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Corrosion Essay

1149 words - 5 pages

1 INTRODUCTION Corrosion is a natural process, from which a modified material will either break down into a more basic state or be broken down (Gundiler et al, 2000). The process causes undesirable traits in the product which may range from a change in appearance to failure of structures. The effect of corrosion as addressed by Ahmad (2006) costs many billions to governments, as an outcomeindustry has developed many ways to combat corrosion. Combating corrosion can be done though two methods, by design and by inhibition. This report examines in sections 2 and 3 examples of corrosion control mechanisms from the standpoint of active and then passive responses. The issues of economic alloy, environment and corrosion control combinations are addressed in section 4. 2 EXAMPLES OF CORROSION CONTROL MECHANISMS In preparation for deployment to an environment, materials have to go through many stages of refinement. The considerations for corrosion protection and design can be illustrated inmany ways. Roberge (2005) emphasised that it must be understood by the designer that materials have a certain life expectancy, in particular he emphasised points on proactive design (see appendix 2). There exist many types of active corrosion management. Most applications for active corrosion management can be separated into two groups by their makeup, inorganic or organic. Organic inhibitors have a hydrocarbon base and are good for generating film coatings as opposed inorganic inhibitors are made from a large variety of chemicals. (NPL, 2003) Active corrosion prevention is considered a permanent protection. Applied before installation active corrosion management requires ongoing operation to protect the material and is specified to last the lifetime gauged by environmental factors. Basic active measures include 2.1 ANODIC PROTECTION A coating is applied which is anodic to the substrate, such as zinc aluminium or cadmium. On steel such coatings are generally called sacrificial coatings. They protect the substrate by acting as a sacrificial anode for the steel which is Cathodic to zinc. 2.2 CATHODIC PROTECTION An electrical system for prevention of rust, corrosion, and pitting of metal surfaces which are in contact with water or soil; this creates a concentration cell generating a DC current similar to ahalf-cell an example is shown in Figure 1. FIGURE 1 PIPELINE WITH ANODIC AND CATHODIC REACTIONS ( AHMAD, 2006) 2.3 CONVERSION COATING Conversion coatings as described in Engineering Fundamentals (2009) are coatings for metals where the metallic surface is converted by the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for corrosion protection, increased surface hardness, and as paint primers. Conversion coatings are usually built up on aluminium alloys, either by anodizing or chromate conversion. 2.4 BARRIER COATING To...

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