Corrosion Allowance

Corrosion Allowance Formula In some cases, regulatory standards or industry best practices may dictate minimum corrosion allowance requirements that must be met.
\( CA \;=\; CR \cdot DL + MS \) (Corrosion Allowance) \( CR \;=\; \dfrac{ CA - MS }{ DL } \) \( DL \;=\; \dfrac{ CA - MS }{ CR } \) \( MS \;=\; CA - CR \cdot DL \)
Symbol	English	Metric
\( CA \) = Corrosion Allowance	\(n\)	\(mm\)
\( CR \) = Corrosion Rate	\(in \;/\; yr\)	\(mm \;/\; y\)
\( DL \) = Design Life	\(yr\)	\(y\)
\( MS \) = Safety Margin	\(dimensionless\)	\(dimensionless\)

Corrosion allowance, abbreviated as \(CA\), is the amount of material in a pipe or vessel that is available for corrosion without affecting the pressure containing integrity. The amount of extra material added to a structure or component in order to compensate for the loss of material due to corrosion over time. Corrosion is a natural process that occurs when metal structures or components are exposed to a corrosive environment, such as seawater or acidic gases.

The corrosion allowance is typically specified by engineers and designers based on the expected lifespan of the structure or component, the corrosivity of the environment, and the type of material used. The amount of corrosion allowance required can vary widely depending on these factors, but it is generally specified as a percentage of the original material thickness.

In all piping systems, the expected corrosion rate may be constant or change with time. External corrosion (atmospheric corrosion, dampness in the soil, or chlorides from insulation) may be a factor. Many companies usually specify a 1/16-inch (0.0625") minimum corrosion allowance for carbon steel and low alloy steel piping if the corrosion rate can be predicted accurately and is less than a defined threshold (3 mils per year or less as an example). For stainless steel, a 1/32-inch (0.0 3125") minimum corrosion allowance is usually sufficient.

Corrosion allowance is an important consideration in the design and maintenance of structures and components that are exposed to corrosive environments. By adding extra material to compensate for the loss of material due to corrosion, engineers and designers can help ensure that the structure or component will last for its intended lifespan without failure due to corrosion.

Factors to Consider When Determining the Corrosion Allowance

Corrosion allowance is an additional thickness applied to the walls of pipes, vessels, or other structures to account for material loss due to corrosion over the anticipated lifespan of the equipment.

Material of Construction - The corrosion resistance of the selected material in the given environment directly influences the required allowance.
Corrosive Environment - Includes fluid composition, corrosivity, presence of impurities, sediments, water, and chlorides.
Operating Temperature - Higher temperatures can accelerate corrosion rates.
Operating Pressure and Fluid Velocity - These affect corrosion under certain conditions.
Expected Service Life - The duration over which the component must remain functional determines the total material loss to accommodate.
Corrosion Rate - Estimated based on material, environment, and historical data, often multiplied by service life.
Type of Corrosion - Uniform/general vs. localized/pitting, as allowance must account for the dominant mechanism.
External Atmospheric Conditions - Humidity, rain, wind, precipitation, and impurities for exposed components.
Protective Measures - Presence of internal linings, external coatings, or cathodic protection, which may reduce the required allowance.
Applicable Codes and Standards - They provide calculation methods and minimum requirements.
Safety Factors - Often applied to double the calculated allowance for uncertainty.
Regulatory or Client Requirements - Input from state agencies, end users, or corrosion engineers based on experience.

By considering these factors, engineers and designers can determine an appropriate corrosion allowance that balances the need for corrosion protection with practical and economic considerations while ensuring the safety and integrity of the equipment or structure over its intended lifespan. It's important to note that the actual calculation and determination of corrosion allowance can vary depending on specific industry standards, engineering practices, and regulatory requirements. Therefore, it's advisable to consult relevant codes, standards, and experienced professionals when determining the appropriate corrosion allowance for a particular application.

Corrosion Allowance Standards

ASME Standards