Materi 5 Basic Corrosion Measurement.ppt

1. 1
CORROSION
Section 2: Basic Corrosion Measurements
By
D.H. Lister & W.G. Cook
Department of Chemical Engineering
University of New Brunswick

2. 2
If a piece of metal:
corrodes to this:
How would we know how much corrosion had occurred?
Clearly, we need before and after measurements ………
but of what?

3. 3
Size?
Length, breadth and thickness before …
• What about after?
Volume before, volume after ….. by liquid displacement ….
• Sensitive enough?
Mass?
Weigh before, weigh after (if piece small enough for reasonable
balance … note that measuring weight losses to a fraction of
a mg in a few grams is the norm in corrosion testing).
To be of any use, a weight loss measurement has to be scaled to
the size of the specimen … corrosion is a surface related
phenomenon.

4. 4
Because corrosion is a surface phenomenon, weight loss is
related to surface area (SA)
For example:
a metal “coupon” having a total surface area of 10 cm2 loses 1
mg when exposed to a particular environment for a month;
which should be a characteristic amount for that metal in that
environment in a month.
2
cm
mg
10
1
corrosion
total 
month
cm
mg
0.1
rate 2


5. 5
• Corrosion engineers traditionally work in units of
dm2, thus:
• BUT … even though mdms give reasonable
numbers for most corrosion situations, the month
is a bad unit … Why?
• So … are often used …. mdd
mdm
10
month
dm
mg
10
dm
cm
100
month
cm
mg
0.1 2
2
2
2



day
dm
mg
2

6. 6
If we know the metal density, we can convert the weight
loss to “penetration”:
for ρ in g/cm3:
Which isn’t a very practical unit.
day
/
cm
10
x
cm
100
dm
mg
1000
g
1
g
cm
day
dm
mg
x 5
2
2
3
2









7. 7
• However, we note that 1 cm = 104 mm, thus:
• Another common unit for corrosion penetraion (in
the US) is the mil … or milli-inch per year
(=0.001 in/year).
yr
m
x
5
.
36
yr
day
365
day
m
1
.
0
x
day
cm
10
x 5 m



m





year
mil
4
.
25
z
yr
m
z 
m

8. 8
NOTE: these rates are averaged over the exposure time, and are
quoted as if corrosion were constant with time … it often is not …
measure rate 1 = measure rate 2 =
1
1
T
M
2
2
T
M

9. 9
Often, our bit of metal ..
after exposure, looks like this …
It is covered with scale.
How would we assess corrosion under these conditions?

10. 10
Scaled (e.g., oxidized) specimens often experience a
weight gain … thus, the high-temperature oxidation of
iron proceeds …
4Fe + 3O2 → 2Fe2O3
(456)g (456 + 616)g
If we know scale composition (e.g., type of oxide) and
weight gain, we can calculate the Fe in the oxide and,
therefore, the Fe corroded.

11. 11
BUT … we can rarely be sure that the scale
accounts exactly for the corrosion … some
might fall off (spall) … some might have
been deposited from the environment …
NOW WHAT?

12. 12
Consider the scaling process…
Weight before = Wo g
Surface area = A dm2
Exposure time = θ days
Find condition; weight after = W1 g

13. 13
We can only be sure that the remaining metal provides an
unequivocal reference, so …
Descale
Descaled weight = W2 g

14. 14
(divided by θ for rate … mdd)
We also have a measure of the weight of scale …
If we assume the scale is composed of Fe2O3, we can
calculate the iron in scale as …








 2
2
0
dm
mg
g
1
mg
1000
A
)
W
W
(
corrosion
Total
2
2
1
dm
g
A
)
W
W
(
scale


g
1
mg
1000
)
16
6
(
)
56
4
(
)
56
4
(
A
)
W
W
(
Fe
scale
in
Fe
of
fraction
A
)
W
W
(
Fe
2
1
scale
in
2
1
scale
in












15. 15
The difference between iron in scale and total
amount corroded is either …
• Iron released to environment … OR …
• Iron deposited from environment
2
2
1
scale
in
dm
mg
A
)
W
W
(
700
Fe



16. 16
How do we descale?
Chemically or electrochemically … usually in weak acids.
BUT … such descaling can also dissolve some of the remaining
metal, so … how do we account for this?
• employ an “inhibitor”
• use a blank …

17. 17
A blank is a “bare” specimen put into the descaling solution
along with the specimen;
during the descaling, if the blank corrodes by:
we apply this blank correction to the specimen measurement
DRAWBACKS? DISCUSS
EXAMPLE.







 2
b
b
dm
mg
A
W
1000
blank
of
loss
weight

18. 18
Note: such corrosion measurements and units are useful for general
or uniform corrosion but they don’t work for localized corrosion
(e.g. pitting or cracking). A component can fail by stress corrosion
cracking (SCC) with no detectable weight loss …

19. 19
• For these situations, we use concepts such as crack
propagation rate or pit propagation rate.
• For example:
• “percent through-wall” for tubes, pipes, vessels, etc.;
• “crack depth” or “pit depth” – usually in mil or mm;
• “propagation rate” – etc.
year
mm
,
year
m
m

20. 20
Examples of Corrosion
General corrosion ….
General corrosion of the reactor vessel head of the Davis-Besse PWR.
(note: this exterior general corrosion was indicative of a much more
serious local corrosion problem due to a leak in the pressure boundary)

21. 21
• Classic crevice corrosion.
In this case caused by a
carbon rubber exhaust
flapper clamped onto a
stainless exhaust flange
plate. Notice that the
worst corrosion is where
the clamp pressed the
rubber the tightest.

22. 22
Stress Corrosion …
Stress corrosion of stainless steel type 304 autoclave. (Mallinckrodt
Chemical Works)

23. 23
• How would you “measure” (estimate) the
corrosion of plant components?
• For example:
• pressure vessels or pressure tubes;
• feeder pipes
• steam generator tubes;
• feedwater pipes and heaters;
• etc.
• DISCUSS …

24. 24
• Methods involve …
• Non-Destructive Evaluation (NDE) or
• Non-Destructive Testing (NDT)
• and include …
• Ultrasonic Testing (UT);
• Eddy Current;
• Acoustic Emission;
• etc.

25. 25
Some useful conversion factors: