Corrosion allowance is the extra thickness of metal added to a component to account for material loss from corrosion over its service life, ensuring it can withstand pressure and structural loads until the end of its designed life. It is calculated based on the predicted corrosion rate, operating conditions, and the equipment’s anticipated life, using data from laboratory experiments or field experience.
According to ASME B31.3 and ASME Section VIII, the minimum corrosion allowance for carbon steel in typical process applications is 1.6 mm (1/16 inch).
A corrosion allowance of ZERO means that the material of a vessel or pipe is not expected to corrode, and no extra thickness is added to its design for corrosion resistance. This is often specified for corrosion-resistant alloys, such as stainless steel, or when the process fluid is noncorrosive, such as in hydrocarbon service. However, a zero corrosion allowance is unusual for carbon steel, and it carries a risk of premature failure if the assumption of non-corrosiveness proves incorrect or if the material is more susceptible to corrosion than anticipated, such as in steam service.
A vessel can corrode even with a zero corrosion allowance if the initial design or operating conditions were flawed, the allowance was based on a prediction that didn’t account for specific corrosive factors, or the material was more susceptible than expected. A zero allowance typically occurs when the vessel contains a non-corrosive fluid or when a different design principle, such as using highly resistant alloys, is applied. However, real-world conditions often lead to corrosion, and the only way to ensure safety is to conduct regular inspections and address it through repairs or other measures.
Reasons for corrosion with zero allowance:
Flawed assumptions: A zero corrosion allowance is often based on the assumption that the vessel will contain a non-corrosive fluid, but actual operating conditions may differ. Even "non-corrosive" fluids can cause corrosion over time.
External corrosion: Some components are subject to external corrosion from atmospheric conditions, even if the internal fluid is not corrosive. This is especially true for components with a low risk category that might not have included an allowance.
Material limitations: Corrosion-resistant alloys are not always immune to all types of corrosion. They are resistant to certain types of corrosion, but not necessarily all of them.
Incorrect initial calculation: The initial calculation for corrosion allowance may have been based on an incorrect prediction of the corrosion rate or a misunderstanding of the operating conditions. This can lead to the vessel being dangerously thin over time. What to do when corrosion is found
Inspect regularly: Even with a zero corrosion allowance, it is imperative to inspect the condition of the vessel regularly. Regular inspections can help identify potential issues before they become serious.
Calculate remaining thickness: Use the current wall thickness and the renewal thickness to calculate the remaining corrosion allowance (RCA) to ensure the vessel is still safe to operate.
Take action: Based on the inspection, take action to address the corrosion. This may include repair, welding, or reinforcement, depending on the severity of the corrosion.