Uniform corrosion monitoring on the piping and tanks of industrial plants is one of the most important maintenance activities for asset integrity management. Unlike pitting (localised corrosion), uniform corrosion reduces wall thickness progressively and evenly — and that is precisely what makes it possible to monitor it systematically with ultrasonic measurements repeated over time at the same reference points.
What uniform corrosion is
Uniform corrosion is an electrochemical phenomenon that attacks the metal surface evenly, producing a distributed loss of thickness. It differs from other mechanisms in the following respects:
- Slow but steady rate: typically 0.1-0.5 mm/year in aggressive conditions, slower in mild environments
- Even distribution over large areas of the surface
- Predictability: once the rate is known, the remaining life can be estimated with remaining-life calculations
- Influenced by temperature, pH, fluid composition, flow velocity and the presence of inhibitors
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Monitoring methodology
Uniform corrosion monitoring rests on three principles:
- Condition Monitoring Locations (CML): predefined fixed points along piping and tanks, uniquely identified and physically marked
- Measurements repeatable over time: the same CMLs are re-measured at regular intervals (typically every six or twelve months)
- Trend analysis: comparing measurements over time gives the actual corrosion rate and allows the remaining life to be predicted
Dakota instruments for monitoring
For uniform corrosion monitoring campaigns, the most suitable Dakota instruments are:
- CX8-DL: professional entry level with a 100,000-reading data logger, ideal for medium-sized campaigns
- CMX1-DL / CMX2-DL / CMX3-DL: A-Scan, 4 GB memory, multiple measurement modes (PE, EE, PETP, CT, PECT)
- MX2-DL: B-Scan with 2D view, useful for large areas
- CMX10-DL: top of the range for intensive campaigns and critical assets
From data to decision: the role of the data logger
The value of monitoring comes from the structured data logger. The workflow is:
- Baseline: first campaign, recording every CML in organised grids
- Subsequent campaigns: repeating the same measurements with the same instrument and the same probe
- Import into DakMaster™: the data is organised by project, asset and CML
- Historical comparison: the software automatically calculates the corrosion rate between two consecutive measurements
- Extrapolation: by comparison with the minimum allowable thickness, the remaining life is calculated and the next inspection date is set
- Formal report production: for internal audits, asset integrity management systems and regulatory authorities
Regulatory framework
Corrosion monitoring falls within the scope of several standards:
- API 570: for in-service piping in process plants
- API 653: for atmospheric storage tanks
- API 510: for pressure vessels (not covered by Dakota, but thickness measurements are part of the requirements)
- EN 14127, ASTM E797: ultrasonic thickness measurement procedures
- Internal company specifications for asset integrity management (RBI — Risk Based Inspection)
Common mistakes to avoid
- Measurements that are not repeatable: without physical marking of the CMLs, measuring “at the same point” becomes an approximation and the trend is distorted
- Changing instrument between campaigns: different instruments or probes can give slightly different readings; good practice is to keep the same equipment for a given programme
- Calibration that is not traceable: without calibration on a certified reference block, the data loses its legal value
- Data not archived in a structured way: scattered Excel sheets with no link to asset management software make any longitudinal analysis difficult
- Purely numerical interpretation: readings that fluctuate with no coherent trend point to measurement problems (coupling, faulty probe) rather than to real corrosion
To set up a uniform corrosion monitoring programme on Your assets, contact our technical department.