Microbes cause Catastrophic Failure of a Duplex Stainless Steel SOx Scrubber

R-TECH investigated the failure of a sulphur oxides (SOx) scrubber system which suffered multiple pin hole leaks after only one month of operation. The material was intended to be manufactured from duplex stainless steel, UNS S32750.

In January 2020, new rules were introduced by the International Maritime Organisation to drastically reduce emissions of sulphur oxides. The International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI specifies a global and a local limit on the sulphur content in marine fuel. In order to conform to these restrictions, there are two options; either use fuels with low sulphur content or apply an exhaust gas cleaning system to reduce the total emission of SOx. The most commonly used cleaning system is where the exhaust gas is cleaned through a cloud of water, thereby removing SOx and reducing the harmful emissions generated by burning fuel. Sulphur oxides dissolve in the water, typically containing calcium hydroxide or calcium carbonate, which neutralise the SOx emissions. The water is subsequently injected into the exhaust gas stream and discharged from the bottom of the scrubber. The scrubbing water must then be cleaned of particulate matter and other contaminants before being discharged out to sea.

The leak site, in-situ, is shown in Figure 1. The area of perforation is shown in Figures 2 to 4. The perforation was larger at the external surface and was associated with a dark brown coloured deposit confined to the area around the hole. At the internal surface, the area of perforation was associated with a black deposit with the remainder of the internal surface exhibiting a thin layer of dark brown coloured deposit. The deposit was found to consist predominantly of sulphur, oxygen, sodium and vanadium with trace amounts of chlorine, potassium, calcium and magnesium. Additional pits were noted along the internal surface of the pipe.

Upon sectioning, the area of perforation exhibited significant undercutting and metal loss, which was scalloped in nature, see Figures 5 to 8. Additional metal loss was evident along the internal surface. The area of perforation and the metal loss for one of the areas had occurred through a seam weld with evidence of selective attack of the austenite phase, see Figures 9 and 10. This also revealed the presence of sigma phase and carbide/nitride precipitation at the delta ferrite phase boundary in the parent material, see Figures 11 to 13.

The nature of the metal loss and the presence of a sulphur bearing species suggests that failure had occurred due to Microbiologically Induced Corrosion (MIC), from the internal surface. Microbiologically induced corrosion (MIC) is defined as corrosion that is influenced by the presence and activities of microorganisms. MIC is found where water is always or sometimes present, especially where stagnant or low flow conditions allow and / or promote the growth of organisms.

Micro-organisms on the surface do not directly attack metal, but their by-products promote several forms of corrosion, including pitting, crevice corrosion and under-deposit corrosion. There are two main types of microbiologically induced corrosion, aerobic and anaerobic. Aerobic bacteria can only survive in the presence of oxygen, while anaerobic bacteria can only survive in its absence. Both of these types of bacteria modify the environment, creating a favourable situation for corrosion to occur, creating crevices, differential aeration zones, or a more aggressive environment. In these cases, local breaks in the passivation layer on the steel surface are produced, resulting in more rapid local corrosion. The environment becomes more acidic under the bacteria colony on the surface, increasing the corrosive attack. Electrochemical cells can also be set up due to differential oxygen concentrations, again promoting localised corrosion.

An aggressive bacterium is the Sulphate Reducing Bacteria (SRB) type where perforation can occur very quickly, in a matter of weeks in some circumstances. The presence of a black, sulphur rich deposit may indicate the presence of iron sulphide which may suggest that the bacteria are of the sulphate reducing type, but this could only be confirmed by culturing a live sample of the bacteria. MIC as a result of sulphate reducing bacteria typically occurs at temperatures between 20 and 30°C, and at a pH between 6 and 8. Sulphate reducing bacteria are anaerobic and their characteristic nature is that they reduce sulphates to sulphides and produce hydrogen sulphide. Sulphide generation can be due to this direct reduction by the SRB, though the generation of sulphide may also be due to complementary activity between different bacteria in a consortium; individuals in a consortium can produce nutrients which are required by other species (including the SRB). Colonies of SRB can form deposits that are then conducive to under deposit corrosion. In most cases, the resultant corrosion is due to differential concentration, though the production of hydrogen sulphide in a crevice region would also accelerate corrosion rates. Corrosion is normally self-limited due to the build-up of corrosion products. However, microbes can absorb some of these materials in their metabolism, thus removing them from the anodic and cathodic sites. The removal of corrosion products then accelerates corrosion.

MIC is associated with pitting which is the predominant cause of fast and unexpected failures of stainless steel materials. MIC tends to selectively attack the austenite phase since those elements present in austenite, such as nickel, increase the likelihood of bacteria attachment and biofilm development. Nickel has been reported to enhance bacterial attachment as it increases the number of bacteria and colonies on the surfaces of materials in both aerobic and anaerobic environments. On the other hand, elements such as chromium and molybdenum (present in higher levels within delta ferrite) reduce the likelihood of bacteria attachment.

Microbiological influenced corrosion is an important factor in aggressive pitting corrosion of duplex stainless steel in SOx scrubbers. Sources of the bacteria can be from limestone and makeup water. The most severe corrosion is observed with sulphate reducing bacteria and the conditions that promote bacteria colonisation may be related to the grade of fuel burned. It appears that units handling high-sulphur fuels may be more susceptible to MIC than units burning low sulphur fuels. The unit should maintain operational usage as much as possible to ensure continued flow through the pipework and thus minimise the likelihood of MIC.

The chemical analysis for all sections examined complied with the UNS S32750 grade. The hardness levels were also considered acceptable. Duplex stainless steel contains high levels of Cr, Mo, and Ni resulting in excellent corrosion resistance and good pitting corrosion resistance. However, these materials are still susceptible to MIC. Significant chromium carbide/nitride precipitation was evident at the delta ferrite phase boundaries of the parent materials, seam welds and circumferential butt-welds. Precipitation of carbides and nitrides can greatly reduce the corrosion resistance of duplex stainless steels. Since some of the main alloying elements of duplex stainless steels i.e. chromium and molybdenum possess a high affinity for carbon, the tendency to form carbides is high. Further, carbides precipitate preferentially at the ferrite/austenite phase boundary since these sites are high in chromium and are associated with a high defect density as expected. ?The presence of chromium rich carbides/nitrides results in the formation of a wide chromium-depleted area adjacent to the precipitate which increases the likelihood of corrosion. Sigma phase was also present in the pipe parent of the pipe to pipe circumferentially welded section. Sigma phase can also increase susceptibility to corrosion and has a detrimental effect on impact toughness.

Carbide/nitride and sigma precipitation are a time and temperature dependent process. The presence of chromium carbides/nitrides and sigma phase in the parent materials indicates that the heat treatment of the pipework during manufacture was inadequate. It is important to keep the time between exiting the furnace and water quenching as short as possible. Carbides/nitrides were also evident in the seam weld which may suggest inadequate heat input during welding. The heat input during welding should be optimised so that the cooling rate is quick enough to avoid the presence of detrimental phases, though not so fast that there remains excessive ferrite in the vicinity of the fusion line.

The following recommendations can be implemented to reduce the likelihood of MIC:

1) The unit should maintain operational usage as much as possible to ensure continued flow through the pipework.

2) When performing a hydrotest ensure that clean water is utilised i.e. demineralised, potable etc.

3) Regardless of water quality, drain, dry and inspect to assure dryness immediately following a hydrotest, i.e. within 3 to 5 days.?

4) Develop and implement a MIC-prevention water treatment program with proper attention, control and monitoring.

5) It is important to keep the time between exiting the furnace and water quenching as short as possible. Further, the heat input during welding should be optimised so that the cooling rate is quick enough to avoid the presence of detrimental phases, though not so fast that there remains excessive ferrite in the vicinity of the fusion line.