Passivation of Stainless Steel — What is It and How Does It Work?

Passivation of Stainless Steel 

Many sanitary processors know that stainless steels such as 304 and 316 are "stainless" and resist corrosion because they are alloys with some key components. Some even know the stainless steel processing equipment forms an interior layer that protects the metal from damaging corrosion. But what's hard to picture is that this protective layer is only one to three nanometers thick. That's only a few atoms deep, but it's enough to provide needed protection if conditions are right and remain stable. 

This unimaginably thin internal coat is known as the passive layer, and passivation is the process by which it forms.

The Chromium Difference

Fast forward several millennia to the mid-1800s, and metallurgists discovered that adding chromium to iron made it more durable and formable for making tools, implements, and other items. Chromium had only been isolated as a material about 50 years previously. Up to the early 20th century, chromium had only been added to iron in quantities of less than 5%. 

It was only when metallurgists added over 5% that they discovered that chromium prevented iron from rusting. Soon after, the formula of 18% chromium and 8% nickel added to iron began coming into ubiquitous use as 18/8 stainless steel for cutlery and cooking equipment. 18/8 is actually included in the modern 300 series of stainless steels along with 304 and 316 stainless steels.

So how does chromium prevent corrosion? 

Through a chemical reaction called passivation. "Passive" basically means non-reactive, and that's what chromium does for steel - shields it from being chemically reactive and therefore corroding. The chromium combines with oxygen to form chromium oxide (Cr2O3 - two atoms of chromium and three of oxygen, though there is another form of chromium oxide with one atom of chromium and one of oxygen).

Although a passive layer forms naturally in alloys with a chromium content of between 10.5% and 12%, stainless steel equipment should be treated with a process called chemical passivation to ensure it's protected immediately and properly.

Citric acid is gaining on nitric acid for passivation use for several reasons:

• It can passivate more kinds of stainless steel alloys, so it can be used for systems composed of different alloys
• It's far less toxic and hazardous, and biodegradable, so disposal is much less of a challenge
• Since it's used as a food additive and is on the FDA's GRAS (Generally Recognized as Safe) list, it's well-suited for use in food and beverage processing
• Some passivation processes remove some nickel and chromium from alloys as well as removing iron, which thins the passive layer. Using citric acid minimizes this potential for this detrimental removal, promoting a thicker oxide layer

It should be noted, though, that citric acid itself doesn't do the passivation - but it does a superior job of preparing surfaces to spontaneously passivate in the ambient air.

The importance of thorough pre-cleaning

It's also important to emphasize that a crucial first step - cleaning - is needed to eliminate contaminants that could compromise the passivation process. During the machining process, iron particles can abrade from the cutting tool and transfer to the surface of the stainless steel workpiece. Other substances such as grease and coolant from the shop environment can land on the stainless steel parts. If not removed, these particles can compromise the passivation process and plant the seeds for corrosion.

While it might seem reasonable to assume that an acid bath would remove grease and other contaminants, it all gets back to the micro-level chemistry with corrosion. Fats react with acids to form gas bubbles that stick to the surface of the metal, interfering with passivation. Degreasers or other appropriate commercial cleansers should be used for an initial cleaning process.

Passive layer maintenance - when it self-heals, and when it doesn't

Once stainless steel processing equipment has been passivized and placed in service, the passive layer can be damaged by being abraded or through expansion or contraction caused by heating and cooling. If enough oxygen is present to combine with the chromium in the alloy (and other conditions are right), the passive layer will "heal itself," which is one of stainless steel's major benefits.

However, chemical reactions can also damage the passive layer and/or keep it from forming successfully or reforming. Suppose your processes use certain chemicals under certain circumstances such as high temperatures. In that case, you won't be able to rely on the passive layer to protect your equipment investment because corrosion will be inevitable.

How often to re-passivate — generally accomplished by running a passivization acid through the system — depends on how demanding processing conditions are and how aggressive the chemicals are that are being processed.

Some processors re-passivate once a year, but others might do it more often because their products, such as those made from tomatoes, are high in chlorides and corrosive acids. Some water used in processing is naturally high in chlorides and hard on the passive layer. There are test kits available from chemical supply firms that will test for free surface iron. If a high level is found, it could be time to passivate.

Stainless passivation vs. pickling

Pickling is a process that's often confused with passivation, but they serve different purposes. They're both aimed at improving the corrosion resistance of stainless steel and other alloys by promoting an effective chromium oxide passive layer.

While underheating during welding can cause poor weld penetration, overheating can negatively affect the physical properties and chemistry of stainless steels and other alloys. It can oxidize the component metals, which causes the metal to take on a range of colors from yellow to brown to blue depending on the temperature they were exposed to and how thick the oxidized layer is. This discoloration is called "heat tint."

In stainless steels and other alloys where chromium has the central role in corrosion resistance, a heat tint area means the chromium has been depleted from the metal's surface and is unavailable to form the passive layer. Therefore, the damaged, oxidized layer must be removed to re-expose the alloy in its original, corrosion-resistant form. 

So while passivation involves creating a new layer, pickling removes a damaged layer, after which passivation can be done.

Like passivation, pickling is done with chemicals - usually nitric or hydrofluoric acid solutions - and the acids are much more aggressive. Pickling can also be done by electropolishing (when metal is immersed in a solution carrying an electrical current that removes a very thin layer of the metal's surface) or by mechanical removal, which may leave small contaminating particles.