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Passivating Stainless Steel & Aluminum

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Corrosion is one of the concerns that keep designers and engineers up late at night worrying.  If the right materials aren’t specified from the beginning, then the durability of a product or structure can become significantly compromised after only a few years, or even sooner.  That’s why the development of modern metals such as aluminum and stainless steel has been so important.

Prior to the 20th century, most metals were extremely vulnerable to corrosion.  This was especially true of traditional carbon steel, which is prone to rust when exposed to moisture.  Matters were even worse for applications and structures located in marine environments.

One of the tremendous features of stainless steel and aluminum is their ability to undergo passivation.  This is a process by which an extra layer of protection is added to the material surface and a naturally occurring phenomenon common to both these metals. Metallurgists have also found ways to boost or elevate the process to make materials that are even more corrosion resistant than normal.

What is passivation?

In chemistry or engineering, the term passivation refers to the process a material undergoes to become less susceptible to environmental corrosion.  This process usually involves the formation of an outer protective layer.  It can involve the application of a micro coating to the surface of the material that leads to a chemical reaction with the base metal, or the spontaneous oxidation that happens when it comes into contact with the air.

Passivation can happen in a number of different scenarios, such as in microelectronics to enhance silicon.  For our purposes, we’re taking a look at the passivation processes for stainless steel or aluminum, when a light coat of protective material, such as metal oxide, is used to add extra corrosion protection.

One more benefit when it comes to passivation of aluminum and stainless steel is that the process will enhance the appearance of the metal, giving it an extra sheen. Technically speaking, the passivation process is a form of corrosion, as the metal reacts with the environment to form a hard, relatively inert surface.  It just so happens that the formation of this layer will help to reduce or nearly eliminate any further corrosion.  This is as opposed to rust, which as it progresses, degrades the surface of the metal and actually hastens, rather than stops, further corrosion.

What the process of passivation for stainless steel?

It’s important to note that the process of passivation is not the same for aluminum and stainless steel, so let’s distinguish between the two metals.  The term passivation is more likely to be associated with stainless steel, so we’ll start there.

While every stainless steel has some natural corrosion resistance built in, that doesn’t mean that every alloy is completely impervious to rust.  For example, it’s not uncommon for small spots of rust to appear on the surface of stainless steel because bits of foreign matter have become embedded in the surface and allowed water molecules to oxidize the iron content.  We refer to this kind of corrosion as rouging, and certain grades are more susceptible than others.

Prior to passivation, the material must be thoroughly cleaned to ensure no contaminants are present.  The metal then goes into an acidic passivating bath that meets the grade’s temperature and chemical requirements, then rinsed and dried. This process removes exogenous iron and creates a passive oxide layer that will inhibit further rust.  For certain stainless steels, such as from the martensitic family, it may be necessary to apply a special process, as they can be more difficult to passivate.

The passivation process for stainless steel will generally adhere to certain industry standards, such as ASTM A 967 and AMS 2700. 

What is the process of passivation for aluminum?

Pure aluminum already undergoes a natural process of oxidation in which a thin surface layer of aluminum oxide will form when it comes into contact with oxygen. This transformation leads to a protective barrier that helps prevent further corrosion. Unfortunately, not every aluminum alloy does a good job of forming this oxide layer, meaning that extra protection might be needed.

If you do not want to plate, paint or coat your aluminum, there are two main options for creating a passivation layer.  The first is known as chromate conversion coating.  Such coatings, which can vary in thickness from 0.00001–0.00004 inches, are amorphous in structure and have a gel-like composition when hydrated with water.  If you work with other metals such as zinc, cadmium, copper, silver, magnesium or tin alloys, you are likely already familiar with chromate conversion.

A second option is known as anodizing.  This is when the aluminum undergoes an electrolytic process in order to thicken the oxide layer.  The anodic coating will create a layer of hydrated aluminum oxide that is more resistant than the natural passivation layer to corrosion.

When should you consider passivation?

Both aluminum and stainless steel have inherent anti-corrosion properties that protect the metal from rust and other deterioration.  Not all alloys have the same level of invulnerability, and even the highest performing metals can be susceptible to imperfections and foreign material leading to a degraded surface that is more prone to corrosion.  The two major benefits of passivation are actually closely linked, the fact that the process ensures your material is as clean as possible while at the same time protecting it from future corrosion (which is often the result of impurities when the metal is not properly cleaned).  If protection against the elements is a primary concern, then passivation is something that you should strongly consider, especially if you are required to use an alloy or grade that is not naturally anti-corrosive.

At Clinton Aluminum, our goal is to help our customers through every step of the procurement process, including material selection and preparation.  As the Midwest’s leading supplier of aluminum and stainless steel, our dedicated technical professionals have plenty of experience helping manufacturers to optimize their production processes and lower their costs. To learn more about passivation, contact us today.

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