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What You Need to Know About Corrosion Resistance

As the bioprocessing industry continues to evolve and improve its processes, we all need to be aware of the challenges this evolution may create for systems and equipment, including the materials used to build these systems and equipment. One of the specific areas to keep an eye on is corrosion resistance. Choosing the right material is a balancing act between service life and cost.

The more corrosion resistant a material is, the costlier it becomes. Finding the right balance between costs (capital outlay and long-term maintenance) and performance is always important. Corrosion-resistant stainless is more expensive because of the increase in certain chemicals in the material, primarily chromium, nickel, and molybdenum.

The technology used to make stainless steel has improved with time, allowing for better control of the chemistry in stainless steel as well as better control of your construction and maintenance costs. So it might be better for you to specify a higher grade of stainless steel than you typically order.

Causes of Corrosion

To better understand your stainless steel options, it is useful to know more about what makes stainless steel resistant to corrosion. Basically, stainless steel gets its corrosion resistance from a passive chromium-oxide film that forms when chromium is exposed to oxygen. While this film can be damaged, it typically repairs itself when exposed to oxygen. Increased levels of molybdenum and nickel also improve corrosion resistance. In addition, heat treatments and surface conditioning can improve resistance.

The environment the stainless steel is exposed to can cause different types of corrosion. In bioprocessing applications, the primary corrosion types for stainless steel are:

  • Pitting corrosionThe passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl is the most common of these, and is found in everyday materials such as salt and bleach. Pitting corrosion can be avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals, or by choosing an alloy that is more resistant to attack. The pitting corrosion resistance can be assessed using the pitting resistance equivalent number (PREN) calculated from the alloy content. (More about this later.)
  • Crevice corrosionStainless steel requires a supply of oxygen to allow the passive chromium film layer to form on the surface. In very tight crevices, it is not always possible for oxygen to reach the stainless steel surface. As a result, the surface is vulnerable to attack.

It is possible to avoid crevice corrosion by minimizing crevices, such as those created by poor product design or by poorly fitting gaskets. As with pitting resistance, crevice corrosion resistance is better in higher-grade alloys.

Not All Stainless Steel Is Created Equal

Over the years, many stainless steel alloys have been developed—dozens of them. Each alloy has its own particular metal composition that enables it to handle specific applications or situations. Although there are a number of elements that are used to create these alloys, three elements in particular are used:

  • Chromium—Provides resistance to oxidizing environments; the more chromium, the more resistance.
  • Nickel—Stabilizes the microstructure of the materials to improve overall general corrosion resistance; supports the chromium layer.
  • Molybdenum—Improves resistance to chlorides and other harsh chemicals; especially good at preventing crevice corrosion.

Table 1, below, shows the chemical composition of five common stainless steel alloys (in percentages). As the table shows, there are two types of 316L commonly used in bioprocessing: EN 1.4404 and EN 1.4435 (EN 1.4404 is the more commonly used material). As shown in the table, the ranges of chromium, molybdenum, and nickel vary slightly for these two 316L alloys. The higher chromium value in EN 1.4435 gives it more corrosion resistance than EN 1.4404.

Element %

316L
EN 1.4404

316L
EN 1.4435

AL-6XN

Hastelloy C-22

Hastelloy C-276

Chromium

16.5 – 18.5

17.0 – 19.0

20.0 – 22.0

20.0 – 22.5

14.5 – 16.5

Molybdenum

2.0 – 2.5

2.5 – 3.5

6.0 – 7.0

12.5 – 14.5

15.0 – 17.0

Nickel

10.0 – 13.0

12.5 – 15.0

23.5 – 25.5

Remainder

Remainder

Iron

Remainder

Remainder

Remainder

2.0 – 6.0

4.0 – 7.0

Carbon

0.03 max

0.03 max

0.03 max

0.015 max

0.02 max

Cobalt

 

 

 

2.5 max

2.5 max

Copper

 

 

0.75 max

0.5 max

0.5 max

Manganese

2.0 max

2.0 max

0.40 max

0.50 max

1.0 max

Nitrogen

0.11 max

0.11 max

0.18 – 0.25

 

 

Phosphorus

0.045

0.045

0.04

0.02

0.04

Silicon

0.75 max

0.75 max

1.0 max

0.08 max

0.08 max

Sulfur

0.015 max

0.015 max

0.001 max

0.02 max

0.03 max

Tungsten

 

 

 

2.5 – 3.5

3.0 – 4.5

Vanadium

 

 

 

0.35 max

0.35 max

Table 1. Chemical compositions (percentages) of a few stainless steels

Several of the elements in Table 1 are specified in ranges. Because of the advanced control over the chemistry of these materials, an alloy can be made to the alloy standard at the minimum or maximum of the allowed range. For example, EN 1.4404 can be made with a chromium composition of 16.5, instead of the 18.5 max. This lower value results in a less costly and easier to fabricate material; however, it also has a lower corrosion resistance. Therefore, it can be important when selecting a material to know the exact composition of the alloy.

PREN comparison

Figure 1. Minimum PREN for each material

Figure 1, above, shows that the combination of chromium and molybdenum levels (further enhanced by nitrogen) have a significant impact on the overall pitting resistance equivalent number (PREN) of a material. Higher-grade alloys, such as 6% molybdenum and super duplex, can have much higher resistance.

Notice that AL-6XN and the Hastelloy alloys in the figure have much higher chromium levels, and therefore higher resistance to corrosion. As products become more corrosion resistant by using these special alloys, they also become increasingly expensive. This is because the raw materials (alloys) cost more, and the alloys are more challenging to work with when fabricating parts or equipment. For example, machining AL-6XN is typically greater than three (3)gg or more times more expensive than EN 1.4404.

Do You Need Increased Corrosion Resistance?

For most applications, EN 1.4404 provides sufficient corrosion resistance with proper maintenance of the chromium-oxide layer. However, it is useful to evaluate where you may benefit from the increased corrosion resistance of other stainless steel alloys.

If you have a hold tank with chemicals that will likely corrode an EN 1.4404 material unless routine passivation (adding a coat of protective material) is performed on a frequent basis, it could be worthwhile to make your vessel (including nozzles, valves, and other accessories) out of a higher-grade material where it is attached to the vessel. For example, fabricating the main body of a valve from EN 1.4435, but using EN 1.4404 for the outlet, could provide a balance between cost and service life.

In other situations, you may benefit from fabricating an entire system from a higher-grade material. It is important to take into account not just the capital costs, but also the long-term maintenance costs and the expected life of the system. These factors are vital in determining the right choice of materials for your systems. This is especially true because the raw materials can be made more consistently and with better control of their chemistry and cost.

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