The Chemistry Behind the Mitigation of Steel Corrosion
Author: Juan Dominguez Olivo, Ph.D., R&D Lab Manager. Zerust Oil & Gas.
Corrosion is a worldwide problem that affects the integrity of assets, poses risks of loss of primary containers, and leads to major economic loss.1 Thereby, if we want to mitigate corrosion, it is important to understand its nature. In this post, the basics of the chemistry of atmospheric corrosion / corrosion under tanks and how to mitigate it will be covered. For the sake of simplicity, let us start with an oxygenated atmosphere with a single contaminant: sulfur dioxide (SO2).
Corrosion under the presence of oxygen (O2) and Sulfur dioxide (SO2)
Sulfur dioxide is a gas typically produced as a combustion byproduct in many operations across diverse industries. The insidious nature of this chemical is that when dry, it does not corrode steel significantly. However, under the right conditions of humidity and temperature, severe corrosion may occur.2
The mechanism associated with the corrosion of steel under the presence of sulfur dioxide is well explained by McLeod and Rogers.2 When SO2 is in the gas phase, it can react with the water vapor in the environment and produce a variety of compounds such as sulfite ions (SO32–), bisulfite ions (HOSO–2), sulfonates (HSO3), and sulfuric acid (H2SO4).3 The complete aqueous chemistry speciation of sulfur dioxide is out of the scope of the current post. For simplicity, it is assumed that SO₂ undergoes the most commonly found speciation under atmospheric corrosion of steel:4
- Homogeneous reaction:
- SO₂ + H₂O ⇌ SO32–+ 2H+
- Cathodic Reactions:
- O₂ + H₂O+ 4e- ⇌ 4OH– (Oxygen evaluation)
- 2H++ 2e– ⇌ H₂ (Hydrogen evolution -out of the scope of this post-)
- Anodic Reaction:
- Fe ⇌ Fe2++ 2e– (Iron dissolution, main corrosion reaction)
- Formation Reactions:
- Fe2++ SO32–⇌ FeSO3 (Iron sulfite formation)
- 2Fe2 + 3O₂ ⇌ 2 Fe2O3 (Iron oxide formation, known as ‘red rust’)
The reaction scheme above assumes that all the reactions are happening inside droplets condensed on the steel surface at sufficiently high humidity levels, as depicted by Figure 1.
Figure 1. Reactions happening inside a small, condensed droplet on the steel surface. Green scheme: iron sulfate formation. Purple scheme: hydrogen evolution. Red scheme: iron oxide formation.
In general, steel corrodes faster when more reactions involving the generation of ferrous ions (Fe2+) from steel are happening. For instance, the formation of iron sulfate implies that Fe2+ is being consumed; the Fe2+ is from the iron of the steel (from the anodic dissolution of iron); therefore, the steel is being corroded by this reaction. The same happens with oxygen and hydrogen evolution (red and purple scheme, respectively).
How can we mitigate corrosion?
If we want to mitigate the corrosion associated with the presence of oxygen and sulfur dioxide, we need to inhibit or at least slow down the reaction rate of the abovementioned schemes. One way to impede corrosion associated with the anodic dissolution of iron is the use of ammonia. Ammonia has been characterized in atmospheric chemistry research as a neutralizing agent of acidic species.3 The main role of ammonia is to convert acidic aerosols into non-volatile compounds such as ammonium sulfite – NH4SHO3 – through the following mechanism proposed by Townsend, et al.3
SO32–+ 2H+⇌ SHO-3
SHO-3 + H+ + NH3 ⇌ NH4SHO-3
In other words, ammonia hinders the acidic speciation of SO2 by neutralizing acidic ions (SHO-3 and H+). Moreover, the alkaline nature of ammonia hinders the oxygen evolution by producing OH- ions:
NH3 + H₂O ⇌ NH4OH ⇌ NH+4 +
Since more OH– ions are produced, the pH increases. At a pH above 9, the oxygen evolution rate has been reported to decrease an order of magnitude with respect to a neutral pH.5 Thus, ammonia also has an inhibitive effect on oxygen evolution.
As the reader might have noticed, the oxygen evolution as well as hydrogen evolution involve the exchange of electrons through an electrolyte (water). Thereby, they are called electrochemical reactions. In a following post, the electrochemical nature of corrosion will be discussed.
- Gangopadhyay, S. & Mahanwar, P. A. Recent developments in the volatile corrosion inhibitor (VCI) coatings for metal: a review. J. Coatings Technol. Res. 15, 789–807 (2018).
- McLeod, W. & Rorgers, R. R. Sulfurous Acid Corrosion of Low Carbon Steel at Ordinary Temperatures – I. Its Nature. Corrosion 22, 143–146 (1966).
- Townsend, T. M., Allanic, A., Noonan, C. & Sodeau, J. R. Characterization of sulfurous acid, sulfite, and bisulfite aerosol systems. J. Phys. Chem. A 116, 4035–4046 (2012).
- Choi, Y. & Nesic, S. Effect of Water Content on the Corrosion Behavior of Carbon Steel in Supercritical CO2 Phase with Impurities. NACE Corros. Conf. 1–15 (2011).
- Ooka, H., Yamaguchi, A., Takashima, T., Hashimoto, K. & Nakamura, R. Efficiency of Oxygen Evolution on Iridium Oxide Determined from the pH Dependence of Charge Accumulation. J. Phys. Chem. C 121, 17873–17881 (2017).
Contact a Zerust Oil & Gas representative today for help with development of a corrosion mitigation solution to fit your needs.