1.4563

Definition and characteristics of acid-resistant steels

Acid-resistant steels are a group of iron alloys engineered for resistance to aggressive chemical environments — acids, salts and media containing sulphur- or chlorine-based compounds. Their primary feature is the formation of a passive oxide film that protects the metal from further corrosion. In practice this protective film is usually chromium-based and, together with appropriate alloying additions, provides a durable barrier against many corrosive reactions.

Acid-resistant steels include several families: ferritic, martensitic, duplex and austenitic. Each of these groups has different mechanical properties, corrosion resistance and machinability. In industrial contexts a combination of corrosion resistance and ductility is most often required — which is why austenitic stainless steels play a key role in chemical and petrochemical installations.

Characteristics of austenitic stainless steels compared with other groups

Austenitic steels are distinguished by an austenite crystal structure (face-centred cubic, FCC), stabilised mainly by nickel. This determines a number of characteristic features:

• high ductility and toughness, facilitating forming and bending,
• relatively low yield strength compared with martensite or ferrite, but good impact properties,
• excellent resistance to general and local corrosion (thanks to high chromium and molybdenum contents),
• absence of ferromagnetism in the fully austenitic condition.

Compared with ferritic and martensitic alloys, austenitics are significantly more resistant to stress corrosion cracking (SCC) in the presence of chlorides, especially if they contain a high nickel content. Duplex steels, by contrast, combine corrosion resistance with increased strength due to their mixed ferritic–austenitic structure, but can be less ductile and more difficult to work.

The role of austenite in the steel structure and material properties

Austenite serves as a “flexible framework” for mechanical and corrosion properties. Its atomic lattice:

• facilitates dislocation movement, which translates into good ductility and high impact resistance,
• promotes an even distribution of alloying elements, which in turn affects the uniformity of the protective oxide layer,
• in the presence of additions such as molybdenum or copper improves resistance to local forms of corrosion.

In practice, austenite stability is achieved through a combination of nickel, manganese and nitrogen; at the same time carbon must be kept low to avoid carbide precipitation, which can cause sensitisation and reduced corrosion resistance in welded zones.

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Beginnings of acid-resistant steel production

The history of acid-resistant steels dates back to the late 19th and early 20th centuries, when there arose a need for materials resistant to industrial acids and marine environments. Adding chromium to iron enabled the formation of a passive oxide layer. Subsequent decades saw the introduction of nickel, molybdenum and other elements, which significantly expanded the range of applications. After World War II the growth of the chemical and petrochemical industries demanded alloys with even better resistance to aggressive media, including saline solutions and concentrated acids.

Origin and standardisation of Alloy 28 in international standards

Alloy 28, also known as UNS N08028, EN 1.4563 or X1NiCrMoCu31-27-4, was developed in response to growing requirements for resistance to sulphuric acid and chlorides at elevated temperatures. It was formulated to combine a high proportion of nickel and chromium with additions of molybdenum and copper, giving it unique corrosion properties.

The material was standardised across various systems:

• in Europe as EN 1.4563 (stainless and acid-resistant steels),
• in the UNS system as N08028,
• chemical descriptions and specifications for pipes, plate and wire appear in ASTM-type standards and equivalents.

These standards specify chemical composition ranges, mechanical requirements and recommendations for heat treatment and testing.

Changes in chemical composition and production technology

Since its introduction, the composition of Alloy 28 has been optimised with regard to element availability, cost and resistance to specific corrosive processes. Advances in metallurgical technology and quality control have enabled reduction of impurities, precise dosing of alloying additions and improved microstructural homogeneity. In recent decades attention has also focused on nitrogen content and control of residual contaminants, which affect strength and resistance to stress corrosion cracking.

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Chromium, nickel and molybdenum content — impact on corrosion resistance

Alloy 28 goes beyond a simple iron–chromium system. Key elements are:

• Chromium (Cr) — in Alloy 28 chromium content is about 26–28%. Chromium forms chromium oxide, which is the basis of the passive corrosion-protective layer. Higher chromium increases resistance to general corrosion and some forms of localised corrosion.
• Nickel (Ni) — Alloy 28 contains approx. 30–34% nickel. Nickel stabilises austenite, increases ductility and resistance to stress corrosion cracking. A high nickel content significantly improves performance in chloride-containing environments.
• Molybdenum (Mo) — typically 3–4% molybdenum. Molybdenum is key to resistance against pitting and crevice corrosion in the presence of chloride ions. Even small additions of Mo substantially raise the PREN (Pitting Resistance Equivalent Number).

This Cr–Ni–Mo combination gives Alloy 28 high resistance in aggressive chemical environments, outperforming typical 300 series stainless steels.

Role of copper and other alloying elements

Copper (Cu) in Alloy 28 (typically 1–2%) plays a specific role: it significantly improves resistance to oxidising acids, especially sulphuric and phosphoric acids. Copper also benefits surface behaviour in reducing environments.

Other elements present in small amounts include:

• Carbon (C) — controlled at a low level (typically below 0.02%) to prevent sensitisation and chromium carbide precipitation.
• Manganese (Mn), silicon (Si) — in minimal amounts, as technological elements; they influence forming and castability.
• Nitrogen (N) — may be present in small amounts and is used as an austenite stabiliser; it increases strength and corrosion resistance.
• Iron (Fe) — the base element of the alloy.

Control of these elements and of impurities (sulfur, phosphorus) is crucial to maintaining the material’s properties.

Microstructural analysis resulting from the unique composition

The microstructure of Alloy 28 is a homogeneous austenite with fine precipitates of possible oxides or process-dependent contaminants. Due to low carbon and the presence of nickel, the structure remains resistant to chromium carbide precipitation during heat treatment, reducing the risk of sensitisation.

Additions of molybdenum and copper affect local phase properties: molybdenum can enhance passive film durability at sites exposed to chlorides, while copper improves surface resistance in acidic environments. Under improper heat treatment undesirable intermetallic phases can form; hence control of temperature and cooling is important to preserve the expected microstructure.

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Tensile strength and impact toughness

Alloy 28 offers a combination of respectable mechanical strength and excellent impact properties:

• typical values for the annealed condition: tensile strength (Rm) in the range of roughly 550–750 MPa,
• yield strength (Rp0.2) typically 200–300 MPa,
• elongation at break above 30–40%, indicating good ductility,
• impact toughness (Charpy) remains good even at low temperatures, thanks to the stable austenitic structure.

These ranges may vary depending on product form (plate, pipe, wire) and thermal history. In practice Alloy 28 is used where a combination of ductility and strength is required alongside corrosion resistance.

Resistance to high temperatures and variable operating conditions

Alloy 28 performs effectively across a wide temperature range. Its oxidation resistance and retention of mechanical properties make it suitable for installations at moderately elevated temperatures.

• Recommended service ranges include temperatures from low (below zero) to several hundred degrees Celsius — depending on the corrosive environment.
• At temperatures above approximately 600–700°C changes in structure (e.g. possible intermetallic phase precipitation) must be considered; for prolonged exposure at high temperature technical consultation and testing are advised.

Alloy 28 also maintains stability under cyclic temperature conditions, provided appropriate design and stress control.

Other important mechanical parameters

Additional practical characteristics include:

• Abrasion resistance — moderate; it is not intended for highly abrasive environments without protective coatings.
• Ductility and formability — very good, facilitating bending of pipes, pressing of plates and installation assembly.
• Density — similar to other stainless steels; the high nickel and chromium content affects cost rather than mass to any significant extent.

Precise mechanical parameters should be verified from supplier certificates and standard specifications for the particular grade and product form.

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Behaviour in acidic environments

Alloy 28 was designed for excellent resistance in acid-containing environments, especially sulphuric and phosphoric acids, where standard stainless steels fail. Copper plays a key role here, improving resistance to reducing acids such as sulphuric acid.

Typical behaviours:

• Concentrated sulphuric acid: Alloy 28 shows much better resistance than standard 316-type steels; it can be used in storage and processing equipment for sulphuric acid.
• Phosphoric acid: similarly, Alloy 28 performs favourably, particularly at higher concentrations and temperatures.
• In oxidising environments resistance depends on concentration and temperature; alloying additions limit chemical aggressiveness.

Reaction to chlorides and susceptibility to crevice corrosion

Chlorides are one of the greater challenges for stainless steels. Thanks to its Cr–Ni–Mo combination Alloy 28 exhibits high resistance to pitting and crevice corrosion in chloride-containing solutions, performing better than 316L.

• The PREN for Alloy 28 typically sits at a high level, indicating significant resistance to crevice corrosion in chloride environments.
• Nonetheless, under extreme conditions (high temperature, very high chloride concentrations, presence of stress) the possibility of crevice or pitting corrosion cannot be excluded. Therefore design should aim to minimise stagnation and areas of restricted flow.

Resistance to stress corrosion cracking and local corrosion

Alloy 28 is valued for resistance to stress corrosion cracking (SCC), particularly in environments containing chlorides and temperatures that favour cracking. High nickel content stabilises austenite and reduces susceptibility to cracking.

Regarding localised corrosion (pitting, crevice) — molybdenum significantly enhances resistance. In practice Alloy 28 is used where reliable long-term protection against localised degradation is required.

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Plastic forming and cutting techniques

Alloy 28 exhibits good machinability and formability, though the high nickel content leads to somewhat lower thermal conductivity and a greater tendency to heat up during machining.

Practical recommendations:

• use moderate cutting speeds and higher feeds, apply robust cooling,
• for forming (bending, rolling) use larger bend radii than for carbon steels,
• laser, plasma and waterjet cutting are common; oxy-fuel cutting is not recommended due to the risk of oxidation and composition changes.

Attention should also be paid to residual stresses after processing, which combined with aggressive media can lead to damage.

Welding specifics and selection of appropriate methods

Welding Alloy 28 is feasible but requires careful selection of filler materials and parameters:

• Welding methods: TIG (GTAW) and MIG (GMAW) are most commonly used; overlay welding is also practised. Welding techniques should ensure low heat input and appropriate control of deposit formation.
• Filler materials: wires and electrodes matched to Alloy 28 composition or compatible nickel-based materials are recommended to avoid galvanic differences and preserve corrosion resistance. Nickel-based fillers or matched austenitic grades are often advised; the exact choice should consider the application.
• Process control: maintain weld cleanliness, avoid contamination, control interpass temperature and limit time spent in temperature ranges that favour intermetallic precipitation.
• Post-weld treatment: surface passivation and, if necessary, solution annealing at about 1040–1150°C followed by rapid cooling are recommended to restore structural homogeneity.

Challenges and recommendations for annealing and heat treatment

Solution annealing is the basic heat treatment for Alloy 28 after welding or forming, restoring a homogeneous austenitic structure and relieving stresses. Recommended procedures include:

• annealing in the range 1040–1150°C,
• rapid cooling (e.g. water quench or forced air) to avoid precipitation of intermetallic phases,
• surface passivation after annealing to restore optimal corrosion resistance.

It is also important to avoid prolonged exposure to temperatures that promote embrittlement (e.g. alpha embrittlement, sigma phase precipitation under certain conditions), although Alloy 28 is less susceptible to these phenomena than some other high-silicon alloys.

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Comparison of EN, AISI, ASTM and other designation systems

Alloy 28 appears under several designations in standards systems:

• EN: 1.4563 (material designation in the European standard for stainless steels),
• UNS: N08028 (Unified Numbering System number),
• AISI/ASTM: references are typically made via UNS equivalents or manufacturer specifications, rather than as a standard AISI grade.

Other chemical designations, such as X1NiCrMoCu31-27-4, convey approximate percentages of the main elements (Ni, Cr, Mo, Cu) following conventions used in European alloy descriptions. When ordering and documenting, it is important to use full numbers and specifications to avoid misunderstandings between systems.

Certificates and industry standards

Materials supplied to the chemical and petrochemical industries must meet specified quality certificates, for example:

• material certificates compliant with EN 10204 (2.1, 2.2, 3.1, 3.2) confirming chemical composition and mechanical test results,
• standards for pipes, fittings and heat exchangers (relevant ASTM or API specifications depending on application),
• manufacturer and plant approvals.

Quality control includes non-destructive testing, hardness measurements, mechanical tests, chemical composition analysis and corrosion resistance tests under specific conditions.

The role of standards in material quality control

Standards provide the framework that ensures repeatability of material properties and minimises design risks. In practice standards define composition limits, annealing conditions, mechanical requirements and test methods, enabling the design of installations with predictable durability. For critical applications such as chemical plants, certification and conformity to standards are essential elements of material acceptability.

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Chemical and petrochemical industry

Alloy 28 is widely used where contact with aggressive acids and chlorides is routine:

• pipelines and storage tanks in the chemical industry, especially for sulphuric and phosphoric acids,
• heat exchangers, columns and process equipment exposed to aggressive media,
• components in installations processing sulphur- and chlorine-containing substances.

In these applications Alloy 28 provides long-term corrosion resistance, reducing replacement frequency and downtime costs.

Construction of installations resistant to aggressive environments

Thanks to its resistance to crevice corrosion and stress corrosion cracking, Alloy 28 is valued in construction of:

• equipment for chemical waste management,
• flue gas desulphurisation (FGD) systems,
• components exposed to saline influences in marine environments where higher resistance than 316L is required.

Practical uses also include gaskets, fittings, pumps and structural elements in environments with variable temperature and aggressiveness.

Other industries using Alloy 28

Beyond chemical and petrochemical sectors, Alloy 28 finds applications in:

• the pharmaceutical industry where cleanliness and resistance to cleaning chemicals are required,
• the electronics and semiconductor industries in locations exposed to aggressive chemical processes,
• marine installations and water treatment where elevated risks of local corrosion exist.

Selection of Alloy 28 stems from the need to combine chemical resistance with ductility and operational durability.

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Influence of environmental conditions on material ageing

Material ageing depends on a combination of factors: temperature, environmental composition, presence of contaminants and stresses. High temperatures accelerate diffusion processes and may lead to precipitation of intermetallic phases or local breakdown of the passive film. Environmental contaminants (e.g. sulphur, nitrogen oxides) and contact with metals of different electrochemical potential can accelerate degradation.

Regular monitoring and appropriate structural maintenance (avoiding stagnation, ensuring drainage and fluid exchange) extend Alloy 28 service life.

Typical damage types and prevention methods

The most common forms of degradation are:

• localised corrosion (pitting, crevice corrosion),
• stress corrosion cracking,
• erosion-corrosion in flow-exposed components.

Prevention methods:

• select the correct material grade according to service conditions,
• design to eliminate stagnation points and local stress concentrators,
• use surface passivation and cathodic protection where economically justified,
• perform regular non-destructive tests and monitor installation condition.

Importance of proper maintenance and monitoring

Systematic oversight of installations made from Alloy 28 reduces the risk of unexpected failures. Recommended actions include visual inspections, ultrasonic thickness measurements, passivity tests and chemical analysis of deposits and process media. Early detection of anomalies allows planned repairs and minimises costs.

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Modern alloy technologies and their impact on material properties

Materials development focuses on optimisation of properties through:

• composition modification (control of nitrogen, copper and non-standard alloying additions),
• innovative production processes (precision casting, powder metallurgy),
• improved microstructural homogeneity using advanced thermal control.

Such approaches enable creation of alloys with enhanced resistance to specific corrosion forms and increased mechanical strength.

Development of applications and new industrial requirements

Industry demands ever higher performance: reduced downtime, safety and compliance with environmental regulations. This drives the development of materials resistant to extreme conditions, such as higher temperatures, aggressive chemical mixtures or variable environments. Alloy 28 and similar alloys will continue to be adapted for new applications where the combination of chemical and mechanical resistance is critical.

Trends in ecology and sustainable material development

Globally there is increased emphasis on sustainability — recycling, reducing use of critical raw materials (e.g. nickel), and applying lower-carbon production processes. The challenge is to maintain alloy properties while reducing environmental impact. Work on recycling high-nickel alloys, optimising their composition and extending service life is key to the sector’s future.

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• Alloy 28 (EN 1.4563, UNS N08028) is an austenitic stainless steel with high contents of nickel and chromium, and additions of molybdenum and copper, designed for very aggressive chemical environments.
• Main advantages: excellent resistance to acids (particularly sulphuric and phosphoric), high resistance to crevice and pitting corrosion and good resistance to stress corrosion cracking.
• Typical composition: Cr ~26–28%, Ni ~30–34%, Mo ~3–4%, Cu ~1–2%, with controlled low carbon; exact values depend on standards and manufacturer.
• Mechanical properties: good Rm and ductility, high impact toughness; parameters largely depend on thermal condition and product form.
• Processing and welding require suitable filler materials and thermal control; solution annealing restores structural homogeneity.
• Applications: chemical and petrochemical industries, process equipment and installations for aggressive acids and chloride-exposed environments.
• Compared with 316L and 904L, Alloy 28 offers higher resistance in extreme conditions, although material costs are higher.
• Longevity and operational safety depend on correct material selection, installation design, regular monitoring and appropriate maintenance.

This guide aims to provide a concise yet comprehensive overview of Alloy 28, its properties, applications and practical aspects related to production and operation.