FACULTY OF CHEMICAL AND METALLURGICAL ENGINEERINGMETALLURGICAL CHEMISTRY (MCI201T)NATIONAL DIPLOMA: METALLURGICAL ENGINNERING (S3)

NAME & SURNAME: MOAKI S.MOAKISTUDENT NUMBER: 209324610ASSIGNMENT TITLE: CORROSION SUBMISSION DATE: 17th May 2014SUPERVISOR: DR.D.J.DELPORT

INTRODUCTIONWHAT IS CORROSION?Corrosion is defined as the chemical deterioration of a material as a result of a reaction with its environment.The material can be metal, ceramic, polymer or composite. The reaction can be chemical, physical, or electrochemical. As a result of corrosion the outcome is weight loss of a material but could also be weakening of a material strength without weight loss. Corrosion of metals happens in a water-base media and is normally electrochemical.There are various problems that corrosion can cause in any metallurgical operations and briefed as follows Maintenance and operating cost Plant shutdowns Contamination of products Loss of valuable products Product liability And effects of safety and reliabilityIn corrosion principles the cost and corrosion resistance of a material are usually the most important properties in the Engineering applications requiring high chemical resistance. Fabricability ease of forming and other mechanical properties are considered in the principles.Physical and chemical, electrochemical, thermodynamics and metallurgical are some of the factors that affect the corrosion resistance of any metal.There are factors that affect the choice of an engineering metal such as cost, fabricability, corrosion resistant, availability, strength and appearance.MECHANISM OF CORROSIONElectro-chemical reactions: water-based media which normally consist of loaded and neutral specimen which are cations, anions, and dissolved gases. When a metal is put into contact with an electrolyte a potential difference exists between the surface of the metal and the electrolyte. This reaction causes electrons to be removed from the metal and then ionised.The are some environmental effects that facilitate corrosion such as1. Effects of oxygen and oxidizers2. Effects of velocity3. Effects of temperature4. Effects of Corrosive concentration5. And of Galvanic coupling. THERE ARE 8 FORMS OF CORROSION BUT OF INTEREST PITTING CORROSION IS DISCUSSED.Pitting Corrosion: it is an example of a non-uniform corrosion resulting from inhomogeneity in metal due to inclusions, coring, and distorted zones. These inhomogeneities set up differences of potential at localised spots to cause isolated holes. The pitting ranges from average large to very micro holes, small holes grow progressively to form cavities into deep penetration of the surface.Pits are described in terms of diameter: depth ratio-for a wide pit the ratio > 1and for anarrow pit the ratio < 1. Pitting is very destructive and causes equipment to fail because of perforation.Pitting is an autocatalytic process-that is corrosion process within the pit produce conditions which are stimulating and necessary for the continuing activity of the pit. The sample below illustrates a steel bolt that was found at an automotive mechanics work-shop.

FIGURE.1

In most cases the metal can pitted by aerated sodium chloride solution. Some dissolution occurs in the pit, while oxygen reduction takes place on the adjacent surfaces. The process is self propagating and self stimulating. The rapid dissolution in the metal tends to produce excess of positive charge in the affected area, resulting in the migration of chloride ions to maintain electro neutrality. Both hydrogen and chloride ions stimulate the dissolution of most metals and alloys and in the process accelerates with time. Oxygen is virtually zero in concentrated solutions no oxygen reduction occurs in the pit. The cathodic oxygen reduction on the surfaces adjacent to the pit tends to suppress corrosion. Pits cathodically protect the rest of the metal surface.

PREVENTETIVE MEASURES OF PITTING Pitting can be stopped by adding molybdenum in the corroded area. Application of sol-gel coatings. Design vessels for complete drainage; avoid sharp corners and stagnant areas. Inspect equipment and remove deposits frequently. Weld instead of rolling in tubes in tube sheets. Use welded butt joint instead of riveted or bolted joints in new equipment. Coating with zinc.