pitting of AISI304 due to Microbiologically Influenced Corrosion

Pitting of Stainless steel drinking water lines due to microbial influenced corrosion

Microbial influenced corrosion (MIC) is a significant issue that affects the durability of stainless steel drinking water lines. This type of corrosion occurs when microbial biofilms form on the surface of the metal, leading to an electrochemical reaction that breaks down the protective layers and can result in pitting. Stainless steel is particularly susceptible to MIC as it contains chromium, which can be oxidized by microbial activity. If left unresolved, this corrosion can lead to structural damage and render piping unsafe for use. Therefore, addressing microbial influenced corrosion is a critical step in maintaining safe and efficient drinking water systems.

Type of materials being used

The most dominant type of steel being used in drinking water applications are austenitic stainless steel AISI304 (EN 1.4301) and 316 (EN 1.4401). Austenitic steels are highly resistant to pitting, but due to their chromium content can be vulnerable to microbial influenced corrosion.

Notwithstanding the fact that stainless steel is a remarkable metal alloy, ferritic chromium steels can be worth considering for certain situations. Contrarily to popular opinion, these may not always perform worse regarding corrosion than austenitic types – there are cases when they prove their value.

Many people unfairly assume that ferritic types are lower quality because they can be magnetized; however, this perception is highly subjective since even duplex stainless steel contains 50% ferrite and still attracts a magnet. Therefore, chromium steel should not be viewed in a negative light as it has its own unique applications.

Within the variety of chromium steel, Boasting 2% molybdenum and 18% chromium, Stainless steel 444 with EN 1.4521 material number is said to provides sublime corrosion immunity. Particularly effectual for heat exchangers, boilers and similar applications, the alloy has been officially accepted by both German and Swedish governments to be utilized in water pipes as well as other drinking water surroundings.

Advantages of chromium steel for drinking water purposes

When compared to austenitic types, ferritic types offer a number of advantages, including:

1. Compared to its austenitic counterparts, ferritic stainless steel has a higher thermal conductivity of 23W(m.K) compared with 12W(m.K). This gives it an advantageous edge when heat transfer is involved as the improved rate increases efficiency and productivity significantly!

2. Boasting a coefficient of expansion that is 1.5x lower than austenitic stainless steels and equal to carbon steel, this element offers an advantageous fusion during welding as well as resistance against thermal fatigue.

3. Ferritic chromium steels are significantly harder and more wear-resistant than austenitic stainless steels and, in addition, their mechanical values are better, which in principle allows lighter construction;

4. The advantages mentioned above afford one the benefit of utilizing significantly fewer tack welds prior to finishing welding.

5. Ferritic stainless steel is insensitive to stress corrosion cracking unlike the austenitic types;

6. In sulphurous gas environments, ferritic chromium steels are preferable to the austenitic grades.

7. It is less susceptible to cold hardening as with the austenitic grades;

8. Chrome steel is considerably cheaper to purchase than the austenitic grades because it lacks the expensive nickel.

Heatexchangers can be affected by Microbial Influenced Corrosion

It is important that steps are taken to prevent MIC in stainless steel drinking water lines. Precautionary measures include the use of clean drinking water without microbial contaminations or other enablers of microbial growth, maintaining a neutral pH level and cleaning pipes regularly. Additionally, it is important to avoid areas of stagnant water, as microbial activity is more likely in these conditions. If microbial influenced corrosion is already present, then more targeted methods such as electrochemical treatments may need to be employed. Furthermore, if the damage has become too extensive then the piping may have to be replaced altogether.

It is also important to monitor water chemistry regularly to detect changes that may indicate microbial or chemical attack of the metal. By following these steps, stainless steel drinking water lines will remain safe and efficient for many years to come.

Even though these measures have been taken, the near weld area can be affected by pitting corrosion. It is strongly recommended to monitor boiler systems and piping on the presence and activity of micro-organisms. Especially for systems placed in North Western Europe where almost no chemical treatment of the water is being done.

You can learn more about Microbial influenced corrosion by following our masterclasses on this topic

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