Chemical resistance terminology for luminaires: decoding data sheets

On a luminaire data sheet, the IP rating stands out. Chemical resistance, on the other hand, hides behind acronyms: PMMA, AL6063, ADC12, 1.4404, EPDM, ISO 9227. In a livestock building, a wine cellar, a swimming pool hall or a coastal site, these references decide whether the installation will last ten years or two seasons. Here is the vocabulary to master.

Why a luminaire deteriorates in a chemical atmosphere

A luminaire assembles families of materials that do not react to the same attacks: metal body, polymer or glass optic, elastomer seals, screws, cable gland, cable. Three mechanisms coexist.

Electrochemical corrosion affects metal parts. It requires a film of water, even invisible, in which the gases of the atmosphere dissolve, making it acidic or alkaline. The rate of corrosion therefore increases sharply with relative humidity: in dry air, the same gas is far less active.

Attack on polymers produces no rust but swelling, clouding or cracking. The most treacherous case is environmental stress cracking: a plastic that is stable at rest crazes when it is mechanically stressed, at a screw tightening point for example, and simultaneously exposed to a chemical agent.

Seal degradation is the most frequent tipping point. A seal that hardens or swells stops doing its job: moisture enters the electronic compartment and failure follows, with no visible sign from outside. A luminaire stated as IP66 is no longer IP66 once its seal has aged chemically.

Four aggressive atmospheres, four different chemistries

Ammonia in livestock buildings

Ammonia (NH3) is released from manure. It is a basic compound: dissolved in the film of water covering surfaces, it raises the pH. This alkalinity penalises aluminium, whose protective oxide layer is stable only within a moderate pH range, of the order of 4 to 8.5 according to usual corrosion data. It also penalises certain polymers: polycarbonate is classed as not resistant to ammonia solutions and dilute bases in resin producers' tables, whereas PMMA is given as resistant.

Hence the ammonia-resistant materials claimed by the luminaires in the poultry and livestock range, such as the AVITUB-B for pig farms, designed to withstand the vapours as well as pressure washing between two batches. The subject is developed in our article on LED lighting in poultry farming.

SO2 and sulphites in wine cellars

Sulphur dioxide used for sulphiting and sulphur wicks combines with water to form sulphurous acid, then, in the presence of oxygen, sulphuric acid. The mechanism is the reverse of that of ammonia: the atmosphere becomes acidic. In a cellar, permanent humidity amplifies the phenomenon and bare or poorly coated metals pit quickly. Solutions for vineyards and cellars therefore favour closed, smooth and chemically neutral bodies, as detailed in our article on lighting for wine cellars.

Chlorine and chloramines in indoor swimming pools

This is the most misunderstood case: it is not the immersed parts that suffer most, but those located high up. Chloramines, which are volatile, evaporate and then condense on surfaces that are not rinsed. As they dry, they leave deposits of chlorides that absorb moisture and form solutions far more concentrated than the pool water. Standard austenitic stainless steels, 304 and 316 included, can then suffer chloride stress corrosion cracking, even at temperatures long considered safe, with failures of suspension parts documented in Europe. For lighting swimming pool halls: qualify fixings and suspensions specifically, and plan for periodic rinsing of exposed surfaces.

Salt spray on the coast

Marine chlorides attack by pitting, a very localised corrosion that perforates the metal while the surface looks sound. The NF EN ISO 12944-2 standard, the reference for classifying environments, describes a scale from C1, a clean heated indoor atmosphere, to C5 and then CX for highly saline coastal areas and offshore atmospheres. Food factories and swimming pools are typically in C3 and C4. Asking which category a product is designed for is more useful than asking whether it "resists corrosion".

The vocabulary of materials

Optics and diffusers

Polycarbonate (PC) resists impacts, dilute acids, ethanol and isopropanol well, but it is sensitive to bases, ammonia, methanol and aromatic solvents, which cause swelling or cracking. PMMA, or acrylic, withstands alkaline environments and ammonia better and keeps its transparency, but remains limited against ethanol, is not resistant to acetone and aromatics, and is more brittle. Glass is the most inert option, at the cost of a fragility to be offset by design.

This logic can be read on the REXSHOCK TUBULAR IP, available in a PMMA or polycarbonate version depending on the dominant constraint, with 316L stainless steel end caps and a pressure-balancing membrane to limit internal condensation.

Bodies and structures

| Material | What it means |

| --- | --- |

| AL6063 | Wrought aluminium of the 6000 series, extruded. Good natural corrosion behaviour, common on linear profiles and heat sinks. |

| ADC12 | Die-casting alloy, rich in silicon and containing 3 to 4.5 % copper. Ideal for moulding complex shapes, but this copper content penalises resistance in corrosive environments: the quality of the coating becomes decisive. |

| 304 stainless (1.4301) | Chromium-nickel austenitic, suited to indoor atmospheres with a low chloride load. |

| 316 / 316L stainless (1.4401 / 1.4404) | Addition of molybdenum, which clearly improves resistance to chloride pitting. Remains exposed to stress corrosion cracking in a swimming pool atmosphere. |

Coatings and surface treatments

Powder coating is never better than its pretreatment. The typical failure on the coast is filiform corrosion, which spreads under the film from a scratch or a drilled edge. The QUALICOAT SEASIDE label addresses this risk by requiring reinforced pretreatment, with a minimum etching rate of 2.0 g/m², higher than the standard requirement.

Anodising creates an oxide layer integrated into the metal, specified in thickness classes from 5 to 25 microns. Professional recommendations point to class 15 for very humid indoor use and class 20 for urban, industrial or coastal outdoor use. Beware: this layer is still alumina, sensitive to both very acidic and very alkaline environments, and can be destroyed by aggressive cleaning.

Seals

Silicone (VMQ) excels at high temperature and withstands steam cleaning, hence its presence in food processing, but it swells in contact with hydrocarbons. EPDM resists aqueous acids and bases, ozone and ultraviolet better, at a lower cost, but it is incompatible with mineral oils and petroleum solvents. Remember this principle: the chemical limit of a luminaire is often that of its seal, not of its body.

Reading a data sheet without being misled

Chemical compatibility is not a box to tick

A statement "resistant to ammonia" means nothing on its own. Chemical compatibility is defined by four parameters: nature of the agent, concentration, temperature, duration or frequency of exposure. A fifth is added for polymers, mechanical stress. The same material can be resistant at 20 °C and not resistant at 60 °C, or withstand brief exposure and give way under permanent exposure.

The tests you can require

• ISO 9227, salt spray. Three variants: neutral NSS with 5 % sodium chloride at 35 °C, AASS acidified with acetic acid, CASS with added cupric chloride at 50 °C. The number of hours is a test parameter, not a score: compare durations obtained with the same variant.
• IEC 60068-2-52, test Kb. Cyclic salt spray alternating spraying and humid storage, typically at 40 °C and 93 % relative humidity. More representative than continuous spray.
• ISO 22479, known as the Kesternich test. Exposure to sulphur dioxide in a humid atmosphere at 40 °C: the relevant test for sulphurous atmospheres.
• ISO 12944-2. Not a test, but the C1 to CX grid used to qualify the site before the product.

Three reflexes before ordering

• Qualify the site before the product. Note the dominant agent, the temperature, the average relative humidity, the frequency of washing and the trade name of the cleaning products used. This last point, often forgotten, explains a large share of premature deterioration.
• Check the whole chain. A 316 stainless steel body combined with ordinary screws, an unsuitable cable gland or a poorly chosen seal is no better than its weakest link.
• Anticipate galvanic corrosion. As soon as two different metals are in electrical contact with an electrolyte, the less noble one corrodes. Aluminium assembled with stainless steel screws deteriorates all the faster as its surface is small compared with that of the steel. Countermeasures: insulating washers, coating of the cathodic part, drainage.

Frequently asked questions

Is an IP69K luminaire necessarily resistant to chemicals?

No. The IP rating measures ingress protection against dust and water, including high-pressure, high-temperature jets for IP69K; the IK rating, impacts. Neither qualifies resistance to detergents or corrosive vapours, which is checked through the stated materials and the corrosion tests provided. See also our article on how to assess the reliability of LED lighting.

Is 316 stainless steel enough everywhere on the coast and in swimming pools?

On the coast, 316 brings a real gain over 304 thanks to molybdenum, which improves pitting resistance. In an indoor swimming pool hall, the situation is different: deposits of chloramines and chlorides on surfaces that are not rinsed expose standard austenitic stainless steels to stress corrosion cracking. For suspended and stressed parts, deal with the question specifically, according to the regulations and recommendations in force.

Polycarbonate or PMMA in a livestock building?

Polycarbonate is listed among the materials not resistant to ammonia solutions in resin manufacturers' tables, whereas PMMA is given as resistant. PMMA is therefore often preferred for the optic, provided its lower impact resistance is offset by the design, or a polycarbonate validated by testing is chosen.

How many hours of salt spray should be required?

There is no universal threshold: the duration only makes sense in relation to the test variant, the acceptance criterion and the corrosivity category of the site. A long duration in NSS and a shorter duration in CASS are not comparable. First define the category of the site under ISO 12944-2, then ask for the corresponding test report.

Having your choice of materials validated

Are you preparing a project in a corrosive atmosphere and want to secure the choice of materials, coatings and seals? Get in touch with our technical partner, the Luminance-Consulting agency, a lighting solutions consultancy since 2013. Its engineers carry out audits of existing installations, photometric studies and checks of compliance with the standards.

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Consult Luminance Consulting · 1 Route de Bartres, 65100 Adé, France

Sources: NF EN ISO 9227 · NF EN ISO 12944-2 · QUALICOAT SEASIDE · Covestro, chemical resistance of polycarbonate · Galvanic corrosion guide