1. Introduction

In modern industry, the reliability of technological equipment is critical for ensuring uninterrupted production processes, safety and economic efficiency. Choosing the right materials plays a key role in achieving these goals. Stainless steel, due to its high corrosion resistance and mechanical properties, is an indispensable material in many industries: from food and pharmaceutical to oil and gas and chemical industries.

However, not all stainless steels are created equal. There is a wide range of brands, each of which has a unique chemical composition and, accordingly, specific characteristics. The wrong choice can lead to premature component failure, costly downtime, and potential hazards to the environment and personnel. This article is devoted to an in-depth analysis and comparison of the three most common groups of stainless steels - austenitic grades 304 and 316, as well as duplex steels - in order to provide engineering and technical personnel with comprehensive criteria for informed material selection.

Understanding the fundamental principles, technical specifications, standards and practical aspects of operating these alloys is essential for optimizing equipment resources and minimizing operating costs. The task of the engineer is not just to choose the material, but to ensure its functionality and durability in the given operating conditions, adhering to current quality standards, such as DSTU, EN, ISO.

2. Fundamental Principles

2.1. Mechanism of corrosion resistance

The main property of stainless steel – its resistance to corrosion – is based on the formation of a thin, passive oxide layer on the surface of the metal. This layer, consisting mainly of chromium oxides (Cr₂O₃), is self-healing under the condition of sufficient oxygen access. The minimum chromium content for the formation of a stable passive layer is about 10.5%. Doping with other elements such as nickel (Ni), molybdenum (Mo) and nitrogen (N) improves the stability of this layer and its resistance to various types of corrosion.

2.2. Structural differences

• Austenitic steels (304, 316): Have a face-centered cubic lattice. This gives them high plasticity, viscosity (especially at low temperatures) and excellent weldability. They are non-magnetic in the annealed state.
• Duplex steels: Characterized by a mixed microstructure consisting of approximately 50% ferrite (body-centered cubic lattice) and 50% austenite. This biphasic structure combines the advantages of both phases: the strength of ferrite and the corrosion resistance and ductility of austenite. Duplex steels are magnetic.

2.3. Influence of alloying elements

• Chromium (Cr): The main element that provides corrosion resistance. Increases resistance to oxidation.
• Nickel (Ni): An austenite-forming element that stabilizes the austenite structure, increases ductility, toughness, and resistance to stress corrosion cracking (SCC) in some environments.
• Molybdenum (Mo): Significantly increases resistance to pitting and crevice corrosion, especially in chloride-containing environments. Also improves resistance to reducing acids.
• Nitrogen (N): Increases strength, stabilizes austenite, and, like molybdenum, increases resistance to pitting and crevice corrosion.

3. Technical Specifications and Standards

The choice of stainless steel should always be based on approved international and national standards, which guarantees the compliance of the materials with the declared characteristics.

3.1. Chemical composition (according to EN 10088-1 / DSTU EN 10088-1)

3.2. Mechanical properties (for EN 10088-2/3)

Note: The values are given for the annealed condition for rolled sheet.

3.3. PREN indicator (Pitting Resistance Equivalent Number)

PREN is a quantitative assessment of the resistance of stainless steel to pitting corrosion in chloride-containing environments. A high PREN value indicates better resistance. Calculation formula: PREN = %Cr + 3.3 × %Mo + 16 × %N.

• 304: PREN 18–20. Low resistance, not recommended for sea water.
• 316: PREN 23–26. Medium resistance, suitable for industrial areas and moderately aggressive environments.
• Duplex 2205: PREN 31–36. High resistance, designed for aggressive chloride-containing environments.

3.4. Applicable standards

• EN 10088 (DSTU EN 10088): A series of standards covering stainless steels, their chemical composition, mechanical properties and delivery conditions.
• ISO 15510: Stainless steels – Chemical composition.
• ASTM A240/A240M: Standard Specification for Sheet, Plate, and Strip of Chromium and Chromium-Nickel Stainless Steels for Pressure Vessels and General Applications.
• ISO 15156 / NACE MR0175: Material requirements for use in the oil and gas industry, particularly for environments containing hydrogen sulphide (H₂S), where duplex steels are often the optimal choice.

4. Selection and Size Guide

The correct choice of stainless steel grade requires careful analysis of operating conditions, including temperature, chemical composition of the environment, mechanical loads and economic factors. Below are the selection criteria and a comparison table.

4.1. Selection criteria

1. Corrosive environment:
   • Chlorides: In the presence of chlorides (>200 ppm), 304 becomes prone to pitting and crevice corrosion. 316 is much better, but for high chloride concentrations and temperatures (>500 ppm, >60°C) or at risk of SCC, duplex steels are required.
   • Acids: 304 is resistant to nitric and some organic acids. 316, due to molybdenum, has increased resistance to sulfuric, phosphoric and acetic acids. Duplex steels show high resistance in a wide range of acidic environments.
   • Alkalis: All brands are resistant to cold alkalis. 316 and especially duplex steels have better resistance in hot and concentrated alkaline solutions.
2. Operating temperature:
   • High temperatures: 304 and 316 have scale resistance up to 870°C and 925°C, respectively. However, prolonged operation in the range of 450-860°C can lead to sensitization (precipitation of carbides), which reduces corrosion resistance, especially for non-L grades. Duplex steels have a limited operating temperature range (usually from -50°C to +280°C) due to the risk of brittle phases.
   • Low/cryogenic temperatures: Austenitic steels (304, 316) retain high ductility and toughness down to cryogenic temperatures (-196°C). Duplex steels become brittle at temperatures below -50°C.
3. Mechanical properties: If high strength and stiffness are required, duplex steels are a better choice because their yield strength is 1.5-2 times higher than 304/316. This allows you to reduce the thickness of the walls of structures, saving weight and material.
4. Cost: 304 is the cheapest of the grades considered, 316 is 40-50% more expensive, and duplex steels are 60-100% more expensive than 304 per kilogram. However, when calculating the full life cycle cost or when taking into account the possibility of reducing material intensity due to high strength, duplex steels can be economically more profitable.

4.2. Material selection matrix

5. Installation and Commissioning Rules

Even the most correctly selected material can undergo premature destruction due to non-compliance with the installation and commissioning technology. The following aspects are critically important for stainless steels:

1. Surface Cleanliness: Before and during installation, contact of stainless steel with carbon steel, copper or other metals that may cause surface contamination and subsequent contact corrosion must be avoided. Use only tools designed for stainless steel.
2. Welding:
   • Protection against oxidation: Welding of austenitic and duplex steels should be carried out in protective atmospheres (argon, gas mixtures) using weld root protection (gas forming). This prevents the formation of scale on the reverse side of the weld, which is a potential site for corrosion initiation.
   • Choice of additive material: For 304 and 316, it is recommended to use low carbon additives (308L, 316L, respectively) to minimize the risk of intergranular corrosion. For duplex steels, filler materials often have an increased nickel content to ensure optimal phase balance (eg 2209).
   • Heat investment: For duplex steels, the heat investment during welding should be controlled (usually 0.5-2.5 kJ/mm) to maintain the optimal ratio of ferrite and austenite.
3. Passivation and cleaning: After welding or machining, the stainless steel surface may lose the passivation layer or become contaminated with iron. Chemical passivation (eg, nitric acid solutions per ASTM A380/A967) and/or etching to remove scale is required to restore corrosion resistance.
4. Avoiding galvanic corrosion: When joining stainless steel with other metals (eg, copper, carbon steel), electrical insulation should be used or materials with close electrochemical potential should be selected.
5. Leakage test: After installation, all systems should be tested for tightness according to the company's internal standards and relevant DSTU/EN norms.

6. Failures and Root Cause Analysis

Despite its high resistance, stainless steel can fail. Understanding typical failure mechanisms and their visual indicators is essential for prompt diagnosis and troubleshooting.

1. Pitting corrosion:
   • Cause: Local destruction of the passive layer in the presence of aggressive ions (mainly chlorides) and oxidants.
   • Visual signs: Small, punctate indentations (ulcers) on the surface, often black or dark brown in color, sometimes with rusty streaks. They can be from micrometers to millimeters in size.
2. Crevice corrosion:
   • Cause: Occurs in limited spaces (cracks) where access to oxygen is impeded, which leads to a local change in the chemical composition of the environment and destruction of the passive layer.
   • Visual signs: Corrosion is concentrated inside or directly near the gap (for example, under gaskets, bolts, in places of connections). It is often accompanied by rusty discharge.
3. Stress Corrosion Cracking (SCC):
   • Cause: Simultaneous action of tensile stresses, aggressive environment (usually chlorides) and elevated temperature. Austenitic steels are very sensitive to SCC.
   • Visual signs: Thin, branched cracks, perpendicular to the direction of applied stress. They can be very difficult to see with the naked eye.
4. Intergranular corrosion:
   • Cause: Precipitation of chromium carbides along grain boundaries (sensitization) during heating in the range of 450-860°C (for example, during welding), which leads to chromium depletion in these areas.
   • Visual signs: "Swelling" of the metal, loss of luster, the appearance of a fine network of cracks or graininess, especially in the heat-affected zone of welds.
5. Erosive corrosion:
   • Cause: Joint action of corrosion and mechanical erosion (friction, cavitation) by the flow of liquid or particles.
   • Visual signs: Thinning of walls, formation of grooves or dimples in the direction of flow, polished surface in areas of intense abrasion.

7. Predictive Maintenance and Condition Monitoring

The use of predictive maintenance methods allows you to identify potential problems at an early stage, preventing emergency failures and optimizing repair schedules.

1. Visual control (VT): Regular inspection of surfaces for signs of corrosion, cracks, deformations or other anomalies. Use of endoscopes for internal surfaces of pipelines and containers.
2. Non-destructive testing (NDT):
   • Capillary testing (PT/LPI): Effective for detecting surface microcracks and defects invisible to the naked eye. It is used to inspect welds and areas with a high risk of SCC.
   • Ultrasonic inspection (UT): Use of ultrasonic flaw detectors to detect internal defects (cracks, pores) and control wall thickness. Austenitic steels require special low frequency sensors to reduce scattering.
   • Radiographic control (RT): It is used for careful quality control of welds, detection of internal defects, such as lack of welds, pores, slag inclusions.
   • Eddy current inspection (ET): It is used to detect surface and near-surface defects in pipes and thin sheet structures.
3. Corrosion monitoring:
   • Corrosion coupons: Installation of material samples (coupons) in the process environment for periodic measurement of corrosion rate.
   • Electrochemical methods: Using sensors to measure corrosion potential or corrosion rate in real time.
4. Analysis of the chemical composition of the medium: Regular monitoring of the chloride content, pH, temperature and other parameters of the technological environment that affect the corrosion rate. For example, increasing the chloride concentration in the water to 500 ppm may require a change from 304 to 316.
5. Thermography: It is used to detect abnormal temperature regimes that may indicate overheating, clogging or other problems.

8. Comparative Matrix of Marks

The following table provides a summary comparison of the main characteristics of stainless steel grades 304, 316 and Duplex 2205, which facilitates selection for specific applications.

9. Conclusions

Choosing the optimal grade of stainless steel for industrial components is a multifactorial process that requires deep engineering knowledge and a systematic approach. AISI 304 and 316 austenitic steels are versatile solutions for a wide range of applications where basic or enhanced corrosion resistance is required in moderately aggressive environments. However, in cases where components are exposed to high chloride concentrations, high mechanical loads or the risk of stress corrosion cracking, duplex steels such as Duplex 2205 become an indispensable solution. Their unique biphasic structure provides a combination of high strength and exceptional resistance to specific types of corrosion.

To ensure the durability and reliability of industrial systems, not only the correct choice of material is critical, but also strict adherence to assembly, welding technologies and the use of effective predictive maintenance methods. UNITEC-D GmbH, as a reliable supplier of high-quality industrial components, offers a wide range of products from all the mentioned brands of stainless steel, certified according to international CE and UkrSEPRO standards, which guarantees their compliance with the highest requirements of the Ukrainian and European markets.

Consult the UNITEC-D e-catalog at https://www.unitecd.com/e-catalog/ for a complete range of stainless steel components and professional advice.

10. Links

1. DSTU EN 10088-1:2018 (EN 10088-1:2014, IDT): Stainless steels. Part 1. List of stainless steels.
2. DSTU EN 10088-2:2018 (EN 10088-2:2014, IDT): Stainless steels. Part 2. Technical conditions of supply of sheets and strips of general purpose.
3. DSTU EN 10088-3:2018 (EN 10088-3:2014, IDT): Stainless steels. Part 3. Technical conditions of supply of semi-finished products, bars, wire rod and profiles of general purpose.
4. ISO 15156-3:2015: Petroleum, petrochemical and natural gas industries — Materials for use in H₂S-containing environments in oil and gas production — Part 3: Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys.
5. ASTM A380/A380M-17: Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems.

Introduction

The reliability and durability of industrial equipment directly depend on the correct choice of materials for its components. In the conditions of aggressive environments, high temperatures and significant mechanical loads, which are characteristic of the Ukrainian industry, stainless steels are a critically important solution. Their corrosion resistance and mechanical properties keep production lines running smoothly, minimizing downtime and repair costs.

Among the wide range of stainless steels, grades 304, 316 and duplex alloys (for example, 2205) are among the most common. Each of them has a unique set of properties that make them optimal for certain applications. Choosing the wrong brand can lead to premature equipment failure, causing significant financial losses and safety hazards. This article is a technical guide for engineers that allows you to make a reasonable approach to the selection of stainless steels, increasing the reliability and operational resource of industrial systems.

UNITEC-D GmbH, as a reliable supplier of high-quality industrial components, understands the importance of accurate selection of materials and provides products that meet the highest quality standards and requirements of Ukrainian and international regulations.

Fundamental Principles

Stainless steels are iron alloys with a minimum chromium content of 10.5%. Chromium forms a passive oxide layer on the surface of the material, which provides basic corrosion resistance. The addition of other alloying elements, such as nickel, molybdenum, nitrogen, allows changing the microstructure and improving the specific properties of steel.

Austenitic Stainless Steels (304, 316)

• Microstructure: The main component is austenite - a face-centered cubic lattice, which gives steel high plasticity, viscosity and ability to cold deformation. Austenitic steels are non-magnetic in the annealed state.
• Doping:
  • Chrome (Cr): Provides corrosion resistance due to the formation of a passive layer.
  • Nickel (Ni): Stabilizes the austenite structure at room temperature, improves plasticity and weldability.
  • Molybdenum (Mo) (for 316): Significantly increases resistance to pitting and crevice corrosion, especially in chloride-containing environments.
• Advantages: Excellent corrosion resistance in a wide range of environments, good formability, high strength at low temperatures.

Duplex Stainless Steels (2205)

• Microstructure: Combination of approximately equal parts of austenite and ferrite (two-phase structure). This combination provides the properties of both phases.
• Alloying: High content of chromium (Cr), molybdenum (Mo) and nitrogen (N) with moderate content of nickel (Ni).
  • Chromium (Cr) and Molybdenum (Mo): Provide high resistance to pitting and crevice corrosion.
  • Nitrogen (N): Increases strength, improves resistance to pitting corrosion and stabilizes austenite.
• Benefits: Significantly higher strength (approximately twice that of austenitic steels), excellent resistance to stress corrosion cracking (SCC) and high resistance to pitting and crevice corrosion.

Technical Specifications and Standards

The choice of stainless steel is governed by national and international standards that set requirements for chemical composition, mechanical properties, heat treatment and delivery conditions.

Basic Standards

• EN 10088 (Series): European Standard for Stainless Steels including:
  • EN 10088-1: List of stainless steels.
  • EN 10088-2: Technical conditions of supply of rolled steel and strips.
  • EN 10088-3: Technical conditions of supply of bars, wire and profiles.
• ISO 15510: Standard defining the chemical composition of stainless steels.
• ASTM A240/A240M: American Standard for Chromium and Chromium-Nickel Stainless Steel Sheets, Plates, and Strips for Pressure Vessels and General Purposes.
• DSTU (Ukraine): National standards, often harmonized with international ones, such as DSTU EN 10088-X.

Chemical Composition (Typical Values, % wt.)

Mechanical Properties (Typical Values for sheet metal up to 16 mm thick)

The PREN indicator (Pitting Resistance Equivalent Number) is important for assessing resistance to pitting corrosion. It is calculated according to the formula: PREN = %Cr + 3.3 × %Mo + 16 × %N. Typical PREN values: for 304 ~18-20, for 316 ~23-27, for Duplex 2205 ~32-38.

Selection and Size Guide

The choice of stainless steel requires careful analysis of operating conditions. It is necessary to take into account not only the aggressiveness of the environment, but also the temperature regime, mechanical loads, weldability requirements and economic feasibility. Below are the selection criteria and a table that simplifies the decision-making process.

Key Selection Criteria

1. Corrosion Resistance:
   • General corrosion: Resistance to uniform destruction of the surface in acidic or alkaline environments.
   • Pitting corrosion: Local destruction in the presence of chlorides. Evaluated by PREN.
   • Crevice corrosion: Occurs in narrow crevices with limited oxygen access, often in chloride-containing environments.
   • Stress Corrosion Cracking (SCC): Failure under the combined action of tensile stresses and an aggressive environment (often chlorides at elevated temperatures).
2. Mechanical Properties: Yield strength and strength required to withstand working loads (pressure, tension, bending).
3. Operation temperature: Maximum and minimum temperatures, effect on strength and corrosion resistance. For example, austenitic steels are prone to sensitization during prolonged stay in the range of 450-850 °C, which reduces corrosion resistance.
4. Weldability: Ease and quality of obtaining welded joints, the need for pre-heating or post-weld heat treatment.
5. Cost: Economic efficiency of the material for a specific application. Duplex steels have a higher initial cost, but can provide lower life cycle costs due to increased durability.

Material Selection Matrix

The following table offers recommendations for choosing a stainless steel brand depending on the operating conditions. This is a simplified guide and critical applications always require a detailed engineering calculation.

Best Practices for Installation and Commissioning

Correct installation and commissioning are critical to ensuring the design life of stainless steel components.

Welding

• Grade 304 and 316: Generally welds well with most standard methods (TIG, MIG/MAG, MMA). It is important to control heat deposition to avoid sensitization (exclusion of chromium carbides along grain boundaries), especially for thick cross-sections. The use of filler materials with a low carbon content (304L, 316L) or stabilized grades (321, 347) helps to avoid this phenomenon. The welding sequence should minimize stress.
• Duplex 2205: Requires stricter control of welding parameters. It is necessary to maintain an optimal balance of ferrite and austenite in the weld zone, which is achieved by controlling heat deposition and using appropriate filler materials, which usually have a higher nickel content. Preheating is usually not required, but interpass temperature control is mandatory. Post-weld heat treatment (annealing) may be necessary to restore the optimal microstructure, but it is often avoided in production, focusing on the quality of the welding process itself.
• Protection: Always use shielding gas for root seam welding and backside protection to prevent oxidation and formation of harmful oxides.

Surface treatment

The surface of stainless steel plays a key role in its corrosion resistance. After mechanical processing or welding, it is necessary to remove all impurities (iron particles, slag, scale) and restore the passive layer.

• Etching: Removal of scale and iron inclusions using acid solutions (for example, a mixture of nitric and hydrofluoric acids).
• Passivation: Restoration of the passive layer using nitric acid or other oxidizing solutions. The process can be performed chemically or electrochemically. Complies with EN 25177.
• Mechanical polishing: Reduces surface roughness, which reduces the risk of crevice corrosion and improves hygienic properties.

Pollution Prevention

Contamination of the stainless steel surface with carbon steel, copper or other metals during installation can be a source of localized corrosion. Use separate tools and equipment for working with stainless steel.

Failure Modes and Root Cause Analysis

Understanding the typical failure modes of stainless steel allows engineers to identify problems early, perform root cause analysis, and develop preventative measures.

1. Pitting Corrosion

• Mechanism: Localized destruction of the passive layer, which leads to the formation of small but deep cavities (pitting). It often occurs in the presence of chloride ions (Cl^-) with sufficient oxidizing potential.
• Visual Indicators: Small pits on the surface that may be hidden by corrosion products.
• Reasons: High concentration of chlorides (for example, in seawater or process solutions), stagnant zones, surface contamination, insufficient PREN level of steel for a specific environment.
• Prevention: Select steel with higher PREN (eg 316 instead of 304, or duplex), improve fluid circulation, regular surface cleaning.

2. Crevice Corrosion

• Mechanism: A form of local corrosion that occurs in narrow gaps (0.025-0.1 mm) between metal surfaces or between metal and non-metal. In such zones, oxygen differentiation is formed, which leads to acidification and local destruction of the passive layer.
• Visual Indicators: Corrosion damage is located exclusively in gaps (under gaskets, in flange connections, under deposits).
• Reasons: Poor quality welds, poor joint design, presence of deposits, inadequate design that promotes fluid stagnation.
• Prevention: Correct engineering design, use of quality welds, use of sealants that do not absorb liquid, selection of steel with higher resistance to crevice corrosion (e.g. duplex).

3. Stress Corrosion Cracking (SCC)

• Mechanism: Combined action of tensile stresses (residual from welding, working) and a specific aggressive environment (often chlorides at a temperature above 60 °C). It leads to the formation of cracks that propagate transcrystalline or intercrystalline.
• Visual Indicators: Macroscopically, they may be invisible, but under the microscope, branching cracks are revealed.
• Reasons: High tensile stresses, presence of chlorides, elevated temperature. Austenitic steels (304, 316) are particularly prone to SCC.
• Prevention: Reduction of stress levels (heat treatment after welding, if possible), use of materials with resistance to SCC (for example, duplex steels or ferritic stainless steels), control of environmental parameters (reduction of temperature, reduction of chloride concentration).

4. Intercrystalline Corrosion

• Mechanism: Occurs during the separation of chromium carbides along the grain boundaries, which leads to the depletion of chromium in the boundary regions and, accordingly, to a decrease in corrosion resistance. It often occurs during sensitization in the temperature range of 450-850 °C (for example, during welding).
• Visual Indicators: Corrosion along grain boundaries, surface becomes rough, delamination may be observed.
• Reasons: High carbon content in steel, being in the sensitization zone.
• Prevention: Use of low carbon steels (L-grades such as 304L, 316L) or titanium or niobium stabilized grades (321, 347).

Predictive Maintenance and Condition Monitoring

The use of predictive maintenance (PM) and condition monitoring (MC) methods allows detection of the initial stages of degradation of stainless steel components, preventing unexpected failures and optimizing repair schedules. These methods comply with the principles of the ISO 17359. standard

1. Visual Review

• Description: The simplest and most affordable method. Regular visual inspection allows you to detect visible signs of corrosion (stains, discoloration, pitting, cracks), erosion, mechanical damage and deposits.
• Frequency: Depends on the criticality of the component and the aggressiveness of the environment, from daily to yearly.
• Standards: DSTU EN ISO 17637 (non-destructive inspection of welds - visual inspection).

2. Ultrasonic Control (UT)

• Description: It is used to detect internal defects (cracks, voids, delaminations), measure the thickness of component walls (pipes, tanks). A decrease in thickness may indicate uniform corrosion or erosion.
• Application: Corrosion thinning monitoring, weld quality control.
• Standards: EN ISO 16810, DSTU EN ISO 17640 (for welded joints).

3. Eddy Current Control (ECT)

• Description: Effective for detecting surface and subsurface cracks, pitting, changes in microstructure, as well as for sorting materials.
• Application: Inspection of heat exchanger tubes, thin-walled components, detection of SCC cracks.
• Standards: EN ISO 17643.

4. Radiographic Control (RT)

• Description: Uses X-ray or gamma radiation to detect internal defects (cracks, pores, inclusions) in welds and cast parts.
• Application: High-precision control of critical welded joints.
• Standards: EN ISO 17636.

5. Infrared Thermography

• Description: Allows you to detect temperature anomalies that may indicate overheating, uneven heat distribution, or blocked flow that can accelerate corrosion.
• Application: Monitoring of heat exchangers, reactors, pipelines.

6. Corrosion Monitoring

• Description: Direct measurement of corrosion rate using special sensors (corrosion coupons, electrochemical probes).
• Application: Continuous monitoring of aggressive environments.

Comparison matrix

This table provides a comparative overview of the key properties and applications of grades 304, 316 and 2205 duplex steel.

Conclusion

Choosing the right grade of stainless steel is a fundamental decision that directly affects the reliability, safety and cost-effectiveness of industrial equipment. Grades 304, 316 and duplex steels offer a wide range of properties that can be adapted to the most demanding operating conditions in Ukrainian industrial production. Careful analysis of conditions, adherence to standards (DSTU, EN, ISO) and application of best installation and maintenance practices ensure long and trouble-free service life of components.

UNITEC-D GmbH supplies high-quality stainless steel components that comply with CE and UkrSEPRO certificates, ensuring high performance and resistance to aggressive environments. For detailed information on the range of products and professional advice on the selection of materials, we invite you to visit our electronic catalog.

View the full range of stainless steel components: https://www.unitecd.com/e-catalog/

Sources and References

1. EN 10088-1: Stainless steels. Part 1: List of stainless steels.
2. EN 10088-2: Stainless steels. Part 2: Technical conditions for the supply of rolled sheets and strips for general use.
3. ISO 15510: Stainless steels — Classification.
4. ASTM A240/A240M: Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications.
5. ISO 17359: Condition monitoring and diagnostics of machines – General guidelines.

1. Introduction: The engineering challenge and the importance of equipment reliability

In modern industrial production, the choice of materials for critical components is a key factor that determines the reliability, durability and efficiency of systems. Due to their high corrosion resistance, mechanical strength and hygienic properties, stainless steels are indispensable in many industries, including chemical, food, pharmaceutical, oil and gas and energy. However, the variety of stainless steel grades, each with unique properties, requires a deep understanding to make optimal engineering decisions. Incorrect material selection can lead to premature failures, expensive repairs, production downtime and, as a result, significant economic losses.

This article is devoted to a detailed review of three main groups of stainless steels widely used in industry: the austenitic grades AISI 304 (EN 1.4301) and AISI 316 (EN 1.4401), as well as duplex stainless steels, in particular 2205 (EN 1.4462). We will review the fundamental principles of their metallurgy, specifications, standards, selection criteria, and best practices for installation, operation, and monitoring. The goal is to provide engineering and technical personnel with comprehensive reference material to ensure the reliability and safety of industrial equipment.

2. Fundamental principles: Metallurgy of stainless steels

Stainless steels are iron alloys with a minimum chromium content of 10.5%, which ensures the formation of a passive oxide layer (Cr₂O₃) on the surface, which protects the metal from corrosion. Alloying elements such as nickel, molybdenum, manganese and nitrogen are added to improve specific properties.

2.1. Austenitic stainless steels (series 300)

These steels are characterized by a face-centered cubic (fcc) crystal lattice, which gives them excellent ductility, weldability and impact toughness at low temperatures. The main alloying element that stabilizes the austenite structure is nickel. They are non-magnetic in the annealed state.

• AISI 304 (EN 1.4301): Standard "18/8" stainless steel containing approximately 18% chromium and 8% nickel. It provides good corrosion resistance in atmospheric conditions, fresh water, organic and some inorganic acids. It is used for food equipment, architectural elements, fasteners. The yield strength is about 210 MPa, the strength limit is 520 MPa.
• AISI 316 (EN 1.4401): Additionally contains about 2-3% molybdenum, which significantly increases its resistance to pitting and crevice corrosion, especially in chloride environments. Resistance to some acids (sulphuric, phosphoric) also improves. This makes it ideal for marine equipment, chemical industry, pharmaceuticals. Yield strength — about 220 MPa, strength — 530 MPa.

2.2. Duplex stainless steels

The name "duplex" comes from their microstructure, which consists of approximately equal parts ferrite and austenite. This two-phase structure is achieved due to the optimal content of chromium (21-26%), nickel (4.5-7%), molybdenum (2.5-4%) and nitrogen (0.08-0.2%). Duplex steels combine the advantages of both phases:

• Ferrite phase: Provides high strength and resistance to stress corrosion cracking (SCR).
• Austenitic phase: Provides good impact toughness and resistance to pitting corrosion.

As a result, duplex steels have almost twice the yield strength (about 450 MPa) compared to 304/316 austenitic steels, while maintaining excellent corrosion resistance, especially in aggressive chloride environments. The EN 1.4462 grade (UNS S31803 or 2205) is the most common duplex steel. They are used in the oil and gas industry, pulp production, desalination of water.

3. Technical characteristics and standards

The choice of stainless steel is governed by national and international standards that set requirements for chemical composition, mechanical properties, test methods and delivery conditions. In Ukraine, DSTU standards are used, which are often harmonized with European (EN) and international (ISO) standards.

3.1. Standardization

• DSTU EN 10088-1: Determines the list of stainless steels by chemical composition.
• DSTU EN 10088-2: Establishes the technical conditions for the supply of stainless steel sheets and strips for general purposes.
• DSTU EN 10088-3: Defines the technical conditions for the supply of semi-finished products, bars, wire and stainless steel profiles for general purposes.
• ISO 3506 series: Adjusts the mechanical properties of stainless steel fasteners (eg ISO 3506-1 for bolts, screws and studs).
• ASTM A240/A240M: Specification for Sheet, Plate, and Strip of Chromium and Chromium-Nickel Stainless Steels for Pressure Vessels and High-Temperature Applications.

3.2. Chemical composition (typical, in % by mass)

PREN (Pitting Resistance Equivalent Number) is an index predicting the resistance of stainless steel to pitting corrosion. It is calculated according to the formula: PREN = %Cr + 3.3 × %Mo + 16 × %N. Higher PREN values ​​indicate better resistance.

3.3. Mechanical properties (typical, for bars ø16mm)

• AISI 304: Yield strength (Rp0.2) ≥ 210 MPa; Strength limit (Rm) 520-720 MPa; Relative elongation (A) ≥ 45%. Brinell hardness (HB) ≤ 215.
• AISI 316: Yield strength (Rp0.2) ≥ 220 MPa; Strength limit (Rm) 520-720 MPa; Relative elongation (A) ≥ 40%. Brinell hardness (HB) ≤ 215.
• Duplex 2205: Yield strength (Rp0.2) ≥ 450 MPa; Strength limit (Rm) 620-800 MPa; Relative elongation (A) ≥ 25%. Brinell hardness (HB) ≤ 290.

4. Guide to the selection and calculation of sizes

Choosing the optimal brand of stainless steel requires a systematic approach that takes into account operating conditions, mechanical loads, temperature conditions and economic aspects.

4.1. Key selection criteria

1. Type of corrosive environment:
• Atmospheric corrosion, fresh water, foodstuffs: AISI 304 is often sufficient.
• Chloride environments (seawater, swimming pools, some chemicals): AISI 316 or duplex steels are required. Chloride concentration >200 ppm at elevated temperatures requires 316; >1000 ppm or high temperatures – duplex.
• Acid environments: Depends on the type and concentration of the acid. Molybdenum-containing steels (316, duplex) are better for sulfuric and phosphoric acids.
• Stress Corrosion Cracking (SCC): For high chloride and elevated temperature (>50°C) environments where tensile stress occurs, duplex steels are significantly superior to austenitic steels.
2. Mechanical properties:
• High loads, pressure: Duplex steels offer almost twice the strength, which allows you to reduce the thickness of the walls or use components of smaller sizes, while maintaining the necessary load-bearing capacity. This can lead to weight and cost savings.
3. Temperature range:
• Cryogenic temperatures: Austenitic steels retain high impact toughness.
• High temperatures: Austenitic steels can undergo sensitization (separation of chromium carbides along grain boundaries), which reduces corrosion resistance. Low carbon (304L, 316L) or stabilized (321, 347) versions are better. Duplex steels have temperature limitations (usually up to 300°C) due to the brittleness of ferrite.
4. Economic feasibility:
• Initial cost: AISI 304 is the cheapest. AISI 316 is 15-30% more expensive. Duplex steels are the most expensive, but their high strength and durability can compensate for this by reducing material or increasing service life.
• Life cycle: It is not only the initial cost that should be considered, but also the costs of maintenance, repair and replacement throughout the lifetime.

4.2. Decision matrix for choosing stainless steel

5. Best Practices for Installation and Commissioning

Even the right choice of material can be compromised by incorrect installation and commissioning. Adherence to established procedures is critical to ensuring the longevity of stainless components.

5.1. Welding

Welding of stainless steels requires control of the thermal regime to prevent deterioration of corrosion resistance and mechanical properties. Welding procedures must comply with DSTU EN ISO 15607 (Specification and qualification of procedures for welding metallic materials).

• Austenitic steels (304, 316): Overheating can lead to sensitization (separation of chromium carbides along grain boundaries), especially for steels with a high carbon content (>0.03%). This reduces resistance to intercrystalline corrosion. To avoid this, low carbon grades (304L, 316L) or stabilized grades should be used, as well as the time spent in the temperature range of 450-850°C should be minimized.
• Duplex steels (2205): Requires precise control of heat input during welding to maintain an optimal ferrite to austenite ratio (typically 40-60% ferrite). Excessive heat leads to excessive growth of ferrite, which reduces impact toughness. Insufficient heat - to excessive austenite. Special welding materials containing additional alloying elements should be used to compensate for losses.

5.2. Surface treatment and passivation

After machining or welding, the stainless steel surface may become contaminated with iron (e.g. from carbon steel tools) or lose the passive layer. This creates areas prone to corrosion. Passivation is a chemical process that restores the protective oxide layer.

• Cleaning: Removal of fats, dirt, oxides.
• Etching: Removal of scale and heat-affected zone after welding.
• Passivation: Treatment with nitric acid or other oxidizing solutions to form a stable passive layer. The process must comply with ISO 16048 (Passivation of Stainless Steel Fasteners) or ASTM A967 (Standard Specification for Chemical Passivation of Stainless Steel Parts).

5.3. Prevention of galvanic corrosion

Galvanic corrosion can occur when two different metals in an electrolyte (for example, a wet environment) come into contact. Stainless steel, being a more noble metal, can cause accelerated corrosion of less noble metals (for example, carbon steel, aluminum). It is necessary to use insulating gaskets or choose metals that are close in the galvanic series.

6. Types of failures and root cause analysis

Understanding the common failure mechanisms of stainless steel is critical to diagnosing, repairing, and preventing future incidents.

• Pitting corrosion: A local form of corrosion that manifests itself in the form of small point depressions (pitting) on ​​the surface, especially in chloride environments. Visual indicators: small craters, often covered with corrosion products. The root cause: destruction of the passive layer in the presence of chloride ions. A higher PREN provides better stability.
• Crevice corrosion: Similar to pitting, but occurs in closed crevices or under gaskets, where limited access to oxygen prevents repassivation. Visual indicators: corrosion damage in the places of contact of parts. Root cause: local decrease in pH and concentration of chlorides in the crevice.
• Intergranular corrosion (sensitization): Corrosion along grain boundaries, caused by the release of chromium carbides during heating (for example, during welding). Visual indicators: cracks along grain boundaries, flaking surface. Root cause: insufficient chromium content near the grain boundaries. Eliminated by using L-grades or stabilized steels.
• Stress corrosion cracking (SCR): Cracking of a metal under the combined effect of tensile stress and a specific corrosive medium (often chlorides at high temperatures). Visual indicators: thin, branching cracks running through the grains. Root Cause: A combination of stress (residual or applied), temperature, and aggressive environment. Duplex steels have much higher resistance to SFR than austenitic steels.
• Corrosion fatigue: Reduction of material strength under the action of cyclic loads in a corrosive environment. Visual indicators: cracks starting at the surface.

7. Predictive maintenance and condition monitoring

Effective predictive maintenance (PR) and condition monitoring (CM) allow you to identify potential problems at an early stage, prevent unexpected failures and optimize service times.

7.1. Monitoring methods

• Visual inspection: Regular inspection of surfaces for signs of corrosion (pitting, cracks, discoloration, cracks). Use endoscopes for hard-to-reach places.
• Non-destructive testing (ND):
• Ultrasonic control (UZK): Detection of internal defects, cracks, changes in wall thickness.
• Eddy current inspection (EDC): Detection of surface and subsurface defects such as fatigue cracks or pitting corrosion.
• X-ray control: Detection of internal defects of welds and castings.
• Corrosion monitoring:
• Corrosion Coupons: Metal samples placed in a system to measure the rate of mass loss (according to ISO 17646).
• Electrochemical monitoring: Measurement of potential or current to assess the activity of corrosion processes (for example, linear polarization resistance - LPR).
• Monitoring of pH and chloride concentration: Control of key environmental parameters affecting corrosion.
• Vibration analysis: For rotating equipment, detecting imbalances or bearing faults that can create additional stresses and accelerate corrosion fatigue.

8. Comparison matrix of stainless steel brands

To make an informed decision, it is important to compare the key properties of different brands. Below is a comparison chart for brands 304, 316, 2205 (duplex) and 2507 (super duplex).

9. Conclusion

The optimal choice of stainless steel grade is fundamental to ensure long-term and reliable operation of industrial equipment. Understanding specific operating conditions – type of corrosive environment, temperature regime, mechanical loads – combined with a detailed assessment of material characteristics allows engineers to make informed decisions.

AISI 304 and 316 grades are universal solutions for many standard applications. However, in aggressive environments, especially those with high chloride content and significant mechanical loads, duplex and super duplex steels offer unmatched resistance and strength, allowing for significant savings in the life cycle of the equipment. UNITEC-D GmbH is a reliable supplier of industrial components made of high-quality stainless steels that meet all international and national standards.

For more information and selection of required components, please visit our electronic UNITEC-D catalog.

10. Links

• DSTU EN 10088-1: Stainless steels. Part 1. List of stainless steels (EN 10088-1:2014, IDT). Kyiv, 2018.
• ISO 3506-1:2009. Mechanical properties of fasteners made of stainless steel - Part 1: Bolts, screws and studs. Geneva, 2009.
• ASTM A967/A967M-17. Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts. West Conshohocken, 2017.
• ISO 15607:2019. Specification and qualification of welding procedures for metallic materials – General rules. Geneva, 2019.
• Sandvik Materials Technology. Stainless Steel Handbook. Sandviken, Sweden. [White paper].