Chloride ions with carbon steels

Galvanised steel provides increased corrosion resistance in carbonated concrete. In concrete with more than 0.4% chloride ion with respect to the cement content, there is an increased risk of corrosion and at high chloride contents the rate of corrosion approaches that of plain carbon steel. In test conditions the rate of corrosion is greater in the presence of sodium chloride than calcium chloride. Fusion-bonded epoxy-coated steel performs well in chloride-contaminated concrete up to about 3.9% chloride ion in content. 

During hydration of cement a highly alkaline pore solution (pH between 13 and 13.8), principally of sodium and potassium hydroxides, is obtained (Section 2.1.1). In this environment the thermodynamically stable compounds of iron are iron oxides and oxyhydroxides. Thus, on ordinary reinforcing steel embedded in alkaline concrete a thin protective oxide film (the passive film) is formed spontaneously [1-3]. This passive film is only a few nanometres thick and is composed of more or less hydrated iron oxides with varying degree of Fe and Fe [4j. The protective action of the passive film is immune to mechanical damage of the steel surface. It can, however, be destroyed by carbonation of concrete or by the presence of chloride ions, the reinforcing steel is then depassivated [5j. 

Steel in cement mortar is in the passive state represented by field II in Fig. 2-2. In this state reinforcing steel can act as a foreign cathodic object whose intensity depends on aeration (see Section 4.3). The passivity can be lost by introduction of sufficient chloride ions or by reaction of the mortar with COj-forming carbonates, resulting in a considerable lowering of the pH. The coordinates then lie in field I. The concentration of OH ions can be raised by strong cathodic polarization and the potential lowered, resulting in possible corrosion in field IV (see Section 2.4). 

The effect of the chromium content of the alloy on corrosion in boiling acids is shown in Table 4.7 along with the data for carbon steel and low-carbon and low-nitrogen 35% Cr alloys. The data show that the corrosion rates of 18 Cr-8 Ni (Type 304) is lower than Type 430 and 446 that is devoid of nickel. The nickel is the alloy probably reduces the rate of hydrogen evolution reaction. The molybdenum in Type 316 alloy was found to be useful in protection from pitting by chloride ions. 

It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels. 

The chloride ion itself is negative and will be repelled by the negatively charged cathode (reinforcing steel). It will move towards the (new external) anode. With the carbon-based anodes it may then combine to form chlorine... 

The chemical corrosion resistance of AAS concrete is veiy high. The hardened paste is completely resistant to sodium sulfate, and has a very high resistance to magnesium chloride and nitrate attack (Tailing and Brandstetr, 1993). AAS concrete also protects the steel reinforcement effectively against corrosion by chloride solutions, mainly because of a very low diffusion rate of chloride ions in the hardened paste (Dhir et al, 1996). However, the carbonation of AAS concrete surfaces by the CO2 of the air progresses faster than in comparable mixes produced with Portland cement. 

Kihira et al. [118] applied EIS to investigate the condition of the rust film formed on the weathering steel, and proposed new corrosion monitoring method based on rust film resistance. Nishimura et al. [119] measured the electrochemical impedance of a carbon steel covered with rust film formed in a wet/dry environment containing chloride ions. They reported that the charge transfer resistance (/ p) increased with the wet-dry cycles of exposure. 

Simm (1984) has studied the conjoint action of carbonation and chloride ions on the corrosion of zinc in mortar. Highly concentrated, high chloride mortars at 100% RH and 25°C can corrode zinc at 100 pm/year, but high carbonation or high chloride on its own causes a loss of zinc of only 5 pm/year. With neither chloride nor carbonation, no corrosion occurred. Sergi et al. (1985) have electrochemically looked at zinc in solutions pH 9.0-14.0 in relation to the use of galvanized steel in concrete. 

Stainless steels (so-called because they do not become stained with rust) are examples of alloy steels, i.e. ones that contain a d-block metal in addition to carbon. Stainless steels have a significant content of the alloy metal and are of high commercial value because of their resistance to corrosion. All contain at least 10.5% (by mass) of chromium, the minimum that renders the steel corrosion-resistant under normal aqueous conditions (i.e. in the absence of acid, alkali or pollutants such as chloride ions). The resistance to corrosion arises from the formation of a thin layer of Cr203 (wl3nm thick) over the surface of the steel. The oxide layer passivates (see Section 10.4) the steel and is self-repairing, i.e. if some of the oxide coating is scratched off, further oxidation of the chromium in the steel necessarily repairs the wound . 

Chloride ion is not as corrosive to carbon steel as to stainless steel. The corrosion rate of carbon steel is unaffected by NaCl concentration in the range of 0-10% at 150-240° C, but the rate increases with... 

Metals Affected. Pitting occurs in most commonly used metals and alloys. Iron buried in the soil corrodes with the formation of shallow pits, but carbon steels in contact with hydrochloric acid or stainless steels immersed in seawater characteristically corrode with the formation of deep pits. Aluminum tends to pit in waters containing chloride ions (for example, at stagnant areas), and aluminum brasses are subject to pitting in polluted waters. 

The importance of concrete cracks in rebar corrosion has also been highlighted by Niirnberger. Both carbonation and chloride ion diffusion, two important processes associated with rebar corrosion, can proceed more rapidly into the concrete along the crack faces, compared with uncracked concrete. Niirnberger argued that corrosion in the vicinity of the crack tip could be accelerated further by crevice corrosion effects and galvanic cell formation. The steel in the crack will tend to be anodic relative to the cathodic (passive) zones in uncracked... 

The type of anode material has an important effect on the reactions encountered on the anode surface. For consumable metals and alloys such as scrap steel or cast iron, the primary anodic reaction is the anodic metal dissolution reaction. On completely passive anode surfaces, metal dissolution is negligible, and the main reactions are the evolution of gases. Oxygen can be evolved in the presence of water, whereas chlorine gas can be formed if chloride ions are dissolved in the electrolyte. The reactions have already been listed in the theory section of this chapter. The above gas evolution reactions also apply to nonmetallic conducting anodes such as carbon. Carbon dioxide evolution is a fiirther possibility for this material. On partially passive surfaces, both the metal dissolution and gas evolution reactions are important. Corrosion product buildup is obviously associated with the former reaction. 

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