Rust at the column base: causes and prevention

Why rust almost always starts at ground level

A lighting column that is still crisp grey or black higher up the shaft may already show brown streaks, blisters in the coating or loose flakes at the base. The zone from roughly 30 cm below to 30 cm above ground level is the most heavily loaded part of any steel column: the steel is alternately wet and dry, sits without oxygen below ground and with plenty of oxygen above it, and salt, soil and mechanical damage all come together there.

That difference in oxygen supply works like a battery. The part just below ground level becomes the anode and dissolves, the part above it the cathode. Zinc that lasts for decades higher up the column can disappear many times faster in the ground-level zone. Once the zinc is gone, the steel rusts from the outside in and the column loses wall thickness exactly where the bending moment from wind is greatest.

Cause 1: de-icing salt, moisture and poor drainage

Along gritting routes, brine splashes against the column base and salt stays behind in the soil. Chlorides sharply accelerate the corrosion of zinc and steel, and salt also attracts moisture, so the base stays damp even in dry weather. The same applies to columns standing in a gutter, in a dip or in paving where rainwater cannot drain away.

The backfill matters too. Rubble and building waste hold water and often contain cement or lime residues; coarse sand lets water drain.

Cause 2: a damaged zinc layer

Hot-dip galvanising to EN ISO 1461 gives a zinc layer that is alloyed with the steel and does not peel; the minimum coating thickness depends on the wall thickness of the steel. Small scratches are not a problem: the surrounding zinc protects the exposed steel cathodically. Larger damage is. Think of a strimmer hitting the same spot every season, lifting chains during unloading, the excavator during erection, or a drilled hole for an extra cable entry that was never touched up.

On a powder-coated (duplex) column you first see damage as a grey patch: the paint is gone, the zinc is still doing its job. Do not leave it. Moisture creeps under the loose paint edge and the zinc layer wears faster at that spot than elsewhere.

Prevention on new columns: heat-shrink sleeve and installation

The standard solution is a thick-walled heat-shrink sleeve with an adhesive lining. You slide it over the column base before erection and shrink it with a gas torch, from the buried section to around 30 cm above ground level. The sleeve seals out salt and moisture and absorbs the knocks from the strimmer. After erection, check that the sleeve is not torn anywhere.

Beyond that: backfill with sand, make sure water can drain away from the base, do not lock the column into concrete or paving without a draining joint, and close the access door properly so the inside stays dry too. We supply the TURRIS steel lighting columns of 4 and 6 metres hot-dip galvanised or additionally powder-coated in RAL 9005, matching the VIA street luminaires and the POLARIS post-top luminaires. With the NEXUS column cut-out in the access door, the connection stays protected as well.

Alternative: an aluminium column

Aluminium forms its own oxide skin, which repairs itself after a scratch. There is no zinc layer to run out, so the ground-level zone is far less critical on aluminium. Along gritting routes, on the coast and in places where touching up is awkward, that is a strong argument for the CIVIS aluminium lighting columns of 4 and 6 metres, supplied on a project basis with price on request.

Do watch out for galvanic corrosion: aluminium should not sit directly against concrete, steel or galvanised anchors. Use an insulating layer or a column shoe and stainless steel fixings.

Inspection and repair of existing columns

Walk your columns at least once a year, preferably in spring after the gritting season. Look for brown deposits, blisters and flakes in the bottom 50 cm, for pitting in the steel and at the access door. If in doubt about the wall thickness, measure it ultrasonically or have the column assessed; a column with a rusted-through base is a safety risk in a storm and must be replaced.

Surface rust can be repaired: wire-brush or sand back to bright metal, degrease, apply a zinc-rich primer or cold galvanising paint and, on duplex columns, put the colour back over it. Then protect the repaired zone anyway, for instance with a wrap-around sleeve or bandage system, because a closed heat-shrink sleeve will no longer fit over a standing column.

Frequently asked questions