spray degreasing and are currently the most widely used degreasing agents.
The main components of low-alkaline degreasing solutions include inorganic alkaline additives, synthetic detergents or surfactants, defoamers, corrosion inhibitors, chelating agents, and water softeners. Alkaline additives such as sodium silicate, sodium tripolyphosphate, sodium phosphate, and sodium carbonate provide the necessary alkalinity, help disperse and suspend contaminants, and prevent re-adsorption of grease. The surfactants used are typically non-ionic polyoxyethylene compounds and anionic sulfonates, which play the key role in grease removal.
Low-alkaline degreasing can be carried out using spray, immersion, or a combination of both methods to ensure effective cleaning.
Common Low-Alkaline Degreasing Solution Formulation and Process Conditions
| Item | Immersion Type (g/L) | Spray Type (g/L) |
| Formulation | ||
| Trisodium Phosphate | 4-10 | 4-10 |
| Sodium Silicate | 0-10 | 0-10 |
| Sodium Carbonate | 4-10 | 4-10 |
| Surfactant | 5-20 | 1-3 |
| Defoamer | — | 0.5-3 |
| Surface Adjusting Agent | 0-3 | 0-3 |
| Free Alkali (points) | 5-20 | 5-15 |
| Process Conditions | ||
| Processing Temperature (°C) | Room Temp – 80 | 40-70 |
| Processing Time (min) | 5-20 | 1.5-3 |
2. Strong Alkaline Degreasing Solutions
Strong alkaline degreasing solutions rely on the saponification reaction between the strong alkali and animal or vegetable oils, turning them into water-soluble soaps to remove grease. This method is a traditional and effective way to remove oils, but it’s not suitable for mineral oils. To tackle mineral oil removal, anionic surfactants like sulfonates are added, which emulsify the oil, making it easier to clean.
However, these degreasing agents operate at high temperatures, which means higher energy consumption and increased corrosion risk to the equipment. Due to these drawbacks, the use of strong alkaline solutions is gradually declining.
3. Neutral Degreasing Solutions
Neutral degreasing solutions are especially suited for non-ferrous metals like aluminum, zinc, and magnesium, which can be prone to corrosion when exposed to degreasing agents with extreme pH levels, particularly alkaline solutions. Using neutral degreasing solutions helps to effectively remove grease without the risk of damaging these sensitive metals.
4. Acidic Degreasing Solutions
Acidic degreasing solutions are composed of non-ionic and anionic surfactants, inorganic acids, and corrosion inhibitors. These solutions work by leveraging the emulsifying, wetting, and penetrating properties of surfactants, along with the mechanical action of hydrogen gas produced by the acid reacting with metal to strip away grease.
These degreasing solutions can be used at low to medium temperatures. At low temperatures, they primarily remove liquid oils, while at medium temperatures, they can handle both oils and fats. Acidic degreasing is particularly suited for the immersion method and offers the dual benefit of degreasing and derusting, which is why it’s often referred to as a “two-in-one” treatment solution.
Common Acidic Degreasing Solution Formulations, Process Conditions, and Characteristics
| Item | Low Temperature Type | Medium Temperature Type | Phosphoric Acid Type |
| Formulation (% by Weight) | |||
| Industrial Hydrochloric Acid (31%) | 20-50 | 0 | 0 |
| Industrial Sulfuric Acid (98%) | 0-15 | 15-30 | 0 |
| Industrial Phosphoric Acid (85%) | 0 | 0 | 10-40 |
| Surfactant (Nonionic & Anionic types) | 0.4-1.0 | 0.4-1.0 | 0.4-1.0 |
| Corrosion Inhibitor | Appropriate Amount | Appropriate Amount | Appropriate Amount |
| Process Conditions | |||
| Processing Temperature (°C) | Room Temp – 45 | 50-80 | Room Temp – 80 |
| Processing Time (min) | Appropriate | 5-10 | Appropriate |
| Characteristics | |||
| Application Range | Very Wide | Very Wide | Moderate |
| Cost | Low | Low | High |
| Efficiency | Relatively High | Relatively High | Low |
| Corrosiveness | High | High | Low |
In the degreasing process, the temperature at which degreasing occurs significantly impacts its effectiveness. Generally, higher temperatures improve degreasing for several reasons. First, increased temperature lowers the viscosity of grease, making it easier to remove. Second, it speeds up chemical reactions, enhancing the overall cleaning process. Third, it boosts the wetting, emulsifying, and dispersing actions of surfactants, leading to better results.
The duration of degreasing is also crucial. Extending the degreasing time improves the cleaning effect, especially when dealing with heavy contamination. On automated production lines, it’s common to use a two-step process—first a spray pre-degreasing, followed by immersion degreasing to ensure thorough cleaning.
When selecting a degreasing agent, it’s important to choose a solution with the appropriate pH level based on the material being treated to prevent corrosion. For instance, the critical pH values for corrosion are zinc at pH 10, aluminum at pH 10, tin at pH 11, brass at pH 11.5, silicon iron at pH 13, and steel at pH 14. Selecting the right pH helps protect the metal from corrosion while ensuring effective degreasing.
Electrochemical Degreasing Methods
Electrochemical degreasing involves using electrolysis to clean the workpiece. In this method, the item to be coated is placed in an electrolyte-filled tank and acts as either the anode or cathode. When a direct current is applied for a short period, it reduces the surface tension at the oil-solution interface. Additionally, gas bubbles released at the electrode surfaces help dislodge the oil film from the workpiece, achieving effective degreasing.
This method is typically used after conventional solvent or chemical degreasing. For steel parts, the electrochemical degreasing solution is usually based on sodium hydroxide, while for materials like aluminum or copper, the solution is often based on sodium carbonate or trisodium phosphate. It’s important to note that surfactants should not be added to electrochemical degreasing solutions, as they tend to produce foam. During electrolysis, this foam can trap hydrogen and oxygen gases at the anode and cathode, creating a significant explosion hazard.
Formulation and Process Conditions for Electrochemical Degreasing Solutions for Different Materials
| Formula & Process Conditions | Steel/Iron | Aluminum & Alloys | Zinc & Alloys | Copper & Alloys |
| Sodium Hydroxide (g/L) | 10-30 | 5-10 | 20-40 | 5-10 |
| Sodium Carbonate (g/L) | — | 10-20 | — | — |
| Trisodium Phosphate (g/L) | — | 15-30 | 20-40 | 10-20 |
| Sodium Silicate (g/L) | — | — | 3-5 | 5-10 |
| Cathode Degreasing Time (min) | 1 | 0.5-1 | 1-3 | 0.5-1 |
| Anode Degreasing Time (min) | 5-10 | — | — | — |
| Temperature (°C) | 80 | 40-60 | 70-80 | 40-50 |
| Current Density (A/dm²) | 2-10 | 5-7 | 2-5 | 5-7 |
Conclusion
Whether using chemical or electrochemical degreasing methods, it’s crucial to thoroughly rinse the workpiece with cold or hot water afterward. This step removes any residual alkaline solution, surfactants, or other contaminants left on the surface. Flowing clean water is ideal for this purpose. To ensure the quality of the rinse, special attention should be paid to the purity and quality of both cold and hot water. Regularly replacing the water in the rinse tanks is essential for maintaining high washing standards and achieving optimal results.
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