Passivation Equipment & Ultrasonic Passivation Systems for Stainless Steel with Nitric / Citric Acid

Have you ever noticed that many day-to-day things seem to get divided into two opposing camps?  Things like asking what’s the best computer quickly become the PC versus Mac debate.  Phones? iPhone versus Android.  So, passivation?  Yup, nitric versus citric.

There are real advantages to each nitric and citric passivation, but that is a topic for other FAQ’s because once you pick one you still have several choices to make.  Choices like: which standard do I use? which method do I choose?

There are two primary standards that address passivation. The SAE Aerospace “Passivation of Corrosion Resistant Steels” (AMS2700) and the ASTM “Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts” (A 967).

Each allows for citric or nitric and each allows for several options within each citric and nitric.  So what’s the difference? Each standard is copyrighted (and protected) and has to be purchased from the publishing agency, so getting into a lot of detail is not allowed. But here’s a short version of the differences in the various methods:

Concentration of the specific acid – each method varies based on how concentrated the citric or nitric acid is – and in the case of nitric if the nitric acid is used alone or with sodium dichromate.

Temperature of the bath – again, each method specifies a temperature range that must be maintained for the duration of the passivation bath.

Time – the length of time a part must remain within the passivation solution is spelled out as well.

What seems complicated becomes really simple.  The standards spell out the concentration, the temperature and the time.   Each method simply varies one or more of those three items.

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.

Typically you will hear the validation process broken down into three distinct parts: the IQ, the OQ and the PQ. Let’s look at each part.

The IQ or Installation Qualification is the first part.  It is developed by describing the machine – what is it? what does it do? etc. 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? etc. – does everything work as intended?

The PQ or Process Qualification is the third part of the passivation test. If the IQ is the theory of how things SHOULD operate and the OQ is the practice of how things DO operate, then the PQ defines how CONSISTENTLY the machine operates.  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. Now when the machine operates within the specified range of time, temperature and/or acidic concentration you know that your parts will meet the specs. 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 the results 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.  An automated system typically eliminates many of the operator variability in the manufacturing process and this process “input” elimination also allows for tighter process output controls.

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 A967, AMS2700, 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.

Passivation is a non-electrolytic process typically using nitric or citric acid which removes free iron from a stainless steel surface. This forms an inert, protective oxide layer that in turn renders the stainless steel more rust-resistant due to lack of iron to react with atmosphere.

During machining manufacturing processes, stainless steel parts may have imperfections from iron being embedded or smeared onto the surface from the machining tool steels. These free irons on the surface of the stainless steel need to be removed to prevent a corrosive reaction can occur between the two different metals.

The passivation process chemically removes these free irons and forms a passive oxide “film” layer which further improves corrosion resistance. When exposed to air, the stainless steel undergoing passivation will form a chemically inactive or inert surface. This is one advantage of using citric acid or nitric acid for passivation.

For more detailed information, please see our “What is Passivation?” page.

When parts are passivated with citric or nitric acid, the parts themselves, as well as the basket they are in, carry some of the acid  into the rinse tank. This “drag out” means that the rinse solution has to be constantly replaced or will simply become more and more acidic over time. The biggest issue is not that the parts will be rinsed off, but that when the parts are withdrawn from the rinse tank, they may have soil redeposited on them. Once the parts are dried this soil can cause spotting on the surface of the otherwise clean parts. A second rinse bath produces a much cleaner final product by rinsing off the soil that is redeposited during the first rinse.

Often times, the second rinse tank includes a heated facility water inlet which constantly overflows the second rinse tank with small amounts of water to ensure water cleanliness.  The second rinse tank overflow is sent to rinse tank 1 and then rinse tank 1 overflows to drain.  This cascade overflow process ensures constant water quality over time no matter the amount of drag out on the parts and baskets.

Both nitric acid and citric acid are effective in passivating many grades of stainless steel, and both methods are described in the industry standards ASTM A967 and AMS 2700.

Citric acid passivation offers the following key differences. Compared to nitric acid passivation, citric acid passivation is:

• Newer
• More environmentally friendly
• Safer.

In the past, some manufacturers avoided citric acid due to potential organic growth and molding issues. Today, modern formulations for biocides prevent organic growth in the solution. See our complete write up on the advancements of citric acid (specifically Citrisurf).

These advancements in citric acid have allowed smaller manufacturers that lacked experience with chemical handling and processing to bring their passivation in-house. For a more detailed comparison, please see our “Nitric vs. Citric Acid Passivation” page.

As a non-electrolytic process, passivation uses solutions like citric and nitric acids instead of an electrical current used in electropolishing to create a inert oxide layer / film and remove free iron and foreign matter from metal surfaces.

Electropolishing, a non-mechanical interactive process, can be used on objects with complex geometries.  The electropolishing  process uses a combination of electrolytic chemicals and an electrical current to carefully eliminate imperfections and contaminants of metal part surfaces. This particular process is an alternative to abrasive fine polishing.

Electropolishing can be used to polish, deburr, and smooth metal components whereby doing so also causes material removal to occur including the free irons on the surface of the part. Electropolishing would be the more appropriate approach when  aesthetics are necessary.  A smoother, polished surface can result from electropolishing.

Passivation, on the other hand, does not change or brighten the surface appearance, and is not an effective method for improving surfaces that have been welded with oxide heat effect scale or heat treated.