9. Atmospheric corrosion

The waterdroplet forms a cell in which the outer ring forms the cathode, by reducing the the oxygen that diffuses into the droplet from the air.

A difference in oxygen concentration

1. surface hydroxylation

   • interaction of water vapour and metal => formation of hydroxyl bonds on surface

2. water absorption

   • water is now adsorbed in a number of monolayer molecules, based on the relative humidity

1. the water film on the metal dissolves atmospheric gases

2. The formed ions (SO3, SO2, NO2) enhance the dissolution of the metal

3. The dissolved ions and metal ions pair and form corrosion products

The layer of corrosion product thickens towards a limit

• wetness

  • relative humidty

  • time of wetness

  • enviroment (rain, fog, sea breeze)

• atmospheric pollutant

• temperature

• sheltered/unshelterd

SO2, nitrogen oxides, Chlorides, phosphates, NH3, HCOOH, CO2, Ozone

• accelerates the rate of physical/chemical processes (e.g. diffusion)

• results in higher solubility of liquid film

They form either sulfuric or nitric acid, which strongly oxidizes the surface and decreases the pH

• increase localized corrosion through permeating the passive barrier

• forming soluble complexes with the metal, which increases uniform corrosion

1. Fe -> Fe2+ + 2e- , Fe2+ + 2OH- -> Fe(OH)2

2. wet period: Fe(OH)2 -> Fe(OH)3

3. dry period: FE(OH)3-> FeOOH +H2O, 2Fe(OH)-> Fe2O3 + 3H2O

source of crevice corrosion

• Corrosion of steel by FeS formation

• acidification of the enviroment

• Hydrogen induced cracking and sulfite stress cracking

Steel forms a stable FeOOH patina which protects the material from further rusting. Needs to be exposed to wet and dry cycle

Cr: surpresses growth of crystalline phase and forms the amorphous layer

Cu: reduces iron solubility, prevents magnetite formation, surpresses growth of crystalline phase

E Fe: -0,45V

E Zn: -0,76V

• Zn forms Zn5(CO3)2(OH)6 at air, which offers very good protection

• when scratched, Zink coating works as a sacrificial annode

(SO4)2- and Cl- can induce pitting, hence it is less protective

Very weak corrosion behaviour

Water enough to induce corrosion