Passivation is a chemical process that helps stainless steel surfaces resist rust and other types of corrosion. Consisting of washing, rinsing and soaking stainless steel parts in an acid solution, the finishing process alone entails multiple rinses. Unlike coatings, nothing is applied to the surface of a workpiece. Rather, it’s a process used to remove the free iron particles that lead to corrosion from a metal or alloy surface.
When it’s done correctly, passivation of stainless steel helps components better resist oxidization that leads to rust. Essentially, passivation forms a film on the alloy’s surface, which acts as a sort of invisible shell that protects against corrosion. Generally, passivation of stainless steel involves using either a nitric or citric acid solution to enhance its corrosion resistance. When looking to outsource passivation services, it’s important to understand the process to determine the best partner.
Outsourcing Passivation of Stainless Steel
Those who are unfamiliar with working with metals and alloys commonly misperceive stainless steel as being corrosion-proof, when it these kinds of steels are really just very resistant to corrosion. In fact, it’s the passive layer that forms on the surface of stainless steel during passivation that protects the alloy’s surface, making it corrosion-resistant. This passive layer must be regularly maintained and fortified to keep corrosion in check on stainless steel surfaces. As such, companies that outsource the passivation of stainless steel for their parts or products should ensure that their provider does the process properly.
5 Tips to Follow When Choosing a Partner for Passivation of Stainless Steel
Choosing any sort of outsourcing partner should always be approached with great care, as the work will be judged by customers of the company selling the product. Any evaluation of a third-party vendor should look less at the price being charged and more at their reputation. This is especially true with such an important process as passivation, as understanding a potential provider’s expertise may make the difference between a product of substandard and exceptional quality. Here are some key areas to consider when seeking to ascertain the expertise of a potential third-party vendor for the passivation of stainless steel.
Documentation is important in processes used by multiple industries. It’s the same concept of “showing your work” that teachers demand from their students on mathematical tests, to ensure they know what they’re doing. Documenting which processes are performed and when shows a third-party vendor is serious about maintaining a high standard for their operations. Not only does documentation show that an outsourcing partner knows what it’s doing, but it also helps them comply with rules and regulations from the Food and Drug Administration (FDA), United States Department of Agriculture (USDA), Department of Defense (DOD), and other regulatory agencies. Every company must follow industry-wide rules to maintain compliance, whether they’re a cosmetics maker, food processor, medical device manufacturer, pharmaceutical company, or any other business.
The American Society for Testing and Materials (ASTM) provides and updates internationally accepted standards for working with various types of materials. As with any other process, the passivation of stainless steel requires metrics against which work can be weighed.
ASTM standards are used to regulate the following sectors and fields:
- Consumer products
- Medical devices and services
- Paint manufacturing
- Plastic Fabrication
- Textile making
ASTM standards for the passivation of stainless steel parts include A967, which includes both precautions and recommendations. The A380 expansion recommends temperatures when utilizing citric acid to passivate stainless steel, and it also includes ASTM B600 which is the specification for descaling and cleaning titanium and titanium alloy surfaces, commonly used in the medical industry when titanium is involved.
Doing the Process Correctly
It’s important that a third-party vendor engaged in the passivation of stainless steel conducts the process in the correct manner and does not take any shortcuts. Some less principled providers will skip parts of the cleaning process, for example, which will make the process less effective for the material being treated. Additionally, the various grades of steel require different durations for treatments and distinctive acid concentrations. Depending on the finish needed, these too must meet specific standards.
Citric Acid Instead of Nitric Acid
The best providers of passivation services will offer processing via citric acid as an alternative to nitric acid, which has been used to passivate iron and iron-based alloys since the 19th century. Using nitric acid for passivation of stainless steel does remove the free iron, yet if a nitric acid solution isn’t diluted sufficiently or prepared incorrectly it can damage base metals within the stainless steel alloy. Systems must be fully purged of nitric acid as well, otherwise, the free iron particles dissolved in it will reattach to surfaces, causing etching. Additionally, nitric acid is considered hazardous waste, so requires a special means of disposal after use.
In contrast, the passivation of stainless steel with citric acid offers a much safer and less toxic means for targeting free iron particles. It also allows the process to run at higher temperatures or longer periods without causing surface etching. Citric acid requires no special equipment or handling after use and can be dumped directly into existing drainage systems. This is due to the manner in which citric acid responds to free iron molecules, binding with them to render the molecules unreactive rather than simply dissolving them. This saves a step in the passivation process, as it doesn’t require the system to be regularly flushed out.
Using the Right Equipment
Passivation of stainless steel surfaces requires special equipment. For example, when passivating newly constructed equipment, pumps must provide sufficient velocity to flush debris from systems prior to commissioning. It additionally requires proper filtering and hoses to circulate the chemical solution through new stainless steel equipment. The passivation of stainless steel components additionally involves tanks and other equipment for washing, rinsing, soaking, and drying parts.
Passivation systems can include:
- Agitated immersion systems for stainless steel parts that can be coupled with other equipment, often used for medical, aerospace, and other precision components.
- Automated systems for components are controlled by a computer and can be programmed via a user interface, with processes like cleaning, rinsing, and drying taking place in a single container.
- Benchtop systems for low production runs or prototyping that allow all steps to be performed by a single piece of equipment.
- Wet bench systems that perform all steps with a single piece of equipment, but allow linkage of electrical and plumbing through a single location.
Solutions for Passivation of Stainless Steel
While no process can make stainless steel completely corrosion-proof, RP Abrasives offers services that get very close. We use filtered, deionized water in passivation lines to keep free iron particles from infiltrating stainless steel surfaces. Besides fastidious cleaning, our passivation of stainless steel surfaces includes ultrasonic capabilities that allow chemical solutions to reach even the tiniest of crannies within a material.
Services RP Abrasives offers capabilities for passivation of stainless steel that include:
- Cleaning components prior to passivation
- Copper sulfate testing for passivated components
- Large complex parts
- Small batch sizes
- Use of citric acid and deionized water in the passivation process
RP Abrasives is ISO 9001:2015 certified, offering superior passivation services for stainless steel and titanium. Our citric acid passivation techniques are environmentally friendly and gentler on stainless steel parts. To learn more about our capabilities for passivation of stainless steel, call us at (603) 335-2132 or contact us for a free quote!