Why corrosion is harder to judge than broken wires

Broken wires can be counted; corrosion can only be estimated. There is no equivalent of a wire count: corrosion changes the wire section, the rope diameter, the lay and the lubrication at the same time, and the worst part usually hides between strands and in the core.

Worse, corrosion hides its own consequences. A film of surface rust may be harmless, while a core already rusted through may show very few broken wires. Corrosion assessment cannot stop at colour; it has to read section loss, diameter change and internal signals.

Visual grading of external corrosion

A rough grading can be done on site. Level one is a surface film: brown colour, wiped off with a rag, no pitting, diameter and lay normal. It usually does not affect service, but re-lubricate and find where the water came from.

Level two is pitting: scattered small craters that do not wipe off, and the wire section starts to fall. Level three is sheet rust or deep pits, wires clearly thinner, gaps between strands opening and the diameter outside tolerance. Level four is corrosion with broken wires, loose strands or an exposed core.

The point of grading is not a label but an action. Level one means re-lubricate and watch; level two means shorter inspection intervals and diameter measurement; levels three and four go to a discard assessment. The exact boundaries follow the applicable standard and the maker's data, and tolerance differs widely by duty. For coating types and classes, see zinc coating on wire rope

Pitting, section loss and diameter

Pitting marks the point where corrosion enters the steel itself. A shallow pit may cost a few percent of the section; a deep pit can cut the effective diameter of a wire by more than half. Breaking force follows the section, so section loss translates directly into strength loss.

Diameter measurement is the most practical quantitative check on site. Measure two perpendicular directions at one section, average them and compare with nominal; take readings at several points along the rope, especially low points, terminations and sheave exits. A smaller diameter may come from outer wear or from core shrinkage caused by internal corrosion, so read it with the corrosion appearance.

What the grease and the coating tell you

Factory lubricant is not only for friction; it is the corrosion barrier between wires and between strands. When the film dries and cracks, is washed out by rain or dissolved by cleaning agents, corrosion starts in those places. Check the colour, the consistency and whether coverage is continuous.

A zinc coating is the other barrier. The zinc corrodes first and protects the steel; when the coating is locally used up, the steel starts to rust. White or grey powder is usually a zinc corrosion product; red-brown means the zinc has already failed locally. Coating mass is measured in grams per square metre to the standard, and the class comes from the product standard.

Signs of internal corrosion

Internal corrosion cannot be seen, but it leaves traces. A local increase in diameter rather than a decrease is a warning: corrosion products swell between strands and in the core and push the rope out. Rust water is another sign, seeping from the core or the termination and leaving red-brown streaks on the surface.

Widening gaps between strands, an uneven lay, a rope that feels stiff or rough when bent can all point to internal corrosion. Opening the rope slightly to look for rust powder between strands and a darkened core is the most direct field check.

Terminations deserve the closest look. Sockets, poured cones and fittings trap water and ventilate poorly, so corrosion often grows inward from the end. Look for rust stains at the fitting exit and, where necessary, open the termination for an internal check.

Why internal corrosion is the dangerous one

The danger of internal corrosion is that it gives no warning. The outer strands look intact, the broken wire count is low and a visual inspection may conclude that the rope is serviceable, while the core and inner wires have already lost load-bearing area. The core carries part of the load and supports the outer strands; when it fails, the structure of the rope collapses suddenly.

Another factor is the combination with fatigue. A rust pit concentrates stress, a crack starts from the bottom of the pit under cyclic load and breaks appear in a short time. By the time broken wires are visible outside, the inside may be far worse. Discard decisions for internal corrosion should therefore be more conservative than a table based on broken wires alone. See the wire rope discard criteria

Opening the rope and non-destructive testing

When internal corrosion is suspected, the direct method is to open the rope at several points: part the strands with a spike or a proper tool and look between them and at the core. Sample points should include low points, the area near terminations, sections that stood idle for long periods and sheave exits. Opening damages the local structure, and the sampled section is normally not returned to service, so plan it in advance.

NDT is suited to a survey of the whole rope. The electromagnetic method measures loss of metallic area (LMA) and gives a continuous curve along the length; internal corrosion shows as a general drop in area. It does not replace visual inspection and opening, but it shows which section is worth opening. For methods and limits, see wire rope NDT

Discard and operating strategy

A discard decision reads the corrosion level, diameter loss, broken wires, lay extension, lubrication and duty together. Where people are lifted, or where heat, chemicals or seawater are present, the tolerance is lower; a fixed guy rope in general duty can be treated more generously, but it still needs a defined inspection interval.

A common mistake is to re-grease a corroded rope and treat the job as done. Grease on the surface slows external corrosion only; it does not reach a rusted core and does not restore lost section. Lubrication is maintenance, not a substitute for a discard assessment.

On the operating side, fix what can be fixed: drain the terminations, avoid water collecting at low points, re-lubricate on schedule, choose galvanized or stainless rope for coastal or chemical sites, and keep the rope ventilated when the machine stands idle. For standards and acceptance items, see the wire rope standards guide

Records and one rule

Corrosion records must be comparable: date, position, corrosion level, diameter readings, broken wire count, lubrication state, the environment (humidity, salt spray, chemicals) and the action taken. Only repeated readings on the same rope at the same points show whether corrosion is accelerating or stable.

In one line: surface rust is read by colour, pitting by depth, internal corrosion by diameter and rust water. Colour can mislead; section cannot. Before deciding whether a corroded rope can stay, ask how its core is doing.