What makes stainless steel stainless

What Makes Stainless Steel Stainless? A Passivated Finish

From actuators to fasteners to landing gear components, exhaust components, and more, stainless steel is used in countless ways across the aerospace industry. It has a number of distinct advantages over other types of material including superior resistance to fire and heat, high strength and impact resistance, and a sleek aesthetic appearance. But it's one of its biggest benefits — corrosion resistance — that many people misunderstand.

What makes stainless steel stainless? The answer to that question is a bit more complicated than people realize, particularly in this context.

The Chemistry of Stainless Steel 

Officially, a steel is considered to be "stainless" if it’s a mostly-iron alloy with 1) at least 10.5% chromium, and 2) less than 1.2% of carbon and other alloying elements. This is important to understand, as you can essentially boost the natural benefits of stainless steel by adding elements like nickel, molybdenum, and chromium.

Chromium is primarily what gives stainless steel its natural corrosion resistance. Therefore, if you wanted to make a stainless steel even more resistant to issues like rust than it already is (which is a top priority in the aerospace industry where these components are regularly exposed to harsh environments), you would therefore focus on adding chromium.

However, this alone is not enough to guarantee a totally stainless performance. 

The Ins and Outs of Passivation

This segues to a process called passivation, which is used to create a protective layer on the surface of stainless steel so that it truly becomes "stainless" as intended.

First, things begin with a thorough cleaning of the material in question, at which point it is then immersed in an acid bath. Either citric acid or nitric acid is used depending on the passivation method being employed. This is important, as it is the acid that removes any free iron from the stainless steel.

The material is then rinsed and, after between one and two days, a passivation layer forms. This is important, as the layer has a higher proportion of chromium than the original stainless steel itself. Think of it as a protective layer that not only looks great, but that also boosts the corrosion and rust resistance exponentially. 

All told, passivation is important when any foreign material might be introduced to the stainless steel in a manufacturing environment. Dirt, dust, and contaminants on equipment (or the shop floor) can literally be microscopic, so even if a component appears clean it is still more susceptible to rust without additional protection.

The same concept holds true if those parts need to be cut or used in conjunction with tools in any way during the manufacturing or even maintenance processes. Cutting a piece of stainless steel could leave behind small pieces of the drill bit that are imperceptible to the human eye that could also cause long-term damage if left totally unchecked. Passivation helps prevent that from happening, allowing stainless steel to truly become stainless in the way that most people think of it.

Additional Considerations About the Passivation Process

This is all critical for components that are being used in the aerospace industry in particular. Essential components don't just need to be resistant to simple corrosion — they also have to fend off high temperature oxidation as well. Their mechanical properties need to be maintained over an incredibly wide temperature range and over long periods of time as well.

According to the National Institutes of Health, a typical commercial aircraft needs to be able to safely and reliably operate over temperatures that can range from -65 degrees Fahrenheit to 122 degrees Fahrenheit at ground level. When that same flight has assumed an altitude of approximately 39,400 feet, that range expands to temperatures as low as negative 122 degrees Fahrenheit.

It needs to be able to do this while traveling at incredible speeds for long periods of time. During all of these conditions, there is no such thing as a "small problem" anymore — particularly as far as the lives of those onboard are considered. Even parts that rust and degrade before their anticipated lifespan can lead to flights being grounded and canceled, adding up to an enormous expense (not to mention frustrating experience) for all involved.

Stainless steel was chosen for a lot of these critical components because they help prevent these situations from happening and a passivated finish goes a long way towards extending those natural properties as far as they can conceivably go.

If you'd like to find out more information about how a passivation layer can improve the long-term value of stainless steel components for the aerospace industry, or if you have any additional questions that you'd like to go over with someone in a bit more detail, please don't hesitate to contact us today.

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