5.6.3 The Determination
of Corrosion
5.6.3.1 Liquid phase corrosion
5.6.3.1.1 Test conditions
No standardised test procedures are known
for the determination of liquid phase corrosion, although
the experimental equipment required for the purpose is relatively
simple. Methods employed include:
- the storage of test samples in the corrosive media at
room temperature (over a test period of between 10 and 100
days),
- storage of the test samples in the reflux cooler at higher
temperatures (over a test period of between 5 and 50 days)
and
- storage in autoclaves at higher temperatures and pressure
(test period 1 to 5 days).
In the first two methods, preliminary conclusions can be
drawn after about a week by then subjecting the samples to
penetration tests.
5.6.3.1.2 Determining the Level of Corrosion
Penetration test
After the exposure period, the washed, dried sample is dipped
in a dye solution. The intensity of coloration is a measure
of the degree of corrosion. The penetration depth can be determined
on samples cut perpendicularly to the surface.
This method, however, fails if there are no wide intergranular
regions between the grains.
A negative penetration test alone does not permit clear conclusions
to be drawn over the corrosion resistance of a material. Further
investigations are necessary.
Dimensional changes
The determination of dimensional changes, e.g. thicknesses,
can be used to examine surface corrosion, but fails in respect
of inter-granular corrosion.
Mass loss  m
The determination of mass loss, ?m in g, in relation to the
area of the component, is the procedure most often applied.
Because of the corrosion Mechanisms at work , ?m values do
not permit quantitative conclusions to be drawn about the
corrosion behaviour of ceramic components.
Corrosion at grain boundaries is usually present in ceramic
materials. Deeper damage – as is visible, for instance,
in the penetration test – cannot be detected in this
way. The result of this is that in the case of, for instance,
Al2O3 > 99.5 %, S and HP materials, hardly any measurable
loss of mass is found. For this reason it is only extremely
rarely that calculations of rates of material removal or component
lifetimes for technical ceramics on this basis are satisfactory.
Loss of flexural strength  B
The determination of bending strength (in association with
Weibull statistics) permits a reliable conclusion to be drawn
about the effect of corrosion processes on the strength and
therefore on the service life of ceramic components.
Loss of hardness HV
The hardness of corroded samples drops significantly. However,
the measured values vary so widely that reliable conclusions
cannot be drawn from this measurement.
Resistance to wear
Abrasion tests on corroded samples permit conclusions about
the service life of components in corrosive media to be drawn.
Appropriate abrasion tests are used .
Examination with microscopes and scanning microscopes
Conclusions can be drawn from corroded samples regarding the
progress of corrosion processes over time and the corrosion
Mechanisms that occur.
|
|
