Stainless Steel vs. Monel, Hastelloy, Incoloy and Inconel: the differences.

When it comes to corrosion-resistant metals, stainless steel (SS) and nickel alloys (Monel, Inconel, Incoloy and Hastelloy) are the two most widely used material groups.

They are all corrosion-resistant, but what are the differences?

Chemical composition

In terms of chemical composition, stainless steel is primarily classified as an iron-based alloy. Alloys such as Monel, Inconel, Incoloy and Hastelloy are nickel-based alloys.

Iron (Fe) is the main component of stainless steel, with iron content reaching more than 60%. In some ferritic and martensitic stainless steels, the iron content can even exceed 80%. Nickel alloys, on the other hand, generally contain less than 50% iron, and in some cases less than 5%.

The nickel content of nickel alloys such as Monel, Inconel, Incoloy and Hastelloy is often above 30% and can exceed 70%. The nickel content of austenitic stainless steels generally does not exceed 30%, while some grades contain virtually no nickel at all. Nickel is essential for maintaining performance at elevated temperatures.

Both stainless steels and nickel alloys contain broadly comparable levels of chromium. Chromium is a highly effective corrosion-resistant alloying element in both stainless steel and nickel alloys.

Molybdenum may also be present in certain stainless steels and nickel alloys. Nickel alloys generally contain higher levels of molybdenum than stainless steel. In addition, nickel alloys can contain up to around 20 different alloying elements, whereas stainless steels typically contain fewer than 10.

Operating temperature

The operating temperature is one of the most significant differences between stainless steel and nickel alloys. Stainless steel is generally used for applications at or around room temperature. Nickel alloys such as Inconel, Incoloy and Hastelloy are primarily used in high-temperature environments.

One of the key alloying elements that enables high-temperature applications is nickel. Because iron cannot maintain a stable structure at elevated temperatures, it is difficult to guarantee the performance of iron-based stainless steel at high temperatures. Its corrosion resistance can consequently decrease.

Nickel, on the other hand, can maintain the austenitic structure of an alloy at elevated temperatures. This is why nickel-based alloys are particularly well suited to high-temperature applications. Although nickel is also present in some austenitic stainless steels, the nickel content is generally too low to provide the same level of high-temperature performance.

Monel grades, by contrast, are often used at room temperature because the corrosion resistance provided by their copper content decreases at elevated temperatures.

Corrosion resistance

The main alloying elements that determine the corrosion resistance of stainless steel and nickel alloys are nickel, chromium and molybdenum.

Chromium forms a passive oxide layer on the metal surface when exposed to oxygen, preventing further oxidation of the material. The difference in chromium content between nickel alloys and stainless steel is not particularly large. Both material groups therefore offer good corrosion resistance.

Higher levels of nickel and molybdenum generally give nickel alloys better corrosion resistance than stainless steel.

Nickel itself is an excellent corrosion-resistant element, whereas iron readily corrodes when exposed to air. Nickel alloys therefore have an inherent advantage in terms of corrosion resistance compared with iron-based stainless steel.

Molybdenum improves the resistance of nickel alloys to non-oxidising acids. Depending on its concentration, molybdenum can provide resistance to hydrochloric acid, phosphoric acid and hydrofluoric acid, as well as sulphuric acid. Molybdenum also significantly improves resistance to pitting corrosion and crevice corrosion.

Overall, nickel alloys offer significantly higher corrosion resistance than stainless steel.

Mechanical properties

Both stainless steels and nickel alloys are available in grades with either relatively high or lower strength levels.

For both material groups, certain grades can achieve significantly higher strength through precipitation hardening (age hardening).

In stainless steel, these are known as precipitation-hardening stainless steels, such as 17-4 PH (AISI 630 / 1.4542).

In nickel alloys, these are referred to as precipitation-strengthened alloys, such as Inconel 718 (2.4668).

The achievable strength of these alloys is significantly higher than that of non-hardenable grades. Precipitation-strengthened nickel alloys retain their high strength at elevated temperatures. The high strength of precipitation-hardened stainless steel, however, decreases significantly at high temperatures.

In practice, this means that precipitation-strengthened nickel alloys are used more frequently for high-temperature applications than precipitation-hardened stainless steels.

What is the difference between Monel, Inconel, Incoloy and Hastelloy?

• Monel has excellent resistance to seawater due to its high copper content, making it particularly suitable for marine applications at normal operating temperatures.
• Inconel and Incoloy contain higher levels of chromium and are suitable for corrosion-resistant applications at elevated temperatures. Incoloy contains more iron than Inconel, making it less expensive, but also generally less corrosion-resistant.
• Hastelloy contains more molybdenum than Inconel, providing even greater corrosion resistance, particularly in aggressive chemical environments.

Stainless steel compared with Monel, Inconel, Incoloy and Hastelloy

Compared with stainless steel, nickel alloys generally offer superior corrosion resistance. Nickel alloys are frequently used at elevated temperatures, whereas stainless steel is often limited to lower operating temperatures if its corrosion resistance is to be maintained.

These superior performance characteristics also mean that nickel alloys are considerably more expensive than stainless steel. The table below provides an overview of the differences. The comparison is based on the highest achievable values for each material group.

Material	Stainless Steel	Monel	Inconel	Incoloy	Hastelloy
Main alloying elements	Iron Chromium	Nickel Copper	Nickel Chromium	Nickel Chromium Iron	Nickel Chromium Molybdenum
Operating temperature while maintaining corrosion resistance	Room temp	Up to 500°C	Up to 980°C	Up to 1150°C	Up to 1200°C
Corrosion resistance (SS = 1)	1,0	1,2	1,5	1,3	1,7
Tensile strength Rm (N/mm²)	400–1300	470–1000	550–1250	450–1050	600–700
Price index (SS = 1)	1	25	27	13	33

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