Galvanised or black malleable iron fittings?

It is the same casting. Galvanised iron (GI) and black malleable iron (MI) fittings come out of the same mould, in the same figure numbers and the same sizes, and are rated to the same pressures. The only difference is a zinc coating - and that difference decides which services each one belongs in.

What galvanising actually is

A finished black fitting is dipped in molten zinc at around 450C. The zinc alloys with the iron at the surface rather than sitting on it as paint would, which is why a scratch does not start a rust line the way a scratch in paint does: the zinc around the damage corrodes in preference to the steel underneath and protects it. That is the whole mechanism, and it is also the whole limitation - once the zinc in an area is consumed, the protection there is finished.

Practically, galvanising adds a coating to the threads as well. Threads are cut before dipping, so a galvanised fitting is machined to allow for the coating thickness. This is why mixing a galvanised fitting with a black nipple can feel wrong on assembly, and why a fitting should not be re-cut on site after galvanising.

Where galvanised belongs

Anything carrying water that people drink, wash in or that sits still for long periods. Potable water distribution, plumbing risers, cold water services, and irrigation and landscape lines all take galvanised iron (GI). The zinc buys years in exactly the condition that kills bare iron: wet, oxygenated and intermittently drained.

Outdoor and buried work, and anything in a humid coastal atmosphere, is the other clear case - which in the Gulf means most plant rooms, roofs and external runs, where the salt-laden air attacks bare iron far faster than it would inland.

Where black is the right answer

Fire protection. Sprinkler and hydrant systems run black, and the reason is not cost: a wet sprinkler system is a sealed, static volume of water with no continuous oxygen supply, so internal corrosion is slow, and the UL Listed and FM Approved figures the specification calls for are the black ones.

Steam, condensate and hot water above roughly 60C. Zinc behaves differently at temperature, and above about 60C the galvanic relationship between zinc and steel can reverse in hot water - the coating stops protecting the iron and can begin to accelerate attack on it. Steam and condensate lines run black for that reason.

Compressed air, gas distribution and oil lines. These are dry or oily services where there is nothing for the zinc to protect against, and in gas work the specification is usually written around the DVGW-approved black figures.

Where it goes wrong

The two expensive mistakes are the same mistake in opposite directions. Galvanised fittings on a steam or high-temperature hot water line will not fail on the day of commissioning; they fail quietly over a year or two, and flakes of zinc find their way into valves and traps. Black fittings on a potable water line rust from the inside, discolour the water and eventually restrict it.

The third mistake is mixing the two on the same run. It works mechanically, but a galvanised fitting hard against black pipe puts two metals in contact in a wet line, and the less noble one gives way first. Standardise on one finish per service.

Both, from one order

Every TUPY figure from 1/4in to 6in is produced in both finishes, to EN 10242 and ISO 49 threads, so a project running galvanised risers and black sprinkler mains can take both from one supplier, one order and one set of mill certificates. The figure numbers do not change between finishes - only the finish column on the order does.

Perguntas frequentes

• Todas as conexões e figuras
• Dimensões e pesos
• Normas e dados técnicos
• Baixar os catálogos
• Class 150 or Class 300 malleable iron fittings?
• Threaded or press-fit? When TUPYPRES pays for itself
• TUPY figure numbers, and what the trade calls them