The issue of corrosion allowance for carbon steel equipment
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When manufacturing carbon steel equipment, a corrosion allowance is generally provided. It doesn’t matter whether this allowance is placed on the inside or outside; it’s sufficient to simply thicken the walls of the equipment accordingly. Assuming a corrosion allowance of 3 mm, once this allowance is exhausted due to the use of the equipment, can it still be used? What procedures are required and what precautions should be taken?
Corrosion allowance is the additional margin added on top of the design value, depending on the degree of corrosion. If the corrosion exceeds the allowance, it may be impossible to achieve the strength specified in the design parameters (without considering the thickness added by the designer as a safety measure). Therefore, if the above situations occur, it is recommended not to use it or to use it at a lower level (special equipment should not be considered in such cases)
First, you need to understand how the corrosion allowance is determined: corrosion rate * design service life. If the level of corrosion exceeds this value, it indicates a risk, and a professional assessment is required to determine whether the material can still be used
So how does the corrosion rate arise? Can it be found?
There are manuals on corrosion rates; these values are obtained by relevant departments through extensive experimental data, not based on arbitrary estimates
For newly manufactured carbon steel equipment, its outer surface needs to be painted to prevent corrosion. Although atmospheric corrosion may occur over time, this margin against corrosion is primarily designed taking into account the internal substances. Over time, it is necessary to regularly measure the thickness of the surface; once there is no more margin against corrosion, there is essentially no safety margin left, and the equipment then needs to be scrapped
The wall thickness of the equipment is generally: theoretical calculated thickness * safety factor + corrosion allowance. In such cases, the strength of the equipment is still sufficient; if it is a pressure vessel, a determination by the boiler inspection agency is required, and it may be necessary to shorten the inspection period (as determined by the inspection rating given by the agency). The key is to ensure good control of process parameters.
Corrosion mainly refers to the corrosion of the medium, that is, the corrosion on the side of the container that is in contact with the medium, usually inside the container.