Three classes, and the difference is coating mass

A, AB and B are the three quality classes for zinc coating on steel wire, first defined in galvanizing standards such as ISO 7989 and now common in EN 10244-2 and the rope standards that refer to it. The class says nothing about rope strength or construction, only how much zinc sits on the surface of each wire.

The same construction at the same diameter can be supplied as class A, AB or B. All three may look the same dull grey or bright finish; the difference is measured as coating mass per unit area in g/m², not judged by colour.

The higher the class, the thicker the coating and the longer protection lasts, but a thick coating is also more easily damaged in bending and rubbing. Higher is not automatically better; read the class together with the duty. Within one project, rope body, terminations and fittings may use different finishes, so check each item separately at acceptance.

Turning coating mass into thickness

Zinc has a density of about 7.14 g/cm³, so 1 g/m² of coating corresponds to roughly 0.14 μm of thickness. Where a standard states 60 g/m², you can read that as about 8 μm of zinc.

Standards usually give minimum coating mass by wire diameter. Fine wires carry a thinner coating and thick wires can carry more; within one class the figure is not a single number, so read the table for the actual wire size. Different makers and standards may lay out the tables differently, so unify the units before comparing.

Coating mass is a minimum, not a nominal. Acceptance is by sampling, weighing or chemical stripping to measure mass per unit area, not by a single thickness reading. A gauge reading on site is an indicator only and does not replace the standard method.

Class A: thin, even and neat

Class A is the thinnest coating, often produced by electro-galvanizing or by drawing wire that was coated first. The coating is even, the surface smooth, dimensional accuracy good, and it suits work where appearance matters.

Corrosion resistance is limited. Class A suits indoor, dry or air-conditioned industrial sites. Under permanent damp, condensation or coastal atmosphere the thin coating is consumed quickly, and the rope may rust before it reaches its fatigue life.

Class A is common on control cables, door and window mechanisms, fitness equipment and indoor decorative cables. Corrosion pressure is low in these duties, so the coating provides short-term protection and a clean finish.

Class AB: the middle ground

Class AB sits between A and B: more coating than A, a smoother surface than B. It suits general outdoor work that is not marine, such as urban atmosphere, ordinary industrial buildings and open-air parking equipment.

Class AB is often used as the default: more durable than A without the price step of B. For most outdoor projects that are neither immersed nor chemical, AB is a common starting point. Where the project sets a service-life target, convert that target into coating mass before comparing, rather than comparing class names.

Confirm the standard when you specify AB. Definitions are not identical across standards; some treat AB as a thickened grade and others list it separately. An order that says AB without naming the standard invites ambiguity.

Class B: heavy coating for severe corrosion

Class B is the heaviest of the three, usually hot-dip galvanized, with coating mass and thickness above A and AB. It suits marine atmosphere, chemical plants, sewage works, port machinery and long-term exposed structures.

The price of a heavy coating is a rougher surface. Rope diameter and the gaps between strands may change slightly, and the coating can flake or powder in bending. On a winch that bends the rope often, a class B coating wears faster, so weigh the trade-off.

Class B is not permanent protection. Zinc is a sacrificial anode, and seawater and salt spray keep consuming it; once bare steel shows, corrosion spreads from that point. In splash zones and under water, class B is often not enough.

Coating thickness and service life

In a given corrosive environment, coating life is roughly proportional to coating mass until the zinc is used up. On the same site, class B usually protects for considerably longer than class A, but not indefinitely.

Corrosion rate depends on the environment and varies widely: a dry indoor site may consume very little zinc each year, while industrial atmosphere and the coast consume much more. When estimating a replacement interval, adjust for the actual site, the maintenance and measured data rather than copying brochure figures.

The coating also protects at damage. Where the surface is scratched or wires rub through to steel, zinc acts as a sacrificial anode for the nearby steel, but only over a limited distance; the larger and deeper the damage, the weaker the protection. A heavy coating buys tolerance as well as time. Mechanical damage in handling and installation can also make the coating fail early at a local spot.

When stainless or plastic impregnation is justified

Where the duty is permanent immersion, splash zone, chemical media or frequent food-plant washing, the coating is consumed faster than is economic. Stainless rope, or a plastic-coated or plastic-impregnated construction, should then be considered.

Stainless is not a case of the highest grade winning. 304 suits ordinary atmosphere and indoor work; 316 contains molybdenum, resists chloride better and suits coastal and washed environments; more severe duty or higher stress may call for duplex or a higher grade. The wrong grade costs more and still corrodes.

Plastic impregnation and plastic sheathing block water from the core and keep lubricant inside, but they do not add structural strength and cannot be counted as extra breaking force. Once the sheath is damaged, water enters through the breach and collects inside, which is harder to see, so inspection must be closer.

Settle the coating class, the environment and the core together; for surface options see galvanized wire rope grades and uses

Ordering and acceptance points

State the class, the standard, the wire diameter range and the coating mass requirement on the inquiry. Galvanized alone is not enough, because the coating mass of A, AB and B differs widely, and so do price and life. Colour can also vary between batches and within one batch, so colour is not an acceptance criterion.

Where third-party inspection is required, state the test method and sampling ratio for coating mass. Test methods differ between standards and results are not directly comparable.

• Class and standard: state A, AB or B and quote the standard number.
• Wire diameter range: coating mass is graded by diameter, so acceptance follows the table.
• Coating mass and test method: state the minimum, the sampling ratio and the test standard.
• Packing and marking: moisture-proof packing, with class, batch and date on the label.

Before replacing galvanized rope with stainless, confirm the strength grade and termination, then compare price. For grade selection see stainless steel wire rope grade selection

To screen finish and construction against the environment and load, use the wire rope selector