Decoding Corrosion: Understanding and Controlling Iron Oxides in Closed-Loop Systems

Fabrice CHAUSSEC¹, Logan MANARANCHE^2
 
1ODYSSEE Environnement, Z.A. belle croix 72510 Requeil, +33 2 43 44 39 33, siege@odymail.fr
²ODYSSEE USA INC., Hollywood, FL 33021, +1 414 243 7063, order@odymail.com

Abstract
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 (α-FeO(OH)) and lepidocrocite (γ-FeO(OH)). Under anoxic conditions, maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) 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.

Keywords: corrosion, closed-loop systems, iron oxyhydroxides, magnetite, maghemite, corrosion inhibitors.

1. Introduction
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.

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:

FeOOH, nH₂O (amorphous, with an undetermined value of n)
α-FeO(OH) (goethite)
γ-FeO(OH) (lepidocrocite)
δ-FeO(OH) (feroxyhyte)
β-FeO(OH) (akaganeite)
Fe(OH)₂

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 (γ-Fe₂O₃) is the only iron oxide that can form.

It is often confused with magnetite (Fe₃O₄) due to their similar black color and because both are ferromagnetic.

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

2. Formation of Corrosion Products in Aqueous Environments

2.1. In the Presence of Oxygen
As with all corrosion in an aerated aqueous environment, the process begins with the reduction of dissolved oxygen, classically expressed as:
2Fe → 2Fe²⁺ + 4e^– (Anodic reaction) 

O₂ + 2H₂O +4e^– → 4OH (Cathodic reaction)

Note: The reduction of H⁺ ions remains possible but is almost negligible due to their low concentration at typical water pH levels, which are generally above 7.
Fe²⁺ ions will immediately oxidize into Fe³⁺ ions, and since their solubility is extremely low at common water pH levels, they will precipitate as ferric hydroxide:

Fe³⁺ + 3OH^– → Fe(OH)₃
This hydroxide is found in various allotropic forms of iron oxyhydroxides:
Fe(OH)₃ → FeO(OH)* + H₂O
*Generally goethite, the most thermodynamically stable form)

2.2. In the Absence of Oxygen
In the absence of oxygen (< 30 ppb), the only thermodynamically possible reduction reactions that can sustain corrosion are [2][3]:
2Fe → 2Fe²⁺ + 4e^– (Anodic reaction)
NO₃^– + H₂O + 2e^– → NO₂^– + 2 OH^–
NO₂^– + 5H₂O + 6e^– → NH₃ + 7OH^– (Cathodic reactions)
SO₄^2- +6H₂O + 8e^– → H₂S + 10 OH^–

Fe²⁺ ions will not oxidize into Fe³⁺; instead, they will precipitate as ferrous hydroxide:
Fe²⁺ + 2OH^– → Fe(OH)₂
 
which undergoes immediate dehydration:
 
2Fe(OH)₂ → Fe₂O₃ +H₂O.
The Fe₂O₃ formed is always maghemite (γ-Fe₂O₃) and never hematite (α-Fe₂O₃), which can only be obtained through “dry” oxidation.

3. Special Case of Magnetite
In the presence of oxygen, at a pH higher than 9, one hypothesis suggests that goethite, formed from Fe(OH)₂, reacts with residual hydroxide to form magnetite [4]:
2α-FeOOH + Fe(OH)₂ → Fe₃O₄ + 2H₂O
In the absence of oxygen, the process follows the Schikorr reaction:
3Fe(OH)₂ → Fe₃O₄ + 2H₂O + H₂

4. Diagnosing Corrosion on field: Identifying Deposits by Magnetic Properties and Color
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.

5. Case Study: Analysis of an Urban Heating Network

Table 1 presents the characteristics of the system

System:	Urban Heating Network
Treatment:	MetaBiSO3 – Caustic Soda 50% – Alcaline PO43-
Make-up Water:	Softened city water 2’245’462 gal/year (8’500 m³/year)
Volume:	528’344 gal (2000 m³)

Table 1: Characteristics of the System

Table 2 presents the water analysis of the water in the circuit

pH	TH (ppm)	P-Alk (ppm)	M-Alk (ppm)	Cl- (ppm)	Conductivity (µS/cm)	Iron (ppm)
9.9	0	55	148	99	770	< 0.1

Table 2: Water Analysis

Figure 2 shows the interior of a pipe from the system.



Figure 2: pictures of the interior of a pipe from the system

Findings:
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).

Explanation:
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.

Solution:
It is crucial to consistently maintain an adequate level of sulfites throughout the network, particularly in cooler zones where oxygen ingress is more likely.

Alternative:
Implement a film-forming treatment using filming amines and alkalizing amines, ensuring corrosion protection that is independent of oxygen concentration in the water.

Conclusion
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.

References
[1] J.P. Labbé, J. Lédion, F. Hui, Infrared spectrometry for solid phase analysis: Corrosion rusts, Corrosion Science 50 (2008) 1228-1234
[2] P. Leroy, Calcium et corrosion, thèse, Université ParisV, 1991
[3] L. Legrand, P. Leroy, Prévention de la corrosion et de l’entartrage dans les réseaux de distribution d’eau, CIFEC, Paris, 1995.
[4] Y. Cudennec, A. Lecerf, Étude des mécanismes de formation des oxy-hydroxydes de fer ; hypothèses de transformations topotactiques.