BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Rate from
- What it is
- How the corrosion rate is found: from the mass a test coupon lost over a known exposure, or from two wall thickness readings taken years apart.
- Units and format
- Choose one of 2 options from the list: Coupon mass loss; Two thickness readings.
- Example
- The form opens with “Coupon mass loss” selected as a worked example. Choose the option that matches your case.
Coupon
What this means: Mass loss
- What it is
- The mass the corrosion coupon lost during exposure, found by weighing before exposure and again after cleaning off corrosion products.
- Units and format
- Enter a number in mg: mg. The form accepts 0 mg to 10000000 mg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120 mg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Milligrams, not grams. Only used for the coupon method.
What this means: Coupon area
- What it is
- The total exposed surface area of the coupon, counting all faces and edges.
- Units and format
- Enter a number in cm²: square centimetres. The form accepts 0.000001 cm² to 100000 cm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 cm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Only used for the coupon method.
What this means: Exposure time
- What it is
- How long the coupon was exposed in the process stream before it was removed.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 1000000 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 720 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- In hours: a 30-day month is 720 hours. Only used for the coupon method.
What this means: Metal density
- What it is
- The density of the coupon metal, used to turn mass loss into a thickness loss rate.
- Units and format
- Enter a number in g/cm³: grams per cubic centimetre (1 g/cm³ = 1000 kg/m³). The form accepts 0.000001 g/cm³ to 25 g/cm³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7.86 g/cm³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Thickness readings
What this means: Earlier thickness
- What it is
- The wall thickness measured at the same location at an earlier inspection; only used with the two-readings method.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Compare readings from the same spot taken with the same method; it must not be thinner than the present reading.
What this means: Years between readings
- What it is
- The time between the earlier and the present thickness readings; only used with the two-readings method.
- Units and format
- Enter a number in years: years. The form accepts 0.01 years to 100 years. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 years as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Wall and design
What this means: Present thickness
- What it is
- The wall thickness measured now at the thinnest point. Remaining life is counted from it, and the two-readings method also uses it for the rate.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Outside diameter
- What it is
- The outside diameter of the pipe or vessel, used in Barlow’s formula for the thickness the pressure needs.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 114.3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Design pressure
- What it is
- The internal design pressure of the pipe or vessel.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 2000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Gauge pressure in bar; the tool converts it to MPa (1 bar = 0.1 MPa).
What this means: Allowable stress
- What it is
- The allowable stress for the material at the design temperature, from the design code that applies.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 138 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Joint efficiency
- What it is
- The longitudinal weld joint efficiency or quality factor, from the design code for the type of seam and its inspection.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower efficiency raises the pressure thickness and shortens the remaining life.
- Get it right
- A fraction of 1, not a percentage.
What this means: Structural minimum thickness
- What it is
- The least thickness the component needs for its own weight, supports and handling, whatever the pressure. The tool uses whichever minimum is larger.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Life required
- What it is
- The service life the component must still deliver, for example to the next major shutdown or the planned end of the plant.
- Units and format
- Enter a number in years: years. The form accepts 0.000001 years to 200 years. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 years as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Inspect at this share of remaining life
- What it is
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Longest inspection interval
- What it is
- The longest time allowed between inspections whatever the calculated life, from your inspection code or policy.
- Units and format
- Enter a number in years: years. The form accepts 0.1 years to 50 years. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 years as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
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
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Remaining life and inspection intervals
In Ultrasonic Testing by the Numbers
What it covers: From survey readings to a decision: minimum required thickness for pipe and vessel shells, corrosion rate, remaining life, its uncertainty, the next inspection date and when to repair or replace.
BIZOMAT 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.
Open the Corrosion Remaining Life and Inspection Interval calculator