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Materials module 2 > Corrosion technology > Flashcards

Corrosion technology Flashcards

1
Q

What is corrosion?

A

The reaction of a material with its environment

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2
Q

What is rust?

A

ferrous metals and alloys

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3
Q

What is oxidation corrosion?

A

non ferrous metals and alloys

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4
Q

What is degradation?

A

polymers, composites & ceramics

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5
Q

What does corrosion involve? (2)

A

involves the movement of electrons; thus an electric current flow in the metal.

Oxygen and water must normally be present for corrosion of metal to occur (but there are exceptions).

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6
Q

What is pitting?

A

Corrosion of metal does not take place evenly over the surface; there are local areas of corrosion which give rise to pitting.

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7
Q

What are films? (3)

A

Films form on the surface of all metals, giving the metal a degree of corrosion resistance

– Materials with adherent (insoluble) films have greater corrosion resistance than materials with soluble films.

– Breakage of the film indicates further corrosion of the bare metal.

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8
Q

What is general corrosion (uniform attack)?

A
  • Corrosion that takes place over a general exposed area with equivalent intensity, resulting in uniform damage or scale deposit on the surface.
  • A typical example would be the rusting of railway tracks.
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9
Q

What is galvanic corrosion?

A
  • Corrosion associated with the current resulting from the coupling of dissimilar electrodes in an electrolyte.
  • When two dissimilar metals are in direct contact, electrically connected by a conductor or by the conductive medium, preferential attack on one, the anodic metal, occurs.
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10
Q

How can you prevent galvanic corrosion? (10)

A

• Avoid Galvanic couples, unless necessary
• Provide for a complete dielectric insulation of one from
the other
• Avoid unfavourable effect of a small anode and a large cathode (Figure above)
• Extend distance between dissimilar metals in conductive medium.
• Design for use of readily replaceable anodic parts.
• Avoid threaded joints between materials far apart in
galvanic series, use brazed fused or welded joints.
• Avoid embedment of dissimilar metals (metal couples) in a porous, moisture absorbing material.
• Specify effective coating of low porosity, especially on the anode.
• Provide for ventilation drying of coupled metals
• Exploit galvanic corrosion in its beneficial form for cathodic protection of critical parts or materials

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11
Q

What is thermogalvanic corrosion?

A

• Corrosion resulting from a galvaniccell caused by a thermal gradient

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12
Q

Why does thermoglavanic corrosion occur?

A

When a thermal gradient exists the metal is differentially polarised and anodic and cathodic zones are created, causing preferential attack to develop.

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13
Q

How can you avoid thermogalvanic corrosion? (3)

A
  • Avoid uneven heating, cooling and formation of hot spots
  • Provide for continuity of insulation or lining
  • Prevent by design, access of differentially heated or cooled liquids from exterior sources.
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14
Q

What is crevice corrosion? (4)

A
  • Crevice corrosion occurs as a consequence of concentration differences of ions or dissolved gases (O2) in the electrolyte soln and between two surfaces, one at least of which is a metal or two regions of the same metal (Figure above).
  • Corrosion occurs at the low conc.locale.

• Oxygen & metal concentration differences are
present in crevices and also in water lines, solids which hinder the diffusion of oxygen/metal ion and set up differences in solution concentration.

• The crevice must be wide enough for the solutions to penetrate, and narrow enough to stagnate the diffusion causing theconcentration difference.

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15
Q

How do you prevent crevice corrosion? (10)

A
  • Reduce crevices in areas of heat transfer or where aqueous environments contain oxygen
    – Avoid sharp corners and stagnant areas in design
    – Design for complete drainage and provide uniform
    environment.
- Prevent ingress of corrodent into crevices
– by improved fit
– by use of impervious joining materials
– by encapsulation
– by enveloping and by sealing.
  • Avoid crevice effect between insulation and substrate
  • Design for easy removal of scale or foreign material and the application of protective coatings.
  • Remove solids in suspension, provide filtration
  • Use welded butt joints instead of bolted or riveted joints
  • Use continuous welds; avoid crevices in lap joints by continuous welding, caulking or soldering
  • Specify sound welding techniques to ensure complete penetration so avoiding porosity and crevices.
  • Avoid fibrous or absorbent packing and gaskets
  • Inhibit the environment in crevices or stagnant areas.
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16
Q

What pitting corrosion?

A

• Very localised corrosion in which appreciable penetration into the metal occurs, resulting in the formation of cavities, similar to crevice corrosion.

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17
Q

How is pitting corrosion caused? (2)

A

Cause: When protective film or layers of corrosion product break down, localised corrosion (pitting) occurs. An anode forms where the film has broken and the unbroken film (or corrosion product) acts as a cathode.

• Tiny pits form starting points for stress concentration that can cause or accelerate stress‐corrosion or corrosion fatigue attack.

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18
Q

How can you prevent pitting corrosion? (5)

A

• Select suitable resistant material.

• Select suitable geometry to prevent aggressive
conditions.

  • Adjust thickness of material to allow for depth of pits.
  • Provide for control of media chemistry (do not rely solely on inhibitors unless attack can be thus completely stopped).
  • Specify protective coating and/or secure formation of protective film.
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19
Q

What is intergranular corrosion?

A

• Select suitable resistant material.
• Select suitable geometry to prevent aggressive
conditions.
• Adjust thickness of material to allow for depth of pits.
• Provide for control of media chemistry (do not rely solely on inhibitors unless attack can be thus completely stopped).
• Specify protective coating and/or secure formation of protective film.

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20
Q

What can happen in some austenitic stainless steels?

A

In some austenitic stainlesssteels, chromium carbides may precipitate at grain boundaries when cooling from welding temperatures. Corrosion attacks chrome‐starved areas.

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21
Q

How can you prevent intergranular corrosion? (3)

A
  • Select materials not subject to grain boundary depletion
  • Select suitable heat treatment
  • Avoid specifying heat treatment (500‐800 °C for SS) or welding in the susceptible range.
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22
Q

What is selective attack (leaching)?

A

The process of extraction of a soluble component from an alloy, by percolation of the alloy with water.

23
Q

How does selective attack (leaching) occur?

A

Cause: Basically, one element of a metal or alloy is singled out for corrosion attack. Common types are dezincification, dealuminification, and graphitic corrosion.

24
Q

What are some examples of selective attack (leaching)?

A

For example, when copper – zinc alloys (brasses) contain less than 85% copper and exposed to wet conditions for prolonged period, zinc may go into solution and the re‐ deposited copper has little strength.

Common cast iron can also act in this way in as much as, in some corrosives, the iron corrodes out leaving just a graphite deposit that virtually crumbles.

25
Q

How can you prevent selective attack (leaching)? (3)

A
  • Select resistant material.
  • Reduce aggressiveness of the environment
  • Use cathodic protection
26
Q

What is erosion-corrosion? (2)

A

Erosion‐corrosion arises from the combined action of chemical attack, and mechanical abrasion, or wear as a consequence of fluid motion

Corrosion reaction is accelerated by kinetic energy of impact of fluid or solid particles in suspension

27
Q

How can you prevent erosion? (10)

A
  • Decrease fluid stream velocity and improved lamellar flow.
  • Streamline the flow, avoid design that creates turbulence or flow restraint.
  • Minimise abrupt changes in flow direction
  • Align pipe sections.
  • Increase thickness of material in vulnerable areas
  • Design for easy renewal by interchangeable parts.
  • Provide filters for abrasive contaminants.
  • Introduce smooth aerodynamic and hydrodynamic flow and avoid rough surfacetextures.
  • Specify suitable materials, coatings or linings • Use cathodic protection (sacrificial anodes)
28
Q

What is cavitation damage? (2)

A
  • Cavitation corrosion is a form of erosion caused by the “implosion” of gas bubbles on a metal surface.
  • It is often associated with sudden variations in pressure related to the hydrodynamic parameters of the fluid (e.g. hydraulic turbine blades, propellors, stirrer blades,etc.).
29
Q

What does cavitation damage cause damage to?

A

• Causing damages to
– protective films (cavitationcorrosion)
– Severe deformation and fracture of surface (cavitation deformation)
– Fatigue of a metal surface(cavitation fatigue)

30
Q

When does cavitation damage occur?

A

• It occurs when a fluid’s operational pressure drops below it’s vapor pressure causing gas pockets and bubbles to form and collapse.

31
Q

How can you prevent cavitation damage? (10)

A
  • Avoid conditions that allow absolute pressure to fall below vapour pressureof liquid.
  • Minimize hydrodynamic pressure difference
  • Reduce vibration transfer
  • Select suitable geometry of surfaces to contain formation, amassment oradverse travel of gaseous bubbles in the liquid.
  • Prevent ingress of dispersed air, if not required for formation of protectivescale.
  • Select resistant material, hardenable by cold working of bubbles.
  • Specify smooth finishes
  • Specify resilient surface coating or lining
  • Use cathodic protection
  • Inject or generate larger air bubbles or gas bubbles to buffer cavitation process.
32
Q

What is high temperature corrosion?

A
  • Corrosion associated with the effect of atmospheric conditions, various gases, molten materials and salts at high temperatures.
  • It depends on the composition of the basic metals, composition of the environmental atmosphere, gases, salts or deposited metals.
33
Q

What is another factor of high temperature corrosion?

A

Exposure time is also a factor, light metals (those lighter that their oxides) for a non‐protective layer that gets thicker as time goes by. This layer, spalls and reforms.

34
Q

How can you prevent high temperature corrosion? (3)

A
  • Select stable materials
  • Where possible adjust temperatures and environment
  • Limit duration of adverse contact
35
Q

What is stress corrosion cracking?

A

Premature cracking of metals producedby the combined effects of a corrosive environment and surface tensile stress

36
Q

How is stress corrosion cracking caused?

A

Cause: Tensile stress increase (stress raisers) due to localised corrosive action, causing them to exceed the metal’s yield point. The metal will eventually fail.

• Non metals are also subject to similar phenomena.

37
Q

What are ways to prevent stress corrosion cracking? (12)

A

• Minimize applied or residual tensile stresses.
• Reduce stress concentration or redistribute
stress.
• Avoid misalignment of sections by riveting, bolting, and welding. Avoid lap welding, butt or fillet welding is preferred.
• Eliminate stress raisers
• Select suitable materials
• Stress relieve after machining, assembling and welding
• Use materials in assembly with similar expansion coefficients
• Induce compressive stresses by shot peening, swaging, rolling, tumbling, etc.
• Electroplate stressed areas
• Select suitable surface coatings
• Use controlled cathodic protection
• Inhibit the corrosive environment

38
Q

What is hydrogen damage?

A

• Hydrogen damage is defined as the reduction in ductility and tensile strength by the admission or inclusion of hydrogen into the metal.

39
Q

What causes hydrogen damage?

A

Cause: the penetration of atomic hydrogen into the metal. De‐carburation is caused by moist hydrogen at high temperatures.

40
Q

Where is the origin of hydrogen found in hydrogen damage?

A

The origin of hydrogen can be found in the cleaning, pickling, cathodic protection, welding, and heat treatment of metal.

41
Q

How can you prevent hydrogen damage? (10)

A

• Select a clean metal
• Select a hydrogen resistant material
• Select low hydrogen welding electrodes and specify welding in dry conditions
• Select correct surface preparation and treatment
• Avoid incorrect picking and plating procedures
• Metallise with resistant metal or use clad metal
• Induce compressive stresses
• Remove hydrogen by baking metal at 93 to 149°C
• Specify impervious protective coatings (rubber,
plastic)
• Avoid cathodic coatings

42
Q

What is fretting corrosion?

A

• Localised deterioration at the interfacebetween two contacting surfaces accelerated by relative motion of sufficient amplitude between them to produce slip.

43
Q

Where does fretting corrosion occur? (2)

A
  • Occurring between surfaces in close contact, usually under fairly heavy load and subject to very slight relative movement, or with different elastic strain between surfaces
  • Local attack may start fatigue cracks, especially where stresses concentrate and mating areasare pitted.
44
Q

how can you prevent fretting corrosion? (8)

A

• Avoid vibration transmitting design.
• Introduce barrier between metals that slip (allow
slip)
• Increase load (but do not overload) to stop motion
• Specify protective coating absorbing) material
of a porous (lubricant
• Isolate moving components from the stationary ones
• Increase abrasion resistance between treating one or both of them.
surfaces, by
• Design for exclusive of oxygen on bearing surfaces
• Select compatible materials

45
Q

What is corrosion fatigue?

A

Failure by cracking caused by alternating stresses in the presence of a corrosive environment.
This results in metal failure occurring substantially below the fatigue limit for non-corrosive conditions

46
Q

What are ways to prevent corrosion fatigue? (11)

A
  • Minimise or eliminate cyclic stressing
  • Increase the strength (reduce stress) of critical sections
  • Reduce stress concentration or redistribute stress
  • Provide for sufficient flexibility to reduce overstressing by thermal expansion and contraction, vibration, shock and ‘working’ of the structure
  • Provide against rapid changes of load, temperature or pressure.
  • Avoid vibration producing or vibration transmitting conditions. Design for reduction of resonance corrosion fatigue.
  • Improve ductility and impact strength
  • Specify stress relieve by heat treatment or by shot peering, swaging, rolling, vapour blasting, tumbling, etc to induce compressive stresses.
  • Specify suitable surface finish for elimination of stress raisers, fretting, scoring and hence corrosion
  • Specify electro‐deposit of suitable coatings (Zn, Cr, Ni, Cu etc) by plating techniques that do not induce or produce tensile stresses
  • Inhibit corrosive environment
47
Q

What is microbial corrosion?

A

• Deterioration of materials caused directlyor indirectly by bacteria, moulds of fungi or in combination.

48
Q

How can microbial corrosion be caused by? (4)

A

– Chemical attack of metals, concrete andother materials, by the products of microbes, namely acids (sulphuric, carbonic) hydrogen sulphide or ammonia.
– Microbial attack of organic materials (organic paint coatings, plastic fittings and linings).
– Depassivation of metal surfaces and induction of corrosion cells.
– Attack of metal by a process in which microbes and the metal co‐operate to sustain corrosion reaction.

49
Q

What are ways to prevent microbial corrosion? (7)

A
  • Analyse accurately the probabilities of contamination
  • Provide for control of media chemistry
  • Inhibit or provide for addition of germicide
  • Construct non‐aggressive surround or secure controlled removal of microbialnutrients.
  • Select suitable protective coating and/or resistant material
  • Use cathodic protection
  • Provide accessibility for frequent cleaning.
50
Q

What is stray current corrosion?

A

Corrosion resulting from D.C flow through paths other than the intended circuit.

51
Q

What is the cause of stray current corrosion?

A

Cause: Bad earth return on electrical equipment, giving rise to leakage current through metal structures and other preferentially conductive paths

52
Q

How can stray current corrosion be prevented? (10)

A
  • All exposed parts or panels of conductive materials should be at ground potential at all times.
  • Insulate cables, components and equipment from frames or structures
  • Provide controlled grounding of all electrical equipment
  • Use non conducting fluids
  • Drain off stray currents with another
  • Apply cathodic protection
  • Embed structures or parts in inert, non‐ conducting medium to prevent access of stray currents
  • Secure electrical continuity of critical conductors (example pipelines)
  • Bond metallic structures to an earthing point.
  • Use expendable targets connected to the anodic sides of insulating joints.
53
Q

How do ceramics and glasses degrade?

A

Most ceramics are already in the oxidised or reacted condition, so are essentially inert to the environment. They are used as corrosion prevention coatings.

54
Q

How are polymers and composites degrade? (5)

A

• Polymers and polymer‐based composites are more susceptible to environmental degradation due by physiochemical mechanisms.

– Physical degradation: abrasive, absorption of gases & liquids (water and oil), and swelling

– Thermal degradation (oxidation)

– Radiation degradation , e.g. UV, can cause molecular bonds in the material to break due to higher excitation levels of orbital electrons.

– Chemical degradation, acids, bases … – Biodegradation

Materials module 2 (10 decks)

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