Types of metallic coatings on steel sheet and their interchangeability
A zinc coating is not the only way to protect steel sheet against corrosion, and it is less and less often the optimal one. EN 10346 defines six types of continuously hot-dip applied coatings, and EN 10152 adds an electrolytically applied coating. They differ in composition, corrosion resistance, behaviour at cut edges, service temperature range and suitability for painting. At the same coating mass, a zinc-magnesium coating loses thickness around 2.2 times more slowly than a conventional zinc coating, which means that ZM310 provides protection comparable to Z600 with almost half the amount of metal.
Knowing these differences stops being academic the moment a specific coating mass is in short supply on the market and the order lead time is pressing.
What the standard defines
EN 10346 covers continuously hot-dip coated flat products intended for cold forming. The coating symbol defines its composition, and the full sheet designation combines it with the steel grade.
| Symbol | Name | Composition to the standard |
|---|---|---|
| Z | zinc coating | minimum 99% zinc |
| ZF | zinc-iron, galvannealed | 8-12% iron, balance zinc |
| ZA | zinc-aluminium, Galfan | around 5% aluminium, balance zinc |
| ZM | zinc-aluminium-magnesium | aluminium plus magnesium 1.5-8%, minimum 0.2% magnesium |
| AZ | aluminium-zinc, aluzinc | 55% aluminium, 1.6% silicon, balance zinc |
| AS | aluminium-silicon, alusi | 8-11% silicon, balance aluminium |
The number after the symbol is the coating mass in grams per square metre, counted for both sides together. Z275 means 275 g/m2, i.e. around 20 µm per side. Available coating masses range from Z80 to Z725 for zinc, from ZM70 to ZM430 for zinc-magnesium, from ZA095 to ZA300 for Galfan and from AS50 to AS240 for aluminium-silicon.
Electrolytically applied coating, covered by EN 10152, is treated separately. Here the designations describe the thickness of each side separately: ZE25/25 means 2.5 µm and 18 g/m2 per side, ZE50/50 5 µm and 36 g/m2, ZE100/100 10 µm and 72 g/m2. Asymmetric variants are possible when one side of the product requires a thicker coating than the other.
Zinc-magnesium coatings
Adding magnesium and aluminium to the zinc bath changes the chemistry of the corrosion products. The resulting layer is more compact and adheres better, and when the coating is damaged, zinc products migrate over the exposed area and restore protection.
The best publicly documented assessment of this effect is a report by the independent Steel Construction Institute, prepared for ArcelorMittal. After six years of atmospheric exposure at non-coastal sites, the average ratio of thickness loss of the Z275 zinc coating to the ZM250 zinc-magnesium coating was 2.2, with a minimum of 1.5 over the entire test period. In the cyclic test the Z600 sample was fully covered with white rust after four weeks, while ZM310 showed only partial coverage after eleven weeks. In the salt spray test ZM310 reached the same rating as Z600 after roughly twice the time.
The report conclusion is clear: in UK atmospheric conditions and for typical light steel construction applications, the protection provided by ZM310 is not inferior to that of Z600, both on the surface and at cut edges.
The edge protection mechanism unfolds over time. In the first weeks the exposed steel core oxidises and rust appears. With further exposure to moisture, coating material spreads over the cut edge. After a year, iron corrosion products give way to white zinc products.
The composition of ZM coatings differs between producers, although all fall within the standard. The ArcelorMittal product contains 93.5% zinc, 3.5% aluminium and 3% magnesium. The voestalpine product contains 2.5% aluminium and 1.5% magnesium, balance zinc. When replacing one product with another, it is worth checking the material data sheet rather than the standard symbol alone.
Aluminium and aluminium-zinc coatings
Where temperature resistance matters, a zinc coating stops being the solution. Zinc remains stable up to around 200 degrees Celsius. Between 200 and 250 degrees the outer layer of free zinc begins to peel, because diffusion of zinc into the iron-zinc alloy layer creates voids. Above 250 degrees the process accelerates and the alloy layers may crack and detach from the steel.
The aluminium-silicon coating behaves differently. According to the producer data sheet it remains stable up to 450 degrees while retaining around 80% heat reflectivity, and withstands up to 650 degrees without peeling. The aluminium-zinc coating keeps a bright surface without discolouration up to 400 degrees and tolerates 650 degrees without heavy oxidation.
The aluminium-zinc coating also stands out for long-term durability. GalvInfo data indicate an advantage over a zinc coating of a mass corresponding to Z275 of around 3.8 times in a severe marine environment, 6.4 times in an industrial environment and 18.7 times in a rural environment. The mechanism is that the zinc-rich microstructure corrodes preferentially and fills the interdendritic spaces of the aluminium layer, so corrosion progresses parabolically rather than linearly as in a zinc coating.
This coating does, however, have clear limitations. It should not be used in contact with fresh, wet concrete, whose alkalinity attacks the aluminium layer, nor in livestock buildings, nor in contact with lead, graphite or copper.
Coatings for painting and appearance
The zinc-iron coating, known as galvannealed, is produced by diffusion annealing immediately after the zinc bath. An iron content of around 8-12% gives a fine-structured surface with very good paint adhesion, which makes it the first-choice material wherever the product is painted. The presence of iron also widens the welding current range and reduces electrode wear, which matters in high-volume production.
Electrolytically applied coating is brighter and more uniform, and its thickness can be controlled precisely and independently for each side. That is why it is used for visible parts with the highest visual requirements, especially painted ones.
Galfan, with its eutectic, lamellar microstructure, acts as a natural lubricant during forming, which gives it an advantage in deep drawing and roll forming. The producer declares corrosion resistance around twice that of a zinc coating, although the conditions of this comparison are not specified in detail in public materials.
When substitution makes sense
In procurement practice the question usually is whether another coating can be accepted instead of the ordered zinc coating mass without losing product properties. The table below sets out the most common situations.
| Original requirement | Possible substitution | Rationale |
|---|---|---|
| Z600 or Z450, external structure | ZM310 or ZM250 | comparable or better durability at a lower coating mass, confirmed by the institute study |
| Z275, product with many cut edges | ZM of similar coating mass | clearly better edge behaviour after cutting |
| Z275, part working above 250 degrees | AS or AZ | zinc coating is not intended for service in this range |
| Z, painted product | ZF | fine-structured surface and paint adhesion |
| Z, visible part with high visual requirements | ZE | uniform, bright surface and precise thickness control |
| Z, deep-drawn or roll-formed product | ZA | microstructure that eases forming |
| Z, contact with fresh concrete | do not substitute with AZ | concrete alkalinity attacks the aluminium layer |
Substitution is not a purely material decision. If the product is subject to conformity assessment, a structural design or a customer specification, a change of coating symbol requires approval by the purchaser. Where there are no such restrictions, flexibility in coating selection can be the difference between on-time delivery and downtime.
Welding zinc coatings
Zinc boils at around 907 degrees Celsius, well below the temperature of the welding arc. The coating in the weld zone evaporates and oxidises in air to zinc oxide, forming fumes inhaled by the operator. The American Welding Society identifies exposure to these fumes as the cause of metal fume fever, a condition with flu-like symptoms that appear a few hours after exposure and usually subside within one or two days.
The exposure limits adopted in the United States are 5 mg/m3 as an eight-hour time-weighted average and 10 mg/m3 as a short-term value. The practical recommendations are unchanged: keeping the head out of the fume plume, local exhaust at the arc, general ventilation and, where technical measures are insufficient, respiratory protection. A separate point is that any welding destroys the coating at the weld and damages it in the heat-affected zone, which requires protection to be restored with zinc-rich paint.
Practical selection criteria
Coating selection follows from four questions: what environment the product works in, how long it must last there, what happens at the cut edges, and whether the product will be painted, formed or heated. The answers rarely point to a single solution, but they usually rule out several.
In recent years the European market has clearly been shifting towards zinc-magnesium coatings, mainly because they achieve the same durability with less metal, and therefore with lower weight and a lower unit cost of coating. For the buyer this also means wider availability of alternatives when a specific zinc coating mass is temporarily hard to obtain.
Available steel grades, coating types and formats of coils, slit strip and sheets are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.
See how we can help
HEN-STOL helps select the coating type and mass for the service conditions of the product and indicates which substitutes are equivalent in a given application and which require agreement with the designer or customer. Send us the specification, and we will check availability and propose the most favourable option.
Frequently asked questions
Can a ZM coating replace Z600?
In atmospheric conditions and for light steel structures, ZM310 provides protection not inferior to Z600, as confirmed by an independent institute study covering both the surface and cut edges. For products covered by a customer specification, substitution requires the customer’s consent.
How thick is a Z275 coating?
A coating mass of 275 g/m2, counted for both sides together, corresponds to around 20 µm per side. The same conversion rule applies to other zinc coating masses.
Up to what temperature can galvanised sheet be used?
Zinc coating is intended for service up to around 200 degrees Celsius. Above 200-250 degrees the free zinc layer begins to peel. At higher temperatures, aluminium-silicon or aluminium-zinc coatings are used.
Which coating is best for painting?
The ZF zinc-iron coating, thanks to its fine-structured surface ensuring paint adhesion. For visible parts with the highest visual requirements, the electrolytic ZE coating is used.
Can galvanised sheet be welded?
Yes, provided that safety requirements for fume extraction and ventilation are met, because zinc evaporates at arc temperature and forms zinc oxide fumes. The coating in the weld zone must be restored, most often with zinc-rich paint.
How does Galfan differ from ordinary galvanising?
Galfan is a zinc-aluminium coating containing around 5% aluminium. Its microstructure eases forming, which gives an advantage in deep drawing and roll forming, and producers declare higher corrosion resistance than a pure zinc coating.
Sources
- EN 10346, Continuously hot-dip coated steel flat products for cold forming - technical delivery conditions
- EN 10152, Electrolytically zinc coated cold rolled steel flat products for cold forming
- The Steel Construction Institute, Review of Magnelis coating performance as corrosion protection ZM310, report RT1999
- GalvInfo Center, GalvInfoNote 1.4, 55 percent Aluminum-Zinc Alloy-Coated Steel Sheet
- American Galvanizers Association, Galvanized Steel Performance in Extreme Temperatures
- American Welding Society, Safety and Health Fact Sheet No. 25, Metal Fume Fever