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Materials – Corrosion Allowance -


By knowing the expected general corrosion rate and the anticipated plant or service life of a part, the designer can calculate the extra thickness required for corrosion resistance of the process equipment being designed.
After determining a wall thickness that meets mechanical requirements, such as pressure, temperature and weight of equipment, an extra thickness called “corrosion allowance” is added to the wall thickness to comensate for the metal expected to be lost over the life of the equipment. Then, because the penetration depth cab very, a corrosion allowance is assigned a safety factor of two.
A tank wall required a 5 mm wall thickness for mechanical considerations. The designer has determined that the corrosion rate will be 0.4 mm/yr and the expected life of the tank will be 10yr. The total corrosion allowance is the corrosion rate per year (0.4 mm x 10 yr = 4 mm).
The corrosion allowance is doubled to 8 mm as a safety consideration.

Corrosion allowance is not only determined by the designer, but also and especially by a state or local agency. The latter often have years of experience with local conditions and especially the weather conditions in a relevant area.

Humidity, temperature, rain, wind, impurities and metal wet times have an effect on the corrosion rate. Corrosion occurs when the relative humidity of the air is 70 to 80%. Corrosion reaction is possible generally when the temperature is above 0°C and the relative humidity is over 80% (the surface is wet). Air impurities that dissolve in condensed water or rain water may accelerate corrosion. Settling of dust and dirt on the metal surface accelerates atmospheric corrosion.

Corrosion rates are expressed in terms of mm per year of surface wastage and are used to provide a corrosion allowance in the design thickness of equipment such as vessels and pipework.
Operators will often use data based on historical experience from plant operations to aid them in determining appropriate corrosion allowances. Alternatively corrosion charts are widely available that give corrosion rates for many combinations of materials of construction and process fluids and normally a range of values will be provided for various process temperatures.
In some instances, particularly where there is a mixture of chemicals present, appropriate data may not exist and corrosion tests may be necessary in order to determine the suitability of equipment. Operators should be able to demonstrate the use of corrosion allowances in equipment specification and design. The sources of data used should be traceable.

As far as I know, there is no corrosion allowance exactly specified in ASME B31.3. Corrosion allowances are normally established by the end user and are somewhat based on personal preferences and industry tradition. 1.5 mm for piping is a common standard, but you are free to set a corrosion allowances you wish, unless a state or local agency has adopted and superceded B31.3. To specify the pipe, add the corrosion allowance to the minimum design thickness and select a pipe schedule that is equal to or greater than the minimum + corrosion allowance.

The corrosion allowance is a measure which is used in the maintenance applications.
A corrosion allowance of 3.0 mm means that e.g. a wall thickness without any problem may be decrease with 3.0 mm.
Mind you, that does not mean than the minimum wall thickness is achieved and the object has reached its end of life.

Example: a piece of equipment has a wall thickness of 10 mm with a corrosion allowance of 3.0 mm. With a wall thickness of 7.0 mm action is required because the specified corrosion allowance is completely consumed.

Some attention points when the corrosion allowance almost of completely is consumed.

  • What is the corrosion rate. Is it linear or progressive occurred.
  • What was the wall thickness during construction.
  • Are there baseline measurements are available.
  • Are the process conditions changed.
  • Is there erosion occurred in combination with corrosion.
  • Can we get the next inspection period.
  • What is the structural wall thickness that is needed.
  • Should we make a recalculation.
  • Wall thickness monitoring during process (Preventive control).
  • Must the equipment to be replaced at the next shut-down.
  • Must be the same material used for eventual replacement.
  • The wall thickness measurements are reliable.
    Paint-and temperature can affect the measurement results.
  • Is the decline occurred internal or external, or a combination of both.
  • Can equipement remain in use or should it be taken out of service.
  • etc. etc.
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