Why luminaire housings corrode.

What you actually see in the field

Corrosion on a luminaire housing presents in a handful of recognisable ways: a white, powdery deposit on aluminium (oxide forming as the material's otherwise protective surface layer breaks down), reddish-brown rust that almost always starts at exactly the point where a fastener, hinge, or joint meets a different metal, and bubbling or flaking paintwork that leaves bare metal exposed. Near the coast or on heavily salted roads, you often see white or greenish salt deposits concentrating around the same locations.

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 three things simultaneously: an anode (a metal that gives up electrons and in doing so dissolves), a cathode (a more noble metal or surface zone that receives those electrons), and an electrolyte — most commonly moisture, and considerably more active when it contains dissolved ions from salt. Remove any one of the three and the 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 — for instance, an uninsulated stainless steel fastener directly in an aluminium housing — the pair acts like a small battery, and the less noble metal is consumed faster than it would have been on its own.

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. Most housings are delivered with a protective surface layer: paint, powder coating, or anodising. That layer ages under the same UV radiation and thermal cycling that affects all other components in a luminaire — year by year it becomes stiffer and more brittle until microcracks form. From that point, it is no longer the surface coating but the underlying metal that is exposed to moisture and salt, and corrosion can begin in earnest.

This is why corrosion rates track the environment in several ways simultaneously: closer to the coast or on heavily salted roads there is more chloride-rich electrolyte available; higher humidity and more freeze-thaw cycles extend the time the surface stays wet; and strong sun accelerates the embrittling ageing of the paint or anodising that would otherwise keep the electrolyte away from the metal.

The difference between initial coating performance — typically demonstrated by salt spray testing to ISO 9227 — and long-term field performance is therefore significant. A coating that survives 500 hours in a salt spray cabinet may still begin to fail in the field after five or six years of UV exposure and thermal cycling, because the test measures resistance to ingress, not resistance to progressive coating degradation under real operating conditions.

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. Die-cast aluminium is shielded by its natural oxide layer but is susceptible to pitting in chloride environments and to galvanic corrosion if it comes into direct electrical contact with a more noble metal without isolation. Stainless steel holds up considerably longer, but is not immune — certain alloys can be pitted in chloride-rich environments over time.

A thermoset composite such as SMC lacks the metallic lattice structure that electrochemical corrosion depends on, which means the specific degradation mechanism simply cannot start in the same way. That does not make the material eternal — composites do age under UV radiation over very long periods — but it eliminates galvanic corrosion and pitting as failure modes entirely — the degradation mechanisms that dominate in coastal and marine environments.

The practical implication for procurement is significant: the material decision made at the specification stage determines the maintenance cycle for the next 20 years. A housing that begins corroding at year 8 in a coastal environment will require unplanned replacement — with all the associated labour, traffic management, and vehicle costs — long before a composite housing in identical conditions would reach the same point.

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

1. Which atmospheric corrosivity class under ISO 12944 is the housing verified for, and by whom?
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. Are salt spray results available to ISO 9227, and over what exposure duration?

Summary

Corrosion is not a random failure but an electrochemical process with three clear prerequisites: an anode, a cathode, and an electrolyte. As long as all three are present simultaneously, 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, making them more susceptible both to self-degradation and to galvanic attack when in contact with more noble metals. A thermoset composite such as SMC lacks entirely the metallic structure that electrochemical corrosion requires, which eliminates that failure mode regardless of how aggressive the environment is.

For procurement, this means that a clear material declaration — and for metallic housings, a stated corrosivity class under ISO 12944 — provides a considerably more reliable basis than the vague claim of "corrosion-protected" on its own.