1.4404

Definition and properties of acid-resistant steels

Steels referred to as “acid-resistant” are a group of iron alloys enriched with elements that form a durable, passive oxide layer on the material surface. This layer – most commonly chromium oxide – inhibits further oxidation and protects the metal core from aggressive chemical environments. In practice the term “acid-resistant” denotes resistance to corrosion in a wide range of aggressive media, including acids, chloride solutions and many industrial reagents.

Key features of acid-resistant steels:

• high resistance to chemical and atmospheric corrosion,
• ability to self-heal the passive layer after damage,
• good ductility and machinability in the annealed condition,
• varied resistance to specific forms of corrosion (pitting, crevice, stress-corrosion cracking).

Characteristics of austenitic stainless steels

Austenitic steels are a type of stainless steel whose crystal structure at room temperature is austenitic (a regular face-centred cubic – FCC – lattice). Austenite is stabilised primarily by nickel and additions of nitrogen. Characteristic properties of these alloys include:

• excellent ductility and toughness even at low temperatures,
• non-ferromagnetic in the annealed state,
• tendency to undergo plastic deformation (they behave as ductile materials) and to work-harden during cold forming,
• good weldability and compatibility with most joining processes.

Austenitic stainless steels – represented by grades such as 1.4404 (AISI 316L) – are versatile in applications requiring both chemical resistance and favourable mechanical properties.

The importance of chromium, nickel and molybdenum in steels

Three elements – chromium, nickel and molybdenum – determine the functionality of austenitic steels:

• Chromium (Cr): the fundamental element enabling the formation of the passive oxide layer. A Cr content ≥ 10.5% is the condition for inclusion in the “stainless” group; typical grades contain 16–18% Cr, which provides good resistance in oxidising and atmospheric environments.

• Nickel (Ni): stabilises the austenitic structure, increases ductility, impact resistance and eases forming. Higher Ni contents allow austenite to be retained over a wide temperature range and reduce brittleness.

• Molybdenum (Mo): important for resistance to corrosion in chloride-containing environments. Molybdenum significantly improves resistance to pitting and crevice corrosion. The presence of Mo makes 316/1.4404 steels a better choice than 304/1.4301 where halides occur.

These three elements interact like orchestral instruments: chromium provides the foundation, nickel gives flexibility, and molybdenum adds “resilience” in aggressive conditions.

The EN system and number 1.4404

The number 1.4404 comes from the European standard EN 10088-1/2, which catalogues stainless steels. This designation is used in Europe and describes a specific steel grade with defined chemical and mechanical properties. In practice “1.4404” is the equivalent of the commercial “316L” in the AISI/ASTM system, with additional requirements related to quality control processes and approvals under European standards.

Interpretation of the symbol X2CrNiMo17-12-2

The symbol X2CrNiMo17-12-2 is the chemical nomenclature used in the EN standard. It consists of elements:

• X – high-alloy steel (indicates an alloyed composition),
• 2 – maximum carbon content ≈ 0.02% (indicates the low-carbon version),
• Cr 17 – approximately 17% chromium,
• Ni 12 – approximately 12% nickel,
• Mo 2 – approximately 2% molybdenum.

In other words, X2CrNiMo17-12-2 describes a steel with low carbon content and characteristic amounts of Cr, Ni and Mo, resulting in good corrosion resistance while retaining an austenitic structure.

AISI standards and their meaning – 316L and 316LS

AISI 316L is the American designation corresponding to the European 1.4404. The letter “L” denotes low carbon, which reduces the risk of chromium carbide precipitation during heat treatment and welding, thereby limiting intergranular corrosion. The designation 316LS is less common and usually indicates a variant with reduced sulphur content (Low Sulfur) or another special composition modification used where improved ductility, cold-rolling properties, or reduced contamination are important in hygienic applications. The “LS” approach is typically associated with the requirements of specific industries, e.g. pharmaceutical or food.

Alloying elements and their percentage ranges

Typical chemical composition of 1.4404 / X2CrNiMo17-12-2 (approximate values):

• C (carbon): ≤ 0.02% (low-carbon version),
• Cr (chromium): 16.5–18.5%,
• Ni (nickel): 10–13%,
• Mo (molybdenum): 2–2.5%,
• Mn (manganese): ≤ 2%,
• Si (silicon): ≤ 1%,
• P (phosphorus): ≤ 0.045%,
• S (sulphur): ≤ 0.015% (LS variants have lower sulphur content),
• N (nitrogen): ≤ 0.11% (sometimes lower for specific specifications).

The AISI 316L version corresponds to this distribution with emphasis on low carbon content, which protects against carbide precipitation during welding and annealing. Special variants, e.g. 316LS, may have additional limits on sulphur or other impurities.

The role of nickel, chromium and molybdenum in corrosion resistance

• Chromium is responsible for forming and regenerating the passive oxide layer. Its presence is essential for classic “stainless” behaviour.
• Nickel stabilises the austenitic structure, which indirectly affects mechanical resistance, formability and low-temperature performance.
• Molybdenum increases resistance to corrosion caused by chloride ions and reduces susceptibility to pitting and crevice corrosion.

Addition of nitrogen also improves pitting resistance and increases strength, often at relatively low cost to altering other alloy parameters.

Austenitic microstructure: construction and characteristic features

The austenitic microstructure is characterised by an ordered atomic lattice in a face-centred cubic (FCC) arrangement. In practice this means:

• a high capacity for plastic deformation without fracture,
• absence of martensitic transformation during cooling, which makes plastic deformations stable,
• tendency to plastically deform and to surface harden as a result of cold working.

An austenitic matrix, in the presence of dispersed carbides or other inclusions, can lead to local deterioration of resistance; therefore control of composition and heat treatment processes is important.

Tensile strength and ductility

316L steels offer a good compromise between strength and ductility. Typical mechanical values for the annealed condition (approximate):

• yield strength R0.2: typically 170–300 MPa (dependent on grade and processing),
• tensile strength Rm: about 480–650 MPa,
• elongation at break A: usually 40% or more for rolled plates and bars.

Thanks to the low carbon content, 316L retains good ductility after welding, which is important in welded structures and piping.

Resistance to high temperatures

Austenitic 316L steels retain strength at moderately high temperatures and delay brittle transitions, but limits must be respected:

• suitable for continuous service at temperatures up to about 400–600°C depending on conditions – above this range the risk of chromium carbide precipitation and loss of intergranular resistance increases unless appropriate heat treatments are used;
• the melting point of the alloy is generally in the range 1375–1400°C;
• solution annealing is used to stabilise and restore resistance after welding at approximately 1040–1150°C, followed by rapid cooling.

Thermal conductivity and linear expansion

Austenitic steels have relatively low thermal conductivity and moderate thermal expansion:

• thermal conductivity is lower than for copper or aluminium, typically on the order of a few tens of W/(m·K),
• the coefficient of linear thermal expansion for 316L is approximately 15–17 × 10^−6 K^−1 in the service temperature range.

These parameters affect the design of heat exchangers and components exposed to cyclic temperature variations, where thermal stresses and expansion must be accounted for.

Reactions to acids and oxidising agents

316L steel shows broad resistance to oxidising acids and many diluted non-oxidising acids. For example:

• it performs well with diluted sulphuric acid and nitric acid (especially at low concentrations),
• it is more resistant than 304 in chloride-containing environments due to the presence of Mo,
• it can be attacked in concentrated sulphuric acids or in the presence of strong reducing agents and halogens.

Each chemical application requires analysis of the specific environment composition, temperature and concentration, as these factors determine corrosion mechanisms.

Resistance to intergranular and stress corrosion

• Intergranular corrosion: High carbon content in austenitic steels leads to chromium carbide precipitation along grain boundaries during heating in the 400–850°C range, which weakens the passive layer and causes local corrosion. “L” versions (316L, 1.4404) with limited carbon minimise this phenomenon, making them suitable for welded constructions.
• Stress corrosion cracking (SCC): austenitic steels are susceptible to stress corrosion cracking in environments containing chloride ions, particularly under the combined effect of mechanical stresses and elevated temperatures. In such conditions it may be necessary to use more resistant grades (duplex, super-austenitic), control chloride concentration, reduce operating temperature or use inhibitors.

Use of molybdenum to protect against crevice corrosion

Molybdenum is key to protection against pitting and crevice corrosion; its influence is assessed, among others, by the PREN (Pitting Resistance Equivalent Number):

• PREN = %Cr + 3.3 × %Mo + 16 × %N.

For 316L PREN typically places it in a range providing moderate pitting protection (depending on actual composition), making this steel a good compromise between cost and resistance in chloride environments.

Methods of producing austenitic steels

Production of austenitic steel grades includes several stages:

• melting in electric arc furnaces (EAF) with additions of scrap and alloying elements,
• refining by vacuum degassing (VD), argon-oxygen decarburisation (AOD) processes for controlled removal of carbon and impurities,
• possible casting by continuous casting and hot rolling,
• solution annealing after plastic working to restore a homogeneous structure and remove stresses.

For medical applications and implants vacuum processes or VIM/VAR (vacuum induction melting / vacuum arc remelting) are often used to achieve exceptionally low impurity levels and a uniform microstructure.

Principles of heat treatment and its effect on structure

Heat treatment is crucial:

• Solution annealing in the 1040–1150°C range dissolves carbides and restores a homogeneous austenitic structure. Rapid cooling (e.g. water quench) prevents re-precipitation of carbides.
• Stabilisation (e.g. addition of Ti or Nb) can protect the steel against carbon precipitates and is used in some specifications.
• Improper annealing, too slow cooling or residence in the 400–850°C range can lead to sensitisation and deterioration of corrosion resistance.

Welding and cutting – specifics and good practice

Welding 316L is common and relatively straightforward due to the low carbon content; however, there are good practices:

• Welding techniques: TIG (GTAW), MIG (GMAW) and stick electrodes are standard. Well-selected parameters and shielding gas minimise oxide inclusions and hardening of the weld bead.
• It is recommended to use filler materials with compatible composition (316L or similar) to avoid local potential differences.
• Post-weld annealing is not always necessary; in most cases the material retains acceptable resistance thanks to the low carbon content, but for critical applications solution annealing is recommended.
• Plasma and laser cutting are common; attention must be paid to edge quality and removal of the oxidised zone, especially where hygienic requirements apply.

Chemical and petrochemical industry

316L/1.4404 steels are widely used in chemical apparatus: pipelines, reaction vessels, heat exchangers and valves. Their advantage lies in resistance to many reagents and good weldability, which facilitates construction of complex pressurised systems. In highly aggressive locations modified variants or more resistant alloys are used, but where a cost/resistance compromise is needed, 316L is a common choice.

Medical and pharmaceutical equipment manufacture

In medicine 316L is the standard for surgical tools, instruments and equipment parts requiring high resistance to disinfectants and sterilisation. For implant applications special variants (316LVM) with stricter metallurgical requirements are usually used. In the pharmaceutical industry 316L surfaces meet hygienic requirements and are easy to clean.

Food industry and food transport

316L steels are used in the manufacture of food processing lines, fermentation tanks, pipelines and food-contact components. Low impurity content and the possibility of passivation make surfaces resistant to deposits and easy to decontaminate, which is key for hygiene and food quality control.

Architecture and construction resistant to atmospheric conditions

Because of aesthetics and resistance to atmospheric corrosion, 316L is used in locations exposed to aggressive environments (e.g. coastal areas), for façades, railings and elements of small architecture. High stain resistance and ease of maintenance make it attractive both visually and functionally.

Differences in chemical composition and properties

• 304 / 1.4301 vs 316L / 1.4404: 304 does not contain molybdenum, so it has poorer pitting resistance in chloride environments. 316L with Mo is better for chemical and marine applications.
• 317L: contains more molybdenum than 316L, improving pitting resistance but increasing cost.
• Duplex steels (e.g. 1.4462): combine austenitic and ferritic features, offering much higher strength and better resistance to stress corrosion cracking in chloride environments. Duplexes can be more difficult to machine and are more expensive.
• Super-austenitic alloys (e.g. 254 SMO): have significantly higher contents of Cr, Mo and N and offer excellent resistance in chloride environments, but their cost is substantially higher.

Advantages and limitations in application

Advantages of 316L/1.4404:

• good compromise of resistance vs cost,
• versatility of applications,
• ease of welding and processing.

Limitations:

• susceptibility to SCC in the presence of chloride ions and stresses,
• not always sufficient in extremely aggressive conditions, where alloys with higher Mo content or nickel-based alloys are preferable.

Cost and availability analysis

316L is widely available and relatively inexpensive compared to super-austenitic or nickel-based alloys. When selecting material it is always worthwhile to perform a total life-cycle cost analysis: service life, maintenance costs, potential failure risk and downtime costs often determine the choice of a more expensive but more resistant grade.

Key European and American standards

Key standards and designations:

• EN 10088 – the European series of standards for composition and properties of stainless steels,
• EN 10204 – formats of material certificates (2.1, 2.2, 3.1, 3.2),
• ASTM/ASME (e.g. A240 for plates and strips, A276 for bars),
• AISI/UNS – grade designations (AISI 316L, UNS S31603),
• ISO and other industry standards depending on application (e.g. food, pharmaceutical, medical standards).

Possession of 3.1 or 3.2 certificates is often required in the chemical and food industries, where declaration of composition and test results is essential.

Test procedures and quality inspection methods

Standard quality tests include:

• chemical analyses (emission spectrometry, OES),
• mechanical tests (tensile, hardness, impact),
• metallographic examinations (microstructure, inclusion control),
• corrosion resistance tests (pitting, crevice, tests according to ASTM/EN),
• dimensional checks and surface inspections (especially for hygienic applications).

Depending on purpose, additional tests are used: passivation, weld leak testing, accelerated condition testing.

The effect of certification on material selection in industry

Certification confirms that the material meets production and service requirements. In the chemical, pharmaceutical industries or when producing pressure equipment, lack of appropriate documentation may lead to rejection of a component, legal liability and risk of failure. Therefore the choice between 316L and alternatives often depends not only on material properties but also on available certifications and compliance with purchaser requirements.

Influence of the operating environment and corrosion

The service life of 316L components depends on:

• concentration and type of chemical aggressors (crucially: chloride ions),
• operating temperature (higher temperature increases SCC risk and accelerates corrosion),
• presence of mechanical stresses and their accumulation,
• quality of welds and surface treatment,
• flow conditions of the medium (erosion-corrosion under turbulence).

Design of installations must consider these factors and anticipate preventive measures such as additional passivation, isolation from chloride sources or use of inhibitors.

Optimising maintenance and repair

Care for longevity includes:

• regular inspections and non-destructive testing (e.g. ultrasonic, penetrant testing),
• cleaning and passivation of surfaces (e.g. with nitric or citric acid to restore the passive layer),
• avoiding contact with materials causing galvanic corrosion,
• immediate removal of deposits and crevices where aggressive environments may accumulate,
• use of appropriate repair procedures and filler materials compatible with the original alloy.

Examples of failures and prevention methods

Common failures include pitting and SCC in seawater cooling pipelines, crevice corrosion in joints, erosion-related losses under turbulent flow and corrosion hotspots where deposits accumulate. Prevention includes:

• selecting a more resistant alloy or applying protective coatings,
• reducing chloride concentration and controlling operating temperature,
• designing for easy cleaning and minimising crevices,
• using low-sulphur (LS) materials where surface properties are critical.

• 1.4404/X2CrNiMo17-12-2/AISI 316L is an austenitic acid-resistant stainless steel with low carbon content, typically containing 16–18% Cr, 10–13% Ni and around 2% Mo.
• The low carbon content (the “L” designation) minimises the risk of intergranular corrosion after welding, making the grade particularly useful in welded constructions.
• Molybdenum improves resistance to pitting and crevice corrosion, and nickel stabilises the austenitic structure; together they create a durable compromise of performance and cost.
• 316LS refers to variants with limited sulphur content or another improved chemical profile, used where better surface properties and ductility are required.
• 316L is used in the chemical, petrochemical, food, pharmaceutical and medical industries and in architecture – wherever corrosion resistance and good weldability are required.
• Limitations of 316L include susceptibility to stress corrosion cracking in the presence of chloride ions and limited resistance in extremely aggressive conditions; in such cases alloys with higher Mo content, duplex steels or nickel alloys are considered.
• Production includes melting, degassing and controlled annealing; correct heat treatment and selection of welding consumables are key to retaining properties.
• EN, ASTM standards and appropriate quality certificates (e.g. EN 10204 3.1/3.2) are important when choosing material for industrial applications.
• Effective maintenance, monitoring of the operating environment and using appropriate welding procedures significantly extend the life of 316L components.

Steel 1.4404/316L is like a reliable craftsman: not always the cheapest, but offering solidity, versatility and predictable behaviour in many environments. Choosing this alloy is a decision based on balancing cost, availability and expected durability in real operating conditions.