THE IMPACT OF STAGNANT WATER ON THE CORROSION PROCESSES IN PIPELINE

Marjan Suban, Robert Cvelbar, Borut BundaraInstitute of metal constructions, Ljubljana, Slovenia

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It is necessary to avoid the occurrence of stagnant water in water supply systems, because it represents a potential source of microorganisms, leading to the development of MIC. The emergence of stagnant water is often result of irregularities in the construction, improper execution of the hydrotest or because of the system itself (e.g. fire protection sprinkler systems). To avoid the occurrence of MIC some recommendations are to be considered:

When designing and constructing water supply system blind branch should be avoided. Design horizontal pipelines to be “self-draining”.Pipeline must be constructed so that the velocity of fluid flow is at least 1,5 m/s.For filling the fire protection sprinkler system use only chemically treated water.To carry out the hydrotest use at least drinking demineralized water. As soon as possible after completion of the hydrotest it is required to drain and dry pipeline.

Introduction

Microbiologically Influenced Corrosion

In the pipeline, which is basically designed to transport liquid, gaseous or solid materials (strewn materials), can often occur stagnant or standing water. Stagnant water or water with low flow rate, form a potential breeding ground for the development of microorganisms. Microorganisms do not represent a risk only to living organisms but they also act on non-living nature, such as various metal and other materials. Bacteria, fungi and algae are microorganisms, which among other things also cause and/or increase corrosion of metals and their alloys. For this type of corrosion the name of Microbiologically Influenced Corrosion or MIC for short is introduced. Similar considerations also apply to systems that are shown in Table 1 and are vulnerable in terms of MIC. The table also presents the microorganisms that are most commonly found as a cause of corrosion.

Table 1: Systems with persistent MIC problems [1]

Sulfate reducing bacteria or SRB are found in a variety of natural environments. They are the most known bacteria that have been found to be involved with MIC problems. The growth of bacteria occurs in an environment that is shown in Figure 1 (left). Bacteria in stagnant or slow moving water form a biofilm on the surface of metal under which the metal corrosion processes run up to 100 times faster than normal corrosion.For the course of chemical reactions in the MIC, microorganisms are required.

2+Anodic reaction: 4Fe -› 4Fe + 8e

+Electrolytic dissociation of water: 8H O -› 8H + 8OH2+

Cathodic reaction: 8H + 8e -› 8H+ 2 2Cathodic depolarization by SRB: 8H + SO -› 4H O + S4 22+ 2

Corrosion product: Fe + S -› FeS2+Corrosion product: 3Fe + 6OH -› 3Fe(OH)2

2Overall reaction: 4Fe + 4H O + SO -› 3Fe(OH) + FeS + 2OH2 4 2

SRB bacteria, therefore, in the case of ferrous alloys causes the formation of iron sulfide FeS. In the case of reaction with zinc (Zn), which represents the corrosion protection of steel, corrosion products are zinc sulfide ZnS and its aggregates, which can be found in biofilm, as is shown in Figure 1 (right).

Figure 1: Components of the environment for the development of MIC (right) and microscopic photo of biofilm in the case of zinc corrosion (left)

ResultsImpact of stagnant water on MIC in water pipeline was studied. Corrosion inside the pipe is much more intense at the lower part of the pipe (see Figure 2). This implies, that the corrosion process started mainly due to stagnant water in not completely drained water supply system. This shows the explicit effect of stagnant water, probably after the hydrotest, or in another time, when the system was for a longer period only partially drained. Smaller amount of stagnant water was very appropriate medium for the microorganism growth and further development of MIC. They dissolve zinc and iron and even lower quality stainless steel (e.g. 18Cr-8Ni austenitic stainless steel AISI 304) is not successful to passivate in such medium.

Figure 2: Corrosion products in the pipe and longitudinally cut pipe

Corrosion products formed in pit were analyzed by EDX method. The graph in Figure 3, showing peaks for iron Fe, zinc Zn, oxygen O and sulfur S. As already shown by equatios in the MIC iron sulfide FeS was formed as a corrosion product. For this reason, EDX analysis of corrosion products detected some amount of sulfur (0,42 weight%), which is characteristic of this type of corrosion. The presence of sulfur in conjunction with zinc was also suggested by the presence of zinc sulfide ZnS in the corrosion products. In addition, we have also found a

-significant amount of chlorine in the corrosion products, probably in the form of Cl ions. Thus, a large amount of chlorides is probably due to the implementation of the chlorine shocks in water supply system. Effects of chlorine on the creation of new corrosion pits and deepening of already formed pits due to MIC, has not been the subject of this investigation.

Figure 3: EDX analysis of the MIC products formed on the galvanized steel surfaces

For comparison we also analyzed water pipe on which we can only found traces of white corrosion (Figure 4 top right). From the analysis of corrosion products on galvanized surface we found that the corrosion products consist only of products of zinc with oxygen bound as zinc hydroxide Zn(OH) (Figure 4). We have also found a trace of iron. As the proportion of iron in 2

these corrosion products is very small, we can conclude that it originates from the lower layers of zinc coating in which can be from 7 to about 20 weight% of iron Fe in the form of various intermetallic phases.

Figure 4: EDX analysis of the corrosion products and photo of pipe section with white corrosion of galvanized steel (top right)

Element App Conc

Intensity Corrn.

Weight% Weight% Sigma

Atomic%

C K 44,47 0,4227 13,59 5,37 27,33 O K 242,97 0,9385 33,44 4,54 50,48 S K 2,70 0,8206 0,42 0,56 0,32 Cl K 6,24 0,7625 1,06 0,68 0,72 Fe K 239,02 0,9161 33,71 4,31 14,58 Zn K 113,90 0,8279 17,77 5,00 6,57 Totals 100,00 100,00

Element App Conc

Intensity Corrn.

Weight% Weight% Sigma

Atomic%

C K 44,47 0,4227 13,59 5,37 27,33 O K 242,97 0,9385 33,44 4,54 50,48 S K 2,70 0,8206 0,42 0,56 0,32 Cl K 6,24 0,7625 1,06 0,68 0,72 Fe K 239,02 0,9161 33,71 4,31 14,58 Zn K 113,90 0,8279 17,77 5,00 6,57 Totals 100,00 100,00

Application/System Problem Components/Areas Microorganisms Pipelines/storage tanks (water, wastewater, gas, oil)

Stagnant areas in the interior Exterior of buried pipelines and tanks, especially in wet clay environments Inadequate drying after hydrotesting

Aerobic and anaerobic acid producers Sulfate reducing bacteria Iron/manganese oxidizing bacteria Sulfate oxidizing bacteria

Cooling systems Cooling towers Heat exchangers Storage tanks

Aerobic and anaerobic bacteria Metal oxidizing bacteria Slime forming bacteria Algae Fungi

Docks, piers, and other aquatic structures

Splash zone Just below low tide

Sulfate reducing bacteria

Vehicle fuel tanks Stagnant areas Fungi Power generation plants Heat exchangers

Condensers Aerobic and anaerobic bacteria Sulfate reducing bacteria Metal oxidizing bacteria

Fire sprinkler systems Stagnant areas Anaerobic bacteria Sulfate reducing bacteria

Upper part of pipe

Lower part of pipe

Pipe interior

Biofilm

ZnS

ZnSAggregates

Corrosion product detail