evaluation of corrosion attack for metals basic and more advanced techniques
TRANSCRIPT
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-