Corrosion investigations and pharmaceutical clean steam systems

2022-12-13 02:32:12 By : Mr. Kevin Yang

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Pharmaceutical clean or pure steam systems comprise a generator, control valves, distribution tubing or piping, thermodynamic or balanced pressure thermostatic traps, pressure gauges, pressure-reducing regulators, pressure-relief valves, and volumetric totalizers. Ferrous Sulphate In Pregnancy

Corrosion investigations and pharmaceutical clean steam systems

The majority of these components are constructed from 316 L stainless steel and contain fluoropolymer gaskets (typically polytetrafluoroethylene, also called Teflon or PTFE), as well as semi-metallic or other elastomeric materials.

These components are prone to corrosion or degradation when in service, which can compromise the quality of the finished clean steam (CS) utility product. The project that is detailed in this article evaluated stainless steel coupon samples from four CS system case studies, carrying out a risk assessment of possible corrosion effects on the process as well as critical utility systems and testing condensate for particles and metals.

To examine the corrosion byproducts, arranging sample coupons of corroded tubing and distribution system components was necessary.9 Each case study assessed various surface conditions. For instance, standard rouge products and corrosion effects were assessed.

The sample surfaces referenced were assessed for rouge deposits by visual inspection, Auger Electron Spectroscopy (AES), Electron Spectroscopy for Chemical Analysis (ESCA), Scanning Electron Microscopy (SEM), and X-Ray Photoelectron Spectroscopy (XPS).

These techniques can reveal the physical and atomic properties of the corrosion and deposits and determine what contributes to the critical utility fluid properties or final product.1

Stainless steel corrosion products come in various forms, such as a ferric oxide rouge layer (brown or red) on the surface under or overlying the ferrous oxide layer (black or grey).2 The rouge layer is crystalline in structure with dynamic potential or the capacity to migrate downstream.

Over time, the ferrous oxide (black rouge) layer may thicken as the deposit becomes more marked; its migratory presence is confirmed by particles or deposits seen on sterilizer chamber surfaces and equipment or vessels after steam sterilization. Laboratory analyses of condensate samples demonstrate the particulate nature of the rouge as well as the number of soluble metals in the CS fluid.4

While there are several causes of such phenomena, the CS generator is generally the main contributor. It is not uncommon to detect ferric oxides of rouge (brown/red) on the surface, with ferrous oxides (black/gray) at the steam discharge, with both migrating slowly throughout the entire CS distribution system.6

The CS distribution system is a branching configuration with several use points, terminating at distant areas or ends of the main header and various branching subheaders. The system can incorporate a sequence of regulators to help initiate a pressure/temperature decrease at specific use points; these could be potential corrosion sites.

Corrosion may additionally occur in hygienically designed traps positioned at various points throughout the system to eliminate condensate and air from the mobile clean steam, past the traps, downstream piping/tubing to drains, or in condensate collectors.

In most cases, reverse migration may be present, with rouge deposits collecting over the traps and growing upstream into adjacent use point piping or sub-headers and beyond; the rouge that forms in traps or other components may be seen upstream from the source while continually migrating both down and upstream.

Rouge can be found in all forms in steam systems, including:

Specific stainless steel components also display moderate to high levels of a disparate metallurgical structure, including delta ferrite. It is thought that ferrite crystal reduces corrosion resistance, even though its content may only be 1%–5%. 

Ferrite is also not as corrosion resistant as an austenitic crystal structure, so it will corrode preferentially. Ferrite can be accurately detected with a ferrite meter or semi-accurately with a magnet and with significant limitations. 

From system setup, when first commissioning and energizing a new CS generator and distribution tubing, numerous factors for corrosion potentiality are present: 

As a function of time, corrosion factors such as these may generate corrosion products as they meet, combine, and overlap with a mixture of ferrous and ferric rouge. Black rouge is typically first observed in the generator; rouge then appears at the generator discharge piping and ultimately throughout the CS distribution system. 

SEM analysis was performed to highlight the microstructure of the corrosion byproducts coating the entire surface with crystalline and other particles. The background or underlying surface on which the particles sit ranges from different gradations of ferrous (Figures 1–3) to the universal sample, a silica/ferrous, tenacious, glassy, uniform deposit (Figure 4). A steam trap bellows (Figures 5–6) was also analyzed. 

Figure 1. SEM of surface: Case 1 (10,000X). Image Credit: Astro Pak Corporation

Figure 2. SEM of surface: Case 2 (4,000X). Image Credit: Astro Pak Corporation

Figure 3. SEM of surface: Case 3 (2,000X). Image Credit: Astro Pak Corporation

Figure 4. SEM of surface: Case 4 (500X). Image Credit: Astro Pak Corporation

Figure 5. SEM of steam trap surface: (4,000X). Image Credit: Astro Pak Corporation

Figure 6. Photo of steam trap/bellows. Image Credit: Astro Pak Corporation

AES testing is an analytical technique applied to establish the surface chemistry of stainless steel and diagnose its corrosion resistance. It also reveals the degradation of the passive film and the reduction of chromium concentration in the passive film as surface degradation occurs due to corrosion. 

AES survey scans (depth profiles of elemental concentrations at the surface) were applied for the characterization of the elemental composition of each sample surface. 

Each site for analysis and SEM magnification was carefully chosen to deliver information from typical regions. Each survey offered information from the top few molecular layers (estimated at 10 ångstroms [Å] per layer) down to the alloy metal depth (200–1,000 Å). 

Numerous amounts of iron (Fe), Cr (chromium), nickel (Ni), oxygen (O), and carbon (C) were recorded in all areas of rouge. AES figures and results are outlined in the Case Studies section. 

General AES results of initial conditions demonstrate heavy oxidation on the sample received with exceptionally high Fe and O concentrations (iron oxide) and low Cr at the surface. This rouge depositing creates particulate release, potentially contaminating the product and product-contact surfaces. 

After removing the rouge, the “passivated” samples display complete restoration of the passive film, with Cr hitting a greater concentration level than Fe, and a Cr:Fe ratio from 1.0 to 2.0 at the surface, with a prevalent lack of iron oxides. 

Various rouged surfaces were analyzed with the application of XPS/ESCA to compare elemental concentrations and oxidation states of the spectra for Fe, Cr, sulfur (S), calcium (Ca),  sodium (Na), phosphorous (P), and nitrogen (N), as well as O and C (Table A). 

Table A: Elemental concentrations. Source: Astro Pak Corporation

There was an evident variance in Cr content from near-passive-layer values to lower values generally found in the base alloy. The Fe and Cr levels discovered on the surface represent the various thicknesses and classes of rouge deposits. XPS testing shows increases in Na, C, or Ca in the rouged surfaces over the clean and passivated surface. 

XPS testing also reveals that a high C content was present in the ferrous (black) rouge as well as Fe(x)O(y) (iron oxides) within the rouge. XPS data was ineffective in understanding the changes occurring on the surface throughout the corrosion process since it assesses the rouge and the base metal simultaneously. Additional XPS testing using more samples is needed to evaluate the results correctly.  

Previous authors also experienced difficulty assessing the XPS data.10 Field observations during removal showed that the C content was high and normally removed during the processing via filtration. SEM micrographs taken prior to and following derouging treatments illustrate the surface damage created by these deposits, including pitting and porosity, which directly affect corrosion. 

XPS results after passivation demonstrate a much greater Cr:Fe content ratio at the surface as the passive film is reformed, reducing the corrosion rate and other detrimental effects on the surface. 

Sample coupons demonstrated considerable increases in the Cr:Fe ratios between “as-received” surfaces and passivated surfaces. Cr:Fe ratios on the as-received sample were tested between 0.6 and 1.0, while the passivated after-treatment ratios ranged between 1.0 and 2.5. Values that represent electropolished and passivated stainless steel range between 1.5 and 2.5. 

The maximum Cr:Fe ratio depth (as established using AES) was between 3 and 16 Å in the after-treatment samples. These offer favorable comparisons to previous research data reported by Coleman2 and Roll.9 All samples had standard Fe, Ni, O, Cr, and C levels on the surface. Low levels of P, Cl, S, N, Ca, and Na were also detected in most samples. 

These residues are common in cleaning chemistries, purified water, or electropolishing. In further analyses, some Si contamination was detected at various levels on the surface and on the austenitic crystals themselves. The source appears to be the silica content of the water/steam, mechanical polishing compounds, or visual sight glasses dissolving or etching within the CS generating unit. 

Corrosion products found in CS systems, as reported, are highly variable. This is due to the variety of conditions across these systems and the placement of various components, such as valves, traps, and other appurtenances, which can bring about corrosive conditions and corrosion products. 

Additionally, replacement components that have not been appropriately passivated are often introduced into the system. Corrosion products are also significantly influenced by the CS generator’s design and water quality. Some generator unit types are reboilers, while others are tube flash evaporators. CS generators typically use terminal mesh screens to eliminate moisture from clean steam, while others use a baffle or cyclone separator. 

Image Credit: Astro Pak Corporation

Some produce an almost consistent ferrous patina within the distribution tubing and overlying ferric rouge. Units with baffles produced a dark ferrous film with ferric oxide rouge beneath, and a second upper surface phenomenon of a soot-like rouge formed, which may be wiped from surfaces more easily. 

Generally, this ferrous, soot-like deposit is significantly more visible than the ferric rouging and with greater mobility. The rouge that builds up in the condensate trail at the bottom of distribution tubing has ferric oxide rouge on top of the ferrous rouge resulting from the elevated oxidation state of iron in the condensate fluid. 

The ferric oxide rouge moves through the condensate traps, visible in drain lines, and the upper layers can be wiped easily from the surface. Water quality has a significant role in rouge product chemistry. 

Higher hydrocarbon levels lead to excess black carbon in the rouge, and greater silica levels lead to higher silica content, which forms a smooth or shiny rouge layer. As previously mentioned, it is known that water-level sight glasses may also erode, releasing its debris and silica into the system. 

Figure 7. As-received surface (2,000X). Image Credit: Astro Pak Corporation

Figure 8. As-received surface (10,000X). Image Credit: Astro Pak Corporation

Figure 9. Derouged and passivated surface (2,000X). Image Credit: Astro Pak Corporation

Rouge is a cause for concern in steam systems as thick layers that generate particles may form. These particles are discovered on steamed surfaces or in steam sterilization equipment. The possible effect on pharmaceutical products is outlined in the following sections. 

The as-received sample SEMs in Figures 7 and 8 display the microcrystalline nature of the class 2 rouge in Case 1. A particularly tight matrix of iron oxide crystals forms on the surface, which appears like a fine-grain residue. The derouged and passivated surface exhibits damage caused by corrosion, leading to a rough and slightly porous surface texture, displayed in Figures 9 and 10. 

Figure 10. Derouged and passivated surface. Image Credit: Astro Pak Corporation

The AES scans in Figure 11 demonstrate the initial condition of the surface in the as-received sample with heavy iron oxide on the surface. The passivated and derouged surface (Figure 12) indicates that the passive film now has an elevated Cr (red line) content above the Fe (black line) at > 1.0 Cr:Fe ratio. 

Figure 11. Case 1 AES scan, as received. Image Credit: Astro Pak Corporation

Figure 12. Case 1 AES scan, derouged and passivated. Image Credit: Astro Pak Corporation

The thin (< 80 Å) chromium oxide passive film is significantly more protective than the hundreds of ångstroms thick iron oxide crystalline film of the base metal and rouge layer, with over 65% Fe content. 

The passivated and derouged surface chemistry is now comparable to a passivated mill-finished material. The rouge in Case 1 is a class 2 rouge that was able to form in place; as the rouge accumulates, particulates that expand in size are produced and then migrate with the steam. 

The corrosion demonstrated, in this case, does not severely pit or degrade the surface. Routine derouging will reduce both corrosive effects on the surface and eliminate the potential for extreme migration of particles that may become visible. 

In Figure 11, AES results display a thick layer near the surface with elevated levels of Fe and O (500 Å iron oxides; lime green and blue lines, respectively) that transitions to alloy levels of Fe, Ni, Cr, and O. The Fe concentration (blue line) is considerably greater than any of the other metals, increasing from 35% at the surface to over 65% in the alloy. 

At the surface, the O level (lime green line) transitions from nearly 50%  to near zero in the alloy at more than 700 Å in depth of the oxide film. The Ni (dark green line) and Cr (red line) levels are extremely low at the surface (< 4%) and increase to normal levels (11% and 17%, respectively) at alloy depth. 

The AES chart in Figure 12 reveals that the rouge layer (iron oxides) has been eliminated, and the passive film has reformed. In the primary 15 Å, the Cr level (red line) is greater than the Fe level (black line), representing the passive film. Initially, the Ni level is at 9% on the surface and increases above the Cr level (± 16%) between 60 and 70 Å, then increases further to the alloy levels at 200 Å. 

Starting at 2%, the carbon level (blue line) drops to zero at 30 Å. The Fe level is initially low (< 15%) and later equal to the Cr level at 15 Å and continues to increase to the alloy level at more than 65% at 150 Å. The Cr level increases from the surface to 25% at 30 Å and reduces to 17% in the alloy. 

The increased O level near the surface (lime green line) is reduced to zero after a depth of 120 Å. This analysis demonstrates a well-developed surface passive film. The SEM photos in Figures 13 and 14 show the rouged, rough, and porous crystalline nature of the surface’s class 1 and 2 ferrous oxide layer. The derouged surface demonstrates the effects of the corrosion in its partially pitted, roughened surface (Figures 18–19). 

The passivated and derouged surface displayed in Figures 13 and 14 could not sustain heavy oxidation. Figures 15 and 16 display a restored passive film on the metal surface. 

Figure 13. As-received surface (1,000X). Image Credit: Astro Pak Corporation

Figure 14. As-received surface (4,000X). Image Credit: Astro Pak Corporation

Figure 15. Derouged and passivated surface (1,000X). Image Credit: Astro Pak Corporation

Figure 16. Derouged and passivated surface (4,000X). Image Credit: Astro Pak Corporation

In Figure 17, there is a dramatic decrease in the Fe concentration (black line) at the near-surface area (< 100 Å). In comparison, an increase in the Cr content (red line) occurs in the first 20 Å to generate a passive film, with a Cr:Fe ratio of around 1.5 in the first 25 Å of the surface, which is around three molecular layers. 

Figure 17. Case 2: AES scan, derouged and passivated. Image Credit: Astro Pak Corporation

The O level (lime green line) stays high in the first 30 Å of the passive film. The C level (blue line) remains high at the surface but drops at the transition between the alloy and surface. 

Figures 18 and 19 demonstrate the size of oxide crystals that can emerge from steam corrosion. These class 2 corrosion products begin as tiny crystal growths on the surface, then grow into particles from 5 to 50 micrometers (μm) in size and sometimes even larger.

Figure 18. As-received surface (500X). Image Credit: Astro Pak Corporation

Figure 19. As-received surface (2,000X). Image Credit: Astro Pak Corporation  

The release of these crystals into the steam may result in migration into the process stream or onto product contact surfaces. Figures 20 and 21 display the conditions of the CS tubing throughout the distribution system. 

Figure 20. Distribution tubing. Image Credit: Astro Pak Corporation 

Figure 21. Pretreatment view. Image Credit: Astro Pak Corporation

Figures 22 and 23 demonstrate that the clean derouged surface is free from the presence of the iron oxide material, with minor pitting and roughness produced from the corrosion of the surface. The SEMs show the surface rouge as inspected. The surface’s ferrous oxide contains a thin nonuniform overlaying film of ferric oxide rouge. 

Figure 22. Derouged and passivated surface (500X). Image Credit: Astro Pak Corporation

Figure 23. Derouged and passivated surface (2,000X). Image Credit: Astro Pak Corporation

Excessive carbon and iron oxide deposits are visible on the AES scan (Figure 24) image of the rouged surface with no passive film. Figure 25 displays the surface post-derouging and passivation, with a recreation of the passive film and loss of the iron oxide film. 

Figure 24. Case 3: AES scan, as received. Image Credit: Astro Pak Corporation

Figure 25. Case 3: AES scan, derouged and passivated. Image Credit: Astro Pak Corporation

The C, O, and Fe levels are incredibly high in the first 50 Å of the as-received sample; this shows that the iron oxide film with elemental carbon is present on the surface, commonly seen in CS rouge of class 2 corrosion with enhanced carbon levels. 

Figures 26–29 reveal a shiny black surface under the microscope. The surface appears to be significantly more smooth than typical rouge crystals due to the amorphous silica, which seems like a glassy coating. 

Figure 26. As-received surface (500X). Image Credit: Astro Pak Corporation

Figure 27. As-received surface (2,000X). Image Credit: Astro Pak Corporation

Figure 28. Derouged and passivated surface (500X). Image Credit: Astro Pak Corporation

Figure 29. Derouged and passivated surface (2,000X). Image Credit: Astro Pak Corporation

Once removed, the surface displays low-level pitting and austenitic metallic crystal edge deformation. Figure 30 shows the appearance of the passive film after iron oxide deposits were eliminated. The Cr:Fe ratio at the surface is greater than 1:1 in the first 20+ Å, as the Cr (red line) and Fe (blue line) combine toward the surface. 

Figure 30. Case 4: AES scan, derouged and passivated. Image Credit: Astro Pak Corporation

The O level (lime green line) is initially high at the surface (at 40%) and suddenly drops to zero at 120 Å. In comparison, the Ni level (darker green line) started at 7%, quickly rising to nearly 15%, then levels off around 10% into the alloy composition under 150 Å. 

It is possible to monitor CS systems for metals in the steam flow and condensate, measuring both the number and dimensions of particles from 5 to > 100 μm. The results in Table B demonstrate the ranges of metal content and particulate in the CS critical utility of three case studies. 

Table B: CS Systems, particulate counts. Source: Astro Pak Corporation

CSG: Clean steam generator; CSD: Clean steam distribution

Particle sizes more than 50 μm are visible contaminants,3 and considerable numbers of particles more than 50–100 μm put an increased risk of contamination on surfaces steamed by this critical utility. 

The removal of ferrous oxide rouge deposits can be performed using organic acids and other variable complexes in the appropriate contact times, concentrations, and temperatures. Different suitable mineral acid treatment approaches include, but are not restricted to: 

The main goal of the derouging process is to eliminate the iron oxide deposits while ensuring the stainless steel substrate surface is protected from additional pitting corrosion. To guarantee that the derouging solutions do not impact the finish of polished surfaces, it is also crucial to avoid aggressive techniques that may cause the removal of the base metal. 

After the rouge and oxide deposit removal, a passivation treatment can restore the passive film by eradicating elemental iron and iron oxides from the first few molecular layers on the surface while preserving the protective chromium oxide layer. Returning to service can lead to a reduction in continued corrosive mechanisms. 

The corrosion byproducts found in clean and pure steam systems — silica, carbon, and iron oxide compounds — are typically present to some extent in every system. Many CS systems do not possess regular maintenance and inspection that could mitigate corrosion and migration of particulates from oxide deposit exfoliation.2 

Corrosion issues are intensified by components with dissimilar metals, poor gasket specifications, and decreased stainless steel surface quality, as well as the unrestrained nature of electrochemical/mechanical polishing materials and methods in combination with poor material handling and lack of regularly performed derouging and passivation techniques.7 

It has been suspected that stainless steel materials are not always delivered at a preferable quality level. Manufacturing processes, in combination with ensuing material handling unit operations and fabrication techniques, determine the corrosion resistance, surface chemistry, and surface finish quality, each of which will impact the quality of the final product. 

There is little credibility in the assertions that these blackish/grey deposits are inevitable, stable, and should be left alone. 

Corrosion produces rouge that is visible as discolored stains on the contact surface of the products and leads to the production of mobile particles that build up on steam sterilized surfaces.

CS rouge contaminants have been detected in final filtration processes, potentially becoming an uncontrolled material in the final process gasses and fluids. While it is acknowledged that the examples presented in this article are case specific, they are not isolated. 

Similar cases are found in other systems, particularly those where corrosion has been left to continue without the appropriate corrective treatment.4 Ultimately, corrosion within CS systems will produce migratory rouge that can be detected and measured in the condensate, the steam, and the interior system surfaces. 

It is, therefore, vital that proper design and maintenance are conducted in the operation of CS generation and distribution systems for high-purity applications. 

Future studies that measure the time, properties, and conditions of rouge development could be compared to particulate generation to measure the risk of product contamination. 

Routine, systematic measurements over two years could monitor corrosion products to highlight changes in particulate release and transport within the evaluated CS system, as well as the surface conditions of the distribution and generation equipment. 

The authors have given credit (posthumously) to Robert W. Evans for his efforts in joining Drew Coleman during the 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, Dr. Brent Ekstrand needs to be thanked for his work in the editing and finalizing of this technical article to enable the message of this article to be more clearly conveyed to the industry. 

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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Last updated: Nov 15, 2022 at 10:37 AM

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