Week 2 Corrosion

Question 1

What is corrosion?

Answer 1

The interaction of a material with its environment, in an engineering context leading to degradation and ultimately the failure of the structure or assembly

Question 2

How does the ore become useful?

Answer 2

carbon intensive process and large energy input. There is a large driving force in the opposite direction for the metal to become a metal. It is a battle against becoming a ore again once it has become a metal.

Question 3

What is pitting corrosion?

Answer 3

Small local area when the surface starts to corrode maybe because it was damaged.

Question 4

What can cause corrosion?

Answer 4

Sunlight or other things that the chemical reacts with.

Question 5

What is the Gibbs free energy change for the electrochemical cell reaction?

Answer 5

ΔGcell = -nFEcell
F = Faraday Constant (96,485 C mol-1)
n = number of electron involved
Ecell = equilibrium cell potential

Question 6

What is the equation for Ecell?

Answer 6

the more negative value becomes the anode value therfore the more positive is the cathos
Ecell=Ecathode-Eanode
This can then create two half cells at equilibrium with each other. This can then form cell notation in pg 4.

Question 7

How can you tell if the reaction is spontaneous?

Answer 7

If the Gcell(Gibbs free energy) is negative

Question 8

How can changes in conditions allow the cell potential to be calculated?

Answer 8

The nernst equation.
Standard cell potentials are at a concentration of 1M, 25°C and a pressure of 1 atmosphere.
If the pressure, temperature or concentration (or partial pressure of any gases) are at different values, the cell potential will change AKA non-standard conditions.

Question 9

What is the nernst equation?

Answer 9

ref pg7 in corrosion 3 pptx

Question 10

What is the reaction quotient(Q) dependent on? and how do you find A?

Answer 10

pH
through the nernst equation, find RT/F first.
It can be found through the fraction of the products and their concentrations over the reactants and their concentrations.

Question 11

What are the main types of corrosion?

Answer 11

• No corrosion
• General/Uniform attack/corrosion
• Intergranular corrosion
• Exfoliation or layer corrosion
• Selective corrosion
• Corrosion fatigue
• Stress corrosion cracking
• Pitting corrosion
• Crevice corrosion
• Bi-metallic corrosion
• Selective corrosion

Question 12

What is bimetallic corrosion also known as? and what is it?

Answer 12

galvanic corrosion
as can be guessed by the name it deals with 2+ metals and if there is a big enough difference between them then there will be a corrosion cell (the anode will corrode)

Question 13

For example, with rivets how would you recommend to design it for best corrosion resistance?

Answer 13

better to have a smaller cathode and large anode rather than the other way around

Question 14

What are the 4 main things corrosion needs?

Answer 14

anode, cathode, electrolyte and electrical connection

Question 15

What value should the potential be around to corrode?

Answer 15

to corrode it would need a small anode and large cathode potential difference >0.3. if the potential difference was <0.3 then no corrosion and if it was a small cathode to large anode then it will not sustain corrosion and it will be limited.

Question 16

what are the 7 main ways to prevent bimetallic corrosion?

Answer 16

1. Select metals close to each other in the electrochemical series.
2. A void the small anode to large cathode situation.
3. Insulate dissimilar metals where possible.
4. Apply coatings with caution.
5. Add corrosion inhibitors to the environments.
6. Design a longer service life into anodic components by making them more
   substantial or allowing for periodic replacement.
7. Install a third metal which is more anodic than both metals of interest.

Question 17

What is pitting corrosion?

Answer 17

Pitting is very localised corrosion which selectively attacks areas of a
metal surface.
In where the metal anode mixes with the hydroxide ions from the cathode and the chlorine ions from the salt cleaning mixture, mixes with the hydrogen atoms a metal hydroxide that will only go down into the material.

Question 18

how can pitting be initiated?

Answer 18

(a) a surface scratch or break in a protective film
(b) an emerging dislocation or slip step, caused by applied or residual
stresses
(c) a compositional heterogeneity such as an inclusion, segregation or
precipitate.

Question 19

What is crevice corrosion?

Answer 19

Corrosion which occurs because part of a metal surface is in a shielded or restricted environment, compared to the rest of the metal which is exposed to a larger volume of electrolyte.

Question 20

What is the mechanism of crevice corrosion and how do you know when it has happened?

Answer 20

only know when the corrosion products have presented themself at the edges but by that point there is already a large amount of damage that has been done.
the mechanism is the fontane-green mechanism

Question 21

how to prevent pitting corrosion?

Answer 21

consider the surface finish - polished surfaces are able to resist pitting corrosion longer.
can be alloyes so by adding Mo to stainless steel for exampled it can increase its resistance to corrosion.

Question 22

how to prevent crevice corrosion?

Answer 22

avoid stagnation of fluids in a system
remove the crevices from the design. use welding for example rather then mechanical fastenings.

Question 23

What is stress corrosion cracking?

Answer 23

The intergranular or transgranular cracking of a metal by the combined
action of a static tensile stress and a specific environment
* Stress corrosion cracking is a delayed failure process in which cracks initiate and propagate under the combined influence of applied stress and corrosive action
* Cracks may occur when localised regions of metal, that were previously protected by surface films such as oxides, become exposed to corrosive media
* The oxide formed can stop crack propagation but can also prevent the crack from healing even if the stress is relaxed

Question 24

What condition should the ideal oxide be in during processing?

Answer 24

 Thin external oxide layer
 No internal oxidation
 Low adherence

Question 25

What condition should the ideal oxide be in in-service?

Answer 25

 Slow-growing external oxide layer  No internal oxidation  Low tendency to spall

Question 26

What are the 4 main microstructures in iron(Steel)?

Answer 26

hematite Fe2O3 magnetite Fe3O4 wustite FeO substrate Fe

Question 27

What is the microstructure made up of?

Answer 27

elemental enrichement, delamination/spallation/blistering, magnetite seam at substrate surface, internal oxidation, precipitates, cracking

Question 28

what is concrete?

Answer 28

Concrete is a composite material made up cement, a fine aggregate (sand), acourse aggregate (gravel) and water.

Question 29

What is the main limitation of concrete and how is this overcome?

Answer 29

As concrete is effectively a ceramic composite, it is strong in compression but suspectable to tensile failure. To overcome this limitation, particularly in buildings, we use reinforcing steel bars as part of the concrete assembly. When concrete is assembled, the steel sits in an alkali environment (pH ~12.5) and does not therefore suffer with corrosion as it is relatively passive (has a protective magnetite layer on the surface).

Question 30

Does the concrete corrode and if so how?

Answer 30

Overtime however interacts with their environment. This will include CO2,salts (chlorides) from the atmosphere (sea water, de-icing salts, road salts), and other acids (rain). These start to penetrate into the concrete. Eventually these species will reach the steel * When these species reach the concrete an electrochemical cell is set up in the steel and localised anode and cathode sites are set up. * As we have seen before in crevice corrosion, the reaction releases hydroxide ions at the cathode and the reaction increases. * Rust (iron oxide) forms on the steel. As the volume of the rust is greater than that occupied by the steel, it causes internal pressure within the concrete which cracks and may fall away. This exposes the steel to the atmosphere. The detrimental species can then penetrate deeper into the concrete and start to corrode further into the structure.

Question 31

How can we protect / prevent corrosion in rebar concrete?

Answer 31

* Could add a sacrificial electrode (for example zinc) connecting them to the steel and allow the zinc to corrode sacrificially. These will disappear over time so they will need replacing over time. * Replacing carbon steel with stainless steel (more corrosion resistant). May stop corrosion issues but it increases the cost massively. * Replacing the steel with glass fibre but initial alkali starting condition is not good for these materials.

Question 32

What are the 4 main methods to control corrosion? and what are they?

Answer 32

1. Cathodic protection – connect the metal to another metal that is more anodic. 2. Barrier protection – apply a protective coating (such as an anodic oxide, inorganic or organic coating etc.) to act as a barrier between the metal and the corrosive environment. 3. Use corrosion inhibitors. Inhibitors are substances added to the corrosive environment that form a thin protective film, either by adsorbing into the surface of the metal to form very thin (monolayer) or thick (>100nm) films, or that react with the metal to form a coating. 4. Use a more corrosion resistant metal.

Question 33

Explain what cathodic protection is

Answer 33

the corrosion rate of a metal in contact with an electrolyte is proportional to the electrode potential, so by reducing the electrode potential to a lower value, the corrosion rate can be reduced. If a cathodic current is supplied to the surface using an external power supply with an auxiliary anode is located some distance away from the metal, this is called “Impressed Current”. The anode can be sacrificial – such as Zn, Mg or Al for steel, or inert such as graphite or platinised titanium. The cathodic current is supplied via a electrical connection to a less noble metal. The anodes can be welded, riveted or bolted to the structure.

Question 34

What are protective coatings?

Answer 34

Protective coatings are very common for corrosion protection for above ground structures. They act as a barrier to the aggressive environment, such as marine or industrial environments. They have additional functions such as increased aesthetics, friction reduction, increased visibility etc.

Question 35

What are the requirement properties of a good coating against corrosion?

Answer 35

* A good adhesion to the substrate. * Low levels of porosity or discontinuities in the coating. * Sufficient thickness. * Low rates of diffusion for corrosive species such as Clions, water etc.

Question 36

What are the main stages of applying coatings?

Answer 36

1. depends if it is a virgin material or an in-service material(more difficult to deal with in situ as old coatings need to be removed - can be expensive). 2. surface preparation - cleaning could be removing scale from hot rolled steel by acid pickling or mechanical removal. Paints may be degreased using alkaline degreasing/high pressure steam/ organic solvent. Then a pre treatment to improve adhesion and protect against coating corrosion underneath. Applying a primer can be expensive but can have better adhesion for the pain layer. The second layer is then applied which contains all the properties you want like from the pain. The final thin layer is for improved aesthetics/protection. The condition of the surface strongly affects the adhesion of the coating.

Question 37

Give some examples of metals and their coatings

Answer 37

Zinc – Galvanizing - the zinc is always going to galvanically corrode in preference to the steel (even if there's pore/pinhole or crack forms) as it has a more negative electrode so the substrate will remain protected from defects. Tin – Tin plate Nickel – plating AKA noble coating rely on being more corrosion resistant that the substrate, steel. If it does scratch though, the steel underneath will corrode as there is a small anode and large cathode. Coating must be free of pores/cracks to provide protection. Chromium - plating Conversion coatings – phosphating - increase the corrosion resistance relying on the metal reacting in a form of controlled corrosion to produce a protective film. Used as a pre-treatment on steel for paint improving adhesion. The mechanism involves the initial dissolution of Fe by the acid(in a bath) followed by precipitation a phosphate such as Zn3(PO4)2. Zn phosphates provide increased corrosion resistance, Fe phosphates provide an excellent surface for paint adhesion and Mn phosphates provide good wear resistance. Conversion coatings – chromating - Chromate coatings are produced by immersing the component in a solution containing chromic acid as a soluble chromate such as sodium dichromate for a short time. The solution will contain additions to de-passify the metal surface (such as chlorides) to allow the conversion reaction to proceed. Hexavalent chromium is actually restricted in the EU. Can have organic coatings.

Question 38

What are paints?

Answer 38

Paints are a filled organic coating applied as liquids, but which harden to form a protective coating. They are very commonly used to protect surfaces from corrosion as well as looking nice.

Question 39

What must a paint system be able to do ?

Answer 39

* To serve as a physical barrier to the corrosive environment. * To serve as a source of corrosion inhibitors and other substances that provide corrosion protection (such as Zn flakes to provide galvanic protection). * Have enough mechanical strength to maintain its structure and not degrade in chemical environments or due to ultraviolet radiation.

Question 40

What must a paint physically consist of?

Answer 40

* A vehicle or binder – the liquid portion in which the pigment and additives are dispersed. It also contains the solvent. It is a continuous polymer phase such as epoxies, urethanes, acrylics, vinyls etc. * A pigment suspended in the binder to provide a colour and protect the binder from degradation in UV radiation and/or Zn powder it flakes for cathodic protection * An organic solvent or water to disperse the pigments and impart physical characteristics such as an appropriate viscosity to allow the paint to be applied. Also contain special additives for UV absorption, fire retarding agents, abrasives for anti slip paints, biocides for anti fouling etc This would be presented in layers like a membrane like the metal -> oxide film-> conversion coating -> primer -> top coat

Question 41

What are some negatives of paints?

Answer 41

However, the binders used are poor conductors of electrons and ions, so establishing a corrosion cell with the environment is not easy. Additionally, paint systems contain corrosion inhibitors in the primer* coat such as zinc phosphates to inhibit anodic or cathodic reactions The primer coat is applied to give good adhesion to the subsequent paint layers as well as provide corrosion resistance.

Question 42

Nuts bolts and washers

Answer 42

Provide a good seal between the nut and the bolt, can be polymer or polymer coated. some are magnetic some aren't - non magnetic are steel/ stainless steel.