Dye penetrant inspection (DPI) for propellers, stainless cargo systems and aluminium

How dye penetrant inspection works

A coloured or fluorescent liquid is applied to a clean surface and left long enough to seep into any crack open to it by capillary action. The excess is then removed from the surface without washing the liquid out of the cracks, and a thin developer is applied. The developer draws the trapped penetrant back out and spreads it into an indication much wider than the crack itself: a red line on a white ground for colour-contrast penetrant, or a glowing line under UV-A light for fluorescent penetrant.

On board, the solvent-removable colour-contrast system in aerosol cans is the usual choice: it needs no water rinse, no power and no darkness. Fluorescent penetrants are more sensitive and earn their place on machined surfaces and fine fatigue cracks where the area can be darkened. Either way, the method finds only what reaches the surface and is open; a crack packed with corrosion products, paint or smeared metal will not take up penetrant, which is why preparation decides the result. The steps below are followed every time, in this order.

• Pre-cleaning to bare metal, then drying so no solvent or water is left in the cracks
• Application of penetrant, and a dwell time set by the procedure for the material and temperature
• Removal of excess penetrant with lint-free cloth lightly moistened with remover, never by flooding the surface
• A thin, even coat of developer
• Inspection under the specified light, and marking of every indication
• Post-cleaning to remove penetrant and developer residues from the part

Propeller blades: severity zones, damage and repairs

Most fixed-pitch propellers on merchant ships are cast in nickel-aluminium bronze or manganese bronze, and the class rules for cast copper alloy propellers, based on IACS UR W24, divide each blade into severity zones A, B and C. Zone A carries the highest operating stresses: on a conventional low-skew blade it lies on the pressure side, running out from the root fillet; on a highly skewed blade it extends further and includes a strip along the trailing edge on the suction side. Zones B and C follow as stresses fall away towards the tip and the edges.

The zones decide what is allowed. On a new propeller, penetrant testing of zone A is normally done in the class surveyor's presence, and any repaired area is tested whatever its zone. Repair welding in zone A is generally not permitted unless the society specially agrees, and then with stress-relief heat treatment; welding in zone B needs prior approval, and repairs in zone C are generally allowed. In service, cracks start at the root fillet, at old repairs and at edges damaged by debris or ice, and cavitation erosion roughens the surface in a way that has to be dressed before penetrant can be read.

Our part is the examination, not the repair. With the ship in dock, blades are cleaned of fouling and any coating in the areas to be tested, examined zone by zone, and every indication is marked and reported by blade, radius and position. When a propeller specialist straightens, grinds or welds a blade, we test before and after the work, so the surveyor sees evidence that the defect was removed and the repair is free of cracks.

Aluminium superstructures and high-speed craft

Aluminium deckhouses, high-speed craft hulls and gangways crack at weld toes, at the corners of openings and where aluminium meets steel through transition joints. Aluminium is soft, and so is bronze, which raises a preparation problem: grinding or wire-brushing can smear metal across the mouth of a crack and close it, leaving a clean result over a real defect.

Preparation on soft metals therefore avoids heavy abrasion, uses brushes and abrasives of a suitable material, and where mechanical work cannot be avoided is followed by a cleaning or etching step that reopens the surface, as the procedure allows. The same caution applies to bronze propeller blades after dressing and to copper-nickel pipe after a joint has been ground.

Welds, valves and fittings, and how indications are judged

Beyond propellers and cargo systems, DPI is used on stainless and duplex weld repairs, on bronze valve bodies and pump casings after damage or repair, on copper-nickel seawater piping, and on steel parts where a yoke cannot be placed, such as threads, small fittings and the inside of tight corners. It can also confirm that a crack excavation in a non-magnetic part is clean before welding begins.

Indications are classified by shape and size. A linear indication, markedly longer than it is wide, usually points to a crack, lack of fusion or a cold lap; a rounded indication points to porosity or a pit. Acceptance follows the specification: ISO 23277 acceptance levels for welds under ISO specifications, ASTM E165 or ASTM E1417 where the specification is American, and the class rules for propellers and hull repairs. Before judging, the technician wipes and re-develops any doubtful indication, because bleed-out from rough surfaces and crevices can mimic a defect.

What a DPI report records

Each report gives the part and its location, the material, the procedure, the penetrant system and its sensitivity, consumable batch details, the surface temperature, the dwell and development times, the viewing conditions and the result of the reference block check. Indications are listed with position, length, type and acceptance status, shown on sketches and photographs. For propellers, positions are given by blade, radius and severity zone so the propeller maker and the class surveyor can find them without repeating the search.

Frequently asked questions