Danger diffusion

The characteristics of the pressure perturbation accompanying spontaneous combustion complete the description of the appearance of dangerous diffusion flames. 

In soils the constituents restrict diffusion so that in general rises to over 5 mm. The removal rate is mostly below 30 /xm a [11-13]. The danger of corrosion in soil is generally local corrosion through cell formation or by anodic influence (see Fig. 2-5) and can lead to removal rates of from a few tenths of a millimeter to several millimeters/year. 

The efficacy of polymers when used to protect metals from corrosive environments is influenced by their efficiency as barrier materials. When applied to metals by some techniques, such as fluidised bed coating, there is always the danger of macro-diffusion through pinholes which are gross imperfections in the surface and which do not have to be visible to be very much greater than the dimension of penetrating molecules. 

Two crystallographic forms of lead azide are important, the ordinary alpha form which is orthorhombic and the beta form which is monoclinic. The densities of these forms are 4-71 and 4-93 respectively. It was for many years believed that the beta form is the more sensitive to friction and impact and accounted for detonations which have occurred in the manufacture and handling of the substance. It is now known that the beta form is in fact no more sensitive than the alpha. Even the alpha form, when present as large crystals, is very sensitive and conditions can arise (particularly when the formation of the lead azide is controlled by diffusion effects) where spontaneous detonation occurs. Although with modern knowledge these hazards can be avoided, pure lead azide is nevertheless a dangerous compound and is now made only for military purposes. 

Depending on the diffusion of chemical danger the production sites have been graded as follows ... 

The last thirty years have seen a flowering of simulation techniques based on explicit treatments of solvent molecules (some references are given above). Such methods provide new insight into the reasons why continuum methods work or don t work. However they have not and never will replace continuum models. In fact, continuum models are sometimes so strikingly successful that hubris may be the most serious danger facing their practitioners. One of the goals of this present chapter will be to diffuse (but not entirely deflate ) any possible overconfidence. 

Hydrofluoric acid. An acid that has some very useful and specific applications, but is also very dangerous, is hydrofluoric acid, HF. This acid reacts with skin in a way that is not noticeable at first, but becomes quite serious if left in contact for a period of time, ft has been known to be especially serious if trapped against the skin or after diffusing under fingernails. Treatment of this is difficult and painful. Concentrated HF is about 50% HF (26 M). It is an excellent solvent for silica (Si02)-based materials such as sand, rocks, and glass. It can also be used for stainless steel alloys. Since it dissolves glass, it must be stored in plastic containers. This is also true for low pH solutions of fluoride salts. 

The usual experimental criterion for diffusion control involves an evaluation of the rate of reaction as a function of particle size. At a sufficiently small particle size, the measured rate of reaction will become independent of particle size. The reaction rate can then be safely assumed to be independent of intraparticle mass transfer effects. At the other extreme, if the observed rate is inversely proportional to particle size, the reaction is strongly influenced by intraparticle diffusion. For a reaction whose rate is inhibited by the presence of products, there is an attendant danger of misinterpreting experimental results obtained for different particle sizes when a differential reactor is used, because, under these conditions, the effectiveness factor is sensitive to changes in the partial pressure of product. 

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