3)+Material+Selection+for+Sour+Environments

=Material Selection for Sour Environments=

In North America, the NACE (National Association of Corrosion Engineers) standard MR 0175 must be followed for pipeline material selection 1. This standard states specific requirements for steels, steel alloys and other common industrial material that must be followed to prevent sulphide stress cracking (SSC), hydrogen embrittlement, and other forms of corrosion in the presence of hydrogen sulphide (H 2 S). This standard discusses many mechanical and chemical material properties, which include the affect of temperature, partial pressure of H 2 S, pH level, hardness, and the crystal lattice structure of the material. This standard was first amended in 1975 and is continually updated to reflect the enhancement of modern technology.

Crystal Lattice Structures: Body Centered Cubic (BCC) vs. Face Centered Cubic (FCC)
Pure metals have crystal structures, which are typically described in terms of an unit cell. The unit cell is illustrated as a square 3D box that contains atoms. Figure 1 2 illustrates the two different crystal lattice structues in which atoms can be arranged. There are eight atoms at the corners of the unit cell and one atom is in the center of the cell. This structure is body centered cubic (BCC). Similar to the BCC structure, there are eight atoms at the corners of the unit cell (see Figure 2 2 ); however, there is no atom at the center of the unit cell. Instead, there are six atoms located at the center of each surface. This structure is face centered cubic (FCC).



Due to structural differences, BCC and FCC structures will have a significantly different effect on the hydrogen entry into metals (please refer to the General Hydrogen Sulphide Corrosion Chemistry page, which discusses the mechanism for the creation of atomic hydrogen). Figure 3,4,5 and 6 are top views of both BCC and FCC structures 3. Comparing Figure 5 with Figure 3, it is illustrated that there are more atoms in the FCC structure for both the first and second layer of the crystal. This means that once an atomic hydrogen is adsorbed in the first layer, it has a greater path opportunity to enter the second layer. Comparing Figure 6 and Figure 4, the distance between atoms are much shorter in the FCC structure. This means that it will take less time for an adsorbed atomic hydrogen atom to jump into the subsurface of the second layer. Overall, this creates a coverage of 1.5 H per first layer metal atom. 3 This means that it is much easier for the atomic hydrogen to reach subsurface sites. Since the adsorbed hydrogen is adsorbed in multiple layers, this is referred to as physical adsorption.



__Legend:__ White – first layer Grey – second layer Black – third layer

Common metals with a BCC structure may include: iron, lithium, chromium, tungsten, and most common unalloyed carbon steels, while metals with a FCC structure may include: aluminum, nickel, copper, silver, gold, platinum, and lead 4. When the temperature of iron is between 910ºC and 1400ºC, it has an FCC structure. However, when the temperature of iron is not between 910ºC and 1400ºC, then it is in its BCC form. It is important to note that the cooling and heating process, whether cold rolled, hot rolled or annealed also determines whether a material is BCC or FCC.

Corrosion Reactions
In an aqueous system, an electrochemical reaction will typically occur between the strongest oxidizing agent (OA) and the strongest reducing agent (RA). The strongest oxidizing agent undergoes reduction and acts as an anode (gain of electrons), while the strongest reducing agent undergoes oxidation and acts as a cathode (loss of electrons). The following five equations 4a are the reduction half reactions for the given materials at 25ºC. The higher (more positive) the electric potential is, the stronger the oxidizing agent.

S (s) + 2H + + 2e - →H 2 S (g) + 0.14V [1] 2H + (aq) + 2e - → H 2(q) + 0.00V [2] Ni 2+ (aq) + 2e - →Ni (s) - 0.25V [3] Fe 2+ (aq) + 2e - →Fe (s) - 0.44V [4] Cr 3+ (aq) + 3e - →Cr (s) - 0.74V [5]

Considering H 2 S corrosion on pure iron the following electrochemical characteristics may be given: Anode: Fe (s) → Fe 2+ (aq) + 2e - + 0.44V [4*] Cathode: S (s) + 2H + + 2e - →H 2 S (g) + 0.14V [1] Net equation: Fe (s) + S (s) + 2H + →H 2 S (g) + Fe 2+ (aq ) +0.58V [6]


 * Note: equation [4*] is the reverse of equation [4]

The net corrosion equation has a positive potential, which means that the reaction occurs spontaneously. This infers that the corrosion of iron takes place. Most pipelines are made of steel, in which iron is the primary material. If there is significant corrosion on the iron, the damage may be catastrophic.

Now, consider two composite material systems: The main component in this pipe is iron, while the minor component is nickel. When the electrochemical reaction occurs between the strongest oxidizing agent (equation [2]) and strongest reducing agent (equation 4*]), the following equations may be written:
 * 1) Nickel-iron piping**

Cathode: 2H + (aq) + 2e - → H 2(q) + 0.00V [2] Neither anode nor cathode: Ni (s) → Ni 2+ (aq) + 2e - + 0.25V [3*] Anode: Fe (s) → Fe 2+ (aq) + 2e - + 0.44V [4*] Net equation: Fe (s) + 2H + →H 2(g) + Fe 2+ (aq ) +0.44V [7]

Equation [7] shows that nickel does not prevent corrosion, instead it allows the corrosion of iron, the primary component of the pipe to corrode. Note that the crystal lattice structure of this piping is FCC.

Similar to the previous the nickel-iron system, a pipe containing iron as the primary component as well as nickel and chromium as a minor component is examined. For this electrochemical reaction, the strongest oxidizing agent (cathode) and the strongest reducing agent (anode) are [2] and [5*], respectively:
 * 2) Nickel-iron-chromium piping**

Cathode: 2H + (aq) + 2e - → H 2(q) + 0.00V [2] Neither anode nor cathode: Fe (s) → Fe 2+ (aq) + 2e - + 0.44V [4*] Anode: Cr (s) → Cr 3+ (aq) + 3e - + 0.74V [5*] Net equation: 2Cr (s) + 6H + →3H 2(g) + 2 Cr 3+ (aq) +2.22V [8]

In the net reaction, equation [8], the chromium will oxidize since it is the strongest reducing agent. Thus, the corrosion of iron is prevented. Note that the crystal lattice structure of this piping is BCC.

Stainless Steel
In industry today, there are many more things to consider when selecting the appropriate material for pipelines, rather than simply looking at the electrochemical behaviour of the materials. In a sour environment, engineers have suggested that stainless steels should be used 5 due to the fact that liquid water is also present. Steel usually contains metal elements, such as iron, carbon, chromium, and molybdenum. In order to be classified as a stainless steel, the steel alloy must contain at least 12% chromium. There are 3 major categories of stainless steels: austenitic, ferritic and martensitic.

Material selection is crucial in preventing corrosion due to hydrogen sulphide. It is the responsibility of an engineer to select the best material possible for a certain application. In the case of hydrogen sulphide corrosion, chemical properties, electrochemical reactions as well as material structural properties must be taken into consideration. The safety of employees and local citizens are of at most importance. Thus, analyzing the difference between materials and crystal structures is important in creating a sour process with the least corrosion possible, which in turn will allow for a safer process where the risk of health problems and death will be decreased.
 * //Table 1: General properties of 3 main types of Stainless Steels 6 //**
 * = Types ||= Predominant Strucutre ||= General ideas of composition 2 ||= Corrosion Resistance, physically properties & price 2 ||
 * = Austenitic Steel ||= FCC || * Minimum of 16% Cr
 * A decent amount of Ni and/or Mn || * Good corrosion resistance to stress corrosion cracking only in the annealed form (otherwise they can not be used in the sour service)
 * Fairly expensive ||
 * = Ferritic Steel ||= BCC || * Cr ranges from 10.5% to 27%
 * Very little or no nickel
 * Mostly ferritic metals || * Excellent corrosion resistance in the sour service (must have no nickel present)
 * Better engineering properties
 * Less ductile than the Austenitic Steel
 * Less expensive ||
 * = Martensitic Steel ||= / || * Very little amount of Carbon and Nickel
 * 12%-14% of Cr (does not exceed 18%) || * Poor corrosion resistance compared to the two above
 * Must be double tempered to use in sour environment
 * Very strong, tough (physically)
 * Very brittle (physically) ||

**The Effect of Chromium** **in Stainless Steel**
Another interesting consideration for material selection is that chromium plays a major role on making the steel stainless. This is because chromium reacts with oxygen ions, forms a passive film 7 and protects the metal from oxidative corrosion. This passive film may be thought of as a thin and invisible oxide layer. One important property of this layer is that it may quickly form a new layer and recover the exposed surface if the passive film is broken. Also, this passive film allows chromium to be stable in the presence of an acid. Also, this means that hydrogen embrittlement has no effect on chromium. Since the passive film requires a lot of oxygen, this material will have a lower corrosion resistance since hydrogen sulphide itself does not have any oxygen. However, there may be oxygen molecules present in the fluid, so this is important to keep in mind as the passive film may help prevent pipe corrosion.

NACE MR 0175
NACE MR 0175 must be followed when selecting proper materials for specific conditions anywhere in North America 1. The attachment is an example is the 2003 version of NACE MR 0175, "Sulfide Stress Cracking - NACE MR 0175" (available from http://www.fisherregulators.com/technical/sulfide/) 9 NACE MR 0175

**References**

1 Mok CM.Corrosion Basic[Internet].[cited 2009 Nov 29]. Available from: [|www.scribd.com/doc/7579434/Corrosion-Basic] 2 Wikipedia. Cubic crystal system[Internet]. [Cited 2009 Nov 29]. Available from: [|http://en.wikipedia.org/wiki/Cubic_crystal_system] 3 Marcus P. 2002. Corrosion mechanisms in theory and practice. 2nd ed. New York: Marcel Dekker Inc. P. 68-73. 4 Talbot D, Talbot J. 1998. Corrosion Science and Technology. Florida: CRC press LLC. P. 18-19. 4a Chemistry 30 - Saskatchewan Learning. STANDARD REDUCTION POTENTIALS FOR HALF-REACTIONS [Internet]. [Cited 2009 Nov 29]. Available from: [|http://www.saskschools.ca/curr_content/chem30_05/appendix/tables_charts/red_potentials.pdf] 5 Eng-Tip Forums. 4140 vs. 4130 [Internet]. [Cited 2009 Nov 29]. Available from: [] 6 Jarvis AR, Bulloch JH. 1992. The effect of nickel content on the environmental assisted cracking (EAC) behaviour of low alloy steels in sour environments—A review. International Journal of Pressure Vessels and Piping. 49(3):271-307

7 Helmenstine AM. Why Stainless steel stainless?[Internet]. [cited 2009 Nov 29]. Available from: [] 8 Steeltalk.com. Composition of Steel, nickel steels, nickel chromium steels, molybdenum steels [Internet]. [Cited 2009 Nov 29]. Available from: [] 9 Sulfide Stress Cracking NACE MR0175 [Internet]. 2008. [cited 2009 November 20]. Available from:[]