PTQ Q3 2026 Issue

Microbial group Corrosion mechanism Impact on the system Sulphate reducing Reduction of sulphates Formation of deep pits bacteria (SRB) to hydrogen sulphide and hydrogen (H₂S) embrittlement Acid producing Excretion of organic Local pH reduction and bacteria (APB) acids (acetic, lactic) dissolution of the metal matrix Iron oxidising Oxidation of Formation of tubercles bacteria Fe₂+ to Fe₃+ and obstructive deposits Main microbiologically influenced corrosion mechanisms

Fuel

Biomass

Water

A : Dispersed as micro-droplets in the fuel – DROPLET B : Emulsion at the oil-water interface – EMULSIFIED C : Ballast water – FREE

Table 1

include increased time required for product certification and, in severe cases, blockage of filters and fuel injection systems. Case study: Mataripe Refinery The Mataripe Refinery, located in the northeastern Brazilian state of Bahia, was experiencing significant operational delays due to diesel contamination. Diesel stored in the tanks showed high turbidity and sludge accumulation at the water-fuel interface, leading to increased settling time required for certification, higher laboratory analysis costs, and more frequent tank cleaning. In addition to microbiological contamination, other poten - tial contributing factors were considered, including water ingress and accumulation, oxidation and thermal degrada - tion of the fuel, contamination by particulates or corrosion products, and operational factors such as inadequate tank drainage or mixing. Despite the consideration of these alternative hypotheses, microbiological contamination was identified as the most likely root cause and therefore prioritised for investigation. To assess the extent of contamination, microbiological analyses were performed to determine total aerobic bacteria (TAB) counts in diesel S10 samples collected from the bottoms of two storage tanks (A and B). Figure 2 illustrates the sam- pling locations of the aliquots used for monitoring. The results indicated high TAB concentrations in both samples (see Table 2 ). This level of contamination is indic - ative of significant microbial activity in the storage system and, according to widely adopted technical guidelines, ele - vated microorganism concentrations in fuel and, in particular, in fuel-associated water are considered a high operational risk, requiring immediate corrective actions.1 , 7 Mitigation strategy The remediation plan combined integrated physical and chemical strategies, aiming both to remove conditions favourable to microbial growth and to effectively eliminate microbial populations already established in the system. Physical actions included free water drainage and intensi - fied tank cleaning procedures, reducing the primary niche for microbial development. In parallel, the proprietary biocide Defender SR 1664B, formulated for slow release of active agents and developed by the Brazilian Dorf Ketal team, was applied to ensure Figure 2 Sampling locations in Mataripe Refinery diesel storage tanks

Analysis of total aerobic bacteria concentration in diesel samples before treatment

Sample

TAB (CFU/mL)*

Diesel from Tank A Diesel from Tank B

8.1 x 10 10 5.7 x 10 9

*CFU = colony forming unities

Table 2

atmospheric moisture entering through tank breather vents contributes to water accumulation. This water layer creates a favourable environment for the growth of microorganisms, such as filamentous fungi, yeasts, and bacteria, including sulphate-reducing species.1 , 3 For growth and multiplication, these microorganisms uti - lise hydrocarbons and fatty acid esters, producing biomass and biosurfactants that stabilise emulsions. This mechanism promotes micelle formation and facilitates contact between microorganisms naturally associated with the aqueous phase and diesel molecules. At the same time, these emul - sions cause undesirable operational effects by hindering water separation.4 As microbial development progresses in storage tanks, MIC, also referred to as biocorrosion, may be triggered. MIC is one of the most severe consequences of microbial growth. Unlike conventional chemical corrosion, it occurs through the direct or indirect action of microorganisms that alter electrochemical conditions at the metal-solution interface. Different biocorrosion mechanisms are described depend - ing on the microbial group, leading to distinct impacts on the metallic structures that comprise diesel storage sys - tems (see Table 1 ).5 , 6 The sludge found in diesel storage tanks is not composed solely of microbial biomass but rather consists of a complex mixture of organic and inorganic materials. Microbial devel - opment contributes to sludge formation through the precipi - tation of corrosion products. Iron sulphide (FeS), generated by the activity of sul - phate-reducing bacteria (SRB), is a black, insoluble solid that precipitates at the bottom of the tank and becomes incorporated into the sludge. Other examples include iron oxides and hydroxides resulting from biologically mediated oxidation processes.3 Together with microbial biomass and biosurfactants that emulsify the system components, these corrosion products are responsible for many of the operational difficulties asso - ciated with microbial contamination. The final consequences

PTQ Q3 2026