Why Do LED Grow Lights Corrode in Greenhouses?

Published 30 October 2026 · XineLam technical notes

A grow light can carry an IP66 label, pass its jet test, and still be scrap metal after two seasons in a working greenhouse. The reason is that the IP code and the corrosion process measure different things. Ingress protection asks whether water gets inside the housing. Corrosion asks what that water contains, how long it sits on a surface, and which two metals it is bridging.

The IP code does not test chemistry

An IP rating follows IEC 60529 and tests solid objects and water: dust-tight, jet-proof, immersible. Nothing in that test involves salt, sulphur dioxide, fertiliser solution or cleaning chemicals, and nothing in it measures how fast a surface loses metal. Enclosure standards such as NEMA 250 address part of the gap — NEMA 4 is watertight, and NEMA 4X adds resistance to corrosion, as the Steel Tube Institute chart summarises. That is why a datasheet can honestly say “IP66” and say nothing at all about whether the fixture will survive a sulphur programme.

The installation rules point the same way. OSHA’s wiring requirements for damp and wet locations call for enclosures that prevent moisture from entering and accumulating, with an air space behind them. Those clauses exist because standing water is the mechanism: a film of water that stays in contact with a joint, a screw thread or a cut edge is where the metal goes.

The four corrosion loads in a greenhouse

Most greenhouse corrosion is not one process but a rotation of four, and a fixture that resists three of them can still fail on the fourth.

Why the stainless screws are often the first failure

The classic greenhouse failure is not a rusted housing; it is a housing whose fasteners have seized or whose threads have corroded. Two mechanisms do most of the damage. The first is galvanic coupling: a stainless screw in an aluminium body forms a small, intense cell when a water film connects them, and the aluminium around the thread is consumed. The second is crevice corrosion: the gap under a gasket or between two profiles holds stagnant water with a different oxygen concentration from the surrounding surface, and that difference drives the attack.

The practical answers are unglamorous. Use isolating washers or a plated fastener where dissimilar metals meet, specify A4 / 316 stainless rather than A2 / 304 in chloride zones, seal or anodise cut edges rather than leaving bare metal at the end of a profile, and design the mounting so water runs off instead of standing in a joint. None of these change the IP number on the box; all of them change whether the fixture can be opened in year three.

What else fails before the extrusion does

• End-cap gaskets. A compressed flat gasket takes a set and does not recover; once water passes, the interior of the profile becomes a trough. Silicone tolerates heat and UV better than EPDM in most horticultural conditions, but both need replacement intervals.
• Driver terminals and PCB edges. Sulphur and chloride reach the driver compartment through the same cable entry that lets cable in. Conformal coating on the board and a potted or gasketed compartment are the defence.
• Powder-coat edges and fastener seats. Coating is thinnest where a part was cut or drilled; that is exactly where a corrosion cell starts. Masking quality matters more than film thickness on the flat faces.
• Reflectors and optical films. Metallised reflectors can haze or lift at the edges long before the LEDs themselves lose output, which shows up as a slow loss of uniformity rather than an obvious failure.

What to write into a specification

1. Both ratings: the IP rating for water, and a corrosion statement — for example NEMA 4X, or a named salt-spray test with hours and method. “Anti-corrosion” without a test is marketing.
2. Materials by name: 6063-T5 anodised extrusion, A4 / 316 stainless fasteners in chloride zones, silicone gaskets, tinned copper conductors.
3. Chemistry on site: tell the supplier whether sulphur evaporators, chlorine-based sanitisers or ammonia sources are used, because gasket and coating choices follow from that answer.
4. Serviceability: fasteners that can be opened after three seasons, spare gaskets available, and a stated replacement interval for seals.
5. Drainage: a mounting angle or channel so condensate leaves the fixture instead of pooling on it.

Maintenance that actually delays corrosion

Inspection twice a year, at the change of season, catches most of it: look at screw heads, end caps and the underside of the driver compartment for white or green deposits, and replace gaskets rather than re-tightening them. Rinse dust and fertiliser residue with clean low-pressure water — a dry brush pushes salt into crevices — and avoid acid descalers on anodised surfaces. Where a fixture is already showing deposits at the fasteners, the useful question is not whether it still lights, but whether it can still be opened for service.