Whenever corrosion-resistant metal materials are mentioned, stainless steel and nickel alloys (Monel, Inconel, Incoloy and Hastelloy) are the two most common materials that people tend to think of.

However, the same corrosion-resistant materials, can you tell the difference between them? Why do their prices vary so much? Which material should you use?

In this article, we will introduce the differences between nickel alloys and stainless steel in detail from six aspects: operating temperature, chemical composition, mechanical properties, corrosion resistance, application and price.

First of all, we need to understand the chemical composition of the two metals.

From the point of view of chemical composition, stainless steel mainly belongs to iron alloy. Alloys such as Monel, Inconel, Incoloy and Hastelloy are mainly nickel alloys.

Therefore, the main component of stainless steel is iron. Its iron content can reach more than 65%. The iron content of some ferritic and martensitic stainless steels even reaches more than 80%. In contrast, nickel alloys tend to contain less than 50% iron. Certain nickel alloys such as Hastelloy C-2000 have even less than 3% iron.

The nickel content of nickel alloys such as Monel, Inconel, Incoloy and Hastelloy is often greater than 30%, and can reach more than 70%. The nickel content of stainless steel is less than 30%. Partially ferritic and martensitic stainless steels are even completely nickel-free.

Both stainless steel and nickel alloys contain around 20% chromium. Chromium is a very effective anti-corrosion element. It is also a major source of corrosion resistance for stainless steel and nickel alloys.

Molybdenum may also be present in some stainless steels and nickel alloys. Nickel alloys tend to contain more molybdenum than stainless steels.

In addition to the above differences, the composition of nickel alloys is often more complex than that of stainless steel. The metal element content of the former can reach more than 20 kinds at most, while the element types of stainless steel are often below 10. In addition, the control of trace elements in nickel alloys is often stricter than that of stainless steel.

The service temperature is the biggest difference between stainless steel and nickel alloy.

In general, stainless steel tends to be used for room temperature applications. Nickel alloys such as Inconel, Incoloy and Hastelloy are basically used in high temperature environments.

One of the most important factors determining this characteristic is the nickel content. Since the iron element cannot maintain a stable structure at high temperature, it is difficult to guarantee the performance of stainless steel with iron as the main element at high temperature. Nickel can perfectly maintain the austenitic structure of the alloy at high temperature. Therefore, only nickel-based alloys can perform well at high temperatures. Although nickel is also present in some austenitic stainless steels, their nickel content is still low.

In addition, Monel alloys are often used at room temperature because the corrosion resistance of copper elements decreases at high temperatures.

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

Chromium forms an oxide film on the metal surface when it comes into contact with oxygen. This layer of oxide film can effectively prevent the alloy from being further oxidized. The difference between the chromium content of nickel alloy and stainless steel is not very large. Therefore, their antioxidant properties are both excellent.

It is nickel and molybdenum that really make the difference between nickel alloys and stainless steels.

Nickel itself is an excellent corrosion-resistant element, unlike iron, which is easily corroded in the air. Therefore, nickel alloys naturally have a better corrosion resistance foundation than stainless steel.

In addition, molybdenum is a metal element with good resistance to reduction corrosion. As mentioned above, nickel alloys tend to have higher molybdenum content. A typical alloy is Hastelloy B-3. It has a molybdenum content of up to 30%.

In summary, nickel alloys are superior to stainless steel in comprehensive corrosion resistance.

When it comes to the mechanical properties of two materials, it is far more complicated than we think. In stainless steel and nickel alloys, there are both higher strength materials and lower strength materials. Therefore, we cannot make a comparison as a whole.

First of all, in stainless steel and nickel alloys, there are some materials that can greatly increase the strength of the material through precipitation strengthening.

In stainless steels, they are called precipitation hardening stainless steel, such as 17-4PH.

In nickel alloys, they are called precipitation strengthening alloy, such as Inconel 718.

The strength of these alloys far exceeds that of ordinary materials. Among them, precipitation strengthened alloys can maintain high strength at high temperatures. The high strength of precipitation hardening stainless steels will fail at high temperatures. In practical applications, precipitation strengthened alloys are used more frequently than precipitation hardening stainless steels.

For ordinary stainless steels and nickel alloys, we also need to discuss the situation.

First, we tend to compare austenitic stainless steels with general nickel alloys. Because their structure is very similar. Of the two materials, nickel alloys can be strengthened by adding more solid solution elements. Therefore its strength tends to be higher than that of austenitic stainless steel. Likewise, it maintains mechanical properties better than austenitic stainless steels at elevated temperatures.

In addition, there is another type of stainless steel called martensitic stainless steel. This stainless steel can be strengthened by quenching. After quenching, martensitic stainless steels will be stronger than common nickel alloys. However, the high strength of this type of stainless steel is also only maintained at room temperature. Martensitic stainless steels also tend to sacrifice some corrosion resistance.

Nickel alloys have a lot of overlap with stainless steel in terms of applications. Because they are all corrosion-resistant alloys. There are three main differences between them in application.

First, as mentioned above, stainless steel is not suitable for high temperature applications. Therefore, nickel alloys are required for most high temperature applications. Such as: engines, reactors of nuclear power plants, deep oil wells, etc.

Secondly, stainless steel has a wide range of applications in the civilian field. Such as: tableware, medical treatment, construction, etc. Nickel alloys such as Monel, Inconel, Incoloy and Hastelloy are basically only used in industrial and military fields.

Finally, the corrosion resistance of stainless steel is limited to oxidizing environments. If the corrosive environment becomes special and complicated, most stainless steels cannot resist this kind of corrosion. Nickel alloys have developed various targeted grades according to different corrosive environments.

Needless to say, the price of nickel alloys is much higher than that of stainless steel. This is determined by the huge difference between nickel and iron prices. Secondly, the price of molybdenum is even several times higher than that of nickel. Nickel alloys, which are generally higher in molybdenum, are therefore more expensive.

In addition, in the normal state, the strength of nickel alloy is higher than that of stainless steel. This also caused an increase in the difficulty of processing nickel alloys. This high processing difficulty is also an important factor in making nickel alloys more expensive.

Finally, due to the more complex composition of nickel alloy and stricter control of trace elements, it also has higher requirements for raw materials. This also increases the price of the alloy.

Both stainless steel and nickel alloys are corrosion resistant materials. Nickel alloys perform better than stainless steel in most cases. An important difference between them is that stainless steel is often used at room temperature, and nickel alloy is often used at high temperature. Of course, the better performance of nickel alloys will inevitably bring higher prices.

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