Protection layers effectiveness

Another effect, which was first observed and utilized in spot reactions on filter paper impregnated with solid reagents, is the protective layer effect . It consists in the enveloping of the surface of a reactive material by a resistant coating produced chemically in such a manner that the underlying... 

Siliconizing is yet another process used especially for coating of the refractory metals Ti, Nb, Ta, Cr, Mo, and W (see Refractories). These metals form siHcides which have a surface oxidation protection layer of Si02. Siliconizing is especially effective on molybdenum against air oxidation up to 1700°C. 

The aggressive ions either break down the protective films or prevent their formation and, in effect, increase the corrosion rate. In presence of chloride and sulfate ions the corrosion attack is more localized and, as a result, causes deep pitting. Inhibitive ions, on the other hand, tend to limit the attack and decrease the corrosion rate by forming protective films. The film is similiar to adherent carbonate-containing rust, which polarizes the anodic areas. When aggressive and inhibitive ions are present together, the aggressive ions, if present in sufficient quantities, interfere with the deposition of the protective layers. 

In dry air the stability of zinc is remarkable. Once the protective layer of zinc oxide formed initially is complete, the attack ceases. Even under under normal urban conditions, such as those in London, zinc sheet 0 -8 mm thick has been found to have an effective life of 40 years or more when used as a roof covering and no repair has been needed except for mechanical damage. The presence of water does, of course, increase the rate of corrosion when water is present the initial corrosion product is zinc hydroxide, which is then converted by the action of carbon dioxide to a basic zinc carbonate, probably of composition similar to ZnCOj 3Zn(OH)2 . In very damp conditions unprotected zinc sometimes forms a loose and more conspicuous form of corrosion product known as wet storage stain or white rust (see p. 4.171). 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

A second effect is due to the local growth of adsorbed macromolecules as soon as the two protective layers begin to interpenetrate. This results in an osmotic repulsion to restore the equilibrium by diluting the macromolecules... 

For trace analysis in fluids, some Raman sensors (try to) make use of the SERS effect to increase their sensitivity. While the basic sensor layout for SERS sensors is similar to non-enhanced Raman sensors, somehow the metal particles have to be added. Other than in the laboratory, where the necessary metal particles can be added as colloidal solution to the sample, for sensor applications the particles must be suitably immobilised. In most cases, this is achieved by depositing the metal particles onto the surfaces of the excitation waveguide or the interface window and covering them with a suitable protection layer. The additional layer is required as otherwise washout effects or chemical reactions between e.g. sulphur-compounds and the particles reduce the enhancement effect. Alternatively, it is also possible to disperse the metal particles in the layer material before coating and apply them in one step with the coating. Suitable protection or matrix materials for SERS substrates could be e.g. sol-gel layers or polymer coatings. In either... 

LOPA is a semi-quantitative tool for analyzing and assessing risk. This method includes simplified methods to characterize the consequences and estimate the frequencies. Various layers of protection are added to a process, for example, to lower the frequency of the undesired consequences. The protection layers may include inherently safer concepts the basic process control system safety instrumented functions passive devices, such as dikes or blast walls active devices, such as relief valves and human intervention. This concept of layers of protection is illustrated in Figure 11-16. The combined effects of the protection layers and the consequences are then compared against some risk tolerance criteria. 

In LOPA the consequences and effects are approximated by categories, the frequencies are estimated, and the effectiveness of the protection layers is also approximated. The approximate values and categories are selected to provide conservative results. Thus the results of a LOPA should always be more conservative than those from a QRA. If the LOPA results are unsatisfactory or if there is any uncertainty in the results, then a full QRA may be justified. The results of both methods need to be used cautiously. However, the results of QRA and LOPA studies are especially satisfactory when comparing alternatives. 

The primary purpose of LOPA is to determine whether there are sufficient layers of protection against a specific accident scenario. As illustrated in Figure 11-16, many types of protective layers are possible. Figure 11-16 does not include all possible layers of protection. A scenario may require one or many layers of protection, depending on the process complexity and potential severity of an accident. Note that for a given scenario only one layer must work successfully for the consequence to be prevented. Because no layer is perfectly effective, however, sufficient layers must be added to the process to reduce the risk to an acceptable level. 

Layer-of-protection analysis (LOPA) A method, based on event tree analysis, of evaluating the effectiveness of independent protection layers in reducing the likelihood or severity of an undesired event. 

In general, risk reduction is accomplished by implementing one or more protective layers, which reduce the frequency and/or consequence of the hazard scenario. LOPA provides specific criteria and restrictions for the evaluation of protection layers, eliminating the subjectivity of qualitative methods at substantially less cost than fully quantitative techniques. LOPA is a rational, defensible methodology that allows a rapid, cost-effective means for identifying the protection layers that lower the frequency and/or the consequence of specific hazard scenarios. 

Newly developed alloys have improved properties in many aspects over traditional compositions for interconnect applications. The remaining issues that were discussed in the previous sections, however, require further materials modification and optimization for satisfactory durability and lifetime performance. One approach that has proven to be effective is surface modification of metallic interconnects by application of a protection layer to improve surface and electrical stability, to modify compatibility with adjacent components, and also to mitigate or prevent Cr volatility. It is particularly important on the cathode side due to the oxidizing environment and the susceptibility of SOFC cathodes to chromium poisoning. 

Figure 4.24 Dynasylan SIVO 121 forms an almost invisible protective layer (top). On untreated wood surfaces and surfaces that are weathered and varnished, Dynasylan SIVO 121 produces a strong hydrophobic and oleophobic effect. (Reproduced from Dynasylan.com)...

---