It is a common misperception that stainless steel is corrosion-proof. The reality is that stainless steel is highly corrosion-resistant. This means that its passive layer – the surface of the stainless steel which protects the rest of the metal – must be regularly maintained and strengthened to ensure many years of reliable service. While we have already written about the importance of developing a data-based schedule of regular, preventative maintenance, it is also important to note that selecting the right passivation provider is just as critical.
This is more than simply “you get what you pay for” because using the wrong provider can result in greater expense than what would be outlaid hiring a firm with an established track record in serving the specific type of equipment in need of passivation.
Here are some critical considerations that need to be taken into account:
The axiom “no job is complete without the proper documentation” holds true for many industries, as well as the agencies overseeing them. The higher the standards needed for operation, the more critical it is to document that the procedure has been performed to meet those standards. Many food, beverage, cosmetics, medical device manufacturers and bio-pharma companies work closely with the FDA, USDA and agencies overseeing animal health to develop procedures to ensure compliance with Current Good Manufacturing Practice (CGMP) regulations. Each step must be recorded and signed off as it occurs. If a facility isn’t able to “show its work”, the likely outcome is that the process must be performed again.
Any process is meaningless without having some metric to measure the work against. ASTM standards A967 and its A380 expansion to include the use of citric acid at specific temperatures are the two standards by which direct how stainless steel shall be passivated to avoid corrosion and contamination.
Doing the Right Process:
Passivating stainless steel is not a “one size fits all” process. Many providers will skip the cleaning process or take shortcuts. If the surface isn’t clean or if rouge remains, the passivation won’t be successful. Additionally, different grades and finishes of steel require different concentrations of acids and duration of treatment in order to meet the specific standards required by regulators.
Using Citric Acid Instead of Nitric Acid:
Many providers still use nitric acid rather than taking advantage of the benefits of citric acid. Nitric acid is extremely hazardous and is non-selective, meaning that it will attack the base metal as well as the free iron. Nitric acid must be fully purged from the system otherwise the free iron it has dissolved will plate back onto the surface where the problem begins again. And, after it has been used, nitric acid will need to be disposed of as a hazardous . By contrast, citric acid is much safer, less toxic, targets the free ironand will not etch the surface even if it is run at longer intervals or at higher temperatures. It can be disposed of using existing drainage systems without the need for special handling or equipment. Unlike nitric acid, citric acid does not simply dissolve the free iron, it chelates or binds with it and renders it chemically non-reactive. As a result, any free iron that somehow doesn’t get flushed out with the chemistry cannot start the corrosive process anew in a different part of the system.
Using the Right Equipment:
Passivation is more than just “plug and play” with off-the shelf equipment. In the case of new construction, for example, using pumps with insufficient velocity will not be able to flush the debris before commissioning, setting up for rejected production batches almost from the start. Astro Pak crews bring their own custom Clean in Place (CIP) skids which contain the variable-speed suitable for the job, along with the necessary hoses and filters enabling them to hook up to any system to ensure that the chemistry is circulated properly.
While it can be tempting to go with the lowest bidder, the total cost of doing so can be much higher. Insufficient passivation can lead to early failure, improper paperwork can result in a shutdown, cutting corners during cleaning can lead to the costly loss of ruined batches – all of which directly hits the bottom line. Short term gain can lead to long term pain.