Abstract<\/strong>
In closed-loop systems, iron corrosion leads to the formation of various products, influenced by the presence or absence of dissolved oxygen. In oxidizing environments, iron hydroxides and oxyhydroxides predominate, notably goethite (\u03b1-FeO(OH)) and lepidocrocite (\u03b3-FeO(OH)). Under anoxic conditions, maghemite (\u03b3-Fe\u2082O\u2083) and magnetite (Fe\u2083O\u2084) formation is observed, with these phases often being confused due to their similar physical properties. Infrared spectroscopic analysis allows for the identification of these compounds and guides treatment strategies. Rigorous management of corrosion inhibitors is essential to mitigate differential aeration corrosion phenomena.<\/p>\n\n\n\n
O2<\/sub> + 2H2<\/sub>O +4e–<\/sup> \u2192 4OH (Cathodic reaction)<\/p>\n\n\n\n Fe3+<\/sup> + 3OH–<\/sup> \u2192 Fe(OH)3<\/sub> Table 1: Characteristics of the System<\/em> Table 2 presents the water analysis of the water in the circuit<\/p>\n\n\n\n Table 2: Water Analysis<\/em> <\/p>\n\n\n\n Fabrice CHAUSSEC1<\/sup>, Logan MANARANCHE2<\/sup>
Keywords<\/strong>: corrosion, closed-loop systems, iron oxyhydroxides, magnetite, maghemite, corrosion inhibitors.
<\/p>\n\n\n\n
1. Introduction<\/strong>
Unlike other common metals that form only a limited number of corrosion products in aqueous environments, iron can lead to the formation of numerous oxides, hydroxides, or oxyhydroxides, depending on the prevailing conditions (water quality, temperature, etc.). However, for simplification, it is essential to distinguish between waters containing dissolved oxygen and those where oxygen is absent or has been previously consumed.<\/p>\n\n\n\n
In oxygenated water (Excluding steam, overheated, or geothermal waters), iron oxides never form first. Instead, iron hydroxides or oxyhydroxides are the primary corrosion products, including:<\/p>\n\n\n\n
FeOOH, nH\u2082O (amorphous, with an undetermined value of n)
\u03b1-FeO(OH) (goethite)
\u03b3-FeO(OH) (lepidocrocite)
\u03b4-FeO(OH) (feroxyhyte)
\u03b2-FeO(OH) (akaganeite)
Fe(OH)\u2082<\/p>\n\n\n\n
Depending on the corrosion rate, these compounds can exhibit varying degrees of crystallinity, ranging from well-crystallized structures to amorphous forms.
In contrast, in the absence of oxygen in water, maghemite (\u03b3-Fe\u2082O\u2083) is the only iron oxide that can form.<\/p>\n\n\n\n
It is often confused with magnetite (Fe\u2083O\u2084) due to their similar black color and because both are ferromagnetic.<\/p>\n\n\n\n
Among the many techniques available for characterizing iron corrosion products, the most “practical” is infrared absorption spectrometry (Figure 1), which highlights distinct spectral profiles for each iron corrosion product [1].
Figure 1: Spectral profiles for each iron corrosion product<\/em><\/p>\n\n\n\n
2. Formation of Corrosion Products in Aqueous Environments<\/strong><\/p>\n\n\n\n
2.1. In the Presence of Oxygen<\/strong>
As with all corrosion in an aerated aqueous environment, the process begins with the reduction of dissolved oxygen, classically expressed as:
2Fe \u2192 2Fe2+<\/sup> + 4e–<\/sup> (Anodic reaction) <\/p>\n\n\n\n
Note:<\/strong> The reduction of H\u207a ions remains possible but is almost negligible due to their low concentration at typical water pH levels, which are generally above 7.
Fe\u00b2\u207a ions will immediately oxidize into Fe\u00b3\u207a ions, and since their solubility is extremely low at common water pH levels, they will precipitate as ferric hydroxide:
<\/p>\n\n\n\n
This hydroxide is found in various allotropic forms of iron oxyhydroxides:
Fe(OH)3 <\/sub>\u2192 FeO(OH)* + H2<\/sub>O
*Generally goethite, the most thermodynamically stable form)<\/em><\/p>\n\n\n\n
2.2. In the Absence of Oxygen<\/strong>
In the absence of oxygen (< 30 ppb), the only thermodynamically possible reduction reactions that can sustain corrosion are [2][3]:
2Fe \u2192 2Fe2+<\/sup> + 4e–<\/sup> (Anodic reaction)
NO3<\/sub>– <\/sup>+ H2<\/sub>O + 2e–<\/sup> \u2192 NO2<\/sub>–<\/sup> + 2 OH–<\/sup>
NO2<\/sub>–<\/sup> + 5H2<\/sub>O + 6e–<\/sup> \u2192 NH3<\/sub> + 7OH–<\/sup> (Cathodic reactions)
SO4<\/sub>2-<\/sup> +6H2<\/sub>O + 8e–<\/sup> \u2192 H2<\/sub>S + 10 OH–<\/sup><\/p>\n\n\n\n
Fe\u00b2\u207a ions will not oxidize into Fe\u00b3\u207a; instead, they will precipitate as ferrous hydroxide:
Fe2+<\/sup> + 2OH–<\/sup> \u2192 Fe(OH)2<\/sub>
which undergoes immediate dehydration:
2Fe(OH)2 <\/sub>\u2192 Fe2<\/sub>O3<\/sub> +H2<\/sub>O.
The Fe\u2082O\u2083 formed is always maghemite (\u03b3-Fe\u2082O\u2083) and never hematite (\u03b1-Fe\u2082O\u2083), which can only be obtained through “dry” oxidation.<\/p>\n\n\n\n
3. Special Case of Magnetite<\/strong>
In the presence of oxygen, at a pH higher than 9, one hypothesis suggests that goethite, formed from Fe(OH)\u2082, reacts with residual hydroxide to form magnetite [4]:
2\u03b1-FeOOH + Fe(OH)2<\/sub> \u2192 Fe3<\/sub>O4<\/sub> + 2H2<\/sub>O
In the absence of oxygen, the process follows the Schikorr reaction:
3Fe(OH)2<\/sub> \u2192 Fe3<\/sub>O4<\/sub> + 2H2<\/sub>O + H2<\/sub><\/p>\n\n\n\n
4. Diagnosing Corrosion on field: Identifying Deposits by Magnetic Properties and Color<\/strong>
In practice, when an anomaly is observed in a closed-loop system, it is essential to first assess whether the corrosion products are magnetic and their color. The presence of ochre-colored deposits generally indicates corrosion in an aerated environment, signaling the presence of iron oxyhydroxides. In contrast, a black coloration suggests a properly sealed circuit, effectively protected by magnetite or maghemite.<\/p>\n\n\n\n
5. Case Study: Analysis of an Urban Heating Network<\/strong><\/p>\n\n\n\n
Table 1 presents the characteristics of the system
<\/p>\n\n\n\nSystem:<\/strong><\/td> Urban Heating Network<\/td><\/tr> Treatment:<\/strong><\/td> MetaBiSO3 \u2013 Caustic Soda 50% \u2013 Alcaline PO4<\/sub>3-<\/sup><\/td><\/tr> Make-up Water:<\/strong><\/td> Softened city water 2\u2019245\u2019462 gal\/year (8\u2019500 m\u00b3\/year)<\/td><\/tr> Volume:<\/strong><\/td> 528\u2019344 gal (2000 m\u00b3)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
<\/p>\n\n\n\npH<\/strong><\/td> TH (ppm)<\/strong><\/td> P-Alk (ppm)<\/strong>
<\/td>M-Alk (ppm)<\/strong>
<\/td>Cl- (ppm)<\/strong>
<\/td>Conductivity (\u00b5S\/cm)<\/strong>
<\/td>Iron (ppm)<\/strong>
<\/td><\/tr>9.9<\/td> 0<\/td> 55<\/td> 148<\/td> 99<\/td> 770<\/td> < 0.1<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
Figure 2 shows the interior of a pipe from the system.
Figure 2: pictures of the interior of a pipe from the system<\/em><\/p>\n\n\n\n
Findings:<\/strong>
The surface is entirely covered with an abundant presence of magnetic iron oxides, displaying both black and ochre colors, distributed unevenly. The oxides vary in shape and size, indicating the likely presence of a mixture of iron oxyhydroxides (ochre) along with maghemite and\/or magnetite (black).<\/p>\n\n\n\n
Explanation:<\/strong>
This section of the network experiences alternating conditions of oxygen presence and absence in the water, leading to the successive formation of iron oxyhydroxides and maghemite\/magnetite. The most probable cause is periodic and recurrent deficiencies in sulfites.
The resulting deposit is neither homogeneous nor protective; instead, it consists of a mixture of different corrosion products. Combined with the presence of oxygen, this situation promotes differential aeration corrosion, which often leads to pitting and potential leaks.<\/p>\n\n\n\n
Solution:<\/strong>
It is crucial to consistently maintain an adequate level of sulfites throughout the network, particularly in cooler zones where oxygen ingress is more likely.<\/p>\n\n\n\n
Alternative:<\/strong>
Implement a film-forming treatment using filming amines and alkalizing amines, ensuring corrosion protection that is independent of oxygen concentration in the water.<\/p>\n\n\n\n
Conclusion<\/strong>
Identifying corrosion products in closed-loop systems is crucial for diagnosing and preventing degradation. The alternation between oxygenated and anoxic conditions leads to mixed deposits, necessitating optimized water treatment program. Combining spectroscopic analysis with tailored treatments is an effective strategy to mitigate corrosion risks.<\/p>\n\n\n\n
1<\/sup>ODYSSEE Environnement, Z.A. belle croix 72510 Requeil, +33 2 43 44 39 33, siege@odymail.fr
2<\/sup>ODYSSEE USA INC., Hollywood, FL 33021, +1 414 243 7063, order@odymail.com<\/p>\n\n\n\n