Introduction
In the world of plumbing, as in industrial and decorative installations, the choice of piping material determines the durability of the installation, its regulatory compliance and, of course, its overall cost. Between black steel pipes, which are valued for their strength, ease of welding and raw aesthetic appeal, and galvanised steel pipes, which are popular for their corrosion resistance thanks to their zinc coating, it can sometimes be difficult to decide without specific technical information.
At Home Invasion, we offer both product families: black steel threaded pipes and galvanised threaded pipes certified to EN ISO 1461. Each solution meets specific requirements in terms of pressure, fluid transported, environment and budget. To help you make an informed decision, we have analysed the manufacturing processes, standards (EN 10255, EN 10241) and actual performance of each of these finishes.
If you would like to understand the dimensions of the tubes, you can consult our complete table of steel tube diameters
This guide is designed to be both educational and comprehensive:
- Detailed presentation of the industrial steps that lead to a so-called "black" or "galvanised" tube.
- Comparison of their behaviour in terms of corrosion, weldability and operating constraints (drinking water, heating, gas, compressed air).
- Practical implementation guide, selection table, best practices for storage, threading and final protection.
Whether you are planning to create an outdoor irrigation network, a high-temperature heating circuit or an exposed loft decor, this guide will give you the keys to selecting the most suitable solution — and to fully exploiting the potential of the EN 10241 threadable tubes available in our shop.
Regulatory overview: EN 10255 & EN 10241
Summary:
- EN 10255: defines round steel tubes for welding or threading (L, M, H series).
- Common steel grades: S195T and S235JR, smooth or BSPT threaded ends.
- EN 10241: specifies compatible threaded fittings (DN 6 to DN 150).
- Indicative working pressures: up to 25 bar for tubes, 16-40 bar for fittings.
- Correspondence with other standards: EN ISO 1461 (galvanised), ISO 7-1 / EN 10226 (threads), EN 1775 (gas).
2.1 - EN 10255: the standard for "plumbing" tubes
EN 10255 covers round non-alloy steel tubes for welding and threading (formerly BS 1387). Three thickness series are provided:
| Series | Abbreviation | BS 1387 equivalent | Example Ø ext. DN 25 |
|---|---|---|---|
| Light | L | Light | 33.7 × 2.0 mm |
| Medium | M | Medium | 33.7 × 3.25 mm |
| Strong | H | Heavy | 33.7 × 4.0 mm |
The tubes may be welded (type W) or seamless (type S). Their external diameters and masses refer to EN 10220 to ensure interchangeability. The usual steel grades S195T or S235JR are supplied as-rolled; no heat treatment is required. The ends are supplied smooth (for welding) or threaded BSPT with screw-on sleeve.
Areas of application: industrial water, heating circuits, compressed air networks, sprinkler systems and gas pipes (in accordance with national regulations). Fordrinking water, an internal coating that complies with health requirements may be required.
2.2 – EN 10241: compatible threaded fittings
EN 10241 defines thick-walled threaded steel fittings – nipples, sleeves, elbows, tees, plugs, etc. – from DN 6 to DN 150. They are machined from EN 10255 tubing or forged, then threaded in accordance with ISO 7-1 / EN 10226-1: BSPT male tapered thread and BSPP female parallel thread.
Examples of common external diameters: 10.3 mm (DN 6), 13.7 mm (DN 8), 21.3 mm (DN 15) up to 168.3 mm (DN 150). Fittings are available in black finish (raw steel lightly oiled) or hot-dip galvanised. Other protective coatings are permitted provided they do not alter the geometry of the thread.
2.3 - Threads, pressures and compatibility
| Element | Reference thread | Indicative operating pressure |
|---|---|---|
| EN 10255 threadable tubes | BSPT ISO 7-1 (tapered) | Up to 25 bar (H series) |
| EN 10241 fittings | BSPT male / BSPP female | 16–40 bar depending on DN and class |
Good to know: the tapered thread ensures a metal-to-metal seal; PTFE tape or string provides the final barrier. On galvanised pipes, the thread is generally machined after galvanising to avoid excess zinc thickness and ensure precision.
2.4 - Correspondence with other standards
| Requirement | Related standard | Purpose |
|---|---|---|
| Zinc protection | EN ISO 1461 | Hot-dip galvanising |
| Threads | ISO 7-1 / EN 10226 | Threaded fittings with thread seal |
| Gas pressure tests | EN 1775 (formerly NF E 29-100) | Combustible gas installations |
By complying with these references, tubes and fittings purchased separately remain fully compatible – dimensionally and mechanically – whether you choose black steel or the galvanised version.
Black steel tubes
Summary:
- Composition: mild steel S195T / S235JR, two manufacturing processes (ERW welded or seamless).
- Finish: black oxide coating + temporary protection (oiling or workshop varnish).
- Advantages: low cost, excellent weldability, mechanical and thermal resistance, industrial look.
- Limitations: susceptibility to corrosion, internal deposits, weight, bare threads, not suitable for drinking water.
3.1 - Composition and manufacturing processes
So-called "black" tubes are rolled from mild carbon steel, grades S195T or S235JR. Example of typical composition for an S195T:
≤ 0.20% C · ≤ 1.40% Mn · ≤ 0.035% P · ≤ 0.030% S – giving a minimum yield strength of 195 MPa and a tensile strength of 320 to 520 MPa.
| Field | Main steps | Common uses (EN 10255) |
|---|---|---|
| ERW welded | Coil unwinding → "U-O" forming → longitudinal HF welding → calibration → hydraulic testing | L / M series – heating, gas, compressed air |
| Seamless | Hot billet perforation → mandrel rolling → calibration → testing | H series – high pressure, steam |
The tubes are delivered with smooth ends (construction weld) or BSPT threaded in the factory; the threading is carried out after cutting, degreased and then plugged for transport. No additional heat treatment is required: delivery as rolled preserves the yield strength and facilitates weldability.
3.2 - "Black" finish: mill scale and temporary protection
The dark appearance comes from a film of scale (iron oxides) formed during heating; it consists of three layers: FeO on the inside,Fe3O4 in the middle andFe2O3 on the outside. This layer is fragile and cracks on impact, allowing moisture to penetrate and rust to form.
- Oiling: mineral oil film applied after tube forming, providing protection for several weeks.
- Black workshop varnish: thin lacquer (epoxy or bitumen) sprayed on, protection during transport and short storage.
These finishes are strictly temporary. On site, the oil is removed before welding and an anti-rust paint or heat insulator is applied to exposed networks.
3.3 - Technical highlights
- Economy: lowest material price in the steel tube range, ideal for tight budgets or secondary networks.
- Weldability: no coating → no de-galvanisation or toxic fumes.
- Mechanical and thermal resistance: withstands high pressures and fluids up to ≈ 350°C (low-pressure steam, heating) without deformation.
- Versatility: hot bending, simple threading, compatible with all EN 10241 fittings.
- Industrial aesthetics: highly sought-after raw finish for loft and retail settings.
3.4 - Limitations and precautions for use
| Potential problem | Consequence | Recommended solutions |
|---|---|---|
| Rapid corrosion (water/condensation) | Loss of thickness, rusty water, sludge | Paint or sheathing after assembly; do not use raw drinking water; closed circuit inhibited |
| Internal deposits (rust, limescale) | Reduced flow, noise, heat loss | Rinsing and filtration during commissioning; inhibitor treatment |
| High weight | More demanding handling than copper/PEX | Hoists or trolleys for large DNs; prefabrication in workshop |
| Unprotected cut threads | Rust spots at joints | Zinc-rich coating or paint after tightening; tow + non-hygroscopic paste |
| Non-compliance with drinking water standards | Fe, Mn migration, bacterial proliferation | Choose stainless steel, galvanised or internally coated steel for DHW/DH |
In summary: black steel remains the most flexible and economical solution for heating, light steam or gas networks where corrosion is controlled. However, it requires serious surface protection and regular maintenance (rinsing, painting) to ensure its longevity.
Galvanised pipes
Summary
- Main process: hot-dip galvanising (EN ISO 1461) with zinc-iron reaction in four protective layers.
- Variants: electro-zinc, zinc flake, metallisation – useful for threading or local touch-ups.
- Advantages: greatly reduced corrosion, internal and external protection, virtually no maintenance, patinated grey appearance.
- Points to note: very hot/acidic water, zinc fumes during welding, "loaded" threads, white rust, Zn/Cu galvanic couples.
Detailed information on the galvanised steel pipes we offer can be found on our dedicated page.
4.1 - Hot-dip galvanising: metal-metal reaction
For EN 10255 / EN 10241 tubes,hot-dip galvanising is almost always used (EN ISO 1461 standard). After degreasing, acid pickling and fluxing, the tube is immersed for a few minutes in a zinc bath maintained at approximately 450°C. The liquid zinc then reacts with the iron to form, from the steel towards the outside:
- Γ-phaseFe3Zn10 (high hardness)
- δ phaseFeZn7
- ζ phaseFeZn13
- η-layer of pure zinc (50–70 µm for walls ≤ 3 mm)
This metallurgical stratification, which is much more resistant than a simple paint, ensures exceptional mechanical adhesion and acts asa sacrificial anode if the surface is cut. After cooling, the tubes are passivated (trivalent chromate or silane) to limit the appearance of "white rust" during storage.
Practical guide: a coating of 600 g Zn/m² (≈ 85 µm) guarantees an average of 30 years of protection in a moderate urban atmosphere before the steel is exposed.
4.2 - Other zinc coating variants
| Process | Typical thickness | Target use | Main advantage |
|---|---|---|---|
| Electro-zinc (electrolytic zinc coating) | 5–15 µm | Machined threads, small parts | Very smooth appearance, tight tolerances |
| Zinc flake (lamellar coating) | 8–12 µm | Fittings, screws | > 1000 hours salt spray, no embrittlementH2 |
| Zn metallisation (thermal spraying) | 60–120 µm | Local touch-ups, welded structures | Cold application, directly on site |
For long tubes carrying water or air, only hot-dip galvanising guarantees complete coverage, including on the inside. The above variants are mainly used to touch up a weld seam or to protect a thread made after the fact.
4.3 – Advantages of galvanised tubes
- Slow corrosion: zinc acts as a sacrificial anode and protects even if scratched.
- Double-sided protection: the inside of the tube is galvanised as well as the outside.
- Minimal maintenance: apart from very hot water (> 60°C), no repainting is necessary for decades.
- Cold water/sprinkler applications: prevents red rust and premature clogging.
- Workshop aesthetics: the initial shine develops into a matt grey patina, which is very popular in industrial design.
4.4 - Points to consider before choosing
| Risk/constraint | Preventive measure |
|---|---|
| Very hot or aggressive water (pH < 6 or > 9): accelerated dissolution of zinc | Limit the hot water temperature; prefer stainless steel or black steel (+ internal epoxy coating) |
| Toxic zinc fumes during welding | Grind the galvanisation 20 mm on either side of the joint; ventilation, FFP3 mask |
| Threads partially "loaded" with zinc | Thread after galvanising or calibrate the tap; remove chips before filling with water |
| White rust (zinc carbonates) in damp storage | Covered and ventilated storage; trivalent passivation after bath |
| Galvanic coupling Zn/Cu (mixed copper installation) | Install dielectric fittings to isolate metals |
These precautions will enable you to get the most out of the galvanised coating without any unpleasant surprises.
The next section provides a comprehensive comparison table of black and galvanised steel, enabling you to quickly select the most cost-effective and safest finish for your project.
Comparison table: black steel vs galvanised steel
- Summary of performance, costs and constraints for each finish.
- Service life: < 2 years without paint for bare steel compared to 40-80 years for an 85 µm galvanised tube.
- Maximum temperature: 350°C for black steel, 200°C for galvanised steel.
- Material cost: black steel is the most economical option; galvanisation adds ~15-30%.
- Typical uses: gas and closed heating for black; water, outdoor and fire protection for galvanised.
| Criterion | Black steel tubes | Galvanised steel tubes (hot-dip galvanised) |
|---|---|---|
| Product standard | EN 10255 (L, M, H series) – grades S195T / S235JR | EN 10255 identical + coating compliant with EN ISO 1461 |
| Original finish | Calamine + oil film or thin varnish | Zinc 50–85 µm (≈ 600 g/m²) on inner and outer surfaces |
| Corrosion protection | None: bare steel rusts quickly in humid conditions | Excellent: barrier + sacrificial anode |
| Time beforefirst maintenance s (ISO C3 atmosphere) |
< 2 years without paint | 40–80 years with 85 µm of zinc |
| Maximum recommended temperature | ≈ 350°C (low-pressure steam) | ≈ 200 °C (above this, zinc becomes brittle) |
| Weldability | Direct (MMA, MAG, TIG) without special precautions | Grind ± 20 mm of zinc, collect fumes |
| Threading | Easy to cut; protect with paste/linen | Thread cutting after bath or calibration + zinc touch-up |
| Material cost | Reference (low cost) | + 15-30% depending on diameter |
| Unit weight | Identical (steel base) | +≈1% (mass of zinc, negligible) |
| Typical uses | Gas, closed heating, loft decoration | Water ≤ 60°C, fire networks, exposed outdoor use |
| Major limitations | Rapid corrosion, not suitable for drinking water | Delicate welding; very hot or acidic water corrodes zinc |
| Maintenance | Periodic anti-rust paint | None until zinc is completely consumed |
Quick read: for a closed circuit (heating, dry compressed air), black steel is unbeatable in terms of budget. As soon as there is circulating water or outdoor exposure, galvanisation multiplies the service life and reduces overall maintenance.
5.1 – Focus: zinc thickness & longevity
- 50 µm of zinc ≈ 40 years without visible rust in a normal urban atmosphere.
- 85 µm of zinc increases durability to 70 years or more.
- The corrosion rate of zinc varies between 0.7 and 2 µm/year in a C3 environment:
thickness ÷ rate= first repair. - Conversely, bare steel loses 20-25 µm/year in a humid environment → paint protection required upon installation.
5.2 - What about electro-zinc?
Electro-zinc plating deposits only 5-15 µm of zinc: sufficient for threads (tight tolerances) or small parts, but insufficient for pipes continuously exposed to water or the elements. This technique is therefore mainly used for local touch-ups (threads or weld beads) rather than for protection over the entire length.
This overview of figures lays the foundations for the technical choice. In the next section, a decision tree will help you select the most cost-effective and reliable finish in just a few clicks, depending on the fluid, the environment and your overall budget.
How to choose? – Pragmatic decision tree
Summary:
- Step 1: identify the fluid (water, gas, air, steam) and its constraints.
- Step 2: analyse the environment (dry interior, humid exterior, corrosive atmosphere).
- Step 3: balance budget, maintenance and aesthetic or hygienic criteria.
- Quick tool: graphical decision tree + four practical cases to validate your choice.
6.1 – Step 1: identify the fluid and its constraints
| Main fluid | Questions to ask | Recommended orientation |
|---|---|---|
| Drinking water | Temperature ≤ 60°C? pH neutral? | Galvanised if the water is not aggressive, otherwise stainless steel or internally coated pipe |
| Heating water (closed circuit) | Inhibitor treatment required? | Black steel + external paint |
| Fire water/sprinkler | Permanent wet network? | Galvanised recommended: limits tuberculation & deposits |
| Natural gas/propane | Internal network? EN 1775 compliant? | Welded or threaded black steel; galvanised not required |
| Compressed air | Dew point < 5 °C? | Dry air → black steel; humid air → galvanised or stainless steel |
| Steam < 350 °C | Pressure < 16 bar? | Black steel (zinc not recommended > 200 °C) |
6.2 – Step 2: Characterise the environment
- Dry indoor environment: low corrosion → painted black steel is sufficient.
- Damp or outdoor environment: condensation/rain → galvanisation preferred (double-sided, no maintenance).
- Corrosive area C4/C5 (seaside, industrial site): salt spray, acid pollutants → galvanised + paint (duplex system) or stainless steel.
6.3 – Step 3: Balance budget and maintenance
| Project profile | Decisive criterion | Recommended finish |
|---|---|---|
| Tight CAPEX / OPEX tolerated | Minimal initial investment | Black steel + site paint |
| Long service life without maintenance | Difficult access, costly downtime | Galvanised (or even duplex): no repainting |
| Aesthetic appeal | Raw/loft look | Black painted or galvanised steel with a patina finish, depending on style |
| Hygiene/strict regulations | Drinking water, health | ACS-certified stainless steel or galvanised |
6.4 – Condensed decision tree
±— Drinking water? — yes — Aggressive water? — yes → Stainless steel
| |
| no → Galvanised
|
Fluid ±— Closed heating system? — yes → Black steel
|
±— Gas? — yes → Black steel (welded or threaded)
|
±— Air/Steam/Other — see Table 6.1
|
±— Outdoor environment? — yes → Galvanised
|
no → Black steel
Pro tip: if water circulates and comes into contact withair → galvanised; if the circuit is closed or containsgas → black steel, unless otherwise specified by health regulations.
6.5 - Quick application examples
| Specific example | Material selected | Why? |
|---|---|---|
| 2½" dry fire hose reel in underground car park | Galvanised | Permanent humidity, maintenance impossible |
| 90/70°C heating loop in boiler room | Black steel M series | Closed circuit, optimised budget, easy welding |
| 10 bar compressed air network with -20°C dryer | Painted black steel | Dry air, low corrosion |
| Pipe-shelf style suspended decoration | Transparent varnished black steel | Industrial look, no fluids |
Good implementation practices
Summary:
- Store tubes away from moisture and avoid any metal-to-metal contact.
- Cut, thread and deburr cleanly; always remove all filings.
- Select the appropriate seal and tightening torque for each DN.
- Prepare the areas to be welded correctly; treat and repaint after welding.
- Apply a final protective coating (paint or duplex system) and check thickness and tightness.
7.1 – Storage and handling
| Key point | Recommended use |
|---|---|
| Location | Dry, ventilated shelter, away from alkaline or acidic powders. |
| Position | Store horizontally on wooden or plastic slats; avoid steel-to-steel contact to preserve the zinc coating. |
7.2 – Cutting and threading
- Lubrication: sulphur-chlorinated oil for black steel; clear mineral oil for galvanised steel.
- Deburring: rotary burr or conical reamer – removes chips and preserves the seal.
- Cleaning: blow gun or dry air rinse; filings are theleading cause of leaks on valves.
To find out how to thread and assemble steel tubes, please refer to our comprehensive tutorial. You can also consult our guide to threading steel plumbing tubes here.
7.3 – Screw assembly
Sealing gaskets
- Standard PTFE: 40 µm, 8-10 turns on DN 15-25, clockwise.
- Flax + paste: use DN ≥ 40 for better filling.
- Gas: G1 certified paste + yellow tape compliant with EN 751-3.
Tightening torque & orientation
- Aim for 2-3 visible threads after tightening BSPT.
- Use socket wrenches (pipe wrenches); do not use Stillson wrenches on galvanised parts (they strip the zinc).
- For a tee, adjust the threaded length for alignment without locknut (½ turn margin with tow).
7.4 – Welding
| Steel | Pre-treatment | Recommended process | Special |
|---|---|---|---|
| Black | Degreasing + light brushing of scale | MAG solid wire G3Si1 Ø 1 mm, 180 A | Fast bead, little slag |
| Galvanised | Grinding of zinc to ± 20 mm; dust recovery | MAG rutile-coated wire Zn < 0.9% | Repaint the area with zinc-rich paint (> 92%) |
Construction tip: for DN ≤ 40, CuSi3 rod brazing (TIG brazing) reduces smoke and shrinkage while remaining compliant with ISO 17672 for low-pressure water/gas.
7.5 – Protection & finish after assembly
Black steel
- Sand the welds and degrease with acetone.
- Zinc phosphate epoxy primer ≥ 50 µm.
- Polyurethane finish: 2 coats, total thickness ≥ 120 µm.
Galvanised
- Clean with Scotch-Brite + neutral ammonia solution.
- Duplex system (C4/C5 areas): special zinc adhesion primer + polyurethane lacquer.
Colour coding
Apply ISO 20560-1 code: yellow for gas, red RAL 3000 for sprinklers, blue RAL 5012 for cold water, etc. Adhesive strips are acceptable if decorative paint is required.
7.6 – Inspections & tests
| Test | Threshold | Method |
|---|---|---|
| Hydraulic test | 1.5 × P service (≥ 10 bar heating) | Progressive increase, hold for 30 minutes, ΔP < 0.1 bar |
| Gas tightness test | 150 mbar or EN 1775 | Differential pressure gauge < 0.01 bar / 5 min |
| Visual inspection of coating | Paint continuity ≥ 98% | 400 lux LED lamp + angle mirror |
| Zinc thickness measurement | ≥ 50 µm on thin walls | Magnetometer gauge (3 points / 50 cm) |
Frequently asked questions
8.1 – Can I paint a galvanised tube?
Yes, provided that the surface is prepared: degrease (hot water + mild detergent), lightly sand with Scotch-Brite, then apply a special zinc primer. Polyurethane or acrylic lacquers will then adhere without blistering. Avoid paints rich in chlorinated solvents: they alter passivation.
8.2 – Is black steel tubing permitted for drinking water?
In France, no: the ACS positive list only authorises stainless steel, copper or steel with an approved internal coating. Bare black steel releases iron/manganese and promotes bacterial growth. For water supply, opt for stainless steel or ACS-certified galvanised pipe (DN > 15).
8.3 – How long does a galvanised network actually last?
In a C3 indoor environment (heated dwelling), an 85 µm coating loses approximately 1 µm/year: allow 60 to 80 years before the steel appears. In slightly hard cold water (pH ≈ 7.5), dissolution is slower; in hot water ≥ 60°C or very soft water, the lifespan drops to 25-30 years. On an unpainted C4 outdoor network, you can expect 30-40 years.
8.4 – How can you prevent stainless steel threads from seizing on steel fittings?
Insert a dielectric fitting (insulating nipple) or, if this is not possible, apply a PTFE paste with a high oil content and tighten moderately. Seizing is mainly caused by galvanic coupling and cold welding between stainless steel and steel threads.
8.5 – Can black and copper pipes be mixed in the same installation?
Yes, but always copper after steel in the direction of flow to prevent galvanic corrosion. Install an insulating fitting if the copper must be upstream, or maintain at least 300 mm of plastic pipe between the two metals.
8.6 – What tightening torque should be applied to a Ø 1" BSPT thread?
There is no universal standard value; the industry recommends manual tightening + 2 to 3 turns with a wrench. For a DN 25 M series steel pipe, this corresponds to ≈ 120 N·m with PTFE tape (slightly less with hemp). Tighten until 1-2 threads are visible; any more than this and you risk splitting a cast iron fitting.
8.7 – How to touch up galvanisation after welding?
Brush off the scale, then apply a zinc-rich paint (≥ 92% Zn metal) in two cross coats, each 50 µm minimum. This touch-up restores cathodic continuity. For areas subject to heavy wear, use spray metallisation.
8.8 – What varnish should be used to preserve the "raw steel" appearance?
Use a single-component, UV-resistant, non-yellowing satin polyurethane varnish. Apply it to a thoroughly cleaned and degreased tube; three thin coats create an invisible film that fixes the anthracite colour of the metal.
Conclusion – getting the most out of black and galvanised steel
In plumbing and interior design, no material is "best" in all circumstances: it all depends on the fluid, the environment and the acceptable frequency of maintenance. Our analysis shows that:
- Black steel: unbeatable when it comes to thermal conductivity, weldability and cost. Ideal for closed networks (heating, dry air, gas) or decorative projects with a raw aesthetic – a simple paint/varnish cycle is then sufficient to control external corrosion.
- Galvanisation: provides unrivalled structural longevity in circuits in prolonged contact with water or exposed to the elements. Its dual action (barrier + sacrificial anode) delays maintenance operations by several decades, reducing the overall cost of ownership despite a moderate initial investment.
- The decision tree (section 6) and best practices (section 7) summarise the logic behind the choice:
→ Water + outside air? → Galvanised.
→ Closed circuit + tight budget? → Black steel.
→ Severe corrosive environment → duplex galvanised or stainless steel.
Trade tip: in a mixed network, alternate between the two ranges if necessary; simply use an insulating connector or follow the order 'steel then copper/stainless steel' to avoid any galvanic coupling.
Now it's up to you!