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Pitting corrosion and differential aeration

Pitting corrosion and differential aeration A report by Dr. Barry White - Principal Chemist of Re-Tech Imbach Total immersion of iron or steel in de-aerated water does indefinitely. On important surfaces such as decorative finishes, ball-bearings, ground surfaces, and die-stamps, not result in corrosion. this soon renders them beyond repair. The reaction: Protection by an efficient water-displacing oil within 48 hours of the first exposure can remove the air/water Fe + 2H O - Fe(OH) + H (gas) 2 2 2 combination from the surface, and prevent it from coming back to the metal, and in this way, rapid action does begin to occur, but the insoluble film of Fe (OH) , can result in almost all of the metal being capable of full ferrous hydroxide, produced in the reaction arrests the reinstatement. process before any noticeable effect has occurred. The cause of corrosion of steel in water is oxygen from the air It is difficult to generalise, since steels of different reacting with the Fe(OH) : composition, or of different heat-treatment histories, corrode at different rates. Temperature and humidity are also important. However, in general only surfaces with 4Fe(OH) + O - 4FeO.OH + 2H O 2 2 2 the most critical applications would be seriously damaged in 24 hours, and for most surfaces the item The FeO.OH produced here is rust, a loose powdery would remain reinstatable for at least a week or two. material affording no protection to the underlying metal. Some items could still be made functional after two or The rate at which corrosion occurs in the presence of three months, although the original smooth, bright water and oxygen depends on the quantity of oxygen appearance would have been thoroughly lost. Clearly available in a surprising way. The more oxygen is the sooner the appropriate remedial action is undertaken, present, the more positive becomes the electrical the higher the proportion of the equipment that can be potential of the metal. saved, and the higher the certainty of success. Correspondingly, at lower oxygen levels, the metal becomes relatively negatively charged. At the more Details: Imbach (UK) Ltd., Linley Lodge Industrial Est., positive sites, the metal becomes passive, covered with a Westgate, Aldridge, Walsall. WS9 8EX. Tel: 0922 54144. thin layer of adherent, coherent Fe(OH) or Fe2O (ferric 2 3 oxide) which prevents any further attack. This does not occur at the more negative sites, which therefore continue to corrode. This is self-propagating because the result of local corrosion at the points under the centres of water droplets is excavation of pits at these sites. Access of oxygen to the bottom of the pits is restricted by the rust in the pit, so that the bottoms of the pits continue to corrode preferentially. The increase of surface energy by roughening within the pits results in water film formation at lower humidity than would be possible on the flat surface. Thus pitting corrosion continues indefinitely, even in the absence of corrosive agents. Acid solutions corrode steel more vigorously than plain water by forming soluble ferrous salts rather than insoluble ferrous hydroxide. For example, in hydrochloric acid, soluble ferrous chloride, FeCI , is formed: Fe + 2HCI - FeCI + H (gas) 2 2 Paper and cardboard usually contain residual acid from manufacture, and for this reason corrosion can be made more severe by contact of the metal with cardboard packaging. Such packaging also maintains a supply of water at the surface of the metal. Corrosion of wet steel can be very rapid in the absence of protective oil or grease, and serious rusting can occur within the first 24 hours. Water has considerable ability to diffuse through plastic films, and even more can penetrate imperfections in plastic packaging. Thus corrosion of steel items such as bearings, inside sealed plastic bags, is frequently observed. The corrosion process is naturally more rapid while the original water remains present in liquid form on the metal surface, than it is after the water has evaporated. It is after the evaporation, however, that the pitting process really localises into deep excavations, and continues ANTI-CORROSIO N September 1990 13 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Anti-Corrosion Methods and Materials Emerald Publishing

Pitting corrosion and differential aeration

White, Barry
Anti-Corrosion Methods and Materials , Volume 37 (9): 1 – Sep 1, 1990
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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0003-5599
DOI
10.1108/eb007279
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Abstract

A report by Dr. Barry White - Principal Chemist of Re-Tech Imbach Total immersion of iron or steel in de-aerated water does indefinitely. On important surfaces such as decorative finishes, ball-bearings, ground surfaces, and die-stamps, not result in corrosion. this soon renders them beyond repair. The reaction: Protection by an efficient water-displacing oil within 48 hours of the first exposure can remove the air/water Fe + 2H O - Fe(OH) + H (gas) 2 2 2 combination from the surface, and prevent it from coming back to the metal, and in this way, rapid action does begin to occur, but the insoluble film of Fe (OH) , can result in almost all of the metal being capable of full ferrous hydroxide, produced in the reaction arrests the reinstatement. process before any noticeable effect has occurred. The cause of corrosion of steel in water is oxygen from the air It is difficult to generalise, since steels of different reacting with the Fe(OH) : composition, or of different heat-treatment histories, corrode at different rates. Temperature and humidity are also important. However, in general only surfaces with 4Fe(OH) + O - 4FeO.OH + 2H O 2 2 2 the most critical applications would be seriously damaged in 24 hours, and for most surfaces the item The FeO.OH produced here is rust, a loose powdery would remain reinstatable for at least a week or two. material affording no protection to the underlying metal. Some items could still be made functional after two or The rate at which corrosion occurs in the presence of three months, although the original smooth, bright water and oxygen depends on the quantity of oxygen appearance would have been thoroughly lost. Clearly available in a surprising way. The more oxygen is the sooner the appropriate remedial action is undertaken, present, the more positive becomes the electrical the higher the proportion of the equipment that can be potential of the metal. saved, and the higher the certainty of success. Correspondingly, at lower oxygen levels, the metal becomes relatively negatively charged. At the more Details: Imbach (UK) Ltd., Linley Lodge Industrial Est., positive sites, the metal becomes passive, covered with a Westgate, Aldridge, Walsall. WS9 8EX. Tel: 0922 54144. thin layer of adherent, coherent Fe(OH) or Fe2O (ferric 2 3 oxide) which prevents any further attack. This does not occur at the more negative sites, which therefore continue to corrode. This is self-propagating because the result of local corrosion at the points under the centres of water droplets is excavation of pits at these sites. Access of oxygen to the bottom of the pits is restricted by the rust in the pit, so that the bottoms of the pits continue to corrode preferentially. The increase of surface energy by roughening within the pits results in water film formation at lower humidity than would be possible on the flat surface. Thus pitting corrosion continues indefinitely, even in the absence of corrosive agents. Acid solutions corrode steel more vigorously than plain water by forming soluble ferrous salts rather than insoluble ferrous hydroxide. For example, in hydrochloric acid, soluble ferrous chloride, FeCI , is formed: Fe + 2HCI - FeCI + H (gas) 2 2 Paper and cardboard usually contain residual acid from manufacture, and for this reason corrosion can be made more severe by contact of the metal with cardboard packaging. Such packaging also maintains a supply of water at the surface of the metal. Corrosion of wet steel can be very rapid in the absence of protective oil or grease, and serious rusting can occur within the first 24 hours. Water has considerable ability to diffuse through plastic films, and even more can penetrate imperfections in plastic packaging. Thus corrosion of steel items such as bearings, inside sealed plastic bags, is frequently observed. The corrosion process is naturally more rapid while the original water remains present in liquid form on the metal surface, than it is after the water has evaporated. It is after the evaporation, however, that the pitting process really localises into deep excavations, and continues ANTI-CORROSIO N September 1990 13

Journal

Anti-Corrosion Methods and Materials – Emerald Publishing

Published: Sep 1, 1990

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