Micro-organisms Destroyed Stainless Steel Installation
The installation was built in stainless steel 304L. It was designed for the production of soft water and was only 9 months old when the first leaks occurred. At first it seemed like bad engineering or construction, as the medium was just soft water fed by city water. It was however, micro-organisms that initiated the destruction of a brand new installation.
The evaluation of the leakage started with cutting one of the leaking pipes in half. Rust stains were observed around the welds. The internal surface shows severe deposit of red-brown corrosion product mostly on the weld and on both sides of the heat-affected zone.
Close to the mouth of the pit there is no deposit present and the metal surface appears shiny and bright. This is a clear indication that a microbial corrosion mechanism may be active. A cross section also reveals pinholes that are quite deep and interconnected.
Every cavity must be in contact with the environment for Microbiological Influenced Corrosion (MIC) to occur. SEM/EDS examination further revealed the presence of Sulphur. Sulphur is detected frequently when sulphate-reducing bacteria (SRB) are present or iron bacteria producing Sulphur.
Biocorrosion and Sulphate-Reducing Bacteria
Although corrosion is an electrochemical process, it is in 30 – 50 percent of the cases caused or accelerated by microbiological activities. In this case we speak of biocorrosion. It causes corrosion problems in often unexpected places and is developing 10 – 100 times faster than normal corrosion. It is therefore not surprising that biocorrosion is seen as one of the biggest enemies for the maintenance of pipelines and storage tanks.
Sulphate-reducing bacteria (SRB) are implicated most frequently in MIC. These bacteria use sulphate as a source of energy and consequently release the very toxic Hydrogen Sulphide (H2S) into their environment. Besides extensive corrosion, H2S causes significant other problems, like the typical rotten egg odour, blackening of equipment and slime formations. They grow under anaerobic conditions (they don’t use Oxygen) in an environment that contains sufficient sulphate and organic material.
Microbial Analyses Can Help
For determining the role of micro-organisms in this corrosion problem, water samples were taken from the system to perform analyses. Based on this examination Sulphur bacteria were detected in relatively high numbers.
The environmental conditions in the water system (Oxygen concentration, pH, temperature and flow rate) were optimal for Sulphur-oxidizing bacteria. These micro-organisms convert Sulphur compounds into sulphate. They can grow in a pH range of 2.0 to 8.5 (most species grow in the range of 5 to 8).
Optimum temperatures are generally between 25oC–40oC. The activity of Sulphur-oxidizing bacteria can cause corrosion and therefore result in metal loss. In this system a temperature was measured of around 25oC–27oC. These temperatures are ideal for microbial growth.
The water analysis demonstrated that micro-organisms perform Sulphur oxidation (also known as Sulphur-oxidising micro-organisms, SOB). These are aerobic and are therefore dependent on Oxygen.
The measured Oxygen concentration of 8 – 26 mg/l in the system is sufficient to meet their needs. If the water flow rate is low (at some locations less than 1 – 3 m3/h and even no flow at weekends) spontaneous growth can occur, allowing them to contribute strongly to the corrosion process. Once micro-organisms are present, it is difficult to totally eradicate them.
Corrosion and Leakagesin Stainless Steel 304L
Based on the above-mentioned chemical, physical and microbiological results, it was proven that the corrosion and subsequent leakages in the stainless steel 304L system are caused by Microbiologically Influenced Corrosion (MIC). It was promoted by low and no flow rates during prolonged periods that encourage microbial growth.
The malefactor in this particular case is the Sulphur Oxidising Bacteria (SOB) that enters the soft water system via the feed (city) water. During periods of no/very low flow rate and at dead ends SOB is able to attach to the internal surface of the pipes and to reproduce, especially on rough surfaces such as close to the welds.
Welding also creates an undesired Oxide layer, leaving the material more sensitive to certain types of corrosion, including MIC. The speed of MIC can be extremely high and therefore it is not exceptional that leakages occur in a period of several months only.
If the system had been thoroughly flushed from the very moment it became operational, MIC probably would not have occurred at all. In that case free-floating organisms do not get the opportunity to attach to the surface and reproduce.
I would like to emphasize that much attention must be paid in maintaining flow rates even when these are fed by city water, or contain chlorides in small amounts. The best quality stainless steel will not survive when the engineering and construction does not involve corrosion awareness.
Whether by lack of flow at for instance dead ends or electrochemical processes such as galvanic corrosion using dissimilar metals or chemical products, most corrosion problems are created on the drawing board. They are aggravated by construction and maintenance activities that do not take into account the risk of corrosion phenomena.
Additional Analyses and Recommendations
As stainless steel 304 does not contain Molybdenum (improves corrosion resistance) it was not the most suitable quality for this water softener system. Even though the welding is carried out with backing gas (argon) to prevent discoloration, the high welding temperatures still affect the corrosion resistant properties of the steel. For this reason it is recommended to pickle and passivate the system before commissioning.
Although the water analyses showed acceptable chloride concentrations, it should not be forgotten that during regeneration, thousands of ppm chlorides are flowing through the system. In particular, these chlorides may cause corrosion on 304 during periods of no flow and dead ends.
316L contains Molybdenum and therefore provides a much better resistance in chloride conditions and would therefore be a more appropriate choice for this purpose and temperatures up to 60oC. But also in the case of 316L, pre-commissioning pickling and passivation is recommended and the system must be engineered, constructed and maintained in such a way that micro-organisms do not get the chance to attach to the surface. To prevent this, sufficient flow rate and regular flushing of drains and possible dead ends (if these cannot be avoided) are essential.
If (in relation to micro-organisms) a sufficient flow rate and flushing cannot be managed and the engineering, construction and maintenance of the system are not based on sufficient corrosion awareness, the corrosion resistant properties of stainless steel 316L will also eventually be affected. In this case protection against MIC and chlorides should be considered.
Plastics can help in some applications. For media in the range of -5°C to 30°C HDPE PE100 piping is recommend, class SDR 17 (corresponding to PN 10). For media in the range of 15°C to 60°C PP piping is recommend, class SDR 11 (corresponding to PN 10).
In this particular case it is assumed the pipes are applied inside the building. For piping with a diameter DN 400 and larger, GRP offers the advantage of a lower price level. In the case of PP and HDPE much of the internal diameter is lost at pressure stages SDR 11 and SDR 17. A smaller diameter may affect the required flow rate though. On the other hand, if the water requirement remains the same, a smaller diameter may increase the flow rate.
New heat-cured coatings on the market offer the possibility of upgrading carbon steel piping to a system that is resistant to most chemicals like salt water, MIC, chlorides. Carbon steel pipes and bends (partly prefabricated) are heat-cured coated in an oven.
For assembly Victaulic couplings are used. As the steel is insulated by the coating, valves of dissimilar metals can be applied without the risk of galvanic corrosion. But in many cases valves can also be coated, or made of PP or HDPE.
Sources used for this article
Exova – Failure analyses
Bioclear – Microbial analyses
Thermopol – Plastics
CP Phenolics – Heat cured coatings
These all are based in the Netherlands and are members of the Corrosion Control Technology Alliance, www.corrosioncontrol.nl
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