KEVOS® MAKEOS · Materials
Corrosion Remaining Life and Inspection Interval
At its measured corrosion rate, how long before this pipe or vessel wall reaches its minimum thickness, and when should it next be inspected?
Remaining-life assessment of a corroding wall: corrosion rate from a coupon’s mass loss (as the corrosion rate tool) or from two thickness readings, the minimum required thickness as the larger of the pressure thickness by Barlow’s formula with your allowable stress and joint factor and your structural minimum, the remaining life to that thickness, and the next inspection date as a share of the remaining life capped at your maximum interval.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Rate from | choice | Whether the rate comes from a corrosion coupon or from two wall-thickness readings. |
| Coupon | ||
| Mass loss | mg | Mass lost by the coupon. |
| Coupon area | cm² | Exposed area of the coupon. |
| Exposure time | h | Time the coupon was exposed. |
| Metal density | g/cm³ | Density of the coupon metal. |
| Thickness readings | ||
| Earlier thickness | mm | Wall thickness at the earlier inspection. |
| Years between readings | years | Time between the two readings. |
| Wall and design | ||
| Present thickness | mm | Wall thickness measured now (the thinnest reading). |
| Outside diameter | mm | Outside diameter of the pipe or shell. |
| Design pressure | bar | Design pressure. |
| Allowable stress | MPa | Allowable stress of the material at temperature (from the code). |
| Joint efficiency | Weld joint efficiency. | |
| Structural minimum thickness | mm | Least thickness for structural integrity, regardless of pressure (from your code or company rule). |
| Life required | years | Years the equipment must still serve. |
| Inspect at this share of remaining life | % | Share of the remaining life at which to inspect next (often half). |
| Longest inspection interval | years | Longest interval your inspection plan allows. |
Method and formulas
- Coupon rate (mm/y) = 87.6 W ÷ (ρ A t), W in mg, A in cm², t in hours (as the corrosion rate tool); or (t₁ − t_now) ÷ years.
- Pressure thickness t = p D ÷ (2 S E) (Barlow); required minimum = the larger of that and the structural minimum.
- Remaining life = (t_now − t_min) ÷ rate; next inspection = min(share × life, longest interval).
- Example: at 0.092875 mm/y the wall reaches 2.5 mm in 37.685 years; inspect again within 5 years.
Limitations
- Uniform corrosion at a constant rate; pitting and erosion need separate assessment.
- Barlow thickness is a screening value; use the design code formula for the equipment.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Plan the next inspection of a corroding pipe or tank
- Judge whether a line can run to the next shutdown
- Turn coupon results into a remaining life
Who it is for
- Inspection engineers
- Maintenance engineers
- Plant engineers
What you get
- Corrosion rate
- Minimum required thickness
- Remaining life and next inspection
Interpreting the result
- A short-term coupon rate can be far from the long-term rate: confirm with thickness monitoring.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1500–1516, 290–294. Topic reference; no source tables or recommended settings are reproduced.
- ASTM G1 Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
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