Corrosion is a deterioration process to form metallic compounds by redox reaction of pure metal, which consumes metal and causes pitting, rusting, cracking and even breakdown of industrial units. The redox reaction can be broken down into two steps. The first step is the electrochemical oxidation of metals which entails the formation of metallic cation. Then corrosion continues as the metallic cation react with oxidant such as oxygen or sulfur. [1] There are three major mechanisms:
Galvanic (Wet) Corrosion
Galvanic corrosion is the most common type of corrosion in aqueous environments. It occurs when two metals are in electrical contact with electrolyte or when one metal comes in contact with a electrolyte. The metal with less electrode potential will act as the anode and the other as the cathode. [1] The anode is the metal with less half cell potential and will experience accelerated corrosion while the cathode is “protected” and ennobled. The Galvanic reaction is driven by electromotive force. [2]
In the reaction for formation of iron rust, oxidant (usually oxygen) and electrolyte (usually water) must be present for iron to rust. A region of the metal's surface serves as the anode, where the oxidation occurs: [3]
(Anodic dissolution of Fe)
The electrons released from iron would then reduce the oxidant (oxygen in this case) and form rust deposit at the cathode, which is another metal or another region of the same iron surface:
The overall equation would be:
In the above reaction, the standard electromotive force is:
The overall mechanism is shown in the image below:
Localized corrosion can be described as a special type of galvanic corrosion where only only material is present. Due to impurities, the composition of a metal may differ at different locations, causing both cathodic and anodic sites to form on the same metal. The localized corrosion phenomenon is explained in the video below: [4]
Acid Corrosion
Acid Corrosion is a special case of Galvanic Corrosion, where only one metal is present and the standard reduction potential is negative (i.e. lower than the hydrogen). The common mechanism is shown by the reaction of iron with acid: [2]
Dry Corrosion
Dry corrosion occurs when the metal directly reacts with oxidants. The presence of water or electrolyte is unnecessary. The extent of corrosion for a particular metal depends on the chemical affinity of the metal towards the oxidant. Oxygen is responsible for the corrosion of most metals. [2]
In petroleum industries, hydrogen sulfide attacks steel and forms porous FeS scale under high temperature. [5]
Standard Reduction Potential (Half Cell Potential) List
Following is the list of standard reduction potential for metals at 25 degree Celsius. When two of them are present in a galvanic cell, the one with higher standard potential would tend to be the cathode and be reduced. The difference between is called the standard electromotive force. [2]
Figure 2. Standard reduction potentials [2]
Possible contributing factors for pipeline corrosion:
The relevant factors for corrosion are: metal, environmental chemicals, temperature and design. Following is a list of possible chemical factors affecting pipeline corrosion: [5]
Hydrogen sulfide: cause galvanic cell corrosion, increase acidity, localized breakdown of FeS results in increased pitting;
Carbon dioxide: increase acidity;
Oxygen:serve as oxidant, 50ppm would be enough to increase the corrosion;
Water: serve as electrolyte in the corrosion reaction; physical impact: turbulence might accelerate the corrosion;
Chlorides: increase the conductivity of water; also serve as oxidant.
In addition to the above corrosion mechanisms, microorganisms living in the pipeline fluid may also cause corrosion through their metabolic processes. Microbially influenced corrosion is a type of corrosion that occurs frequently in pipelines and has been divided into three stages: [6]
Stage I: Beginning of film and bio-film formation with hydrogen permeation by redox reaction. This stage is dominant by general galvanic corrosion.
Stage II: Stabilization of film and bio-film and the metal is ennobled;
Stage III: Detachment of film and bio-film, which leads to localized galvanic corrosion.
Internal Pipeline Corrosion Mechanisms
Corrosion is a deterioration process to form metallic compounds by redox reaction of pure metal, which consumes metal and causes pitting, rusting, cracking and even breakdown of industrial units. The redox reaction can be broken down into two steps. The first step is the electrochemical oxidation of metals which entails the formation of metallic cation. Then corrosion continues as the metallic cation react with oxidant such as oxygen or sulfur. [1] There are three major mechanisms:
Galvanic (Wet) Corrosion
Galvanic corrosion is the most common type of corrosion in aqueous environments. It occurs when two metals are in electrical contact with electrolyte or when one metal comes in contact with a electrolyte. The metal with less electrode potential will act as the anode and the other as the cathode. [1] The anode is the metal with less half cell potential and will experience accelerated corrosion while the cathode is “protected” and ennobled. The Galvanic reaction is driven by electromotive force. [2]
In the reaction for formation of iron rust, oxidant (usually oxygen) and electrolyte (usually water) must be present for iron to rust. A region of the metal's surface serves as the anode, where the oxidation occurs: [3]
(Anodic dissolution of Fe)
The electrons released from iron would then reduce the oxidant (oxygen in this case) and form rust deposit at the cathode, which is another metal or another region of the same iron surface:
The overall equation would be:
In the above reaction, the standard electromotive force is:
The overall mechanism is shown in the image below:
Localized Corrosion
Localized corrosion can be described as a special type of galvanic corrosion where only only material is present. Due to impurities, the composition of a metal may differ at different locations, causing both cathodic and anodic sites to form on the same metal. The localized corrosion phenomenon is explained in the video below: [4]
Acid Corrosion
Acid Corrosion is a special case of Galvanic Corrosion, where only one metal is present and the standard reduction potential is negative (i.e. lower than the hydrogen). The common mechanism is shown by the reaction of iron with acid: [2]
Dry Corrosion
Dry corrosion occurs when the metal directly reacts with oxidants. The presence of water or electrolyte is unnecessary. The extent of corrosion for a particular metal depends on the chemical affinity of the metal towards the oxidant. Oxygen is responsible for the corrosion of most metals. [2]
In petroleum industries, hydrogen sulfide attacks steel and forms porous FeS scale under high temperature. [5]
Standard Reduction Potential (Half Cell Potential) List
Following is the list of standard reduction potential for metals at 25 degree Celsius. When two of them are present in a galvanic cell, the one with higher standard potential would tend to be the cathode and be reduced. The difference between is called the standard electromotive force. [2]
Possible contributing factors for pipeline corrosion:
The relevant factors for corrosion are: metal, environmental chemicals, temperature and design. Following is a list of possible chemical factors affecting pipeline corrosion: [5]
Hydrogen sulfide: cause galvanic cell corrosion, increase acidity, localized breakdown of FeS results in increased pitting;
Carbon dioxide: increase acidity;
Oxygen:serve as oxidant, 50ppm would be enough to increase the corrosion;
Water: serve as electrolyte in the corrosion reaction; physical impact: turbulence might accelerate the corrosion;
Chlorides: increase the conductivity of water; also serve as oxidant.
Microbially Influenced Corrosion:
In addition to the above corrosion mechanisms, microorganisms living in the pipeline fluid may also cause corrosion through their metabolic processes. Microbially influenced corrosion is a type of corrosion that occurs frequently in pipelines and has been divided into three stages: [6]
Stage I: Beginning of film and bio-film formation with hydrogen permeation by redox reaction. This stage is dominant by general galvanic corrosion.
Stage II: Stabilization of film and bio-film and the metal is ennobled;
Stage III: Detachment of film and bio-film, which leads to localized galvanic corrosion.
Details are discussed in the MIC page.
[1] C. Raymond. Chemistry. New York: McGraw-Hill, 2007, pp. 796-825.
[2] S. Zumdahl et al, Chemistry. Houghton Mifflin Company. 5th ed, vol. 18.2000.
[3] askIITians. Electrochemical Series [Online]. Available: http://www.askiitians.com/iit-jee-chemistry/physical-chemistry/electrochemical-series.aspx
[4] CorrConnect. Corrosion Microcell [Online]. Available: https://www.youtube.com/watch?v=hfmD1RyUWgY
[5] Mitigation of Internal Corrosion in Oil Effluent Pipeline Systems [Online]. Available: http://www.capp.ca/getdoc.aspx?DocId=155641&DT=PDF
[6] S. Kakooei et al , Mechanisms of Microbiologically Influenced Corrosion: A Review , World Applied Sciences Journal, vol 4, no 17, pp. 524 - 531