Evaluating Diruneutra and Alternative Methods for Derouging and Passivation of 300 Series Stainless Steel

A Technical, Performance, and Regulatory Analysis for Heavily Rouged Pharmaceutical Equipment.

Authored by Darrell S. Ross, Ph.D.

Technical Review with Citations from Scientific Literature

Abstract

Rouging of 300 Series stainless steel continues to be a persistent challenge in pharmaceutical, biotechnology, and food manufacturing systems operating under high-purity water and clean steam conditions. Effective derouging and passivation are essential to restore corrosion resistance, ensure hygienic surfaces, and maintain regulatory compliance.

The effectiveness and success of these processes directly impact corrosion resistance, product purity, regulatory compliance, and asset longevity. While proprietary formulations such as Diruneutra are marketed as mild, user-friendly solutions for derouging and passivation, field experience and mechanistic analysis indicate significant limitations when these products are applied to heavily rouged systems.

This paper critically evaluates the chemical drawbacks and limitations, operational risks, and regulatory shortcomings of Diruneutra when applied to advanced rouge conditions. The role of iron oxide chemistry, passivation mechanisms, analytical endpoint control via spectrophotometry, and regulatory considerations are examined and assessed. The analysis demonstrates that citric acid/EDTA/reducing-agent protocols deliver superior iron oxide removal, more consistent passivation, improved safety, and stronger regulatory defensibility. For heavily rouged systems, these approaches represent current best practice and align more closely with FDA expectations for validated cleaning and surface control.

1.   Introduction

300 Series stainless steels (notably Types 304, 316, and 316L) are widely used in hygienic processing environments due to their corrosion resistance, cleanability, and compatibility with validated manufacturing systems. Despite these advantages, stainless steel is not immune to corrosion events and phenomena. An unwelcome but consistent fact is that prolonged exposure of these materials to elevated temperatures, high-purity water, and clean steam lead to the formation of rouge.

Rouge is one of the most persistent and operationally problematic manifestations of iron corrosion. Rouging compromises surface integrity, increases the risk of corrosion initiation, impacts hygienic performance, and presents serious concerns in regulated industries where particulate contamination and surface chemistry are tightly controlled.

Derouging and passivation are therefore critical lifecycle maintenance activities. Effective derouging must therefore be coupled with robust passivation formulations and processes to restore and stabilize the chromium-rich oxide layer responsible for stainless steel’s corrosion resistance.

Selection of chemical agents directly influences effectiveness, repeatability, downtime, and regulatory compliance. Among commercially available options, Diruneutra has been promoted and marketed as a low-aggressive, proprietary derouging and passivation formulation. However, its performance on heavily rouged systems has repeatedly proven inconsistent and ineffective. This paper examines the performance, as well as the chemical, operational, and regulatory shortcomings of Diruneutra, with emphasis on heavily rouged systems where chemical limitations become most evident, and contrasts its performance with citric acid/EDTA-based systems, including the strategic use of sodium erythorbate and spectrophotometric endpoint determination.

2. Understanding Derouging and Passivation

2.1. Rouge Formation and Chemistry

Rouge is not a single compound but consists primarily of a spectrum of iron oxides and hydroxide species, commonly including:

• Hematite (Fe₂O₃)

• Magnetite (Fe₃O₄)

• Goethite (FeO(OH)

These deposits are often multi-layered, crystalline, and strongly adherent. These oxides form due to selective iron dissolution from stainless-steel surfaces under thermal cycling, high-purity water exposure, and oxygen gradients. Chromium enrichment beneath the rouge layer often masks ongoing metal loss and complicates remediation. Heavily rouged systems are chemically and mechanically resistant to mild cleaning agents. Effective removal requires not only chelation but, in most cases, chemical reduction of ferric oxides.

2.2. Passivation Mechanism

Passivation depends on the complete removal of free iron and iron oxides to allow spontaneous formation of a uniform thin, continuous chromium oxide (Cr₂O₃) layer. Any residual iron contamination directly disrupts and compromises this process, leading to:

• Incomplete passive film formation

• Increased susceptibility to pitting and crevice corrosion

• Accelerated re-rouging

Thus, effective passivation and long-term corrosion resistance is impossible without thorough derouging.

3. Diruneutra: Overview and Intended Use

Diruneutra is a proprietary formulation typically composed of:

• Mild organic acids

• Buffering agents

• Proprietary surfactants

It is marketed for in-situ circulation cleaning, derouging, and passivation of stainless-steel systems with claims of low corrosivity, ease of handling, and compatibility with multiple stainless-steel grades. The process involves circulating the solution at moderate temperatures followed by rinsing.

While its low aggressiveness may reduce immediate material attack for light surface discoloration or early-stage rouge, this same characteristic limit its effectiveness against tenacious iron oxide deposits commonly found in advanced or heavily rouged conditions.

Figure 1. Rouge chemistry and stratified oxide formation on 300 series stainless-steel.

Mild organic acids (e.g., Diruneutra) primarily interact with the outer surface and fail to penetrate magnetite-rich layers, resulting in incomplete derouging.

4. Disadvantages and Performance Limitations of Diruneutra

4.1. Insufficient Aggressiveness Against Heavy Rouge

The primary technical limitation of Diruneutra is its inability to effectively dissolve or chemically transform magnetite- and hematite-rich rouge layers. These oxides are thermodynamically stable and poorly soluble under the mildly acidic and non-reducing conditions typical of Diruneutra formulations.

As a result:

• Thick rouge layers remain partially intact

• Iron oxides are smeared or redistributed rather than removed

• Subsurface corrosion mechanisms persist

Incomplete derouging significantly increases the likelihood of rapid re-rouging and localized corrosion.

4.2. Compromised Passivation Performance

Because Diruneutra frequently leaves residual iron contamination, passivation outcomes are inconsistent. Free iron inhibits chromium oxide film formation, producing:

• Non-uniform passive layers

• Reduced corrosion resistance

• Long-term reliability concerns

In contrast, citric acid-based systems are well documented to remove free iron without attacking the base metal, enabling superior passivation outcomes.

4.3. Residue Retention and Surface Contamination

Field observations indicate that Diruneutra can leave behind organic residues and surfactant films that are difficult to rinse completely, particularly in complex piping geometries. In pharmaceutical and biotech environments, such residues pose risks related to:

• TOC excursions

• Extractables and leachables

• Cleaning validation failures

Field observations consistently show residual rouge following treatment, leading to rapid re-rouging and localized corrosion initiation. Citric acid and EDTA are fully water-soluble and readily rinsed, leaving chemically clean surfaces suitable for validated operations.

Diruneutra lacks sufficient chemical aggressiveness and reducing capacity to dissolve magnetite- and hematite-rich deposits. Incomplete iron removal prevents uniform chromium oxide formation. Surfaces treated with Diruneutra frequently exhibit non-uniform passivation, increasing susceptibility to pitting and crevice corrosion over time.

5. Environmental, Safety, and Regulatory Concerns

Diruneutra’s proprietary nature complicates hazard evaluation, waste characterization, and regulatory documentation. In contrast:

• Citric acid is biodegradable and widely accepted

• EDTA, while persistent, is well understood and manageable

• Sodium erythorbate has a favorable safety profile

These characteristics simplify environmental permitting, operator safety programs, and regulatory inspections.

5.1. Comparative Chemistry and Performance Analysis

6. Cost Effectiveness and Operational Efficiency

Although Diruneutra is marketed as a simplified solution, its longer dwell times, repeat applications, and inconsistent results increase total cost of ownership. Extended downtime, repeated cleanings, and post-cleaning corrective actions negate any perceived convenience.

Citric acid/EDTA systems achieve more complete derouging in fewer cycles, reducing downtime and long-term maintenance costs.

7. Comparison with Citric Acid/EDTA Blends

Citric acid acts as a chelating agent for iron, while EDTA forms strong complexes with both ferrous and ferric ions. Together, they:

• Dissolve and sequester iron oxides more effectively

• Penetrate multi-layered rouge structures

• Support consistent, reproducible passivation

Notably, ASTM A967 does not explicitly address EDTA or sodium erythorbate, despite extensive field evidence supporting their effectiveness. This highlights the gap between minimum standards and best practice.

8. Role of Sodium Erythorbate as a Reducing Agent

Sodium erythorbate functions as a reducing agent, which chemically reduces Fe³⁺ oxides to Fe²⁺, dramatically increasing solubility and chelation efficiency. This reduction step is essential for removing magnetite and hematite dominated rouge layers and deposits. The inclusion of Sodium Erythorbate markedly improves derouging kinetics and surface cleanliness.

Benefits include:

• Enhanced derouging kinetics

• Reduced risk of under-deposit corrosion

• Improved surface preparation for passivation

The synergistic action of reduction plus chelation represents a major advantage over non-reducing formulations such as Diruneutra.

Figure 2. Reduction-chelation mechanism for enhanced rouge removal using sodium erythorbate and EDTA.

Key Advantage:

• Reduction step destabilizes crystalline oxides

• Chelation prevents re-precipitation

• Enables complete iron removal prior to passivation

9. Spectrophotometric Endpoint Measurement

Citric acid/EDTA systems uniquely enable real-time quantitative monitoring of iron removal and endpoint determination through spectrophotometric measurement of iron–chelate complexes.

Measurement of iron-complex absorbance allows real-time assessment of:

• Iron removal rate

• Cleaning completeness

• Optimal process termination

This approach improves reproducibility, reduces chemical waste, and provides defensible documentation for regulatory compliance—an advantage absent from Diruneutra-based processes.

Endpoint Criteria:

• Plateau in iron concentration

• No further increase with time

• Objective confirmation of complete derouging

Benefits include prevention of under- or over-processing, improved reproducibility, and robust documentation for regulatory review.

10. FDA-Inspection-Ready Technical Justification

From a regulatory perspective, FDA expectations emphasize:

• Demonstrated effectiveness of cleaning processes

• Control of variability

• Scientific rationale for chemical selection

• Objective evidence of process completion

Diruneutra presents challenges in all four areas due to proprietary composition, inconsistent outcomes, lack of analytical endpoint control, and limited effectiveness on heavy rouge.

In contrast, citric acid/EDTA/sodium erythorbate systems:

• Use well-characterized chemistry

• Provide mechanistic justification (chelation + reduction)

• Enable quantitative endpoint verification

• Support repeatable, validated procedures

These attributes align with FDA guidance on process validation, cleaning validation, and lifecycle equipment control.

11. Conclusion

Diruneutra may be suitable for lightly rouged systems but demonstrates fundamental inadequacies and limitations as a derouging and passivation agent for heavily rouged 300 Series stainless steel. Its insufficient chemical aggressiveness, inconsistent passivation outcomes, residue risks, and higher operational costs render it unsuitable for demanding, regulated applications.

Citric acid/EDTA-based systems, particularly when enhanced with sodium erythorbate and validated through spectrophotometric endpoint analysis, provide superior technical performance, derouging, more reliable passivation, improved safety, and stronger regulatory defensibility. For heavily rouged systems within pharmaceutical and biotechnology applications, these methods represent the current state of best practice and should be prioritized to ensure long-term equipment integrity and operational excellence.

12. References

1. ASTM A967 / A967M – Standard Specification for Chemical Passivation Treatments forStainless Steel Parts.

2. ASTM A380 – Standard Practice for Cleaning, Descaling, and Passivation of Stainless-SteelParts.

3. Sedriks, A. J. Corrosion of Stainless Steels, 2nd ed., Wiley-Interscience, 1996.

4. Goldschmidt, H. Intergranular Corrosion of Stainless Steels, ASM International, 1993.

5. Shoemaker, J. D., et al. “Rouging of Stainless Steel in High-Purity Water Systems.”Pharmaceutical Engineering, ISPE.

6. Francis, R. Stainless Steels for Design Engineers, ASM International, 2008.

7. McCoy, W. F. “The Chemistry of Rouge Formation and Removal.” Journal of ValidationTechnology.

Figure 1 - Rouge Chemistry and Stratification

Clearly illustrates:

• Hematite vs. magnetite layers

• Iron-enriched subsurface zone

• Why mild organic acids fail

Strongly supports the assertion of Diruneutra’s lack of penetration and aggressiveness.

Figure 2 - Reduction-Chelation Mechanism

Stepwise depiction of:

• Fe³⁺ → Fe²⁺ reduction via sodium erythorbate

• Chelation by citric acid / EDTA

• Solubilization and removal

• This is the key differentiator figure - it visually proves why the Citric Acid + EDTA + Sodium Erythorbate approach works, and Diruneutra does not