A Guide to Ultrasonic Passivation of Stainless Steel
Stainless steel is used in a vast range of industries and products. It's recyclable, easy to clean, and durable, making it an ideal material for everything from medical implants to sculptures to rail cars. One of the core essential characteristics of stainless steel is its ability to resist corrosion. Indeed, this property can be the differentiator in the structural integrity of a bridge or the safety of an aircraft.
While inherent, this property isn’t guaranteed or permanent. As such, it’s often necessary to boost the anti-corrosive properties of stainless steel using a process called passivation. The passivation process can be applied to various types and sizes of stainless steel parts and may be carried out using several different methods.
In this post, we explain more about why ultrasonic passivation is necessary and what it entails. We also compare two popular passivation methods: ultrasonic nitric passivation and ultrasonic citric passivation.
Ultrasonic Cleaner for Passivation of Stainless Steel
What Is Ultrasonic Passivation?
Ultrasonic passivation overcomes the issue of corrosion by restoring the chromium oxide layer.
First, the component is thoroughly cleaned to completely remove all contaminants from the surface of the object. Ultrasonic cleaning is the ideal method here because it enables you to ensure every surface of the part is precision-cleaned using the proper time, temperature, and detergent.
After rinsing, a special solution is used (usually containing nitric acid or citric acid) to return the corrosion resistance quality to the steel surface. Note that ultrasonic passivation does not involve stripping or coating, but rather facilitates a surface reaction that enables the chromium oxide layer to naturally reform.
The parts are rinsed again before going through a drying process, after which they are ready for use.
Why Is Ultrasonic Passivation Necessary?
Passivation is a very specialized type of cleaning for stainless steel parts and componentry, that prevents corrosion. One of the most valuable properties of stainless steel is its resistance to corrosion. However, while stainless steel has inherent anti-corrosive properties, it is not completely immune to compromise. And depending on the use case, the slightest sign of corrosion could signal dire consequences, both in terms of safety and costs.
Stainless steel is covered in an extremely thin layer of chromium oxide. This layer forms automatically as soon as the metal is exposed to air and protects the steel from corrosion by shielding it from atmospheric oxygen. Theoretically, if the layer is damaged, it should reform automatically upon exposure to oxygen. However, in reality, the layer often contains contaminants that compromise the steel.
Indeed, the primary cause of stainless steel corrosion is contamination. The contamination can come from various sources, for example, from tools used to repair or machine the steel part. Components may undergo surface damage as a result of maintenance or handling, and parts are often exposed to dust or dirt that settles on the exterior. Sulfides used to facilitate machining are another common source of contamination.
Any foreign material that sits on or becomes embedded in the steel surface can compromise the integrity of the chromium oxide layer. It could even directly introduce corrosion onto the metal, a particular concern with iron or sulfide contamination.
Passivation is a process that restores the chromium oxide layer to its original form. This process has a key role in a broad range of industries, including manufacturing & fabrication, aerospace, pharma & medical.
Ultrasonic Nitric Passivation vs. Ultrasonic Citric Passivation: Which Is Best?
This is the most traditionally used type of ultrasonic passivation, but it does have its drawbacks. It involves handling nitric acid which is a hazardous substance that emits toxic gases. Plus, elevated temperatures are often required for processing which adds another level of risk. As such, specialized safety equipment is required for ultrasonic nitric passivation.
Nitric acid solutions are harmful to the environment which is far from ideal, particularly in today’s green-centric world. In addition, they can cause corrosion over time, reducing the lifespan of your ultrasonic passivation equipment. Another drawback to using nitric acid is that the passivation process can take a relatively long time, often up to several hours.
With all that said, nitric acid passivation does have its benefits. The raw material itself is very inexpensive and provides excellent passivation of most stainless steel grades.
Citric acid passivation is a newer method that offers a host of benefits. Citric acid is far less hazardous to use than nitric acid and requires minimal safety equipment. You also won’t have to worry about solutions corroding equipment. Citric acid still provides excellent passivation and has a faster processing time than nitric acid, typically 5–20 minutes. It can be used at room temperature so there’s no need to apply heat.
CItric acid is environmentally friendly and a lower concentration is required for effective passivation. It does cost more than nitric acid, although these costs are offset by the ability to use a more dilute solution, as well as relatively low costs associated with maintenance, safety, and waste disposal.