1. ENT 487 ENVIRONMENTALLY ASSISTED CRACKING IN METALS
DR. HAFTIRMAN
LECTURE 12
WED, 8 OCTOBER 2008

2. ENVIRONMENTALLY ASSISTED CRACKING IN METALS
Environmentally assisted cracking (EAC) is a common problem in a variety of industries.
In the petroleum industry, for example, EAC is pervasive (merembes). Offshore platforms are susceptible to corrosion-assisted fatigue.
Equipment in refineries and petrochemical plants are exposed to a myriad of aggressive environments that lead to stress corrosion cracking and hydrogen embrittlement.
Similar problems exist in other settings, including fossil and nuclear power plants, pulp and paper plants, ships, bridges, and aircrafts.
Environmental cracking can occur even when there are no visible signs of corrosions.

3. CORROSION PRINCIPLES ELECTROCHEMICAL REACTIONS
All corrosion processes involve electrochemical reactions. Figure illustrates a simple electrochemical cell. The anode and cathode are physically connected to one another and are immersed in a conductive medium called an electrolyte. Atom from the anode material give up electrons, resulting in ions being released into the electrolyte and electrons flowing to the cathode.

4. ELECTROCHEMICAL REACTIONS
Note that the corrosion cell forms an electrical circuit. There is a voltage drop,ΔE, between the anode and cathode. Over time, the anode is consumed( corrodes), as it releases ions into the electrolyte.

5. ELECTROCHEMICAL REACTIONS
In cases where the two electrodes in an electrochemical cell are different metals, the anode is the metal that has a higher propensity to give up electrodes (oxidize).
For example, in an electrochemical cell with gold and iron electrodes, iron would be the anode because it oxidizes more readily than gold.

6. ELECTROCHEMICAL REACTIONS
An electrochemical cell need not include a bond between dissimilar metals. A single metal in contact with an electrolyte may be sufficient to form a corrosion cell, depending on the respective chemical compositions of the metal and electrolyte.
For example, consider a coupon of iron immersed in hydrochloric acid (HCl).

7. ELECTROCHEMICAL REACTIONS
The chemical reaction is
The iron is consumed by this reaction and hydrogen gas (H2) is generated. If we consider only the interaction between iron and hydrogen, the above reaction can be written in the following forms:
Therefore, iron reacts with hydrogen ions to form iron ions, atomic hydrogen, and hydrogen gas. This reaction can be divided into two parts:

8. ELECTROCHEMICAL REACTIONS
Iron is oxidized to iron ions and hydrogen ions are reduced to H atoms that can be either be absorbed by the electrode or recombined and evolve into electrolyte as hydrogen gas.
The former is an anodic reaction and the latter is a cathodic reaction.
An oxidizing indicates a reducing or cathodic reaction.

9. ANODIC AND CATHODIC REACTIONS
Figure schematically illustrates the anodic and cathodic reactions occur at the same physical location.
Every corrosion process consist of an anodic and cathodic reaction.

10. ANODIC AND CATHODIC REACTIONS
The anodic reaction normally involves the oxidation of a metal to its ion. The general form for the anodic reaction is given by
Where n is number of electrons produced, which equals the valence of the iron.
Most metallic corrosion processes involve one or more of the cathodic (reduction) reactions.

11. CATHODIC REACTIONS Hydrogen evolution:
Oxygen reduction (acid solution):
Oxygen reduction (neutral or basic solutions):
Metal ion reduction:
Metal deposition:

12. CATHODIC REACTIONS The overall reaction is
Ferrous hydroxide, which is the product of the above reaction, is unstable in oxygenated water. It oxidizes to ferric hydroxide, which is known to the layperson as rust:

13. Note Both water and oxygen are required to corrode steel.
Steel that is completely submerged in water normally corrodes very slowly because the cathodic reaction is starved for oxygen.
Steel corrodes most quickly when there is an ample supply of both moisture and oxygen, such as in a climate with high relative humidity and frequent rain showers.
The corrosion rate is also accelerated if steel is coupled galvanically to a more noble metal.

14. Note
Consider a steel structure in a seawater environment, such as an offshore platform. The most aggressive environments occur just above and below the water surface.
In the splash zone above the surface, both oxygen and water are plentiful. Within the first few feet below surface, the water is oxygen rich because wave motion traps air bubbles.
The relatively simple situation is complicated by tight crevice geometries, the presence of additional dissolved ions in the electrolyte, and the imposed cathodic protection.

15. Corrosion Current an Polarization
Since corrosion is an electrochemical process, the magnitude of the electric current in the corrosion cell is a fundamental measure of the corrosion rate. The corrosion current can be reduced by inhibiting either reaction, or by reducing the conductivity of the electrolyte.
When an electrochemical reaction is retarded by one or more environmental factors, it is said to be polarized. There are three types of polarization: activation polarization, concentration polarization, and resistance polarization.
Activation polarization refers to processes that are controlled by the rate of the reaction at the metal-electrolyte interface.
Concentration polarization occurs when the rate-limiting step is diffusion of ions in the electrolyte.

16. Corrosion Current an Polarization
Resistance polarization is a consequence of the resistivity of the electrolyte. A reaction can also be polarized by an externally applied current (galvanic polarization) or potential (potentiostatic polarization).
Resistance polarization is a major factor in the corrosiveness of seawater compared to tap water and de-ionized water.
Seawater is very conductive because there is a ample supply of sodium and chlorides ions, while de-ionized water has relatively low electrical conductivity.
Normal tap water falls somewhere between the extremes.

17. ELECTRODE POTENTIAL AND PASSIVITY
A key factor that controls the corrosion current is the electrode potential. The simple corrosion cell in previous Figure, which showed an electric potential drop (ΔE) between the anode and cathode. The elctrode potential refers to the half-cell potential of the electrode. It is define as the potential difference between the electrode of interest and a reference electrode, such as a standard hydrogen electrode (SHE). The magnitude of the electrode potential is a function of the chemical composition of the electrode and the oxidizing power of the electrolyte.
The oxidizing power is a function of the reagents that are present as well as their concentration.
Normally, the corrosion current increases exponentially with increasing electrode potential. However, many technologically important materials ( steel, aluminum, and titanium alloys) exhibit a more complex behavior call passivity.

18. POLARIZATION DIAGRAM OF A METAL THAT EXHIBITS PASSIVITY EFFECTS

19. POLARIZATION DIAGRAM OF A METAL THAT EXHIBITS PASSIVITY EFFECTS
Figure illustrates the typical behavior of a metal that exhibits passivity effects. There are three distinct regimes: active, passivity, and transpassive. In the active region, a small increase in the electrode potential causes a large increase in corrosion rate. A plot of electrode potential vs the logarithm of current density is a straight line in the active region. As electrode potential is increased further by any of the polarization processes, the current density exhibits a sudden decrease at the beginning of the passive region. The corrosion rate in the passive region is typically 3 to 6 orders of magnitude slower than one would predict by extrapolating the trend in the active region.
In the passive region, a surface film that acts as a protective barrier forms on the surface. This surface film remains stable over a wide range of electrode potential. The surface film breaks down in the transpassive region due to the presence of very powerful oxidizers. The highly protective surface films are very thin, perhaps tens of nanometers. Such films are easily damaged by mechanical means, but quickly reform to prtect the metal from corrosion.