What is corrosion rate?
Corrosion rate tells us how fast a metal is wearing away because of chemical or electrochemical reactions with its environment. It helps you compare materials, choose protection methods, and plan maintenance before leaks, failures, or safety issues happen.
Why it matters
- Safety: Slower corrosion means fewer failures and accidents.
- Cost control: Helps set inspection intervals and avoid unplanned shutdowns.
- Design: Guides material selection and corrosion allowance in new projects.
- Compliance: Supports reporting for quality, reliability, and audits.
Common units
- mpy (mils per year): Thousandths of an inch lost per year.
- mm/y (millimetres per year): Millimetres lost per year.
Quick conversion:
1 mpy = 0.0254 mm/y
1 mm/y ≈ 39.37 mpy
Faster, real-time estimation (electrochemical)
In operating systems you can estimate corrosion rate without waiting for long exposures:
- LPR (Linear Polarization Resistance): corrosion rate ≈ B/RpB/R_pB/Rp, where RpR_pRp is polarization resistance and BBB is a material/environment constant.
- Corrosion probes: electrical resistance (ER) probes and electrochemical sensors provide trend data online.
These methods are great for trend monitoring and chemistry control, while weight-loss coupons remain useful for absolute rates and deposit analysis.
What affects corrosion rate?
- Material: composition, microstructure, heat treatment.
- Environment: pH, temperature, chloride/sulfide content, dissolved oxygen, flow velocity.
- Design: crevices, dissimilar metals (galvanic couples), stagnant zones.
- Surface condition: scale, deposits, weld quality, roughness.
- Protection in place: coatings/linings, inhibitors, cathodic protection, and how well they’re maintained.
How to reduce corrosion rate
- Choose the right material: stainless steels, nickel alloys, non-metals where justified.
- Use protective barriers: paints, metallic coatings, linings, wraps.
- Control chemistry: adjust pH, remove oxygen, dose corrosion inhibitors.
- Apply cathodic protection: sacrificial anodes or impressed current (for tanks, pipelines).
- Improve design: eliminate crevices, avoid water traps, insulate dissimilar metals.
- Manage flow and temperature: reduce erosion, avoid hot spots.
- Inspect and monitor: coupons, probes, UT thickness checks, and data trending.
Frequently asked questions
Q1. What is a “good” corrosion rate?
It depends on the asset and risk. For example, thin instruments may need very low rates, while thick piping can tolerate higher rates. Always compare the rate to the available corrosion allowance and your inspection interval.
Q2. How often should I measure?
For new systems or after a change (chemistry, temperature, inhibitor), start with more frequent checks (weekly/monthly trends), then adjust once stable.
Q3. What if the rate is increasing?
Check recent process changes, verify inhibitor concentration, inspect for deposits/oxygen ingress, and review flow conditions. Consider switching to more corrosion-resistant materials or adding protection.
