GI vs MI Fittings: Galvanised and Malleable Iron Threaded Fittings Explained

"GI fittings" and "MI fittings" are ordered every day on building services projects, usually from the same supplier and often from the same shelf, yet the two terms describe different things: one is a coating, the other is a material. Understanding the difference tells you what you are actually buying, which standard to quote, which thread to ask for and where each fitting belongs. This article sets that out for the buyer who has to write a clear purchase order and the engineer who has to approve it.

One is a material, the other is a finish

MI stands for malleable iron. It is a cast iron that has been heat treated (annealed) so that the brittle carbides of the as-cast iron break down into nodules of temper carbon in a ferritic matrix; that structure is what makes it tough enough to be threaded, wrenched and mildly shock-loaded without cracking. ASTM A197 covers cupola malleable iron, the grade traditionally used for pipe fittings, and ASTM A47 covers ferritic malleable castings for general engineering use from ambient to about 400 C. In Europe, EN 10242 is the fitting standard and names the malleable iron requirements within it.

GI stands for galvanised iron. It describes a zinc coating applied by hot-dip galvanising, in which the fitting is immersed in molten zinc and zinc-iron alloy layers bond to the surface. A GI fitting is therefore almost always a malleable iron fitting that has been galvanised, and an MI fitting in the trade sense is the same casting left black (ungalvanised) or sometimes black-varnished. The question is not GI or MI as if they were two materials; it is whether the malleable iron fitting is galvanised or black.

The standards that define the fitting

In the metric world, EN 10242 specifies the design and performance of threaded malleable iron fittings with either a black or a hot-dip galvanised surface, for general fluid and gas service within the pressure and temperature limits set in the standard. It covers thread sizes 1/8 to 6 (DN 6 to DN 150), is written mainly for use with non-alloy steel tubes to EN 10255, and relies on sealing materials from the EN 751 series for the threaded joint.

In the American system, ASME B16.3 covers malleable iron threaded fittings in Classes 150 and 300. Class 150 is the standard pattern used for water, air and low-pressure steam; Class 300 has heavier walls and bands for higher pressures. The two systems are not interchangeable in detail: thread form, dimensions and pressure classes differ, so a purchase order should state which standard the fitting must meet and which thread it must carry. For the pressure and temperature limits, refer to the standard quoted.

Threads: ISO 7-1 and NPT are not the same

Most malleable iron fittings sold in the Gulf carry ISO 7-1 (BSPT) threads. This is a 55 degree Whitworth-form thread; the external thread R is always tapered, and the internal thread may be tapered (Rc) or parallel (Rp). Pressure tightness is made on the thread itself with a jointing compound or tape. ISO 228 G threads look similar but are parallel and do not seal on the thread; they must not be used where the thread is the seal.

NPT to ASME B1.20.1 is a 60 degree thread with a taper of 1 in 16 on diameter, and it also requires a sealant or PTFE tape for a leak-tight joint. A 55 degree BSPT thread screwed into a 60 degree NPT fitting may appear to engage but will not seal reliably. When steel pipe is threaded on site, the die set must match the fittings, and when imported equipment arrives with NPT connections, transition nipples or adaptors are needed.

Galvanising: what the coating standard promises

Two standards are commonly cited for the zinc coating. ISO 1461 is the general hot-dip galvanising standard for fabricated iron and steel articles; ASTM A153 covers hot-dip zinc coating on hardware, in Classes A to D, and is written for items that are centrifuged after dipping to remove excess zinc, which is exactly how small threaded fittings are processed so that the threads stay usable.

Coating thickness depends on the thickness of the article and on whether it is centrifuged. Under ISO 1461, non-centrifuged castings 6 mm and thicker require a minimum local coating of 70 microns and a mean of 80 microns, while centrifuged threaded articles over 6 mm require 40 microns local and 50 microns mean, and those 6 mm and under 20 and 25 microns respectively. Where coating thickness matters, write the standard and the class on the order and ask for the test report. The coating on the thread is thin and is partly cut away when the joint is made, so GI fittings protect the body of the fitting, not the thread flanks; a compatible jointing compound protects the exposed thread.

Temperature is the other limit. The American Galvanizers Association recommends continuous service below about 200 C for conventional galvanised coatings; between 200 and 250 C the outer zinc layer can peel, although the zinc-iron alloy layers continue to protect the steel. For hot condensate and steam lines above that range, black fittings are the normal choice.

Where each belongs

GI fittings go wherever water is present or the fitting is exposed: potable and domestic water, cooling and condenser water, irrigation, drainage vents, outdoor and plant-room pipework, and galvanised steel sprinkler systems where the specification calls for galvanised pipe and fittings. Black MI fittings go on fuel gas, oil, compressed air, steam and closed heating circuits, where there is little oxygen or moisture to drive corrosion and where the coating would add cost without benefit, and on black steel sprinkler systems. Keep the fitting consistent with the pipe: galvanised fittings on black pipe or black fittings on galvanised pipe create a corrosion cell at every joint and complicate inspection.

Threaded malleable iron fittings of either finish share the limits of threaded joints generally. They suit low-pressure, low-temperature services without vibration; where vibration, cyclic loading, hazardous fluids or higher pressures apply, forged steel threaded fittings to ASME B16.11 in Classes 2000, 3000 and 6000, or welded connections, are the correct step up. Refer to the fitting standard and the piping code for the pressure and temperature limits of a given class.

Buying checklist

• State the standard: EN 10242 or ASME B16.3, and the class or design symbol.
• State the thread: ISO 7-1 (Rc or Rp as required) or NPT.
• State the finish: hot-dip galvanised to ISO 1461 or ASTM A153 with the class, or black.
• Match the pipe: galvanised fittings with galvanised tube, black with black.
• Check the markings on delivery: size, manufacturer, standard and class should be cast or stamped on the fitting.
• Request mill or test certificates for the castings and the coating where the project requires them.

Key takeaways

• MI is the material (malleable iron to ASTM A197, A47 or EN 10242); GI is a hot-dip zinc finish on that material.
• GI fittings suit water and exposed services; black MI fittings suit gas, oil, air, steam and closed circuits.
• ISO 7-1 (55 degree BSPT) and NPT (60 degree, 1 in 16 taper) threads are not interchangeable; both need a sealant.
• Coating thickness is set by ISO 1461 or ASTM A153 and depends on article thickness and centrifuging; threaded articles carry thinner coatings than castings.
• Keep galvanised coatings below about 200 C in continuous service.
• For vibration, cyclic or hazardous duty, move up to forged steel fittings to ASME B16.11 or to welded joints.

Frequently asked questions

Can GI fittings be used on a black steel pipe system?

It is not good practice. Mixing finishes creates a galvanic pair at each joint and makes it impossible to tell at a glance whether a system is protected. Use black fittings on black pipe and galvanised fittings on galvanised pipe unless the specification states otherwise.

Are GI fittings suitable for hot water?

Yes, for domestic and heating hot water within the fitting standard's temperature limits. The galvanised coating is the constraint on much hotter services; continuous service should stay below about 200 C, which is well above any building hot water system but relevant to steam and condensate.

What is the difference between a GI fitting and a galvanised forged steel fitting?

The GI fitting is a malleable iron casting to EN 10242 or ASME B16.3 with a zinc coating; a galvanised forged steel fitting is a forged carbon steel part to ASME B16.11 in Class 2000, 3000 or 6000, which is heavier, stronger and rated for higher pressures. The forged fitting is specified where the pressure class, vibration or fluid rules out cast malleable iron.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company supplies GI threaded fittings and accessories and MI threaded fittings and accessories through its GI and MI fittings range, together with forged threaded fittings for higher-duty lines. Send us the schedule with sizes, finish, thread standard and fitting standard and our team will confirm availability and specification before quoting; contact us to start.