The Rusting of Steel A UNILEVER LABORATORY EXPERIMENT NUMBER 9 lA 540.78 UNII The Rusting of Steel If a drop of water containing salt is placed on an abraded steel surface in air, a corrosion reaction takes place which results in the pitting of the steel and the production of rust. The overall reaction may be represented by: 2 Fe +^02 + 3H 2 0 ———) 2 Fe (OH) 3 The reaction is electrochemical. The salt does not take part in it, but provides a path for the current. There is an anodic reaction leading to solution of the iron: 2Fe ———) 2Fei++4e and an initial cathodic reaction involving the absorption of oxygen: 0 2 +2H 20 +4e ——) 4 OH So the first product of reaction is given by: 2Fe2 + +4 OH - ———)2Fe (OH) 2 This is oxidized further to rust as follows: 2Fe2 + ———) 2Fe 3 + +2e 40 2 +H 2 0 +2e ———) 20H Hence, 2Fe +j0 or Fe3 + +30H +60H ——>2Fe (OH) 3 —->Fe (OH) 3 (rust) The anodic and cathodic areas on the surface may be distinguished by adding ferroxyl indicator (phenol phthalein plus potassium ferricyanide) to the water. Small pink and blue spots appear and persist until all the dissolved oxygen is used up. This is known as the primary distribution of anodes and cathodes. The formation of hydrate ions then ceases at the centre where oxygen replenishment is negligible, but is maintained at the periphery, which consequently turns pink. At this stage the ferrous ions, which are deep blue, concentrate in the centre of the drop. This is known as the secondary distribution. In time the ferrous ions react further with aerated water to form brown rust as a ring between the anodic and cathodic areas. If freshly boiled water has been used for the solution the secondary distribution will be seen from the outset. If oxygenated water is used the primary distribution persists for a long period. If oxygen is blown onto the centre of the drop the distribution is reversed and the centre becomes cathodic. Thus whilst oxygen is necessary for the corrosive attack to take place, it is those areas where there is least oxygen that become anodic, with dissolution of the metal. The setting up of an electrochemical current can be demonstrated if an electromagnet is available. This must be set with the poles vertical. The specimen carrying the drop is placed between the poles, and the drop is observed through a lens or low-powered microscope against a dark background, with side illumination. Usually the drop carries small particles of dust, and from the movement of these it will be evident that the drop rotates in the magnetic field when the current is switched on. Chemicals and Apparatus Corrosion/indicator solution: Add 0-5 ml of 1% alcoholic phenol phthalein and 3 ml of freshly prepared 1% potassium ferricyanide to 100 ml N/10 sodium chloride solution. Mild steel plate 75mm x 75mm x 5mm Procedure Take the steel plate and free it, if necessary, from rust. Then abrade the surface to be treated with coarse emery cloth, and place a drop of the corrosion/indicator solution on the abraded surface. Primary Anodic, corrosion Cathodic, no corrosion Secondary Anodic, pit develops Rust N23774 Unilever Unilever is an international company with head offices in London and Rotterdam. Unilever interests include: Washing products Toilet preparations Food Edible fats Oil milling and animal foods Chemicals Perfumes and flavours Plantations Corrosion is one of the most costly problems common to all kinds of industry and can affect everything from a large structural steelworks to a small piece of highly specialized continuous processing plant. Unilever's study of corrosion technology and its related fields arises from the company's interests in a wide range of liquid and bulk materials. Research is directed not only to corrective measures for tarnishing and erosion by these materials but to preventive measures which will not harm the many different kinds of products, particularly food products, processed by associated companies. Copyright Unilever Limited 1969 First Published by Information Division, Unilever, 1969 Reprinted 1970 Reprinted 1972 Reprinted 1974 Designed by Tony Stanford, MSIA Printed in England by W. J. Pollock & Co. Ltd. Unilever Laboratory Experiment Number 9 Copies of this leaflet may be obtained by science teachers from Unilever Education Section, Unilever House, P.O. Box 68, London EC4P 4BQ.
