Abstract
A n environmental test cell and associated equipment are described, in which direct visual and photomicroscopical information on the initiation of simulated atmospheric corrosion as it occurs in situ have been obtained.
The environmental flow rate and angle of presentation to the metal surface, temperature, relative humidity, and sulphur dioxide concentration are controlled and monitored at values typical of natural conditions. Special emphasis has been placed on corrosion initiation associated with deposited particulate matter related to atmospheric pollution. Inspection of the particulate matter and induced corrosion has been undertaken by SEM/EDAX and attempted by Auger spectroscopy. Examination using ammonium sulphate or ammonium chloride particles separately showed that, whilst the critical humidity for corrosion initiation on iron relates to the humidity over the saturated solution of the salt in each case, thereafter the humidity permitting continued corrosion is most probably affected by the equilibrium humidity over the appropriate soluble corrosion product. Other work demonstrated that sulphur dioxide promotes both the development of aerial corrosion protuberances in ammonium chloride initiated corrosion and the appearance of localised corrosion around sand particles. More generally, the amount of sulphur dioxide presented to iron specimens was found to be a more important corrosion parameter than its atmospheric concentration at relative humidities over 90%. Such considerations are relevant to the corrosion protection of exposed plant and buildings, where design aspects should take account of consequential air flow patterns (flow rate and direction), so minimising both turbulence and deposition of particulate matter.
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