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Evaluation of corrosion attackfor metals
Basic and more advanced techniques
Two principal ways to quantitatively assess corrosion attack gravimetrically
M LM C P
M etal M etal M etal
C orrosionproducts
B eforeexposure
After
exposure
Afterpickling
z·M C P
WI{
WI – weight increaseML – mass loss = corrosion attack = standard method
Quantitative techniques
• Mass Loss (ML)
– Normal balance, standard 10 x 15 cm samples (outdoor)
– Microbalance, small 1 x 5 cm samples (indoor)
– Resistance sensors
– Cathodic reduction (only copper and silver)
• Weight Increase (WI)
– Normal balance, standard 10 x 15 cm samples (outdoor)
– Microbalance, small 1 x 5 cm samples (indoor)
– Quartz Crystal Microbalance (QCM)
Resistance sensors
corrosion protected reference track
corroding track
reference track
Corrosion =
refi
si
s
refi R
R
R
Rt
,
,1=
Quartz crystal sensors
mN
ff
q
20
f= change in frequency, 0f=resonance frequency N = frequency constant (1670 kHz mm for a
transversal wave in AT-cut quartz)
q = density (2,648 g/cm3 for quartz) m= mass change/area, assuming film growth on one side of the crystal
On-line corrosion monitoring of indoor atmospheres
Electrical resistance sensors considered most promising, improvement necessary for corrosivity classification use
Quartz crystal microbalance Electrical resistance sensors
• used in practice • not as common under atmospheric conditions
• sufficiently sensitive for indicating changes• slightly vulnerable• mechanically vulnerable
• affected by temperature• unaffected by humidity• affected by humidity• unaffected by dust etc.• affected by dust etc.• large spread in results• signal disturbances
• metals subject to even corrosion only
Corrosion logger
Activities in short• Corrosion sensors
More advanced techniques
IR-microscope objective
In-situ experimental set-up
In-situ FTIR-microspectroscopy
Schematic drawing of the newly developed experimental set-up for in situ FT-IR microspectroscopy
Gas in Gas out
Rubber spacer
Metal sample
Teflon spacer
Metal holder
ZnSe window
NaCl particle
Infrared light
Schematic illustration of the model experiment
Single NaCl-particle
Cu
Humid air, 80%RH Electrolyte droplet
Cu
•A secondary spreading effect was observed outside the original water droplet, which was much larger at <5 ppm CO2 than at 350 ppm CO2.
RH: 802%; Exposure time: 6 hours.
350 ppm CO2 5 ppm CO2
Secondary spreading area Original water droplet
Original water droplet
•Hydroxyl ions(OH-) were formed in the secondary spreading area due to the cathodic reaction.
Ex situ FT-IR microspectra obtained at the secondary spreading area. (RH: 802%; <5ppm CO2;Exposure time: 6 hours.)
4000 3500 3000 2500 2000 1500 1000 500
- lo
g (
R/R
0 )
Wavenumber ( cm-1 )
0.012OH-
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