In lay terms, the passivation process removes "free iron" contamination left behind on the surface of the stainless steel from machining and fabricating. These contaminants are potential corrosion sites that result in premature corrosion and ultimately result in deterioration of the component if not removed.
In addition, the passivation process facilitates the formation of a thin, transparent oxide film that protects the stainless steel from selective oxidation corrosion. So what is passivation?
Is it cleaning? Is it a protective coating? It is a combination of both. How is passivation performed? The process typically begins with a thorough cleaning cycle. It removes oils, greases, forming compounds, lubricants, coolants, cutting fluids and other undesirable organic and metallic residue left behind because of fabrication and machining processes. General degreasing and cleaning can be accomplished many ways, including vapor degreasing, solvent cleaning and alkaline soaking.
Imagineering Enterprises, Inc. The company, founded in , is a QS company. Its clientele includes the automotive, aerospace, medical, agricultural and computer industries. Processes offered include electroless nickel plating, phosphate coatings, passivation, bonded solid film lubricants, specialty paints and chemical conversion coatings. Imagineering also provides customer-specific quality-assurance tests, non-destructive test methods, salt spray and humidity tests.
After removing organic and metallic residues, the parts are placed into the appropriate passivation solution. Although there are many variations of passivating solutions, the overwhelming choice is still the nitric-acid-based solutions. Recently, there has been substantial research performed to develop alternative processes and solutions that are more environmentally friendly, yet equally effective.
Although alternative solutions containing citric acid and other types of proprietary chemistry are available, they have not been as widely accepted commercially as nitric-acid-based solutions. The three major variables that must be considered and controlled for the passivation process selection are time, temperature and concentration. Typical immersion times are between 20 min and two hours.
Typical bath temperatures range between room temperature and F. Agitated Immersion Passivation Systems. In the aerospace and medical device industries, many high-precision manufacturers face additional guidelines, specifications, regulations and accreditation standards when passivating their products.
Use of an automated passivation system ensures tight, documented process control parameters to meet validation requirements. Within the medical device world there is a need to validate the passivation process. But what does that mean, and how does that work? Validation is the process of insuring that the passivation process you use will reproduce repeatable and predictable results every time a batch of parts is run through the process.
By validating the process you are able to forego subjecting every part to testing to prove that it is properly passivated. The IQ or Installation Qualification is the first part. It is developed by describing the machine — what is it? It also looks at what the components on the machine are, gauges, switches, PLC, etc. It provides a description of the machine and its parts — what is it and how does it work?
The OQ or Operational Qualification is the second part. It essentially help you verify the IQ — does the machine operate as it is supposed to? Do the components do what they are designed to do? The PQ or Process Qualification is the third part of the passivation test. You create a DOE Design of Experiments that tests the equipment at the top end and bottom end of allowable ranges and run parts to verify the results across the entire range of the variables.
That is the goal of the passivation validation process. Process control and stability are critical aspects to regulated medical device and aerospace processes. It is important to ensure that a process has input and output variable limits which are defined and fully tested during process design, Equipment Qualification IQ , Operational Qualification OQ and Process Qualification PQ validation testing. Setting up a proper DOE Design of experiments to test these limits is also important as it result of the DOE will give statistical confidence intervals of the limits.
Being that operators and employees perform various process operations different no matter how instructed in work instructions, the variation of operators must also be captured during process qualification PQ validation.
For example, in our automated passivation system , the elimination of relying on an operator to move the parts basket from stage to stage ensures that the parts remain in the appropriate wash, rinse, acid passivation, etc solutions for the process defined times and in accordance with the proper ASTM A, AMS, etc specification.
If a parts basket is immersed in the acid passivation solution too short or long duration, the passivation can likely fail and be outside specification limits. Inquire today for more information on how your company can benefit from our passivation equipment and process design. For one medical device manufacturer, the design of a citric passivation solution faced two major challenges: The manufacturing process required performing a water-break test after cleaning parts, before proceeding to passivation.
This meant that they could not use a fully automated passivation solution. After thorough cleaning, the stainless steel part is ready for immersion in a passivating acid bath. Any one of three approaches can be used—nitric acid passivation, nitric acid with sodium dichromate passivation and citric acid passivation.
Which approach to use depends on the grade of stainless steel and prescribed acceptance criteria. More resistant chromium-nickel grades can be passivated in a 20 percent-by-volume nitric acid bath Figure 2. As indicated in the same table, less resistant stainless grades can be passivated by adding sodium dichromate to the nitric acid bath to make the solution more oxidizing and capable of forming a passive film on the surface.
Another option, used in place of nitric acid plus sodium dichromate, is to increase the concentration of nitric acid to 50 percent-by-volume. The sodium dichromate addition and the higher nitric acid concentration both reduce the chance of undesirable flash attack.
The procedure for passivating free-machining stainless steels also shown in Figure 2 is somewhat different from that used with the non-free-machining stainless grades. That is because the sulfides of sulfur-containing free-machining grades are partially or totally removed during passivation in a typical nitric acid bath, creating microscopic discontinuities in the surface of the machined part.
Even normally efficient water rinses can leave residual acid trapped in these discontinuities after passivation. This acid can then attack the surface of the part unless it is neutralized or removed.
To effectively passivate the free-machining stainless steels, Carpenter has developed the A-A-A alkaline-acid-alkaline process that neutralizes trapped acid. This method of passivation can be accomplished in less than 2 hours. Here is the step-by-step procedure:. Then rinse the part thoroughly in water. After removing the part from this bath, flush it with water, then immerse it in the sodium hydroxide solution for another 30 minutes.
Rinse the part again with water and dry it, completing the A-A-A method. The benefits of this method are shown in Figure 3. Citric acid passivation has become increasingly popular with manufacturers who want to avoid the use of mineral acids or solutions containing sodium dichromate, along with the disposal problems and greater safety concerns associated with their use.
Citric acid is considered environmentally friendly in every respect. Although citric acid passivation offers attractive environmental advantages, shops having success with mineral acid passivation and suffering no safety issues might want to stay the course. There may be no real need to change if those users have a clean shop, well-maintained and clean equipment, coolant free of iron-containing shop dirt, and a process that yields good results.
Passivation treatment in citric acid baths has been found useful for a large number of stainless steel families, including several individual stainless grades, as summarized in Figure 4. The conventional nitric acid passivation methods from Figure 2 are included for convenience. Note that the older nitric acid formulations are in volume percent, while newer citric acid concentrations are in weight percent.
The passivation treatment varies depending on chrome content and machinability characteristics of the grades in each family. Note the columns referring to Process 1 or Process 2. As shown in Figure 5, Process 1 involves fewer steps than Process 2. Factors causing this attack included excessive bath temperature, excessive immersion time and bath contamination. The ultimate choice of passivation method will depend on the acceptance criteria imposed by your customer.
Tests are often performed to evaluate the surface of passivated parts. It is important that the test method be matched to the grade under evaluation. In both home and commercial settings exposure to things like cleaning solutions, bleach or salt oceanic environments all will contribute to the need for regular passivation of the stainless steel.
Historically nitric acid has been used to passivate stainless steel, but recently a safer and more effective means using citric acid has been introduced. We keep the product in stock for fast delivery to your location. Citrisurf Stainless Steel Passivation Products.
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