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-