Most enclosure quotations state the finish in one line: powder coated, RAL 7035. That line decides whether a cabinet lasts three years outdoors or ten, and whether the earth studs actually earth anything. Finish is only 5–15% of enclosure cost, yet it causes most corrosion failures and warranty claims.
This guide compares the six finishes used on sheet metal cabinets and enclosures, and shows which one to specify for a given service environment.
1. Six finishes at a glance
Salt spray figures are reference ranges for neutral salt spray (NSS, 5% NaCl, 35 °C, ISO 9227). Results depend on pre-treatment, film thickness and rating method, so treat them as selection guidance, not a warranty figure.
| Finish | Substrate | Thickness | NSS reference | Cost | Typical use |
|---|---|---|---|---|---|
| Powder coating | CRS, galvanized, aluminium, stainless | 60–120 µm | 500–1000 h | Low | Indoor and outdoor cabinets |
| Wet paint | Same | 25–60 µm | 240–500 h | Medium | Small runs, special or metallic colours |
| Zinc plating | Cold rolled steel | 5–25 µm zinc | 72–240 h | Low | Mounting plates, brackets, hidden parts |
| Galvanized sheet Z275 | Steel | 275 g/m² (~20 µm/side) | 300–600 h | Low–med | Outdoor and coastal substrate |
| Anodizing (sulphuric) | Aluminium alloys | 5–25 µm (hard 25–50) | 500–1000 h+ | Medium | Aluminium housings, heat sinks |
| Passivation | Stainless 304 / 316L | Nanometres to 1 µm | 48 h → 200–500 h+ | Low | Stainless cabinets, semiconductor, food |
| Blasting / brushed | All metals | No added thickness | No gain alone; needs passivation or coating | Low | Weld dressing, decorative texture |
2. Powder coating: the default answer
Sequence: pre-degrease, degrease, two-stage rinse, acid pickling or phosphating, DI rinse, dry-off, electrostatic spray, cure at 180–200 °C for 15–20 minutes.
- Pre-treatment drives about 70% of service life. Powder sprayed straight after blasting gives weaker adhesion; phosphating or a chrome-free silane/zirconium pre-treatment lifts results by a full grade.
- Match the powder to the environment. Epoxy bonds best but chalks under UV; polyester for outdoors; epoxy-polyester hybrids indoors; polyurethane where UV and chemicals both matter.
- Thicker is not better. 60–80 µm indoors, 80–120 µm outdoors; above about 150 µm expect cracking and heavy edge build.
- Mask before coating. Cured powder insulates. Earth studs, EMC bonding faces, threaded holes and gasket faces must be masked; serrated washers are a supplement, not a primary earth path.
- Limits: no gradients, colour changes need a full line flush, and rework means stripping the part.
3. Zinc plating and galvanized steel
- Two routes: plate cold rolled steel after fabrication, or start from electro-galvanized SGCC or hot dip galvanized DX51D+Z sheet.
- Read the designation. Z275 is 275 g/m² total, about 20 µm per side; Z120 and Z180 step down. More zinc lasts longer but cracks more at tight bends.
- Cut edges are the weak point. Cutting, bending, welding and tapping break the zinc layer, and a cabinet built from bare galvanized sheet usually rusts at cut edges within one to two years. Use galvanized sheet plus powder as a duplex system and touch up welds with zinc-rich paint.
- White rust is not red rust. White rust is surface zinc corrosion product and wipes off; red rust means the steel itself is corroding.
- Best value: mounting plates, DIN rails, brackets and cable ducts, which need no coating.
4. Anodizing: right for aluminium, three mistakes
- Sulphuric anodizing gives 5–25 µm, clear, black or dyed. Hard anodizing gives 25–50 µm at HV 300–500 for rails and wear surfaces.
- Mistake 1 — the film insulates. Parts that must earth cannot be fully anodized. Mask those areas, or use conversion coating, which is thin and still conductive.
- Mistake 2 — the film grows the part. About half the oxide grows outward, so precision bores and slots need 0.01–0.02 mm allowance.
- Mistake 3 — colour varies with alloy. 6061, 5052 and 6063 come out differently. Specify same-heat, same-load production and a colour tolerance.
5. Passivation: mandatory for stainless steel
Stainless resists corrosion through a chromium-rich passive film a few nanometres thick. Welding and machining deplete chromium at the surface and add iron contamination — that is the source of rust at weld beads. Skipping passivation means giving up corrosion resistance.
- Pickling paste is cheapest for local weld dressing, but residue must be rinsed thoroughly.
- Tank pickling suits small parts in batches and needs bath maintenance.
- Electrolytic passivation is fast, uniform and dimensionally neutral — the usual method for stainless enclosures.
- Citric passivation is preferred for food, medical and semiconductor work.
Typical gain: polished 304 rusts within tens of hours of NSS, passivated 304 reaches 200 h or more, and good electrolytic passivation reaches the 500 h range; 316L goes higher. Passivation only works on a clean surface, and passivated stainless should not be cleaned with chlorine-based agents. In coastal installations 304 fails far more often than 316L.
6. Blasting, brushing and ultrasonic cleaning
- Blasting removes weld discolouration, scale and rust and improves adhesion; stainless must be passivated afterwards, or embedded media becomes pitting sites.
- Brushing creates a decorative grain; stainless still needs passivation, brushed aluminium is anodized.
- Ultrasonic cleaning removes oil and chips before coating or passivation, and is controlled for semiconductor and gas cabinets.
7. Five questions that decide the finish
- Corrosivity category? Indoor dry (C1/C2), general outdoor (C3), coastal or heavy industrial (C4), chemical or marine (C5). This outranks colour.
- Substrate? Aluminium must be anodized or conversion coated; stainless must be passivated. There is no substitute for either.
- Earthing or EMC bonding? If yes, plan the masked and conductive areas up front.
- Cosmetics? Specify the RAL number, gloss level and colour tolerance.
- Batch and lead time? Liquid paint suits small multi-colour runs; powder is cheaper at stable single-colour volume.
8. Category and recommended specification
| Category | Typical environment | Recommended specification | NSS target |
|---|---|---|---|
| C1 / C2 indoor | Switch rooms, control cabinets | Powder coated CRS at 60–80 µm, or zinc plated sheet | 240–480 h |
| C3 general outdoor | Outdoor cabinets, EV charger housings | Galvanized sheet plus polyester powder at 80–120 µm | 500–720 h |
| C4 coastal / industrial | Coastal sites, chemical parks, ports | Galvanized sheet with powder topcoat, or 304 electrolytic passivation | 720–1000 h |
| C5 chemical / marine | Offshore, aggressive acid or alkali | 316L electrolytic passivation, or duplex system with heavy-duty topcoat | 1000 h+ |
9. How to read a salt spray report
- Standard: ISO 9227 and GB/T 10125 are equivalent, ASTM B117 is the US reference. NSS (5% NaCl, 35 °C, pH 6.5–7.2) is the default; AASS and CASS are stricter.
- Rating: protection rating (Rp) grades substrate corrosion by area percentage, appearance rating (Ap) describes coating changes. Hours without a rating tell you nothing.
- Do not over-read it: 1000 h of salt spray is not 1000 days outdoors; aluminium and stainless correlate poorly with real exposure, so carbon steel experience cannot be extrapolated; and whether discolouration counts as a failure must be agreed in writing.
10. Common defects and their causes
- Blistering or flaking — residual oil, rust or rinse water; control degreasing, rinses and dry-off.
- Craters, fish eyes — oil or water in compressed air, or silicone contamination; fit oil-water separators.
- Orange peel — excessive film build or wrong cure profile; hold 60–120 µm, follow the cure curve.
- Colour variation — mixed powder lots; use one lot per batch and agree a colour tolerance.
- Salt spray failure — incomplete phosphate film, low thickness, poor edge coverage; check thickness, touch up edges.
- Zinc cracking at bends — over-thick zinc or too small a radius; keep the radius at least the material thickness.
- Rust at stainless welds — no passivation, chromium-depleted zone, iron contamination; pickle or passivate after welding.
11. What to state on your RFQ
- Service environment and expected corrosion life
- Colour (RAL), gloss level and colour tolerance
- Film thickness requirement and test method
- Areas to be masked: earth studs, threaded holes, gasket faces, precision fits
- Substrate, and whether a duplex galvanized-plus-powder system is required
- Salt spray standard, duration, rating, third-party reports yes or no
12. FAQ
What finish should a standard enclosure use?
Powder over cold rolled or galvanized steel for indoor cabinets. Outdoors, galvanized sheet with polyester powder at 80–120 µm. Coastal and chemical sites need 304/316L electrolytic passivation or a duplex system.
Powder coating or wet paint?
Powder gives higher film build, better wear resistance and no solvents. Wet paint is thinner, more flexible and handles special or metallic colours in small runs. Most of the corrosion difference comes from film thickness and pre-treatment, not the method.
Can a powder coated part still be earthed?
No, cured powder is an insulator. Earth studs and EMC bonding faces must be masked, or the part must use a conductive finish such as conversion coating.
Is 500 hours of salt spray good?
For indoor and general outdoor service, yes. Coastal and chemical sites should target 720–1000 h with a duplex system. Read the protection rating, not just the hours.
Why does stainless steel still rust?
Welding destroys the chromium-rich passive layer, and iron contamination plus chlorides finish the job. Passivate after welding and specify 316L for coastal projects.
Can we weld or tap after powder coating?
No. Welding burns off the coating and tapping drags chips across it. All welding and tapping must happen before surface treatment, which is the last operation.
About Xingyuan Electric
Xingyuan Electric (Suzhou) Co., Ltd. is based in Lili Town, Wujiang District, Suzhou, China, with over 5,000 m² of production space and a 52-person team. We run 20+ machines for laser cutting, CNC bending, welding and clinching, and provide powder coating, wet painting, anodizing, electrolytic passivation and ultrasonic cleaning in house, keeping the whole chain from blanking to final finish under one roof. Standard lead time for cabinets and sheet metal parts is 7–15 days, and we build to drawing or sample for customers worldwide. Send us your service environment, substrate and required corrosion life, and our engineering team will return a specific process route and parameters.