The importance of the cleanliness of a system’s stainless steel surfaces is paramount, but it may come as a surprise to most that the smoothness of those surfaces is just as important. It only takes a few moments of thought to realize that a smooth surface is directly related to the cleanliness of the system. A rough or uneven surface can trap foreign objects. Items which become lodged in a surface can serve as a haven for bacterial colonies which, in turn can contaminate the product. Even if no such colonization occurs, the material itself can break loose and travel further down the system to cause further damage or render the product unusable.
For this reason, product contact stainless steel surfaces used in manufacturing or processing are designed to minimize surface area where objects or product can become trapped. This is easy to see on a macro scale in various systems and environments, such as in the construction of food preparation areas. The ends of tubing are sealed, seams are smoothed, corners are rounded for ease of cleaning and so on. But, inside of machinery, it is far more difficult to see – or appreciate – the importance of a smooth surface.
Smoothness is important
While the food preparation example above is mostly concerned with details visible to the naked eye, the challenge of smoothness extends on down to the microscopic level. A small scratch in the surface can not only serve as a place for bacteria to flourish, the physical damage to the steel’s protective passive layer can allow that point of impact to be the starting point for corrosion that can eventually penetrate deeper into the metal.
Within a pharmaceutical or food processing system, the risks are increased exponentially because of the complexity of the system itself as well as the reality that the surfaces in contact with the product are hidden from view. Whether it’s the inside of holding vessels, or the piping systems through which the product or water flows, or within the processing tanks themselves, there must not be a location where material can catch and accumulate.
Trouble right from the start
The very process of building a system can present risks to product down the road. A gouge a few microns deep that was left by a mechanical polishing wheel smoothing out a weld can serve as a toehold for an equally microscopic organism. Further, polishing grit might become embedded within the metal itself. While these threats may appear miniscule, they can quickly grow to become serious issues if not pre-emptively addressed.
At the very least, mechanical damage that penetrates through the passive layer of chromium oxide exposes the underlying iron-rich metal below it. That passive layer is only a few angstroms (0.0001µ) thick, so it doesn’t take much to compromise it. Once exposed, the free iron ions in the metal will interact with any moisture in the atmosphere, if not within the product, to form iron oxide, commonly known as rust. The threat is not just limited to the site of the damage. Iron oxide particles can be carried further down the stream until they come to rest in a location far removed from their point of origin. When this free iron comes to rest upon the surface and accumulates, it becomes rouge.
The rouge itself creates a rough area on the metal’s surface. And, like the gouge described in the example above, it becomes a location where other foreign objects can accumulate. The iron crystals will grow over time, expanding the patch of rouge, which facilitates the accumulation of more foreign debris and continuing the cycle. The crystals can break or become detached and travel even further downstream, potentially forming yet another patch of rouge, if they don’t contaminate the product itself.
As if the threat of iron oxide spreading through a system and creating contamination sources was not enough, there is the added threat of biological colonization as well. As mentioned above, a microscopic bacteria or other biological element can find safe harbor in a location of surface damage, but areas of rouge can be colonized as well. The risk to the product is not just from the iron component, but from colonies shedding off patches during the manufacturing process, completely negating the sanitary precautions put in place for the feed water and ingredients like buffers and media.
If left unchecked, these colonies can form a protective film over themselves which makes them extremely resistant to the normal chemicals used to clean the system between batches. As a result, a manufacturer can lose several production runs worth of product before the source of the problem is identified.
Mechanical polishing removes metal on a macroscopic level to leave what appears to be a smooth surface. It should be followed up with an electrochemical polishing which removes metal and debris from the surface on a microscopic level. This is done using a careful application of chemistry and electricity to run a charge through the stainless steel, releasing a dense layer of gaseous oxygen just above the surface. The electrical current will flow from the raised peaks more so than the valleys, leaving an even and smooth surface.
Even after being put into service, if mechanical or chemical damage occurs or is suspected, mechanical and electropolishing of the affected area can be performed to return the surface to its required smoothness, measured in terms of Roughness Average (Ra).
In cases where rouge is detected, and especially where bioburden is present, the process becomes more involved. First, the organic colonies must be cleaned from the system using appropriate chemistries and processes. Cleaning solution is pumped through the system followed by a high purity water rinse. In most instances, multiple cycles of this process are required. Once the bioburden has been removed, the rouge is then treated and removed using a different set of chemicals which not only loosens it from the surface but flushes it out of the system. Only after that has taken place can electropolishing be performed. If there are scratches, pits or gouges, they would need to be addressed with mechanical polishing beforehand.
Finally, once the surface smoothness has been restored, a passivation treatment is performed. There is still the matter of the stainless steel’s damaged passive layer. This layer can be compromised by the presence of the rouge and the removal of the iron crystals can weaken it further. The electropolishing process does partially restore the chromium oxide layer to some degree, but not to the superior levels obtained with a proper passivation treatment. To be clear, passivation does not alter the steel’s physical smoothness, but rather removes free iron atoms from the alloy’s surface so that its corrosion resistance is greatly enhanced.
The bottom line
Ensuring that a processing system’s internal stainless steel surfaces meet the users Ra requirement is essential to maximizing its efficiencies while mitigating contamination risk to the product. Regular testing for the presence of rouge can allow the problem to be addressed before it can potentially expand into a risk of biological contamination. Proactive measures can ensure extended periods of smooth operation.