Chapter 4: Corrosion testing
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1. Chapter 4: Corrosion testing
Nima Safara
4-1 to 4-8
Picture Reference: http://www.mistesting.co.uk/corrosion_testing.html
2. Introduction
• Many tests are made every year but the value
and reliability of the data obtained depend on
details involved.
• Most tests are made with a specific objective in
mind, some are educational and some are made
for practical proposes.
• well-planned and executed tests usually result in
reproducibility and reliability.
• Corrosion tests and application of the results are
considered to be a most important aspect of
corrosion engineering.
3. Classification
1. Laboratory tests.
2. Pilot-plant or semi works tests.
3. Plant or actual service test.
4. Field tests.
4. 1-Laboratory tests, including
acceptance or qualifying tests
• These tests are characterized by small
specimens and small volumes of solutions
• Best that can be done is the use of actual
plant solutions or environment
• It is not good idea to build a plant just based
on laboratory tests
5. 2-Pilot-plant or semi works tests
• These tests are usually the best and the most
desirable
• Tests are made in small-scale plant that
essentially duplicates the intended large-scale
operation
• Pilot plants are usually run long enough to ensure
good results
• One possible disadvantage is that conditions of
operation may be widely varied in attempting to
determine optimum operation
6. 3-Plant or actual service test
• Involves in a particular service or a given plant
7. 4-Field tests
• Involves field tests designed to obtain more
general information
• Example of field tests are atmospheric
exposure of a large number of specimens in
racks at one or more geographical locations
and similar tests in soils or seawater.
8. Purposes:
main justification for corrosion testing
1. Evaluation and selection of materials for a
specific environment or a given definite
application
2. Evaluation of new or old metals or alloys to
determine the environments in which they are
suitable
3. Control of corrosion resistance of the material or
corrosiveness of the environment
4. Study of the mechanisms of corrosion or other
research and development purposes
9. Materials and Specimens
• First step in corrosion testing concerns the
specimens themselves.
• If complete information on the materials is
not known, the data obtained may be
practically useless.
• If particular shapes are involved,
representative material should be tested.
10. Size and shape of specimen
• Flat samples are usually preferred because of
easier handling and surface preparation.
• Specimens 1/16 to ¼ inch thick, 1 inch wide, and
2 inch long are commonly employed in laboratory
tests.
• For wrought specimens, a large ratio of rolled
area is desirable.
• one reason of using thin specimens is,
experiment shows the cut edge might corrode
twice as fast as the rolled surface and accordingly
results in misleading answers.
11. Small specimens
• Small samples permit more accurate weighing
and measuring of dimensions
• Short time tests
• Low corrosion rate
12. Large Samples
• Large sample are desirable when studying
pitting corrosion because of the probability
factor involved
13. Surface preparation
• Surface of the test specimen should be
identical with the surface of the actual
equipment to be used in the plant.
• A common and widely used surface finish is
produced by polishing with No. 120 abrasive
cloth or paper or its approximate equivalent.
• This is not a smooth surface, but it is not
rough.
14. Surface preparation
• Excessive heating of the specimen should be
avoided, a good general rule is that the
specimen could at all time be held by naked
hand
• The edges must be machined, filed, or ground
to remove the severely cold-worked metal and
subsequently finished similarly to the
remainder of the specimen.
15. Surface preparation
• For soft metals like lead and lead alloys
rubbing with a hard eraser until obtaining a
bright surface is recommended.
• Scrubbing with pumice powder and other fine
abrasives is sometimes used on magnesium,
aluminum and their alloys.
16. Measuring and Weighing
• Since surface area enters in the formula for
calculation of the corrosion rate, the results
can be no more accurate than the accuracy of
measurement of surface area.
• After measuring, the specimen is degreased,
dried and weighted to nearest 0.1 mg
17. Consideration on Exposure Techniques
1. The corrosive should have easy access to the
specimen.
2. The support should not fail during the test.
3. Specimens should be insulated or isolated
electrically from contact with another metal
unless galvanic effects are intended
18. Consideration on Exposure Techniques
4. The specimen should be properly positioned
if effects of complete immersion, partial
immersion, or vapor phase are being studied
5. For plants tests, the specimens should be as
readily accessible as possible.
19. Corrosion tests: Boiling tests
• The specimen is held in a glass cradle to
permit circulation of the corrosion.
• The flask is an ordinary 1000 ml wide-mouth
Erlenmeyer.
• The condenser is called an acorn or finger type
condenser.
• An important consideration for boiling tests is
to be sure that sufficient heat is available to
cause boiling is all the flasks.
