Corrosion

Introduction

  • Corrosion is defined as the chemical deterioration of a material over time

Dry Corrosion

  • Dry corrosion occurs when the metal reacts with oxygen in the air.
  • This causes an oxide layer to form over the metal, which damages its surface properties.
    • This oxide layer is usually known as rust
    • Since dry corrosion requires contact between air and the metal, as the oxide layer forms corrosion eventually stops as the layer prevents contact(see picture)
    • This stopping is known as passivation.
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  • In some cases, the oxide layer is porous, and thus corrosion can continue deep into the material.
    • This is known active corrosion
  • Dry corrosion is sensitive to temperature: it reacts much faster under an application of heat.
  • Most engineering materials undergo dry corrosion, though the rate varies on the material.

Wet Corrosion

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  • Wet corrosion is an electro-chemical phenomenon, which occurs in galvanic cells.
  • Its process is as follows:
    • When two metals are in contact with a liquid containing salts, an electric potential is formed between the two metals.
    • This causes the more reactive metal to lose electrons in a process known as oxidation.
    • When it does so, the more reactive metal then loses ions to the less reactive metal and corrodes.
  • This method can be used to protect against corrosion as well, in what is known as electroplating. (If Zinc is used, it is known as galvanising)
    • By purposely letting the more reactive metal corrode, and accelerating the process by applying a Voltage, the less reactive metal is "coated" with a layer of the less reactive metal.
    • This coating then corrodes before the less reactive metal as it is in contact with the environment now.
    • The coating often creates an electric charge while corroding, which also prevents the protected material from corroding
  • The use of sacrificial anodes, which are very reactive metal blocks to simply corrode instead of the parent metal is enough to protect it at times.

Stress Corrosion

  • This is a combination of fatigue and corrosion that result in a larger loss of strength than the sum of their parts.
  • This can occur if a constant load is applied in an corrosive environment such as seawater or acid.
  • Stress corrosion cracks(SCC) are branch-like cracks that form well under yield stress of the material.

Preventing corrosion

Coating

  • Some metal alloys (stainless steels for example) create a protective oxide layer instead of corroding when exposed to air.
    • In stainless steel's case, the layer provides a smooth and shiny surface.
    • Aluminium is another metal that does this.
  • However this coating can be scraped off by shear forces or cyclical loads.
  • This oxide layer can be encouraged through anodising, which is when the item is connected to an electrical source.
    • Anodised Aluminium is often used in windows and door frames
  • Some paints can prevent corrosion by providing a layer that would corrode slower than the parent metal.
    • As it's on the outside, it corrodes first.
    • Of course, this requires maintenance and re-applications.
      • Due to the nature of the paint, if it is scraped off the material will actually corrode faster than without it.
  • Phosphoric acid can also be used to remove rust and replace it with a corrosion resistant film that can be painted.

Metal Coatings

  • Similar to painting, this mainly involves dumping a layer of metal onto what you want to protect.
  • It can be done in many different ways:
  • Hot-dipping: Exactly what it sounds like. The metal to be coated is cleaned and then dipped into molten metal.
    • A very popular application of this is hot-dip galvanising, in which iron/steel/aluminium is dipped into molten Zinc.
      • It creates a layer of Zinc Carbonate, that is extremely corrosion resistant.
      • You can often see it as the checkered grey stuff.

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