Corrosion

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Corrosion

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The destructive result of a chemical reaction between metal or alloy and its environment.

Same energy needed to extract metal from it's ore is emitted by the chemical reaction during corrosion. It returns the metal to the minerals that they were extracted from

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Examples of corrosion protection

Back 2

paint(cars,equipment), sacrificial anode(boat, fence posts), passivation (anodizing), alloy

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4 parts of corrosion

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Anode - Oxidation (electron release)

Cathode - Reduction (electron distribution)

Electrical Connection - electron exchange

Electrolyte - Conductive medium (ion exchange)

**Remove any of the above to stop corrosion**

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Galvanic Series

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list of metals and alloys in order of reactiveness. metals less reactive and will become cathode if galvanically coupled with a metal more reactive. Sea water is the electrolyte used in the series. Most Reactive: Mg, Zn, Al

Least Reactive: Platinum, Ag, graphite

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Corrosion Penetration Rate (CPR)

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the thickness of material lost per unit of time given in mpy (mils per yr) or mm/yr.

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CPR equation

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CPR = (KW)/(pAt)

K- Const dependent on exposed area A.

534 if in in squared or 87.6 if in cm squared

W - weight lost after time t (milligrams)

t- time (hrs)

p - density (g/cm cubed)

A - exposed area (in or cm squared)

Front 7

Effects on Corrosion

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pH of the environment can cause metal to become active or passive. Size of cathode effect the rate of corrosion. stress can alter the corrosion potential (usually inc). Alloy composition and phase distribution can influence anode and cathode formation

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Passivity

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The formation of a highly adherent and very thin oxide film on some metal surfaces. Serves as a protective barrier from further corrosion.

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Uniform Attack

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Oxidation/Reduction occurs randomly across the surface. Variations in alloy composition or other defects can cause areas to become reactive and become the anode

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Uniform Attack (example and prevention)

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Example:

- rusting of steel or iron

- tarnishing of silverware

Prevention:

- protective coating

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Galvanic Corrosion

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Two different metals electrically connected in an electrolyte. The more active metal will be the anode and the less active the cathode

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Galvanic Corrosion (example and prevention)

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Example:
- Cu and steel plumbing
- sacrificial anode
Prevention:
- Remove electrical connection
- selection of metals close on galvanic series
- Large Anode with a small cathode

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Crevice Corrosion

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Solution in crevice is stagnant and oxygen depleted this causes concentration differences of dissolved gasses or ions and a change in the pH. Crevice becomes a small anode and surrounding area is a large cathode

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Crevice Corrosion (example and prevention)

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example:

- A washer between a bolt and the surface

Prevention:

- Non-absorbng gasket

- weld the pieces

- complete drainage

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Pitting Corrosion

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Will typically start on some localized defect on the surface. This form of corrosion happens from the top surface down with gravity causing the corrosion to be driven down. Its process is similar to crevice corrosion where the solution at the bottom of the pit is stagnant

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Pitting Corrosion (Example and Prevention)

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Example:

- aluminum soap dish

Prevention:

- Polishing the surface improves resistance

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Intergranular Corrosion

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Small precipitates can form at certain temps removing one alloying element causing the depleted areas to become anodic. Occurs along grain boundaries (areas of highest energy). It is also known as Weld decay as it happens in the heat affected zone (HAZ)

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Intergranular Corrosion (example and

prevention)

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Example:

- Weld decay in the heat affected decay of some welded stainless steels

Prevention:

- High temp heat treatment to redissolve

particles

- Lower Carbon concentration in stainless steel

- Alloys with metals that will preferentially form precipitate

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Selective Leaching

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In solid solution alloys when one element is corroded leaving a porous shell of the other element. Happens when the solute element is more active than the solvent element

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Selective Leaching (Example and Prevention)

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Example:

-Dezincification, Zn corrodes leaving pure Cu

-Graphitic corrosion where the iron corrodes out of the cast iron leaving graphite (buried cast iron pipe)

Prevention:

-select different alloy

- for graphitic corrrosion, graphite must be continuous

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Erosion Corrosion

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Combined action of corrosion and mechanical abrasion. Surface films are eroded away exposing underlying metal for further corrosion. All metals are susceptible to this type of corrosion especially metals with passivation layers

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Erosion Corrosion (Examples and Prevention)

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Example:

- A pipe elbow in your home plumbing

- Slurry delivery tubing in lapping and polishing tools

Prevention:

- Redesign to minimize fluid turbulence

- Remove particulates or bubbles from the fluid stream

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Stress Corrosion

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Happens only when both a stress and a corrosive environment are present. The stress can be residual stress or stresses due to thermal cycling. Small cracks will form and grow perpendicular to the applied stress. This usually ends up with a brittle type fracture

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Stress Corrosion (Example and Prevention)

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Example:

- Brass will become susceptible in an amonia

solution

- Stainless steels will become susceptible in a

solution with Cl ions

Prevention:

- Reduce applied stress

- Anneal part to remove residual stresses

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Hydrogen Embrittlement

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Atomic Hydrogen (H not H2) present

insterstitially in the metal. This reduces the ductility and tensile strength and causes brittle fracture due to rapid crack growth. Concentration as low as several parts per million can cause failure

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Hydrogen Embrittlement (Example and

Prevention)

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Example:

-Arc weld with damp (flux coated) weld parts

- Electroplating in presence of a hydrogen enviro

Prevention:

- Remove source of Hydrogen

- Baking at high temp to remove the Hydrogen

- Annealing to reduce tensile strength

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Scale Formation - Pilling - Bedworth ratio

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ratio of the volume of the metal to the volume of the oxide

= (AoPm)/(AmPo) A- area P- density

o - oxide m - metal

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Scale types

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Ratio output

< 1 volume smaller, porous, unprotective

1 - 2 volume similar, film with compressive stresses , protective

> 2 volume greater, large compressive stresses causing cracking and flaking of oxide film, unprotective

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Degradation of Ceramics

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ceramics are compounds of metals an nonmetals it is like they are already corroded. basically immune to corrosion in most environments. Can be dissolved by simple chemical dissolution. Ceramics are very wear resistant

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Degradation of Polymers

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Do not corrode but deteriorate with interaction with environment. Bond rupture due to heat, radiation or chemicals

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Bond rupture

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By radiation (UV, X-ray, e- beam) is caused by the radiation energy ionizing atoms causing them to loose a bond. O2 and O3 react with double bonds. Bonds can break due to thermal energy

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Swelling

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Degrades a polymer's properties by the liquid absorbing into the polymer structure, squeezing between molecules(chains) and inc space between molecules and weakening the secondary bonding. this also lowers the glass transition temperature (Tg)

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Dissolution

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is swelling taken to the point where the liquid absorption causes the chains to completely

separate