Customers across the country trust their aluminum parts to Global Metal Finishing for clear and color anodizing, chromate conversion coating, precision hardcoat anodizing, and custom services like acid etch and vapor honing. But did you know that our comprehensive metal finishing services also include stainless steel passivation? 

From aerospace to energy, from marine to medical, you can rely on Global Metal Finishing to passivate your machined stainless steel parts.

What Is Passivation?

Passivation is a chemical treatment method for machined stainless steel. It strengthens the alloy’s protective oxide layer by removing disruptive contaminants and maximizing the oxide coating across the metal’s entire surface area.

Oxygen exposure is the catalyst for stainless steel’s invisible oxide coating, which protects the alloy from corrosion. Unfortunately, precision machining disrupts this naturally occurring shield by exposing sulfides and introducing iron particles to the alloy’s surface. Without passivation, these contaminants invite rust and diminish the lifetime effectiveness of your stainless steel part.  

Passivation restores the alloy to its inherently corrosion-resistant state, first by removing the sulfides and errant iron particles from the part’s surface, then by enhancing the steel’s chromium oxide “micro coating” with an oxidizer.

Common Applications for Chemical Passivation

Passivation is a critical metal finishing step for stainless steel parts destined for heavy use and corrosive, closed environments. Driveshafts for electric motors, medical robots for surgical procedures, and parts intended for harsh marine or aerospace conditions are excellent candidates for passivation.

Benefits of Passivation

Our customers choose to passivate their stainless steel parts because:

• It maximizes corrosion resistance without the need for plating
• Passivated parts retain their original dimensions—no warping or expansion occurs 
• It prevents iron-triggered contamination reactions with other alloys or materials
• It prevents rust and rust discoloration
• It heals any inclusions of the alloy’s natural chromium oxide layer
• The surfaces of passivated parts remain exceptionally clean

Understanding the Chemical Passivation Process

There are three passivation methods: nitric acid passivation, nitric acid with sodium dichromate passivation, and citric acid passivation. Within each passivation method are seven types, each with its own temperature and acid concentration requirements.

Before a part can undergo a passivating bath, detailed specifications are required, including the steel grade and the end customer’s acceptance criteria. With this information, we can ensure the part receives the appropriate treatment.

Some alloys can tolerate only citric acid passivation, while others require varying concentrations of nitric acid to achieve their passivation specifications. Highly resistant steel grades, for example, can be passivated in a low-percentage nitric acid bath, whereas less-resistant alloys require increased nitric acid or the addition of sodium dichromate. 

Our Two-Step Passivation Process

Chemical passivation occurs in two steps.

First, because iron enables corrosion, we use acid to dissolve any free iron or iron compounds from the part’s surface. This process does not harm the part’s surface or alter its dimensions.

Second, to create a uniform chromium oxide protective layer over the part’s surface, we use an oxidizer to force-convert any surface chromium metal into oxide form.

Regardless of the method used, passivation demands strict attention to these four factors: chemistry, concentration, temperature, and time.

Inexact passivation can cause surface-damaging “flash attacks” that discolor the part and render it useless. Properly passivated alloys will look like regular steel but perform better and last longer.

After passivation, we perform grade-specific testing to evaluate the surface integrity of the passivated parts. We consider a passivation process successful if it has removed the sulfides and free iron and enhanced the part’s corrosion resistance.

Choose Global Metal Finishing for Passivation of Stainless Steel 

When you’re ready to send your precision machined parts to Global Metal Finishing for passivation, including comprehensive details in your quote request can help expedite your project. 

Ideally, we need to know the desired passivation method and type and the alloy specifications. For instance, a customer may indicate that they want method AMS 2700E Method 1, Type 6 for 304 Stainless Steel. We’ll verify that the method and type are compatible with the alloy and get to work quickly. 

Sometimes, customers provide us with the alloy specifications but don’t indicate the method or type of passivation they want. In these cases, we can backtrack to make those determinations ourselves. 

If you can’t supply us with alloy specifications, we’ll recommend a passivation method that successfully passivates most alloys. However, we can achieve optimal results when we know your alloy specifications from the start.

When you need top-tier passivation of stainless steel, we’re ready to answer your questions and get your project off the ground. Request a quote to get started.