Enclosure materials compared

The five materials at a glance

Values are from producer datasheets, except the carbon steel density and the cutting ratings, which are one enclosure maker's figures. The modulus column explains a lot: for the same wall thickness, a polycarbonate wall is roughly 80 times more flexible than a steel one (200 ÷ 2.4, our arithmetic), which is why molded plastic enclosures use thicker walls and ribs.

Corrosion and chemical resistance

Stainless steel resists corrosion because of a thin chromium-rich passive film that re-forms by itself in air. NEMA's corrosion test for Type 4X puts the enclosure through 600 hours of salt spray and then another 200 hours, compared against an AISI 304 stainless steel specimen. That specimen is the test's benchmark, not a guarantee for any design: the rating depends on how the whole enclosure is built. Where chlorides are present (salt air, brine, chlorinated sanitizers), 316L is the safer grade: see 304 vs 316L stainless for enclosures and the stainless steel enclosure guide.

Powder-coated carbon steel is protected by its coating. Powder-coating practice treats the pretreatment as part of the corrosion system: steel is typically given an iron or zinc phosphate, and zinc phosphate is the standard for high-performance work. Where the coat is drilled, cut or chipped, the bare steel underneath needs touching up, or it is open to the environment. Salt-spray hours quoted for coatings are comparisons, not life predictions: ASTM B117 itself says that performance in natural environments "has seldom been correlated with salt spray results when used as stand alone data."

Aluminum forms a dense, insulating oxide film that is stable over roughly pH 4 to pH 9. The Aluminum Association describes the 5xxx (aluminum-magnesium) series as having good resistance to corrosion in the marine environment and lists 5052 among electronics alloys. The same oxide film is why caustic cleaners are a problem: extrusion guidance is simply "avoid contact with strong alkalis."

Polycarbonate resists dilute acids and most industrial oils and greases, but its producer's chemical-resistance guide lists alkalis, ammonia and amines as aggressive, and acetone, MEK and aromatic solvents as causing swelling. Fiberglass (glass-fiber-reinforced polyester) is used in petrochemical, water treatment and marine settings and with many solvents, alcohols and moderately strong acids and alkalis. For strong alkalis, enclosure makers' own resistance tables disagree, so check the specific chemical and concentration. The details are in polycarbonate vs fiberglass enclosures.

Sun, UV and outdoor exposure

• Bare stainless and aluminum have no coating to fade.
• Powder coat: the chemistry matters. Powder-coating industry guidance says epoxy-containing powders, hybrids included, cannot be used where appearance must hold up outdoors; entry-level polyesters begin to fade, lose gloss and chalk after 12 to 18 months of continuous exposure; "superdurable" polyesters withstand about five years before colour and gloss change. Ask which powder an outdoor enclosure uses.
• Polycarbonate needs a UV-stabilized grade outdoors. One enclosure maker notes that clear polycarbonate exposed to UV will, over some years, turn yellowish and lose impact resistance.
• Fiberglass surfaces degrade slowly under UV until glass fibers show through ("fiber bloom"). UV-inhibited formulations slow it.

For plastics, the outdoor rating to look for is the UL 746C "f1" rating: suitable for outdoor use with respect to UV light, water exposure and immersion. Sun also adds heat load, so an outdoor plastic enclosure needs the heat check below as well.

Weight, stiffness and impact

Aluminum is about one third the weight of stainless steel (2.7 against 7.9 g/cm³). At the same 1.5 mm wall, a square metre of 304 sheet weighs about 11.9 kg and a square metre of 5052 aluminum about 4 kg (our arithmetic from the densities). That matters for pole mounting and mobile equipment.

Plastics are lighter still. Polycarbonate has very high impact strength: a UV-stabilized grade's datasheet gives a puncture energy of 60 J at 23 °C and 65 J at −30 °C. Its notched impact value, though, falls from 70 to 15 kJ/m² between those temperatures, so notches and sharp corners matter more in the cold. Fiberglass is about five times stiffer than polycarbonate (11–12.7 GPa against 2.4 GPa). Impact ratings for finished enclosures are product-specific: ask for the IK code under IEC 62262.

Heat and temperature limits

Metals conduct heat far better than plastics: thermal conductivity is about 55 W/m·K for unalloyed steel, 15 W/m·K for 304 at room temperature, 138 W/m·K for annealed 5052 aluminum, 0.2 W/m·K for polycarbonate and 0.30–0.36 W/m·K for electrical-grade sheet molding compound. But an enclosure sheds heat mainly from its outer surface into the air, and enclosure heat calculations use a surface coefficient instead:

Reference A is a distributor's enclosure-sizing white paper; B and C are two enclosure makers' technical documents. They agree on painted steel and plastic and disagree on stainless. One enclosure maker's thermal note adds that unfinished aluminum and stainless run warmer than painted enclosures because they radiate heat less efficiently, while non-metallic enclosures behave much like painted metal. Treat bare-metal figures as approximate and keep a margin; see enclosure heat calculation.

Temperature limits are a plastics question. One enclosure maker gives about −50 °C to +120 °C for its polycarbonate and typically −50 °C to +150 °C for fiberglass-reinforced polyester; another rates a polycarbonate line at −40 °C to +80 °C long term. Use the range published for the specific enclosure. For metal enclosures, the same maker notes that the limit is set by the gasket or paint, not the metal.

EMI shielding, wireless signals and grounding

Metal enclosures (steel, stainless, aluminum) form a conductive boundary that reflects and absorbs interference. Plastics are electrical insulators and offer negligible EMI protection unless they are plated or given a conductive coating. The same property works the other way for wireless: putting a radio inside a steel box is, as one instrumentation columnist puts it, the same as putting it in a Faraday cage, while polycarbonate and fiberglass let Wi-Fi, cellular and other signals through. With a metal enclosure the antenna goes outside; specify that opening with your other cutouts.

Grounding differs too. In the NEC as quoted by code commentators, 314.3 limits non-metallic boxes to non-metallic wiring methods unless internal bonding is provided between all metal entries, and 314.4 requires metal boxes to be connected to an equipment grounding conductor per 250.148. These sections are written for boxes: apply them as your authority having jurisdiction does, and check the edition it enforces.

Cutting, drilling and modification

• Stainless: work hardens. Keep tools sharp, keep a positive feed and do not let the tool dwell. Use tools reserved for stainless, or iron picked up from carbon-steel work will rust on the surface.
• Powder-coated steel: touch up the paint around every new hole so no bare steel is left exposed.
• Aluminum: rated moderately difficult in one maker's tables, like mild steel. Deburr, and use stainless or aluminum fasteners rather than plain steel.
• Polycarbonate: easy to machine with sharp tools. Cool with air or a light water mist; petroleum cutting oils and aggressive coolants can cause stress cracking.
• Fiberglass: abrasive. A dull tool is the most common cause of chipped holes; carbide-tipped or diamond-tipped hole saws at high speed and low feed work best. Extract the dust and wear eye and respiratory protection.

Every opening must then be closed by a fitting rated at least as high as the enclosure: see cutouts, cable glands and HMI windows.

Which material for which job

Frequently asked questions

Sources

1. NEMA, "NEMA FAQs: Enclosures" (Q14 salt spray: 600 h vs G90, then 200 h vs AISI 304 for Type 4X)
2. ATI, Types 302/304/304L/305 datasheet (density, modulus, thermal conductivity)
3. ATI, Types 316/316L/317/317L datasheet (density)
4. Outokumpu, Steel grades, properties and global standards (Table 12)
5. Kaiser Aluminum, alloy 5052 datasheet
6. Aluminum Association, alloy series (5xxx: resistance to corrosion in the marine environment; 5052 in electronics)
7. Aluminum Association, electronics and appliances
8. Profilgruppen, corrosion resistance of aluminium (oxide film stable pH 4–9; avoid strong alkalis)
9. Covestro, Makrolon 2807 polycarbonate datasheet
10. Covestro, "Makrolon: the chemical resistance"
11. Altraset AS8125 electrical SMC datasheet (specific gravity, tensile modulus)
12. LyondellBasell, Premi-Glas SMC datasheet (density, thermal conductivity)
13. nVent HOFFMAN, Technical Information (material properties, heat transfer rates)
14. nVent HOFFMAN, Spec-00488 (unfinished aluminum and stainless run warmer; non-metallic similar to painted)
15. AutomationDirect, "How to Select and Size an Enclosure" (heat transfer coefficients by material)
16. Rittal, Thermal Management Solutions white paper (heat transfer coefficients)
17. Fibox, polycarbonate enclosure declaration (long-term operating temperature)
18. Fibox, enclosure materials
19. ASTM B117, Operating Salt Spray (Fog) Apparatus
20. Powder Coated Tough (Powder Coating Institute), powder coating strategies for corrosion resistance
21. Powder Coated Tough, "Pretreating for Outdoor Performance" (zinc phosphate standard for steel)
22. Powder Coated Tough, "A Guide to Powder Coatings for Extreme Service Environments"
23. CompositesWorld, UV surface degradation of fiberglass (fiber show-through)
24. UL Solutions, Understanding the UL 746 series (f1 / f2 outdoor ratings)
25. Interference Technology, new techniques for shielding EMI
26. Control Global, "Enclosures are more than a box"
27. Interstate Plastics, Polycarbonate fabrication guide (coolants and stress cracking)
28. Stahlin, cutting and drilling fiberglass enclosures (tools, speeds and feeds)
29. Mike Holt, NEC Article 250 commentary (314.3 non-metallic boxes, 314.4 and 250.148)
30. Euro Inox, Pickling and Passivating Stainless Steel (self-passivation)
31. SSINA, Machining stainless steels
32. BSI, IEC 62262 (IK code)