AMME1362 Chap.13 Corrosion and Material Degradation
Corrosion and Material Degradation
Corrosion is a coupled electrochemical process with anodic oxidation, cathodic reduction, an electronic path and an ionic path. Material pairing, electrolyte chemistry, temperature, flow, surface films and area ratio determine where attack concentrates.
Prevention should interrupt the diagnosed mechanism through material choice, coatings, design, environment control or cathodic protection, then use inspection or monitoring to verify that degradation has slowed.
What this chapter covers
- 01
Anodic and cathodic reactions
- 02
Galvanic coupling
- 03
Area-ratio effects
- 04
Uniform and localised attack
- 05
Stress and erosion corrosion
- 06
Prevention and monitoring
Worked example · free
Compare anodic current density
- 1Current density equals total current divided by exposed anodic area.
- 1The ratio of current densities is 10 divided by 2, which equals 5.
- 1The 2 cm² anode has five times the current density and more intense local loss if other conditions match.
Key terms
- Anodic reaction
- Oxidation that releases electrons and consumes metal at an active site.
- Cathodic reaction
- Reduction that consumes electrons at a coupled site.
- Passivity
- Reduced reaction rate associated with a protective surface film.
- Pitting corrosion
- Highly localised attack producing cavities that can penetrate with limited total mass loss.
- Area ratio
- The exposed anodic area compared with coupled cathodic area. For a given current, concentrating oxidation onto a small anode raises current density and can intensify local penetration at coating defects or fasteners.
- Electrolyte path
- The ion-conducting route completing a corrosion cell between anodic and cathodic reactions. Moisture, dissolved species, deposits and crevice geometry determine whether this path exists and how transport varies across the surface.
- Cathodic protection
- Electrochemical control that supplies electrons so the protected structure behaves cathodically. Successful use requires adequate current distribution, compatible materials and monitoring at shielded or remote regions rather than assuming every exposed point is protected.
Corrosion and Material Degradation FAQ
Why can the cathode remain undamaged?
The cathode consumes electrons through reduction while metal oxidation occurs at the anode. It can therefore enable material loss elsewhere without itself being the location of dissolution.
Why is localised corrosion especially dangerous?
A small total mass loss can create a deep pit, perforate a pressure boundary or initiate a crack. Average thickness or mass change can hide the maximum local penetration.
How should a prevention method be chosen?
Map the anodic site, cathodic site, electron path and electrolyte, then interrupt a controllable link. Verify the result using a signal matched to the predicted mode, such as local depth, current or film stability.
Exam move
Draw every corrosion scenario as a four-link cell. Label the anodic oxidation site, cathodic reduction site, electronic path and ionic path. If one link cannot be located, state the missing information rather than naming a mechanism from appearance alone. Add the service environment, temperature, flow and any protective film around the diagram.
Practise half-reaction bookkeeping at a qualitative level by following metal atoms into ions and electrons through the conductor. The cathode consumes electrons and may remain intact, so do not point to the undamaged region and call it uninvolved. For galvanic cases, identify which material is active in the stated environment and avoid treating a series measured elsewhere as universal.
Work area-ratio examples with the same total current and different anodic areas. Calculate current density and explain why a small anode coupled to a broad cathode concentrates loss. Apply the reasoning to a coating scratch, fastener or repair patch. Separate total mass loss from maximum penetration.
Build a diagnosis table for uniform attack, pitting, crevice corrosion, intergranular attack, stress corrosion cracking and erosion-corrosion. Record location, necessary conditions, likely evidence and an alternative explanation. A cavity below a deposit may be a crevice rather than an isolated pit; a branched crack can have competing fatigue and environmental histories.
Use sectioning and composition evidence to distinguish them. Match controls to cell links. Material substitution changes electrochemical or film behaviour; a barrier coating interrupts electrolyte contact; drainage removes stagnant solution; inhibitors alter reaction kinetics; cathodic protection supplies electrons. For each method, write a credible failure mode.
Coatings fail at edges, inhibitors require concentration control and protective current may not reach shielded regions. Include maintenance changes because a new fastener or weld deposit can create a fresh couple. Create environment-control scenarios involving oxygen removal, temperature reduction, flow redesign and chemistry adjustment. Predict the intended rate change and one process consequence.
Excessive velocity can remove films even when stagnation is reduced, so direction alone is not enough. Finish with a monitoring plan tied to the proposed mode: maximum pit depth for local attack, thickness mapping for distributed loss, crack inspection for environmentally assisted fracture, or current and potential checks for protection systems. The final answer routine is location, cell, mode, intervention and verification.
End by stating system consequence, since a tiny perforation or crack can matter more than larger uniform mass loss.
