Understanding Passivation: Processes, Practical Uses, and Advantages

When Is Passivation Used?

Passivation is a rapid and automatable surface treatment process, making it suitable for various applications. Some of the scenarios where passivation is a viable and efficient solution include:

1. Before putting stainless steel components into use
2. After carrying out mechanical machining operations.
3. After welding
4. When new components are added to existing components
5. Passivation must be performed after contamination
6. Perform preventive maintenance as a form of upkeep.

Two Primary Functions of Passivation

1. Free iron particles or residue may be left on stainless steel parts during machining, grinding, forming, or stamping operations. Additionally, shop dirt and debris may also contribute to this contamination. Passivation is necessary when parts undergo fabrication through cold forming or machining processes, as these processes can introduce contaminants such as shop dirt or iron particles from cutting tools and fixtures onto the part’s surface. If left untreated, these free iron particles can rapidly corrode, creating the appearance of rust on the stainless steel itself.
2. To protect the components, a thin passive oxide film is developed on their surface. This film, 100,000 times thinner than human hair, acts as a physical barrier, preventing corrosion or damage to the stainless steel material underneath. The term “passivation” is derived from this passive oxide barrier.

Procedure of Passivation

1. Alkaline cleaning removes all contaminants, oils, and foreign materials using detergent cleaners such as sodium hydroxide, Micro-90, or Simple Green.
2. In high-precision industries, water rinsing is commonly carried out with DI (Deionized) water or RO (Reverse Osmosis) water.
3. A nitric acid or citric acid (CitriSurf) immersion bath completely dissolves any free irons and sulfides and accelerates the formation of a passive film or oxide layer.
4. Water rinse: Commonly done with deionized water in high-precision industries.
5. Second water rinse: Commonly done with deionized water in high-precision industries.
6. Dry components.
7. Test sample parts according to specification standards utilizing salt spray, copper sulfate, or high humidity chamber exposure testing.

Factors Contribute to The Effectiveness of Passivation

1. Chromium content is essential for the formation of the oxide layer.
2. Immersion duration should be correct.
3. Surface grain boundaries should be taken into consideration.
4. Remember to maintain ideal conditions such as temperature and solution strength.
5. Understand the unique demands of various types of metals.

Passivation Test Options

Various passivation test options are available to verify the correct passivation of stainless steel, depending on the passivation standard. The ASTM A967 standard includes the following passivation tests:

1. Water Immersion Test – The process requires a minimum of 24 hours and a distilled water tank.
2. High Humidity Test – Ensure the item spends at least 24 hours in a humidity cabinet.
3. Salt Spray Test – Make sure to allow a minimum of 2 hours in the salt-spray testing room.
4. Copper Sulfate Test—This rapid test takes only 6 minutes but can’t be used with steel grades containing less than 16% chromium or with parts for food processing.
5. The Potassium Ferricyanide-Nitric Acid Test – This is also known as the Ferroxyl Test, and it is a quick and highly accurate test. The test solution must be prepared daily, and careful attention must be paid to safe chemical handling. This test cannot be used with specific steel grades or parts intended for food processing.
6. Free Iron Test—This is an alternative to the Water Immersion and High Humidity Tests, particularly suitable for huge parts that do not fit in a tank or cabinet.

Tips to Watch for with Passivation of Metal

1. Passivation can be seen as a form of controlled corrosion. The acid bath dissolves or corrodes free iron on the surface in a uniform, controlled manner. If not adequately controlled, uncontrolled corrosion can happen in a “flash attack.” During the flash attack, the metal develops a dark, heavily etched surface – exactly the corrosion the passive layer aims to avoid.
2. Keeping the acid solution free of contaminants is essential to prevent flash attacks. Often, the solution can be refreshed simply by refilling the acid bath with a fresh solution. It’s recommended that the acid solution be replaced regularly to avoid the build-up of contaminants. Additionally, using a higher grade of water, such as RO water or DI water with fewer chlorides than tap water, may also help resolve issues with flash attacks.
3. It is essential to clean stainless steel components thoroughly before immersing them in an acid bath. Any grease or cutting oil left on the parts can create bubbles that disrupt the process. To address this, you should use a degreaser or switch detergents to guarantee that the part is entirely impurities-free. Additionally, in certain instances, thermal oxides from heat treating or welding might need to be removed by grinding or pickling before passivation.
4. Remember not to mix different grades of stainless steel in the acid bath simultaneously. This can cause galvanic corrosion, where the less noble metal corrodes faster when in contact with dissimilar metals in the solution.