Stainless steel’s anti-corrosive properties make it an ideal material for an expansive range of applications. However, the surface of the metal can still end up developing a coating of rust or corrosion due to free iron molecules on the material’s surface. This is a common problem in manufacturing as iron residue from fabrication, water, or any other number of sources present in industrial facilities can contaminate stainless steel surfaces.
Stainless steel should undergo passivation—a process that dissolves free iron molecules. Removing free iron from the surface makes that part more resistant to corrosion by creating a protective microscopic oxide layer with enhanced resistances to the many environmental factors that can initiate the corrosion process. Iron molecules can also gather on parts throughout their service life, possibly requiring repeat passivation to restore a part’s anti-corrosive properties.
Passivation helps preserve stainless steel’s smooth, rust or corrosion-resistant surface. The passivation process uses an acid bath to strip away contaminants and free iron molecules, so it’s not something that can be conducted without proper training and equipment. Incorrectly passivating stainless steel can actually induce corrosion rather than prevent it.
The Process of Passivation
While stainless steel is already resistant to rusting and corrosion, it’s not immune to collecting iron particles from nearby equipment or as tools leave microscopic residue behind. Common sources of free iron molecules include:
- Iron contamination in the air from various industrial processes
- Natural iron content in water used for cleaning parts
- Reusing abrasive blast media previous used on iron parts
- Transferred molecules from machine tools, material handling equipment, storage containers, etc.
Free iron from various environmental factors is relatively unavoidable, but these molecules should not be confused with the alloyed iron contained within the stainless steel itself. While stainless steel has an outer layer of chromium to protect it, the chromium itself is made of about 60–70% iron as well. As such, the steel needs to be treated to keep that passive layer intact.
During stainless steel production, mills add sulfur to the stainless steel to make it easier to machine. This sulfur, along with the iron residue that can form during fabrication, actually eats into the passive layer of chromium and leaves the material vulnerable to corrosion. Passivation eliminates these risks.
Passivation Step by Step
The passivation process follows these steps:
- The stainless-steel item first undergoes cleaning first with an alkaline-based cleaner. This removes surface contaminants, including iron molecules, grease, and oils that may have been left on the part.
- Next, the part is rinsed with deionized (DI) water and then placed in an acid bath to dissolve the iron—typically nitric or citric acid. This is when the passive layer forms on the part.
- A second rinse with DI water follows, and then the part is dried.
- A final testing stage tests the material’s resistance to salt sprays and materials such as copper sulfate.
Passivation protects stainless steel from rust, extending the usable service life of parts and components made from this material. In addition, passivation doesn’t affect the surface of the steel itself, unlike some other surface finishing options that may eat into or strip away the steel. It only strips away iron and contaminants on the surface of the steel.
Nothing is introduced into or onto the steel—the passive layer would form naturally in optimal circumstances. As a bonus, FZE uses a citric acid process which is eco-friendly and relatively safe to use.
Which Industries Does Passivation Help?
Realistically, passivation helps any industry that uses stainless steel. In particular, however, passivation is necessary for:
- Automotive, Motorcycles, Trucking
- Agriculture and Dairy
- Food and Beverage
- Medical/Dental/Pharmaceutical devices and equipment
- Oil & Gas, Petroleum
- Power generation
- Transportation (DOT, trucking equipment)
- Water and wastewater equipment
Work With FZE Manufacturing
FZE stands out as a leader in passivation. We have been around since 1974, offering a range of manufacturing expertise for almost five decades. We have a range of certifications that display our dedication to efficiency and quality in all of our processes and services, such as:
- Six Sigma (Black Belt)
- ISO 9001:2015
- Kaizen/5S and Lean manufacturing
Our broader range of services includes Just-in-Time (JIT) manufacturing, inventory management, supply-chain optimization, and cost reduction. Onsite consultations and inspection services are also available, including optical monitoring and visual inspections. We can also accommodate quick turnaround times and rush orders and have a proven track record of on-time delivery performance.