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Isolated scc and Colony scc

Cracks may form as isolated cracks or within colonies. An example of an isolated crack is shown in the photos. Cracking within colonies may result in the cracks on the periphery of the colony being deeper then the cracks at the center of the colony. This may be observed because the effect of cracking within a colony environment may act as a stress relieving mechanism causing reduced crack growth for cracks located in the center of the colony while the cracks on the periphery continue to grow.

  1. Read the information about “Applied coatings” and then the statements and circle t (True) or f (False) Applied coatings

Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the structural material. Platings usually fail only in small sections. In this case, it is necessary to plate with a more active metal such as zinc or cadmium.

The corrosion control system for an oil and gas pipeline consists of two parts: the external coating on the pipeline and the cathodic protection. The primary purpose of the coating is to protect the pipe surface from its external environment. Coatings are also increasingly used to protect the internal surface of pipelines. Over the years the pipeline has been protected by various polymeric coatings. In the 1950s and 1960s, coal tar or asphalt coatings were applied. In the mid-1950s, mill-applied extruded polyethylene coatings were introduced. From the early 1960s to the early 1980s, polyethylene tape coatings were field applied.

1. Painting is the only anti-corrosion treatment

T

F

2. Anti-corrosion elements are obstacle materials

between corroded surface and structural material

T

F

3. Platings are excellent in small sections.

T

F

4. The corrosion control system includes two parts

T

F

5. A coating does not protect the pipe surface.

T

F

6. Coatings protect both external and internal pipeline

surfaces.

T

F

7. Polyethylene coatings were first applied in the 70s.

T

F

15. Match the questions about “Cathodic protection” on the left with the answers on the right. Cathodic protection

If the environment is controlled corrosion inhibitors can often be added to it. These form an electrically insulating and/or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in the coating. Chemicals that inhibit corrosion include some of the salts in hard water, chromates, phosphates.

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks and offshore oil platforms.

The application of cathodic protection (CP) for steel gas pipelines started in the 1930s. This technology was rapidly adopted by the oil and gas pipeline industry to protect the pipeline in conjunction with polymeric coatings. For effective CP, the potential of the steel surface is polarized (pushed) more negative until the metal surface has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For galvanic CP systems, the anode material corrodes under the influence of the steel, and eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference in electrochemical potential between the anode and the cathode.

  1. When can corrosion inhibitors be applied?

  2. What do corrosion inhibitors form?

  3. What is the main effect of corrosion inhibitors?

  4. What chemicals can be inhibitors?

  5. What is CP?

  6. What does CP protect?

  7. When was CP application first used?

    1. salts in hard water, chromates.

    2. metal structure-steel pipelines, tanks.

    3. 1930s.

    4. less sensitive to scratches / defects.

    5. if environment is controlled.

    6. surface cathode is an electro-chemical cell.

    7. an electrically insulating / chemically impermeable coating.

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