4)+Hydrogen+Sulphide+Corrosion+Prevention+Methods

= Hydrogen Sulphide Corrosion Prevention Methods=

Corrosion related to sour service, or the presence of hydrogen sulphide (H 2 S), is a crucial consideration in the oil and gas industry. Hydrogen sulphide is a very corrosive and toxic chemical that may lead to death if an individual is exposed to high concentrations. Also, corrosion caused by hydrogen sulphide may require sour gas production to be stopped, while damaged materials are either fixed or replaced; thus, costing a company both time and money. The oil and gas industry has invested millions of dollars to combat hydrogen sulphide corrosion ever since the sour gas began to be produced. On top of the NACE MR 0175 standard created by the National Association of Corrosion Engineers, outlining specific materials and material processes that may or may not be used the sour service industry 1, other methods of hydrogen sulphide prevention may include cathodic protection, galvanization and chemically altering the fluid that is in contact with the material itself through chemical inhibitors 2.

> material with less corrosion potential will experience corrosion at a much faster rate than the material with more corrosion potential 3. Galvanization is a localized form of cathodic protection where the strongest reducing agent acts as a sacrificial anode 5.
 * **Cathodic Protection** - A technique used to reduce corrosion by making the material act as a cathode, while having a second material in direct contact that is more easily oxidized, which acts as the anode and the site of corrosion. For example, this means that the second material like zinc, will undergo the majority of the corrosion rather than the material that is that being preserved such as the iron pipe line 3.
 * **Galvanization** - A process in which two different metals or metal alloys are placed in the same electrolyte fluid. The metal or metal alloy that has less corrosion potential typically coats the material that is to be protected and acts as an anode by polarizing itself toward a higher potential (positive shift); while the metal or metal allow that has more corrosion potential acts as a cathode by polarizing itself toward a lower potential (negative shift) 3 . This means that the
 * **Chemical Inhibitors** - One of the most effective methods of hydrogen sulphide prevention by adding a chemical inhibitor such as Catamin AB into the fluid 6 . In a high concentration of hydrogen sulphide, an iron sulphide layer coats the material where the Catamin AB is adsorbed stably to. Catamin AB has vertically aligned molecules that is similar to the monomolecular film, which prevents the steel piping from dissolution and sulphide film growth.

Cathodic Protection & Galvanization
For a common pipe that contains iron, the oxidation electrochemical half reaction and reductive agent is given as: Fe(s) → Fe 2+ (aq) + 2e - +0.44V [1] 7 In the presence of hydrogen sulphide, the reduction electrochemical half reaction and oxidizing agent for hydrogen may be given as: 2H + + 2e - → H o + H o 0.00V [2] 7 In this electrochemical reaction, equation [1] acts as an anode, while equation [2] acts as the cathode.

Combining equations [1] and [2] gives the general electrochemical equation: Fe + 2H +  → Fe 2+  + H o  +H o  +0.44V [3]  Equation [3] is very important in understanding the corrosion process of iron in the presence of hydrogen sulphide. Equation [3] has a positive net potential of +0.44V, which indicates that this reaction occurs spontaneously (electrical current is not required for this reaction to occur). Also, since the iron ion is a product, this illustrates that corrosion of iron is occurring, such that solid iron is becoming an iron ion in solution.

In order to reduce the corrosion of the iron piping, cathodic protection may be implemented. This may be done by putting a second metal or metal alloy that has lower corrosion potential (or stronger reducing agent) in direct contact with the iron and hydrogen sulphide 3 . Some metals that have a lower corrosion potential are those that have an electrical potential lower than Fe (-0.44V). These metals may include, but are not limited to zinc, aluminum and magnesium (see equations [4], [5] and [6] below). These three metals have a lower electrical potential than iron because they are more willing to donate electrons 7 ; thus, they will become a sacrificial anode (see figure 3). This simply means that these three metals will corrode before the iron begins to corrode; therefore, the corrosion of the iron in the piping will be prevented.

Zn(s) → Zn 2+ + 2e - +0.763 V [4] Al(s) → Al 3+ + 3e - +1.66 V [5] Mg(s) → Mg 2+ + 2e - +2.37 V [6]


 * Note: Since these reactions undergo oxidation, the reactions have been flipped to accommodate the loss of electrons; thus, the potentials are positive.


 * Note: As defined above, the cathodic protection and galvanization processes are very similar, thus, the above chemistry very similar both processes. Galvanization is simply a localized form of cathodic protection 5.

Disadvantages for Cathodic Protection and Galvanization
Theoretically, cathodic protection would be a possible solution to preventing the corrosion of iron in iron pipes. However, there are two major disadvantages for cathodic protection: As described in the general hydrogen sulphide corrosion chemistry section, atomic hydrogen may be produced in the presence of hydrogen sulphide. Once formed, one atomic hydrogen may bond with a second atomic hydrogen to form hydrogen gas (as illustrated in equation [7]) 7, or, an atomic hydrogen may be absorbed by the pipe where the atomic hydrogen may embrittle the pipe's crystalline structure. If the atomic hydrogens form a hydrogen gas molecule, the gas bubble will float away without harming the pipe. However, during the cathodic protection process a percentage of the hydrogen molecules will not be completely converted into hydrogen gas; thus, atomic hydrogens may diffuse into the pipeline's crystalline structure 5 (see Figure 4).
 * 1) Cathodic protection and galvanization may encourage hydrogen embrittlement.**

H + H → H 2 (g) [7]






 * 2) Very expensive and difficult to implement**. It may be very difficult to replace the sacrificial anode throughout the pipe, determine when the sacrificial anode has been completely eroded, as well as establish when and where iron corrosion has taken place (if any) after the sacrificial anode has been completely eroded 5 . The sacrificial anode typically undergoes rapid corrosion 3 . For example, if a 300km pipe is being used to transport aqueous hydrogen sulphide the following questions should be determined by an engineer: How should the sacrificial anodes be spaced, or should the sacrificial anode be continuous for the 300km? If the sacrificial anode is placed one meter from the pipe inlet, will corrosion occur 300km down the line? If the sacrificial anode is placed throughout the pipeline, how much would it cost the company to stop production, dig up and open the pipe to insert a replacement sacrificial anode? How can the rate of corrosion of the sacrificial anode be monitored throughout the entire pipe? Can the sacrificial anode be assumed to corrode uniformly? How often does the sacrificial anode need to be replaced? 5

Chemical Inhibitors - Catamin AB
A corrosion inhibitor is a chemical that may be added to a liquid or gas stream to decrease the rate at which corrosion is occurring. Generally, chemical inhibitors display relatively high efficiencies in the range of 90-96% if the most efficient inhibitor is available 5 . One example of an anodic inhibitor which slows down the rate of sour gas corrosion is dimethyl alkyl benzyl ammonium chloride [C //n //<span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">H <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">2 // n // <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">+1 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">N+(CH <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">3 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">) <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">CH <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">C <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">6 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">H <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">5 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">]Cl <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: super;">– <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">(where //<span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">n // <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">= 10–18), commonly known as catamin AB <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: super;">6 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">. Catamin AB is a quaternary ammonium compound (QAC), which, like all anodic inhibitors slows down the rate of corrosion by forming a thin film layer on the metal surface. This layer prevents the metal from being oxidized, and thus corroded. A common property of QAC’s is that their inhibiting action is improved in the presence of anions (such as HS <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: super;">- <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">). Experiments have been conducted and the following data was given by Figure 6 below:

Steel 70S2KhA in solutions containing NaCl (0.5%) and CH <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt; vertical-align: sub;">3 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">COOH (0.25 g/l) as a function of concentrations of hydrogen sulphide and catamin AB. Test duration 1.5 h

These results show that with high concentrations of hydrogen sulphide, the catamin AB inhibitor is most effective. This results because the adsorption of catamin AB is enhanced when there is a layer of iron sulphide film on the surface metal (i.e. a more stable catamind AB film is formed). The adsorption of catamin AB, protects the steel pipe surface from corrosion by providing a protective (nanodimensional) film which prevents the redox half reactions from occurring 6. By preventing the transfer of electrons between the H 2 S solution and the steel pipe, catamin AB prevents the degradation/corrosion of the pipe 5.

**Electrochemical Behaviour of Iron in a Catamin AB and Hydrogen Sulphide Solution**
The electrochemical potentials were studied in the prementioned experiment by comparing the 0.5% NaCl and acetic acid solution (acidified to a pH of 3.6) with catamin AB to the same solution with hydrogen sulphide present. It was determined that the electrochemical potentials were lower (more negative) in the solution with the hydrogen sulphide present (~ -0.46V) compared to the solution without the presence of hydrogen sulphide (~ -0.34V) 6. By comparing electrochemical potentials, these results may be explained through electrochemistry. From equation [8], the reduction half reaction of iron is -0.44V. The solution with hydrogen sulphide present has a more negative potential than the iron, while the solution without hydrogen sulphide present has a more positive potential than the iron half reaction. This indicates that the solution with hydrogen sulphide is a strong reducing agent and is oxidized since the potential is more negative 2. Similar to cathodic protection and a sacrificial anode, the solution with the hydrogen sulphide will be oxidized before the iron; thus, the corrosion in the iron will be prevented. For the solution without the hydrogen sulphide present, the potential is more positive than the potential of the iron half reduction reaction; therefore, this solution will not prevent the iron from corroding because the iron will be the stronger reducing agent and will be oxidized 7. Thus, in the presence of hydrogen sulphide, catamin AB slows the anodic reaction since the corrosion of the ferrous steel takes place in the region of most active potentials 6.

Solution without hydrogen sulphide in the presence of catamin AB* -0.36V (Strongest oxidizing agent, reduced) [7] Fe 2+ (aq) + 2e - → Fe(s) -0.44V [8] Solution with hydrogen sulphide in the presence of catamin AB* -0.46V (Strongest reducing agent, oxidized) [9]
 * Note: The reduction half reactions for the solutions above were not found; however, the experimental potentials were given.

**Le Chatelier's Principle of Chemical Equilibrium Related to the Concentration of Catamin AB**
Another interesting link to previous studies is through the use of Le Chatelier's principle related to chemical equilibrium. It was found that in this experiment, by increasing the concentration of hydrogen sulphide the corrosion rate of iron will also increase; however, increasing the concentration of catamin AB will decrease the corrosion rate of iron 3.

A general oxidation equation for the solution described in equation [9] may be written as: Ct(s) → Ct p+ (aq) + ze - +0. 46V [10]
 * Note: Ct represents the nanodimenstional film formed by adsorption of catamin AB, z represents the number of electrons and p represents the ionic charge of the film.

Combining equation [8] with the general oxidation equation of a solution with hydrogen sulphide with catamin AB (equation [10]), the overall electrochemical equation is spontaneous due to a positive overall potential and may be written as: Ct(s) + Fe 2+ (aq) → Fe(s) + Ct p+ (aq) +0.02V [11]
 * Note: equation [11] is used for demonstration purposes only. The actual oxidation reaction was not found.

According to Le Chatelier's principle, an increase of catamin AB favours the products of equation [11], which will produce more catamin AB ions; thus, the corrosion of iron will be prevented and the corrosion of the catamin AB film will occur. Also, according to Le Chatelier's principle, as the catmin AB film is corroded, the reaction would favour the reactant side, meaning the iron would corrode. However, this does not happen because equation [11] is spontaneous; thus, the reaction must occur favouring the products 5.

**Structural Formation of the Catamin AB Film**
Since there is a nonuniform charge distribution associated with catamin AB (as a quaternary ammonium compound), the preferred structural arrangement is vertical during the adsorption of catamin AB 6. The positive charge of catamin AB is formed on the nitrogen molecule. Due to the nonuniform charge distribution of catamin AB, the molecule is considered polar. Therefore, it may be assumed that catamin AB is adsorbed on the iron surface when the hydrogen atoms bond between alkyl radicals, thus forming a film layer on the iron in a vertical arrangement (as illustrated in Figure 7 below) 6.

There are two reasons that may explain why the film layer prefers a vertical arrangement: >
 * Molecules that bond with other molecules typically are regioselective such that the most stable arrangement of the product is formed. The vertical arrangement (also known as para- directing in terms of the aromatic directing group), is more stable than the ortho- or meta- directing groups simply due to steric hindrance (see figure 8 below) 7 . The physical shape and size of the molecule is very important. If catamin AB were to be adsorbed to the iron, directly beside the existing substituent (ortho position), then the molecule would be very unstable. However, if the catamin AB were to be adsorbed to the iron directly across in a vertical arrangement (para position), then the molecule would be stable because the large molecules will not be in each others physical space 6.
 * The vertical alignment of catamin AB may also be explained through the nonuniform charge distribution. The nitrogen atom in the molecule carries a positive charge, which in combination with the iron sulphide layer increases the adsorption of catamin to the surface. In the iron sulphide layer, sulphur molecules carry a negative charge, which attracts the positive charge of the nitrogen molecule 7 . This, in turn, promotes the formation of the protective layer that prevents corrosion. Also, the tetrahedral shape of the nitrogen atom results in the most hydrogen bonding sites when catamin AB is vertically aligned.

Other Definitions

 * Reduction Reaction** - Implies the gain of electron(s) by a species; thus, a decrease in oxidation number, decrease in oxygen and an increase in hydrogen 7.


 * Oxidation Reactio**n - Implies the loss of electron(s) by a species; thus, an increase in oxidation number, and an increase in oxygen 7.


 * Reducing Agent** - Are electron donors and the species is oxidized 7.


 * Oxidizing Agent** - Are electron acceptors and the species is reduced 7.