Illumination corrosion

A use is for internally illuminated signs in which any color can be introduced. Its dii sional stability recommends it for many optical uses. Acrylics are modified by copoly-mf ition to improve impact strength at the loss their extreme transparency.. An example is aci -modified polyvinyl chloride sheet, which is tougher than aciylonitrile-butadiene-styrene and polycarbonate and is suitable for corrosion-resistant pans, aircraft parts and materia idling equipment. 

The electrochemistry of single-crystal and polycrystalline pyrite electrodes in acidic and alkaline aqueous solutions has been investigated extensively. Emphasis has been laid on the complex anodic oxidation process of pyrite and its products, which appears to proceed via an autocatalytic pathway [160]. A number of investigations and reviews have been published on this subject [161]. Electrochemical corrosion has been observed in the dark on single crystals and, more drastically, on polycrystalline pyrite [162]. Overall, the electrochemical path for the corrosion of n-EeS2 pyrite in water under illumination has been described as a 15 h" reaction ... 

The band edges are flattened when the anode is illuminated, the Fermi level rises, and the electrode potential shifts in the negative direction. As a result, a potential difference which amounts to about 0.6 to 0.8 V develops between the semiconductor and metal electrode. When the external circuit is closed over some load R, the electrons produced by illumination in the conduction band of the semiconductor electrode will flow through the external circuit to the metal electrode, where they are consumed in the cathodic reaction. Holes from the valence band of the semiconductor electrode at the same time are directly absorbed by the anodic reaction. Therefore, a steady electrical current arises in the system, and the energy of this current can be utilized in the external circuit. In such devices, the solar-to-electrical energy conversion efficiency is as high as 5 to 10%. Unfortunately, their operating life is restricted by the low corrosion resistance of semiconductor electrodes. 

Although it presents an obstacle in practical applications, the photoanodic corrosion of colloids has often been used to obtain information about the interaction of dissolved compounds with the photo-produced charge carriers, as it was found that solutes can influence the rate of the dissolution. Both promoting and retarding effects were observed The rate of dissolution is readily followed by recording the decrease in the intensity of the absorption spectrum of the colloid upon illumination, or more precisely, by determining the yields of metal and sulfate ions in solution. 

Chlorides are responsible for the pitting corrosion of steel parts. Normal carbon steel can stand 1000 ppm of chlorides (=1000 g M 3), but stainless steel starts to corrode severely from 100 ppm on Attention for ladders, illumination sets etc. 

Thus, although the potential required for polarization would be much larger at n-type semiconductors than at illuminated p-type semiconductors and despite the fact that not all n-type semiconductors can be used because of corrosion (or reduction) of semiconductor materials themselves, the use of n-type semiconductors to examine C02 reduction seems to be indicated because the cathodic current is much larger (the electron is the major carrier for n-type semiconductors), approaching that of metal electrodes, compared to the photocurrent obtained at illuminated p-type semiconductors,... 

The ability to manipulate the anodic corrosion problem using high concentrations of redox active electrolyte also makes possible the sustained oxidation of Br" at illuminated metal dichalcogenide-based cells, Figure 1.(15) The use of high concentrations of electrolyte has proven valuable in situations involving other photoanode materials, notably n-type Si.(36,37)... 

There is also an etched layer of Si on the surface under illumination as illustrated in Figure 18. This etched layer is mainly due to photo-induced corrosion. As a result of the photo induced dissolution the top surface of PS layer recedes with time. The rate of dissolution depends on doping, F1F concentration, current density and illumination intensity. 

The dissolution of PS during PS formation may occur in the dark or under illumination. Both are essentially corrosion processes, by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The material in the PS, which is distant from the growing front is little affected by the external bias due to the high resistivity of PS and is essentially at the open circuit potential (OCP). Such corrosion process is responsible for the formation of micro PS of certain thickness (stain film) in HF solutions containing oxidants under an unbiased condition. 

Illumination generates holes within the material of PS and causes photo corrosion of PS that is much faster than that in the dark. Depending on illumination intensity and time, the pore walls in a PS can be thinned to various extents by the photo induced corrosion. This corrosion process is responsible for the etched crater between the initial surface and the surface of PS as illustrated in Figure 28. It is also responsible for the fractal structure of the micro PS formed under illumination. 

The corrosion behavior of semiconductors can, in principle, be described within the framework of the same concepts as for metals (see, for example, Wagner and Traud, 1938), but with due account for specific features in the electrochemical behavior of a solid caused by its semiconducting nature (Gerischer, 1970). One of the main features is photosensitivity related to a change in the free-carrier concentration under illumination. Photosensitivity underlies the phenomenon of photocorrosion. 

Fig. 14. Diagram explaining the effect of light on the potential and rate of corrosion of a semiconductor (a) n-type and (b) p-type.

Though processes occurring under photopassivation have not so far been understood in detail, they may be related with certainty (Izidinov, 1979) to the acceleration, under illumination, of one of the two conjugated reactions, which constitute the overall process of electrochemical corrosion. Depending on the initial state of corroding silicon, either the anodic (at the active surface) or the cathodic (at the passive surface) partial reaction is accelerated. This leads to the shift of the potential, and the system jumps over the maximum of the polarization curve from one stable state to the other. 

Among the methods of anodic and chemical etching of semiconductors, widely used both in the production of semiconductor devices and in investigations (see, for example, Schnable and Schmidt, 1976 Turner and Pankove, 1978), the so-called light-sensitive etching is of great importance. It is based on the variation, under illumination, of the concentration of minority carriers, which often determines, as was shown above, the rate of anodic dissolution and corrosion of semiconductors. 

Here corrosion occurs even in darkness. In the simplest case where the partial cathodic reaction proceeds exclusively through the conduction band and the anodic reaction through the valence band, the corrosion rate is limited, as was shown in Section 8, by the supply of minority carriers to the surface, irrespective of the type of sample conductivity. Therefore, in darkness the corrosion rate is low. Illumination accelerates corrosion. This case is similar to case (a), but with the difference that the role of anodic polarization is played by chemical polarization with the help of an oxidizer introduced into the solution (see Section 13 for examples). 

The etching process may be characterized by an amount of light energy received by a sample for the optimal etching time. For some semiconductor materials this amount is equal to approximately 1 J/cm2. The minimum operating illumination intensity, at which the rate of photoetching exceeds noticeably the rate of dark corrosion, is 10"4 W/cm2. 

When an n-CdS electrode is suddenly illuminated with light capable of producing holes in the CdS, j, would almost immediately reach some large value (equal to or less than the saturation current of curve 1) and then decay to the steady state value of curve 1 as the steady state value of N is approached according to equation 3. If such a transient does not occurthe oxidized corrosion site acting as a recombination state is not the controlling factor in the photocurrent onset. 

Figure 10. Scheme of competing reactions at illuminated CdSe surface. The process on the extreme left is the main rate-limiting step, while the one on the right can lead to photoelectrode corrosion. 

Paint is necessary on interior steel surfaces of ships to prevent corrosion. In addition, properly selected paints augment illumination, assist in sanitation, and provide decoration. 

---