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Introduction

UNS N08028 (Sanicro 28) is a highly corrosion-resistant, iron-nickel-chromium-molybdenum alloy with an ultra-low carbon content, originally developed in Sweden during the 1960s. It offers excellent resistance to a wide range of aggressive environments, particularly those containing hydrogen sulfide (H₂S), chlorides, phosphoric acid, and sulfuric acid.

Available Products and Specifications

Equivalent Grade

Standard	Grade
ISO	4563-080-28-I
EN Number	1.4563
EN Name	X1NiCrMoCu 31-27-4
W. Nr.	1.4563
DIN	X1NiCrMoCuN 31 27 4
SS	2584
AFNOR	Z1NCDU31-27-03

Related Product Standards

Standard	Grade
ISO	ISO 9723
JIS	JIS G 4305, JIS G 4303
EN	EN 10088-3
ASTM	ASTM B575, ASTM B622, ASTM B829
ASME	ASME SB 575, ASME SB 622

Available Product Forms

UNS N08028 (Sanicro 28) is available in multiple forms suitable for a wide applications:

• Pipe and Tube: Ideal for handling aggressive chemicals and high-temperature processes.
• Plate and Sheet: Used in structural and mechanical applications where corrosion resistance is needed.
• Bar and Rod: Used for machining into custom components or structural parts.
• Wire and Strip: Ideal for fabrication, electrical, and mechanical uses.
• Forging Stock: Forged into complex shapes for heavy-duty applications.

Applications

Thanks to its exceptional corrosion resistance, Sanicro 28 is a versatile alloy suited for extremely demanding environments. Below are key application areas where Sanicro 28 has demonstrated outstanding performance:

1. Phosphoric Acid Production

Sanicro 28 is the most widely used metallic material for evaporator tubes in the wet-process phosphoric acid industry. With over a decade of proven service in many units, it has replaced graphite heat exchangers, which were prone to frequent tube failures and production downtime.

2. Sulfuric Acid Service

Sanicro 28 is highly suitable for piping and heat exchangers in sulfuric acid concentrations ranging from 40–70% (deaerated) and above 85%. Its corrosion resistance in 98% H₂SO₄ is comparable to that of Alloy C, making it a cost-effective and reliable alternative in acid environments.

3. Oil and Gas Industry

Sanicro 28 is widely used in sour gas wells for:

• Production tubing, casing, and liners.
• Corrosive oil wells, particularly in cold-rolled, high-strength form.
• Solution annealed piping in surface treatment facilities.
• Wirelines for deep well tool deployment and instrument control.

Its resistance to H₂S and chlorides makes it ideal for high-pressure, high-temperature (HPHT) environments.

4. Fluoride-Bearing Media

In the phosphate fertilizer industry, Sanicro 28 performs excellently in handling fluoride-bearing off-gases and gypsum recovery systems. It surpasses even higher-alloyed CrNiMo grades in terms of long-term stability and resistance in these aggressive media.

5. Nuclear Power Plants

Due to its superior resistance to stress corrosion cracking (SCC), pitting, and crevice corrosion, Sanicro 28 is chosen for heat exchangers and critical piping systems in nuclear power facilities.

6. Seawater and Chloride-Containing Cooling Systems

Sanicro 28’s high pitting and crevice corrosion resistance makes it ideal for:

• Seawater piping systems
• Seawater-cooled heat exchangers
• Chloride-bearing industrial cooling circuits

It has successfully replaced nickel alloys, Cu-Ni, bimetallic tubes, and coated carbon steel, which failed due to corrosion. The alloy also withstands shutdown periods up to one month without the need for system drainage or flushing, as long as the water temperature stays below 30°C (85°F).
Note: For even higher resistance in seawater, SAF™ 2507 duplex stainless steel may be more suitable.

Corrosion Resistance Properties

UNS N08028 (Sanicro 28) offers superb corrosion resistance, making it an ideal choice for applications in harsh environments. Its key corrosion-resistant properties include:

• Pitting and Crevice Corrosion Resistance: Excellent resistance to pitting and crevice corrosion in chloride-rich environments, which is common in seawater and acidic solutions.
• Stress Corrosion Cracking: High resistance to stress corrosion cracking (SCC), particularly in environments with chlorides and high temperatures.
• Oxidation Resistance: Outstanding oxidation resistance, especially in high-temperature applications, where oxidation can significantly degrade other materials.

Physical and Thermal Properties

UNS N08028 (Sanicro 28) exhibits impressive physical and thermal properties, making it suitable for high-temperature and high-stress applications:

Property	Value
Density	8.0 g/cm³
Melting Point	1350–1410°C
Thermal Conductivity	14.5 W/m·K
Specific Heat	420 J/kg·K at 25°C
Modulus of Elasticity	200 GPa

Chemical Composition

The chemical composition of UNS N08028 (Sanicro 28) is carefully controlled to provide its exceptional properties. It typically contains:

Element	Content Range
Nickel (Ni)	31–35%
Chromium (Cr)	27–32%
Molybdenum (Mo)	3.5–4.5%
Iron (Fe)	Balance
Copper (Cu)	0.5–1.5%
Silicon (Si)	0.5–1.0%
Manganese (Mn)	1.5–2.0%
Carbon (C)	Max 0.02%
Sulfur (S)	Max 0.01%

This precise composition ensures exceptional resistance to corrosion, high mechanical strength, and excellent performance in various aggressive environments.
PREN Number: PREN=%Cr + 3.3 x %Mo + 16 x %N=38.55 ~ 46.85

Mechanical Properties

UNS N08028 (Sanicro 28) provides outstanding mechanical properties, making it ideal for structural applications and pressure-containing components:

Property	Value
Tensile Strength	620 MPa (min)
Yield Strength	290 MPa (min)
Elongation	40% (min)
Hardness	240 HB (max)

These mechanical properties ensure that the alloy can perform well under both static and dynamic loading conditions.

Heat Treatment

UNS N08028 (Sanicro 28) is typically delivered in the annealed condition. The recommended heat treatment for this alloy involves heating to 1040–1100°C followed by rapid cooling, which helps to relieve any residual stresses and optimize its corrosion resistance and mechanical properties.

Forming

UNS N08028 (Sanicro 28) can be easily formed using conventional forming techniques. It exhibits good ductility and can be cold worked, bent, or drawn into various shapes. Hot working is performed at temperatures between 1100°C and 1150°C, followed by rapid cooling to maintain the desired microstructure.

Welding

Welding of UNS N08028 (Sanicro 28) is highly feasible using standard welding techniques such as TIG, MIG, and SMAW. It is essential to use a filler metal with a compatible composition to preserve the alloy’s properties and prevent defects such as cracking or reduced corrosion resistance in the weld zone.

Corrosion of Welds

While UNS N08028 (Sanicro 28) has excellent corrosion resistance, it is crucial to carefully control the welding process to avoid weld-related corrosion issues. Post-weld heat treatment or proper cleaning techniques are recommended to reduce the risk of corrosion in welded areas.

Descaling, Pickling, and Cleaning

After forming or welding, UNS N08028 (Sanicro 28) should undergo pickling or descaling to remove surface oxides and other impurities. A common method for pickling involves using a mixture of nitric acid and hydrofluoric acid, followed by thorough rinsing and neutralization. This process enhances the material’s corrosion resistance and restores its surface quality.

Surface Hardening

Surface hardening techniques, such as nitriding or shot peening, can be applied to UNS N08028 (Sanicro 28) to improve its mechanical properties. These processes enhance wear resistance and surface strength without compromising corrosion resistance, making the alloy suitable for applications with high wear and tear.

Conclusion

UNS N08028 (Sanicro 28) is a highly versatile and corrosion-resistant alloy ideal for use in demanding industries such as oil and gas, chemical processing, marine, and power generation. Its exceptional resistance to pitting, stress corrosion cracking, and oxidation, combined with high strength and excellent formability, makes it a go-to material for applications exposed to aggressive chemical environments and elevated temperatures.

By understanding the properties, applications, and processing techniques of UNS N08028 (Sanicro 28), engineers and professionals can confidently select this alloy for critical applications, ensuring long-lasting performance and reliability in some of the harshest conditions. Whether you’re working in high-corrosion environments, demanding manufacturing processes, or structural applications, UNS N08028 (Sanicro 28) provides the durability and performance needed to meet stringent requirements.