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Corrosion Process Inside Steel Fire Sprinkler Piping

Sprinkler contractors, facilities managers, and their technical advisors sometimes have to deal with corrosion inside pressurized, water-based, metal fire sprinkler piping systems. Corrosion causes pinhole leaks as well as insoluble corrosion residue buildup (obstructive growth) that increases pipe friction losses. Corrosion deterioration can be attributed to microorganisms in water, i.e., to microbiologically influenced corrosion (MIC).

Corrosion processes
Corrosion processes involve paired, mutually dependent electrochemical reactions between metal and certain reactive chemical species dissolved in ordinary fresh water.  Reactions occur at the metal-water interface and tend to speed up with increasing temperature. Concentrations of individual corrosive chemical species in fresh water charged into fire sprinkler piping systems from public water supplies usually range from 1 to 200 mg/L.

For example, the solubility of oxygen in water is about 8 mg/L at 60°F and atmospheric pressure. 10 Spontaneous electric charge transfer occurs at the atomic scale of dimensions during corrosion reactions, e.g., dissolved oxygen receives electrons from iron metal during the process of oxygen corrosion of low-carbon steel pipe. External electrical factors are not likely to play a role in corrosion processes if the piping system is electrically grounded.

Corrosion reactions dissolve metal. Following electron transfer, metal particles with positive electrical charge (ions) are expelled into the water. Wasting of metal one particle at a time produces pits, craters, and sometimes penetrations during time spans ranging from months to years. Corrosion processes tend to localize in crevices and underneath aggregations of insoluble substances. Insoluble substances that might be found inside steel fire sprinkler piping systems include scale from hard water of high carbonate or bicarbonate alkalinity, sediments, chips and filings from drilling and sawing during fabrication, and corrosion residues. Several corrosion processes that can occur inside water-based, metal fire sprinkler piping are described below. Each process can proceed independently of any other, provided the necessary chemical ingredients are available and that physical conditions are suitable. Each corrosion process is described in more detail elsewhere.

  • Oxygen corrosion is due to oxygen dissolved in water reacting with metals. Discussions of oxygen corrosion often use rusting of iron by dissolved oxygen as an example of one of many possible metal-water corrosion processes. Figure 1 illustrates orange-brown rust (hydrated ferric iron oxide) that forms spontaneously on low-carbon steel pipe and cast-iron fittings exposed to outdoor conditions. Oxygen dissolved in atmospheric moisture fuels such rusting. The electrochemical reactions causing rust formation also can occur inside pressurized, water-based, low-carbon steel fire sprinkler piping if the water contains dissolved oxygen.
  • Acid corrosion is caused by hydrogen ions from dissolved acids reacting with metals Ordinary fresh water usually is considered acidic when the chemical characteristic, "pH," is below the neutral pH value of 7. However, water chemists consider waters to be acidic only when pH drops below 4.4. This apparent contradiction in the definition of acidic water is due to alkalinity that arises from hydrolysis associated with dissolved bicarbonates and several other chemical species. The pH of most public water supplies charged into fire sprinkler piping systems ranges between 7.3 and 8.4, so it is not likely that acidified water is charged into a piping system. However, concentrations of acidified water can arise spontaneously in certain localities inside a closed piping system via hydrolysis of hydrated metal ions. Localities that favor spontaneous acidification include crevices and regions underneath aggregations of insoluble substances. Trace concentrations of chloride ions and sulfate ions play a role in hydrolysis. Acidified water also can be generated by the metabolism of acid-producing bacteria or sulfate-reducing bacteria living in biofilms that might be growing inside a piping system.
  • Acid-oxygen corrosion occurs in oxygenated acid solutions and is due to an electrochemical reaction in which hydrogen ions and dissolved oxygen team up to waste metal. Acid-oxygen corrosion in iron-water systems usually is a speedier process than either oxygen corrosion or acid corrosion acting alone.
  • Hypochlorite corrosion occurs when certain chlorine-based disinfectants found in most public water supplies react with metal. Concentration of disinfectants usually ranges from 1 to 4 mg/L of "free available chlorine." Chlorine and hypochlorite disinfectants hydrolyze water to produce biocidal hypochlorous acid molecules in acidified water and hypochlorite ions otherwise. Both of these chemical species can corrode low-carbon steel and most other structural metals.
  • Microbiologically Influenced Corrosion is another segment of the corrosion world and is traceable to certain microorganisms. For example, bacteria are a category of single-cell microorganisms that frequently participate in biologically mediated corrosion. They live in aggregations of wet, viscous, gelatinous substances called biofilms. Several bacterial species and many millions of miniscule creatures might thrive communally in a biofilm where chemical and physical conditions are favorable. Biofilms trap organic matter and certain dissolved chemical species that serve as nutrients for bacteria. The metabolic processes of certain bacteria produce waste products such as organic acids that are the source of hydrogen ions that can cause acid corrosion of most metals.) Interestingly, bacterial metabolism is understood in terms of electrochemical reactions, as are the foregoing metal-water corrosion processes.

Pinhole leaks
Pinhole leaks usually spray with no forewarning and cause water damage to the surroundings. Figure 2 illustrates a pinhole leak that sprayed, whereas Figure 3 shows a seeping pinhole leak on the verge of spraying. Remedy usually involves immediately sealing pinhole leaks with temporary encirclement sleeves and replacing degraded sections of piping.

Pinhole leaks are caused by one or more ongoing electrochemical corrosion processes that usually are localized in crevices or underneath aggregations of insoluble substances. A spraying pinhole leak might take several months or several years to develop, depending on pipe wall thickness, ambient temperature, and availability of chemical ingredients for corrosion reactions. Reactions are focused so precisely that the process could be called chemical drilling.

Outbreaks of several pinhole leaks nearby one another in an older or a thin-wall piping system sometimes occur when fresh supplies of water are introduced frequently into a completely drained piping system, e.g., during construction to expand or rehabilitate a piping system that has been in service for several years. Introduction of water replenishes the reactive chemical ingredients that dissolve ("leach") metal from the interior pipe wall. Air that is not vented during charging can become trapped and compressed at high elevations. The increased solubility of oxygen in water at typical gage pressures of 50 to 160 pounds per square inch (340 to 1,100 kPa) in a fire sprinkler piping system provides ample dissolved oxygen to fuel oxygen corrosion in such localities. Oxygen corrosion is a likely contributor to the formation of pinhole leaks.

Image of Corrosion Process Inside Steel Fire Sprinkler Piping

Insoluble corrosion residues
Corrosion reactions cause buildup of insoluble corrosion residues on the interior pipe wall over months and years, gradually increasing pipe friction losses. Residues form when metal ions react spontaneously with certain other dissolved chemical species, including oxygen. Figure 4 illustrates dried-out residues in a steel pipe section long after removal from a fire sprinkler piping system. These residues are probably iron compounds, e.g., oxides, hydroxides, and carbonates. Regions where residues have built up on the interior pipe wall can be located, and wall thickness can be measured, by applying non-destructive techniques such as ultrasound to the exterior pipe surface. Residue thickness also can be measured using ultrasound. Residues can be flushed from a piping system using weakly acidified solutions that disrupt and carry away most insoluble aggregations that build up on the interior pipe wall. Cleansing with solutions that neutralize acidity follows such flushing. More detail about the causes of buildup and clearing away of insoluble aggregations appears elsewhere.

The Friction Loss Formula for fire sprinkler piping indicates that frictional resistance increases exponentially with a linear decrease in actual internal diameter. Consequently, the gradual buildup of obstructive growth due to insoluble corrosion residues such as those shown in Figure 4 can increase pipe friction losses. Dramatic increases can occur in small-diameter pipe because pipe friction losses vary inversely with diameter (d), i.e., with d -4.87. For example, analysis using the Friction Loss Formula shows that pipe friction doubles if internal diameter decreases from 2.00 to 1.75 inches (50 to 44 mm); friction quadruples if internal diameter decreases from 1.00 to 0.75 inches (25 to 19 mm). Although the Friction Loss Formula does not apply accurately to a nonuniform buildup of obstructive growth like that in Figure 4, it is clear that gradual buildup increases pipe friction losses to a value greater than that calculated by designers from the actual internal diameter of uncorroded pipe.

The Friction Loss Formula also indicates that pipe friction losses depend on the surface texture of the interior pipe wall, which is represented quantitatively by the friction loss coefficient, C. Pipe friction losses are proportional to 1/C 1.85. "C" becomes smaller as the surface texture roughens. Consequently, pipe friction losses can increase significantly as obstructive growth due to insoluble corrosion residues gradually build up and roughen the interior pipe wall.

Figure 5 shows wet, black, sulfurous-smelling residues found inside a freshly removed steel pipe section. These residues probably contain the insoluble black oxide of iron, magnetite, which forms when dissolved oxygen is scarce.) The sulfurous odor suggests that these residues probably also contain sulfide ions (low pH) or bisulfide ions (high pH). These ions form when sulfate ions present in the water are used by sulfate-reducing bacteria as terminal electron acceptors. The residues in Figure 5 probably contain some insoluble black ferrous iron sulfides that are produced by the reaction between dissolved iron and sulfide ions.

Corrosion control measures
The following measures can minimize iron-water corrosion inside pressurized, water-based, low-carbon steel fire sprinkler piping. These measures are discussed more fully elsewhere.

  • Reducing the frequency of filling a piping system and venting trapped air tend to minimize oxygen corrosion.
  • Inspecting a piping system externally using nondestructive evaluation techniques such as ultrasound can locate regions where insoluble substances have built up on the interior pipe wall and also regions where corrosion has reduced pipe wall thickness to unacceptable values.
  • Flushing a piping system with a solution that disrupts and carries away insoluble aggregations and biofilms that build up on the interior pipe wall minimizes underdeposit corrosion that causes pits, craters, and sometimes penetrations. Shaking out chips and filings from drilling and sawing during fabrication also minimizes underdeposit corrosion.
  • Maintaining pH in the range 8.3 to 8.5 so that water cannot become acidified via hydrolysis of hydrated metal ions minimizes acid corrosion.
  • Chemically treating water to minimize chloride and sulfate concentrations minimizes acidification via hydrolysis of hydrated metal ions. Minimizing sulfate concentration also minimizes acidification due to metabolism of sulfate-reducing bacteria.
  • Minimizing organic matter in input water and lubricants on interior pipe surfaces reduces the metabolic activity of bacteria that produce acids upon using organic matter for nutrients, e.g., acid-producing bacteria such as Clostridia and Thiobacillus, as well as sulfate-reducing bacteria.
  • Providing continuous, adherent, nonporous coatings on a metal surface can block electrochemical reactions between corrosive chemical species dissolved in water and metal.
  • Providing a corrosion allowance, i.e., more metal thickness than is needed to support structural loads, increases the service time before corrosion causes undesirable consequences.
  • Keeping an operations log provides a database that can help sprinkler contractors, facilities managers, and their technical advisors evaluate corrosion durability and reliability of fire sprinkler piping systems. A log might record renovations and rehabilitations, changes in water chemistry, results of nondestructive inspections, and general maintenance procedures.