Environmental effect

• 1. Atmosphere corrosion-

  -Corrosion in air (O2,H2O) with possible pollutants (SO2,H2S)

  • Corrosion nature: General corrosion( consumes a large quantity of materials
  • Different atmospheres: Dry air, damp/moist, wet airs
  • Rural
  • Urban
  • Industrial
  • Marine- high conc of cl- damage passive film- accelerate corrosion- change from dry/wet worse than just wet
  • geothermal area- S containing enviroment
• 1. Atmosphere corrosion

  Macro- weather, rainfall- near sea,more rainfall/ wind than locations

  micro- localized position city/industrial pollution

  Applications: city planning,infrastructure,plant location choice

  corrosion in Nz:

  -strong sea influence

  -Wind direction - west worse than east

  -Geothermal corrosion =bad

  Control of atmospheric corrosion:

  • material selection
  • Coating
  • SS for longterm outdoor
  • low alloys reduce the atm corrosion- cheaper than SS
• 2.Corrosion in Soils

  • sewerage/ pipelines/cables buried underground
  • soil contains mixture of organic and inorganic --> not uniform

  Influenced by: H2O,O2, salt, plants

  Corrosion mechanism:

  • anodic processes: depends on water content- wet soli= similar to solution - dry/air-filled= atm corr
  • Galvanic Corrosion:different O2 &H2O, E=different, corrosion current=diff

  Prevention:

  • similar to atm corr- mat selection,coating,electrochemical protection
  • avoid electrical systems nearby
  • regular check of pipelines
• 3. Sea water & fresh water corr

  high Cl-,high conductivity,highly corrosive

  Why seawater corr faster than freshwater

  • weak polarisation=fast
  • cl- penetrate passive films
  • high conductivity
  • pitting/crevice corrosion increase
  • galvanic corrosion increases
  • erosion corrosion increases
  • bio/microbiological corrosion- sea animal stick

  deeper in sea- low O2

  Bad between splash- high tide/low tide

  Protection:

  • mat selection
  • coatings- need to resist force and seawater
• 4. Microbiological Corrosion

  • Reproduction of bacteria
  • Temp increases
  • humidity increases
  • acid/ph decreases
  • O2 decreases
  • fouling also increases fuel

  Prevention:

  • Cleaness on surface- once biofilm formed- easy corrosion
  • Design structure so easy to clean
  • chemical to destroy biofilms may pollute seawater
  • Ship paints have huge market but face severe challenge
• 5. Corrosion in concrete

  • reinforced with steel- steel causes cracking
  • salts & chemical enter concrete
  • crack allow moisture &salt to reach metal

  Prevention

  • make steel deeper-water cannot penetrate
  • high grade concrete
  • inhibitors
  • cracks sealed if not severe
  • testing is a problem in early stage
  • cathodic protection
• 6. Corrosion in industrial

  chemical processing plant, petrochemical

  corrosive agents- caustics, acids

  conditions- high pressure, high temp

  Influencing factors:

  1. Oxidising acid (HNO3)- easy to form passive film
  2. Reducing acid (HCL) - no passivation- difficult to handle

  Caustic solutions--> weaker than acids BUT SCC may happen

  • oxide/hydroxides may form- protective
  • Driving force< in acidic solution

  High Temp, High Pressure H2O-->SCC main form

  • not pure H2O that's corrosive but the impurities such as Cl- - cause damage to high grade SS