Properties of Materials & Corrosion

1. Properties of Materials & Corrosion Eng. ShadiSawalha

2. Corrosion Definition • material or metal deterioration or surface damage in an aggressive environment • a chemical or electrochemical oxidation process, in which the metal transfers electrons to the environment and undergoes a valence change from zero to a positive value z. • M→ M+z + ze Chemical Engineering Department

3. Corrosion Environments • May be a liquid, gas or hybrid soil-liquid • Are called electrolytes since they have their own conductivity for electron transfer • They may be with positive charge or negative which called cations and anions recpectively Chemical Engineering Department

4. Electrochemical Reactions • Anodic Reaction: which is an oxidation reaction and occurs on the anode( electron loss), where anode has a negative pole • Cathodic reaction: which is a reduction reaction and occurs on the cathode ( electron gain), where cathode has a positive pole • M→ M+z + ze (anodic) • M+z+ ze → M (cathodic) Chemical Engineering Department

5. Electrochemical cell Chemical Engineering Department

6. Classification of Corrosion • General Corrosion: This is the case when the exposed metal/alloy surface area is entirely corroded in an environment such as a liquid electrolyte (chemical solution, liquid metal), gaseous electrolyte (air, etc.), or a hybrid electrolyte (solid and water, biological organisms, etc. Chemical Engineering Department

7. General Corrosion • Atmospheric Corrosion on steel tanks, steel containers, Zn parts, Al plates, etc.. • Galvanic Corrosion between dissimilar metal/alloys or microstructural phases (pearlitic steels, α−β copper alloys, α−β lead alloys). • High-Temperature Corrosion on carburized steels that forms a porous scale of several iron oxide phases. • Liquid-Metal Corrosion on stainless steel exposed to a sodium chloride environment. • Molten-Salt Corrosion on stainless steels due to molten fluorides LiF, BeF2 etc.). • Biological Corrosion on steel, Cu– alloys, Zn– alloys in seawater. • Stray-Current Corrosion on a pipeline near a railroad.

8. Atmospheric corrosion • This is a uniform and general attack, in which the entire metal surface area exposed to the corrosive environment is converted into its oxide form, provided that the metallic material has a uniform microstructure. • See the following examples: Chemical Engineering Department

9. Aqueous corrosion of iron in sulfuric acid Corrosion of Zn in dilute sulfuric acid solution Chemical Engineering Department

10. Atmospheric corrosion of a steel structure is also a common example of uniform corrosion, which is manifested as a brown-color corrosion layer on the exposed steel surface. This layer is a ferric hydroxide compound known as Rust. The formation of Brown Rust is as follows Chemical Engineering Department

11. In general, the oxidation process can be deduced using a proper Pourbaix diagram, as schematically shown in Figure below. This diagram is a plot of electric potential of a metal as a function of pH of water at 25°C Chemical Engineering Department

12. Prevention of Uniform Corrosion • material having a uniform microstructure • Coating or paint, • Inhibitor(s) for retarding or suppressing corrosion. These are classified as adsorption-type hydrogen-evolution poisons, scavengers, oxidizers, and vapor-phase, 4) cathodic protection, which is an electrochemical process for suppressing corrosion in large steel structures. Chemical Engineering Department

13. Chemical Engineering Department