Technical Analysis of QQ-P-35 and ASTM A-967 Passivation Standards for 300 Series Stainless Steels:History, Shortcomings, Ambiguities, and Recommendations for Improved Corrosion Resistance

By Darrell S. Ross, Ph.D.

Abstract

Passivation of austenitic stainless steels is a critical procedure for ensuring long-term corrosion resistance in demanding service environments. While QQ-P-35 and ASTM A-967 have historically served as foundational references for chemical passivation practices, both demonstrate substantial technical limitations when evaluated against current metallurgical understanding and modern processing requirements. This article provides an expanded examination of the gaps, ambiguities, and under-specified parameters within these standards, with particular emphasis on citric acid-based treatments, pH control, inclusion removal, temperature effects, and advanced analytical verification methods. Drawing on metallurgical principles and contemporary process technology, this analysis proposes a more rigorous framework for achieving consistent and reproducible passivation of 300 series stainless steels.

Introduction

Passivation of stainless steel is predicated on the controlled removal of free iron, exogenous contaminants, and surface imperfections that inhibit the formation of the chromium-rich passive film central to corrosion resistance. Stainless steels in the 300 series—especially Type 304, 316, and their low-carbon and molybdenum-modified counterparts—depend on a stable austenitic microstructure and a chromium oxide layer for durability in aggressive environments.

QQ-P-35, originally issued as a U.S. military specification, was intended to provide prescriptive procedures for chemical cleaning and passivation. Over time, it became a de facto industry reference. ASTM A-967, introduced to replace it, sought to modernize passivation practices by offering multiple chemical treatment options and harmonizing approaches for civilian, industrial, and international use.

Despite these intentions, ASTM A-967 maintains a largely procedural orientation. Many crucial variables—such as solution chemistry, pH, process temperature, and passivation dwell time—remain broadly defined or entirely unspecified. As stainless steel metallurgy has advanced and applications have become more demanding, the limitations of these standards have become increasingly evident.

This expanded analysis critically examines these deficiencies and introduces recommendations based on metallurgical principles, process chemistry, and modern analytical methods.

Historical and Structural Limitations of QQ-P-35 and ASTM A-967

Evolution and Persistence of Legacy Practices:

QQ-P-35 was formulated in an era when nitric acid dominated steel cleaning and passivation. Its procedures reflect assumptions consistent with mid-20th-century metallurgy: relatively simple alloy compositions, less sophisticated surface engineering, and limited understanding of inclusion behavior. As industries adopted QQ-P-35 broadly, many of its practices became ingrained—even after the underlying metallurgical landscape had shifted.

ASTM A-967, though developed to replace QQ-P-35, preserved much of its conceptual framework. While offering multiple “types” of passivation treatments, it did not fully exploit the opportunity to introduce rigorous, chemistry-specific guidance. This has resulted in continued reliance on generic, legacy methods rather than optimized, evidence-based procedures.

Ambiguities Inherited From Prior Standards:

Both standards share three overarching deficiencies:

• Lack of precise process parameters (for citric acid in particular).

• Over-reliance on qualitative validation tests.

• Insufficient attention to metallurgical phenomena such as inclusions, delta ferrite, and magnetism.

In industries where corrosion resistance is mission-critical—such as biopharmaceutical manufacturing, aerospace, and food processing—these shortcomings can have significant implications for product reliability and regulatory compliance.

Critical Parameters in Citric Acid Passivation

Citric acid’s rise as an environmentally preferable alternative to nitric acid has exposed weaknesses in ASTM A-967’s generalized process structure. Citric acid behaves fundamentally differently from mineral acids, requiring more specific control of pH, concentration, temperature, and chemical additives to achieve reliable passivation.

Chemical Concentration

Citric acid is not self-limiting in its reaction behavior; its effectiveness depends on maintaining sufficient ligand concentration to chelate iron and other metallic contaminants. Concentrations below 1% by weight are nearly inert with respect to rouge removal, while concentrations in the 2–10% range enable effective metal ion complexation.

However, neither standard addresses:

• Threshold concentration for iron chelation.

• Concentration shifts due to drag-out or metal loading.

• The impact of citrate complex stability on passivation kinetics.

  
  

The absence of these details encourages inconsistent industrial practice and hampers process reproducibility.

Inclusion Behavior and Removal

Sulfide Inclusions (MnS)

Both standards focus on manganese sulfide inclusions, which are well-documented initiation points for pitting corrosion due to their electrochemical activity and dissolution behavior in acidic media.

Non-Sulfide Inclusions: An Overlooked Threat

Modern 300 series stainless steels frequently incorporate aluminum deoxidation practices, introducing aluminum oxide and mixed aluminate inclusions.

These inclusions:

• Are resistant to nitric and citric acid.

• Exhibit lower local corrosion resistance.

• Become primary initiation sites once sulfides are removed.

This shift in inclusion landscape is not acknowledged in either standard.

Alkaline Dissolution Requirements

Dissolving aluminum inclusions requires highly specific chemistry—typically alkaline solutions engineered for aluminum oxide solubility.

This means:

• Citric acid alone is insufficient.

• Nitric acid accelerates localized attack without removing the inclusion.

• EDTA-based blends or multi-stage chemistries are required for comprehensive inclusion removal.

Ignoring these realities compromises the corrosion resistance of modern stainless steel components.

Temperature Control and Kinetic Considerations

Temperature profoundly influences citric acid passivation:

• Below ~120°F (49°C), citric acid is comparatively inert.

• Reaction rates increase substantially between 120°F–200°F (49–93°C).

• Excessive temperature can lead to citrate precipitation, increased viscosity, or surface roughening.

Because ASTM A-967 offers no chemistry-specific temperature guidance, operators may select temperatures that are either ineffective or detrimental.

A more rigorous standard would specify temperature windows tailored to solution chemistry, alloy grade, and inclusion profile.

Problems With Sodium Hydroxide for pH Adjustment

Using NaOH generates sodium citrate, which exhibits:

• Poor solubility at elevated pH.

• Tendency to precipitate.

• Risk of redeposition onto stainless surfaces.

• Fouling of pumps, filters, piping, and wastewater systems.

Advantages of Ammonium Hydroxide

Ammonia avoids these issues through:

• High solubility of ammonium citrate.

• Improved clarity of process solution.

• Reduced risk of surface contamination.

• Volatility that facilitates post-process removal.

Despite the significance of pH control, neither standard addresses the selection of pH adjusters or precipitation risks, representing a major technical omission.

Process Monitoring and Determination of Passivation Completion

The reliance of ASTM A-967 on qualitative, post-process validation is increasingly inadequate.

Limitations of Traditional Testing

• Water-break tests detect only gross organic contamination.

• Copper sulfate tests identify iron presence but are surface-sensitive and easily misinterpreted.

• Salt spray tests require lengthy exposure and are not quantitative.

• 
  

These methods cannot determine:

• When the passivation reaction has reached completion.

• Whether inclusions remain.

• Whether solution chemistry has deteriorated.

• If the oxide formed is sufficiently chromium-rich.

  
  

Spectrophotometric Monitoring: A Quantitative Solution

Spectrophotometric analysis enables:

• Real-time measurement of metal ion concentration.

• Quantification of iron removal rates

• Detection of citrate degradation.

• Verification of passivation endpoint using objective metrics.

• Traceability for quality systems and audits.

Spectrophotometry represents a significant advancement that ASTM A-967 currently does not incorporate, leaving high-reliability industries to develop their own internal controls.

Advanced Passivation Chemistries

Contemporary research demonstrates the advantages of blended systems that combine:

• Citric acid (primary chelating ligand).

• EDTA (enhanced complexation of iron, nickel, chromium, and aluminum).

• Sodium erythorbate or similar reducing agents.

These blends:

• Remove a broader range of inclusions.

• Promote uniform oxide formation.

• Improve resistance to localized corrosion.

• Provide reproducible results across diverse stainless grades.

ASTM A-967 does not reference or acknowledge these modern chemistries, despite their demonstrated superiority in controlled studies and industrial applications.

Metallurgical Considerations Absent From the Standards

Magnetism and Microstructural Integrity

Austenitic stainless steels should exhibit negligible magnetism. Measurable magnetic response indicates the presence of:

• Delta ferrite.

• Strain-induced martensite.

• Autogenous weld transformations.

These phases possess inferior corrosion resistance and can significantly alter passivation behavior. Neither standard provides guidance for assessing magnetism or evaluating microstructural anomalies.

Delta Ferrite Formation and Associated Risks

Delta ferrite may form during:

• Casting.

• Welding.

• High-temperature thermal cycles.

Its persistence correlates with:

• Increased pitting susceptibility.

• Heterogeneous oxide formation.

• Reduced chromium availability in adjacent regions.

Its role in passivation efficacy is rarely acknowledged in existing practice, despite longstanding metallurgical evidence.

Conclusions and Recommendations

A thorough evaluation of QQ-P-35 and ASTM A-967 reveals systemic gaps that hinder consistent, high-performance passivation of 300 series stainless steels. To address these deficiencies, the following measures are recommended:

• Define precise concentration, pH, temperature, and contact-time parameters for citric acid-based systems.

• Introduce quantitative validation methods, especially spectrophotometric monitoring.

• Expand inclusion removal guidance beyond sulfide inclusions to modern metallurgical realities.

• Recognize advanced blended chemistries and their superior performance.

• Address magnetism, delta ferrite, and microstructural deviations as factors influencing corrosion resistance.

• Provide specific guidance for pH adjustment, including the explicit advantages of ammonia over sodium hydroxide.

  
  

Updating ASTM A-967 to incorporate these elements would substantially improve the reliability, consistency, and scientific foundation of stainless steel passivation practices.