INCONEL ALLOY 22 VS ALLOY C276: WHICH SHOULD YOU CHOOSE?

Corrosion is the enemy of metal components. The elevated corrosion resistance of nickel alloys is just one reason they’re an outstanding choice for critical, high-stress and/or high-temperature applications where component failure can prove catastrophic. But even among this top-performing group, some alloys stand out. Today we’re taking a look at two very similar alloys that have been specially formulated to resist corrosion: Inconel Alloy 22  and Alloy C276.

CORROSION IS COMPLICATED

In the simplest layperson’s terms, corrosion is when a metal becomes oxidised through exposure to a harsh substance. The metal deteriorates, becoming weak and brittle, and will eventually break. Corrosion is one of the leading causes of component failure. This can have expensive and dangerous consequences when it comes to critical applications such as waste processing, nuclear energy production, and the aerospace industry.

Corrosion is hard to prevent and predict because it’s influenced by so many factors. Metals can be corroded by oxidising agents such as chlorine, oxygen and hydrogen peroxide, as well as different forms of acid: hydrochloric, sulphuric, phosphoric, acetic and formic. Heat can accelerate corrosion, and it can also be aggravated by the presence of contaminants – which may go undetected.

What makes nickel alloys so resistant to corrosion? The clue is in the name. When nickel is exposed to corrosive media, it forms a layer of nickel oxide that effectively protects the metal from further damage. This property continues to work even at extremely high temperatures. That’s the superpower of nickel alloys.

ALLOYS 22 AND C276: CHROMIUM IS THE KEY

In many ways, alloys 22 and C276 are very similar. They’re both nickel-molybdenum-chromium alloys that display truly exceptional anti-corrosion properties across a range of media and temperatures. Both alloys are prime choices for components that will be exposed to extreme heat and harsh chemicals. Flue gas desulphurisation, waste fluid treatment, chemical processing, and pulp & paper processing are just some of the applications they share.

One thing these two alloys have in common is the addition of a powerful anti-corrosion ingredient: chromium. Chromium enhances the inherent corrosion resistance of alloys in a similar way to nickel. When exposed to oxygen – in practice, when the system is put into operation for the first time – it reacts to form a fine protective layer of chromium oxide. This layer is stable and non-reactive, so when it comes into contact with chemical substances, it’s unlikely to be affected. So effectively, it provides the alloy with an additional protective barrier against corrosion.

In view of this, it’s not surprising that all stainless steels contain a minimum of 10–11% chromium. Alloy 22 has a higher proportion at 22%–22.5%, while alloy C276 has 14.5%–16.5%. This is one major reason that they both perform so well in corrosive environments. And the different proportion of chromium is also one of the biggest differences between them. We’ll return to this further down.

A FURTHER REINFORCEMENT: MOLYBDENUM

While nickel and chromium are strong all-rounders when it comes to corrosion resistance, molybdenum has a specific strength. While this metal doesn’t resist oxidising media, it does contribute extra protection when it comes to reducing media such as sulphuric and hydrochloric acids. These are common components in applications like sanitary, chemical and paper/pulp processing.

When matched with chromium, molybdenum acquires a new dimension, helping to fend off crevice and pitting corrosion and other forms of localised corrosion damage. But this only applies in combination: molybdenum alone is not enough to enhance an alloy’s resistance in oxidising environments, and often undermines it.

Both alloy 22 and alloy C276 contain molybdenum alongside chromium. Alloy 22 has 12.5%–14.5% molybdenum, while C276 has a higher share at 15%–17%.

THE BENEFITS OF TUNGSTEN

Tungsten performs much the same set of functions as molybdenum, making the alloy markedly more stable. But at extreme temperatures, it comes into its own – reinforcing the alloy’s resistance to stress corrosion cracking as well as oxidation, carburisation, nitridation and halogenation. Alloy 22 has 2.5%–3.5% tungsten, while C276 has 3%–4.5%.

SIMILARITIES AND DIFFERENCES

By now, we’ve seen that alloy 22 and alloy C276 have substantial similarities. They both contain an elevated share of chromium compared to a standard stainless steel, and they both gain extra corrosion resistance from the addition of molybdenum and tungsten. And, of course, they both benefit from the heat resistance and anti-corrosion properties of nickel. It’s not surprising that they share so many applications.

And when it comes to reducing environments – involving media like formic acid, oxalic acids and phosphorous acid – you’ll find that the two alloys perform in much the same way. There’s little if anything to choose between them, so you can let yourself be guided by other factors, like pricing, forms and availability.