How to Overcome Corrosion with Super Alloys™ AL-6XN® and Hastelloy® C-22®

Sanitary and high-purity processors understand that their equipment has a tough job. It has to stand up to a range of harsh factors, year after year, that cause system corrosion and failure if the material it’s constructed of isn’t suitable.

For example, 316/316L (UNS S31603) stainless steels is widely considered the “workhorse” materials of construction for high purity and sanitary tube, process components and equipment for food, beverage, personal and home care, pharmaceutical, and biotech processing. Certain factors can push commonly used stainless steels past their capability to resist corrosion:

Super Alloys™: When Processing Conditions Demand More

Many modern processing environments call for better corrosion resistance than austenitic stainless steels like 316 and 316L can provide. Corrosion-resistant Super Alloys™ AL-6XN® and Hastelloy® C-22® are excellent alternatives for the design and construction of systems operating under harsh conditions.

Both contain higher levels of chromium, molybdenum, and nickel than the 316 stainless steel. The increased chromium and molybdenum add resistance to pitting and crevice corrosion. 

• Molybdenum provides higher resistance against reducing environments such as formic and phosphorous acid mediums, and higher nickel content boosts immunity to chloride stress corrosion cracking.

In this article, we will discuss the differences between corrosion-resistant Super Alloys AL-6XN and Hastelloy C-22 to help educate you in selecting the right material for your application. To begin this discussion — and provide a comparison between applications for Super Alloys — let’s look first at some standard Stainless-Steel materials used in processing and its limitations.

Stainless Steel: Corrosion-Resistant, but Not Corrosion-Proof

We’ll begin with the science behind standard steels. Steel is “stainless” – that is, doesn’t “stain” due to corrosion in specific circumstances – because it forms a self-repairing, protective layer referred to as a passive film. 316 stainless steel contain 16-18% chromium, which reacts with oxygen in air or process solution to form a chromium oxide layer – the passive film. Over time it will continue to re-form and provide a barrier to corrosion as the metal is scratched or otherwise degraded as part of regular use.

316 and 316L are austenitic stainless steels – they get their austenite structure primarily from the addition of nickel (about 11.5% of the total composition). They also contain small amounts of other austenite stabilizing elements like carbon, manganese, and nitrogen. Resistance to reducing acids and pitting and crevice corrosion is increased by adding molybdenum – about 2.1%. 316L differs from 316 by its lower carbon content, decreasing the susceptibility to intergranular corrosion. 

Corrosion Hazards in Sanitary and High-Purity Processing

All stainless steels and nickel alloys have a threshold beyond which they will likely corrode. If a corrosive attack occurs, the corrosion's mode and severity can tell you a lot about whether the right or wrong alloy is being used for the job.

Decades of experience with stainless steel processing equipment used in sanitary and high purity applications have shown that the most common modes of corrosive attack are:

• Crevice corrosion and pitting in the presence of chloride-containing processing solutions
• Stress corrosion cracking in chloride-containing environments that are exposed to temperatures above 50°C/122°F

Crevice corrosion is most likely to happen in aqueous chloride salt-containing solutions, which are almost ubiquitous in sanitary and high-purity processing:

• Sodium chloride (NaCl) is found in nearly all food and beverage product in varying quantities
• Potassium chloride (KCl) found in soups, sauces, and sports drinks
• Magnesium chloride, found in soymilk and peanut butter
• Calcium chloride, found in detergents and cleaners

In solution, the chloride ions (Cl-) and the hydrogen ion (H+), will concentrate inside the crevice, creating an acidic chloride environment that can react with stainless steels like 316 and cause a localized breakdown of the passive film and rapid attack inside the crevice. The likelihood of this form of corrosion increases as temperature rises.

C-22 is a nickel alloy with almost twice the quantity of nickel and molybdenum as AL-6XN and about the same amount of chromium. It also contains 3.5% tungsten to boost resistance to corrosion from chlorides further. It has high ductility, good weldability, and is easily fabricated into industrial components.

Designed to tolerate extremely corrosive conditions, C-22 provides superior protection from pitting and crevice corrosion, stress corrosion cracking in high-concentration chloride/high-temperature solutions, and mineral acids.

Mineral acids are a group of corrosive inorganic acids that includes hydrochloric, nitric, and sulfuric acids. The food, beverage, home and personal care, pharmaceutical, and chemical industries use hydrochloric acid, for example, for a variety of purposes, including:

• As an additive to adjust the pH of water, food, and drugs
• In the production of gelatin, fructose, citric acid, lysine, aspartame, and hydrolyzed vegetable protein
• In household cleaning products like disinfectants and tile cleaners
• To produce inorganic chemical compounds
• To purify table salt

Mineral acids’ corrosive properties are sometimes responsible for the products’ effectiveness - like toilet cleaners and descaling products household electric water kettles and clothes irons – and the reason the products contain them. But the corrosive characteristics that make these products work are also what create extreme and ongoing demands on the equipment that makes them.

Sodium hypochlorite, commonly used to clean and sanitize processing equipment, is also extremely harsh. Household bleach is around 5.25% sodium hypochlorite. The hypochlorite ion (OCl-) is highly aggressive and will cause pitting and crevice corrosion in most stainless-steel grades in a 5% solution at room temperature. At higher temperatures, stress corrosion is likely to occur.

C-22 is also strongly resistant to both oxidizing and reducing environments. High chromium content (20%-22.5%) boosts corrosion resistance in oxidizing media like wet chlorine because it creates a and maintains a protective layer. C-22 includes around 3.5% tungsten, which provides superior resistance to reducing environments. Neither AL-6XN nor standard steels contain tungsten. C-22’s molybdenum content also protects against reducing environments – it includes 12.5% to 14.5% molybdenum, around twice the amount in AL-6XN, and three to four times that in 316 stainless steel.

Table 1: Chemical Composition of Hastelloy C-22, AL-6XN and 316L

Note: All values in table are allowable maximum unless otherwise stated

C-22 and AL-6XN: Choosing the Right Alloy

Choosing the right corrosion-resistant Super Alloy is a vital part of project design for high-purity and sanitary processing equipment. The material must have sufficient corrosion resistance to withstand the service environment and cleaning regimens used. If they don’t, the system will be vulnerable to decreased service life, unexpected shutdowns, increased maintenance costs, and safety hazards. 

Choosing an over alloyed material (higher corrosion resistance than conditions may require) could increase project cost, but having a safety margin could also protect you in case of an unintended or intentional change in production processes that imposed more aggressive conditions.

So how do you know when the demands of your processes mean moving on from a 316/316L stainless steel to AL-6XN or the C-22 – and which is the right one for your situation?

A multitude of chemicals, temperatures, and pHs, in infinite combinations, characterize sanitary and high-purity processing environments, making it challenging to address selection guidelines for all situations fully.

The most widely encountered corrosive environments, though, involve chloride-bearing aqueous solutions at near neutral pH. In the presence of chlorides, stainless steels are most susceptible to pitting on boldly exposed surfaces and crevice corrosion in confined spaces. The environment becomes more corrosive, and the potential for pitting on boldly exposed surfaces and crevice corrosion, with:

• Rising chloride content
• Rising temperature
• Falling pH
• Oxidizing conditions such as encountered with oxidizing sanitizers such as chlorine or ozone.

Even under the most concentrated chloride levels (up to 100,000 mg/l - a 10% chlorine solution), AL-6XN will resist cracking at temperatures up to approximately 120°C/250°F. When service conditions involve chloride levels and temperatures that put AL-6XN at risk of cracking, C-22, which is immune to chloride stress corrosion cracking, is the right choice.

Step Two – Assess Pitting and Crevice Corrosion Risk

After assessing the risk of stress corrosion cracking, look at the potential for pitting and crevice corrosion in your processing environment. In near-neutral pH environments, you can use the maximum service temperature and chloride level for this.

Because crevice corrosion initiates more readily than pitting corrosion, it’s critical to know whether your system has crevices — it’s typically very difficult to eliminate all crevices. Unless you’re certain the system is crevice-free, it’s recommended to choose your construction material based on the crevice corrosion threshold.

Table 2 below gives threshold temperatures for initiating pitting and crevice corrosion at various chloride concentrations for 316/316L stainless steel and 6% Moly superaustenitic grades such as AL-6XN. It shows that if a proposed service environment has a maximum chloride content of 1,000 mg/l chloride and a maximum service temperature of 40°C/100°F, the 316/316L grade is expected to pit and crevice corrode because the threshold temperatures for crevice corrosion and pitting at 1,000 mg/l chloride is well below 40°C/100°F. 

 Under these conditions, AL-6XN is a good candidate because at 40°C/100°F it will resist crevice corrosion and pitting above that chloride level.

Table 2: Threshold Temperatures for Initiating Pitting and Crevice Corrosion

Data from Outokumpu Corrosion Handbook, Tenth Edition, Outokumpu Stainless AB, 2009

If you find that a material is unlikely to pit in your process environment but may be susceptible to crevice corrosion, you could still use that material if you can verify your system is crevice free.

When your environment is too aggressive for AL-6XN, for example 20,000 mg/l chloride at 50°C/122°F, it’s time to consider using C-22.

C-22 has outstanding resistance to localized corrosion and has been shown to resist crevice corrosion up to 55°C/131°F and pitting corrosion up to 100°C/212°F in a 6% ferric chloride solution, which corresponds to a chloride content of 39,000 mg/l and a pH of approximately 2.