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A Passivation Primer: Using Ultrasonic Cleaning to Protect Aerospace Components From Rust

Aerospace Passivation

No matter what type of aerospace technology you're working with or what your exact activities are‚ manufacturing, refurbishment, maintenance or operations‚ you'll be handling numerous stainless steel components. From tubing on rocket engines to the sinks and toilets in airliner lavatories, from landing gear to piston engine manifolds to simple fasteners, the ability of stainless steel to resist corrosion and maintain its mechanical properties under a wide range of stresses and temperatures makes it indispensable. In this field, corrosion is an enormous cost and safety concern, and as they say, rust never sleeps. Unfortunately, even stainless steel can be compromised and become subject to corrosion. This mainly occurs through contamination. Here are some common sources:

  • Contamination from the cutting surfaces of tools used to machine or repair the components, which can leave behind microscopic portions of a blade or bit.
  • Damage from handling and/or maintenance.
  • Exposure to common dirt or shop dust.
  • Sulfide exposure at the surface of stainless alloys with relatively high amounts of sulfur in the mix for ease of machining.

These contaminants damage your stainless steel in two main ways. First, any damage or contaminant disrupts the amazingly thin layer of chromium oxide that protects the metal from atmospheric oxygen and therefore from corrosion. This layer is so thin‚ one-ten-millionth of an inch thick, or about 1/100,000th as thick as a human hair‚ as to be invisible to the naked eye. The chromium oxide layer forms automatically when the steel is exposed to air, so theoretically it will reform over the steel's surface immediately. That brings us to the second way the contaminants compromise the steel. Any material left behind, embedded in or lying on the surface, can not only compromise the layer but introduce corrosion directly onto the metal. This is especially true if the contaminant contains iron, and even shop dust will often contain tiny amounts. Sulfide exposure has the same effect. The good news? A passivation bath of nitric or citric acid can reestablish the chromium oxide layer. But prior to that, it's essential that the part is perfectly clean. For that task, there's no substitute for ultrasonic cleaning.

Let's look at the clear advantages of ultrasonic cleaning in this area:

Hand cleaning can be especially expensive in an aerospace environment and may not remove all contamination

If you pay an employee to clean the part, they will almost certainly be a highly paid specialist who could be using their advanced technical skills elsewhere in your operation. Instead, placing batches in an ultrasonic cleaner allows you to make them spotless in minutes.

Mechanical washers can miss contaminants

Like hand washing, spray washers can't necessarily reach every surface of a component, especially if improper loading prevents a spray stream from reaching it. But the bubbles formed in ultrasonic cavitation reach any spot that liquid will reach, eliminating the possibility of missed spots‚ or scratches and dings‚ from human error.

Ultrasonic cleaning eliminates grease and oils

Cutting oils and other greasy substances can prevent passivation because they react with the chemical bath and form bubbles that keep the acid away from the surface. That means pitting or rusting later on. Ultrasonic cleaning will remove these oils without error.

Ultrasonic cleaning prevents flash attack

Some contaminants can cause a sudden change in the passivation bath's chemical makeup known as a flash attack.‚ The result is parts that are darkened and may also suffer from etching. That means rework at best and ruined components at worst. However, ultrasonic cleaning reaches each surface and removes any contaminants, preventing a flash attack. Today's aviation and spaceflight operations depend on corrosion-resistant components. To deliver them you need a cleaning process you can depend on, and ultrasonic cleaning delivers unsurpassed results.