The cleanliness of a system’s stainless steel surface is essential; however, its smoothness is equally important, as they are correlated.
A rough or uneven surface can trap foreign objects. Items stuck on a surface can serve as a shelter for bacterial colonies, eventually contaminating the product. Even when no such colonization happens, the material can break loose on its own and move further down the system rendering the product unsuitable for use or causing further damage.
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For this reason, product contact stainless steel surfaces employed in processing or manufacturing are created to reduce the surface area where products or objects can become stuck. On a macro scale, this can be observed in different environments and systems, for example, in the development of food preparation regions. The seams are smoothed, tubing ends are sealed, and corners are curved for easy cleaning, etc. However, the significance of a smooth surface is more challenging to recognize or see in the interior of machinery.
Smoothness is Important
Although the example of food preparation focuses primarily on materials that are observable to the naked eye, the difficulty of smoothness exists on the microscopic level. On the surface, a small scratch not only functions as a haven for bacteria to grow, but the physical damage to the protective passive layer of steel can enable corrosion that can ultimately enter deeper into the metal.
The complications within the system and the fact that surfaces in contact with the product are concealed from view means that the risks are increased exponentially inside a food processing or pharmaceutical system. Materials should never be able to catch and accumulate, whether within holding vessels, the piping systems, or inside the processing tanks themselves.
Trouble Right from the Start
The very process of building a system can present risks to products later down the line. A gouge a few microns deep left by a mechanical polishing wheel that was employed to smooth out a weld can serve as a prime environment for an equally microscopic organism.
Polishing grit may also become embedded inside the metal. Such risks may seem tiny, but they can rapidly flourish into serious concerns if not immediately addressed.
Mechanical damage that enters via the passive layer of chromium oxide reveals the iron-rich metal underneath. The passive layer has a thickness of just a few angstroms (0.0001 μ), which makes it easy to compromise.
After being exposed, the free ions of iron in the metal will interact with atmospheric moisture to produce iron oxide or rust. This risk is not restricted to the damage site.
Until iron particles come to rest in a position detached from their origin point, they can travel further downstream. The free iron becomes rouge when it rests on a surface and accumulates.
On the surface of the metal, the rouge forms a rough area. In the instance of a gouge, it becomes a point where other foreign objects can accumulate. The iron crystals will grow over time, expanding the rouge and enabling the buildup of foreign debris.
The crystals could break or be separated and move downstream, possibly developing further rouge or polluting the product.
Biological Threat
There is also an additional risk of biological colonization. A microscopic bacteria or any other biological component could rest in an area of surface damage and colonize rouge areas.
The threat to the product is not only from the iron component but also from colonies shedding patches in manufacturing, negating the hygienic protections for the feed water and constituents such as media and buffers.
These colonies can build a protective film upon themselves, making them exceedingly resistant to the regular chemicals employed to clean the system between batches. Consequently, a manufacturer could lose numerous production runs worth of product before the source of the problem is found.
Chemical Solution
Mechanical polishing eradicates metal to leave a smooth surface on a macroscopic level. Electrochemical polishing should follow at a microscopic level, removing debris and metal from the surface.
This is performed by carefully using chemistry and electricity to run a charge via the stainless steel, releasing a dense layer of gaseous oxygen over the surface. The electrical current will flow from the elevated peaks more than the valleys, leaving a smooth and even surface.
After being put into service, if chemical or mechanical damage occurs, electro- and mechanical polishing of the damaged area can be done to return the surface to its needed smoothness, quantified in terms of Roughness Average (Ra).
The process is more involved in circumstances where rouge is found — particularly where bioburden is available. Firstly, the organic colonies must be cleaned using the right processes and chemistries. The cleaning solution is pumped via the system after rinsing with high-purity water. In most cases, several cycles of this method are required.
The rouge is treated and removed after the bioburden has been cleared, using a different set of chemicals, loosening it from the surface and flushing it out of the system.
Electropolishing is done after this process. Early addressing with mechanical polishing would be needed if there are gouges, pits, or scratches.
Finally, a passivation treatment is carried out after the restoration of surface smoothness, but there is still the matter of the damaged passive layer of stainless steel. The presence of the rouge compromises this layer, and it can be weakened further by removing the iron crystals.
The electropolishing process partially restores the chromium oxide layer to some extent, but not to the higher levels of an appropriate passivation treatment. The physical smoothness of steel is not altered by the passivation, which instead removes free iron atoms from the surface of the alloy so that its corrosion resistance is enhanced.
The Bottom Line
It is essential to ensure that the internal stainless steel surfaces of a processing system meet the Ra requirement of the users to increase its competencies while mitigating contamination threats to the product. Consistent testing for the availability of rouge can identify the issue before it can develop into a biological contamination threat. An extended timespan of seamless operation can be assured with proactive measures.
This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.
For more information on this source, please visit Astro Pak Corporation.