Corrosion Mapping: Understanding Localised Wall Loss

Why isolated readings can leave a gap

Localised deterioration may fall between monitoring points. Conversely, one low point may be part of a small feature rather than broad thinning. The engineering implications depend on the remaining wall, extent, geometry and location of the affected area.

Mapping should be selected to answer that spatial question. It is not proof that the rest of the vessel is unaffected. Clearly distinguish the mapped area from adjacent surfaces that were not examined.

Define the area and resolution

The scope should identify the component, orientation, boundaries and location reference. Coverage density needs to suit the feature being investigated and the information required for assessment. A coarse grid can miss a narrow feature; a detailed map of the wrong area may still fail to answer the question.

Surface condition, access and geometry can limit measurement quality. The record should show where valid readings could not be obtained and why. A visually attractive colour map is only as reliable as the acquisition and interpretation behind it.

For localised wall loss, define ultrasonic thickness testing coverage around the spatial information the reviewer needs.

Connect the pattern to its possible mechanism

The pattern may help direct further investigation, but wall-loss data alone does not establish every damage mechanism. Provide operating history, contents, deposits, external exposure and prior repairs to the reviewer. Where another mechanism is suspected, a different examination technique may be needed.

Related non-destructive testing can complement mapping when the question extends beyond thickness. Material identification and flaw detection remain separate objectives; adding a method should resolve a defined uncertainty rather than simply increase the number of tests.

• Map position and boundaries on a drawing or photograph.
• Retain measurement units, orientation and valid coverage.
• Identify minimum readings without losing their spatial context.
• Record unexamined zones and measurement limitations.
• Link the map to the equipment history and assessment reference.

Complement mapping with non-destructive testing where the condition question extends beyond remaining wall thickness.

Make the output suitable for a decision

Ask the engineering reviewer what data format and coverage are needed before examination starts. Additional evidence may be necessary to judge fitness for service or define a repair. Do not convert a minimum thickness or colour band into an operating limit without an appropriate assessment basis.