Introduction: Galvanised and Stainless Steel Metal Conduits
A metal conduit is a flexible protective sheath for electrical cables, used in conditions of vibration, mobility, and mechanical stress. The two main versions based on strip material are galvanised steel and stainless steel. Both types are spirally wound metal strips, often with a PVC polymer coating for tightness. The difference in material determines corrosion resistance, service life, and permissible operating environments.
Galvanised metal conduit is made from steel strip coated with zinc. It is cheap, widely available, and suitable for most industrial and domestic applications. Stainless steel conduit is made from alloyed steel with a high chromium content, providing resistance to aggressive environments, high temperatures, and seawater. For detailed information on flexible metal conduits of different types, see the article flexible metal conduit, and about sealed versions — in the material sealed metal conduit. A comparison of sealed and unsealed options is given in the article or sealed.
Comparison Table: Galvanised vs. Stainless Steel Metal Conduits
| Parameter | Galvanised Metal Conduit | Stainless Steel Metal Conduit |
| Strip material | Steel, galvanised by Sendzimir process | Stainless steel AISI 304, 316 |
| Corrosion resistance | Medium; zinc deteriorates in aggressive environments | High; resistant to acids, alkalis, seawater |
| Temperature range | −40 to +100 °C (with PVC coating up to +60 °C) | −60 to +400 °C (without coating), with PVC up to +60 °C |
| Tensile strength | High | Very high |
| Flexibility | High | Medium, slightly stiffer |
| Tightness | Possible with PVC coating IP65–IP67 | Possible with PVC coating IP65–IP67 |
| UV and weather resistance | Medium; zinc oxidises | High; no coating required |
| Cost | Lower | 3–5 times higher |
| Service life | 10–15 years in normal environment | 25–30 years and more |
| Main applications | Dry rooms, concealed wiring, general machinery | Food, chemical, marine, high‑temperature |
Material and Corrosion Resistance
Galvanised metal conduit is produced from cold‑rolled steel strip with a zinc coating applied. Zinc protects the steel from corrosion, but it gradually breaks down in humid, acidic, or saline environments. In dry heated rooms, such conduit lasts for decades without visible changes. When laid outdoors, in basements, or in plants with acid or salt fumes, the zinc layer quickly thins, exposing the steel and causing rust.
Stainless steel conduit is made from alloyed steel with a chromium content of at least 10.5%. A self‑healing oxide film forms on the surface, protecting against corrosion even with mechanical scratches. Grades AISI 304 and AISI 316 withstand food acids, seawater, alkalis, and high temperatures. Therefore, such conduits are installed in food production, pharmaceuticals, on ships, and in the chemical industry. The price of stainless steel is several times higher, but it is justified in conditions where galvanised steel fails.
Flexibility, Strength, and Bending Radius
Both types of metal conduit have a spiral construction that provides flexibility. Galvanised conduit, made from softer steel, bends more easily, with a bending radius of 3–4 outer diameters. This is convenient for installation in tight spaces, behind equipment, and in moving parts. Stainless steel is stronger and stiffer, so its bending radius is slightly larger — 4–5 diameters. At the same time, stainless steel holds its shape better, does not crush under pressure, and withstands higher tensile loads.
Tensile strength is higher for stainless steel conduit due to the alloyed steel. If the cable route is subject to significant tensile forces, for example, during vertical installation with a heavy cable mass, stainless steel is preferable. Galvanised conduit is also strong, but under excessive load it may deform or tear at the seam. For most normal conditions, its strength is sufficient.
Tightness and IP Protection
Tightness of metal conduit is achieved by applying a polymer coating of PVC compound. This version provides IP65–IP67 protection, completely preventing ingress of water and dust. Galvanised conduit with PVC coating is the most common sealed product: it is cheap and covers the needs of wet rooms, outdoors, and underground laying. Stainless steel with PVC coating is used where, in addition to tightness, chemical resistance or high temperature is needed. It is important that the PVC coating itself limits the upper temperature to +60 °C, so for hot environments conduits without a polymer layer or with silicone coating are used.
For connection to equipment enclosures, flanged or threaded fittings are used. Sealed metal conduit with a flange allows quick and secure attachment to a cabinet, junction box, or machine. The flange is sealed with a rubber gasket, maintaining the protection rating. For stainless steel conduits, flanges are also made of stainless steel, eliminating galvanic coupling.
Temperature Range and Operating Environment
Galvanised metal conduit without polymer coating operates in the range from −40 to +100 °C. With PVC coating, the upper limit drops to +60 °C, and for low‑temperature modifications the lower limit is −40 °C. For most temperate climates this is sufficient. Stainless steel conduit without coating withstands from −60 to +400 °C, while with coating it is limited by the polymer temperature. High‑temperature stainless steel conduits are used in metallurgy, near furnaces, steam lines, and exhaust systems.
The operating environment is the decisive factor. In a neutral atmosphere, galvanised steel lasts a long time. In the presence of sulphurous gases, chlorides, or sea salt, zinc deteriorates, so stainless steel is used. In food production, where the conduit is washed with alkaline solutions, galvanised steel quickly darkens and corrodes, while stainless steel remains inert. Where sanitary steam cleaning is required, steel conduits made of AISI 316 also withstand repeated washing.
Areas of Application
Galvanised metal conduit is a universal product for dry and moderately humid rooms, concealed wiring, and protection of cables from mechanical damage. It is used in residential and office buildings, warehouses, and light industry. It is suitable for cable routes in machine tools, suspended ceilings, and behind drywall. Due to its low price, it protects most non‑critical lines.
Stainless steel metal conduit finds application in the food, pharmaceutical, chemical, oil and gas industries, as well as in marine facilities and subways. Where aggressive environments, high humidity, salt spray, and frequent temperature fluctuations are present, galvanised steel fails, and the choice leans toward stainless steel. High‑temperature versions without coating protect cables near boilers, drying chambers, and furnaces. Stainless steel corrugated conduit is also used as a flexible insert for high pressure, although that is a separate category.
When to Choose Galvanised Metal Conduit
- Dry and moderately humid rooms with temperatures up to +60 °C.
- Concealed electrical wiring in walls, floors, under suspended ceilings.
- Cable routes in standard machine tools, conveyors, without aggressive media.
- Budget projects where cost savings are important without compromising basic protection.
- Open laying in areas without chemical exposure.
When to Choose Stainless Steel Metal Conduit
- Food processing plants with wet cleaning and detergents.
- Chemical and pharmaceutical industries with aggressive fumes.
- Marine vessels, ports, dock facilities with salt spray.
- High temperatures above +100 °C, e.g., near furnaces and steam lines.
- Facilities with high sanitary requirements and frequent washing.
Typical Mistakes in Selection
- Using galvanised conduit in a marine climate: zinc degrades within 2–3 years, leaving the cable unprotected.
- Using galvanised conduit in food production: corrosion contaminates the product and fails sanitary inspection.
- Overheating PVC coating above +60 °C: the plastic melts, losing tightness.
- Connecting stainless steel conduit to galvanised fittings: creates a galvanic couple that accelerates corrosion.
- Skimping on stainless steel in a high‑temperature zone: galvanised at +200 °C loses zinc and deforms.
- Purchasing stainless steel conduit without a certificate of steel grade: low‑quality steel may be sold under the guise of AISI 304.
Frequently Asked Questions
What is the difference between galvanised and stainless steel metal conduit?
The strip material: galvanised uses steel with a zinc coating, stainless uses alloyed steel. Stainless is resistant to corrosion, high temperatures, and aggressive environments, but costs more.
Can galvanised conduit be used outdoors?
In a temperate climate — yes, with a sealed PVC coating. In marine or chemically active air, galvanised corrodes quickly; it is better to use stainless steel.
Which metal conduit withstands high pressure?
Special corrugated stainless steel hoses designed for pressure are used for gas and hydraulic systems. Electrical metal conduit is not intended for that purpose.
What does a diameter of 20 mm for metal conduit mean?
This is the nominal outer diameter, selected with a 30–50% margin over the cable diameter. For medium‑section cables, 20 mm is the most common size.
Is PVC coating necessary on stainless steel conduit?
Not always. Without coating, stainless steel works at high temperatures and in aggressive environments where polymer is not needed. If tightness is required, PVC‑coated versions are used.
What is the service life of galvanised and stainless steel conduit?
Galvanised — 10–15 years in normal environments; stainless — 25–30 years or more. In aggressive conditions, the difference is even more pronounced.
Conclusion
The choice between galvanised and stainless steel metal conduit comes down to the operating environment and budget. Galvanised steel covers the needs of most domestic and general industrial applications; stainless steel is indispensable in aggressive, humid, and hot environments. The extra cost for alloyed steel pays off through service life and the absence of failures. When deciding, analyse the chemical composition of the environment, temperature, humidity, and sanitary requirements. The right choice ensures cable protection for decades.
