E3: CORROSION

Cathodic Protection

corrosion prevention method that works by making the metal you want to protect the cathode.(Sacrificing another metal to protect yours)

• prevent your structure from losing electrons by making it the cathode.

Example of Cathodic Protection

1. Sacrificial Anode (Galvanic) Protection

   1. more active metal (anodic) is connected directly to the metal you want to protect

   2. the sacrificial metal corrodes (oxidizes) instead, saving the protected metal.

      1. Zinc to save steel

2. Impressed Current Cathodic Protection

   1. Uses DC power source to supply electrons to the protected metal

   2. an inert anode is placed nearby and current is applied

   3. the protected metal becomes a cathode and corrosion is stopped and slowed.

      1. Giving your metal a steady stream of protective electrons

Design changes to prevent or reduce corrosion:

• crevice control: avoid tight gaps where moisture and ions get trapped

• no metal mixes: avoid contact between dissimilar metals

• corrosion allowance: extra material thickness to account for expected material loss

• welding practice: use proper techniques to avoid residual stress and microstructural changes

• stagnant areas: eliminate zones with poor fluid flow that can lead to oxygen depleted conditions and crevice or pitting corrosion

• hot spots: prevent localized heating through uniform heating or cooling, improve heat transfer

What is Anodic Protection?

*force the metal into a passive state by making it an anode, where it forms a stable, protective oxide film

• apply a carefully controlled voltage to maintain the metal in its passive region

• works well for stainless steel, carbon steel in acid or chromium alloys

Only useful if the metal naturally forms a passive oxide layer

needs precise control (too much voltage → will corrode faster)

passivation: metals that form a very thin stable oxide layer on its surface that protects it from further corrosion.

What is Cathodic Protection?

*make the metal you’re protecting act as a cathode so it does not corrode

• supply electrons to the metal to keep it in a reduced state

• usually used for underground pipelines, tanks, ships, marine structures

overprotection leads to hydrogen evolution (hydrogen gas forms on a metal)

Environmental Control

• pH: maintain a neutral or controlled pH to minimize acidic corrosion reactions

• temperature: lower system temperatures to slow down corrosion reaction rates

• velocity: optimize fluid velocity to prevent stagnation without causing erosion or turbulence induced corrosion

• concentration: avoid high concentrations of corrosive species that accelerate degradation

• inhibitors: add chemicals that form protective films or neutralize corrosive agents in the environment

• cleaning: regularly remove deposits biofilms or corrosion products that can create localized corrosion zones

• aeration: control oxygen levels (either limit or maintain it uniformly)

Material Selection

• Coatings: apply a barrier layer to isolate the metal from corrosive environments

• Cladding: bond a corrosion resistant metal layer to a cheaper structural core for dual protection

• Heat treatment: modify the microstructure to relieve internal stresses and reduce susceptibility to corrosion

• diffusion treatments: introduce elements like nitrogen or chromium at the surface to enhance corrosion and wear resistance

• corrosion data: select materials based on experimental or published data showing performance in specific environments

• surface finish: use smooth, polished surfaces to reduce crevice formation and limit sites for corrosion initiation.

What is a sacrificial anode?

a more reactive metal that is intentionally attached to a less reactive metal so it corrodes instead.

• zinc

• magnesium

• aluminum

How does galvanizing work to protect against corrosion?

galvanizing: the process of coating steel or iron with a thin layer of zinc to protect it from corrosion

1. barrier protection: the zinc coating acts as a physical shield, keeping air, moisture and corrosive chemicals away from the steel surface.

   1. as long as the zinc layer stays intact, the steel doesn’t rust.

2. Sacrificial (Galvanic) Protection

   1. zinc is more reactive (anodic) than steel.

   2. if the zinc layer gets scratched or damaged, zinc will corrode instead of the steel

   3. the zinc gives up electrons to the exposed steel, keeping it in a protected cathodic state.

What is cladding and how does it help?

the process of bonding a thin layer of corrosion-resistant metal onto the surface of a less expensive or less resistant metal (usually steel) to combine strength with corrosion protection.

• not a coating - its permanently bonded metal layer, often applied by welding, rolling, or explosion bonding.

how:

• acts as a corrosion-resistant barrier (it protects underlying metal in highly aggressive conditions)

• cladding is thicker and stronger than typical coatings - more resistant to scratches, wear and chemical attack

• cost effective- gives corrosion resistance of expensive alloys without using them throughout.

What is the difference between this and coating?

cladding: corrosion resistant metal layer is permanently bonded to the surface

• much thicker and more durable

• mechanically stronger

• extremely harsh environments

coating: thin layer of material that is applied to the surface

• easier and cheaper

• used in milder environments