Have you ever received stainless steel castings that were supposed to be smooth but were covered in rust spots, putting you at risk of having to return them? This is because the manufacturer failed to properly pickle and passivate the stainless steel. This article will explain the importance of acid pickling and passivation in stainless steel investment casting and help you avoid this situation.
What is Pickling and Passivation
Although pickling and passivation are often discussed together, they are actually two distinct processes. Understanding the specific roles each plays in stainless steel castings will help you appreciate the importance of these two processes, particularly in applications that require high surface quality and corrosion resistance.
What is Pickling for Stainless Steel Casting
The pickling of stainless steel essentially involves using an acid liquor to remove contaminants from the surface of stainless steel castings, such as scale formed by oxygen reaction during high-temperature pouring, iron contaminants, oil residues, and other surface impurities. Commonly used pickling solutions include nitric acid (HNO₃), hydrofluoric acid (HF), and sulfuric acid (H₂SO₄).
What is Passivation for Stainless Steel Casting
Passivation of stainless steel refers to the process of using an oxidizing chemical liquor to promote the formation of a chromium-rich passivation film on the surface of a casting from which contaminants have been removed. This reinforces the protective layer naturally formed by the stainless steel, thereby further enhancing its corrosion resistance. Common passivation liquors include nitric acid and citric acid; citric acid is gradually replacing nitric acid due to its environmental benefits.
Pickling vs Passivation
| Pickling | Passivation | |
| Primary Objective | Remove oxide scale | Form a passive film |
| Metal Removal | Yes | Minimal |
| Main Agents | HF+HNO₃ | HNO₃ or citric acid
|
| Object | Contaminated layer | Surface of stainless steel |
| Improvement of Corrosion Resistance | Indirect | Direct |
| Sequence | First | After |
Pickling and Passivation Method
There are many methods for pickling and passivation, and each method will cause different results. Knowing these methods will help you choose the best that suits your stainless steel components when making a purchase.
Dipping Method
The dipping method is the most common method of pickling and passivation. This method involves completely submerging the stainless steel casting in a chemical solution to ensure the casting’s surface comes into full contact with the solution. Because this method is convenient to use and produces consistent results, it is suitable for most stainless steel castings with complex shapes.
Spraying Method
The spraying method involves applying a chemical solution to the casting surface using a spray gun or spray system. This method is suitable for large castings that exceed the dimensions of the pickling tank and for which immersion would be too costly, such as large valve bodies and pump housings. Although this approach reduces chemical consumption, the solution coverage is less uniform than the dipping method.
Brushing Method
The brushing method involves manually applying a chemical solution to the joints or crevices of castings using a brush. It is suitable for repairing welds, performing localized rework, and treating small areas. Although this method is convenient, it is not very efficient because it requires careful manual application; therefore, it is not suitable for the pickling and passivation of large-scale parts.
Electrolytic Method
The electrolytic method refers to the process of pickling and passivating using an electric current to further improve their surface quality. This method is commonly used in industries such as medical and food processing, where components must meet high standards for precision and surface quality.
Vapor Method
The vapor method involves treating the surface of parts with acidic vapor. It is suitable for parts with complex internal structures that are difficult to reach with liquid, such as those with narrow, elongated channels and complex internal cavities.
Pickling and Passivation Process of Investment Casting
In investment casting, the processes of pickling and passivation also require careful attention. It is not simply a matter of pickling followed by passivation; there are several intermediate steps involved. Understanding these steps will give you a deeper understanding of the pickling and passivation, and will also help you better assess the standardization of a manufacturer’s processes when making purchases.
The following are all the steps involved in pickling and passivation.
Shot Blasting—Degreasing—Rinse—Pickling—Rinse—Neutralization—Rinse—Passivation —Final Rinse—Drying
Key Steps of the Process
- Shot Blasting: This is the first step in the treatment of stainless steel castings. High-speed shot is used to impact the surface, removing scale and residual mold residue, improving surface finish, and preparing the casting for subsequent chemical treatment.
- Degreasing: This refers to the removal of grease from the surface of castings. It prevents grease contamination that could prevent the acid solution from making uniform contact with the casting surface, which could result in incomplete pickling.
- Rinse: This refers to rinsing the surface with clean water to remove any chemical residues left over from the previous step, thereby preventing cross-contamination between different chemical solutions and reducing the risk of yellowing on the casting’s surface in subsequent processes.
- Pickling: Acid is used to remove contaminants on the surface, making it cleaner and preparing it for subsequent passivation. If the acid concentration, temperature, and duration are not properly controlled during the process, overpickling can easily occur, meaning that not only is the surface scale removed, but the interior of the casting is also corroded.
- Neutralization: This refers to the process of using an alkaline solution, such as sodium carbonate, to remove acidic residues left on the surface after pickling. This not only prevents these acidic substances from continuing to corrode the metal but also reduces the risk of subsequent rusting.
- Passivation: Passivation refers to the process of forming a chromium-rich passive film on the surface of castings using a chemical solution. This step is critical to the entire process because the resulting oxide film is key to ensuring the corrosion resistance of stainless steel parts and reducing the risk of rust.
- Drying: Finally, moisture must be removed from the surface of the stainless steel parts using methods such as hot-air drying or an oven to prevent this residual moisture from causing the casting surface to turn yellow.
Benefits of Pickling and Passivation
Pickling and passivation offer numerous benefits for stainless steel castings. Understanding these benefits will help you appreciate the importance of these processes, encourage you to pay more attention to whether castings have undergone surface treatments such as pickling and passivation, and enable you to identify higher-quality stainless steel castings.
Benefits of Pickling
Most of the benefits of pickling are visible and involve improvements in the surface quality of castings.
- Remove Surface Contaminants
This is the primary benefit of pickling. During the investment casting process, particularly during heat treatment, a layer of scale and iron contaminants will form on the surface of stainless steel castings. These impurities not only affect the appearance but also compromise corrosion resistance and may even lead to rust formation on the casting surface. Pickling could effectively remove these impurities and improve the surface finish of castings.
- Eliminate Surface Defects
During the casting and heat treatment processes, variations in the thickness of castings can lead to differences in the rate of oxidation across different parts of the component. As a result, color variations often appear on the surface of stainless steel castings, affecting their appearance. Pickling can eliminate these color variations, restoring the surface to a smooth finish while improving its surface finish and facilitating subsequent surface treatments such as polishing.
- Prepare for Passivation
Pickling is a critical preliminary step for passivation, as passivation relies on a clean metal surface. If surface scale is not thoroughly removed, the chromium-rich passivation film formed will be uneven, which in turn will compromise the part’s corrosion resistance.
Benefits of Passivation
The primary benefit of passivation for parts is that it improves their internal chemical properties, which are invisible.
- Enhance Corrosion Resistance
This is the most fundamental function of passivation. After passivation treatment, a dense, uniform chromium-rich passivation film forms on the surface of the component. This film effectively prevents corrosive media such as air and moisture from contacting the stainless steel casting. It not only enhances the component’s operational stability but also extends its service life. These advantages make it suitable for working environments, such as marine and chemical industries, which are in contact with corrosive liquids.
- Prevent Rust
Stainless steel can still rust when contaminants such as iron are present on its surface. However, passivation could effectively remove these contaminants and further strengthen the protective layer on the stainless steel surface. This reduces the possibility of contamination and corrosion during subsequent processing or transportation.
- Improve Chemical Stability
After passivation, the surface of stainless steel becomes more stable. This stability makes it more resistant to corrosion from acidic, alkaline, and other corrosive media, ensuring that the performance of components will not be compromised by rust or corrosion. These properties make it widely used in the chemical industry, the pump and valve industry, and various types of industrial equipment.
- Boost Hygiene Performance
After passivation, the surface of stainless steel is cleaner, and the presence of a chromium-rich passivation layer makes it resistant to the adhesion of contaminants. This characteristic makes it suitable for industries such as food processing and medical, which require higher levels of cleanliness and stricter hygiene standards.
Applications Requiring Pickling and Passivation
In industries with extremely high requirements for corrosion resistance, hygiene standards, and component lifespan, pickling and passivation are essential steps. Understanding which industries require pickling and passivation is extremely valuable in helping you decide whether to adopt these processes.
Stainless steel castings used in corrosive environments
Pickling and passivation significantly enhance the corrosion resistance of parts, enabling stainless steel components to operate continuously for extended periods in environments containing corrosive media. Under this benefit, pickling and passivation are essential for stainless steel parts used in the marine, chemical, and pharmaceutical industries, especially for CF8M (316) and CF8 (304) stainless steel castings.
For example
Stainless Steel Castings used in a sanitary environment
Pickling and passivation also make the surface cleaner, making it less prone to adhering contaminants and less likely to release contamination. Therefore, pickling and passivation are essential for industries such as food processing and healthcare, where parts must meet extremely high hygiene standards.
For example
- Valve body castings
- Pump components
- Food machinery parts
- Pipe fitting castings
- Filter housings
- Mixer parts
Problems Caused by Improper Pickling and Passivation
Without proper pickling and passivation, the chromium-rich passivation layer will not form a uniform covering on the surface of the stainless steel casting. Understanding the common consequences of improper pickling and passivation will help you recognize the importance of these and identify which problems are caused by improper treatment, enabling you to quickly judge the root cause.
Rusting
If passivation is incomplete, the layer of iron contaminants on the casting’s surface will not be thoroughly removed. Under the influence of factors such as ocean transport and high temperatures, these iron ions will undergo an electrochemical reaction with the high chromium content in the stainless steel, causing the surface to gradually rust. Eventually, the surface will become covered with red rust, or even extensive rust stains.
Contamination
If pickling is not thorough, chemical residues left on the casting surface during the investment casting process will not be completely removed. When the casting is used in industries with extremely high hygiene standards, such as the food or medical industry, these substances will gradually dissolve into the liquid, causing contamination and leading the entire batch of food or medicine to be discarded.
Pitting corrosion
If proper pickling is not performed, the layer of scale on the casting’s surface will not be thoroughly removed. The metal beneath this scale reacts with oxygen in the air, resulting in a severe deficiency of chromium. This makes the metal extremely susceptible to penetration by corrosive media, leading to corrosion. Furthermore, incomplete passivation prevents the formation of a chromium-rich passivation film.
Both of these factors make castings extremely susceptible to pitting corrosion in marine or industrial environments containing chloride ions. This means pinhead-sized black holes will suddenly appear on the surface, rapidly spreading across the entire surface, leading to fluid leaks in pump and valve castings.
Verify Pickling and Passivation Quality in ASTM A380 & ASTM A967
Given that the consequences of improper pickling and passivation are so severe, is there any way to test the quality of these processes? The answer is definitely yes. International standards specify several methods for quality inspection, and understanding these methods will enable you to verify with the manufacturer whether proper pickling and passivation have been performed.
Water Immersion Test
This is the most basic test, which involves alternately immersing stainless steel castings in distilled water and air within 24 hours, then observing whether localized rust spots appear on the casting surface.
High Humidity Test
This test involves exposing stainless steel castings to 100% humidity and high temperatures for 24 hours, after which the castings are inspected for sudden rusting caused by surface iron ions. This is done to verify whether the passivation process has completely removed the iron ions.
Salt Spray Test
Place the casting in a salt spray chamber and subject it to continuous spraying for at least 24 hours; the duration can be gradually increased based on the specific working environment. Afterward, inspect the casting’s surface for rust. This is the gold standard for determining whether the part will rust during maritime transport or in a marine operating environment.
Ferroxyl Test
This is a specific reagent for iron ions, capable of rigorously testing whether passivation meets the required standards. If the solution turns blue within a few seconds, it indicates that the passivation is substandard. Conversely, if it does not turn blue, it indicates that the surface purity has reached 100%. However, because the residual chemicals are extremely difficult to remove and contain trace toxins, it is recommended for use in industrial castings and is not recommended for use in the food or medical industries.
Final Thoughts
Trust you have a thorough understanding of the importance of pickling and passivation. If you are looking for a high-quality stainless steel investment casting foundry, please contact us. Pingheng Machinery uses rigorous pickling and passivation processes, along with strict quality control, to custom-manufacture the castings you need.
FAQ
Q1. What are the International Standards for Pickling and Passivation?
The primary standard for the pickling of stainless steel is ASTM A380, and the primary standard for the passivation of stainless steel is ASTM A967. Both of these standards are globally recognized and widely adopted.
Q2. Can pickling and passivation be performed in the same workshop as other manufacturing processes?
No, pickling and passivation essentially involve chemical reactions with strong acids. Conducting these processes alongside other manufacturing operations could quite possibly contaminate other equipment. A separate workshop or facility is required for them. As a leading stainless steel investment casting foundry, Pingheng Machinery offers in-house passivation facilities.
Q3. Can there be a pause between steps such as pickling, neutralization, and rinse?
No. If there is a pause after pickling, it can easily lead to over-pickling. That is when the acid solution remains on the surface of the casting; as it evaporates in the air, its concentration becomes extremely high, resulting in a rough, darkened surface.
Q4. When Should Pickling and Passivation Be Performed?
There are three situations.
If you plan to machine the parts yourself, the best time to perform these processes is after shot blasting and before packaging.
If you outsource all machining to a factory, pickling and passivation should be performed separately. To prevent the acid solution from damaging the surface tolerances of the castings during pickling, it is recommended to pickle the raw castings first, machine them, and then perform passivation for the completed casting.
If your parts require welding, we recommend that you perform pickling and passivation only after all welding and grinding are completed. Otherwise, the high temperatures generated during welding will destroy the passivation layer.
Q5. Besides pickling and passivation, are there other factors that affect surface quality?
Yes, factors such as mold quality, shell-making processes, and heat treatment can all affect the surface quality of castings. If you’d like to learn more about surface quality, please click.
“The Ultimate Guide to Surface Finish in Stainless Steel Investment Casting”
Q6. Is improper pickling and passivation the only reason why stainless steel rusts?
No. While it may be the direct cause of rust on stainless steel, it is by no means this is the only factor. Rust on stainless steel can also be related to chemicals it comes into contact with, improper welding, and microscopic voids within the casting. If you’d like to learn more about rust on stainless steel, please click “Why does Stainless Steel Rust?”
Q7. How does pickling affect the dimensional tolerances of investment castings?
Since pickling is essentially a chemical corrosion process, according to the ASTM A380 standard for pickling, the amount of metal removed from the surface of stainless steel during pickling typically ranges from 0.005 mm to 0.02 mm.
