1.4432

Definition of acid-resistant and stainless steels

Stainless steels are a group of iron alloys whose common characteristic is the ability to form a protective passive layer on the surface — a thin, durable layer of chromium oxides that prevents further corrosion. When the alloy contains at least about 10.5% chromium, a passive layer forms, making an otherwise susceptible material resistant to many forms of corrosion. The term “acid-resistant” usually refers to grades that show particularly good resistance to aggressive chemical environments, including acids and chloride ions.

Austenitic steels are a subgroup of stainless steels in which the equilibrium microstructure is austenite — a phase based on a face-centred cubic (FCC) lattice. As a result, these alloys are typically very ductile, well weldable and retain good mechanical properties even at low temperatures.

Characteristics of austenitic steels

Austenitic stainless steels are distinguished by several key features:

• High ductility and malleability — allows forming of thin components and complex shapes.
• Excellent weldability — do not require hardening after welding; low tendency to cold cracking.
• Resistance to general and localised corrosion — especially after the addition of molybdenum (improved pitting resistance).
• No hardenability by heat treatment — they are not hardened by thermal treatment; strength increases are achieved by cold working.
• Non-ferromagnetic in the annealed condition — essentially non-magnetic, although cold working can introduce some magnetic properties.

Differences between austenitic, ferritic and martensitic steels

Fundamental differences stem from the dominant microstructure and intended use:

• Austenitic (e.g. 1.4432 / AISI 316L): contain nickel, have an austenitic structure, are ductile, well weldable, resistant to general corrosion and pitting. Used where ductility and chemical resistance are required.
• Ferritic (e.g. 1.4016, 1.4509): contain chromium, little or no nickel, have a ferritic (BCC) structure, are ferromagnetic, less amenable to plastic deformation, often cheaper; resistant to stress corrosion cracking in some conditions but have poorer ductility and weldability than austenitics.
• Martensitic (e.g. 1.4021, 1.4057): contain more carbon and are heat-treatable; after heat treatment they achieve high strength; they are magnetic and less corrosion-resistant than austenitics. Used where high strength and wear resistance are needed.

Austenitic steels combine ease of processing with high corrosion resistance, making them very versatile in industrial applications.

Development of stainless steel in industry

The history of stainless steel begins in the early 20th century, when the need for a rust-resistant material became pressing — from weaponry to kitchenware. In 1913 Harry Brearley in England made the first practical steps towards “stainless” steel by experimenting with adding chromium to iron. In subsequent decades the development of alloy chemistry (particularly the addition of nickel) and advances in steelmaking technology led to the family of austenitic steels, with the classic 18/8 grade (about 18% Cr, 8% Ni), known as 304.

As industry encountered increasingly aggressive environments — marine, chemical and petrochemical — there was a need to improve resistance to localised corrosion. The response was the addition of molybdenum, which gave rise to grades such as 316. Over time standards and norms unified the designations, and trade names gave way to EN numbering (e.g. 1.4432) and American AISI/ASTM designations (316L).

Origin and application of grade 1.4432

Grade 1.4432, also known as X2CrNiMo17-12-3 and equivalent to AISI 316L, developed as a variant of 316 with a reduced carbon content (hence the letter L – low carbon). Lower carbon content prevents the precipitation of chromium carbides during welding, reducing the risk of intergranular corrosion in the heat-affected zone. In practice 1.4432 became the standard for applications requiring both good corrosion resistance and reliable weldability — from chemical and marine installations, through the food and pharmaceutical industries, to architectural and medical components.

Main elements and their role in the alloy

The main constituents of 1.4432 (X2CrNiMo17-12-3 / AISI 316L) and their roles can be summarised as follows:

• Chromium (Cr, ~16–18%) — key to forming the passive oxide layer, providing resistance to general corrosion.
• Nickel (Ni, ~10–14%) — stabilises the austenitic structure, increases ductility and toughness.
• Molybdenum (Mo, ~2–3%) — improves resistance to localised corrosion (pitting, crevice corrosion), especially in chloride-containing environments.
• Carbon (C, ≤0.03% in the L version) — low level minimises chromium carbide precipitation, reducing the risk of intergranular corrosion.
• Manganese (Mn), silicon (Si), phosphorus (P), sulphur (S), nitrogen (N) — trace elements that affect machinability, strength and alloy stability; their contents are strictly controlled.

Role of chromium, nickel and molybdenum

• Chromium: forms and regenerates the passive layer. Without sufficient chromium the surface protection is inadequate and the steel corrodes easily. Practically, a minimum of about 10.5% is necessary to achieve adequate stainless properties, while engineering grades typically use 16–18% for improved durability.
• Nickel: determines austenite stability. Without nickel the steel becomes ferritic or duplex, losing ductility and weldability. Nickel also increases resistance to embrittlement at low temperatures.
• Molybdenum: is a “specialist” for pitting and crevice corrosion resistance. In chloride or halide-containing environments, molybdenum significantly increases material lifetime. Its presence is the main distinguishing factor of 316L compared with 304.

Importance of low carbon content (L version)

The low carbon content in 316L (≤0.03%) has practical consequences:

• Reduced risk of sensitisation: during welding chromium carbides can form in heat-affected zones (between 400–850°C). High concentrations of carbides cause local chromium depletion, leading to intergranular corrosion. The L version minimises this phenomenon.
• Better weldability: removes the need for very expensive methods to avoid sensitisation; standard procedures can be used with appropriate precautions.
• Slight reduction in strength compared with higher-carbon variants, but this practically does not affect most applications where corrosion resistance and weldability are the priority.

Corrosion resistance and behaviour towards acids

1.4432 / AISI 316L is characterised by very good resistance to general corrosion in industrial and marine atmospheres, as well as in many chemical solutions. Thanks to molybdenum it displays better resistance to pitting and crevice corrosion than standard 1.4301 (304). In practice it performs well in environments containing chlorides, oils and many oxidising acids, such as concentrated nitric acid under certain conditions.

At the same time it should be remembered that 316L is not resistant to all acids and concentrations. For example, in concentrated reducing acids (such as concentrated HCl) or in environments with very high temperatures and concentrations of chloride ions, the steel may undergo rapid corrosion. Therefore material selection should be based on analysis of the specific service environment.

Tensile strength and ductility

Typical mechanical values after annealing (indicative figures):

• Yield strength Rp0.2: approximately 170–310 MPa (depending on processing and condition)
• Tensile strength Rm: typically 480–620 MPa
• Elongation A5: usually >40% (in the annealed condition)
• Hardness: approx. 150 HB (depending on processing)

These parameters make 316L a material with a good compromise between strength and ductility — easily formable, yet sufficiently strong for many structural applications.

Resistance to high and low temperatures

• Low temperatures: austenitic 316L steels retain excellent ductility and toughness even at very low temperatures; therefore they are used where good properties in cryogenic conditions are required.
• High temperatures: austenitic steel is not intended for long-term service at very high temperatures, where sensitisation and oxidation may occur. For high-temperature applications there are specialised heat-resistant grades. However, short-term or moderate temperatures of a few hundred degrees Celsius are acceptable, provided heat treatment and design rules are observed.

Methods of producing the alloy

Production of 316L involves several stages:

• Melting in electric arc furnaces (EAF) and induction furnaces: controlled melting conditions ensure a uniform chemical composition.
• Refining and vacuum degassing (VAR, ESR) for specialised materials (e.g. implants) to limit impurities and gas inclusions.
• Hot and cold rolling: giving semi-finished products (sheets, strips, tubes) the required dimensions and mechanical properties.
• Recrystallisation annealing: relieving internal stresses after mechanical processing and restoring the austenitic structure.

Heat treatment and its effect on structure

Austenitic steels are not hardened by conventional heat treatments. Main operations:

• Solution annealing: typically 1040–1120°C, followed by rapid cooling (e.g. water quench). Restores a homogeneous structure, dissolves carbides and removes effects of cold work.
• Avoiding temperature ranges of 400–850°C: prolonged exposure in this range can lead to chromium carbide precipitation and sensitisation.
• Quenching and tempering: not applicable in the classic sense — they are not hardened by thermal microstructural changes, but by plastic deformation.

Welding and forming of components

• Welding: 316L is excellently weldable by TIG, MIG/MAG, covered electrodes, plasma. Low carbon content reduces the need for special procedures to prevent intergranular corrosion. Filler metals should match the base grade (316L) to maintain chemical and mechanical compatibility. For critical constructions electrodes and wires with low carbon and added molybdenum are often used.
• Forming (bending, stamping, drawing): good ductility facilitates production of complex shapes. Thicker sheet may require a larger bend radius. Cold working increases strength but reduces ductility; finishing operations often require re-annealing.

From a production standpoint, 316L is a “jack-of-all-trades” — easy to work and predictable in behaviour, provided quality control and protection against contamination are observed.

Chemical and petrochemical industry

In chemical and petrochemical installations 316L is used for:

• heat exchangers,
• pressure vessels and pipelines for aggressive media,
• valves, pumps and fittings,
• process tanks.

Molybdenum provides resistance to aggressive salts and chlorides, and the low carbon content facilitates welding of large assemblies without loss of resistance.

Food and pharmaceutical sectors

In the food and pharmaceutical industries cleanliness and ease of sterilisation are priorities. 316L is used for:

• production lines, tanks and apparatus,
• equipment for processing milk, beer and beverages,
• fittings and components in contact with food,
• pharmaceutical installations where compatibility with strong detergents and CIP (clean-in-place) processes is required.

Thanks to good chemical resistance and the ability to passivate, surfaces are easy to keep clean.

Marine and offshore industry

In marine constructions 316L is used for:

• ship equipment components,
• onboard piping systems,
• nearshore components and devices operating in saltwater environments.

Although in very aggressive marine zones (e.g. splash zones, turbulent currents) alloys with higher resistance (duplex, super-austenitic) are often used, 316L is a popular choice where a compromise between cost and durability is required.

Construction and sanitary-heating installations

In construction and installations 316L is used for:

• architectural elements exposed to the weather,
• cladding, handrails and facades,
• sanitary, heating and drainage installations in environments of increased chemical aggressiveness.

Its aesthetics combined with durability make it popular in projects requiring both appearance and strength.

Resistance to oxidising and non-oxidising acids

• Oxidising acids (e.g. concentrated HNO3): 316L tolerates oxidising acids well over a wide range of concentrations; passivation is effective and the oxide layer rebuilds. In practice many installations using nitric acid employ 316L.
• Non-oxidising acids (e.g. HCl): 316L has limited resistance, especially at high concentrations and temperatures. In aggressive conditions more resistant materials or protective coatings are recommended.

Behaviour in chloride-containing environments

Chlorides are a challenge for stainless steels. Chloride ions promote pitting and crevice corrosion. Thanks to molybdenum 316L has a significant advantage over 304, but there remains a risk of corrosion at:

• high temperatures and high chloride concentrations,
• stagnant fluids and the presence of crevices where oxygen access is limited,
• applied stresses and mechanical loading.

The PREN (Pitting Resistance Equivalent Number) is used to assess pitting resistance; one simplified formula is: PREN = %Cr + 3.3×%Mo + 16×%N. For 316L PREN typically lies in a range considered moderate, making the steel suitable for many applications but not the best choice where chlorides dominate aggressively.

Risk of stress corrosion cracking and mitigation methods

Stress corrosion cracking (SCC) is a significant threat, especially in chloride-containing environments and at elevated temperatures. To minimise the risk:

• low-carbon (L) or stabilised grades (with additions of Ti, Nb) are used,
• operating temperatures are limited and prolonged exposure in the sensitisation range is avoided,
• residual stresses are reduced through appropriate heat treatment or stress-relief annealing,
• coatings or corrosion inhibitors are applied in more aggressive conditions.

Differences compared with 1.4301 (AISI 304)

• Composition: 316L contains molybdenum (~2–3%), which is absent in 304 (1.4301). 304 typically has 18% Cr and 8–10% Ni, whereas 316L has ~16–18% Cr, 10–14% Ni and Mo.
• Corrosion resistance: 316L is superior, especially against pitting and chloride environments.
• Cost: 316L is more expensive due to nickel and molybdenum additions.
• Applications: 304 is sufficient in less aggressive environments (household, standard industrial applications), while 316L is used where higher chemical resistance is required.

Comparison with ferritic and martensitic steels

• Ferritic: cheaper, magnetic, better resistance in some high-temperature conditions, but poorer ductility and weldability.
• Martensitic: harder and heat-treatable — used where knives, shafts, springs are needed; not as corrosion-resistant as 316L.
• Austenitic 316L: the best combination of ductility, corrosion resistance and weldability for many engineering uses.

Pros and cons in practical applications

Advantages of 316L:

• high corrosion resistance,
• excellent weldability,
• good formability,
• durability and surface aesthetics.

Disadvantages:

• higher material cost than 304,
• unsuitable in extremely aggressive chloride environments without additional protection,
• heavier than alternatives (e.g. some plastics) where weight is critical.

EN and ASTM requirements

The most important standards and documents concerning 316L and its semi-finished products:

• EN 10088 — European standard for stainless steels; contains classification and chemical compositions of grades (including 1.4432).
• EN 10204 — standards for inspection documents supplied with the material (e.g. 2.1, 2.2, 3.1, 3.2).
• ASTM A240 — plates, sheets and strips of stainless steel used in pressure vessels and resistant elements.
• ASTM A276, A479, A312 — standards for bars, tubes and items for specialised applications.
• AISI — historical American designation (AISI 316L), often used interchangeably with EN numbers.

Quality certificates and quality testing

In production and critical applications the following are often required:

• Material certificates (EN 10204 3.1) confirming chemical composition and mechanical test results.
• Non-destructive testing: ultrasonic (UT), radiographic (RT), magnetic (MT) — depending on requirements.
• Corrosion resistance tests: pitting tests, crevice tests, tests in solutions of specific media.
• Quality certificates and management systems: ISO 9001, industry-specific certifications (e.g. PED for pressure equipment).

Good practice requires strict documentary control and testing for components operating in critical conditions.

Proper cleaning and surface maintenance

• Regular rinsing with water — in marine or dusty environments; removes salt deposits and contaminants.
• Mild detergents — wash with gentle agents, free of chlorine, avoiding aggressive cleaners containing hydrochloric acid.
• Avoid steel cleaning tools — wire brushes and scouring pads made of steel can cause iron contamination, leading to localised corrosion (contact with carbon steels).
• Passivation — chemical treatment of the surface (e.g. nitric acid or citric acid) can renew the passive layer and remove contaminants.
• Polishing and electropolishing — improve aesthetics and reduce fouling; electropolishing additionally strengthens passivity.

Avoiding corrosion threats and mechanical damage

• Control the working environment — analyse the composition of the medium, temperature and stresses.
• Design without crevices and dead spaces — eliminate areas that accumulate salts and dirt.
• Limit residual stresses — use appropriate heat treatments and welding techniques.
• Protect against contamination by carbon steel — separate materials during transport and assembly.

Storage and transport rules

• Dry, ventilated storage — avoid contact with moisture and chemicals.
• Separate storage from carbon steels — minimise contact and contamination risk.
• Protect surfaces with film or protective coating — especially for finished-surface items.
• Appropriate securing during transport — use non-metallic spacers and securing straps.

Careful maintenance and correct operational practices significantly extend the service life of components made from 316L.

Modern production technologies

• Better composition control through spectroscopic analyses and vacuum melting reducing contaminants.
• Additive processes (3D printing from powders) increasingly used to produce complex 316L components, allowing waste reduction and shorter production times.
• Coatings and surface technologies: electropolishing, antibacterial and hydrophobic coatings increase the functionality of steel in medical and food applications.

Composition modifications for improved properties

• Low-sulphur and low-oxygen versions for demanding implant applications (316LVM).
• Nitrogen control additions — improving strength and PREN without excessive cost increases.
• New high-PREN formulations — increasing pitting resistance, competing with duplex grades in aggressive environments.

Applications in modern technological sectors

• Biomedicine: implants, surgical instruments; 316LVM and specialised variants are used where ultra-high purity and biocompatibility are required.
• Renewable energy: components of wind turbines and offshore installations where atmospheric resistance is critical.
• Semiconductor and speciality chemical industries: benefits from cleanliness and passivation capability.

Innovations aim to increase durability and functionality while reducing costs — hence work on improving surface properties and implementing additive manufacturing methods opens new design possibilities.

Steel 1.4432 (X2CrNiMo17-12-3, AISI 316L) is a versatile yet refined material — combining corrosion resistance, good weldability and ductility. Thanks to additions of chromium, nickel and molybdenum it offers significantly better protection against pitting and crevice corrosion than standard 304 grades. The low carbon content in the L variant is crucial for welded constructions, minimising the risk of intergranular corrosion.

Typical applications include chemical, petrochemical, food, pharmaceutical, marine industries and construction. Selection of this material should be preceded by analysis of the working environment — especially the presence of chloride ions, temperatures and stresses — and should take into account standards and quality certificates. Modern production technologies and composition modifications continuously broaden the application range of 316L, and developments in surface treatment and additive manufacturing open new construction possibilities.

In practice 316L is like a reliable companion in demanding conditions: durable, unobtrusive and tailored to many industrial needs, although in the most extreme environments it is sometimes worth considering alternatives (duplex, super-austenitic alloys).