Basic Corrosion Theory
Corrosion is an electrochemical reaction composed of two half cell reactions,
an anodic reaction and a cathodic reaction. The anodic reaction releases electrons,
while the cathodic reaction consumes electrons. There are three common cathodic
reactions, oxygen reduction (fast), hydrogen evolution from neutral water (slow),
and hydrogen evolution from acid (fast).
The corrosion cell
The corrosion cell can be represented as follows:
M → Mn+ + ne-
i.e. for iron and steel: Fe → Fe2+
- M stands for a metal and n stands for the number of electrons that an atom of the
metal will easily release.
O2 + 4 H+ + 4e- →
2H2O (oxygen reduction in acidic solution)
1/2 O2 + H2O + 2e- →
2 OH- (oxygen reduction in neutral or basic solution)
2 H+ + 2e- → H2
(hydrogen evolution from acidic solution)
2 H2O + 2e- → H2
+ 2 OH- (hydrogen evolution from neutral water)
Each half-cell reaction has an electrical potential, known as the half-cell electrode
potential. The anodic reaction potential, Ea , plus the cathodic
reaction potential, Ec , adds up to E, the cell potential. If
the overall cell potential is positive, the reaction will proceed spontaneously.
Every metal or alloy has a unique corrosion potential in a defined environment.
When the reactants and products are at an arbitrarily defined standard state, the
half-cell electrode potentials are designated Eo
. These standard potentials are measured with respect to the standard hydrogen electrode
(SHE). A listing of standard half-cell electrode potentials is given in Table 1.
Selected half-cell reduction potentials are given in Table 1. To determine oxidation
potentials, reverse the direction of the arrow and reverse the sign of the standard
potential. For a given cathodic reaction, those anodic (reversed) reactions below
it in the table will go spontaneously, while those above it will not. Thus any metal
below the hydrogen evolution reaction will corrode (oxidize) in acidic solutions.
Cathodic reaction: 2H+ + 2e- → H2 (hydrogen
Two possible anodic reactions:
Cu → Cu2+ + 2e- (above cathodic rxn in table - will
Zn → Zn2+ + 2e-
(below cathodic rxn in table - spontaneous corrosion)
Thus, in the presence of H+
ions, Zinc (Zn) will spontaneously corrode while copper (Cu) will not.
Table 1. Standard Electromotive Force Potentials
Source: Handbook of Chemistry and Physics, 71st ed, CRC Press, 1991
Standard Potential, eo (volts vs. SHE)
Au3+ + 3e- → Au
+1.498 (Most Noble)
O2 + 4H+ + 4e- → 2H2O
+1.229 (in acidic solution)
Pt2+ + 2e- → Pt
NO3- + 4H+ + 3e- → NO + 2H2O
Ag+ + e- → Ag
O2 + 2H2O + 4e- → 4OH-
+0.401 (in neutral or basic solution)
Cu2+ + 2e- → Cu
2H+ + 2e- → H2
Pb2+ + 2e- → Pb
Sn2+ + 2e- → Sn
Ni2+ + 2e- → Ni
Co2+ + 2e- → Co
Cd2+ + 2e- → Cd
Fe2+ + 2e- → Fe
Cr3+ + 3e- → Cr
Zn2+ + 2e- → Zn
2H2O + 2e- → H2 + 2OH-
-0.828 (pH = 14)
Al3+ + 3e- → Al
Mg2+ + 2e- → Mg
Na+ + e- → Na
K+ + e- → K
-2.931 (Most Active)
Table 1 can be used to show that copper will corrode in nitric acid solutions (oxidizing)
and aerated water. Similarly, aluminum (Al), magnesium (Mg), sodium (Na) and potassium
(K) will react spontaneously with water in neutral or basic solutions.
Why corrosion cells form
Corrosion cells are created on metal surfaces in contact with an electrolyte because
of energy differences between the metal and the electrolyte. Different area on the
metal surface could also have different potentials with respect to the electrolyte.
These variations could be due to i) metallurgical factors, i.e., differences
in their composition, microstructure, fabrication, and field installations, and
ii) environmental factors
. Carbon and low alloy steels are the most widely used material in the oilfield.
Stainless steels (Fe-Cr-Ni), and nickel-base corrosion resistant alloys (CRA), such
as Incoloys (Ni-Fe-Cr), Inconels (Ni-Cr), Hastelloys (Ni-Cr-Mo-Fe-Co) etc., are
also used in highly corrosive environments.
i) Metallurgical factors
Steel is an alloy of iron (Fe) and carbon (C). Carbon is fairly soluble in liquid
iron at steel making temperatures, however, it is practically insoluble in solid
iron (0.02% at 723C), and trace at room temperature. Pure iron is soft and malleable;
small amounts carbon and manganese are added to give steel its strength and toughness.
Most of the carbon is oxidized during steelmaking. The residual carbon and post-fabrication
heat treatment determines the microstructure, therefore strength and hardness of
steels. Carbon steels are then identified by their carbon contents, i.e., low-carbon
or mild steel, medium carbon (0.2- 0.4 % C), high-carbon (up to 1% C) steels, and
cast irons (>2 % C). American Iron and Steel Institute (AISI) designation 10xx
series represent plain carbon steels, last two digits indicating the carbon content.
For instance, AISI 1036 steel, commonly used in sucker rods, contain 0.36% carbon.
Low alloy steels contain 1-3% alloying elements, such as chromium-molybdenum steels,
4140 (1% Cr-0.2% Mo-0.4% C), for improved strength and corrosion resistance. American
Petroleum Institute (API) specifications also provide guidelines for strength and
chemical composition of oilfield steels.
During equilibrium solidification of steel, individual grains of almost pure iron
(ferrite), and grains richer in iron carbide (cementite, Fe3C) within ferrite form.
The lamellar carbide structure with ferrite is known as pearlite. If, however, steel
is rapidly cooled to room temperature (quenched), carbon is retained in a highly
strained matrix known as martensite. This structure is very hard and brittle and
is not suitable for most engineering applications. The microstructure of fast cooled
steels, for instance after welding or hot-rolling, are modified by reheating steels
to a critical temperature range and controlled cooling, i.e., tempering, normalizing,
The microstructure of a low-carbon pipe steel is shown (magnified 100X) in (a)
transverse and (b) in longitudinal sections, where light grains are ferrite and
the dark grains are pearlite. Other impurities in iron may also migrate to grain
boundaries forming micro-alloys that may have entirely different composition from
the grains, hence may have different corrosion properties.
In a corrosive environment, either grains or the grain boundaries having different
composition can become anodic or cathodic, thus forming the corrosion cells. Hydrogen
evolution reaction can take place on iron carbide, and spheroidized carbon in steels,
and graphite in cast irons, in acidic solutions with relative ease; areas denuded
in carbon become anodic and corrode preferentially. Therefore, post-weld heat treatment
of steels is critical in order to prevent corrosion of the heat affected zone (HAZ),
sensitization and intergranular corrosion in stainless steels.
Other metallurgical factors include improper heat treatment for stress relief after
hot rolling, upsetting, or excessive cold working; slag inclusions, mill scale,
water deposited scale and corrosion product scales, nicks, dents and gouges on the
metal surface. Scars caused by pipe wrench, tongs, and other wellhead equipment
on sucker rods and tubing would become anodic and corrode downhole. Likewise, new
threads cut into pipe will be anodic and corrode in the absence of suitable corrosion
Deformation caused by cold bending or forcing piping into alignment will create
internal stresses in the metal. The most highly stressed areas will become anodic
with respect to the rest of the metal. Hammer marks, nicks and gauges will also
act as stress raisers and may cause fatigue failures.
ii) Environmental factors
Sections of the same steel may corrode differently due to variations in the concentration
of aggressive ions in the environment. For instance, a casing or a pipeline could
pass through several formations or soils with different water composition, hence,
sections of the casing or the pipe could experience different rates of corrosion.
Similarly, a pipeline crossing a river will be exposed to higher concentration salts
as compared to dry land. It is difficult to predict the effect of higher salt concentrations
but, generally, sections of steel exposed to higher salt concentrations become anodic
Differences in the oxygen concentration on the metal surface (differential aeration
or differential oxygen concentration cells) cause particularly insidious forms of
corrosion. A common example is corrosion of pipes under paved roads, parking lots,
Lack of oxygen under the pavement render that section of the pipe anodic, hence
pipe corrodes preferentially. Similarly, loose backfill placed into ditch to cover
a pipeline is more permeable to oxygen diffusion; the topside of the pipe will become
cathodic, and the bottom resting on undisturbed soil will become anodic and corrode.
Crevice and pitting corrosion mechanisms in aerated systems
can also be explained by differential concentration cells.