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Acid Passivation and The Benefits it Provides
If you've ever dealt with rusting metal, you know that corrosion can be extremely damaging. It can cause seemingly sturdy looking cars to fall apart, it can lead to metal parts bending and breaking prematurely, and it can just be generally ugly. Fortunately there are methods to prevent rusting, corrosion, and damage from the elements. One of those methods is referred to as passivation, and one way to do so is by using an acid—acid passivation. But first, let's learn: what is passivation and how it is done?
Passivation means a material becomes "passive," which is to say that it becomes less vulnerable to elements such as air and water. Essentially, passivating metal means the metal becomes stronger, and its appearance is preserved. The way this is accomplished is by applying a light coating to the material in order to protect it. Generally speaking, passivation refers to applying a metal oxide to a metal so that it has an outside shell to protect it against corrosion.
But acid passivation is a completely different procedure. Typically involving nitric or citric acid applied to stainless steel or other metals, this process involves exposing metals to acid that then essentially dissolves a layer of the metal and makes the product smoother overall. This process creates a barrier between the metal and the environment.
There are reasons why companies that perform passivation choose one acid over another. The most popular acids to use are nitric and citric acid. Nitric acid emits noxious vapors into the atmosphere and contributes to the production of smog, while citric acid does not do either of those things. Using nitric acid for acid passivation requires providing special handling equipment to employees, as well as safety devices to ensure no one gets injured or burned. Citric acid also has the benefit of not needing to be classified as hazardous waste. That is, it's much easier to dispose of citric acid than it is to dispose of nitric acid. Since citric acid removes free iron and iron oxides on metal without pulling significant amounts of other metals, like nickel, chromium, and other heavy metals, the waste removal costs are far reduced.
When metals go through the process of acid passivation, the chromium at the surface of the metal reacts with the oxygen in the air to create a protective layer called chromium oxide. Another term for passivated metal is "unreactive." In other words, once metals go through the passivation process, they are less susceptible to damage from the elements. The layer of chromium oxide is very, very thin—just a few molecules thick—but it is enough of a barrier to prevent oxygen from reaching the metal underneath. Oxygen coming into contact with iron in steel can create rust, which, over the long-term, significantly weakens the steel.
So what kind of metals can be passivated? Steels with 11 percent or more of chromium are capable of forming the passive layer of chromium oxide. It's also important to note that even if a stainless steel surface is scratched after acid passivation, more chromium will be exposed, and combined with surface oxygen, the passivated surface will reform. However, if something becomes embedded in the steel, the passive layer cannot reseal, and corrosion will occur at that site.
Another way to passivate metals is through a process called electropolishing, which is an electro-chemical process that does a more thorough job of cleaning, smoothing, and protecting the metals.
Passivation is typically the final step in manufacturing stainless steel parts. Since welding exposes the metal's alloy structure, it needs to be passivated to restore the corrosion resistance.
As for the benefits to be reaped from passivation, either through acid passivation or electropolishing, it prevents rusting and corrosion, first and foremost. This is especially important for steels used in highly corrosive environments, like on boats, oil rigs, and airplanes. In addition to preventing rust and corrosion, passivating metals removes imperfections on the surface, so parts that flex, twist, and bend are made to last longer. Minor surface defects on parts that move can become major surface defects over time, especially if there is also corrosion present, and the parts can snap or break prematurely if they haven't been passivated.
Making sure that metals have gone through a passivation process can ensure they last much longer and resist corrosion and damage from the elements surrounding their daily use. There are many companies who perform the service, and clients generally include businesses in the aerospace industry, the automotive industry, and consumer appliances and the electronics industries, as well as the food and beverage industry, the pharmaceutical and dental industries, and medical industry. Acid passivation is just one way of protecting the metals you use or your company uses against corrosion.