2b (45)

b)    Chloride attack

The chloride ions may be present in the concrete if they are added to the mix (admixtures, water or aggregates). Fortunately it is not common. The most freąuent is that chlorides penetrate from outside, due to placing the structure in marinę environment.

c)    Stress Corrosion Cracking

The SCC is a special type of corrosion which is produced when mechamcal stresses act simultaneously to some specific aggressive agents. This type of corrosion may then develop in prestressed or postensioned wires.

3. Principles of Steel Corrosion in Concrete

a)    Primary Electrochemical Processes

For Steel in concrete, as the passive film is degraded by chloride ions or the pH reduced by carbonation, the metałłic Fe at the anodę is oxidized to form ferrous ions ( Fe²⁺)\

Fe —*■ Fe¹⁺ + 2e The electrons released at the anodę flow through the Steel to the cathodic areas, as illustrated in Figurę 2. The above reaction is initially balanced by cathodic reaction of dissolved oxygen (0₂) to forming hydroxyl ions (OH): O2 + 2 H₂0 + 4 e' —*■ 4 OH The anodic product Fe²~ reacts with the cathodicalły formed hydroxyl ions to produce a ring of white precipitate of ferrous hydroxide (Fe(OH)₂): Fe² + 20H —> Fe(OH)₂

b)    Secondary Processes

The Fe(OH)₂ can be fiirther converted into hydrated ferric oxide ( Fe₂0₂ ^x H₂0\ also known as ordinary red-brown rust, and black magnetite (Fe₂0%) preceded by the fonmation of green hydrated magnetite (Fe₂0₃ ^x H₂0):

4 Fe(OH)₂ + 02-> Fe₂0₃ x H₂0 + 2 H₂0 6 Fe (OH) ₂ + 02-^2 Fe₂0₃ x H₂0 + 4 H₂0 Fej0₃ x H₂0 —* Fe₃0_Ą + H₂0

Carhcće Proc*s»    ?:

O- - ' HjD --f-4 OK"    F» — Fe⁵* - li'

Cuiien: źlo\V

Figurę 2. Mechanism of corrosion of Steel in concrete.

The composition of rust on iron may be expressed as a generał formula:

m x Fe(OH)₂ + n xFe(OH)3 + p x H₂0 where the values of m, n and p vary considerably, depending on conditions such as pH of the solution, the oxygen supply and moisture content. Since the volume of rust products is much higher (about 4 to 6 times) than that of the iron as shown in Figurę 3, the formation of rust products will lead to cracking and spalling of the cover concrete when expansive stress exceeds the tensile strength of the concrete, and reduction of Steel reinforcing cross section may lead to structure faiłure.

Figurę 3. The relative volumes of iron and its corrosion reaction products.