This article presents current research on the problem of rouge in clean steam generators and their distribution systems, as well as possible deleterious effects on capital equipment and final drug products.
By: Drew C. Coleman and Daryl L. Roll
Figures 26–29 show how a shiny black surface appears microscopically. The surface is much smoother than typical rouge crystals due to the amorphous silica that appears like a glassy coating. Once removed, however, the surface reveals its low-level pitting and austenitic metallic crystal edge deformation.
Figure 30 reveals establishment of the passive film after iron oxide deposits were removed. The Cr:Fe ratio at the surface is slightly greater than 1:1 in the first 20 Å, as the Fe (blue line) and Cr (red line) merge toward the surface. The O level (lime green line) starts high at the surface (at 40%) and drops to zero at 120 Å, while the Ni level (dark green line) begins at 7%, rises quickly to nearly 15%, then then levels off at about 10% into the alloy composition below 150 Å.
CS systems can be monitored for metals in the condensate and steam flow, measuring the number and size of particles from 5 to > 100 µm. The results presented in Table B show ranges of metal content and particulate in the CS critical utility of three case studies. Particle sizes above 50 µm are visible contaminants,3 and significant numbers of particles greater than 50–100 µm present a high risk for contamination on surfaces that are steamed by this critical utility.
Ferrous oxide rouge deposits may be removed using organic acids with chelant combinations (and other variable complexes) in the proper concentrations, contact times, and temperatures. Other advocated mineral acid treatment approaches include, but are not limited to:
The objective of the derouging process is to remove the iron oxide deposits while protecting the stainless steel substrate surface from any additional pitting corrosion. To ensure that polished surface finishes are not damaged by the derouging solutions, it’s also important to avoid aggressive techniques that can remove base metal. Following the rouge and oxide deposit removal, a passivation treatment can restore the passive film by removing elemental iron and iron oxides from the first few molecular layers in the surface while maintaining the protective chromium oxide layer. This can minimize continued corrosive mechanisms upon return to service.
The corrosion byproducts encountered in clean and pure steam systems—carbon, silica, and iron oxide compounds—are present to some degree in every system. Many CS systems lack proper routine inspection and maintenance that could control corrosion and particulate migration from oxide deposit exfoliation. 2 problems are exacerbated by poor gasket specifications, components with dissimilar metals, and decreased stainless steel surface quality, as well as the uncontrolled nature of mechanical/electrochemical polishing materials and methods combined with poor material handling and lack of routine derouging and passivation techniques.7
It has also long been suspected that stainless steel materials are not necessarily delivered at a desirable quality level. Manufacturing processes, combined with subsequent material handling unit operations and fabrication techniques, establish the surface chemistry (chromium oxide content of the passive film), corrosion resistance, and surface finish quality, which all affect the final product.
Claims that these grayish/black deposits are stable, inevitable, and should be left alone have very little credibility. Corrosion produces rouge that is evidenced as discolored stains on product contact surfaces, and generates mobile particles that accumulate on steam sterilized surfaces. CS rouge contaminants have been found in final filtration processes, becoming a potentially uncontrolled material in the final process fluids and gasses. While we acknowledge that the examples presented here are specific cases, they are not unique. They are similar to cases found within other systems, especially those where corrosion has been left to continue without proper corrective treatment.4
Finally, corrosion within CS systems will generate migratory rouge that can be identified and measured in the steam, the condensate, and on the system interior surfaces. Proper design and maintenance is critical in the operation of CS generation and distribution systems for high-purity applications.
Future research measuring the time, conditions, and properties of rouge development could be compared to particulate generation to establish risk of product contamination. Systematic, routine measurements over a two-year period could track corrosion products to illustrate changes in particulate release and transport within the CS system studied, as well as the surface conditions within the generation and distribution equipment.
The authors would like to give credit (posthumously) to Robert W. Evans for his efforts in joining Drew Coleman during initial research and drafting phase of this article. He envisioned the need to bring to the industry the effects of corrosion and subsequent rouge products in CS systems with their potential for detrimental process or product contamination. In addition, we would like to thank Dr. Brent Ekstrand for his editing efforts in finalizing this technical article to more clearly bring our message to the industry.