Demagnetisation, Post-Cleaning and Safety

Outcomes

After this chapter, the learner should be able to:

• identify when demagnetisation is required;
• explain reversing-and-reducing field demagnetisation;
• select an AC or reversing-DC method;
• verify residual field against an agreed limit;
• control electrical, chemical, UV-A and mechanical hazards; and
• release a component in the specified final condition.

13.1 Why residual magnetism matters

Demagnetisation may be required before testing if an old field interferes with a new technique, between incompatible shots, or after testing when residual field could affect service or later processing. Examples include:

• attraction of ferromagnetic debris to bearings or moving parts;
• interference with compasses, sensors or electronic systems;
• arc blow during welding;
• swarf retention during machining;
• interference with cleaning, plating or coating; and
• a contractual final-field limit.

Demagnetisation is not automatically required for every component. The inquiry, order, procedure or product requirement should state the final condition. The source notes 0.3 mT as a common industrial value but explicitly does not identify it as a universal ISO limit. Use only an agreed, controlled maximum.

13.2 Principle of demagnetisation

A single reverse field can move B through zero only while that field remains; removing it can leave remanence in the opposite direction. Effective demagnetisation applies a field that repeatedly reverses while its amplitude falls toward zero. Each reversal traces a smaller hysteresis loop until domains are left in a substantially cancelling arrangement.

The supplied source describes approximately 10 to 30 reversals as a training concept. Equipment design, material coercivity, component shape and original waveform control the actual cycle.

13.3 AC removal methods

Withdrawal through an AC coil: Energise an aperture coil, pass the component slowly through it along the intended axis, continue moving beyond effective field influence, and only then switch off. The source describes about 1.5 m withdrawal and an east-west coil axis as a training arrangement. Confirm site magnetic influences and equipment instructions.

Reducing AC in a stationary coil: Where withdrawal is impractical, electronically reduce AC to near zero with the component stationary.

AC yoke withdrawal: For local fields on large structures, energise the yoke, move it smoothly along and away from the surface, and switch off after leaving the effective field. The source describes approximately 450 mm withdrawal as an example. Repeat in a consistent direction if necessary.

AC is effective for surface-dominated residual fields and magnetically soft parts. Deep DC magnetisation in complex or high-coercivity components may require reversing DC.

13.4 Reversing DC or rectified-current methods

Keep the component in a coil or magnetic-flow circuit and apply alternating polarity with successively smaller amplitude. The supplied source illustrates reductions to about 50% of the preceding step. Continue to the equipment's controlled minimum, then verify the result at all relevant locations.

Long components may need the cycle repeated along their length. Complex shapes can contain local residual fields of different direction, so one convenient meter reading is insufficient.

13.5 Residual field verification

Use a calibrated residual-field meter, Hall-effect instrument configured for residual measurement, or another method accepted by the procedure. Record:

• instrument and calibration status;
• measurement unit;
• surface location and probe orientation;
• maximum observed positive and negative value; and
• acceptance limit and result.

Mechanical field indicators can show a residual field but may have limited range and directional sensitivity. A compass is qualitative unless the procedure provides a validated method.

13.6 Chemical and carrier safety

Review the current safety data sheet for each cleaner, contrast coating, particle aerosol, concentrate and carrier. Control:

• inhalation and ventilation;
• skin and eye contact;
• ignition sources and flash point;
• spills and environmental release;
• incompatible storage;
• waste and contaminated rags;
• pressurised containers; and
• component/material compatibility.

Use the hierarchy of controls. Personal protective equipment supports, but does not replace, ventilation, isolation and substitution.

13.7 Electrical, thermal and mechanical safety

MT equipment may be low voltage but can deliver very high current. Inspect leads, insulation, contacts, interlocks and earthing. Prevent arcing, short circuits and contact heating. Observe duty cycle and allow cooling. Secure heavy components and control pinch points at bench heads, yoke legs and permanent magnets.

Consider implanted medical devices and magnetically sensitive equipment in the work area. Establish exclusion or medical-review requirements through the site's risk process and equipment information rather than relying on a generic distance.

13.8 UV-A safety

Do not stare into a UV-A source. Prevent direct and reflected exposure, use guards and follow the lamp manufacturer's safe-use information. Check external filters for cracking or damage and avoid hot housings. Protective eyewear must be compatible with fluorescent inspection; a lens that blocks the excitation or emitted indication light can impair detection.

The source contains exposure values and time estimates, but lamp output and occupational limits evolve. This draft does not adopt those figures as current exposure limits. The organisation's approved risk assessment and applicable work-health requirements control.

13.9 Work-area hazards

Field MT can add working-at-height, confined-space, hot-surface, poor access, noise, weather and simultaneous-operations hazards. Dry powder in wind, solvent aerosol in a confined space and an electrical yoke on a wet structure each require a different control plan. Stop if viewing or safe access cannot be maintained.

13.10 Common traps

• Demagnetising without a defined acceptance limit.
• Measuring residual field at only one easy location.
• Switching off an AC coil while the component remains inside its field.
• Using an AC method on a deeply DC-magnetised complex part without checking effectiveness.
• Treating UV-A as harmless because it is less energetic than UV-B.
• Relying on gloves while ignoring ventilation and ignition sources.

Chapter summary

Demagnetisation is an engineered final-condition process. Reversing and reducing fields progressively collapse the hysteresis loop. The result is verified against an agreed limit. Chemical, electrical, UV-A, thermal, mechanical and access hazards are controlled throughout the examination and release sequence.