What is Passivation?
Passivation is a chemical treatment for stainless steel and other alloys that enhances the ability of the treated surfaces to resist corrosion.
There are many benefits of passivated equipment and systems:
- Passivation removes surface contamination
- Passivation increases corrosion resistance
- Passivation reduces risk of product contamination
- Passivation allows you to extend system maintenance intervals
Composition of Stainless Steel
To understand passivation of stainless steel, it is critical to look at stainless steel itself. All stainless steels are alloys of iron, nickel and chromium. Chromium makes up at least 10% of the metal. It is this element that gives stainless steel its resistance to corrosion. Often steel-makers add molybdenum to enhance chromium’s protective characteristics for highly corrosive or high temperature applications.
Beyond the chemical makeup of the metal, the composition varies in the different layers that make up stainless steel. At the surface is the passive layer, sometimes referred to as the passive film, is responsible for providing corrosion resistance. It is a very thin layer of highly stable metal atoms that do not easily corrode or rust. It is only a few atomic layers in thickness where the ratio of chromium to iron (Cr/Fe) is at least 1.5-to-1. The chromium binds with oxygen to create a chemically inert, “passive” surface.
The illustration below shows the very thin passive layer at the outer surface of the metal:
Below the passive layer is the transition area where nickel is in higher concentrations. Like the passive film, it is only 3 to 4 atomic layers in thickness. The nickel in this section protects the passive film by preventing chemical reactions with the iron in the layer below. It also acts as a protective barrier to the base metal which makes up the majority of the stainless steel. The percentages of chromium, nickel and iron vary by the intended use of the item.
Why Do You Need to Passivate?
Although passivation occurs naturally in corrosion resistant and chromium rich alloys, given the proper conditions, a new stainless steel vessel or part needs to be passivated prior to being placed into service. Fabrication, machining and welding leave behind contaminants such as metal oxides, inclusions, fabrication debris and tramp iron, thereby compromising the metal’s natural ability to resist corrosion.
How Does Passivation work?
Chemical passivation enhances the chemistry of the passive layer by increasing the ratio of very stable chromium atoms to the more reactive iron atoms in the upper three to five atomic layers of the metal’s surface. This enrichment can be quantitatively measured using a variety of methods, including Auger Electro Spectroscopy (AES), Electron Spectroscopy for Chemical Analysis (ESCA), and Energy Dispersive X-ray Analysis (EDX). The most commonly used data generated from these techniques to evaluate the passive layer is the chromium to iron ratio and the depth of enhancement.
Trust the Experts in Passivation
Astro Pak has been perfecting its passivation chemistries and processes for almost 60 years. Our proprietary Ultra Pass® passivation process is renowned for its environmentally friendly blend of high-purity ingredients, including chelants that bind with metal ions rendering them incapable of redepositing on surfaces. As a result, the material handling requirements and other impacts on-site are greatly reduced.
Beyond that, the UltraPass® passivation process provides superior results, capable of producing an optimized passive layer on the surface of stainless steels, yielding unmatched corrosion resistance. Our studies have shown that the chrome to iron ratios achieved by UltraPass® passivation far exceeds ASME BPE recommended minimums and meets or exceeds the requirements of many industries including semiconductor, aerospace, pharmaceutical and biotechnology.
On-Site & Shop Passivation
Astro Pak has almost 60 years of experience perfecting our passivation chemistries and processes. We bring that expertise, along with our state-of-the-art equipment and highly trained personnel, on-site to your facility to perform preoperational or maintenance cleaning and passivation on your systems and equipment.
For components, parts and sub-systems that can be serviced out-of-place, Astro Pak’s nationwide shop facilities are well suited to affordably meet your cleaning and passivation requirements. We have state-of-the-art facilities that can accommodate parts of all sizes and volumes, including dip tanks up to 40ft and overhead bridge cranes with 4 ton lifting capacity for oversized parts. Additionally, we offer pick-up and delivery services as well as cGMP compliant documentation packages at the end of each job. Our highly skilled technicians and controlled environments ensure consistent quality and compliance to industry and customer-specific standards.
ASTM A380 defines passivation of stainless steel as “the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film.” In addition to nitric acid, citric acid can be used. Citric acid offers numerous advantages as it is less toxic and the service can be performed on-site with fewer environmental risks.
ASTM A380 and A967 serve as the most common passivation specifications. However, certain applications require going further. For example, the FDA’s Current Good Manufacturing Processes (cGMP) directs drug makers on how to ensure quality and purity. Additional guidelines apply to aviation parts and surgical implants, among others.