#Risk of Micro-biologically Influenced #Corrosion (MIC) Of Stainless Steel due to Presence of Stagnant Residual Water

Recently on my previous project I face MIC phenonena on SS 316L CIP Tank.Pits observed on 6 mm thick. tank base, Man Hole & on Shell up to stangent water level. Tank found leaking post 2 months of commissioning. See below snaps,

Microbiologically-Influenced Corrosion (MIC), also known as microbial corrosion or biological corrosion, is the deterioration of metals as a result of the metabolic activity of microorganisms.

There are about a dozen of bacteria known to cause microbiologically influenced corrosion of carbon steels, stainless steels, aluminum alloys and copper alloys in waters and soils with pH 4~9 and temperature 10^oC~50^oC.

These bacteria can be broadly classified as aerobic (requires oxygen to become active) or anaerobic (oxygen is toxic to the bacteria). Sulphate reducing bacteria (SRB) is anaerobic and is responsible for most instances of accelerated corrosion damages to ships and offshore steel structures. Iron and manganese oxidizing bacteria are aerobic and are frequently associated with accelerated pitting attacks on stainless steels at welds.

Industries hardest hit with high cost of repairs and replacements, plus lost revenues due to process downtime, are Onshore and offshore oil and gas industry, chemical process and nuclear power generation.

Pinhole leaks due to Microbiologically Influenced Corrosion (MIC) of 300 series stainless steel piping system have often occurred even before plant start- up; there are many cases of such failures in the industry. The problem is typically caused by residual water left in the system for extended period of time following hydro testing, flushing or commissioning during construction or extended shut downs. When such water is remain for a prolonged period, microbial activity can create an environment in which rapid chloride pitting corrosion occurs at much lower chloride concentrations then would normally be considered possible for the affected grade of stainless steel (e.g. 304,316 or 321) at ambient temperature. Although this topic has been widely discussed in oil & gas industry still there is general lack of awareness among the wider engineering disciplines. Consequently, it is the intent of this blog to heightened awareness & understanding of this problem & thereby assists to reduce the probability of this type of failure occurring in the future.

Background

MIC Failures caused by stagnant water - MIC commonly results when residual water is left in stainless steel system after hydro static testing or similar activities, and perforation of 3 mm thick piping in such circumstances has occurred in as little as 1 month. 
Some examples are summarized below,

Case #1- Potable Well Water for Hydro test

The failure of 304 SS piping that was hydro tested with potable well water containing about 250 ppm chloride. During hydro testing, the water was continuously chlorinated to prevent MIC, but the lines were not completely drained after testing & sat idle for 5 months. Within 24 hours of being placed in service, leaks were observed & radiography confirms that 80 % of welds contained extensive subsurface pitting damage. A powdery reddish brown deposit was found on internal pipe surface & failure mechanism was diagnosed as MIC.

Case #2- Samples from Different Power Plant, USA

The examination of twelve pipe sections of 304, 304L SS from different power plants in USA. The sections were each from horizontal pipes in which hydro test water had been left stagnant for several months before the plant was put in to service. For the 11 samples that had suffered from pitting corrosion, the common characteristics included pitting in the bottom third of the pipe, pits predominantly at or adjacent to welds, reddish mounds of deposits above each pit & extensive sub surface damage beneath pinholes visible at the surface.

Mechanism of MIC in SS in Natural Waters

The case studies above show that Iron Oxidising Bacteria (IOB), Manganese Oxidisng Bacteria (MnOB) & Sulphate Reducing Bacteria (SRB) are each often implicated in the MIC of SS caused by stagnant waters (such as residual hydro test water).

Stagnant & low flow conditions promote attachment of bacteria to metal surface. Attachment is followed by colonization & formation of discrete deposits, starting as a film (bio film) & frequently developing as bulky deposits (biomound). Composition of deposits is significantly different & more corrosive then the bulky water environment. Thus under deposit corrosion occurs by formation of concentration cells. And corrosion is accelerated because the oxygen that is essential to the stability of passive film is prevented from diffusing to the occluded sites.

Conclusion –

The occurrence of pinhole leaks due to microbiologically Influence corrosion (MIC) in ASS (Type 304, 321, 316) piping system is not a new problem. This mode of failure can often result when stagnant water s left inside the piping system for an extended period (i.e > 1 month). Consequently, this problem especially affects large projects with long layup times between hydro testing / flushing / commissioning & startup.

Typical features of this type of MIC include:

•    Pin hole leaks, usually found in the bottom (between 5 – 7 "O" clock ) piping & other locations where there   is trapped stagnant water (i.e. socket weld connections, non drain able expansion loop)

•    The pits are often located at or immediately adjacent to circumferential welds & are sometimes only found at welds.

•    The internal surface of the piping is often covered with a reddish brown or slimy deposit.

•    The pits have a typical ink- well shape, such that they are often difficult to see & the full extent of sub – surface damage is only revealed by probing the surface with a sharp object or Radiography.

•    Failure can occur in water with a composition that cause corrosion of SS 300 series SS at ambient temperature (i.e. as low as 100 ppm Cl-) & it must be noted that evaporation can lead to concentration above the nominal level in the in put water. (Shell DEP 74.00.10.10-Gen states that “ Hydro test water shall not contain a concentration of chlorides, either present originally or resulting from evaporation, that may precipitate cracking or pitting”)

•    The rate of penetration is of the order of 1 mm/month, resulting in perforation of standard schedule piping in 3 – 6 months from introduction of water to the system.

Prevention –

Failure due to residual water from hydro testing can be successfully avoided by diligent management of water quality & post – hydro test draining & drying.

•    Eliminate /minimize crevices in fabrication.

•    Avoid heat tint scale in pipe weld HAZ with good inert gas purging. Where unavoidable, remove heat tint scale by grinding / abrasive blasting / Pickling. Heat tint oxides make it poor barrier to any corrosive media. In addition, the chromium depleted layer of base metal beneath the heat tint is significantly less resistant to localized corrosion then the base metal. The corrosion of the heat – tint oxide can also activate the metal beneath it in a manner similar to free iron particles on a surface.

•    Use non – wicking gaskets at flange connections.

•    All introduced water shall contain < 2 ppm chloride & be free from any suspended solids.

•    Draining shall be done immediately. Inadequate draining will lead to ineffective drying.

•    System must be blown dry to ensure complete removal of water at “hang up” points such as welds.

•    Drying shall be done within 48 hours of introducing water to the system. Use of specialized contractors with high capacity dry air blowing units is recommended. 

•    The system shall be considered dry when blown air at the system out lets has a measured dew point < 0 Deg. Centigrade.

•    Slope horizontal piping to make them self draining.

•    High risk factors would include, but not limited to; long layup times, inadequate drain points, high ambient temperatures, local contaminants (such as dust & sand in a coastal environment), and limited availability of required quality water. If any of these factors present, then advice should be sought from Corrosion Engineer.