3.2.3 Corrosion Environment

Corrosion may appear in any of three forms: atmospheric corrosion, galvanic corrosion or stress corrosion cracking. Atmospheric corrosion describes corrosion that is caused by the chemical action of compounds in the environment on the surface of the material. The most common form is water and water soluble pollutants, or substances encountered in processing equipment. Corrosion is controlled by material selection and surface treatment.

Alloys of magnesium and aluminum are more anodic than most other commonly used metals, and give up metal to less active alloys, such as those in the iron and copper groups. Galvanic corrosion is commonly controlled by three means, either alone or in combination:

  • Selecting compatible materials
  • Excluding electrolyte from the contact surfaces
  • Separating the contact surfaces with a material that is either a non-conductor or is compatible with both.

Stress corrosion cracking occurs in some alloys when they are under continuous load for long periods of time in the presence of a specific chemical substance. The stress levels may be well below the yield point. It can be controlled either by excluding the corrosive environment (protective coatings), by designing the affected area of the forging to acceptable stress levels, or by utilizing stress corrosion resistant thermal or mechanical treatments. (For example, T653 or T23 for aluminum alloys.)

The problem is generally associated with metals that are susceptible to corrosion. For example, magnesium alloys containing more than 1.5% aluminum, in normal atmospheres, are subject to stress corrosion cracking at stress levels of 48 MPa (7 ksi) and higher. Stress corrosion cracking may also occur in corrosion resistant alloys, such as brasses with more than 15% zinc, in the presence of ammonia. Certain aluminum alloys, when aged to peak strength (T6) are susceptible to stress corrosion cracking. Stress corrosion cracking can often be minimized by selecting an alternate material.

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Corrosion may appear in any of three forms: atmospheric corrosion, galvanic corrosion or stress corrosion cracking. Atmospheric corrosion describes corrosion that is caused by the chemical action of compounds in the environment on the surface of the material. The most common form is water and water soluble pollutants, or substances encountered in processing equipment. Corrosion is controlled by material selection and surface treatment.

Alloys of magnesium and aluminum are more anodic than most other commonly used metals, and give up metal to less active alloys, such as those in the iron and copper groups. Galvanic corrosion is commonly controlled by three means, either alone or in combination:

  • Selecting compatible materials
  • Excluding electrolyte from the contact surfaces
  • Separating the contact surfaces with a material that is either a non-conductor or is compatible with both.

Stress corrosion cracking occurs in some alloys when they are under continuous load for long periods of time in the presence of a specific chemical substance. The stress levels may be well below the yield point. It can be controlled either by excluding the corrosive environment (protective coatings), by designing the affected area of the forging to acceptable stress levels, or by utilizing stress corrosion resistant thermal or mechanical treatments. (For example, T653 or T23 for aluminum alloys.)

The problem is generally associated with metals that are susceptible to corrosion. For example, magnesium alloys containing more than 1.5% aluminum, in normal atmospheres, are subject to stress corrosion cracking at stress levels of 48 MPa (7 ksi) and higher. Stress corrosion cracking may also occur in corrosion resistant alloys, such as brasses with more than 15% zinc, in the presence of ammonia. Certain aluminum alloys, when aged to peak strength (T6) are susceptible to stress corrosion cracking. Stress corrosion cracking can often be minimized by selecting an alternate material.

Return to Table of Contents

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Corrosion may appear in any of three forms: atmospheric corrosion, galvanic corrosion or stress corrosion cracking. Atmospheric corrosion describes corrosion that is caused by the chemical action of compounds in the environment on the surface of the material. The most common form is water and water soluble pollutants, or substances encountered in processing equipment. Corrosion is controlled by material selection and surface treatment.

Alloys of magnesium and aluminum are more anodic than most other commonly used metals, and give up metal to less active alloys, such as those in the iron and copper groups. Galvanic corrosion is commonly controlled by three means, either alone or in combination:

  • Selecting compatible materials
  • Excluding electrolyte from the contact surfaces
  • Separating the contact surfaces with a material that is either a non-conductor or is compatible with both.

Stress corrosion cracking occurs in some alloys when they are under continuous load for long periods of time in the presence of a specific chemical substance. The stress levels may be well below the yield point. It can be controlled either by excluding the corrosive environment (protective coatings), by designing the affected area of the forging to acceptable stress levels, or by utilizing stress corrosion resistant thermal or mechanical treatments. (For example, T653 or T23 for aluminum alloys.)

The problem is generally associated with metals that are susceptible to corrosion. For example, magnesium alloys containing more than 1.5% aluminum, in normal atmospheres, are subject to stress corrosion cracking at stress levels of 48 MPa (7 ksi) and higher. Stress corrosion cracking may also occur in corrosion resistant alloys, such as brasses with more than 15% zinc, in the presence of ammonia. Certain aluminum alloys, when aged to peak strength (T6) are susceptible to stress corrosion cracking. Stress corrosion cracking can often be minimized by selecting an alternate material.

Return to Table of Contents

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