Dijana Hadziselimovic is Laboratory Manager with Life Sciences Technical Services, STERIS.

Dijana Hadziselimovic

Intravenous therapy; drip chamber in focus against a colorful background | Image Credit: ©Anna Jurkovska -stock.adobe.com

Good Manufacturing Practice for Medicinal Products for Human and Veterinary Use

, Annex 1 and Annex 15 provide guidance that must be followed in the design, qualification, and monitoring of cleaning processes to mitigate cross-contamination risks which includes cleaning direct, indirect, and non-direct product contact surfaces as detailed in the site’s contamination control strategy (CCS). Understanding the critical cleaning parameters (CCPs) for removal of silicone oil from direct and indirect product contact surfaces, qualifying an analytical test method, and setting practical, achievable, and justifiable limits are important to the CCS. This article explores the historical use of silicone oil in the performance of primary drug product packaging; design of experiment test results for selection of the cleaning agent and CCPs; assessment of visible residue limits and Fourier-transform infrared spectroscopy surface analysis; and setting health-based exposure limits for silicone oil using a non-uniform distribution model, and a previously published permitted daily exposure.

Siliconization of parenteral drug packaging has been in use as a lubricant for the past 50 years.

 The primary functions of the lubricant are to ensure machinability of the closure with metallic machinery, reduction of insertion force as the closure is fitted in the parenteral packaging, and integrity of the seal between the closure and primary packaging.

 Siliconization can refer to the use of silicone oil, silicone emulsion, and air-curable silicone. This article focuses on silicone oil that includes silicone fluid, siloxane, dimethicone, polydimethylsiloxane, and other terms which represent the base formula structure of the liquid silicone lubricant of (CH

There is a requirement to develop and maintain a contamination control strategy (CCS) to minimize risks of microbial, particulate, and endotoxin/pyrogen contamination of sterile drug product manufacturing.

 The CCS must include a rationale for the selection, application, and removal of any process aids, such as silicone oil, from direct, indirect, and non-direct product contact surfaces. Residual silicone oil on indirect product contact surfaces, such as the stopper bowl, transfer chutes, star wheels, and other equipment parts, can result in visual residue and sticky surfaces. This could reduce the efficacy of the cleaning and sterilizing process, which can impact the functionality of the siliconization process (machinability, insertion, and seal integrity) and increase particulate and microbial risk of the primary packaging system.

A validated sterilization process with steam or vaporized hydrogen peroxide (VHP) bio-decontamination process begins with a clean and dry surface. The use of laboratory testing and/or field trials are the preferred cleaning development tools. The effects of residual lubricant on the sterilization process have been investigated with healthcare no-rinse lubricants.

 The historical approach to cleaning silicone oil is to use a formulated alkaline detergent containing surfactants at elevated temperature in an automated parts washer or clean-in-place system; however, there are several manufactures utilizing a manual, water-only, sodium hydroxide (NaOH), or alcohol-only cleaning process at ambient to warm temperatures that can be a concern due to the low solubility of silicone oil.

This article explores the development of a cleaning process through laboratory testing, ease of surface detection of silicone oil and cleaning agent residue, and application of practical, achievable, and scientifically justified limits based on health-based exposure limits (HBEL).

Standard cleanability practices were used including a variety of cleaning methods such as agitated immersion (AI), spray wash (SW), cascading flow (CF), and manual cleaning (MC). In addition to this, visual residue limit (VRL) testing was used to determine an acceptable VRL for specified silicone oils. Lastly, bench top Fourier-transform infrared spectroscopy (FTIR) was used to determine the presence of silicone oil on stainless steel surfaces.

 Representative 304 stainless-steel coupons (3×6”) were used throughout the study.

The critical cleaning parameters have been well defined in literature; they include time, action, cleaning chemistry, concentration, temperature, water quality, surface, soil load and condition, and environmental factors.

 Laboratory testing was performed to investigate changes in the critical parameters listed in 

 using 3 different silicone oils widely used in the pharmaceutical industry. Environmental factors such as room temperature, humidity, and particulate levels, which primarily affect dirty and clean hold times, were not investigated in this study.

The following procedure was used throughout the study unless specifically noted:

Weighed dry, clean stainless-steel coupons on an analytical balance (±0.1 mg) to obtain the pre-coating weight.

Coated coupons with 1-3 ml of the sample.

Conditioned the sample on the coupon as indicated in each section.

Weighed the conditioned coupon on an analytical balance to obtain the pre-cleaning weight.

Cleaned each coupon by AI, SW, CF, or MC with a wipe as indicated in 

At specified time intervals, removed and visually observed each coupon for cleanliness.

Rinsed each side of coupon with tap water for 10 seconds at a flow rate of 0.5 gal/min.

Rinsed each side of coupon with deionized water and examined for a water break-free surface.

Dried, visually inspected, and then weighed coupons on an analytical balance to obtain the post-cleaning weight.

Table 2: Effect of Cleaning Agent, Dirty Hold Time (at ambient temperature), Concentration, and Temperature on Cleaning Time Required per Method

Table 3: Effect of Soil Load on Cleanability (Performed with AI at 80 °C)

Table 4: Effect of Cleaning Agent on Manual Cleaning Via Polyester Wipe

Table 5: Acceptance Criteria Used in Laboratory Testing

Two silicone oils (Ompi EZ Fill and 360 Dow Corning [DC] Medical Fluid) were evaluated in the laboratory tests. The following procedure was used throughout the study unless otherwise noted:

Diluted the silicone oils to 1, 2, 3, and 5 mg/mL using laboratory-grade, purified water ( Milli-Q, MilliporeSigma) water.

Applied 1 µL of the diluted detergent and 20 µl of purified water onto a stainless-steel coupon over a 1 cm

Air-dried the spiked coupons at ambient temperature for at least 16 hours; visually observed coupons as dry prior to testing.

Observed the coupons under a variety of conditions as described in 

Two analysts observed the visual residue on the coupons for each test.

The following measurement tools were used:

Lux meter (Cal-light 400, Cooke Corporation) to confirm light intensity.

Standard manual protractor to measure the viewing angle.

Recorded results as either visible (V) or not visible (NV) for each analyst.

Figure 1: Measured Angle of Coupon (Figure courtesy of the authors)

Table 6: VRL Testing Under Different Lighting, Angle, and Distance Conditions for Specified Silicone Oils on Stainless Steel

Test coupons were prepared and cleaned using a formulated alkaline detergent containing potassium hydroxide (KOH), NaOH, and deionized (DI) water. After cleaning, the coupon surfaces were dried and subsequently sampled by swabbing. The collected samples were analyzed by benchtop FTIR (Cary 660 FTIR spectrometer, Agilent) equipped with an attenuated total reflectance (ATR) crystal to evaluate the presence of silicone oil residues, allowing for direct analysis without additional sample preparation.

Effect of cleaning agent, concentration, time, and temperature in automated cleaning applications. 

To evaluate the effects on cleaning efficacy, the following cleaning agents were tested at different concentrations and temperatures:

1-5% v/v formulated alkaline detergent containing NaOH

1-5% v/v formulated alkaline detergent containing KOH

1-5% v/v formulated neutral detergent containing surfactants

Formulated 1:64 quaternary ammonium-based disinfectant

Formulated acidic detergents containing citric acid and oxalic acid were also tested but were not effective in initial cleanings and therefore not tested further. Lower temperatures, such as 25 ºC and 45 ºC, either did not clean the residue or did not clean in a sufficient time (less than 30 minutes).

When changing the critical parameters, the following general trends were seen to shorten cleaning time (see 

Increasing the cleaning solution concentration

Using SW rather than other methods due to impingement.

Increased dirty hold times did not affect the cleanability when using formulated alkaline detergents, while neutral detergents required increased concentration or temperature to achieve passing results.

Figure 2: Trace Residue on SW DI Water at 80°C. (Figure courtesy of the authors)

 Soil conditioning of the silicone oils onto the coupons was conducted in 2 different ways:

Air-drying at ambient temperature for a specified amount of time.

The baking at 121 °C significantly increased the conditions required to clean the surface.

Figure 3: Residue on AI 0.5M NaOH at 80°C. (Figure courtesy of the authors)

 The lab also investigated soil load, attempting to simulate campaigning in industry. Coupons were coated with either 1 or 10 layers of designated silicone oil and air dried for 120 hours. The 10-layers coupons required increased concentration, temperature, and cleaning time to achieve the same results as the 1-layer coupons (

Many applications in the industry include manual cleaning of silicone oil. A common example is the cleaning of silicone oil from stopper bowls.

A variety of cleaning agents were used for manual cleaning, with a focus on operator safety when using specified cleaning agents. Formulated alkaline detergents outperformed formulated neutral detergents, as well as formulated quaternary ammonium disinfectant, IPA, NaOH, and DI water. All tests were performed with a polyester wipe rather than a nylon brush as the brush would smear the residue. Prepared residue had a 24-hour dirty hold time with 1 layer application.

The VRL is determined by the point in which 2 analysts can no longer see the residue at any given condition with the unaided eye. Silicone oil was visible by 2 separate analysts under all conditions when applied at 1 µg/cm

). No further concentrations were tested as this was the lowest concentration.

Coated coupon: Absorbance bands characteristic of silicone oil residue, most notably in the ~1000–1100 cm⁻¹ region, which are absent in the control coupon.

Coupon cleaned with hot water (80 °C, SW): Though less intensity than the coated coupon, distinct silicone‑related peaks remained.

Coupon cleaned with 0.5 M NaOH (80 °C, AI): A pronounced absorbance band persisted in the ~1000–1100 cm⁻¹ region.

Coupon cleaned with 1% formulated alkaline detergent (80°C, AI): A profile comparable to that of the control coupon, with no distinct residual silicone oil signatures.

These results demonstrate that water alone and 0.5 M NaOH were insufficient to remove the coating residue, whereas the formulated alkaline detergent achieved effective cleaning under the conditions evaluated.

Figure 4: FTIR-ATR Spectra of a Non-Coated and Coated Sample Compared to Coupons Cleaned with NaOH or a Formulated Alkaline Detergent (Figure courtesy of the authors)

Figure 5: FTIR-ATR Spectra of a Non-Coated and Coated Sample Compared to Coupons Cleaned with 80°C Water or a Formulated Alkaline Detergent (Figure courtesy of the authors)

Case Study: Identifying Practical Cleaning Conditions for DC360 Residue

A pharmaceutical manufacturer was interested in determining the cleaning parameters needed to remove silicone/dimethicone (DC360) residue from stainless steel surfaces. Controlled coupon testing was used to identify these parameters using AI, SW, and CF. Representative stainless-steel coupons were soiled with 1–2 g of sample, air‑dried at ambient temperature for 120 hours (

), and then cleaned by AI, SW, or CF. Performance was judged using defined acceptance criteria as per 

Figure 6: Stainless-Steel Coupon Coated with Silicone Oil and Stoppers (Figure courtesy of the authors)

Across all 3 cleaning methods, a consistent set of conditions achieved removal on stainless steel:

Baseline condition: CIP 100 detergent at 4% v/v, 75 °C, for 15 minutes.

Addendum testing with high-pressure spray at 44 psi (3.0 bar), CIP 100 detergent at 3% v/v, 75 °C, for 5 minutes.

Addendum testing with hot water pre-rinse at 75 °C for 1 minute, followed by CIP 100 detergent at 3% v/v, 75 °C, for 3 minutes with SW and CF.

The pharmaceutical manufacturer successfully reproduced and validated the recommended cleaning approach as in the addendum testing with hot-water pre-rinse.

Establishing residue limits for silicone oil residues

In this example, a residue limit for silicone oil on stopper bowls was established using the VRL and permitted daily exposure (PDE) value, to determine the potential silicon remaining on stainless-steel surface after visual inspection. When establishing residue limits, the risk of leaving those residues on the surfaces should be assessed and understood. For direct product contact surfaces, the risk is associated with cross-contamination of product A to a subsequently produced product B. In the case of silicone oil on indirect surfaces within a filling line, those risks are mostly related to:

An increase in the risk of particle contamination due to sticky surfaces, which may increase the risk of microbial contamination.

For direct product contact surfaces, residue limit calculation includes a PDE value, which is an exposure or dose that is unlikely to cause an adverse health effect if an individual were to be exposed by any route at or below this amount daily for a lifetime.

 For indirect surfaces, the residue limits may include a calculation with a PDE value and confirmation via:

Articles

This article explores the historical use of silicone oil in the performance of primary drug product packaging; design of experiment test results for selection of the cleaning agent and CCPs; assessment of visible residue limits and Fourier-transform infrared spectroscopy surface analysis; and setting health-based exposure limits for silicone oil using a non-uniform distribution model, and a previously published permitted daily exposure.

Assessment of Silicone Oil Cleaning on Indirect Surfaces in Aseptic Manufacturing

Stainless steel pipe fittings, clamps, and connectors arranged in industrial. Precision manufacturing, mechanical assembly, and hygienic processing systems for food and pharmaceutical industries. | Image Source: ©Artinun -stock.adobe.com

Type III or Class III rouge (magnetite) appears as a black oxide in water systems and production equipment exposed to sanitization, pasteurization, or steam sterilization conditions. Remediation of type III rouge often requires electropolishing or mechanical polishing of the stainless-steel surface, resulting in significant labour cost, safety risk to operators with confined space entry, downtime of the equipment or system, and a reduction in thickness of the stainless steel. Raman spectroscopy is a non-invasive analytical method that can quickly identify the presence of magnetite (Fe

) in rouge. Based on the identification of magnetite, a detergent additive can be combined with a formulated citric/oxalic or a phosphoric/citric acid blend to transform insoluble magnetite to a more soluble form of iron oxide. This approach to type III rouge remediation utilizing a detergent blend, spray device, and recirculating pump reduces downtime, safety risk, material loss, and overall cost of maintenance.

A common surface in production equipment and piping is 300 series stainless steel. This surface is compatible with most pharmaceutical and biopharmaceutical processes; however, certain conditions can lead to discoloration and/or rouge on the surface. Rouge is a corrosion product on or in the surface of the iron alloy. Rouge is generally classified as Type I–III or Class I–III.

 Type I rouge (migratory rouge) is an iron oxide that is loosely attached to the surface and can be wiped off the surface. The origin of this rouge is often from an upstream source. Type II rouge (in situ rouge) is difficult to wipe off and often contains underlying damage to the surface. Examples of Type II rouge include galvanic, intergranular, pitting and stress corrosion. Common acid blends, such as citric/oxalic or phosphoric/citric solutions, can be effective at removing Type I and II rouge.

 These treatments do not restore original surface roughness. Electro- or mechanical polishing may be required to correct surface damage due to Type II rouge from pitting corrosion. Type III rouge is often associated with high temperature and purity water systems. Type III rouge is challenging to remediate due to the presence of magnetite (Fe

). Magnetite removal often requires aggressive chemicals or electro- or mechanical polishing. An acid detergent with phosphonate can be used alone or in combination with a citric/oxalic or phosphoric/citric acid blend to improve the solubility of magnetite and the removal of Type III rouge.

Understanding when and which acid detergent (or detergent additive) to use in rouge remediation can be difficult. The selection of the acid detergent should consider effectiveness to remove the rouge, substrate compatibility, rinse profile, stability and shelf life, analyzability, toxicity, disposal constraints, personnel safety, toxicity, foaming characteristics, microbial effectiveness, quality of manufacture, long-term availability, quality of manufacture and overall cost.

 Raman spectroscopy can be a powerful tool in identifying the oxide compound(s) present and provide direction to successful remediation. Alternative spectroscopy and microscopy techniques may be applicable for surface characterization and corrosion monitoring, such as X-ray diffraction (XRD) and scanning electrochemical microscopy (SECM).

 Raman uses inelastic light scattering, where the incident light (from a high-intensity laser light source) transfers energy to molecular vibrations, and the scattered light collected at the detector provides structural information relating to the molecule. Each molecule/compound will have a unique spectrum. Magnetite displays a primary peak with Raman spectroscopy between 662 and 670 cm

 A secondary peak is often observed at 532 and 550 cm

. A common roadblock in successful rouge remediation is the presence of magnetite. A quick identification of magnetite in a rouge surface using Raman spectroscopy can be helpful in selecting the right chemical treatment and cleaning parameters.

This article reviews the use of Raman spectroscopy in a series of case studies to quickly identify the iron oxide compound(s), including magnetite and hematite, on the surface of representative endcaps or piping spool pieces. The article also provides a strategy using a detergent additive to remove magnetite without performing costly electro- or mechanical polishing.

A Raman microscope with a 532 nm laser (DXR3, Thermo Scientific) was used to analyze selected sample areas. Areas that visually exhibited the greatest amount of residue were selected and focused on using the confocal microscope prior to analysis. The samples were analyzed as is (without cleaning, modification, etc.) using the following conditions in 

After analysis of the selected sample areas, a spectral library search (Wiley Science Solutions KnowItAll Spectral Library) was performed for each Raman spectrum.

For reference and comparison throughout the following case studies, the Raman spectra of hematite (Fe

Hematite exhibits peaks at approximately 220, 290, 400, 600, and 1300 cm

Figure 2. Raman Spectrum of Magnetite (Fe

A large multinational biopharmaceutical drug manufacturer indicated that visible rouge was unable to be removed from a tank using 10% v/v citric/oxalic acid detergent at 70 to 80 

C. Citric/oxalic and phosphoric/citric acid blends are commonly recommended for rouge removal.

 The nature of the rouge was investigated using Raman spectroscopy to identify the oxide composition and determine the reason for the difficulty in the chemical removal. Raman spectroscopy is an analytical tool that can be used in the laboratory and modified for use on the manufacturing floor.



. The following swab samples of the rouge were analyzed using Raman spectroscopy (See 

TA-9620 Pre-Derouge Swab Bottom Visible Ring

TA-9640 Rouge Observation Top Spot Near Sight Glass

Figure 3. Raman Spectra of Swab Samples. Figure courtesy of the authors.

. A spectral library search (Wiley KnowItAll Spectral Library) was performed for each Raman spectrum. All the residue samples exhibited a best match to a mixture of iron oxide compounds—hematite (Fe

). A representative search result is shown in 

Figure 4. Raman Spectra of Iron Oxide Mixture Reference (Mixture of Fe

) and TA-9720 Bottom of the Tank Outlet. Figure courtesy of the authors.

. All the Raman spectra from the selected swabs were similar to each other and exhibited peaks around 200, 300, 400, 700, and 1300 cm

, which can be attributed to an iron oxide mixture of Fe

. The general recommendation based on the swab analysis by Raman spectroscopy was to increase the concentration of the citric/oxalic agent detergent to 15% v/v and add a detergent additive at 15% v/v with recirculation through a device at 60 to 80

C for 1 to 3 hours to solubilize and remove the rouge containing magnetite (Fe

A large multinational biopharmaceutical blood product drug manufacturer submitted two end caps (one smaller and one larger) for Raman spectroscopy analysis. Both end caps exhibited brown discoloration consistent with rouge (

Figure 5. Small End Cap. Figure courtesy of the authors.

Figure 6. Large End Cap. Figure courtesy of the authors.

With 10x magnification, the small end cap exhibited orange, blue and purple/brown discolored areas. Areas of each colour were selected for Raman analysis to determine if there were any differences in the Raman spectra. Representative areas of analysis for the three different colours observed on the end cap’s surface are marked by a red square in the images below (

Figure 7. Small End Cap – Orange Area (10x mag). Figure courtesy of the authors.

Figure 8. Small End Cap – Blue Area (10x mag). Figure courtesy of the authors.

Figure 9. Small End Cap – Purple/Brown Area (10x mag). Figure courtesy of the authors. 

The three different colored areas of the end cap were analyzed, and the Raman spectra were found to be comparable to each other. A comparison of the spectra is shown in 

Figure 10. Raman Spectra of Analyzed Areas of Small End Cap. Figure courtesy of the authors.

. A magnified image of the residue present on the large end cap is shown in 

. The small area marked by a red square is representative of the area selected for Raman analysis.

Figure 11. Large End Cap – Brown Area (50x mag). Figure courtesy of the authors.

. The Raman spectrum of the brown area from the large end cap is shown in 

Figure 12. Raman Spectrum of Large End Cap. Figure courtesy of the authors.

. The three Raman spectra of the small end cap and the Raman spectrum of the large end cap were comparable to each other, with peaks at approximately 660, 600, 496, 408, 292, and 223 cm

.

When a spectral library search was performed (Wiley Spectroscopy KnowItAll Spectral Library), the residue from the end caps appeared to be a mixture of hematite (Fe

). A comparison of the Raman spectra of the residue from the small and large end caps to the hematite and magnetite Raman spectra is displayed in 

Figure 13. Raman Spectra of End Caps, Magnetite, and Hematite. Figure courtesy of the authors.



The end cap spectra matched well to the hematite spectrum; however, the peak around 665 cm

 had a greater intensity than the hematite reference spectrum, suggesting the presence of magnetite, which has a high intensity peak around 665 cm

 The residue was effectively removed from stainless steel by agitated immersion using a solution of 15% v/v formulated phosphoric/citric detergent plus 15% v/v detergent additive at 80 °C for 24 hours (

Figure 14. Derouge and Passivate End Cap Samples. Figure courtesy of the authors.

A large multinational biopharmaceutical drug product manufacturer submitted pipe samples with suspected rouge for Raman spectroscopy analysis. The pipe samples exhibited a brown discoloration consistent with rouge throughout its outer surface.

. Three different areas on the pipe were analyzed using Raman spectroscopy. The spectra of the three areas were found to be comparable to each other. A comparison is shown below in 

Figure 15. Raman Spectra of Analyzed Areas of Pipe. Figure courtesy of the authors.

. A spectral library search was performed (Wiley Spectroscopy KnowItAll Spectral Library), and the Raman spectrum of the brown discoloration from the pipe had a best match to the spectrum of a mixture of hematite (Fe

). A comparison of the Raman spectrum of the hematite and magnetite mixture and the Raman spectrum of the brown residue from the pipe is shown in 

Figure 16. Raman Spectra of Iron Oxide Mixture and Brown Residue From Pipe. Figure courtesy of the authors.

 The brown discoloration was effectively removed compared to untreated controls from stainless steel by agitated immersion using a solution of 15% v/v formulated phosphoric/citric acid detergent plus 15% v/v formulated detergent additive or a 15% v/v formulated citric/oxalic acid detergent plus 15% v/v formulated detergent additive at 80 °C for 4 hours. The surface was tested with an electrical pen test kit to confirm a passive surface following treatment. Testing with 15% v/v formulated phosphoric/citric acid or the formulated citric/oxalic acid detergent at 80 °C for 24 hours was not successful in removing the discoloration shown in 

 alongside untreated controls (left side) and successfully derouged and passivated pipe samples (right side).

Figure 17. Case Study 3 Heat Exchanger Pipe Samples, Left to Right:

Sample 3, Cleaned with 15% formulated phosphoric/citric acid detergent

Sample 4, Cleaned with 15% formulated citric/oxalic acid detergent

Sample 5, Cleaned with 15% formulated phosphoric/citric acid detergent plus 15% detergent additive

Sample 6, Cleaned with 15% formulated citric/oxalic acid detergent plus 15% detergent additive

A general requirement of pharmaceutical drug manufacturers is that equipment shall be constructed so that surfaces that contact components, in-process materials, or drug products shall not be reactive, additive or absorptive so as not to alter the strength, quality, integrity, identity and purity of the drug product.

 The presence of discoloration or rouge on direct and indirect product surfaces has been cited by regulatory authorities.

Guidelines for Good Manufacturing Practice for Medical Products for Human and Veterinary Use 

(Annex 1) state in section 2.5 that process equipment, utilities, raw materials and premises are potential sources for microbial and particulate contamination. Stainless steel maintenance and cleaning of direct, indirect, and non-direct surface must be considered within any contamination control strategy.

 A risk-based strategy to stainless-steel maintenance utilizing International Council for Harmonisation (ICH) Q9 

 has two primary principles: the evaluation of risk to quality from rouge should be based on scientific knowledge and ultimately linked to the protection of the patient and product, and the level of effort, formality and documentation should be commensurate with the level of risk.

 Stainless steel commonly used in pharmaceutical and biopharmaceutical manufacturing can be discoloured or rouged based on several factors, including the nature of the in-process solution, product, medias, buffers, reagents and water quality.

Through a series of case studies, the authors used Raman spectroscopy to identify the presence of magnetite on various process and utility system samples (swabs, end caps and spool pieces). Type III rouge (magnetite) presents considerable challenges to chemical remediation and stainless steel maintenance programs. Based on the identification of magnetite, a readily available detergent additive can be combined with a formulated citric/oxalic or phosphoric/citric acid detergent commonly used for derouging and passivating stainless steel. The two-detergent approach transforms the insoluble magnetite to a more soluble form of iron oxide for removal without polishing or a secondary passivation step. This science-based approach to type III rouge remediation incorporates a detergent additive, spray device and recirculating pump to reduce equipment downtime, operator safety risk, material loss from polishing and overall cost of maintenance and compliance to current good manufacturing practices.

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21 CFR Part 211: Current Good Manufacturing Practices for Finished Pharmaceuticals

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 A risk-based approach to stainless steel equipment maintenance. 

Type I–II rouge on 300-series stainless steel is often removable with citric/oxalic or phosphoric/citric blends, whereas Type III rouge is driven by magnetite (Fe₃O₄) formed under high-temperature, high-purity water conditions and resists standard derouging. Raman spectroscopy rapidly differentiates hematite from magnetite via characteristic peaks, enabling targeted chemistry selection. Across tanks, end caps, and piping, adding a phosphonate-containing detergent additive to common acid detergents improved magnetite solubilization and removal, avoiding electropolishing, reducing downtime and confined-space risk, and supporting GMP-aligned contamination control.