Why luminaire housings corrode.

What you actually see in the field

White deposits on aluminium, reddish-brown rust from ferrous parts, and coating blisters or flaking warrant investigation. Appearance alone does not identify the deposit composition or the cause. Inspect joints, fixings, damaged surfaces and areas where water or salts can accumulate.

What all these signs share is that they mark the end of a process, not the beginning. The surface has already been exposed to moisture for a long time before the change becomes visible to the eye.

The mechanism — what corrosion actually requires

Metallic corrosion is an electrochemical process that requires a closed circuit with four parts. An anode is a metal that gives up electrons and in doing so dissolves. A cathode is a more noble metal or surface zone that receives those electrons. An electrolyte, most commonly moisture, carries ions between the surfaces. Electrons also need a conductive path through the metal or an electrical connection between different metals. The process becomes considerably more active when the moisture contains dissolved ions from salt. Break any part of the circuit and this electrochemical process stops.

This is also why two dissimilar metals in contact with each other can be far worse than either metal on its own. In the galvanic series, metals such as zinc and aluminium sit towards the reactive end, while stainless steel and copper sit closer to the noble end.

When a less noble metal comes into electrical contact with a more noble metal in the presence of moisture, the pair acts like a small battery. An uninsulated stainless steel fastener directly in an aluminium housing is one example. The less noble metal is consumed faster than it would have been on its own. (TWI)

Aluminium normally protects itself through a thin, dense oxide layer that forms naturally on exposure to air. Chloride ions — from sea salt or road salt — can break down that layer locally and initiate what is known as pitting corrosion, where attack concentrates at small points rather than spreading evenly. Similar localised attack — crevice corrosion — frequently develops under gaskets, washers, and other concealed joints where moisture is trapped and cannot dry out.

The role of environment — and the ageing of surface coatings

An electrolyte on its own is not enough — it needs a route to the metal. Organic coatings such as wet paint and powder coating differ from anodising. Their long-term performance can be affected by UV radiation, temperature cycling, moisture, mechanical damage and the quality of pretreatment, film thickness and adhesion. If an organic coating loses continuity, the underlying metal can be exposed to moisture and salt.

Anodising is an anodic oxide layer on aluminium, not paint. Its performance depends on factors including the alloy, layer thickness, sealing, defects and actual exposure. The environment can therefore affect organic coatings and anodic oxide layers through different mechanisms; not every surface layer can be described as inevitably becoming more brittle until microcracks form.

A corrosion test under ISO 9227 does not verify the IP66 ingress-protection rating.

The fastener problem

One of the most overlooked corrosion risks in street luminaires is not the housing itself but the fasteners and brackets used to assemble and mount it. A zinc-coated steel bolt driven into an aluminium housing creates a classic galvanic cell the moment moisture bridges the joint. The aluminium acts as the anode and is consumed at an accelerated rate, producing the characteristic white oxide powder around the fastener head — and, over time, structural weakening of the joint.

The fix is either to use fasteners of the same or compatible metal, or to isolate dissimilar metals with non-conductive washers or coatings so that no continuous electrical path exists between them. In practice, this detail is often specified inconsistently: a housing may be rated for corrosive environments while the fasteners supplied with it are not.

How material choice changes the picture

Which material the housing is made from determines how this process unfolds — but no common metal is entirely unaffected. Untreated or damaged steel rusts relatively quickly without its own protection. Hot-dip galvanised steel is protected by the zinc layer until that layer is consumed, at which point the underlying steel corrodes in the same way as untreated steel.

Aluminium has a naturally protective oxide layer, but chlorides can cause pitting and contact with nobler materials can increase galvanic attack. Stainless steel can also suffer pitting or crevice corrosion. Compare specified alloys and surface conditions under the same exposure.

The non-metallic SMC material itself has no metallic lattice structure of the kind required for metal-based electrochemical corrosion. Galvanic corrosion and pitting therefore cannot occur in the SMC material itself. This limitation does not extend to metallic fasteners or other components in the complete luminaire. The suitability of the SMC housing still depends on the exact material grade, the complete design and the actual UV, heat, moisture, chemical and mechanical exposure.

For procurement, four questions are more useful than simply asking whether the housing is "corrosion-protected":

1. For metallic substrates with protective paint systems within the scope of ISO 12944, which relevant current corrosivity category has been used, and is the selection of the complete coating system for the actual substrate documented?
2. What material is the housing itself made from, and does it contain metal that can be subject to galvanic corrosion?
3. If metal components are present — fasteners, hinges, locks — are they isolated from other metals, or in direct electrical contact with them?
4. When testing to ISO 9227 is cited, which test specimen, method variant, exposure duration, assessment criteria and result are stated in the test report?

Summary

Corrosion is not a random failure but an electrochemical process with four parts: an anode, a cathode, an electrolyte and an electrically conductive path between anode and cathode. As long as the circuit remains closed, the process continues — and the environment determines how often and how long that condition is met.

Metals such as zinc and aluminium sit towards the reactive end of the galvanic series compared with stainless steel, making them more susceptible both to degradation and to galvanic corrosion when in contact with more noble metals. A thermoset composite such as SMC lacks the metallic structure required for electrochemical corrosion. Metallic, galvanic and electrochemical corrosion therefore cannot occur in the SMC housing itself; other ageing mechanisms must be assessed based on the material grade and exposure.

For procurement, a clear material declaration provides a considerably more reliable basis than the vague claim of “corrosion-protected” on its own. For metallic substrates within the scope of ISO 12944, the relevant corrosivity category should be linked to a documented selection of protective paint system for the actual substrate; this is not a general classification of all luminaire housings. (ISO 12944-2)

Coastal street lighting: material selection and Smögen

Sources and further reading

• TWI — What is Galvanic Corrosion and How Can it be Prevented?
• AMPP — What is Corrosion? (electrochemical cell)
• ISO 9227:2022 — Corrosion tests in artificial atmospheres — Salt spray tests
• ISO 12944-2:2017 — Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 2: Classification of environments