Unprotected/protected coatings

Unprotected steel corrodes at a rate which is generally assumed to be 0.1 to 0.2mm per annum. Factors that influence the actual rate of corrosion include the maintenance program applied by the owner - particularly preservation of protective coatings, efficiency of cathodic protection systems in ballast tanks, corrosive properties of the cargo carried and environmental factors such as temperature and humidity. Under extreme conditions it has been known for the annual rate of corrosion on unprotected steel exposed on both surfaces to approach 1mm. 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

Over the next couple of days observe the pennies. The nail in contact with the zinc should accumulate much less rust compared to the unprotected nail. The nail not in contact with zinc will show signs of oxidation in just a few hours. Appreciable rust will accumulate on this nail. The zinc oxidizes on the other nail, but the oxidation results in a protective coating of zinc oxide forming on the zinc. 

Commercially available are various types of aluminium front surface mirrors to suit different requirements. For the visible spectral range, there are standard mirrors such as Alflex A . If improved reflection is required, a multiple film mirror Alflex B can be used. Both types of mirrors are provided with a hard and resistant dielectric protection coating. Such mirrors were first made by Hass et al. [73, 74]. The aluminium film on the surface mirror Alflex is even protected by an interference film system, which also enhances the reflectance for the visible range. In the visible and infrared, the spectral curve of the reflectance is approximately the same for Alflex A as that of an unprotected aluminium surface. With a mirror type Alflex B. the increase in reflection in the visible, with a maximum at 550 nm, can be clearly seen in Fig. 12. If required, this maximum can also be shifted to other wavelengths in the visible spectrum. 

Field tests include those in which specimens are surrounded by aggressive soils, atmospheres, or waters (seawater). Atmospheric and water tests, which are performed both on unprotected and on protected (coated) materials, require special methods (DIN 50 917 1979). Here it is important that control specimens be tested simultaneously in order to predetermine the corrosive conditions at the testing site. 

Tests of the bare, unprotected metal are useful to establish a minimum basis of performance and comparative ranking. However, the applicability to the end use and prediction of service life require evaluation of the protective coatings and maintenance procedures to be used in service. 

Zinc sulfate is soluble in water and may be washed away by rain leaving the zinc unprotected, producing a high corrosion rate. The protective coating may also be damaged by abrasion and erosion. 

Figure 40.16 Mixed oxide inorganic UV protective coatings prepared by epoxy assisted sol-gel procedure used for protection of a colored surface, (a) Photodegradation behavior, (b) Unprotected and protected coatings.

Many cellular plastic products are available with different types of protective faces, including composite metal and plastic foils, fiber-reinforced plastic skins, and other coatings. These reduce but do not eliminate the rate of aging. For optimum performance, such membranes must be totally adhered to the foam, and other imperfections such as wrinkles, cuts, holes, and unprotected edges should be avoided because they all contribute to accelerated aging. 

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. 

A Pt catalyst was applied by dry and wet techniques. By means of sputtering using a mask process protecting parts of the microstructure, the micro channel bottom was coated selectively. In addition, an y-alumina layer was applied by the sol-gel technique. Initially, the whole micro structure was covered by such a layer. Then, photoresist was applied and patterned so that only the channel part remained covered. After removal of the exposed photoresist and unprotected y-alumina, only the channel bottom was coated with y-alumina. 

Consequently, mirror optics are more common, in particular in the mid-IR. The mirrors used are usually aluminium- or gold-coated flat or curved substrates. While near-IR mirrors are usually protected by thin SiO-layers, in the mid-IR unprotected mirrors have to be used. Disadvantages of mirror optics are the elevated space consumption and the higher prices in comparison to refractive optics, especially comparing non-standard mirrors against non-standard lens. In total, mirror optics are so preferable to fibres and refractive optics, at least in the mid-IR, that in some technical applications they are used to replace waveguides to transport IR radiation between source, sensor head and spectrometer. 

Fig. 37 Linear chain formation of DNA-coated paramagnetic polystyrene colloids with the different self-protection schemes displayed in Fig. 33. By using an external magnetic field, DNA-functionalized particles were brought together into linear chains, after which the temperature was lowered below the association temperature for beads, and the field turned off. (a) Representative microscopy picture of the resulting chain structures immediately after switching off the magnetic field, (b-d) Chains after 1 h at the specified temperature for particles functionalized with sticky end sequences able to form both loops and hairpins (b, c) or only loops (d). The degree of aggregation of chains in (d) is intermediate between the unprotected, branched chains in (b) and the perfectly linear, protected chains in (c). Adapted with permission from [157]...

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