Corrosion section

With underground installations in the soil, it must be ensured that no water can penetrate in gaps between cathodically protected and unprotected parts since the cathodically unprotected side of the coupling can be destroyed by anodic corrosion. Sections of pipe behind the insulator must be particularly well coated. 

J G N Thomas, BSc, PhD, ARCS, DIC Formerly Corrosion Section, Division of Materials Applications,... 

Weld corrosion (Section 9.5) Crevice corrosion at butt welds due to poor penetration has already been discussed and was shown in Fig. 9.1(a). Conversely, if there is a large weld bead protruding in the pipe bore, erosion/cor-rosion can occur downstream due to the turbulence produced over the weld bead (Fig. 9.8). In either case, the fault probably lies in the incorrect spacing of the butts at welding. 

From time to time astronomical estimates are made of the annual destruction of metals, particularly iron and steel, by corrosion (Section 1.1). Paint is one of the oldest methods used for delaying this process and consequently it is somewhat surprising that its protective action has only recently been systematically examined. 

Describe corrosion and means for protecting iron from corrosion (Section 12.14). 

Sulfuric acid is a stronger acid than sulfurous [pAa(l) < 0, p7fa(2) = 1.99 at 25 °C and infinite dilution] rain as acidic as pH 2.1 has been recorded at Hubbard Brook, New Hampshire, and the pH of water droplets in clouds can be as low as 1.5 (for comparison, the pH of rainwater saturated with atmospheric CO2 is about 5.6 at 15 °C). Acid rain destroys building materials (especially marble), kills fish and vegetation, accelerates metallic corrosion (Sections 16.5 and 16.7), and can be directly harmful to humans (e.g., it causes the alligator skin condition reported in Cubatao, Brazil). Sulfate rain is not completely without redeeming features, as many soils (e.g., in southern Alberta, Canada) are sulfur-deficient. On balance, however, its acidity is unacceptable, and sulfur oxide emissions must be controlled at the source. Several control measures are possible ... 

Dust (especially from industrial activities) and salt spray will also exacerbate atmospheric corrosion (Section 16.4). In enclosed industrial premises, atmospheric corrosion could be minimized by preventing noxious emissions, filtering the air to remove particulate matter, and scrubbing the air with water to remove SO2 and other objectionable gases, although the humidity should itself be kept as low as possible (e.g., steam leaks should not be tolerated). On the global scale, however, the cost to the public of atmospheric corrosion could be substantially reduced by sharply limiting SO2 and, to a lesser extent, NO. emissions from power plants, smelters, automobiles, and other industrial functions. This is an aspect of the acid rain threat (Chapter 8) that is usually overlooked. 

Further, the above methods measure the equivalent global corrosion rate of a large area. Corrosion in reality is often local, i.e., takes place only at tiny areas of the surface. It is necessary to have a local probe, not only of the rate, but of the mechanical properties. Something about this approach will be given in the discussion of local corrosion (Section 12.5). 

The section on metallic corrosion clearly indicates that the conventional method of classification of environments into marine, industrial, and rural no longer is adequate. More specific information is needed about the actual chemical components in the atmosphere as well as humidity and other factors. Specific environments also are addressed in the metallic corrosion section. For example, the automotive environment in the northeastern United States is particularly severe because of the combination of acid deposition and the use of road de-icing salts. These factors exert a synergistic effect on the corrosion behavior of auto-body steel and on exterior anodized aluminum automobile trim. 

For artefacts recovered from marine sites, they are often covered in concretions. These are hard layers of calcareous deposit derived from decaying shells of aquatic animals (e.g. barnacles, mussels, etc.) or hardness salts present in seawater. The latter is present as soluble bicarbonate ions (HC( )3 ) in sea or fresh water. At cathodic sites on the metal surface, there is a rise in the local pH due to the production of OH- during the reduction of dissolved oxygen gas (see Equation (10) in corrosion section). This results in the precipitation of solid calcium carbonate (CaC03) scale on the cathodic sites according to the following reaction ... 

Definition of Corrosive Section 2(i) of the FHSA defines the term corrosive as any substance which in contact with living tissue will cause destruction of tissue by chemical action. CPSC regulations elaborate on this definition by providing as follows ... 

When more and less noble materials are placed in contact, the more noble material offers an extra area for the cathodic reaction. Therefore flie total rate of the cathodie reaction is increased, and this is balanced with an increased anodic reaction, i.e. increased dissolution of the less noble material (galvanic corrosion. Section 7.3). If the more noble material (the cathodic material) has a large surface area and the less noble metal (the anodic metal) has a relatively small area, a large cathodic reaction must be balanced by a correspondingly large anodic reaction concentrated in a small area. The intensity of the anodic reaction, i.e. the corrosion rate (material loss per area unit and time unit) becomes high. Thus, the area ratio between the cathodic and the anodic materials is very important and should be kept as low as possible. It should be mentioned that in a galvanic corrosion process, the more noble material is more or less protected. This is an example of cathodic protection, by which the less noble material acts as a sacrificial anode (see next section). 

Reduction of metal ions must be taken into consideration in some cases. A typical example is reduction of copper. Equation (6.1). (This is further dealt with under galvanic corrosion. Section 7.3.)... 

We have already determined that the chloride ion is a catalyst to corrosion (Section 3.2.3). As it is negatively charged we can use the electrochemical process to repel the chloride ion from the steel surface and move it towards an external anode. This process, called electrochemical chloride extraction (ECE), desalination or chloride removal, uses a temporary anode and a higher electrical power density than CP, but is otherwise similar (Figure 7.1). Preparation in terms of concrete repair, power supplies etc are similar to those for impressed current cathodic protection except that the power supply is temporary and may be from a temporary source such as a generator. The output is larger, up to 50 V and 2 A-m. ... 

CORROSION (SECTION 20.8) Electrochemical principles help us imderstand corrosion, imdesirable redox reactions in which a metal is attacked by some substance in its environment. The corrosion of iron into nist is caused by the presence of water and oxygen, and it is accelerated by the presence of electrolytes, such as road salt The protection of a metal by putting it in contact with another metal that more readily undergoes oxidation is called cathodic protection. Galvanized iron, for example, is coated with a thin layer of zinc because zinc is oxidized more readily than iron, the zinc serves as a sacrificial anode in the redox reaction. 

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