Chapter 4: Corrosion testing Nima Safara 4-1 to 4-8Picture Reference: http://www.mistesting.co.uk/corrosion_testing.html
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.
Classification1. Laboratory tests.2. Pilot-plant or semi works tests.3. Plant or actual service test.4. Field tests.
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
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
3-Plant or actual service test• Involves in a particular service or a given plant
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.
Purposes:main justification for corrosion testing1. Evaluation and selection of materials for a specific environment or a given definite application2. Evaluation of new or old metals or alloys to determine the environments in which they are suitable3. Control of corrosion resistance of the material or corrosiveness of the environment4. Study of the mechanisms of corrosion or other research and development purposes
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.
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.
Small specimens• Small samples permit more accurate weighing and measuring of dimensions• Short time tests• Low corrosion rate
Large Samples• Large sample are desirable when studying pitting corrosion because of the probability factor involved
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.
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.
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.
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
Consideration on Exposure Techniques1. 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
Consideration on Exposure Techniques4. The specimen should be properly positionedif effects of complete immersion, partialimmersion, or vapor phase are being studied5. For plants tests, the specimens should be asreadily accessible as possible.
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.