Hastelloy® C-22® or Hastelloy® C-276®: Which Is Right for Your Processing Environment?

Hastelloy® C-22® vs C-276®: Both made for tough conditions

Here we’ll explore two highly corrosion-resistant alloys - Hastelloy C-22 and Hastelloy C-276. These alloys are often used in processing that places heavy demands on equipment – involving harsh chemicals, high temperatures, and the need for continuous cleaning to ensure product purity and prevent contamination. 

Sanitary processing, whether it be for food and beverages, personal care and cleaning products, or pharmaceuticals, falls squarely in the “demanding” category. It has to do with the chemistry of the products used in and made by the equipment, and the chemistry of the materials used to construct the processing system.

“Hastelloy” is a trademark name that applies to around 20 high-performance alloys that all have nickel as a primary ingredient. 

Many are highly resistant to corrosion by hydrochloric, sulfuric, phosphoric, acetic, and formic acids, and to chlorine – all of which are common in sanitary processing.

The Hastelloys fall into two groups:

• Nickel-molybdenum (Ni-Mo) alloys
• Nickel-molybdenum-chromium (Ni-Mo-Cr) alloys

Both C-22 and C-276 are Ni-Mo-Cr alloys. Let’s first look at the critical role of chromium in enhancing corrosion resistance.

When this protective layer is formed, the alloy surface is referred to as “passive” because it’s unlikely to react chemically or electrochemically with what’s put through the processing equipment.

It is also chemical and electrochemical reactions between processor contents and the equipment construction material that leads to corrosion.

Not only does the passive layer itself not react, but it also acts as a barrier between the contents inside the processor and the system’s underlying metal, preventing it from corroding. Passivation can be removed with strong reducing agents that destroy the protective oxide layer on the metal.

In nominal terms, C-276 is 16% chromium, and C-22 is 22% – a significant difference. We’ll discuss the practical differences this makes to their respective performances below.

Nickel: strength and stability

Over two-thirds of all the nickel produced goes into making stainless steel, and about three-quarters of all stainless steels, including 304 (about 8% nickel) and 316 (10% nickel), contain it. Nickel lends formability, weldability, and ductility. 

It also adds resistance to: 

• Reducing acids
• Caustic solutions
• Stress corrosion cracking

Under certain processing conditions, including the presence of chlorides, the protective chromium passive layer can become chemically reactive and begin to break down, exposing the inside of the system to localized corrosion - particularly pitting and crevice corrosion. Nickel has no role in initiating that process but slows down the progression once it has begun. It also allows more chromium to be incorporated into an alloy, thereby increasing corrosion resistance.

Molybdenum: versatility in enhancing alloy qualities

Molybdenum is a key alloying element in high-performance nickel-based alloys that fall into two categories:

• Corrosion-resistant alloys
• Alloys that need to withstand high-temperature conditions

In sanitary processing, temperatures, in general, don’t exceed boiling (212°F/100°C), so the temperature-resistant qualities don’t really apply. Corrosion resistance is the quality with which sanitary processing is primarily concerned. 

These are some of the main corrosion-fighting benefits of molybdenum:

• It resists non-oxidizing conditions (reducing acids where the primary corrodent is the hydrogen ion rather than dissolved oxygen), including hydrochloric and sulfuric acids, common in sanitary processing.
• Molybdenum doesn’t provide corrosion resistance to oxidizing environments - in fact, it typically results in a higher corrosion rate. Because of this, nickel alloys that contain high levels of Mo and little or no Cr are not recommended for oxidizing acids or solutions containing higher levels of oxidizers such as chlorine.
• In oxidizing environments, it joins with chromium to protect against localized s such as pitting and crevice corrosion, especially in the presence of chlorides and non-oxidizing acids.
• It dramatically enhances the resistance of nickel to reducing agents such as oxalic acid.

Tungsten

Tungsten’s primary benefits (increased resistance to oxidation, nitridation, carburization, halogenation, and stress corrosion cracking) apply in temperatures above those involved in sanitary processing. At temperatures applicable to sanitary processing – generally between room temperature and boiling – it performs many of the same functions as molybdenum, including supporting metallurgical stability.

Nominal quantities (percent by weight) of Cr, Ni, Mo and W in C-22 and C-276

Oxidation and reduction: the foundation of corrosion

We discussed earlier how chromium reacts with oxygen to form the protective passive layer, and after that, has a tendency to stabilize and stop reacting with other elements in the environment and thereby stop corrosion. We would think of this as beneficial oxidation, while corrosion is harmful oxidation. Creating the conditions for the stability that preserves the passive layer is a lot about choosing the right alloy for the environment.

When metals react with their environments, two basic electrochemical processes are going on – oxidation and reduction - which result in atoms leaving the metal surface as ions and either remaining as dissolved metal ions in the process stream or combining with other substances to form accumulated corrosion products.

Oxidation is the process of an atom or lower valance ion giving up one or more electrons. It will only occur when the electrons from the oxidation reaction can be transferred to another atom or ion by a corresponding reduction reaction. A typical example of this is the corrosion of iron in moist air or a neutral water environment. 

Under these conditions, the Fe will oxidize to form a soluble Fe2+ion and two electrons. The dissolved O2 in the water will accept the electrons and be reduced to form OH- ions. The reactions for this corrosion process are summarized below.

Fe = Fe2++ 2e- (oxidation reaction)
O2 + 2H2O + 4e- = 4OH- (reduction reaction)
2Fe + O2 + H2O = 4OH- + Fe2+ (overall corrosion reaction)

When products of this corrosion reaction, OH- and Fe2+, become sufficiently concentrated, they will combine and precipitate out as ferrous hydroxide Fe(OH)2, which in the presence of oxygen will oxidize to Fe(OH)3, which is the final corrosion product known as rust.

The species in the reduction reaction that accepts the electrons is the oxidant or oxidizing agent. It’s being reduced and becoming more negative because it accepts the electrons from the oxidized species. The two most common oxidants responsible for the corrosion of stainless steels and nickel alloys are dissolved oxygen in neutral and basic solutions and the H+ ion in acidic solutions.

Comparison of the Aqueous Corrosion Resistances of C-22 and C-276
(in acidified 6 wt.% ferric chloride)

The C22 alloy also provides corrosion resistance over a much broader range of oxidizing and reducing environments, and using it to construct your processing system will give you a highly robust system for resisting corrosion in a wide range of chemical environments.