Corrosion Allowance

Definition - What does Corrosion Allowance mean?

Corrosion allowance describes an extra measurement added to the thickness of the wall. This helps estimate the expected metal loss throughout the lifespan of certain equipment. Since the depth of penetration may vary from case to case, corrosion allowance has a safety factor equivalent to 2.

By knowing the expected rate of corrosion and the expected service life of a plant or parts, industries can compute the additional thickness needed for corrosion resistance of equipment in the design process.

Corrosionpedia explains Corrosion Allowance

Corrosion allowance must be determined once the wall thickness has been identified. The wall thickness should comply with various requirements such as temperature, pressure and weight. This allowance should not only be measured by project engineers, but also by local or state agencies. Local agencies have more experience with local conditions in a particular area.

Factors that affect the rate of corrosion include:

  • Temperature
  • Humidity
  • Wind
  • Rain
  • Impurities

Corrosion usually takes place when the environment's temperature is over 0°C and the humidity is at 80% on a damp surface.

The rate of corrosion is expressed in mm/year, based on wastage of a certain surface. This is utilized to determine corrosion allowance when designing the thickness of equipment such as pipe works and vessels. Operators will make use of information according to past experiences or operations. This is to help them in identifying the correct corrosion allowance calculations. Corrosion charts also play a role in giving corrosion resistance rates for several construction material combinations and process fluids. In general, values in a broad range will be given for various temperatures for processing.

In some cases, suitable data may not be present and numerous corrosion tests are required in order to identify the appropriateness of certain equipment. There is no specific value for corrosion allowance—what exist are guidelines, but they can be based on the end user or personal preferences.

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