At first glance, galvanised and black steel pipes look very similar.
Both are made of steel, they are produced in similar dimensions, they can have threaded joints and they are used in various technical installations.
The main difference is not in the base material itself but in the protection of the surface against corrosion.
A black steel pipe is practically unprotected steel, while a galvanised pipe is covered with a layer of zinc whose task is to slow down the corrosion of the base steel material.
What is a black steel pipe?
The name "black pipe" does not denote a special kind of steel.
It is the usual name for a steel pipe that has no protective zinc layer.
The dark colour of the surface comes from the way it is produced, from the oxide layer and from the condition of the steel surface.
The base material is still steel.
If such steel is exposed to water and oxygen, corrosion sets in over time.
That is why black steel pipes are used where the working conditions and the designed purpose allow it, while for drinking water installations they traditionally did not have the same role as galvanised pipes.
What does it mean that a pipe is galvanised?
A galvanised steel pipe has a layer of zinc on its surface.
In classic hot-dip galvanising, the steel item is exposed to molten zinc, and a protective layer is formed that is metallurgically bonded to the base steel surface.
Zinc has two important roles.
The first is physical:
it separates the steel from water, moisture and oxygen.
The second is electrochemical:
zinc is less noble than steel and can corrode before it does.
Because of that, even smaller local damage to the zinc layer need not immediately lead to the same degree of steel corrosion as on a completely unprotected surface.
Why is zinc a good protection?
If the protection were only an ordinary coating, every scratch would immediately expose the steel to corrosion.
With zinc there is also so-called sacrificial protection.
In an electrochemical pair, zinc has a greater tendency to oxidise than steel.
That is why, under suitable conditions, the zinc will corrode first and thereby protect the neighbouring steel surface.
This is a principle similar to the one for which a magnesium anode is used in an enamelled water heater, although the specific materials and conditions differ.
So does a galvanised pipe not rust?
It does.
Galvanising does not make steel indestructible.
It only slows corrosion down considerably.
Over time the zinc can:
- dissolve;
- wear away;
- be mechanically damaged;
- be chemically attacked;
- be covered with deposits;
- be lost locally.
When the protective layer is no longer sufficient, corrosion of the base steel begins.
That is why in old buildings it is not unusual to find galvanised water pipes that after several decades have corroded heavily on the inside.
A pipe can look good on the outside and be bad on the inside
With old galvanised water installations, one of the main problems is that the external appearance does not necessarily show the condition inside.
Inside, over the years, the following can develop:
- corrosion;
- mineral deposits;
- corrosion products;
- a reduction of the clear cross section of the pipe.
A pipe that nominally had a certain internal diameter can, after several decades, have a considerably smaller free area for flow.
The consequence is a weak water flow.
So with an old galvanised installation the problem is sometimes not low mains pressure, but simply a pipe whose internal cross section is almost closed by deposits.
Why are old threaded joints hard to take apart?
Galvanised steel installations were traditionally often made with threaded joints.
After many years the thread is acted upon by:
- corrosion;
- mineral deposits;
- remains of the sealing material;
- mechanical stress.
The joints can practically "grow together".
That is why dismantling an old galvanised installation is sometimes not a simple unscrewing of a fitting.
If great force is applied, it can happen that:
- the fitting cracks;
- the old pipe turns inside the wall;
- a new leak opens;
- a neighbouring joint gives way.
With old installations it is therefore necessary to assess not only the part we want to replace, but also the condition of the surrounding pipework.
What happens when we cut a thread on a galvanised pipe?
This is an important detail.
The pipe is protected by zinc on its surface.
When we machine a new thread on it, the tool removes part of the material and exposes the base steel on the crests and flanks of the thread.
At that place we no longer have the same continuous galvanised layer as on the undamaged part of the pipe.
That is why a threaded joint is one of the more sensitive zones of a galvanised installation.
Still, that does not mean that every cut thread must immediately corrode and fail.
The life of the joint depends on:
- the quality of the pipe;
- the quality of the thread;
- the way it is sealed;
- the composition of the water;
- the temperature;
- the presence of oxygen;
- the material of the fitting.
And what about a black fitting on a galvanised pipe?
Mechanically, the thread can fit perfectly.
So it is possible to screw a black steel fitting onto a galvanised pipe and get a watertight joint.
But by doing that we have introduced a piece of unprotected steel into the water installation.
That part is in contact with water and is subject to corrosion far more directly than a galvanised fitting.
For that reason, when repairing a galvanised water installation, a fitting of the appropriate material and purpose is chosen as a rule, not just any fitting that has the same thread.
The fact that two parts can be joined mechanically does not mean that they are an equally good solution in terms of material.
Does black pipe corrode the same way in every system?
No.
The corrosion of steel depends greatly on the environment.
Different conditions exist in:
- a drinking water system;
- a closed heating system;
- a gas installation;
- a compressed air system;
- the outdoor atmosphere.
For example, in a well sealed heating system the amount of dissolved oxygen can be limited after an initial period, so the corrosion conditions differ from an installation into which fresh, oxygen-rich water constantly enters.
So the material of the pipe is not chosen only by the question:
"Does steel rust?"
but according to the specific medium and working conditions.
Why are galvanised pipes used less in new domestic water installations today?
Galvanised steel was the standard solution for water installations for decades.
Today there are systems that are:
- lighter;
- faster to install;
- more resistant to internal corrosion;
- simpler to work with;
- more suitable for concealed installation.
That is why various plastic and multilayer systems, copper and other solutions are often used in new buildings, depending on the purpose.
That does not mean that every old galvanised installation is automatically faulty.
But when repairing it, one has to take into account that we are often working with pipes several decades old.
Why does age matter for a small repair?
Replacing a single valve on a new installation is usually a local job.
On an old galvanised installation the same operation can open a far bigger problem.
When unscrewing a valve, it can happen that:
- a corroded pipe in the wall is set in motion;
- a weakened thread is damaged;
- a leak opens at a neighbouring joint;
- it turns out that the pipe wall is already very thin.
With old steel installations there is therefore not always a clear boundary between a "small repair" and the beginning of replacing a larger part of the pipework.
A tradesman cannot give the metal back the thickness that corrosion has already taken away.
Galvanised pipe and drinking water
Historically, galvanised steel pipes were widely used for distributing sanitary water.
However, in old installations the following become significant over time:
- internal corrosion;
- deposition;
- a drop in flow;
- a change in water quality;
- a growing risk of mechanical failure.
That is why, with a very old galvanised installation, there is often not much economic sense in endlessly replacing individual fittings if the rest of the system is already at the end of its service life.
Sometimes it is more correct to replace the problematic branch or the whole installation.
The essential point
A galvanised and a black steel pipe are basically very similar products.
The main difference is:
black steel → base material practically without galvanised anti-corrosion protection
galvanised steel → a steel pipe protected by a layer of zinc
Zinc does not prevent corrosion for ever, but it slows it down considerably and gives the steel additional electrochemical protection.
So the choice between black and galvanised steel is not a question of the colour or the look of the fitting.
It is a question of:
in what environment the material will work, what the installation is intended for and how different materials will age under those conditions.
And it is exactly when different metals are joined that another important problem appears, galvanic corrosion.
