As defined in MIL-STD-753C, the passivation process is the final treatment/cleaning process used to remove iron from the surface of corrosion resistant steel parts such that a more uniform formation of a passive surface is obtained thus enhancing corrosion resistance.
Stainless steel is different from other metals in that as you get closer to the surface the composition of the metal actually changes. In the passivation process, free iron is removed from the surface into solution, leaving behind a higher chromium level. A good chrome to iron ratio is usually considered to be 1.5 to 1 or higher.
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 the 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.
Current Passivation Methodologies
Astro Pak employs many different methods of passivation, including:
- Tank Immersion – Usually done at an Astro Pak site and is advantageous for treating all the fabrication surfaces at the same time for uniformity of finish and optimum corrosion resistance.
- Circulation – Chemical solution is circulated through a system of pipework and is particularly recommended for piping that will carry corrosive liquids.
- Spray Application – Can be done at Astro Pak or at a customer site and advantageous for on-site treatment but proper acid disposal and safety procedures are essential.
- Gel Application – Manual treatment can be accomplished by brushing on pastes or gels to the surface. It is useful for spot treatment of welds and other intricate areas that require manual detail.
Astro Pak Services and Capabilities
Ultrapass passivation is provided in our state of the art cleanroom facilities and at Clients high purity locations. Nitric acid as well as citric acid chelant passivation modalities can be utilized based on site specific requirements. Cleaning and passivation can be provided for pre-commissioning of new construction or maintenance and shutdown efforts. Immediate response for emergency outages and critical utilities are routinely provided. Passivation is performed in conjunction with pickling of weld area defects, derouging of corrosion coloration films, and new additions or replacement equipment. Professional consulting by our known industry experts with many years of experience, benefits our clients with their difficult issues.