Actions during service 163
It is well known that when steel is exposed to normal environmental
conditions (at least in most places on earth), it will corrode because of the
reaction of iron (which is the main component of steel) with water and oxygen. Corrosion of steel is a complex electrochemical reaction process with
many influencing factors. For a detailed treatment of general corrosion processes, reference is made to specialised literature. More simply, it can be
said that the corrosion process requires an anode where electrochemical
oxidation takes place and a cathode where electrochemical reduction takes
place. Furthermore, negative and positive particles have to be transported
through the electrically conducting corroding steel and the surrounding
electrolyte solution.
When steel is placed in a water solution (electrolyte), a certain number of
iron atoms will go into solution as iron ions, leaving some electrons behind
in the steel:
Fe → Fe 2+ + 2e −
(5.13)
This is the anode reaction or oxidation, occurring at the anode (negative
pole). The electrons (e − ) will be transported through the steel toward a
cathode (positive pole), where a cathode reaction or reduction takes place,
which in the presence of water and oxygen can be described as follows:
O 2 + 2H 2 O + 4e − → 4OH −
(5.14)
A classical illustration of a steady corrosion process is the electrochemical macrocell, consisting of two different metals which are electrically connected and immersed in an electrolyte solution, as illustrated
in Figure 5.37. One of the metals becomes the anode, while the other
becomes the cathode. At the anode, metal ions go into solution. Thus, this
metal is oxidizing or corroding. The other metal remains sound. Here,
the electrons coming from the corroding metal react with oxygen and
water, forming hydroxyl ions OH − . The macrocell clearly illustrates that
a corrosion process is the combination of chemical transformations and
the transfer of electrical charges, justifying the term ‘electrochemical’
process.
In the case of steel reinforcement in concrete where the concrete pore
solution acts as an electrolyte, normally no second type of metal is present
except in case of cathodic protection, which however is beyond the scope
of this textbook. Nevertheless, anodes and cathodes can occur on the same
steel surface, due to heterogeneities in the steel (metallurgical segregation
or different grain orientations), or due to local variations in the electrolyte (variations in concentration or aeration) (Rosenberg et al. 1989). As
anode and cathode can be close together, the term microcell is typically
used instead of macrocell.
It is well known that when steel is exposed to normal environmental
conditions (at least in most places on earth), it will corrode because of the
reaction of iron (which is the main component of steel) with water and oxygen. Corrosion of steel is a complex electrochemical reaction process with
many influencing factors. For a detailed treatment of general corrosion processes, reference is made to specialised literature. More simply, it can be
said that the corrosion process requires an anode where electrochemical
oxidation takes place and a cathode where electrochemical reduction takes
place. Furthermore, negative and positive particles have to be transported
through the electrically conducting corroding steel and the surrounding
electrolyte solution.
When steel is placed in a water solution (electrolyte), a certain number of
iron atoms will go into solution as iron ions, leaving some electrons behind
in the steel:
Fe → Fe 2+ + 2e −
(5.13)
This is the anode reaction or oxidation, occurring at the anode (negative
pole). The electrons (e − ) will be transported through the steel toward a
cathode (positive pole), where a cathode reaction or reduction takes place,
which in the presence of water and oxygen can be described as follows:
O 2 + 2H 2 O + 4e − → 4OH −
(5.14)
A classical illustration of a steady corrosion process is the electrochemical macrocell, consisting of two different metals which are electrically connected and immersed in an electrolyte solution, as illustrated
in Figure 5.37. One of the metals becomes the anode, while the other
becomes the cathode. At the anode, metal ions go into solution. Thus, this
metal is oxidizing or corroding. The other metal remains sound. Here,
the electrons coming from the corroding metal react with oxygen and
water, forming hydroxyl ions OH − . The macrocell clearly illustrates that
a corrosion process is the combination of chemical transformations and
the transfer of electrical charges, justifying the term ‘electrochemical’
process.
In the case of steel reinforcement in concrete where the concrete pore
solution acts as an electrolyte, normally no second type of metal is present
except in case of cathodic protection, which however is beyond the scope
of this textbook. Nevertheless, anodes and cathodes can occur on the same
steel surface, due to heterogeneities in the steel (metallurgical segregation
or different grain orientations), or due to local variations in the electrolyte (variations in concentration or aeration) (Rosenberg et al. 1989). As
anode and cathode can be close together, the term microcell is typically
used instead of macrocell.