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HOW TO OVERCOME CORROSION WITH SUPER ALLOYS™ AL-6XN® AND HASTELLOY C-22

In sanitary and high-purity processing, equipment has a tough job to do. It has to withstand a range of harsh factors, year after year, that can cause corrosion and failure if the materials it's constructed of are not right al6xn vs hastelloy.

For example, 316/316L stainless steels (UNS S31603) al6xn vs 316l are widely considered the “workhorse” materials of construction for high purity and sanitary tubes, process components and equipment for food, beverage, personal and home care, pharmaceutical, and biotech processing. The corrosion resistance of commonly used stainless steels can be pushed to the limit by certain factors 6xn:

These conditions include:

  • Temperatures that are high
  • A low pH value
  • Products being processed contain chemicals
  • Products used to clean equipment contain chemicals

It's for this reason that engineers must take into account long-term corrosion resistance when designing systems, as well as the correct selection of construction materials.

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PROCESSING CONDITIONS DEMAND MORE FROM SUPER ALLOYSTM

There are many modern processing environments where corrosion resistance is more important than austenitic stainless steels such as 316 and 316L. For the design and construction of systems that operate under harsh conditions, corrosion-resistant Super AlloysTMAL-6XN and Hastelloy C-22 are excellent alternatives.

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

Higher molybdenum content increases resistance to chloride stress corrosion cracking, and higher nickel content increases resistance to formic and phosphorous acid environments.

To help you choose the right material for your application, we will explain the differences between corrosion-resistant Super Alloy AL-6XN and Hastelloy C-22 in this article. To begin this discussion — and provide a comparison between applications for Super Alloys — we need to look at some standard Stainless Steel materials used in processing and their limitations.

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CORROSION-RESISTANT, BUT NOT CORROSION-PROOF: STAINLESS STEEL

Firstly, let's take a look at the science behind standard steel. Steel is "stainless" because it forms a self-repairing, protective layer known as a passive film. In 316 stainless steel, 16-18% of the chromium reacts with oxygen in the air or process solution to form chromium oxide - the passive film. During regular use, the metal will re-form and provide a barrier to corrosion as it scratches or otherwise degrades.

A stainless steel with an austenitic structure is 316 and 316L – nickel (about 11.5% of total composition) is added as a primary component of their austenite structure. As well as carbon, manganese, and nitrogen, they contain small amounts of other austenite stabilising elements. Molybdenum – about 2.1% – increases resistance to reducing acids, pitting, and crevice corrosion. 316L has a lower carbon content than 316, which lowers intergranular corrosion susceptibility. 

CORROSION HAZARDS IN HIGH-PURITY AND SANITARY PROCESSING

There is a threshold beyond which stainless steels and nickel alloys will corrode. The mode and severity of corrosion can tell you a lot about whether the right alloy has been selected for the job.

Based on decades of experience with stainless steel processing equipment used in sanitary and high purity applications, the following modes of corrosion are most common:

Corrosion and pitting in crevices when chloride-containing solutions are used

When chloride-containing environments are exposed to temperatures over 50°C/122°F, stress corrosion cracking may occur

CREVICE CORROSION

A crevice corrosion occurs when the surface passive layer breaks down due to design or fabrication defects or deposits that settle in processing equipment.

Joints, under or between flanges, and gaskets or other contact areas, such as valve seats, can have engineered or "designed in" crevices or gaps.

Tight deep crevices are more likely to be attacked than wide shallow crevices because of their geometry.

Aqueous chloride salt solutions, which are almost ubiquitous in sanitary and high-purity processing, are most likely to cause crevice corrosion:

Nearly all food and beverage products contain sodium chloride (NaCl) in varying amounts

Sports drinks, soups, and sauces contain potassium chloride (KCl)

Peanut butter and soy milk contain magnesium chloride

Detergents and cleaners contain calcium chloride

In solution, the chloride ions (Cl-) and the hydrogen ions (H+), will concentrate inside the crevice, causing an acidic chloride environment which will react with stainless steels like 316 and cause a localised breakdown of the passive film, resulting in rapid corrosion within the crevice. With increasing temperatures, this form of corrosion becomes more likely.

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PITTING CORROSION

It is most likely to occur in aqueous chloride-containing solutions with increasing temperatures. Pitting corrosion occurs as deep cavities in a material's surface. As a result of inclusions, mechanical defects, or places with weld heat tint on the surface, pits will initiate.

When pits are developed, they grow very similarly to crevice corrosion. Essentially, the pit's interior becomes more acidic and enriched with chlorides, exacerbated pit growth. Pitting is easier to prevent than to stop once it begins.

STRESS CORROSION CRACKING

This type of corrosion appears as cracks in materials exposed to corrosive environments and high temperatures - such as chlorides and temperature higher than 50°C/122°F.

There are two sources of tensile stress: service conditions such as high pressures or residual stresses associated with welds.

Cracks caused by stress corrosion are usually found adjacent to welds, where service stress and residual stress combine to cause the most stress corrosion.

ALUMINUM ALLOY AL-6XN AND HASTELLOY C-22

THE AL-6XN HAS EXCELLENT CORROSION RESISTANCE AND WORKABILITY

Superaustenitic stainless steels such as AL-6XN are characterised by a relatively high amount of chromium (20-22%), about 6% molybdenum, and a relatively high amount of nitrogen (0.18-0.25%).

Furthermore, it provides better corrosion resistance, is approximately 50% stronger (based on yield strength) than 300-series austenitic stainless steel, and is workable and weldable.

AL-6XN CASE STUDY

A global producer of food and beverage products contacted CSI with reports of frequent leaks in 316L stainless steel tubing and fittings. According to CSI, the producer processes ultra-high temperature fruit juices and its lines are experiencing pitting corrosion and stress corrosion cracks.

When 316L is exposed to high-acidic, high-temperature environments, its passive layer is damaged, allowing corrosion to occur, resulting in pits that eventually become visible. As opposed to the weld areas, these pits became initiation sites for stress corrosion cracks in the base metal, indicating that 316L was not suited for the processing task.

In order to solve the corrosion problem, the producer upgraded from 316L to AL-6XN after CSI found its root cause. The producer experienced no further material failures as CSI provided all the necessary tubing and fittings from existing stock, saving millions of dollars.

CORROSION PROTECTION FOR THE HARDEST ENVIRONMENTS WITH HASTELLOY C-22

Among stainless steels, AL-6XN has an unparalleled combination of corrosion resistance and excellent fabrication properties, but there are limits.

As a result of extreme temperatures, high chlorides, and acidic conditions, AL-6XN is susceptible to pitting, crevice corrosion, and stress corrosion cracking in very aggressive environments.

If AL-6XN is not suitable for the environment, Hastelloy C-22 offers significant corrosion resistance improvements.

In C-22, nickel and molybdenum are almost twice as abundant as in AL-6XN, and chromium is about the same as in AL-6XN. There is 3.5% tungsten added to enhance corrosion resistance from chlorides. It is easily fabricated into industrial components because of its high ductility, good weldability, and high ductility.

Due to its ability to withstand extremely corrosive conditions, C-22 is particularly resistant to pitting and crevice corrosion, stress corrosion cracking, and mineral acid degradation.

There are many uses for mineral acids, including hydrochloric acid, nitric acid, sulfuric acid, and nitric oxide. The food, beverage, household, pharmaceutical, and chemical industries use hydrochloric acid, for example, for:

Water, food, and drugs can be adjusted with this additive

The production of gelatin, fructose, citric acid, lysine, aspartame, and hydrolyzed vegetable proteins

Cleaning products for the home, such as disinfectants and tile cleaners, contain these chemicals

Inorganic chemical compounds are produced by this process

Salt is purified to be used in cooking

A mineral acid’s corrosive properties are sometimes responsible for the effectiveness of some products - such as toilet cleaners, descalers, electric kettles, and clothes irons - and the reason they are contained in them in the first place. In addition to creating extreme and ongoing demands on the equipment used to make these products, their corrosive characteristics also contribute to their success.

Cleaning and sanitising processing equipment with sodium hypochlorite is extremely harsh, as well. At room temperature, the hypochlorite ion (OCl-) will cause pitting and crevice corrosion in stainless steel grades. Household bleach contains around 5.25% sodium hypochlorite. Stress corrosion is likely to occur at higher temperatures as well.

As well as being highly resistant to oxidising and reducing environments, C-22 has a high chromium content (20%-22.5%), which creates and maintains a protective layer in oxidising media like wet chlorine. The C-22 alloy contains approximately 3.5% tungsten, which makes it stronger against reducing environments. AL-6XN and standard steels do not contain tungsten. Its molybdenum content protects it against reducing environments as well - it contains 12.5% to 14.5% molybdenum, around twice as much as AL-6XN, and three to four times that of 316 stainless steel.

Hastelloy C-22, AL-6XN, and 316L chemical compositions

Alloys C-22 and AL-6XN: Choosing the right one

Choosing the right corrosion-resistant Super Alloy for high-purity and sanitary processing equipment is a crucial part of the project design. Corrosion-resistant materials are required to withstand service environments and cleaning regimens. They will be at risk of reduced service life, unexpected shutdowns, increased maintenance costs, and safety hazards if they fail. 

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

How do you know when to move from 316/316L stainless steel to AL-6XN or C-22 - and which is the best for your needs?

Chemicals, temperatures, and pHs, in infinite combinations, characterise sanitary and high-purity processing environments, making it difficult to address selection guidelines for all scenarios.

Most corrosion occurs from chloride-bearing aqueous solutions with pH near neutral. In the presence of chlorides, stainless steels are susceptible to pitting on boldly exposed surfaces as well as crevice corrosion in confined areas.

Environmental conditions become more corrosive, and pitting on boldly exposed surfaces and crevice corrosion become more likely, with:

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

ASSESS THE RISK OF STRESS CORROSION CRACKING

An assessment of the potential for stress corrosion cracking is necessary for the selection of appropriate materials. Stress corrosion cracking can result in rapid catastrophic system failures.

It should never be used without a thorough review of your process requirements and demands by a qualified corrosion specialist because chlorides, tensile stress, and temperatures above 50°C/122°F can cause stress corrosion cracking of 316/316L stainless steel.

As an alternative to 316/316L, AL-6XN is excellent when stress corrosion cracking is a concern.

In stainless steels containing chlorides, the threshold temperature for cracking in AL-6XN is considerably higher than that of 316/316L.

AL-6XN resists cracking at temperatures up to 120°C/250°F even under the most concentrated chloride levels (up to 100,000 mg/l - 10% chlorine solution). C-22, which is immune to chloride stress corrosion cracking, is the right choice when service conditions involve chloride levels and temperatures that may cause AL-6XN to crack.

THE SECOND STEP IS TO ASSESS THE RISK OF PITTING AND CREEVICE CORROSION

Assess the risk of stress corrosion cracking in your environment before considering pitting and crevice corrosion. You can determine the maximum temperature and chloride level in near-neutral pH environments.

Because crevice corrosion initiates more readily than pitting corrosion, it's crucial to know if your system has crevices — eliminating all crevices is typically very challenging. It's recommended to select a construction material based on its crevice corrosion threshold unless you're sure the system is crevice-free.

The table below gives threshold temperatures for pitting and crevice corrosion in 316/316L stainless steel and 6% Moly super austenitic grades such as AL-6XN at various chloride concentrations. If the maximum chloride content of a proposed service environment is 1,000 mg/l chloride and the maximum service temperature is 40°C/100°F, the 316/316L grade is likely to pit and crevice corrode, since the temperature threshold for crevice corrosion and pitting at 1,000 mg/l chloride is well below 40°C/100°F. 

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

Temperatures that initiate pitting and crevice corrosion are listed in Table 2

You may still be able to use a material that may be susceptible to crevice corrosion in your process environment if you can verify that your system is crevice-free.

For environments with too much chloride, for example 20,000 mg/l at 50°C/122°F, it's time to consider C-22.

It has been shown that C-22 is highly resistant to localised corrosion, and has been shown to withstand crevice corrosion in 6% ferric chloride solutions at 55°C/131°F and pitting corrosion at 100°C/212°F.

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