Corrosion vapour

To prepare gas for evacuation it is necessary to separate the gas and liquid phases and extract or inhibit any components in the gas which are likely to cause pipeline corrosion or blockage. Components which can cause difficulties are water vapour (corrosion, hydrates), heavy hydrocarbons (2-phase flow or wax deposition in pipelines), and contaminants such as carbon dioxide (corrosion) and hydrogen sulphide (corrosion, toxicity). In the case of associated gas, if there is no gas market, gas may have to be flared or re-injected. If significant volumes of associated gas are available it may be worthwhile to extract natural gas liquids (NGLs) before flaring or reinjection. Gas may also have to be treated for gas lifting or for use as a fuel. 

Table 18.18 summarises the reported vapour corrosion effects of a variety of plastics references to the source of the information is given at appropriate places. 

Table 18.18 Summary of vapour corrosion effects of plastics...

Little comprehensive work has been carried out on contact corrosion, but some results on a range of polymers have been reported by Czech workers". In general, plastics that give rise to vapour corrosion (Table 18.18) will also cause contact corrosion. Some qualification is needed to this statement, however, as much depends on the type of contact and the other ingredients in the polymer, e.g. a paint may give good protection to the metal to which it is applied, but the vapour may cause corrosion of adjacent metal items within an enclosed space. 

Table 18.19 list woods whose aggressiveness by vapour corrosion has been quoted in a survey, together with typical pH values of the aqueous extracts of these woods reported in another investigation . 

Table 18.19 Relative corrosivity of woods by vapour corrosion...

While certain reservations must be kept in view (i.e. there is not necessarily a correlation between pH and corrosivity, and different samples of the same species of wood show a wide scatter of pH values, which might well be even wider if differences in duration of seasoning were taken into account), the results of vapour corrosion tests nevertheless indicate a general correlation between quoted pH values and the corrosiveness of wood vapours. It may reasonably be concluded that a strongly acid wood, pH less than 4-0, is potentially dangerous, and a less acid wood, pH more than 5 0, is likely to be relatively safe. 

Volatile acid hardeners such as hydrochloric acid and formaldehyde (which oxidises to formic acid) present in glues in plywood contribute to vapour corrosion, as can varnishes and paints. ... 

Vapour corrosion The best way to pack articles made of metals susceptible to vapour corrosion is in boxes made of metal or of those plastics which do not themselves emit corrosive vapours. If wood cannot be avoided, then the less corrosive kinds should be chosen. Dryness, good ventilation and the inclusion of water-vapour barriers should be sought. Other obvious measures are the avoidance of susceptible metals and the use of protective treatments and paints. 

This effect is so pronoimced that the presence of a single silicon-halogen bond in a molecule is sufficient to make its vapours corrosive... 

Cyg] Oxygen-water vapour corrosion test Corrosion of NdFeB magnets... 

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