Light Fitting Materials for external Applications: what to look out for...

Does stainless steel really stain less?

December 1st, 2014 Dr. Walter Wendel

1. Housing materials for commercial exterior light fittings

Most common materials for the housings and other hardware are stainless steel, aluminium,

galvanized steel and sometimes plastic. And this is also the ranking from best to rarely

acceptable. Bronze is rarely used these days because of the high cost and reserved for

special applications.

Let’s start with stainless steel (S/S):

Does it really not stain? It depends - of course, because there is not one grade S/S but

several. The most common are 304, 316 and 904.

The 304 is the most used S/S in the entire building industry and is a cost effective entry

grade which will tarnish (“tea staining”) under certain conditions like hot weather and

presence of salt or chlorine. In most cases not a

structural problem but a cosmetic one, but can lead

also to leak problems for IP67/68 rated fittings.

The 316 grade is a better grade which in most cases

will not develop tee staining. It’s also called “marine

grade” S/S. Very suitable for exterior and underwater

light fittings. In hot climates in presence of sea water it

still can develop tea staining.

The 904 grade is a high alloyed material suitable for all applications in extreme weather e. g.

hot and ocean water or underwater lights in saline environment.

WIBRE commercial pool light 72W LED, available in 2

different stainless steel grades.

An additional protection for any corrosion is a

surface treatment called e-polish (electro polish) for

stainless steel. It enhances the appearance and

improves corrosion resistance.

Aluminium for housings and hardware

Aluminium is used in light fitting design either as

casted aluminium, extrusions or sheet metal. Many alloys are commonly used. The

corrosion resistance is good because aluminium spontaneously forms a thin but effective

oxide layer. Especially copper free alloys show a very good corrosion resistance. Marine

grade aluminium (5000/6000 series) are suitable even in applications where salt water is

present. Chlorine water, e g from chlorinated pools can create pitting corrosion especially at

higher temperatures. Galvanic corrosion will occur if aluminium is in contact with another

metal which is nobler, for example brass or if in contact with stainless steel and chlorine

water is presence.

HESS City Elements, versatile light poles made from

marine grade aluminium with optional additional

undercoating for salty air applications.

Galvanized steel is only acceptable without

additional protection if there is no salt water or

salty air condition. Also acid rain will slowly

start corrosion. The best protection gives the

hot dipped galvanized steel with thick layers of

zinc. This is a material typical for brackets.

Special paint primer with additional protection

is available.

Plastics are used for some cheaper underwater light fittings, not so much for commercial

outdoor applications. There are many different plastics - often they have a problem with the

UV resistance and tend to get brittle over time or loose surface material. In combination with

elevated temperatures from lamps or control gear the degradation is accelerated.

Chlorinated salt water will also affect common plastic housings over time.

2. Lens materials: glass, acrylic, polycarbonate

Basically 3 different lens material are used for light fitting lenses: glass, acrylic and

polycarbonate

Toughened glass is the oldest material for lenses, the

advantages are scratch resistant, optical neutral, and longevity

with nearly no degradation. Thick glass panels have a high

impact resistance, thin ones are delicate.

Plastic materials are Acrylic (Trade names e. g. “Perspex”,

“Plexiglass” or generic PMMA) and Polycarbonate (Trade

names “Lexan”, “Macrolon”). Unfortunately not all advantages

are combined in one plastic material, see table below.

Left: HESS City Element Module with glass lens and Xicato LED

Material Optical light transmission 3mm thickness

Density g/cm3

Resistance against Yellowing

Impact resistance

Scratch resistance

Manufacturing

Glass 92% 2.5 +++ + +++ +

Polycarbonate 89% 1.2 + 1) +++ + +++

Acrylic 92% 1.2 ++ ++ ++ ++ 1) non UV stabilized, a UV stabilizer will mitigate the yellowing over time, some use a UV protection

coating

3. Gaskets, cables, cable glands, electronics, screws

These days’ silicone rubber gaskets are used, preferable with heat treatment to avoid

evaporation. They have excellent heat, saltwater and UV resistance and stay flexible.

For outdoor and underwater application good quality cable glands made from brass or stainless

steel are preferred over plastic glands because of the achievable compression

of the rubber gasket inside.

Cables for under water fitting should be special underwater cables. For the

fitting construction it is important that the cable connection inside the fitting must

be protected against water intrusion and also the effect of water wicking thru

damaged cables and getting into the fitting must be intercepted.

With the availability of electronic control hear for HID and the necessary use for LED potted

gear is preferred over unspotted components. The potting material will not only help with the

even heat dissipation but also eliminate corrosion problems possible coming from condensation.

At least the electronic boards should be protected with a conformal coating.

For underwater fittings an external control gear is necessary for pool and spa applications and

preferred for all other underwater application. Safety and service are the reasons for that.

Critical parts are suitable fasteners like screws and springs. These should be made of 316

stainless steel and in the case of screws either Teflon coated to prevent seizing in the screw

holes or special stainless steel anti-galling lubricant has to be applied.

4. Connectors

Some outdoor and most underwater fittings cannot be opened to connect a cable. In these

cases either the fitting has to be ordered with the required cable

length or watertight cable connectors have to be used. These

inline cable connectors are available in IP67 and IP68 ratings and

also available in versions to connect three cables if there is a

need for a loop thru to the next fitting. These connectors are WIBRE connector with resin

available in plastic and also in metal versions. The IP68 rated versions for permanent

underwater usage have to be filled with silicone or special soft resin.

Installations with RGB fittings require multi-pole connectors, which again are also available

for loop thru applications.

An alternative are bronze, aluminium or stainless steel junction boxes. A good solution are

flush mounted to the ground types for easy accessibility of the connectors. The IP68 rated

versions for permanent underwater usage have to be filled with silicone or special soft resin.