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Microbial corrosion: Are stainless steels resistant?

Stainless steels are believed to be very resistant to corrosion, so much that various classes of this ferrous materials are among the immediate remedies against varying forms of corrosion. When it comes to microbial corrosion (MIC), the alloying elements in stainless steels – particularly molybdenum and chromium – are the elements that render these steels MIC-resistant.
 
However, any factor that could destroy the protective, anti-corrosion film of chromium oxide on the stainless steels will be also detrimental to the steel. In this article, we will briefly mention some situations where stainless steels could become vulnerable to corrosion, and how to properly mitigate it.

By Dr. Reza Javaherdashti, General Manager, Eninco Engineering B.V.

Corrosion is a thermodynamically favorable reaction. This seemingly simple sentence means that corrosion is as natural as breathing—it cannot be stopped. What can be done about corrosion is to control it, meaning that we can only bring its severity (measured by ‘corrosion rate’) down to the levels that are too slow to pose any threat against the assets and equipment.

You may have noticed from the above paragraph that we also put emphasis on the part that is most important to the overall system. It is a fact that in any given plant or piece of equipment, corrosion of few parts is of importance: this is what we refer to as 20-80 law. This means that, when applied to a corrosion and integrity management context, most accidents can be expected to occur in just a few assets; while corrosion in the chassis of a car is certainly important, it cannot be more important than corrosion in the brake system.

In any plant, corrosion can be a serious issue for three categories of equipment:

1. Production equipment and assets (process assets);

2. Auxiliary assets; and,

3. Safety assets.

There are only five measures that can be applied to control corrosion. With the technology and science we currently possess, there is no sixth or seventh measure. The five anti-corrosion measures can be categorized as below:

1. Physical means – the application of protective coatings where the interaction between the anode/cathode and with electrolyte outside is severed. 

2. Mechanical measures – pigging within a pipe that would serve to remove scale and debris to prevent formation of under deposit corrosion with operating mechanisms.

3. Chemical measures – the use of corrosion inhibitors and, in the case of microbial corrosion (MIC), the use of biocides.

4. Electrical measures – the use of cathodic and/or anodic protections. In the former, corrosion potentials are pushing down as much as possible. In the latter, the corrosion potential is kept high enough so that potentials will be held within the limits of a passivation area, and thus the metal is always in its passivated form.

5. Materials selection and design measures – this means either by switching into materials with higher corrosion resistance by modification of design, corrosion will be controlled.

The various grades of stainless steel

One of the most widely used ferrous materials in industrial sectors across the globe is stainless steel and its various grades. In fact, if you consider stainless steel type 304, 18-8 with 18% Fe (iron), 8-10% Ni (nickel), and 20% Cr (chromium) as the ‘basic’ stainless steel, by simply adding or removing elements, you can create different grades of stainless steel with different capabilities.

By adding Ti (titanium) to overcome sensitization, you make stainless steel 321. To reduce the sensitization, you can lower the carbon content; to improve the pitting resistance, you can add in Mo (molybdenum), which will result in types 304L, 316L and 317L. By adding Cu (copper), Ti and Al (aluminum), while lowering the Ni content for precipitation hardening, you can get precipitation-hardening stainless steel. Likewise, if you take all the Ni content from stainless steel 304, and lower the Cr martensitic, you will get types 403, 410 and 420.


Microbiologically influenced corrosion

In all the variations you can get by ‘playing’ with the alloying elements, what makes stainless steels—particularly ‘austenitic’ stainless steels—considered ‘stainless’ is that it requires  the addition of two alloying elements: Cr and Mo. Chromium serves to form a protective chromium oxide film on the steel substrate to make it resistant towards corrosive electrolytes. It is mainly the effects of molybdenum on bacteria that can keep the stainless steel strong against MIC.

In fact, one of the most significant reasons why industries prefer stainless steels over carbon steel is that the former is much more resistant to corrosion than the latter. However, if stainless steels are corrosion resistant to electrochemical corrosion and MIC, why is there a history of cases involving corroded stainless steel?

A very quick review of examples related to MIC and non-MIC corrosion cases can clearly reveal how frequent and how easily stainless steels can become vulnerable. However, not all types of stainless steels exhibit the expected corrosion resistance performance against MIC. Stainless steel type 304, for example, does not show the same performance as 316L or even SAF2205. In addition, when MIC is blended with conditions that favor stress corrosion cracking (SCC), this can even cause the premature failure of duplex stainless steels, let alone austenitic stainless steels.

Industrial vulnerability to MIC

MIC could be a serious issue in almost every industry where the metal comes into contact with untreated, stagnant water that also contains enough nutrients to allow growth and activity of bacteria. The bacteria activity could be either on process assets, auxiliary assets or in safety assets. In chemical processing and petrochemical industries, for instance, MIC can be an issue in stainless steel tanks, pipelines, flanged joints – particularly in welded areas after being hydrotested, as well as heat exchangers and underground fire water rings.

What was said for petrochemical or chemical processing industries can be taken as a general pattern for other industries, as well as power plants, water and wastewater treatment, offshore platforms, and ports. The main point is to always bear in mind that good housekeeping must happen, in terms of (1) doing the appropriate corrosion management, (2) continuous monitoring, and (3) obtaining proper training.

A corrosion management plan

What makes stainless steels become more vulnerable to corrosion in general, and MIC in particular? If you have done your corrosion management homework well enough, having corrosion-related bacteria (CRB) in the stainless steel environment is not likely to induce corrosion.1 One of the important factors to consider throughout proper corrosion management is the selection of suitable materials. In the case Dr. Reza Javaherdashti tested, stainless steel type 316L was used in an environment inoculated with known species of CRB, and it was observed that no corrosion occurred. This finding was in accordance with NACE (now, the Association for Materials Protection and Performance [AMPP]) related standards regarding MIC, that clearly states finding bacteria in the environment is not the exclusive factor based on which one can judge the MIC possibility.

There are certain circumstances in which stainless steels can become vulnerable to all types of corrosion. Three such circumstances are:

1. The protective chromium oxide film vanishes: the ‘stainlessness’ of steels is a result of a protective film that is formed on these ferrous materials. If the protective film is either dissolved chemically or damaged physically/mechanically, then the underlying ferrous substrate will be exposed to the corrosive electrolyte outside and therefore will be highly likely to be corroded away. Examples of conditions by which the film is damaged are environments where physical-mechanical erosion is to be expected, and/or the environment dissolves the coating.

2. Concentration of chlorides: this normally happens during hydrostatic testing with seawater as the hydrotest medium. A rather common practice in hydrostatic testing is that, after it is over, a wet lay-up regime is applied. In an ideal situation, the wet lay-up is the seawater that has been used for the hydrotest is well-treated with a carefully designed corrosion inhibitor-biocide cocktail, and then let to rest until the time the pipeline is to be commissioned. Alternatively, the line is emptied from the hydrotest water and rinsed and re-filled with fresh water, of which all chemical anti-corrosion measures has been taken care of. In real life conditions, however, neither is done. Typically the seawater remains untreated of undertreated in the line for a long time. During this time, the dissolved chlorides – under the effect of surface temperature and the activity of temenos-forming (biofilm-forming)2 bacteria – are locally concentrated enough to cause chemical disruption of the protective oxide film on the steel. The net effect is local failure of the steel, and particularly if the material of construction has been stainless steel. Dr. Javaherdashti has witnessed such failed cases for stainless steel pipes on which hydrotests had been performed. The best way to avoid such circumstances is to not let the chloride-containing water become stagnant, so that the chloride concentration increases locally. Emptying the hydrotest water as soon as the test is over is the best remedy.

3. Improper post-weld treatment: normally stainless steels do not have an easy fabrication process, due to the severe cold working that must happen, or the specific welding process they go through, due to the migration of chromium from the bulk to the grain boundaries and thus creating sensitizations unless the steel has been treated for such cases. Welding is a very important factor that can render the heat affected zone (HAZ) of stainless steel quite vulnerable to types of corrosion damage. One possible explanation could be due to the input heat, which is carried out during thermal welding procedures, where a gradient is formed as a result. The nearby alloying elements are then concentrated in HAZ; this makes the area ideal for sessile bacteria, which prefer landing on the spot, to ‘slurp’ the required elements necessary for their growth and acting as nutrients from the HAZ. Therefore, to control MIC in welding, it is necessary to apply proper post weld treatment (PWT) to release the stresses im-posed by putting in heat via thermal welding.

Final words

Stainless steels are very significant in advancing the industrial civilization; however, they do have their own weak points. These weak points could become too visible when these materials become exposed to corrosive conditions, especially those created by CRB. The best thing we can do in addition to upgrading stainless steels’ anti-corrosion features – by adding more alloying elements to induce such properties – and the same time skyrocket the process, could be relatively much cheaper option: having a smart corrosion management in place!3


References

1. Reza Javaherdashti, “Microbiologically influenced corrosion—An engineering insight,” Springer-Verlag, UK, 2nd edition, 2017.

2. Reza Javaherdashti, “Some thoughts about misconceptions surrounding the term ‘biofilm’,” Corrosion Engineering, Science and Technology, Vol.55, pp: 681-684, June 2020.

3. Reza Javaherdashti, “Management of corrosion: A smarter, more innovative approach towards corrosion management,” to be published by Wiley, USA, 2021.


About the author

Dr. Reza Javaherdashti holds a PhD in Materials Science – Corrosion. He has more than 25 years of experience and has successfully delivered 400+ projects in corrosion management, problem-shooting, expert-witnessing and Root Cause Analysis in various industries across the globe. He is a certified corrosion and MIC lecturer by ASME (U.S.A.) and Society of Petroleum Engineers (U.S.A.). He has performed 5,000+ hours of teaching corrosion management and MIC to various industries (from oil and gas to mining, aviation, and chemical industries).

 

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