One of the most frequently asked questions is with regard to how long will passivation lasts. However, there are too many variables and differences between systems for there to be just one correct answer.
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A better question, perhaps, would be to ask what is involved in the system's environment that may cause the passive oxide film to degrade. Robert Schuck, Senior Technical Sales Manager at Astro Pak, can provide an answer to this question. He proclaims that stainless steel will remain passivated until it is acted upon.
Several internal and external factors are responsible for damaging the microscopically thin passive layer that protects the surface of stainless steel. These can be broken down into five categories:
- Elevated Temperature
- Physical Damage
It is important to note that significant overlap between these categories is possible, such as the product causing physical damage, but these serve as a helpful base.
1. Elevated Temperature
The passive layer is largely self-sustaining at ambient temperature. Many manufacturing processes, however, require higher temperatures. The corrosion rate doubles every 10 degrees centigrade above ambient.
The result is that a system that operates hotter – with all other considerations being equal - will need to be re-passivated earlier than one that runs at ambient temperatures.
Steam systems are especially vulnerable due to the large amount of heat involved. In addition, any solids dissolved in the water can precipitate out and act as the starting point for corrosive activity.
Chlorine-based chemicals are used in everything from drinking water to cleaning products. The chemical properties that allow these chemicals to perform well in these applications are the same ones that will enable them to degrade the passive layer, exposing the underlying metal and corrosion.
Astro Pak technicians cite numerous examples where chlorine in the process fluid or the systems rinse water became the catalyst for deteriorating the passive film.
On multiple occasions, janitorial crews supplied by the owner have, unknowingly, damaged equipment by spraying chlorine bleach during routine cleaning.
This is perhaps the broadest category, given that it accounts for the manufacturing process that a system was designed for and the cleaning and other maintenance processes within the system.
As a result, there is a significant overlap with risks in other categories. As previously explained, increased heat and chlorine presence can damage the passive layer.
The passive layer will therefore suffer when cleaning routines involving the frequent use of chlorine chemicals are required. Any cleaning process that uses manual tools or entry within the system risks damage, as does any water left to stand within the system between production batches.
The most surprising example of a destructive process is a poorly designed Clean in Place (CIP) system where spray balls shoot water out at velocities high enough to erode the passive layer, and the metal itself becomes pitted as a result. The spray balls in a properly engineered system are designed with this in mind.
4. Physical damage
Physical damage to the passive layer and the underlying metal can occur in various ways. One example is the mechanical interaction of moving parts with the product and the vessel's surface (described below).
Repair work - welds, for example - that are not properly passivated act as a source of rouge on the surface, which can be a starting point for further corrosion and a contaminant to the product.
Effects can result from tools that have been dropped or damage caused during entry into the system – incidents that are frequently overlooked when they occur. Even if the system maintenance crew is careful and makes sure to use non-chlorine cleaners, the surface can still be damaged by using abrasives.
In some cases, the product can cause damage to the system that is being used to produce it. The physical state of the ingredients can also impact 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.
Several other factors can affect the lifespan of the passive layer. One of the more obvious ones is the frequency of use. A system that is rarely used and carefully maintained will have a much longer re-passivation interval than a system in constant use.
The surface finish also plays a role in the passive layer's success. Adhesion is specifically reduced between product and surface when that surface is smooth; therefore, less force is required to remove any residue.
This also decreases the likelihood of corrosive material staying 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 dramatically reduce 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 acceptable amount of impurities and the lowest acceptable amount of beneficial elements certainly costs less but, as a result, performs worse in service than an alloy of the average composition of the same grade.
Determining the Solution for a Specific System
There is no straightforward, universal rule for determining a generic passivation schedule. Simple steps such as avoiding chlorides and abrasives in cleaning are straightforward and can certainly reduce the risk of damage for little or no cost.
To answer the question regarding how long passivation takes, however, analysis of that particular system is required to see how the passive protective layer is affected by its operation.
Implementation of a regular testing regimen is crucial, as well as consulting with Astro Pak. Wiping for rouge, visual inspection, and other steps can identify corrosion risks before damage occurs.
Documenting these tests is key to spotting patterns. That data is then used to help form a science-based passivation regimen as an essential part of ongoing maintenance. By following the schedule, a system owner can gain certainties regarding operational budgeting and planning.
About Astro Pak Corporation
Astro Pak is the leader in providing Passivation, Precision Cleaning and High-Purity Chemical Cleaning services for a wide variety of “cleanliness sensitive” critical systems and components. Many of our customers have external agencies that drive their cleaning requirements, such as the FDA, NASA and others. We service such industries as Pharmaceutical, Biotechnology, Aerospace, Laser, Semiconductor, Water Treatment and more, including Industrial markets. Our services and products leverage decades of experience to deliver the most effective chemistries and techniques, resulting in increased equipment longevity, reduced corrosion-related downtime and regulatory compliance in client facilities.
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