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Understanding API Publ 939 (1994): Evaluating the Effects of Operating Parameters on Sulfur Compound Corrosivity
A Comprehensive Guide to the API Publication on Sulfur Corrosion in Refining Operations
Scope and Background
API Publ 939 (1994), officially titled Evaluation of the Effects of Operating Parameters on the Corrosivity of Sulfur Compounds, is a foundational publication from the American Petroleum Institute (API) that addresses high-temperature sulfur corrosion in petroleum refining processes. This publication focuses on fluid catalytic cracking units (FCCUs) and related equipment where sulfur-bearing streams are processed at elevated temperatures. It synthesizes experimental data and field observations to establish how key process variables influence the severity of sulfidation (high-temperature sulfur corrosion) in carbon steel and low-alloy steels.
The scope of API Publ 939 covers temperature ranges from approximately 250°C to 450°C (482°F to 842°F), sulfur contents typical of refinery streams (0.5–3 wt% total sulfur), and the presence of hydrogen sulfide (H2S) and organic sulfides. It provides empirical correlations and recommended practices for predicting corrosion rates based on operating conditions, enabling engineers to select appropriate materials and implement effective monitoring strategies. Although not a mandatory standard, this publication is widely used as a reference in refinery corrosion management programs and risk-based inspection (RBI) assessments.
Technical Requirements and Key Findings
API Publ 939 details the influence of several operating parameters on the corrosivity of sulfur compounds. The key parameters identified include temperature, sulfur concentration, H2S content, process stream velocity, and material composition. The publication presents corrosion rate data derived from both laboratory tests and plant experience, typically expressed in mils per year (mpy) or millimeters per year (mm/y).
Influence of Temperature and Sulfur Content
Temperature is the most critical variable; general corrosion rates increase significantly above 260°C (500°F) and peak around 350–370°C (662–698°F) in typical sulfidation environments. At higher temperatures (above 400°C), scale formation may become protective depending on alloy composition. Total sulfur content alone is not sufficient to predict corrosivity—the type of sulfur compound (e.g., H2S, mercaptans, sulfides) and its decomposition behavior are also essential.
Material Performance Data
The publication includes comparative corrosion data for carbon steel (C.S.), 5Cr-0.5Mo, 9Cr-1Mo, and Type 304/316 stainless steels. Higher chromium content generally improves resistance to sulfidation, but effects can vary with temperature and gas composition. The table below summarizes typical corrosion rate trends from API Publ 939 for selected materials in an FCCU environment.
| Material | Temperature Range (°C) | Total Sulfur (wt%) | Typical Corrosion Rate (mpy) | Remarks |
|---|---|---|---|---|
| Carbon Steel (C.S.) | 260–370 | 1–2 | 20–80 | High variability; not recommended above 315°C for prolonged service |
| 5Cr-0.5Mo (1.25Cr-0.5Mo tested) | 315–400 | 1–2 | 10–40 | Improved resistance; but may pit under high H2S conditions |
| 9Cr-1Mo | 315–425 | 1–2 | 5–15 | Commonly used for heater tubes and transfer lines |
| Type 304 SS | 370–450 | 1–3 | 1–5 | Excellent resistance; requires careful welding and stress relief |
Tip: When using API Publ 939 corrosion rate predictions, always apply a safety margin of at least 20% to account for variability in actual plant conditions, especially under transient operating states.
Flow Velocity and Geometry Effects
The publication also highlights the role of turbulence and high-velocity flow in accelerating corrosion, particularly at bends, tees, and orifice plates. Erosion-corrosion synergies become significant when velocities exceed 20 m/s (65 ft/s) in vapor-liquid mixtures. API Publ 939 provides velocity limits and recommends the use of chromium-containing alloys or coatings in regions susceptible to impingement.
Implementation Highlights and Practical Guidelines
Implementation of API Publ 939 findings involves integrating its corrosion rate models into a refinery’s corrosion control document (CCD) and equipment fitness-for-service evaluations. Key implementation steps include:
- Data Collection: Gather actual operating temperature, pressure, and process stream composition data for each unit. The publication provides baseline corrosion rates but advises location-specific adjustments.
- Material Selection: For new units, economic trade-offs between carbon steel with monitoring and higher-alloy materials (e.g., 9Cr-1Mo) can be evaluated using the corrosion curves in the publication.
- Inspection Planning: Use the predicted corrosion rates to set initial ultrasonic thickness (UT) inspection intervals, typically every 3–6 years for moderate corrosion environments.
- Modification Control: If a unit is converted to process higher-sulfur feedstocks or if operating temperatures are increased, API Publ 939 can be used to reassess the expected damage and upgrade materials accordingly.
Warning: Do not apply API Publ 939 data directly to non-FCCU environments without verifying applicability. The corrosion behavior in hydrogen service (e.g., hydroprocessing) or wet sour service differs fundamentally from the high-temperature sulfidation addressed by this publication.
Compliance and Regulatory Considerations
API Publ 939 is a technical publication, not a code or standard that mandates compliance per se. However, its recommendations are often incorporated into mandatory documents through references in jurisdictional regulations and industry codes such as API 510 (Pressure Vessel Inspection Code), API 570 (Piping Inspection Code), and API RP 581 (Risk-Based Inspection Methodology). Regulators and third-party inspection agencies may require that operators demonstrate use of recognized and generally accepted good engineering practices (RAGAGEP) for corrosion management; API Publ 939 qualifies as such a reference.
Good Practice: To demonstrate compliance, document the rationale for material selection and inspection intervals based on API Publ 939 corrosion rate projections. Include the publication revision (1994) and any site-specific adjustments in the corrosion control document.
Important: While API Publ 939 provides valuable guidance, it should not replace site-specific condition monitoring. Unexpected corrosion mechanisms (e.g., naphthenic acid corrosion, creep) may co-exist, and the publication does not cover those. Always combine its use with industry standards such as NACE SP0775 for comprehensive corrosion management.
As of 2026, API Publ 939 (1994) remains a foundational reference for refinery corrosion engineers. Despite its age, the core corrosion data and parametric trends are still considered reliable for typical FCCU sulfidation scenarios. Recent updates to related API publications (e.g., API 939-C for sour water stripping, API 939-E for fouling) complement but do not obsolete the 1994 edition.
Frequently Asked Questions
Q: Is API Publ 939 a standard or a mandatory regulation?
A: It is a technical publication providing recommended practices and data. It is not a mandatory code, but it is frequently cited as a recognized good engineering practice in refinery corrosion management programs and by regulatory bodies when establishing compliance with API inspection codes.
A: It is a technical publication providing recommended practices and data. It is not a mandatory code, but it is frequently cited as a recognized good engineering practice in refinery corrosion management programs and by regulatory bodies when establishing compliance with API inspection codes.
Q: Can API Publ 939 be used for predicting corrosion in hydroprocessing units?
A: No. The publication specifically addresses high-temperature sulfidation in FCCU environments. Hydroprocessing units involve hydrogen and H2S at elevated pressures, which lead to different corrosion mechanisms (e.g., hydrogen attack, hydrogen blistering, and high-temperature hydrogen attack) that require separate references (e.g., API RP 941).
A: No. The publication specifically addresses high-temperature sulfidation in FCCU environments. Hydroprocessing units involve hydrogen and H2S at elevated pressures, which lead to different corrosion mechanisms (e.g., hydrogen attack, hydrogen blistering, and high-temperature hydrogen attack) that require separate references (e.g., API RP 941).
Q: How should I adjust the corrosion rates for my specific unit if conditions differ from the publication’s examples?
A: API Publ 939 provides scaling factors for temperature and sulfur content, and encourages the use of linearization between data points for intermediate conditions. For best accuracy, correlate with at least two non-intrusive thickness measurements taken over a known service interval to derive a site-specific rate.
A: API Publ 939 provides scaling factors for temperature and sulfur content, and encourages the use of linearization between data points for intermediate conditions. For best accuracy, correlate with at least two non-intrusive thickness measurements taken over a known service interval to derive a site-specific rate.
Q: Are there any newer publications that replace API Publ 939?
A: No direct replacement exists. API has issued related publications (e.g., API 939-C, 939-E) that cover adjacent topics, but the 1994 edition of API Publ 939 remains the definitive source for FCCU sulfidation predictions. Some companies use proprietary models based on API data, but the original publication is still widely referenced in industry standards.
A: No direct replacement exists. API has issued related publications (e.g., API 939-C, 939-E) that cover adjacent topics, but the 1994 edition of API Publ 939 remains the definitive source for FCCU sulfidation predictions. Some companies use proprietary models based on API data, but the original publication is still widely referenced in industry standards.
Last updated: 2026. This article is for informational purposes and does not substitute for direct application of API Publ 939 (1994) by qualified professionals.