Pyrophoric tendency

The pyrophoric tendency of zirconium in contact with red fuming nitric acid has already been mentioned. 

Note and emphasize any hazards, such as explosive or pyrophoric tendencies and toxicity, in a separate paragraph introduced by the heading Caution . Include precautionary handling procedures, special waste disposal procedures, and any other safety considerations in adequate detail so that workers repeating the experiments can take appropriate safety measures. Some ACS journals also indicate hazards as footnotes on their contents pages. 

Specimens of zirconium and its alloys can be covered with a layer of pyrophoric film when they are tested under certain specific conditions, e.g., a stagnant solution of 77.5 % sulfuric acid and 200 ppm ferric ion at 80 C for ten days [7]. Treating in hot air or steam can eliminate this pyrophoric tendency. This takes 20 to 30 min at 250 C. It should be noted that most corroded specimens do not have a pyrophoric layer. The formation of pyrophoric films is possible, but not guaranteed when both the following conditions exist ... 

All phosphoms oxides are obtained by direct oxidation of phosphoms, but only phosphoms(V) oxide is produced commercially. This is in part because of the stabiUty of phosphoms pentoxide and the tendency for the intermediate oxidation states to undergo disproportionation to mixtures. Besides the oxides mentioned above, other lower oxides of phosphoms can be formed but which are poorly understood. These are commonly termed lower oxides of phosphoms (LOOPs) and are mixtures of usually water-insoluble, yeUow-to-orange, and poorly characteri2ed polymers (58). LOOPs are often formed as a disproportionation by-product in a number of reactions, eg, in combustion of phosphoms with an inadequate air supply, in hydrolysis of a phosphoms trihahde with less than a stoichiometric amount of water, and in various reactions of phosphoms haUdes or phosphonic acid. LOOPs appear to have a backbone of phosphoms atoms having —OH, =0, and —H pendent groups and is often represented by an approximate formula, (P OH). LOOPs may either hydroly2e slowly, be pyrophoric, or pyroly2e rapidly and yield diphosphine-contaminated phosphine. LOOP can also decompose explosively in the presence of moisture and air near 150° C. 

A hazard may arise with a chemical because of its tendency to decompose spontaneously or to react violently on contact with other common chemicals, as illustrated in Figure 7.1. The case of pyrophoric chemicals is summarized in Chapter 6 some dangerous reactions of compressed gases are mentioned in Chapter 9 other cases are summarized here. 

Investigation into the effect has been mainly devoted to reactions with red fuming nitric acid . It seems that in red fuming nitric acid a preliminary reaction results in the formation of a surface deposit of finely divided metallic titanium ignition or pyrophoricity can then be initiated by any slight impact or friction. The tendency to pyrophoricity increases as the nitrogen dioxide content of the nitric acid rises from zero to maximum solubility at about 20%, but decreases as the water content rises, the effect being nearly completely stifled at about 2% water. 

The decomposition yields the metal and hydrocarbons. The TMA reaction has a tendency to leave carbon incorporated in the metal. Both TEA and TIBA have very low vapor pressure at room temperature and are consequently difficult to use. DMAH is generally the preferred precursor. Deposition temperature range is 200-300°C and pressure up to 1 atm (Note these alkyls are pyrophoric.)... 

Transmission electron microscope ( ) images of such n-Al powders indicate the presence of a thin passivation layer of aluminum oxide (A1203) which provides stability to it in the air. Without this layer, A1 nanoparticles would be pyrophoric and also have tendency to agglomerate to form bulk A1 metal. In order to protect this n-Al powder further, some researchers have suggested its coating with self-assembled nanolayers using perfluoroalkyl carboxylic acid [90]. 

Combustibility is also influenced by the amount of oil present the minimum reaction temperature (i.e., the minimum temperature required to cause onset of spontaneous combustion) is lowest in the region where the clay is saturated with 50-60% oil and rises (i.e., becomes less pyrophoric) as the oil content is further reduced to 40% and then to 30%. What these data suggest is that spent clay should be stripped of as much oil as possible before the resultant filter cakes are exposed to the atmosphere. The incorporation of ethoxyquin (a well-known antioxidant used to control spontaneous combustion in fish meal) into the clay/oil mass at the 100-ppm level did reduce the tendency to undergo spontaneous combustion however, whether this approach is practical and how it could be accomplished in plant operations is an open question. 

Crystal structure rophoric tendency (see p. 1614) a-Co (he spontaneous xagonal [49 latent ). . . /8-Co (cubic) not pyrophoric... 

Reactivity Characteristic of substances that have a tendency to undergo violent chemical change (examples are explosives, pyrophoric materials, water-reactive substances, or cyanide- or sulfide-bearing wastes)... 

H2SO4 + 2OO ppm Fe at 80°C consisted of y-hydride, Zr02, Zr(S04)2/ and fine metallic particles. The combination of hydride and fine metallic particles is suggested to be responsible for the pyrophoricity. Treating in hot air or steam can eliminate this tendency ... 

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