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Corrosion

This unit addresses aspects of the following syllabus outcomes:

H6.1 domonstrates skills in research and problem-solving related to engineering

H6.2 domonstrates skills in analysis, synthesis and experimentation related to engineering

Source: Board of Studies (1999) Stage 6 engineering studies syllabus preliminary and HSC courses. Sydney: Board of Studies

What is Corrosion?

You are out in the backyard and you are using a nice steel hammer. You become distracted and leave it outside overnight. When you find it the next day a thin brown film has formed on the surface which you know to be ‘rust’, a specific product of corrosion in iron and steel.The dew forming on the hammer overnight has allowed corrosion to occur quickly. At this stage the corrosion will not be catastrophic but you can see how quickly the process begins. Or, your old car is showing bubbling under the paintwork along the door sills, when you scratch away the paint, you find a corrosion cell full of rust, and a hole in your door sill. Corrosion is degrading your car.

Corrosion may be considered to be the degradation of a material by chemical means. Although all materials will degrade in some way, the term corrosion is usually applied to metals, hence metallic corrosion.When considering metals, corrosion is essentially the reverse of the refining process. When metals are extracted from the earth they are refined into pure metals from the various ores (metal oxides, hydroxides, carbonates, etc).As a result of corrosion, the metal reacts with chemicals in the environment and progressively returns to the combined form, rust is a form of iron oxide.

There are two main mechanisms of corrosion direct chemical attack which generally produces a uniform film over a surface, like the hammer in the example above.The other mechanism is electrochemical corrosion which generally operates in cells, like the car door example. Most serious corrosion results from an electrochemical process. It is chemical in nature and involves a transfer of electrons from one place to another. When corrosion occurs some part of a structure becomes the anode while another part becomes the cathode. The anode loses electrons while the cathode gains them. The result is that the anode will progressively be dissolved. If we place a piece of copper in a beaker of dilute sodium chloride (salt solution) connected to a piece of iron then the iron will be attacked and the copper will gain the electrons the iron loses.

Figure 1: Electrochemical cell

Figure1: An electrochemical cell of copper and iron.

In the example above the iron became the anode because iron is more reactive than copper. Whenever two metals in electrical contact are put together it will always be the more reactive metal that is dissolved. To assist the engineer a table called the Galvanic Series is prepared. This table allows the engineer to see which material is more reactive than another, in the presence of a specific electrolyte - in this case, seawater. It should be noted that there are variations in the rating of materials depending on the type of electrolyte involved. Another table the EMF series has slight variations in the positions of metals, but it does not take into account alloying elements on a metal.

Table1: The Galvanic series in Seawater

Anodic Magnesium
  Magnesium alloys
  Zinc
  Galvanised Steel
  Aluminium Alloys
  Mild Steel, Cast iron
  410 stainless Steel (active)
  50%Pb-50%Sn solder
  316 stainless steel (active)
  Lead
  Tin
  Cu-40%Zn Brass
  Nickel base alloys (active)
  Cu-35%Zn Brass
  Aluminium Bronze
  Copper
  Nickel base alloys (passive)
  Stainless Steels (passive)
  Silver
  Titanium
  Graphite
  Gold
Cathodic Platinum

For the engineer of civil structures, corrosion can be a huge problem. Some materials corrode in a destructive way; some corrode in a way that protects them from further corrosion. Coupled with this, the civil structure is usually exposed to weather conditions which promote the ingress of water and the sun which also degrades coatings that are designed to protect the metal.

Corrosive Enviroments

If we take a steel object, coat it in thick paint and put it in a dry and dark place it is unlikely to corrode. This is an unrealistic environment for most engineered objects particularly civil structures. Destructive corrosion can, in most instances, be minimised, but it requires careful planning and often continual maintenance to keep the systems that protect the material from corrosion effective. Some environments are more corrosive than others, for example a building situated near the sea will be exposed to salt spray from the ocean which will add to the likelihood of corrosion compared to a similar building in the far west of the state. Likewise the existence of rain acting on most civil structures means water can get into locations that are likely to promote corrosion.

Corrosion Systems

We are aware corrosion occurs, but how does it occur? What situation must the engineer avoid if we are to avoid catastrophic results due to corrosion?

Wet Corrosion

This form of corrosion occurs when a conductive liquid (electrolyte) is introduced into the environment. This can set up an electrochemical cell caused by differential concentration within the electrolyte which allows some form of corrosion.

Uniform Attack (Direct chemical attack)

Uniform attack occurs as a form of corrosion when a material is exposed to the environment. Usually the presence of air and moisture. The metal reacts with the environment to form a corrosion product all over the surface and not in localised cells. This is particularly prevalent in steels where the different phases act as the anode and cathode contributing to the corrosion reaction. Another good example is the uniform corrosion on copper roofing.

Figure 2: Uniform Corrosion

Copper dome on St. Thomas church Lewisham. Source: J.Gibson

Galvanic Corrosion (also called Composition Cells)

Occurs when dissimilar metals are placed in electrical contact with one another in the presence of an electrolyte. In a civil structure the electrolyte may come from the ground water, or from rain that is high in pollutants or from water with dissolved salts in it. Here the Galvanic Series becomes important.

If a stainless steel bolt is used with a mild steel plate then the stainless steel bolt will become cathodic, as it is less active than the mild steel.The mild steel plate will become anodic and start to corrode. Couple this with the presence of an electrolyte either due to rain or moisture in the air and corrosion of the mild steel plate will occur quite seriously.If this is not addressed by either a redesign or by some coating on the mild steel the result will be failure of the joint.If a copper pipe has a steel sleeve placed over it – what will be the result?

Corrosion can be demonstrated quite well using small samples of material in an electrolyte. The only disadvantage is that it takes a time to see some results.The results can be accelerated if you use saltwater instead of tap water.

To see a galvanic cell in action a piece of tinplate is ideal.Tinplate is mild steel sheet coated with tin to protect it from corrosion. When the tinplate is cut into a small strip it exposes the steel plate at the edges and the tin is in electrical contact with the steel. If we consult Table 1 we can see that mild steel is more anodic than tin, therefore the mild steel will become the anode and the tin the cathode. Place the tinplate in a glass or beaker with a mild salt water solution as the electrolyte.What happens? Record the results and explain what has happened.

Galvanic cells can also occur within the grains of multi phase alloys; this type of corrosion is called intergranular corrosion. Intergranular corrosion occurs when one phase of the metalstructure becomes anodic compared to an adjacent area. Multi phase alloys almost always have lower resistance to intergranular corrosion than single phase materials. Intergranular corrosion occurs in steels, because ferrite is anodic compared to cementite. Another example are zinc alloys where a second phase of impurities may have collected at the grain boundaries, making the grain boundaries anodic compared to the remainder of the grains.

Concentration Cells

These occur when there is a differential in the composition of an electrolyte. Thus corrosion occurs without the need for dissimilar metals. If there is a differential in the level of oxygen in an electrolyte the area of high oxygen concentration will become the cathode, while the area of low oxygen concentration becomes the anode. A classic example is when the bottom of a car door rusts, because the electrolyte that accumulates at the bottom of the door has varying oxygen levels within it, which leads to corrosion.

This is also a problem in any civil structure whenever water may accumulate in crevices or joints.Typically this crevice corrosion is caused by the varying composition of oxygen within the electrolyte.

Figure 3: Oxygen Concentration

Figure 2: A crevice showing the locations of the anode and cathode caused by differences in oxygen concentration.

Figure 4: Aircraft Panel

Serious crevice Corrosion in an Aircraft panel. Source: J. Gibson

Stress Cells

If a material or object becomes stressed then a corrosion cell may be set up. For example if a nail is bent and then placed in an electrolyte then the area that was bent will have higher levels of mechanical stress and become the anode, while the areas away from the bend with lower stress will become the cathode.This means that situations where corrosion should not occur may in fact become corrosion situations due to varying stress levels.

Get two unplated mild steel nails, bend one into a right angle, and leave the other as purchased. Place each one into a mild salt solution and leave them there for a few days.Observe the results.Take careful note if the unbent nail has corrosion near the head where some stress from forming may exist.

'Concrete Cancer'

Concrete Cancer is a rather emotive name for a process called spalling, where a reinforced concrete structure becomes compromised by the corrosion of the steel reinforcing within the concrete.The addition of steel reinforcement to concrete greatly adds to its ability to span greater distances and carry greater tensile loads.This is because the steel within the concrete can carry the tensile stresses much more effectively than the concrete.

When steel reinforcing is placed within concrete careful planning and design usually means that the steel does not contact the outside air nor does it corrode in-situ due to air pockets in the concrete.

Spalling occurs when steel reinforcement becomes exposed to the atmosphere, or is contacted by salt rich water percolating through the concrete mass. Once this occurs the steel will begin to corrode. As the corrosion occurs, the corrosion product, rust, forms on the steel. The corroded steel can occupy up to 7 times the volume of the non-corroded steel, this causes the concrete to be stressed in tension and results in the concrete breaking away from the steel.The corrosion weakens the steel, but also, as the concrete cracks it breaks the bond between the steel and the concrete, hence seriously reducing the load capacity of the reinforced concrete.As the corrosion process continues the concrete will break away from the structure exposing more steel to the corrosive environment and weakening the structure.

Three factors can cause spalling: poor design, poor manufacturing and weathering of the concrete. Poor design means that the steel reinforcing has been placed in the concrete such that it is exposed to the air already. Poor manufacturing can occur when a slab is poured the steel reinforcing is not properly placed within the slab, or it may be touching the ground underneath the slab. The slab may be porous, thus allowing water to percolate through it, dissolving salts, and reacting with the steel. Finally weathering occurs as the concrete is exposed to the atmosphere.

Spalling may also be made worse by the Alkali Silica reaction.

Once concrete is poured, it is the alkali silica reaction that develops the concrete strength. This reaction draws moisture out of the concrete to form the gel that binds the concrete together. Unfortunately, if the levels of alkali are too high then the reaction between the alkali and the acidic silica may cause the concrete to crack. Once this occurs the concrete will expose the steel reinforcing to the ingress of water and then more destructive deterioration can occur.

Figure 5: Concrete cancer

Concrete Cancer in Verandah Slab

One solution to spalling is to use a different material for reinforcing. Bamboo is one option, in some parts of the world it is a readily available and renewable resource and it has a favourable strength to weight ratio when compared to steel. There are, however, problems with the bamboo bonding to the concrete due to moisture absorption and bamboo shrinkage. Also due to its low modulus of elasticity the bamboo will deflect up to 50 % more than steel. Finally the bamboo will take up 10 times more volume than the steel, 5% of the concretes volume compared to 0.5% for the steel.

It is possible to use glass or polymer fibres to reinforce concrete but these are only used as a last resort or in applications where maximum tensile strength may not be needed.

Passivity

Some metals when they corrode actually form an oxide layer that protects them from further corrosion. Good examples of this are stainless steel, aluminium and titanium. These metals react with the oxygen in the atmosphere to form a non-porous oxide layer that protects the material from further corrosion. Additionally this oxide layer forms quickly and heals itself if scratched. In the case of passive metals corrosion is desirable as it protects the reactive metal from corroding any further.

Conversely iron and steel are examples of materials that form a porous oxide coating. The well known rust (iron oxide) that forms when iron or steel corrodes actually exposes the material to further corrosion and eventually eats away at the material. Stainless steel is a high alloy steel that has chromium and nickel added. The reason stainless steel is “stainless” is because the chromium in the alloy forms a passive layer over the surface of the stainless steel. If Austenitic stainless steel is welded it is possible to form chromium carbides which will mean areas of the alloy have reduced Chromium to form oxides, hence that area will not have a passive layer to protect the steel from corroding.

It should be noted that in some cases passive materials will not always be protected, for example if aluminium is put into a situation where it is the more anodic material it may readily corrode. Also if typically passive materials are put into a corrosive environment where their oxide layer cannot form they may rapidly degrade.

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Protecting Civil Structures

Since many civil structures are made using steel it is essential to find ways to protect them. It is impractical in terms of cost and engineering properties to use stainless steel in lieu of mild or high tensile steels. So we must investigate ways of protecting the steel structures.

Paint

One of the most obvious ways is to paint the civil structure. This is what was done to the Harbour Bridge; in fact over 3000 litres of paint are required to cover the bridge. Since 1932 the Sydney Harbour Bridge has repeatedly been painted and as such it has never suffered from serious corrosion. Another example, although not in the field of civil structures, are the millions of cars around the world. Nearly all cars use steel panels and these are protected from corrosion by paint. How well the steel is protected by primers and undercoats often decides how well the paint protects the car.

Metals Coatings

It is possible to coat metals that are susceptible to destructive corrosion with metals that will protect them from further corrosion. Probably the best known coated material is galvanised steel, which is steel coated with zinc. Zinc is a good choice for two reasons: it forms a passive layer that prevents further corrosion and it is anodic when compared to steel, so in the event of the underlying steel becoming exposed the zinc will still corrode to protect the steel.

Table 2 below outlines the way metal coatings may be applied.

Table 2: Methods of Metal Coating

Coating method Description
Hot dipping Where one metal is cleaned in acid then dipped into the coating material which is molten.
Electroplating This involves the use of an electric current to deposit and layer of protective metal onto a surface; metals often used for plating are gold, silver and chromium. 
Cladding Another way to apply a protective metal coating and it involves rolling the coating onto the base material, Alclad, which is aluminium coating over reactive duralumin, is a good example of this.
Spraying Here an electric arc melts a zinc electrode and is then blasted by air onto the surface top be coated.
Sherardising Zinc powder is deposited onto a heated object.  Used when hot dipping may clog fine details such as threads.

Sacrificial Anodes

This is a protection method where anodes that are placed onto a structure deliberately to make the structure cathodic.This method of protection is used extensively on ships where the steel hull needs to be protected from the corrosive sea water, particularly around the propeller as the bronze propeller is cathodic compared to the steel hull. Zinc blocks are mounted to the hull, they are anodic compared to the steel hull, and hence they will corrode in preference to the steel. This could also be done on exposed metal civil structures.

Impressed EMF (Electromotive Force)

This method of protection involves passing a DC current through the civil structure that tends to oppose the loss of electrons from the object that typically would be the anode. This makes the object cathodic resulting in little or no corrosion at all.

 Figure 6: EMF Source

Figure 3: An impressed EMF used to protect a submerged pipe from becoming anodic.

Good design

The last way to avoid corrosion is to design the structure such that concentration cells and composition cells are avoided. If mixed materials are used spacers and insulating washers of polymeric materials may be used.Alternatively designing a structure so that water can get away and not pool is also important. Finally to avoid corrosion, crevices should be avoided where possible. These should be filled so water cannot seep into them and cause corrosion that is out of sight.

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Activity 1

With reference to Table 1 answer the following question: a submerged brass pipe is to be soldered; will the solder or the brass become the anode? Explain why.

Answer

Activity 2

With the aid of a diagram explain why galvanised steel does not rust even if the zinc coating is scratched.

Answer

Activity 3

Discuss materials that could be used in lieu of carbon steel for use in concrete reinforcement, so that the concrete is less susceptible to concrete cancer?

Answer

Activity 4

Why does concrete cancer cause the concrete to spall (crack and break away)?

Answer

Activity 5

The steel pier shown below is immersed in the water to support a jetty. Identify where the cathode and the anode will be on the pier. Explain your choices.

Figure 7: Steel pier

Answer

Activity 6

An aluminium structure has been riveted unwittingly with austenitic stainless steel rivets. The structure is then used at a salt water pool; determine will happen to the structure in terms of corrosion.

Answer

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References

Askeland, D., (1989), The Science and Engineering of Materials, Chapman and Hall, London

Avner, S., (1983), Introduction to Physical Metallurgy 2nd Edition, McGraw Hill, Tokyo

Copeland, P. L. (2001). Engineering Studies: The Definitive Guide Volume 2. Anno Domini 2000. Helensburgh

Various, (2002). Engineering Studies HSC Course Stage 6 – Civil Structures. Open Training and Education Network.

Higgins, R., (1997), Materials for the Engineering Technician 3rd Edition, ArnoldLondon

http://www.inbar.int/publication/txt/INBAR_Technical_Report_No16.htm (external website)(23/10/05)

http://www.rainforestinfo.org.au/good_wood/nont_bld.htm (external website) (23/10/05)

http://www.bbc.co.uk/dna/h2g2/A4014172 (external website) (13/10/05)

http://www.anppainting.com.au/html/concrete.html (external website)

Corrosion Doctor (external website)

Sydney Harbour Bridge (external website)

Smith, W., (1990), Principles of Materials Science and Engineering,McGraw Hill New York

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