Nitrates cracking

Pouring water over equipment is a change from normal operating practice. It should therefore be treated as a modification. For a description of nitrate cracking of mild steel, see Reference 4. 

In a crankcase-lubricated engine, the oil compartment acts as a sink for heavy molecular incomplete combustion products such as PAH, which can be present at up to 1000 times original concentrations. The lubricating oil is altered by nitration, cracking of polymers,... 

Usually the nitrator is tightly enclosed in the cart by a cement layer. This is necessary to isolate and to prevent the nitrator from any damage. Should the jar-nitrator crack because of too high a temperature, the cement layer prevents the reaction mixture from flowing off sometimes it can even be recovered. 

In nitrates, cracking of low-carbon steels occurs along preexisting active corrosion paths associated with ferrite grain boundaries (Ref 125). 

Each isomer has its individual set of physical and chemical properties however, these properties are similar (Table 6). The fundamental chemical reactions for pentanes are sulfonation to form sulfonic acids, chlorination to form chlorides, nitration to form nitropentanes, oxidation to form various compounds, and cracking to form free radicals. Many of these reactions are used to produce intermediates for the manufacture of industrial chemicals. Generally the reactivity increases from a primary to a secondary to a tertiary hydrogen (37). Other properties available but not Hsted are given in equations for heat capacity and viscosity (34), and saturated Hquid density (36). 

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

Unfortunately, the water contained nitrates, which caused stress corrosion cracking of the mild steel reactor. The reactor was removed for repair, and the temporary pipe that replaced it later failed and caused the explosion (see Section 2.4). 

Nitrate-induced cracking is well known to metallurgists but was not well known to other engineers at the time. Before you poured water over equipment—emergencies apart—would you ask what the water contained and what its effect would be on the equipment ... 

Certain environments containing nitrate, cyanide, carbonate, amines, ammonia or strong caustic, due to the risk of stress corrosion cracking. Temperature is an important factor in assessment of each cracking environment ... 

Sulphate in general appears to behave very similarly Hatch and Rice have shown that small concentrations in distilled water increase corrosion more than similar concentrations of chloride". In practice, high-sulphate waters may attack concrete, and the performance of some inhibitors appears to be adversely affected by the presence of sulphate. Sulphates have also a special role in bacterial corrosion under anaerobic conditions. Both sulphates and nitrates are acceptable in low-pressure boiler feed water as they are believed to be of value in controlling caustic cracking. 

The exceptions in Fig. 8.6 to only ductile failure occurring at potentials high enough to form Fe203 involve nitrates and high temperature water. In both of those systems cracks grow from pits, and within the pit-crack... 

Most of the early work carried out in relation to these aspects of the problem used nitrates as the cracking environment where low-strength steels have been the objects of interest. Consequently most of what follows refers to cracking in boiling concentrated nitrate solutions except where otherwise stated. The medium and higher strength steels, such as involved in sour oil well equipment and other applications, are more frequently tested in chloride- or sulphide-containing environments related to service conditions, but the failure of these steels is dealt with elsewhere (see Section 8.4). 

Fig. 8.14 Effect of carbon content of very low carbon steels quenched from 920°C on cracking in a calcium nitrate-ammonium nitrate solution (after Long and Uhlig")...

Fig. 8.21 Current density dilTerences between fast and slow sweep rate polarisation curves and stress corrosion cracking suspectiblity as a function of potential for a C-Mn steel in nitrate, hydroxide and carbonate-bicarbonate solutions...

For carbon steels, however, a full stress-relief heat treatment (580-620°C) has proved effective against stress-corrosion cracking by nitrates, caustic solutions, anhydrous ammonia, cyanides and carbonate solutions containing arsenite. For nitrates, even a low-temperature anneal at 350°C is effective, while for carbonate solution containing arsenite the stress-relief conditions have to be closely controlled for it to be effective . 

Low-carbon and chromium-nickel steels, certain copper, nickel and aluminium alloys (which are all widely used in marine and offshore engineering) are liable to exhibit stress-corrosion cracking whilst in service in specific environments, where combinations of perhaps relatively modest stress levels in material exposed to environments which are wet, damp or humid, and in the presence of certain gases or ions such as oxygen, chlorides, nitrates, hydroxides, chromates, nitrates, sulphides, sulphates, etc. 

An environment containing HjS, cyanides, nitrates or alkalis may produce stress-corrosion cracking in highly stressed structures and these should be first stress relieved by heating to 650°C. 

Preparation of 2-nitro-2-methyl-1 -propanol-nitrate. 50g of the 2-nitro-2-methyl-1-propanol purified as above were sifted into a mixt of 58.5g of 90.5% nitric acid.and 241,5g of 103.1% sulfuric acid (oleum) while the well-agitated mixt was maintained close to 15°. Stirring was continued for 1 hour and the mixt allowed to stand for hour. No separation of the nitration product occurred and the mixt was poured over cracked ice. An oily layer separated. The mixt was transferred to a separatory funnel, washed with water, Na bicarbonate, and twice with water to neutrality. The separated diluted spent acid together with the washings were thoroughly extracted with ether, the ether extract washed to neutrality and added to the main product. 

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