Passivation

The passivate process is designed to remove foreign metals and oils (usually from machining) from the surface of stainless steel. The end finish will not change the dimension of the part nor the overall appearance of the base metal. Passivation purifies the surface of a machined part and therefore improves corrosion resistance.

QQ-P-35C is the Federal standard for passivation:

• Type I - Low temperature
• Type II - Medium temperature
• Type III - High temperature
• Type V - Anodic - For high carbon martensitic (440) steels

This Federal standard was cancelled in 1997 and has been superceded by ASTM A967 and AMS 2700. Avers Machine and Gear passivates all parts to these specifications using the citric process.

When stainless steel products are manufactured, free iron is transferred to the surface of the material from the steel cutting, stamping and forming tools used in the manufacturing process. Free iron can also be imparted on the surface by polishing or blasting operations that utilize the same polish or blast media between both mild steel and corrosion resistant steel grades. Free iron readily oxidizes, forming visible rust on the surface of the product. Passivation of stainless steel is a chemical treatment with a specific acid formulation that removes free-iron or other surface contamination from the stainless steel while simultaneously promoting the formation of a passive chromium/nickel oxide layer to act as a barrier to further corrosion.

It is important to note that stainless steel is corrosion resistant but not corrosion proof. The degree of corrosion resistance of a stainless steel alloy is a function of the alloying composition, heat treatment, internal stresses and passivation treatment. An example of this phenomenon is 303 free-machining stainless steel which has notably less corrosion resistance than 304 stainless steel due to the higher concentration of sulfur and phosphorous that imparts the desired machinability of the 303 grade. As a general rule, the higher the nickel and chromium content in the alloy, the more corrosion resistance it will have.

Benefits of Passivation

Contaminant Removal - Passivation removes free iron and other surface contaminants from stainless steel parts, eliminating the contamination left behind by metal forming processes like machining and stamping.

Removal of Marks - Removal of black marks left on stainless steel during forging and forming or heat tint left behind by welding.

Improved Corrosion Resistance - By restoring stainless steel parts to mill condition, passivation improves their corrosion resistance. The contaminants left on the surface of a part are initiation sites for corrosion to take hold.

The Passivation Process

The first step of passivation is to ensure that the component is free of chemicals that may adversely affect the chemical process. Technicians inspect for the presence of:

• Grease
• Oil
• Lubricants
• Forming Compounds
• Cutting Fluids
• Wax Markings
• Metallic Residue

During this time, the piece is also inspected for metal quality and the strength of any welds.

After the component is inspected and properly cleaned, it is placed in a bath of citric acid-based solution. The piece is left to soak in the chemical solution for a strictly controlled amount of time, leaving the surface smoother and more resistant to natural corrosion from exposure to air and fluids.

Applications Of Passivation

Passivation is a commonly used corrosion control method for countless products and industries. Some of the many industries that can benefit from the process are:

• Filter Housings
• Food & Beverage Processing
• Forgings & Castings
• Stainless Steel Framework
• Medical & Pharmaceuticals
• Pipes & Piping
• Springs & Stampings
• Tower Internals
• Stainless Steel Vessels
• Water Treatment & Distribution
• Marine & Aerospace

Any industry that requires contaminate-free, corrosion-resistant parts and components can reap the benefits of having high quality passivation.