What is Passivation and Why Does it Matter for Stainless Steel?

It’s no secret that despite its name, stainless steel isn't always immune to staining. While the alloy contains elements such as chromium, nickel, and molybdenum that provide resistance to rust and corrosion, it's still susceptible to discoloration and surface defects if not properly treated. This is especially crucial in industries like food and beverage, where product purity and cleanliness are paramount.

Stainless steel used in these environments often includes elements like iron and chromium, but iron is the culprit when it comes to rust formation. This brings us to passivation – a vital process for enhancing the corrosion resistance of stainless steel.

Understanding Passivation

Passivation is a chemical process that establishes a thin, protective oxide layer on stainless steel, shielding it from corrosion. This barrier primarily consists of chromium oxide, which forms when the chromium in the steel reacts with oxygen. Unlike iron oxide (rust), which weakens and corrodes the material, chromium oxide is stable and highly resistant to further corrosion.

During manufacturing and installation, the stainless steel surface can be compromised by scratches, tool marks, and exposure to contaminants. These small defects can trap free iron, which reacts with air to form iron oxide, or rust. This surface contamination can lead to rouging – the discoloration that appears as red or copper streaks on stainless steel. Rouging isn’t just a cosmetic issue; it can reduce the lifespan of the equipment and, in food and beverage applications, it can even introduce unwanted flavors.

Passivation vs. Acid Washing for Beer Stone: Understanding the Difference

Brewers often confuse passivation with an acid wash intended to remove beer stone, a common buildup found in brewing equipment. While both processes involve the use of acids, they serve different purposes and utilize significantly different acid concentrations.

Beer stone is a calcium-based deposit that forms on the interior surfaces of brewing equipment due to the interaction of calcium and oxalic acid from hops. Acid washing, typically using a 2% nitric acid or phosphoric acid solution, is performed to dissolve these deposits. While phosphoric acid is effective at removing beer stone, it does not passivate stainless steel. In contrast, the concentration of nitric acid used for beer stone removal is much lower than that required for passivation.

The Difference in Nitric Acid Concentration

In passivation, the goal is to create a protective chromium oxide layer on stainless steel, which requires a much higher concentration of nitric acid—typically around 20%. This concentration is strong enough to dissolve free iron and other surface contaminants but must be handled with care due to its corrosive nature.

On the other hand, acid washing for beer stone uses a weaker solution, typically 2% nitric acid, which is sufficient for removing calcium oxalate deposits but does not provide the corrosion resistance or protective oxide layer achieved through passivation. Brewers may mistakenly believe that performing an acid wash passivates their equipment, but in reality, these are two distinct processes.

The Three Steps of Passivation

1. Cleaning the Surface

   
   Before passivation can occur, it’s essential to clean the stainless steel of oils, greases, and other contaminants. Any residues left behind can interfere with the passivation process, preventing the formation of a uniform oxide layer. Common cleaning agents include degreasers or detergent cleaners like sodium hydroxide or citric-based solutions.
   
   

2. Acid Treatment

   
   Once the surface is clean, an acid solution is applied to remove free iron from the surface. Traditionally, nitric acid at 20% concentration has been the go-to option, but citric acid at 10% concentration is gaining popularity due to its environmental benefits. Citric acid is biodegradable, less hazardous to handle, and doesn’t produce harmful byproducts during disposal. This makes it an ideal choice for environmentally conscious operations. The acid dissolves the iron particles, leaving behind a clean, chromium-rich surface.
   
   

3. Oxide Layer Formation

   
   After the acid treatment, the stainless steel is left to dry in an oxygen-rich environment. This is when the magic happens – the chromium in the alloy reacts with oxygen to form a thin, yet robust chromium oxide layer. For nitric acid treatments, this process takes a minimum of 24 hours, while citric acid-treated equipment should be allowed to air dry for at least 72 hours.

The Impact of Passivation on Gaskets, Pump Seals, and Non-Stainless Metals

While passivation is essential for protecting stainless steel surfaces, it can be particularly harsh on other components like gaskets, pump seals, and non-stainless metals. The acids used in the process, especially at the high concentrations required for effective passivation (such as 20% nitric acid), can degrade non-metallic materials over time. Gaskets and pump seals, often made from rubber or silicone, are especially vulnerable to chemical breakdown, leading to premature wear and potential equipment failure.

Additionally, any non-stainless metals in the system can corrode or be damaged by the acids during passivation. These materials should be carefully isolated or removed before the passivation process to prevent damage. It's important to replace or inspect seals and gaskets after passivation to ensure they remain in good working condition and won’t compromise the integrity of the system.

Nitric vs. Citric Acid Passivation: Which Is Better?

Historically, nitric acid has been the standard for passivation. However, citric acid has emerged as a safer and more efficient alternative. CitriSurf® solutions, for example, provide several advantages over nitric acid, including:

• Safer handling: Citric acid is non-toxic and biodegradable, reducing environmental and health risks.
• Lower cost: Disposal of citric acid is less costly than nitric acid, which requires expensive waste treatment.
• Better performance: Citric acid forms a water-soluble complex with free iron, preventing iron from reprecipitating onto the surface – a common issue with nitric acid.

For these reasons, many industries are transitioning to citric acid passivation, especially in food and beverage production, where safety and environmental impact are critical considerations.

Why Passivation is Essential for Food and Beverage Equipment

In food and beverage manufacturing, stainless steel equipment must remain in top condition to avoid contamination and preserve product quality. Passivation ensures the formation of a durable, corrosion-resistant barrier that extends the lifespan of stainless steel equipment and minimizes the risk of rust contamination. Moreover, passivated surfaces reduce maintenance needs and help equipment withstand the rigors of daily use.

When investing in new stainless steel equipment, it’s crucial to confirm that it has been properly passivated before use. Not only does this protect your investment, but it also ensures the safety and quality of the products you produce.

Looking for Citric Acid in Canada?

If you need help sourcing citric acid for your passivation needs in Canada, I recommend reaching out to Stellar Solutions for guidance on purchasing CitriSurf products.

Stellar Solutions
stellar@citrisurf.com
(847) 854-2800
http://citrisurf.com
4511 Prime Parkway
McHenry IL 60050
USA

Cheers!