2)+General+Hydrogen+Sulphide+Corrosion+Chemistry


 * General Hydrogen Sulphide Corrosion Chemistry**

The corrosion of metals in a pipeline or drilling system is due to an acidic aqueous environment. The end cause of corrosion in the system is a reduction of mass of the metal. The solid metal is oxidized to form an ion, which is then carried off in the solution. The reduced mass has negative effects towards the stability of the metal itself. This effect is the main concern of corrosion, as it creates a situation of reduced load capacity from a decreased cross section.

The mechanism for the corrosion of metals is fairly simple and well known. For the purposes of the page, iron will be studied as the major corrosion component, as steel is a common material used in industry and is often subject to corrosion. Most often the solution is aqueous. In the case of sour solution (one with H 2 S ), a source of hydrogen ions could be from hydrogen sulphide itself.

In water, hydrogen sulphide dissociates as a weak acid:

H 2 S (aq) ↔ H S - (aq) + H+(aq) ↔ S 2- (aq) + H 2  + (aq) [1] 4. Visually, this can be represented in a pH curve. At different pH's, H 2 S, H S - and S 2- will be present in different amounts. At low pH's (< 4), H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">S will be the predominant. At high pH levels (> 10), <span style="font-family: arial,helvetica,sans-serif;"><span style="font-family: Arial,Helvetica,sans-serif;">S <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: super;">2-  will be the major ion present. Each species will affect the chemistry differently, and thus the pH has great importance on corrosion. 4



Once these ions are formed, an environment is set up for the spontaneous transfer of electrons. Iron will readily donate electrons to the hydrogen ions, as can be seen by the potentials of each half reaction. Anodic reaction: Fe (s) → Fe 2+ (aq) + 2e - Ered. = +0.44V [2] Cathodic reaction: 2 H + + 2e - → 2H⁰ Eox. = 0.00V [3] This creates an overall redox reaction of: Fe (s) + 2 H + → Fe 2+ (aq) + 2H⁰ E = +0.44V [4] It is from this overall reaction that we can see that this corrosion will convert solid iron to aqueous iron (II). The reduction of iron solid mass will cause a weakening of the steel due to a decrease in cross sectional area. In the presence of hydrogen sulphide, reactions 2 and 3 may also be catalyzed be interactions with hydrogen sulphide ions with the iron itself. The anodic reaction is catalyzed by the following proposed mechanism.

Fe + <span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">S + <span style="font-family: Arial,Helvetica,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;"><span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">O ↔ <span style="font-family: arial,helvetica,sans-serif;"> <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H S -  ) ads + <span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">3 <span style="font-family: Arial,Helvetica,sans-serif;">O  <span style="color: black; font-family: Arial,Helvetica,sans-serif; vertical-align: super;">+ [5] 4    <span style="display: block; font-family: arial,helvetica,sans-serif; text-align: center; vertical-align: super;"> <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H S -  ) ads → <span style="font-family: arial,helvetica,sans-serif;"><span style="font-family: Arial,Helvetica,sans-serif;">Fe(H S -  ) + + 2e - [6] 4 <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H S - ) + + <span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">3 <span style="font-family: Arial,Helvetica,sans-serif;">O  <span style="color: black; font-family: Arial,Helvetica,sans-serif; vertical-align: super;">+ → Fe 2+ + <span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">S + <span style="font-family: arial,helvetica,sans-serif;"><span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">O  [7] 4 Where the subscript "ads" denotes an adsorbed complex on the iron metal surface. Similarly, the cathodic catalyzed mechanism is:

Fe + <span style="font-family: Arial,Helvetica,sans-serif;">H S - ↔ <span style="font-family: arial,helvetica,sans-serif;"> <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H S - ) ads [8] 4     <span style="display: block; font-family: arial,helvetica,sans-serif; text-align: center; vertical-align: super;"> <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H S -  ) ads + <span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">3 <span style="font-family: Arial,Helvetica,sans-serif;">O  <span style="color: black; font-family: Arial,Helvetica,sans-serif; vertical-align: super;">+ ↔ <span style="font-family: arial,helvetica,sans-serif;"> <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H– S–H  ) ads + <span style="font-family: Arial,Helvetica,sans-serif;">H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">O [9] 4 <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H– S–H ) ads + e - → <span style="font-family: arial,helvetica,sans-serif;"> <span style="font-family: Arial,Helvetica,sans-serif;">Fe(H S -  ) ads + <span style="color: black; font-family: Arial,Helvetica,sans-serif;">H ads⁰ [10]4 These catalytic reactions not only aid in the corrosion of the metal, converting the solid metal to an ionic form, but will also become a factor in hydrogen embrittlement.

=Hydrogen Embrittlement=

Hydrogen embrittlement (HE) is a materials and corrosion related problem. HE occurs readily in wet or aqueous H 2 S environments, which are often seen in the oil industry. In these corrosion processes, hydrogen generated can be detrimental to steel equipment, which creates a hazardous and expensive issue. 2

From equation 4, in the absence of H 2 S, the nascent hydrogen (atomic hydrogen) would further react to form harmless hydrogen gas. The gas would then be carried away in the solution. However, with hydrogen sulphide ions present, the reaction is inhibited. The hydrogen sulphide ions act as a negative catalyst or poison towards the combination reaction of nascent hydrogen to form molecular hydrogen. As nascent hydrogen is much smaller than molecular hydrogen, it is able to penetrate the lattice structure of the metal much easier. This allows some nascent hydrogen to be adsorbed into the metal's crystalline structure. This adsorbance into the metal's structure has negative effects on the physical properties of the metal. 3 From equation 10 the effect of hydrogen sulphide and its ions on hydrogen can be seen. Through the catalyzed reaction, atomic hydrogen is produces adsorbed to the iron. This creates a situation where hydrogen can more easily become incorporated into the lattice structure of the metal. This incorporation is what causes the effect of hydrogen embrittlement.

The process, known as hydrogen embittlement, results in a decrease in the ductility or toughness of a metal due to the presence of atomic hydrogen. HE has been separated into two classes: Internal and environmental hydrogen embrittlement. For the discussion of corrosion, environmental hydrogen embrittlement will be of main consideration.

There are several different proposed mechanisms for hydrogen embrittlement, each of which is supported by certain experimental data. No single mechanism has been accepted outright, however. Three common mechanisms are listed and explained below. 5


 * 1) Hydride induced embrittlement
 * 2) <span style="color: black; font-family: Arial,sans-serif; font-size: 10pt;">Hydrogen-enhanced decohesion mechanism, HEDE (brittle fracture)
 * 3) <span style="color: black; font-family: Arial,sans-serif; font-size: 10pt;">Hydrogen-enhanced localized plasticity mechanism, HELP (ductile fracture)

**1. Hydride Induced Embrittlement 5 **
Hydride induced embrittlement is one of the well established embrittlement mechanism as it has extensive experimental and theoretical support. It consists of a growth of hydrides within the metal structure, which causes cleavage. It consists of many new hydrides forming in the stress field ahead of a crack as opposed to individual hydrides continuing to grow. This is the main cause of hydrogen embrittlement for elements that form hydrides such as V, Nb, Ti, and Zr.

**2. Hydrogen-enhanced decohesion mechanism (HEDE) 5 **


HEDE is one of the oldest models used to describe changing properties due to hydrogen atoms, and was proposes in 1941 by Zapffe and Sims. In short, the proposed mechanism suggests that hydrogen adsorbed into the metal lattice weakens the atomic binding forces. This is illustrated in fugure 2, which depicts the hydrogen entering the lattice structure, and shows the weakening of the atomic bonds between the metal atoms in the presence of this hydrogen. This is caused, in a large part, due to the fact that hydrogen solubility is increased in tension fields of the metal. Tension fields occur, for example, at the tip of a crack or in areas of internal tension. At the tip of a crack, for instance, any applied stress would create a tension field. This tension field would increase the solubility of hydrogen, and thus weaken the metal due to hydrogen embrittlement. The weakened metal would then crack further. This is how the propagation of cracks is described by this method.

**3. Hydrogen-enhanced localized plasticity mechanism (HELP) 1,5 **


In metal lattices, irregularities called dislocation are present. Dislocations can be described as a termination of a plane of atoms within the lattice of a metal. The presence of dislocations strongly influences many of the properties of materials. The HELP model, as in the HEDE model, is also based on the fact that in tension fields, hydrogen solubility is increased. With increased hydrogen, the mobility of dislocations in the structure is increased. The dislocations tend to move towards the area of high tension. With these two factors combined, the dislocations migrate towards the high stressed area. The concentration of dislocations causes a decrease in yield strength of the metal, and thus crack propagation can occur. The concentration of dislocations is the cause of hydrogen embrittlement, as defined by this mechanism.

Summary
The corrosion of metals in the presence of H 2 S can be summarized as a two step process. First, an electrochemical reaction occurs due to the metal in the presence of an acidic environment. Next, as a combined result of the produced hydrogen and the presence of H 2 S in the system, the structure of the metal alloy is weakened. This weakening can produce fracture or failure of the substance under lower than usual stress conditions. This process is called hydrogen embittlement.