Corrosion II / Objectives 1.Define activation polarization and concentration polarization.

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Corrosion II / Objectives 1.Define activation polarization and concentration polarization.

Corrosion II / Objectives 1.Define activation polarization and concentration polarization. 2.Explain galvanic corrosion.

Corrosion II / Objectives 1.Define activation polarization and concentration polarization. 2.Explain galvanic corrosion. 3.Describe basic general rules for good corrosion preventing design.

Corrosion II / Objectives 1.Define activation polarization and concentration polarization. 2.Explain galvanic corrosion. 3.Describe basic general rules for good corrosion preventing design. 4.Calculate the cell potential for a redox pair.

Corrosion II: Protection Methods 1.Material Selection 2.Inhibitors - Remove oxidizing agents - Interfere with a step in activation

Protection Methods 3. Cathodic & Anodic Protection – supply electrons to structure to be protected a. External power supply

Protection Methods 3.Cathodic & Anodic Protection – supply electrons to structure to be protected b. Galvanic coupling (sacrificial anode)

Anode Consumption/Current Anode Current capacity (lb/A-year) Mg18 Zn25 Al-Sn16-20

Anode Consumption/Current Required Currents Structure MediaVelocityCurrent Pipeline Fresh water flowing 5-10 mA/ft 2 Piling Salt water tidal 6-8 mA/ft 2 Reinforcing rod Concretestatic mA/ft 2 In general: polarize to a potential of – 0.85 volts with respect to copper/copper sulfate reference electrode.

Protection Methods 4. Coatings A. Metallic – Often dual function cladding flame spraying electro-deposition hot-dipping vapor deposition

Protection Methods 4. Coatings B. Organic

Corrosion Protection 1.Weld rather than rivet

Corrosion Protection 1.Weld rather than rivet 2.Containers should be designed for easy draining

Corrosion Protection 1.Weld rather than rivet 2.Containers should be designed for easy draining 3.Design for easy replacement of expected failures

Corrosion Protection 1.Weld rather than rivet 2.Containers should be designed for easy draining 3.Design for easy replacement of expected failures 4.Avoid stresses in corrosion exposed components

Corrosion Protection 1.Weld rather than rivet 2.Containers should be designed for easy draining 3.Design for easy replacement of expected failures 4.Avoid stresses in corrosion exposed components 5.Avoid electrical contact between dissimilar materials

Corrosion Protection 1.Weld rather than rivet 2.Containers should be designed for easy draining 3.Design for easy replacement of expected failures 4.Avoid stresses in corrosion exposed components 5.Avoid electrical contact between dissimilar materials 6.Avoid heterogeneity (metals, vapor spaces, heat, stress)