Lift shaft corrosion: design, failure modes and the fix

A lift shaft is the wettest room in a building that has no plumbing in it. Water arrives as vapour, not as a leak: warm air from the lobby meets a concrete wall that has been cold since the previous night, and the steel frame at the bottom of the shaft is the coldest surface in the enclosure. Condensation forms on it, runs down the guide rail bracket, and sits in the box section of the counterweight frame until the next warm spell evaporates it.

The damage shows up two years later as a rust streak under a powder-coated frame, a seized bracket bolt, or a paint blister at a weld toe. None of those is a coating failure in the ordinary sense. Each is a design decision that put steel where water collects and trusted a 70 µm film to hold it back.

This guide covers lift shaft corrosion as an engineering problem: the environment that drives it, the failure modes that repeat across shafts, and the finish and drainage rules that put on the drawing. The part families involved are collected in the elevator and lift components hub.

Why does steel corrode in a shaft that never gets wet?

Because the steel surface spends hours below the dew point. Corrosion rate is governed by time of wetness, the share of the year in which a film of water or a high-humidity layer sits on the metal. ISO 9223 uses that, with sulphur and chloride deposition, to classify atmospheric corrosivity.

The dew point arithmetic is simple and worth doing once for a project. At 30 °C and 70% relative humidity, air has a dew point of about 24 °C. A shaft wall at 22 °C after a cool night is below that, so moisture condenses on it as soon as the lobby doors open in the morning. The same happens on a counterweight frame, a guide rail bracket and the inside face of a controller enclosure.

Indoor shafts usually fall into ISO 12944-2 category C3, the same category as an urban exterior, because of that condensation cycling. A shaft with a sump that holds water, a leaking roof drain or a coastal site is C4. The category is a specification decision, not a guess, and it drives the coating system and the material.

The shaft zones, and what each one needs

Corrosion risk inside a shaft is not uniform. The table separates the zones and the finish we would specify for each. The right-hand column is the limit we work to under our seven-tank line.

The pit row matters most because it is the one designers leave as "paint as standard". A frame that stands 150 mm above a sump floor with a 1:50 slope and two 10 mm drains behaves. The same frame sitting flat on the floor with no drain sits in a puddle for the life of the building.

Which failure modes repeat?

Six, and each is visible before the finish is applied if the drawing review looks for it.

Edge and weld corrosion

Cut edges have no coating until the pretreatment and powder stages cover them, and the film thins over a sharp edge. A laser-cut edge with a 0.2 mm burr holds less powder than a flat face. Break the edge, deburr before pretreatment, and measure DFT on the edge as well as the face.

Crevice corrosion at bracket interfaces

Two steel faces bolted together with a gasket line at the perimeter trap a film of water in the middle. Oxygen is consumed in the gap and the steel at the centre becomes anodic, which concentrates attack exactly where it cannot be seen. A bracket that is powder coated on the mating face and bolted dry will corrode at the bolt line. A gasket or a bead of sealant that excludes water is the fix; a thicker coat is not.

Water trapped in closed sections

A counterweight frame built as a closed box is a container. Condensate forms on the inside face overnight and has nowhere to go. Two 10 mm holes at the lowest point and one at the highest point give it a path, and they cost nothing at the laser. Frames without them bleed rust from the weld seam within two or three seasons.

Galvanic corrosion at dissimilar joints

A stainless bolt in a mild steel bracket, or an aluminium sill on a steel landing, makes a cell. The potential difference between stainless and mild steel is around 0.5–0.6 V in a wet environment, and the steel is the anode. The bolt does not corrode; the bracket around it does. A nylon washer and a plastic sleeve isolate the fastener, and the drawing has to say so.

Abrasion at guide contact

A counterweight frame runs on guide rails with sliding or roller guides. The contact face loses coating to abrasion, and the exposed steel then rusts and abrades faster. Specify a harder powder or a replaceable wear pad, and keep the coated face out of the sliding interface.

Sump splash and wicking

Water does not need to reach a joint to travel through it. A lap joint with a 0.1 mm gap wicks water 50 mm upwards by capillary action, and the paint line does not stop it. Seal the lap, or put the lap where water cannot stand, or design the joint as a drain rather than a trap.

Related reading: the Elevator buffer bracket: design, materials and manufacturing and the Elevator cabin panel: design, materials and manufacturing take the same problem further. See the process and inspection scope on the capability pages, or send the drawing through the RFQ route.

How thick does the coating need to be?

Thick enough that the thinnest point on the part still blocks water and oxygen, and that is almost never the flat face. Powder builds 60–90 µm on a face and drops to 30–40 µm over an edge or into a fold. The figure to specify is a minimum on the edge, not a nominal on the face.

The arithmetic for galvanised sheet is worth knowing because it decides whether the zinc or the powder is doing the work. IS 277 quotes coating mass as grams per square metre over both faces. Zinc has a density of 7.14 g/cm³, so 1 µm of zinc weighs 7.14 g/m². Z120 sheet carries 120 g/m² total, which is 60 g/m² per side, or about 8.4 µm of zinc. Z275 gives 137.5 g/m² per side, about 19 µm.

In a C3 shaft, zinc loss runs at roughly 0.5–1 µm a year for a clean zinc surface. Eight micrometres of zinc is therefore a five-to-ten-year sacrificial layer, and it is fully consumed at a cut edge where the exposed steel strip is only 2 mm wide. That is why a galvanised sheet part still needs a powder barrier in a damp shaft: the zinc buys time at the edge, the powder keeps the water off the face.

Powder thickness has a ceiling as well. Above about 120 µm the film becomes brittle, loses impact resistance, and shrinks enough at a sharp corner to crack during handling. The working window is 60–90 µm for a shaft part, with 80–90 µm at the pit and 60–80 µm above.

What drainage detail goes on the drawing?

Drainage is the cheapest corrosion control available and the one most often left to the fabricator. Four rules cover most frames.

1. Lowest point, two holes. 10 mm diameter, at opposite ends of the lowest face of each closed section, so one can let air in while the other lets water out.
2. Highest point, one vent. 10 mm, or the section air-locks and the drain holes do nothing.
3. A slope, not a flat. 1:50 on any face that can collect water, formed into the part rather than packed on site.
4. No upward-facing pockets. A channel that opens upwards is a planter. Cap it, drain it, or turn it over.

The holes go on the flat pattern, before the part is cut, and they are shown on the drawing with a note that they are functional, not optional. We have seen a well-specified frame arrive at site with the drain holes filled by powder because nobody told the booth they were functional. Mask them, or run a reamer through after cure.

What changes when the shaft is coastal or has standing water?

Two things: the material and the system. A coastal shaft moves to ISO 12944-2 C4, and the zinc or the barrier has to carry more. The usual choices are:

• Galvanised sheet with a powder coat. IS 277 Z275, powder at 80–90 µm, edges and weld toes checked. This is the build we run most often for damp shafts.
• Mild steel with a zinc-rich primer and a powder topcoat. A different system with a different application sequence, and only where the primer is compatible with the powder.
• Stainless for the small parts. Brackets, clips and fishplates in 304 where the part is small and the cost is bearable. 304 is adequate for a normal shaft; 316L is the choice where chlorides are present. The stainless grade guides cover the difference in forming behaviour.
• Aluminium for sills and cladding. Aluminium does not rust, but it corrodes at a steel interface, so the isolation detail matters more, not less.

What none of these fix is a frame that sits in water. If the pit floods, the finish is a maintenance item regardless of the system, and the design answer is a sump pump, a raised frame or a drained pit floor.

How the finish is applied at Ganesh

Shaft parts run the same route as the rest of our coated work. Fabricated frames are shot blasted in the 16 ft × 16 ft chamber where the specification calls for a profile, then go through seven-tank pretreatment: alkaline degrease, rinse, derusting, rinse, activation, zinc phosphating and a final rinse with passivation. The phosphate conversion coat is what gives the powder a chemical key and what stops corrosion creeping under a damaged film.

Powder is applied in the 18 ft × 14 ft electrostatic booth and cured in the oven. Coating thickness is checked with a DFT gauge on faces, edges and weld toes, and adhesion with a cross-hatch test on a sample from each batch. Frames that will be welded after coating are masked at the weld zones, because powder does not survive a MIG arc and the burnt film becomes a corrosion site.

Counterweight frames, guide rail brackets, car frames and sills for TKE and Eros Elevators run through this line. The part families and their tolerances are on the elevator parts page, and the paint shop capability page carries the booth and oven envelope.

What the counterweight frame drawing has to settle

• Material and sheet grade, with the zinc coating mass if galvanised sheet is specified.
• Drain and vent hole positions, marked functional, with a mask note for the booth.
• Weld map, showing which seams are seal welds and which are stitch welds, because a stitch weld leaves a crevice.
• Finish schedule, with the shaft zone, the ISO 12944-2 category, the DFT range and the edge minimum.
• Isolation details for stainless fasteners and aluminium interfaces, with the washer and sleeve part numbers.
• Post-coating operations, including weld zones to be masked and touch-up paint for site damage.
• Handling notes, because a lifted frame that lands on its drain edge dents the hole shut.

The counterweight frame design guide covers the structural side of the same part: frame stiffness, guide shoe mounting and the tolerances that keep a counterweight running quietly. Corrosion and structure meet at the fixings, so both drawings should be checked together.

When powder is the wrong answer

Powder is a barrier, not a sacrificial coating, and a barrier fails at a defect. In three situations it is the wrong primary protection.

Hot dip galvanising to ISO 1461. The standard covers a dipped coating with a specified minimum thickness and a defined edge coverage. That is a different product from galvanised sheet plus powder, and we do not run a dipping line. If the specification says ISO 1461, we say so at quotation, and the part is either dipped by a subcontractor or redesigned around a coated build.

Immersion or frequent standing water. A pit that holds water for weeks needs a system designed for immersion, usually a heavy-build epoxy or a dip, not a decorative powder. A powder film under continuous immersion blisters from the edge inwards.

Abrasion at a sliding contact. A coated guide face will wear through. The answer is a replaceable pad in a harder material or a stainless wear strip, not a thicker coat.

We do not test coated parts to a salt-spray hours claim, and we do not publish one. What we do is control the pretreatment chemistry, measure DFT and adhesion, and tell you when the environment on the drawing needs a system we do not apply. A frame that is specified honestly for its shaft will outlast one that carries a coating claim it cannot support.

Send the frame drawing with the shaft environment and the finish callout through request a quote, and the review will come back with the drainage and coating changes that the shaft needs, before the first frame is cut.