Junction Box Material Selection: Steel, Stainless, Aluminium and Plastics
Junction Boxes Anjue Metal | Electrical | 10 min read | Updated Oct 13, 2026
A junction box looks like the simplest thing in a project, and it is where a surprising number of failures start. The box that rusts away on a coastal wall, the plastic box that goes brittle in the sun and the steel box that is wrong for the washdown bay all have one thing in common: the material was chosen before the environment was described. The material is the box.
Describe the Environment First
| Environment | Common choice | Watch out for |
|---|---|---|
| Dry, indoor, sheltered | Coated steel or ABS | Cost, not much else |
| Indoor, damp or unheated | Coated steel or polyester | Condensation inside the box |
| Sheltered outdoor | Coated steel or polyester | UV and temperature cycling |
| Exposed outdoor | Stainless, aluminium or GRP polyester | UV, salt and water |
| Coastal or marine | Stainless 316L or GRP polyester | Chlorides; grade matters |
| Washdown or chemical | Stainless or GRP polyester | Cleaning agents and hot water |
| Hot environment | Metal, not plastic | Plastic softening and ageing |
| High mechanical risk | Steel or stainless | Impact and stiffness |
Coated Steel
- The economical default for indoor and sheltered positions.
- Its life depends on the coating system, not on the steel alone.
- Cut edges and fixing holes are where corrosion starts; specify how they are protected.
- Easy to form, thread and repair, which matters in service.
- Not the right answer where chlorides or washdown are constant.
Stainless Steel
- Chosen for corrosion, washdown and hygiene, and for long service life.
- 304 covers many situations; 316L where chlorides are present.
- Not immune to attack; the wrong grade in a chloride environment still pits.
- Fabrication matters: contamination from carbon steel tooling can start corrosion.
- Decide the surface finish with cleaning and appearance in mind.
Aluminium
- Light, and useful where weight is the constraint.
- Corrodes in a different way from steel and can suffer in chloride environments unless protected.
- Watch galvanic corrosion where it meets steel or stainless in a wet location.
- Softer threads, so fixings need care.
- A reasonable choice for indoor and dry outdoor positions with a suitable finish.
Polyester, GRP and ABS
- Do not corrode, which is why they are common for small outdoor boxes.
- UV stabilisation decides whether the box survives sunlight.
- Temperature tolerance is lower than metal; check the site's extremes.
- Stiffness and impact are limited compared with steel, especially at larger sizes.
- The gasket and the lid material matter as much as the body.
Comparing the Options
| Material | Corrosion | UV | Impact | Temperature | Relative cost |
|---|---|---|---|---|---|
| Coated steel | Depends on coating | Good | Good | Good | Low |
| Stainless 304 | Very good | Excellent | Very good | Excellent | Medium |
| Stainless 316L | Excellent | Excellent | Very good | Excellent | High |
| Aluminium | Good with finish | Excellent | Moderate | Very good | Medium |
| GRP polyester | Excellent | Good | Moderate | Moderate | Medium |
| ABS / PC | Very good | Depends on grade | Low to moderate | Limited | Low |
Gaskets, Entries and the Details That Decide Life
- Match the gasket material to the temperature and the chemicals, not only to the protection class.
- Provide entry plates or defined entries, so that the box is not drilled badly on site.
- Keep the lid sealing surface clear of damage and of coating build-up.
- Protect fixings against corrosion, or the box will fail at its fasteners first.
- Keep drainage in mind where a box is mounted on a horizontal surface.
Checks on the Build
| Check | Method | Why it matters |
|---|---|---|
| Material and grade | Certificate or marking check | Corrosion performance |
| Finish and cut-edge protection | Visual | Corrosion starting points |
| Gasket fit and compression | Fit test | Ingress protection |
| Entry plate fit | Fit check | Cable entry |
| Threads and fixings | Fit test | Assembly and service |
| Marking and labelling position | Visual | Identification |
How We Approach Junction Boxes
- We fabricate bodies, lids, entry plates, gasket channels and mounting parts in steel, stainless and aluminium, and we work to the finish specification you set.
- We will match the material and grade to the environment you describe, and tell you when a choice and an environment do not go together.
- We do not supply the plastic mouldings, the gaskets, the glands, the terminals or the electrical contents, and we do not certify the protection class of the finished assembly.
- Our plastic references above are given as comparison guidance, not as products we mould in house.
- Every quotation states which operations are completed in our plant.
Send the environment, the service life you expect and the entry arrangement, and the box can be made from the material that will still be there at the end of it.
Frequently Asked Questions
There is no single best material, only a best material for an environment. Coated steel is fine in many sheltered outdoor positions; stainless is the right answer where corrosion or washdown is real; aluminium suits weight-sensitive installations; and glass-reinforced polyester and UV-stabilised plastics are widely used for small outdoor boxes because they do not corrode at all. The decision should be made from the environment and the service life you expect, not from a generic rule.
When the environment is hot, when the box is large and needs stiffness, when impact or mechanical protection matters, or when the finish has to survive mechanical abuse. Plastics also behave differently when they burn, and some installations require a metal box for that reason. Plastic is excellent within its limits; the problem is using it outside them.
For coated steel, yes, and often more. A coating system chosen for the environment is what keeps the steel alive, and a decorative finish will not do the job. The preparation, the coating type and the thickness matter, and so does protection of cut edges, which are usually where corrosion begins.
Grade follows the environment. The common 304 covers many outdoor and washdown situations, and the molybdenum-bearing grades such as 316L are chosen for chloride exposure, coastal air and certain chemicals. Stainless is not indestructible: the wrong grade in a chloride environment can still show tea staining or pitting, which is why the site matters more than the material name.
We fabricate the box, the cover, the gasket provisions, the entry and the mounting parts to your drawing, including the finish specification you set. The glands, the terminals and the electrical contents are supplied by others. Every quotation states which operations are completed in our plant.
Explore Related Capabilities
- Waterproof Junction Boxes — IP-rated boxes for cables and terminals
- Stainless Steel Fabrication — Components fabricated in 304 and 316L
Have a Drawing to Quote?
Send STEP, DXF or PDF drawings and we reply with engineering feedback and a detailed quotation within 24 hours on working days.