One of the most commonly asked questions heard by Astro Pak team members is “how long will passivation last?”. The truth is that there are so many variables as well as differences between systems that there is no “right” answer.
Instead, a more accurate question would be to ask “what is involved in the system’s environment that may degrade the passive oxide film?” In the words of Astro Pak Sr. Technical Sales Manager Robert Schuck, “stainless steel will remain passivated until it is acted upon.” There are a number of internal and external factors that damage the microscopically thin passive layer that protects the surface of stainless steel. Broadly, these can be broken down into five categories:
- Elevated Temperature
- Physical Damage
It should be noted that there can be significant overlap between these categories, such as the product being produced causing physical damage (more on that later), but these serve as a useful basis.
Chlorine-based chemicals are used in everything from cleaning supplies to drinking water. The same chemical properties that make these chemicals so useful are the same ones which allow them to degrade the passive layer, exposing the underlying metal and corrosion. Astro Pak technicians can cite numerous examples where chlorine, present in the process fluid itself or in the system’s rinse water, became the catalyst for deteriorating the passive film. And, on more than one occasion, owner supplied janitorial crews have inadvertently damaged equipment by spraying chlorine bleach during routine cleaning.
2. Elevated Temperature
The passive layer is largely self-sustaining at ambient temperature. However, many manufacturing processes require higher temperatures. For every 10-degrees centigrade above ambient, the corrosion rate doubles. As a result, a system that operates hotter, will need to be re-passivated sooner than one that runs at ambient temperatures with all other considerations being equal. Steam systems are particularly vulnerable due to the large amount of heat involved. Additionally, any dissolved solids in the water can precipitate out and serve as the starting point for corrosive activity.
This is perhaps the broadest category as it covers not only the manufacturing process a system was designed for, but also the cleaning and other maintenance processes that take place within the system. As a result, there is a lot of overlap with risks appearing in other categories. As mentioned before, the presence of chlorine can damage the passive layer, as does increased heat. Similarly, if the cleaning routine regularly involves chlorine chemicals, the passive layer will suffer. Any cleaning process that requires the use of manual tools or entry within the system introduces risk of damage as does any water left to stand within the system between production batches.
Perhaps the most surprising example of a process being destructive is an improperly designed Clean in Place (CIP) system where spray balls shoot out water at such high velocities that the passive layer is literally eroded away and the metal itself becomes pitted. In a properly engineered system, the spray balls are designed to avoid this.
It is ironic that in some cases the product being made can be damaging to the system making it. The physical state of the ingredients can also factor into the longevity of the passive layer.
In an example of how interconnected process and product can be in determining re-passivation, As the ingredients were mechanically mixed, the beads scoured the surface of the stainless steel tank, scratching away the passive layer and causing premature corrosion.
5. Physical Damage
There are many possible sources of physical damage to the passive layer and the underlying metal. The mechanical interaction of moving parts with product and the vessel’s surface as described above is just one example.
Repair work, such as welds, which are not properly passivated serve as a source of rouge on the surface which can not only be a starting point for further corrosion, but as a contaminant to the product. Dropped tools, or damage caused during entry into the system are often overlooked at the time they occur, only for the effects to appear later. Even if the system maintenance crew is careful to use non-chlorine cleaners, they can still cause harm to the surface by using abrasives.
There are a number of other factors which will affect the lifespan of the passive layer. Frequency of use is one of the more obvious ones as a system that is rarely used, and carefully maintained, will have a re-passivation interval far longer than a system being used 24/7.
The surface finish also plays a role in the success of the passive layer. Product does not adhere as easily to smoother surfaces, and as a result, less force is required to remove any residue. This also reduces the chance of corrosive material remaining in contact with the stainless steel surface. A well electropolished surface typically allows longer time intervals between re-passivation.
Perhaps the most insidious factor that can severely lessen the longevity of passivation is the material used to make the vessel or system. Each stainless steel grade has a range for each element in its composition. Material with the highest permissible amount of impurities and lowest permissible amount of beneficial elements certainly costs less, but performs worse in service than an alloy of the average composition of the same grade.
Finding the Answer for Your System
As can be seen, there is no “hard and fast” rule to determine a generic passivation schedule. Certainly, simple steps such as avoiding the use of chlorides and abrasives in cleaning are easily done and reduce the risk of damage for little or no cost. But, in order to answer the “How long…” question, it requires an analysis of that particular system to see how its operation affects the protective passive layer.
In addition to consulting with Astro Pak, a regular testing regimen must be implemented. Visual inspections, wiping for rouge, and other steps can catch corrosion risks before damage occurs. Documenting these tests is essential to identifying patterns. That data is used to help form a science-based passivation regimen as a critical part of ongoing maintenance. And, by following the schedule, a system owner can gain a great deal of certainty both in terms of operational planning and budgeting.